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A novel photosynthetic-antenna-reaction-center model compound, comprised of BF2 -chelated dipyrromethene (BODIPY) as an energy-harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole-shifting agent, and phenylimidazole-functionalized fulleropyrrolidine (C60 Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy-level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the (1) ZnP* (formed either as a result of singlet-singlet energy transfer or direct excitation) or (1) C60 * of the coordinated fullerene resulting into the formation of the Fc-(C60 (.) (-) Im:ZnP(.) (+) )-BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc(+) -(C60 (.) (+) Im:ZnP)-BODIPY radical ion pair that persisted for 7-15 μs, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC60 , as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long-lived charge-separated state with the help of the present "antenna-reaction-center" model system has been successfully demonstrated.

Achieving ultrafast light-induced charge separation requires carefully selected donor and acceptor entities often held together in a closer proximity. In the present investigation, two tetrads featuring a near-IR-emitting sensitizer, BF2-chelated dipyrromethene (azaBODIPY), covalently linked to a fullerene through the central boron atom and two electron-donor entities, namely, N,N-dimethylaminophenyl, or bithiophene in close proximity to the fullerene entity were designed and synthesized, and the results were compared to the earlier-reported bisferrocene–azaBODIPY–fullerene tetrads in this series. The tetrads synthesized by establishing a multistep procedure exhibited typical spectral, redox, and photo reactivities of the individual components with some degree of intramolecular interaction. The X-ray structure of one of the precursor triads was also solved as part of this investigation. Ultrafast photoinduced electron transfer was witnessed in the case of both tetrads by femtosecond transient absorption s...

A novel multimodular donor-acceptor polyad featuring zinc porphyrin, fullerene, ferrocene, and triphenylamine entities was designed, synthesized, and studied as a charge-stabilizing, photosynthetic-antenna/reaction-center mimic. The ferrocene and fullerene entities, covalently linked to the porphyrin ring, were distantly separated to accomplish the charge-separation/hole-migration events leading to the creation of a long-lived charge-separated state. The geometry and electronic structures of the newly synthesized compound was deduced by B3LYP/3-21G(*) optimization, while the energy levels for different photochemical events was established using data from the optical absorption and emission, and electrochemical studies. Excitation of the triphenylamine entities revealed singlet-singlet energy transfer to the appended zinc porphyrin. As predicted from the energy levels, photoinduced electron transfer from both the singlet and triplet excited states of the zinc porphyrin to fullerene followed by subsequent hole migration involving ferrocene was witnessed from the transient absorption studies. The charge-separated state persisted for about 8.5 μs and was governed by the distance between the final charge-transfer product, that is, a species involving a ferrocenium cation and a fullerene radical anion, with additional influence from the charge-stabilizing triphenylamine entities located on the zinc-porphyrin macrocycle.

New multi-modular donor- acceptor conjugates featuring zinc por- phyrin (ZnP), catechol-chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C60), or naphthalenedi- A (NDI) have been newly designed and synthesized as photosynthetic an- tenna and reaction-center mimics. The X-ray structure of triphenylamine-BDP is also reported. The wide-band captur- ing polyad revealed ultrafast energy- transfer (kENT = 1.0 � 10 12 s � 1 ) from the singlet excited BDP to the covalently linked ZnP owing to close proximity and favorable orientation of the enti- ties. Introducing either fullerene or naphthalenediimide electron acceptors to the TPA-BDP-ZnP triad through metal-ligand axial coordination result- ed in electron donor-acceptor polyads whose structures were revealed by spectroscopic, electrochemical and computational studies. Excitation of the electron donor, zinc porphyrin re- sulted in rapid electron-transfer to co- ordinated fullerene or naphthalenedi- A yielding charge separated ion- pair species. The measured electron transfer rate constants from femtosec- ond transient spectral technique in non-polar toluene were in the range of 5.0 � 10 9 -3.5 � 10 10 s � 1 . Stabilization of the charge-separated state in these multi-modular donor-acceptor polyads is also observed to certain level.

A near-IR-emitting sensitizer, boron-chelated tetraarylazadipyrromethane, has been utilized as an electron acceptor to synthesize a series of dyads and triads linked with a well-known electron donor, ferrocene. The structural integrity of the newly synthesized dyads and triads was established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a 'molecular clip'-type structure for the triads wherein the donor and acceptor entities were separated by about 14 Å. Differential pulse voltammetry combined with spectroelectrochemical studies have revealed the redox states and estimated the energies of the charge-separated states. Free-energy calculations revealed the charge separation from the covalently linked ferrocene to the singlet excited ADP to yield Fc(+)-ADP(•-) to be energetically favorable. Consequently, the steady-state emission studies revealed quantitative quenching of the ADP fluorescence in all of the investigated dyads and triads. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of photoinduced electron transfer in these donor-acceptor systems by providing spectral proof for formation of ADP radical anion (ADP(•-)) which exhibits a diagnostic absorption band in the near-IR region. The kinetics of charge separation and charge recombination measured by monitoring the rise and decay of the ADP(•-) band revealed ultrafast charge separation in these molecular systems. The charge-separation performance of the triads with two ferrocenes and a fluorophenyl-modified ADP macrocycle was found to be superior. Nanosecond transient absorption studies revealed the charge-recombination process to populate the triplet ADP as well as the ground state.

A multimodular donor-acceptor system composed of three triphenylamine entities at the meso-positions of a zinc porphyrin macrocycle and a quinone at the fourth meso-position was newly synthesized and characterized. The triphenylamine entities acted as energy transferring antenna units in addition of improving the electron donor ability of the zinc porphyrin. Appreciable electronic interactions of the triphenylamine and quinone entities with the porphyrin π-system were observed. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier HOMO over the triphenylamine and the porphyrin macrocycle while the LUMO to be fully localized over the quinone entity. Free-energy calculations suggested photoinduced electron transfer from the singlet excited zinc porphyrin to the directly linked quinone to be exothermic and this was experimentally confirmed by the time-resolved pump probe and up-conversion techniques. In the investigated system, the ET reaction path was found to depend upon the excitation wavelength. That is, when Zn porphyrin was predominantly excited, a rapid charge separation followed by equally fast charge recombination was observed. However, excitation of the peripheral TPA substituents resulted in an extremely long-lived CS state with triplet spin character via the TPA triplet and Zn porphyrin triplet states.

