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Measuring the dynamics of neurochemical‐regulated immunity, particularly in the gut, has been a growing interest over the last several years because of its important implications in gastrointestinal inflammation, neurodegeneration, and even depression. Sympathetic noradrenergic nerves innervate the gastrointestinal tract and resident immune organs, including the mesenteric lymph nodes (MLN) and Peyer's patches. Previous research has suggested that neuronal inputs in the MLN release norepinephrine (NE) at neural‐immune synapses to regulate immune function. The current immunological techniques do not have the appropriate temporal or spatial resolution to monitor this dynamic process in real‐time, within specific regions of intact lymphoid organs. Monitoring dynamic neural signaling within intact immune organs, in real‐time, would facilitate a deeper understanding of neuroimmune communication and would allow the mechanism of rapid immunomodulation to be elucidated. Here, we overcome this technological barrier by coupling real‐time neurochemical detection using fast‐scan cyclic voltammetry (FSCV) in live MLN slices from C57BL/6 mice. We have discovered rapid, spontaneous catecholamine transients in the T‐cell zone of the MLN which are on the order of a few hundred nanomolar, rapid (a few seconds), and frequent (every 20‐s). We demonstrate that the β2‐adrenergic receptor and the classic catecholamine transporters (DAT and NET) play a minor role in transient regulation in the MLN suggesting that regulation at the neural‐immune synapse is quite complicated and further mechanistic studies are needed. Overall, these findings provide direct evidence for rapid neurochemical events in the MLN which could have a major impact on our understanding of neurochemical‐regulated immunomodulation in the gut. [ABSTRACT FROM AUTHOR]

A series of vertically assembled photoanodes, consisting of 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(III) porphyrin (AlPorF3), a pyridine appended electron donor (PTZ-Py, PTZ = phenothiazine; TTF-Py, TTF = tetrathiafulvalene), and semiconductor TiO2, have been fabricated by exploiting the unique axial properties of AlPorF3. The new photoanodes were characterized by steady-state and transient spectroscopic techniques. Transient-absorption studies show that in the absence of a donor, both the photoanodes (AlPorF3–TiO2 and AlPorF3-Ph–TiO2) exhibit electron injection from AlPorF3 into the conduction band of TiO2 and the injection efficiencies are strongly dependent on the linker. Faster electron injection and recombination is revealed when AlPorF3 is directly bound to TiO2. Although a secondary electron donor is coordinated to AlPorF3 (viz., Donor-Py-AlPorF3–TiO2 and Donor-Py-AlPorF3-Ph–TiO2), the primary charge separation occurs in the form of electron injection from AlPorF3 to TiO2 followed by a secondary process involving photooxidation of the donor (PTZ and TTF) with AlPorF3˙+. The estimated electron injection lifetimes and the AlPorF3˙+ decay lifetimes strongly depend on the electron richness of the donor; the higher the electron density of the donor, the faster the electron injection and photooxidation witnessed. The photoanodes with TTF (TTF-Py-AlPorF3–TiO2 and TTF-Py-AlPorF3-Ph–TiO2) show faster injection and shorter decay lifetimes of AlPorF3˙+ over their PTZ counterparts (PTZ-Py-AlPorF3–TiO2 and PTZ-Py-AlPorF3-Ph–TiO2). The observed trends suggest that the strong secondary electron donor enhances the injection and the subsequent photooxidation processes in the investigated photoanodes. The successful mimicking of a sequential charge-separation process makes aluminum(III) porphyrins potential sensitizers for the construction of photoanodes, especially for photocatalytic and dye-sensitized solar cells for conversion and storage of solar energy. [ABSTRACT FROM AUTHOR]

Phosphorous(v) corrole has been employed to build stable, donor–acceptor complexes capable of undergoing photoinduced energy and electron transfer events. Two triads, the first one with naphthalene axial entities (Triad 1) and the second one with pyrene axial entities (Triad 2), were newly synthesized for this purpose. The X-ray structure of Triad 1 revealed the presence of two naphthalene entities bound to the phosphorous centre without causing steric crowding. The electrochemical and computational studies revealed the electron deficient nature of the phosphorous corrole in the triads. Excited state events, monitored by both time-resolved emission and transient absorption studies, revealed several interesting results. In the case of Triad 1, efficient energy transfer from singlet excited naphthalene to corrole was observed, however, no photo events from excited phosphorous corrole were evident. Interestingly, in the case of Triad 2, although an energy transfer process from singlet excited pyrene to corrole was weak, evidence of photoinduced charge separation leading to the formation of pyreneṖ+-PCṖ− was witnessed from femtosecond transient absorption studies when either pyrene or corrole was selectively excited. The importance of axial bound donor entities on phosphorous(v) corrole in promoting photoinduced energy and electron transfer events is borne out from the present study. [ABSTRACT FROM AUTHOR]