A series of molecular triads, composed of closely positioned boron dipyrrin-fullerene units, covalently linked to either an electron donor (donor(1)-acceptor(1)-acceptor(2)-type triads) or an energy donor (antenna-donor(1)-acceptor(1)-type triads) was synthesized and photoinduced energy/electron transfer leading to stabilization of the charge-separated state was demonstrated by using femtosecond and nanosecond transient spectroscopic techniques. The structures of the newly synthesized triads were visualized by DFT calculations, whereas the energies of the excited states were determined from spectral and electrochemical studies. In the case of the antenna-donor(1)-acceptor(1)-type triads, excitation of the antenna moiety results in efficient energy transfer to the boron dipyrrin entity. The singlet-excited boron dipyrrin thus generated, undergoes subsequent energy and electron transfer to fullerene to produce a boron dipyrrin radical cation and a fullerene radical anion as charge-separated species. Stabilization of the charge-separated state in these molecular triads was observed to some extent.

Singlet excitation transfer in self-assembled dyads formed via axial coordination of imidazole appended free-base tetraphenylporphyrin, H2PIm , to either a magnesium phthalocyanine, MgPc , or a magnesium naphthalocyanine, MgNc is investigated in non-coordinating solvents using spectroscopic (optical and mass), electrochemical, and time-resolved transient absorption techniques. The newly assembled dyads are fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral data are found to be in the range of 103–104 M-1 for the 1:1 dyads suggesting fairly stable complex formation. However, the anticipated 2:1 complex for Mg coordination is not observed under the present solution conditions. Electrochemical and computational studies suggested that photoinduced electron transfer to be a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. However, selective excitation of H2PIm entity in these dyads resulted in rapid excitation transfer and the position of the imidazole linkage on the H2P entity seem to direct the overall efficiency of excited energy transfer. Kinetics of energy transfer is monitored by transient absorption measurements using pump-probe technique and is compared with the earlier reported H2PIm:ZnPc and H2PIm:ZnNc dyads. The time constants are in the order of 0.2–18 ps depending upon the type and relative orientation of the donor and acceptor entities of the dyad indicating ultrafast excitation transfer, and agree fairly well with theoretically estimated values assuming Förster energy transfer mechanism.

An adenine-templated molecularly imprinted polymer (MIP) film, deposited on a poly(bithiophene) barrier film, served as the recognition element of a piezomicrogravimetric (acoustic) chemosensor. A 10MHz AT-cut shear-thickness-mode bulk-acoustic-wave quartz crystal resonator with Pt film electrodes was used as the signal transducer. Adenine electrooxidation was prevented by the barrier film. The MIP film was deposited by electrochemical co-polymerization of two functional monomers of bis(bithiophene) derivatives, bearing either the 18-crown-6 or dioxaborinane substituent, in the presence of the adenine template. A strong base solution was then used to extract the template. Completeness of the template removal was substantiated by the UV-vis, XPS, DPV, and EIS measurements. The chemosensor performance was evaluated with the piezoelectric microgravimetry detection at QCM under FIA conditions using a carrier acetonitrile-water (1:1, v:v) mixed solvent solution. The linear dynamic concentration range extended from at least 0.1 to 1mM for the 35 microL/min flow rate, and 100 microL volume of the injected adenine solution. The chemosensor selectivity allowed for discrimination of the adenine analyte from structurally and functionally related interferants, such as 2-aminopurine, guanine, and ascorbic acid. The determined from the FIA kinetic studies stability constant of the MIP-adenine complex, (18+/-2.4)x10(4)M(-1), was much higher than that of the MIP-(2-aminopurine), (650+/-90)M(-1), MIP-guanine, (122+/-11)M(-1), and MIP-(ascorbic acid), (92+/-10)M(-1), complexes. The concentration limit of detection was as low as 5 nM adenine for the 35 microL/min flow rate, and 1 mL volume of the injected sample solution.

Supramolecular ferrocene−fullerene constructs in which the donor, ferrocene linked to a benzo-18-crown-6 entity (Fc-crown), was self-assembled with the acceptor, fullerene bearing one or two alkyl ammonium ions (NH3+−C60), yielding dyads or a triad, respectively. The newly formed conjugates were characterized by spectroscopic (fluorescence, electospray ionization-mass, and 1H NMR) and electrochemical methods. The adopted crown ether−alkyl ammonium ion binding strategy resulting in stable donor−acceptor conjugates was also supported by the computational studies performed at the DFT B3LYP/3-21G(*) level in addition to the binding constants obtained from fluorescence quenching studies. The experimentally calculated free-energy changes indicated exothermic light-induced charge-separation process. Accordingly, efficient photoinduced charge-separation processes were confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. The rates of charge recom...

Using a diacetylamidopyridine/uracil complementary hydrogen-bonding motif, a novel bis(zinc porphyrin)−fullerene supramolecular triad is constructed and characterized. The geometry of the triad ded...

Supramolecular triads composed of free-base porphyrin, fullerene, and ferric porphyrin were constructed by using “covalent-coordinate” binding strategy. For this, a free-base porphyrin was covalently linked to a fullerene entity bearing a pyridine ligand, and was subsequently utilized to coordinate ferric porphyrins bearing different peripheral substituents. The newly formed triads were characterized by spectral methods and the structures were deduced from DFT B3LYP/3-21G(*) methods. Free-energy calculations performed with use of the redox potential and emission data suggested the occurrence of sequential electron transfer from singlet excited free-base porphyrin to the covalently linked fullerene, followed by an electron transfer from fullerene anion radical to ferric porphyrin, ultimately generating free-base porphyrin cation radical and ferrous porphyrin as the electron-transfer products, anticipating the generation of long-lived charge-separated species as a consequence of distant separation between t...

Supramolecular triads composed of zinc naphthalocyanine ( ZnNc ) as an electron donor, pyridylfullerenes as primary electron acceptors, and either ferrocene ( Fc ) or N , N -dimethylaminophenyl ( DMA ) entities as second electron donors were constructed via metal-ligand axial coordination. The formation of the supramolecular triads was monitored using optical absorption methods and the measured binding constants revealed a moderately stable complex formation. The B3LYP/3-21G(*) optimized structures showed the disposition of the three entities of the triads to be in a linear fashion. The redox behavior of the different components was studied using cyclic voltammetry in o-dichlorobenzene containing (n- C 4 H 9)4 NClO 4. The donor ability evaluated from the oxidation potentials of the electron donors followed the trend: ZnNc < Fc < DMA . Efficient electron transfer from the excited singlet state of zinc naphthalocyanine to the fullerene entity was observed in all of the studied triads in o-dichlorobenzene by fluorescence quenching measurements. Clear evidence, that the charge-separation process formed the radical ions in the triads, was obtained from a nanosecond transient absorption spectral technique, indicating the contribution of the second electron donor in prolonging the charge-separated states.

DFT calculations were performed on covalently linked porphyrin–quinone and porphyrin–hydroquinone dyads, and quinhydrone (1:1 quinone–hydroquinone) paired self-assembled supramolecular complexes. The frontier HOMO's and LUMO's of the B3LYP/3-21G(*) and PBEPBE/6-31G(d) optimized structures in the ground state were utilized to probe the site of electron transfer (porphyrin or quinone centered redox reactions) and the results were compared to the experimentally determined site of electron transfer. These studies revealed good agreement between the experimental results and computational predictions. The computed HOMO–LUMO energy gap also followed the trends of the electrochemical results. In the optimized structure of quinhydrone supramolecules, the quinone–hydroquinone entities were stacked on one another with an inter-planar distance of ∼3.1 A. The calculated O–H–O distances suggested the presence of hydrogen bonds. The prediction of the structure and site of electron transfer suggests the usefulness of DFT calculations for probing the self-assembly process and redox behavior of structurally complex molecular and supramolecular systems. The B3LYP/3-21G(*) calculated association energy (the energy of the quinhydrone complex relative to the sum of the energies of the individual entities) for the quinhydrone formation was found to range between −7 and −15 kcal/mol, suggesting quinhydrone as a viable self-assembly mechanism to form supramolecular complexes. The B3LYP/3-21G(*) results were further improved by the PBEPBE/6-31G* method with solvation. Calculations performed at larger basis sets including 6-31G*, 6-31+G*, 6-31++G*, 6-311G*, 6-311G (d,p), and 6-311G (2df, dp) either at HF, BLYP, B3LYP, B3PW91, PBEPBE and MPW1PW91 methods also predicted negative association energies for the simple quinone:hydroquinone (quinhydrone) formation, with smaller association energies at the higher basis sets presumably due to reduced BSSE. Adding the basis set superposition error (BSSE), computed by counterpoise method, over-corrected the B3LYP/3-21G(*) association energy, but adding 50% of BSSE converged rapidly to the large basis set result. Larger association energies were obtained for the porphyrin–quinhydrone assemblies using PBEPBE/6-31G* calculations, presumably due to larger intermolecular ‘dispersion’ forces predicted with the PBEPBE method.

Photochemical properties of pyrene and fluorene appended fulleropyrrolidine triads (AH1–C60–AH2; AH1 = pyrene and fluorene; AH2 = naphthalene and phenyl) are reported. Electrochemical studies using cyclic voltammetry technique and DFT calculations at B3LYP/3-21G(∗) method revealed that the charge-separated states in pyrene and fluorene appended triads are pyrene + – C 60 - – AH 2 and fluorene + – C 60 - – AH 2 , respectively; however, no such charge-separated states could be established for naphthalene and phenyl appended triads. As demonstrated from the time resolved fluorescence, upon excitation of AH moiety in nonpolar solvents, energy transfer predominantly occurred from the singlet excited fluorophore to the C60 moiety, whereas in polar DMF charge-separation also contributed to the fluorescence quenching. Additionally, charge separation also occurred from the singlet excited C60 to the pyrene or fluorene entities of the triads in DMF. The rates and quantum yields of charge separation obtained by time-resolved emission studies were around 109 s−1 and 0.9–0.6 for pyrene–C60–AH2 and fluorene–C60–AH2 triads. Nanosecond transient absorption spectral studies performed by using 355 nm laser light on the triads, exhibited transient bands corresponding to the C 60 - and pyrene + or fluorene +, thus establishing the occurrence of electron transfer in these triads in DMF. The rates of charge recombination obtained by monitoring the decay of the C 60 - were found to be around 106 s−1 in DMF which resulted in the lifetimes of the radical ion pairs up to 1000 ns indicating charge stabilization in pyrene–C60–AH2 and fluorene–C60–AH2 triads. The formations of long-lived charge-separated states, pyrene + – C 60 - – AH 2 and fluorene + – C 60 - – AH 2 in DMF, were rationalized by evaluating the Marcus parameters from the temperature dependence of the charge-recombination rate constants.

The first example of a porphyrin-quinonoid donor–acceptor triad featuring (tetraphenylporphinato)zinc(II) moieties covalently attached to an oxoporphyrinogen through its macrocyclic nitrogen atoms is reported. This arrangement of chromophores results in an interesting interplay between the electron-donating zinc–porphyrin(s) and the electron/energy accepting oxoporphyrinogen. The optical absorption of the triad reveals features corresponding to both the donor and acceptor entities. The geometry and electronic structure of the triad deduced from B3LYP/3-21G(*) calculations reveal an absence of inter-chromophoric interactions and localization of the HOMO on one zinc–porphyrin group and the LUMO on the oxoporphyrinogen scaffold. The electrochemical redox states of the triad were established from a comparative electrochemistry of the triad and the reference compounds. Both steady-state and time-resolved emission studies revealed quenching of the singlet excited state of zinc–porphyrin in the triad, and the free-energy calculations performed using Weller’s approach indicate the possibility of electron transfer from the singlet excited zinc–porphyrin group to the oxoporphyrinogen in polar solvents. Time-resolved fluorescence studies reveal excited state energy transfer from zinc–porphyrin to oxoporphyrinogen in nonpolar solvents, while nanosecond transient absorption studies combined with time-resolved fluorescence studies in polar solvents are indicative of the occurrence of photoinduced charge separation from the singlet excited zinc–porphyrin to the oxoporphyrinogen. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Fullerene, C(60) was functionalized to possess one or two fluorophore entities. The fluorophore-fullerene dyads thus synthesized contain either a naphthalene, pyrene, or fluorene entity while the triads contain either a pyrene or fluorene entity in addition to a naphthalene entity. The redox behavior of these dyads and triads were probed by cyclic voltammetric technique, while the geometry and electronic structures were deduced from ab initio B3LYP/3-21G(*) method. Steady-state emission studies revealed the occurrence of energy transfer from the singlet excited fluorophore to the fullerene entity in the case of the dyads while the occurrence of step-by-step sequential energy transfer is envisioned in the case of the triads. A better 'antenna-effect' owing to the extended range of excitation wavelength to induce energy transfer to the appended fullerene has been achieved in the case of the triads.