A high potential donor-acceptor dyad composed of zinc porphyrin bearing three meso-pentafluorophenyl substituents covalently linked to C60, as a novel dyad capable of generating charge-separated states of high energy (potential) has been developed. The calculated energy of the charge-separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin-fullerene dyad. Intramolecular photoinduced electron transfer leading to charge-separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto- to nanosecond transient absorption techniques. The high energy stored in the form of charge-separated states along with its persistence of about 50-60 ns makes this dyad a potential electron-transporting catalyst to carry out energy-demanding photochemical reactions. This type of high-energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light-to-fuel products. [ABSTRACT FROM AUTHOR]

Subphthalocyanine (SubPc), a unique ring-reduced member of the common phthalocyanines family, although known for its higher absorptivity, reveals narrow absorption with peak maxima around 570 nm thus limiting its utility in light-energy-harvesting applications. In the present study, by peripheral thio-aryl substitution of SubPc macrocycle, the spectral properties have been modulated to extend the absorption and emission well into the visible/near-IR region. Additionally, for α-ring-substituted derivatives, facile oxidation of SubPc was witnessed, thus making these derivatives better electron donors. Next, the preparation of donor-acceptor dyads containing the well-known electron acceptor C60 connected to the central boron atom of SubPc was accomplished by making use of the 1,3-dipolar cycloaddition reaction. Control experiments and free-energy calculations using the redox and spectral data suggested that the observed fluorescence quenching of SubPc in these dyads is due to electron transfer. Accordingly, transient spectral studies performed both in polar and nonpolar solvents conclusively proved electron transfer to be the quenching mechanism in these dyads. The measured rate constants by fitting kinetic data revealed efficient charge separation and charge recombination processes, suggesting that these dyads could be useful materials for the construction of light-to-electricity or light-to-fuel production devices. [ABSTRACT FROM AUTHOR]

A new series of multi-modular donor–acceptor systems capable of exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques. In this series, the electron donor was a BF2-chelated dipyrromethene (BODIPY) appended with two styryl linkers carrying two electron rich triphenylamine or phenothiazine entities. Fulleropyrrolidine linked at the meso-position of the BODIPY ring served as an electron acceptor. As a result of extended conjugation and multiple electroactive chromophore entities, the bis-styryl BODIPY revealed absorbance and emission well-into the near-infrared region covering a 300–850 nm spectral range. Using redox, computational, absorbance and emission data, an energy level diagram was constructed that helped in envisioning the different photochemical events. Spectral evidence for photoinduced charge separation in these systems was established from femtosecond and nanosecond transient absorption studies. The measured rate constants indicated fast charge separation and relatively slow charge recombination revealing their usefulness in light energy harvesting and optoelectronic device building applications. The bis(donor styryl)BODIPY–fullerene systems populated BODIPY triplet excited states during the process of charge recombination. [ABSTRACT FROM AUTHOR]

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states. [ABSTRACT FROM AUTHOR]

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 1010 s−1 and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems. [ABSTRACT FROM AUTHOR]

Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60, possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the b-pyrrole position was newly synthesized, as were the component dyads TPACor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from 1TPA* to the corrole and subsequent electron transfer from 1corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor·+ in Cor-C60 and TPA-Cor·+ in TPACor-C60) and fullerene radical anion (C60 ·-), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS, was found to be about 1011 s-1, suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS, the rate of charge recombination, kCR, was also found to be rapid (kCR ≈1010 s-1), and both processes followed the solvent polarity trend DMF>benzonitrile> THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of 3corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of 3corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy chargeseparated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates. [ABSTRACT FROM AUTHOR]