Photoactive supramolecules composed of electron donor and electron acceptor entities are important for light energy harvesting applications. In the present study, a Star Wars Tie Fighter Ship shaped supramolecular triad was constructed by self-assembling two zinc naphthalocyanines to a fulleropyrrolidine bearing two pyridine entities using an axial coordination approach. Optical absorption and emission studies revealed stable complex formation, and the experimentally determined free-energy change revealed the possibility of electron transfer from singlet excited zinc naphthalocyanine to the fulleropyrrolidine. The picosecond time-resolved emission technique was utilized to evaluate the kinetics of charge separation while nanosecond transient absorption spectral studies provided evidence for electron transfer quenching. The measured charge-separation rate, k CS and quantum yield, Φ CS were found to be 5.7 × 109 s −1 and 0.93 in toluene, respectively, indicating an efficient process within the supramolecular triad. The charge recombination rate (k CR ) of the supramolecular ion-pair calculated from the nanosecond transient absorption technique was found to be 3.5 × 107 s −1 yielding a lifetime for the radical ion-pair (τ RIP ) of about 30 ns. Changing the solvent from the noncoordinating toluene to the coordinating benzonitrile or THF destroyed the supramolecular structure, and under these experimental conditions, only intermolecular electron transfer from the triplet excited zinc naphthalocyanine to fulleropyrrolidine could be observed. Under these conditions, the measured electron transfer rates, k et , T inter , were found to be 2.6 × 107 M −1. s −1 in benzonitrile and 1.2 × 107 M −1. s −1 in THF, respectively.

Slow evaporation of a mixture of organic solvents containing meso-tetraphenylporphyrinato-magnesium(II), MgTPP and N -methyl-2-(4'- N -imidazolylphenyl)-fulleropyrrolidine, C 60 Im yielded, air stable, dark purple crystals. Determination of the structure by single crystal X-ray diffraction revealed two forms of slightly varying self-assembled via axial coordination MgTPP : C 60 Im dyad. The Mg - N distance of the newly formed coordinate bond was found to be 2.131 Å which was close to the porphyrin ring Mg - N bond distances. The edge-to-edge distances were found to be 9.024 and 9.269 Å, respectively, for the two structure forms. The center-to-center distance between the magnesium and fullerene was 13.296 Å irrespective of the structural forms. Additional intermolecular type interactions between the magnesium porphyrin and fullerene entities that are not axially coordinated were also observed. DFT calculations performed at the B3LYP/3-21G(*) level on the fullerene and porphyrin segments suggested that the charge transfer type π–π interactions are primarily responsible for the intermolecular association of the presently studied donor-acceptor dyad.

Spectroscopic, redox, and electron transfer reactions of a self-assembled donor-acceptor dyad formed by axial coordination of magnesium meso-tetraphenylporphyrin (MgTPP) and fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) were investigated. Spectroscopic studies revealed the formation of a 1:1 C(60)Im:MgTPP supramolecular complex, and the anticipated 1:2 complex could not be observed because of the needed large amounts of the axial coordinating ligand. The formation constant, K(1), for the 1:1 complex was found to be (1.5 +/- 0.3) x 10(4) M(-1), suggesting fairly stable complex formation. The geometric and electronic structures of the dyads were probed by ab initio B3LYP/3-21G() methods. The majority of the highest occupied frontier molecular orbital (HOMO) was found to be located on the MgTPP entity, while the lowest unoccupied molecular orbital (LUMO) was on the fullerene entity, suggesting that the charge-separated state of the supramolecular complex is C(60)Im(*-):MgTPP(*+). Redox titrations involving MgTPP and C(60)Im allowed accurate determination of the oxidation and reduction potentials of the donor and acceptor entities in the supramolecular complex. These studies revealed more difficult oxidation, by about 100 mV, for MgTPP in the pentacoordinated C(60)Im:MgTPP compared to pristine MgTPP in o-dichlorobenzene. A total of six one-electron redox processes corresponding to the oxidation and reduction of the zinc porphyrin ring and the reduction of fullerene entities was observed within the accessible potential window of the solvent. The excited state events were monitored by both steady state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene, upon coordination of C(60)Im to MgTPP, the main quenching pathway involved electron transfer from the singlet excited MgTPP to the C(60)Im moiety. The rate of forward electron transfer, k(CS), calculated from the picosecond time-resolved emission studies was found to be 1.1 x 10(10) s(-1) with a quantum yield, Phi(CS), of 0.99, indicating fast and efficient charge separation. The rate of charge recombination, k(CR), evaluated from nanosecond transient absorption studies, was found to be 8.3 x 10(7) s(-1). A comparison between k(CS) and k(CR) suggested an excellent opportunity to utilize the charge-separated state for further electron-mediating processes.

The effect on the electrochemical oxidation and reduction potentials of 5,10,15,20-tetrakis(benzo-15-crown-5)porphyrin (TCP) and its metal derivatives (MTCP; M = Mg(II), VO(IV), Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Ag(II)) upon potassium ion induced dimerization of the porphyrins was systematically performed in benzonitrile containing 0.1 M (TBA)ClO(4) by differential pulse voltammetry technique. The HOMO--LUMO energy level diagram constructed from the electrochemical data revealed destabilization of the HOMO level and stabilization of the LUMO level upon dimer formation while such a perturbation was larger for the HOMO level than the LUMO level. The geometry and electronic structure of a representative ZnTCP and its dimer, K(4)(ZnTCP)(2), were evaluated by the ab initio B3LYP method utilizing a mixed basis set of 3-21G(*) for Zn, K, O, and N and STO-3G for C and H. The inter-porphyrin ring distance of the dimer calculated from the optimized geometry agreed with the spectroscopically determined one, and the calculated HOMO and LUMO frontier orbitals revealed delocalization on both of the porphyrins rings. The metal-metal distances calculated from the triplet ESR spectra of the K(+) induced porphyrin dimers bearing paramagnetic metal ions in the cavity followed the trend Cu--Cu < VO--VO < Ag--Ag. However, the spectral shifts resulting from the exciton coupling of the interacting porphyrin pi-systems revealed no specific trend with respect to the metal ion in the porphyrin cavity. Additionally, linear trends in the electrochemically measured HOMO--LUMO gap and the energy corresponding to the most intense visible band of both MTCP and K(4)(MTCP)(2) were observed. A reduced HOMO--LUMO gap predicted for the dimer by B3LYP/(3-21G(), STO-3G) calculations was confirmed by the results of optical absorption and electrochemical studies.

Supramolecular triads have been constructed by using covalently linked zinc porphyrin−ferrocene(s) dyads, self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads were characterized by optical absorption, computational, and electrochemical methods. The calculated binding constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced charge-separation and charge-recombination processes were examined in the dyads and triads by means of time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin−ferrocene(s) dyads, upon photoexcitation, efficient (ΦCS = 0.98) to moderate (ΦCS = 0.54) amounts of electron transfer from the ferrocene to the singlet excited z...

Stable with defined distance and orientation, self-assembled supramolecular triads composed of N,Ndimethylaminophenylfullerene-pyridine bound to zinc porphyrins by a newly developed “two-point” binding strategy are reported. For this, zinc porphyrin was derivatized to bear “hydrogen-bonding” functionalities, carboxylic acid or amide groups, and C60 was functionalized to bear a ligating group, pyridine, and a second electron donor, N,N-dimethylaminophenyl group. The supramolecular triads formed by self-assembly of the zinc porphyrin and fullerene derivatives via the two-point binding method were characterized by spectroscopic and electrochemical techniques and were modeled by using ab initio computational methods. Evidence for axial coordination of the pyridine entity to the zinc metal center and H-bonding between the pyrrolidine ring nitrogen and the pendant carboxylic acid or amide groups was obtained. In the supramolecular triads, the second electron donor, N,N-dimethylaminophenyl, promotes efficient charge separation upon excitation of the zinc porphyrin to yield the radical ion pairs. The radical ion pairs thus generated undergo slow charge recombination to yield relatively long-lived (30-40 ns) charge-separated states.

The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of covalently linked porphyrin−C60 dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II) derivative are functionalized at the ortho or para positions of one of the aryl groups to bear a fulleropyrrolidine entity through a flexible ethylene dioxide bridge to probe the effect of intramolecular electron transfer phenomena. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized dyads exhibit seven one electron reversible redox reactions within in the potential window of the solvent and the measured redox potentials and UV−visible absorption spectra reveal charge−transfer interactions between the electron donor, porphyrin, and the electron acceptor, fullerene entities. The geometric and electronic structures of the dyads probed by ab initio B3LYP/3-21G(*) methods also revealed the existence of charge−transfer interactions. The excited state photochemical events are monitored by both ...

A new C60-uracil adduct capable of hydrogen bonding, via complimentary base pairing, of adenine, adenosine, and adenosine 5′-triphosphate (ATP) was synthesized and characterized by UV-visible and 1H NMR spectroscopy, cyclic voltammetry and differential pulse voltammetry, as well as ESI-mass spectrometry. Molecular modeling by ab initio B3LYP/3–21G(*) calculations revealed the Watson–Crick type base pairing. Stable “expanded liquid” Langmuir films of the C60-uracil–adenine, C60-uracil–adenosine and C60-uracil–ATP complexes were prepared and characterized by isotherms of surface pressure versus area per molecule as well as the Brewster angle microscopy imaging. The area per molecule at infinite adduct dilution in the film was dependent on composition of the subphase solution and increased in the order: water < adenine < adenosine < ATP solution. Comparison of experimental and calculated areas per molecule and dipole moment components normal to the subphase–air interface indicated prevailing horizontal orientation of the complexes in the films. The Langmuir films were transferred, by using the Langmuir–Blodgett technique, onto quartz slides and characterized by the UV-visible spectroscopy.

Supramolecular triads of the type (donor-1)−acceptor:(donor-2) composed of free-base porphyrin, fullerene, and zinc porphyrin, have been formed by a “covalent-coordinate” approach. Toward this, two diads, namely, 5-(3‘-(2‘ ‘-(3‘ ‘‘ or 4‘ ‘‘-pyridyl)fulleropyrrolidinyl-N)ethoxyphenyl)-10,15,20-triphenylporphyrin, bearing C60 as acceptor and free-base porphyrin, H2P, as donor were first synthesized. The diads and self-assembled supramolecular triads, which were formed by coordinating the pyridine group located on the diads to zinc tetraphenylporphyrin, ZnP, have been characterized by semiempirical PM3, electrochemistry, and steady-state and time-revolved spectroscopic techniques. Subpicosecond and nanosecond transient absorption spectral studies of diads revealed the occurrence of the electron transfer from the H2P moiety to the C60 entity via the excited singlet state of H2P. Clear evidence for the formation of triads in o-dichlorobenzene was obtained from the steady-state and time-resolved fluorescence me...

Spectroscopic, redox, and photochemical behavior of self-assembled donor−acceptor dyads formed by axial coordination of zinc tetraphenylporphyrin, (TPP)Zn, and fulleropyrrolidine bearing either pyridine or imidazole coordinating ligands were investigated. The UV−vis, 1H NMR, and ESI-mass spectral studies, as well as computational studies, revealed supramolecular 1:1 dyad formation between the electron donor [(TPP)Zn] and the electron acceptor, fulleropyrrolidine entities. The determined formation constant K values followed the order o-pyridyl ≪ m-pyridyl ≃ p-pyridyl ≪ N-phenyl imidazole entities of the fulleropyrrolidine. The evaluated thermodynamic parameters revealed stable complexation with complex dissociation enthalpies ranging between 26 and 32 kJ mol-1. The 1H NMR studies revealed axial coordination of the pyridine or imidazole ligands to the central zinc of (TPP)Zn, while the ESI-Mass spectral studies performed in CH2Cl2 matrix revealed the expected molecular ion peak of the self-assembled dyads. ...

Synthesis and physicochemical characterization of a molecular triad comprised of ferrocene, C60, and dinitrobenzene entities is reported. Electrochemical studies revealed multiple redox processes involving all three redox active, ferrocene, C60, and dinitrobenzene entities. A total of eight reversible redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO4 are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triad is elucidated by using ab initio B3LYP/3-21G(*) methods. In the energy optimized structure, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, whereas the first LUMO orbitals are mainly on the C60 entity with small orbital coefficients on the dinitrobenzene entity. The subsequent LUMO's track the obs...

The acid−base equilibria of 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines in aqueous micellar media of a sodium dodecyl sulfate surfactant are examined experimentally and modeled theoretically by using ab initio 3-21G(*) methods. The pKa values, for 2-(n-alkyl)fulleropyrrolidines and N-methyl-2-(n-alkyl)fulleropyrrolidines, determined both by HCl and NaOH titration with potentiometric detection of the end point, are equal to 6.3 ± 0.1 and 7.5 ± 0.1, respectively, and are independent of the alkyl chain length within a range of C1 to C8. The obtained results indicate that the fullerene cage fused to a pyrrolidine ring increases the acidity of the protonated pyrrolidine nitrogen mainly due to the induced electronic effects and, to a smaller extent, by structural effects. The 3-21G(*) estimated free energy difference, ΔE (with solvation corrections incorporated based on the Cramer-Truhlar SM5.4 solvation model), between the protonated and unprotonated fulleropyrrolidines, and the ...

Semiempirical calculations using AM1 and PM3 have been performed on the zinc(II) and magnesium(II) complexes of nine structural isomers of tetrapyrrole macrocycles such as porphyrin, porphycene, corrphycene and hemiporphycene, N-confused porphyrin and other isomers that have not been synthesized. The optimized geometry and the bond parameters obtained compare favorably with results obtained from X-ray and spectral studies. Heats of formation, ionization potentials, HOMO-LUMO energy differences, dipole moments, and the splitting of HOMOs and LUMOs of the metal complexes of each of these isomers are also reported and compared with experimental results. The “four-orbital model” of Gouterman remains valid for the investigated structural isomers. The present study represents an unusually appropriate opportunity to study, via molecular orbital methods, the interaction between various metal ions, and the electronic and geometrical environment of the central cavity of the closely related isomeric macrocycles. The major outcome of this study is the verification of the expected differential behavior of metal ions employed in the present study as a sophisticated probe of cavity properties which also suggests that this procedure can be extended to other metal complexes. This study also serves as an interesting prototype for more elaborate ab initio calculations. However, such calculations on the presently investigated macrocyclic systems may have to be performed at a higher level than MP2 or DFT to account for the unusual delocalization, as suggested by a recent study by Schaefer and co-workers on delocalized [10]annulene (H.M. Sulzbach, H.F. Schaefer, W. Klopper and H.P. Luthi, J. Am. Chem. Soc., 118 (1996) 3519).

Charge-transfer interactions of octaethylporphycenatozinc(II), ( OEPc ) Zn with 2,6-dichloro-3,5-dicyano-1,4-benzoquinone, DDQ, in non-aqueous solvents are reported. Both optical absorption and cyclic voltammetry studies reveal the formation of stable charge-transfer complexes between ( OEPc ) Zn and DDQ. New redox couples corresponding to reduction of the charge-transfer complex have been electrochemically detected. The formation of charge-transfer complexes between ( OEPc ) Zn and doubly reduced DDQ is examined and the present electrochemical studies reveal the possible existence of such complexes in solution. Based on semiempirical AM1 and PM3 calculations, interaction through the zinc(II) metal center of ( OEPc ) Zn and cyanide nitrogen of the quinone is proposed.

Synthesis of two water-soluble pyrrole-β brominated porphyrins, the tetrachloro salt of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(N-methyl-4-pyridinio)porphyrin [tetrakis(N-methyl pyridyl)-β-octabromoporphyrin, (Br8TMpyP)H2] and the tetrasodium salt of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(phenyl sulfonate)porphyrin [tetrakis(4-sulfonatophenyl)-β-octabromoporphyrin, (Br8TPPS)H2] and their zinc(II) derivatives have been reported. The investigated porphyrins are highly non-planar due to the presence of eight bromo substituents at the pyrrole-β positions and four aryl groups at the meso positions. At I = 0.1 M and T = 23 °C, the two sulfonated derivatives, (Br8TPPS)H2 and (Br8TPPS)Zn show a small tendency to aggregate while such behavior is almost negligible for the N-methyl pyridyl derivatives. Like the unbrominated water-soluble zinc(II) porphyrin derivatives, the pyrrole-β brominated zinc(II) porphyrins, with one or two water molecules as axial ligand(s) in aqueous solution, undergo ligand exchange reactions with nitrogeneous bases. The calculated equilibrium constant, K, for this reaction is found to be smaller in magnitude when compared to that obtained for the corresponding unbrominated zinc(II) porphyrin derivatives. The equilibrium constant values parallel the basicity of the axial ligands.

A novel donor–acceptor pentad featuring subphthalocyanine and fullerene as the primary electron donor and acceptor, and three phenothiazine entities as secondary hole transferring agents, have been newly synthesized and characterized as an photosynthetic reaction center model compound. Occurrences of ultrafast photoinduced electron transfer (PET) and slower charge recombination are witnessed in the pentad from the femtosecond and nanosecond transient absorption studies.

The microdynamics of a 2: 1 A1C1,-1-ethyl-3-methylimidazolium chloride (MEICI) roomtemperature melt are studied using I3C and 27Al NMR relaxation methods. The I3C and 27Al NMR correlation times for MEI' and A1,CI; are compared with viscosity measurements corrected for temperature. The Dual Spin Probe (DSP) method is used to calculate an 27Al nuclear quadrupole coupling constant (x) of 4.86 MHZ for A12C1; in the 2: 1 melt. Theoretical (ab initio-gas phase) calculations at the HF/3-21 G(d) and HF/6-31 (d) levels yield 27Al nuclear quadrupole cou lin constants of 8.015 and 11.211 MHZ for A12CI;. Finally, a linear reported. relationship between Pg A1 correlation times of A12C1; and reciprocal conductivity is

We have performed semiempirical molecular orbital calculations, using the Austin Model 1# (AM1) and Parametric Method 3# (PM3) Hamiltonians of the program mopac for the geometric, electronic, and vibrational structure of (ZnO)n (where n = 1–16), spheroids, and (MX)12 analogs, where M = Be, Mg, Zn, Cd, Hg, Pb, Sn, Ge, Si, C when X = O, S and where M = B, Al when X = N, P. Of these the symmetrical (ZnO)n (where n = 12 and 16) clusters are the most stable. All of the n = 12 analogs, including C24, Si24, and (SiC)12, were studied under Td(Oh) symmetry, where they were found to have stationary points on the electronic potential energy surface. Ab initio Hartree-Fock calculations done on (BeO)12, (BN)12, (MgO)12, C24 and (ZnO)12, using the program gaussian 92, confirm this result. All compounds were vibrationally stable except, unsurprisingly, (CO)12, which has six negative eigenvalues of the Hessian matrix under both AM1 and PM3, while (BeO)12 and (GeS)12 have three negative eigenvalues under PM3 but are stable under AM1. Properties computed include bond lengths, bond angles, bond orders, heats of formation, atom charges, valence electron orbital energies including HOMO and LUMO band separation energy, and vibrational normal mode frequencies.

Donor-acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron-transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor-acceptor dyads, revealing excitation-wavelength-dependent photochemical properties demands multimodular, photosynthetic-reaction-center model compounds. Here, we successfully demonstrate donor- acceptor excitation-wavelength-dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF2 -chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured "closely" and "distantly" positioned donor-acceptor systems. The near-IR-emitting BF2-chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation-wavelength-dependent, photoinduced, electron-transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc(+)-ADP-C60(.-) charge-separated state upon C60 excitation, and Fc(+)-ADP(.-)-C60 formation upon ADP excitation is demonstrated.

Covalent functionalization of diameter sorted SWCNTs with porphyrins (MP), and photochemistry to establish nanotube diameter-dependent charge separation efficiencies are reported. The MP-SWCNT(n,m) [M=2H or Zn, and (n,m)=(7,6) or (6,5)] nanohybrids are characterized by a variety of spectroscopic, thermogravimetric, TEM imaging techniques, and also by DFT MO calculations. The thermogravimetric, Raman and fluorescence studies reveal the presence of a moderate number of porphyrins on the SWCNT surface. The MO results suggest charge separation (CS) via the excited state of MP. Time-resolved fluorescence studies reveal quenching of the singlet excited state of the MP with SWCNT(n,m), giving the rate constants of charge separation (k(CS)) in the range of (4-5)×10(9) s(-1). Nanosecond transient absorption measurements confirm the charge-separated radical cation and the radical anion as [MP(.+)-SWCNT(.-)] with their characteristic absorption bands in the visible and near-IR regions. The charge separated states persist for about 70-100 ns thus giving an opportunity to utilize them to build photoelectrochemical cells, which allowed us to derive the structure-reactivity relationship between the nature of porphyrin and diameter of the employed nanotubes.

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (k(ENT)) was monitored by performing transient absorption and emission measurements using pump-probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental k(ENT) values on the order of 10(10)-10(11) s(-1), which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed.

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene-porphyrin-SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π-π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H(2)), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-π and π-π interactions between the MP(alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor-acceptor interactions between them in the ground and excited states. The donor-acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)(4) in two-layered nanohybrids (MP(alkyl)(4)/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)(4) ((1)MP*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) × 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP˙(+)(alkyl)(4)/SWCNT˙(-) and/or MP˙(-)(alkyl)(4)/SWCNT˙(+) for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via(1)ZnP* in C(60)Im→ZnP(alkyl)(4)/SWCNT is observed to form C(60)˙(-)Im→ZnP˙(+)(alkyl)(4)/SWCNT and C(60)˙(-)Im→ZnP(alkyl)(4)/SWCNT˙(+) charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k(CR)) in the range of (6-10) × 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.

A 'molecular clip' featuring a near-IR emitting fluorophore, BF(2)-chelated tetraarylazadipyrromethane (aza-BODIPY) covalently linked to two porphyrins (MP, M = 2H or Zn) has been newly synthesized to host a three-dimensional electron acceptor fullerene via a 'two-point' metal-ligand axial coordination. Efficient singlet-singlet excitation transfer from (1)ZnP* to aza-BODIPY was witnessed in the dyad and triad in nonpolar and less polar solvents, such as toluene and o-dichlorobenzene, however, in polar solvents, additional electron transfer occurred along with energy transfer. A supramolecular tetrad was formed by assembling bis-pyridine functionalized fullerene via a 'two-point' metal-ligand axial coordination, and the resulted complex was characterized by optical absorption and emission, computational, and electrochemical methods. Electron transfer from photoexcited zinc porphyrin to C(60) is witnessed in the supramolecular tetrad from the femtosecond transient absorption spectral studies. Further, the supramolecular polyads (triad or tetrad) were utilized to build photoelectrochemical cells to check their ability to convert light into electricity by fabricating FTO/SnO(2)/polyad electrodes. The presence of azaBODIPY and fullerene entities of the tetrad improved the overall light energy conversion efficiency. An incident photon-to-current conversion efficiency of up to 17% has been achieved for the tetrad modified electrode.

The synthesis, structure, electrochemistry and photodynamics of a BF(2)-chelated azadipyrromethene-fullerene dyad are reported in comparison with BF(2)-chelated azadipyrromethene without fullerene. The attachment of fullerene resulted in efficient generation of the triplet excited state of the azadipyrromethene via photoinduced electron transfer.

A molecular dyad and triad, comprised of a known photosensitizer, BF(2)-chelated dipyrromethane (BDP), covalently linked to its structural analog and near-IR emitting sensitizer, BF(2)-chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular-clip-type structure for the triad, in which the BDP and ADP entities are separated by about 14 A with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge-separated states. Such calculations revealed a charge separation from the singlet excited BDP ((1)BDP*) to ADP (BDP(.+)-ADP(.-)) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from (1)BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to (1)ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10(11) s(-1)) in these molecular systems. The kinetics of electron transfer via (1)ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge-separation process from BDP to (1)ADP*. These findings suggest the potential of the examined ADP-BDP molecules to be efficient photosynthetic antenna and reaction center models.

A self-assembled supramolecular triad as a model to mimic the light-induced events of the photosynthetic antenna-reaction center, that is, ultrafast excitation transfer followed by electron transfer ultimately generating a long-lived charge-separated state, has been accomplished. Boron dipyrrin (BDP), zinc porphyrin (ZnP) and fullerene (C60), respectively, constitute the energy donor, electron donor and electron acceptor segments of the antenna-reaction center imitation. Unlike in the previous models, the BDP entity was placed between the electron donor, ZnP and electron acceptor, C60 entities. For the construction, benzo-18-crown-6 functionalized BDP was synthesized and subsequently reacted with 3,4-dihydroxyphenyl functionalized ZnP through the central boron atom to form the crown-BDP-ZnP dyad. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity of the dyad via ion–dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational and electrochemical methods. Steady-state fluorescence and excitation studies revealed the occurrence of energy transfer upon selective excitation of the BDP in the dyad. Further studies involving the pump–probe technique revealed excitation transfer from the 1BDP* to ZnP to occur in about 7 ps, much faster than that reported for other systems in this series of triads, as a consequence of shorter distance between the entities. Upon forming the supramolecular triad by self-assembling fullerene, the 1ZnP* produced by direct excitation or by energy transfer mechanism resulted in an initial electron transfer to the BDP entity. The charge recombination resulted in the population of the triplet excited state of C60, from where additional electron transfer occurred to produce C60•−:crown-BDP-ZnP•+ ion pair as the final charge-separated species. Nanosecond transient absorption studies revealed the lifetime of the charge-separated state to be ∼100 μs, the longest ever reported for this type of antenna-reaction center mimics, indicating better charge stabilization as a result of the different disposition of the entities of the supramolecular triad.

The ab initio structures and energies of a series of rotamers of 1-nitro-2-propanol have been calculated at the HF/6-31G(d) level. Correlation has been included at the MP2/6-31G(d)//HF/6-31G(d) level. The PES surface for rotation of the OH about a dihedral angle theta confirms the formation of an intramolecular hydrogen bond in the monomer which is stabilized by 5.4 kcal/mol at the MP2/6-31G(d)//HF/6-31G(d) level. Dimer formation via double intermolecular hydrogen bonds was found to be feasible by molecular mechanics and refined by ab initio computations. It is shown that a double intermolecular hydrogen bonded structure is stabilized by 12.9 kcal/mol relative to the monomer molecules at the HF/3-21G(d) level

The high field1H,19F, and13C NMR spectra of haloperidol (HP) in CDCl3 solution were recorded and analyzed with the aid of both homonuclear (H, H-COSY, H, H-COSYLR) and heteronuclear (H,C-COSY) chemical shift correlation experiments. Through space interactions between (1) the piperidine ring and the chlorinated phenyl ring and (2) the fluorinated phenyl ring and a methylene group were identified using phase sensitive Nuclear Overhauser and Exchange Spectroscopy (NOESY). The NOESY results are consistent with a planar structure rather than a folded version of haloperidol.

Singlet-singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H(2)PIm, to either zinc phthalocyanine, ZnPc, or zinc naphthalocyanine, ZnNc, dyads is investigated in noncoordinating solvents, o-dichlorobenzene and toluene, using both steady-state and time-resolved transient absorption techniques. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational, and electrochemical methods. The binding constants measured from optical absorption spectral data were found to be in the range of 10(4)-10(5) M(-1) for the 1:1 dyads, suggesting fairly stable complex formation. Electrochemical and computational studies suggested that photoinduced electron transfer is a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. Selective excitation of the donor free-base porphyrin entity was possible in both types of dyads formed by either of the ZnPc or ZnNc energy acceptors. Efficient singlet-singlet energy transfer was observed in these dyads, and the position of imidazole linkage on the free-base porphyrin entity, although flexible, seems to have some control over the overall efficiency of excited energy transfer process. Kinetics of energy transfer was monitored by performing transient absorption measurements using both up-conversion and pump-probe techniques. Such studies revealed ultrafast singlet-singlet energy transfer in the studied dyads with time constants on the order of 2-25 ps depending upon the type of the dyad.

A melamine piezomicrogravimetric (acoustic) chemosensor using a molecularly imprinted polymer (MIP) film has been devised and tested. The MIP films were prepared by electropolymerization of the melamine complexed by the functional monomer of the bis(bithiophene) derivative bearing an 18-crown-6 substituent 4. The structure of the MIP-melamine complex was visualized by the DFT B3LYP/3-21G(*) energy optimization calculations. The sensitivity and selectivity of the MIP film was improved by cross-linking the polymer with the bithianaphthene monomer 5 and the presence of the porogenic ionic liquid in the prepolymerization solution. After electropolymerization, the melamine template was extracted from the MIP film with an aqueous strong base solution. The measurements of UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), DPV, and EIS as well as scanning electrochemical microscopy (SECM) imaging confirmed extraction of the melamine template from the MIP film and then rebinding of the melamine analyte while the film relative roughness and porosity was determined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging, respectively. The analytical as well as kinetic and thermodynamic parameters of the chemosensing were assessed under flow-injection analysis (FIA) conditions with piezoelectric microgravimetry (PM) detection. The linear concentration range for melamine detection was 5 nM to at least 1 mM with a limit of detection of approximately 5 nM. The chemosensor successfully discriminated the cyanuric acid, cyromazine, and ammeline interfering agents.

A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic antenna-reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied. Boron dipyrrin, zinc porphyrin, and fullerene respectively constitute the energy donor, electron donor, and electron acceptor segments of the antenna-reaction center mimicry. For the construction, first, boron dipyrrin was covalently attached to a zinc porphyrin entity bearing a benzo-18-crown-6 host segment at the opposite end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational, and electrochemical methods. Selective excitation of the boron dipyrrin moiety in the dyad resulted in energy transfer over 97% efficiency creating singlet excited zinc porphyrin. The rate of energy transfer from the decay measurements of time-correlated singlet photon counting (TCSPC) and up-conversion techniques agreed well with that obtained by the pump-probe technique and revealed efficient photoinduced energy transfer in the dyad (time constant in the order of 10-60 ps depending upon the conformer). Upon forming the supramolecular triad by self-assembling fullerene, the excited zinc porphyrin resulted in electron transfer to the coordinated fullerene yielding a charge-separated state, thus mimicking the antenna-reaction center functionalities of photosynthesis. Nanosecond transient absorption studies yielded a lifetime of the charge-separated state to be 23 micros indicating charge stabilization in the supramolecular triad. The present supramolecular system represents a successful model to mimic the rather complex "combined antenna-reaction center" events of photosynthesis.