The axial bonding ability of aluminum(iii) porphyrin (AlPor) has been exploited to synthesize the vertically linked dyad ‘aluminum(iii) porphyrin–gold(iii) porphyrin’ (AlPor-Ph–AuPor+) and the two corresponding self-assembled triads ‘tetrathiafulvalene–aluminum(iii) porphyrin–gold(iii) porphyrin’ (TTF-py→AlPor-Ph–AuPor+ and TTF-Ph-py→AlPor-Ph–AuPor+). The unique topology of these triads provides an excellent opportunity to investigate the sequential electron transfer in the perpendicular direction to the AlPor plane where the AlPor acts as a photosensitizer and primary electron donor while the AuPor and TTF serve as an electron acceptor and donor, respectively. The ground state properties of the dyad and triad suggest that there are no direct intramolecular interactions between the oppositely disposed AuPor and TTF units of the triad. However, the NMR and UV-visible absorption studies of the dyad reveal intermolecular interactions in non-coordinating solvents due to the coordination of counterion PF6− to the Al center of AlPor. Steady-state and femtosecond transient absorption studies of the dyad show that the lowest excited singlet state of AlPor (1AlPor*) is strongly quenched by ultrafast electron transfer to AuPor+ with a time constant of 3.16 ps. The resulting charge separated state (AlPor+Ṗ–AuPorṖ) decays to ground state biexponentially with time constants of 27.26 and 2557 ps. Analogously, upon photo-excitation the triads also produce the same primary radical pair (AlPor+Ṗ–AuPorṖ). However, the formed radical pair is further involved in a rapid hole transfer from AlPor+Ṗ to TTF to form a stable final radical pair TTF+Ṗ–AlPor–AuPorṖ. The lifetime of the charge separated state exhibits an increase from 27.26 ps in AlPor-Ph–AuPor to 1393 ps in TTF-py→AlPor-Ph–AuPor+ and 1484 ps in TTF-Ph-py→AlPor-Ph–AuPor+. These results reveal successful charge stabilization in the self-assembled supramolecular reaction center mimics constructed via the axial linkage strategy. [ABSTRACT FROM AUTHOR]

A new approach to probe the effect of graphene on photochemical charge separation in donor-acceptor conjugates is devised. For this, multimodular donor-acceptor conjugates, composed of three molecules of pyrene, a subphthalocyanine, and a fullerene C60 ((Pyr)3SubPc-C60), have been synthesized and characterized. These systems were hybridized on few-layer graphene through π-π stacking interactions of the three pyrene moieties. The hybrids were characterized using Raman, HRTEM, and spectroscopic and electrochemical techniques. The energy levels of the donor-acceptor conjugates were fine-tuned upon interaction with graphene and photoinduced charge separation in the absence and presence of graphene was studied by femtosecond transient absorption spectroscopy. Accelerated charge separation and recombination was detected in these graphene-decorated conjugates suggesting that they could be used as materials for fast-responding optoelectronic devices and in light energy harvesting applications. [ABSTRACT FROM AUTHOR]

Novel photosynthetic reaction center model compounds of the type donor2-donor1-acceptor, composed of phenothiazine, BF2-chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X-ray structures of three of the phenothiazine-BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N-substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C-substituted analogue. The geometry and electronic structures were obtained by B3LYP/6-31G(dp) calculations (for H, B, N, and O) and B3LYP/6-31G(df) calculations (for S) in vacuum, followed by a single-point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO-1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy-level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ-BODIPY•+-C60•-; subsequent hole shift resulted in PTZ•+ -BODIPY-C60•- charge-separated species. The return of the charge-separated species was found to be solvent dependent. In nonpolar solvents the PTZ•+-BODIPY-C60•- species populated the ³C60* prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The ¹BODIPY* generated radical ion-pair in these triads persisted for few nano-seconds due to electron transfer/hole-shift mechanism. [ABSTRACT FROM AUTHOR]

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 (C60Im) 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 ¹ZnP* (formed either as a result of singlet- singlet energy transfer or direct excitation) or ¹C60* 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. [ABSTRACT FROM AUTHOR]

A multimodular donor-acceptor tetrad featuring a bis(zinc porphyrin)-(zinc phthalocyanine) ((ZnP-ZnP)-ZnPc) triad and bis-pyridine-functionalized fullerene was assembled by a 'two-point' binding strategy, and investigated as a charge-separating photosynthetic antenna-reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis-pyridine-functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi-Hildebrand plot by using the optical data was found to be 1.17×105 M−1, whereas a plot of 'mole-ratio' method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6-31G* optimization, whereas the energy levels for different photochemical events was established by using data from the optical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet-singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer ( kENT) in the case of the triad was found to be 7.5×1011 s−1 in toluene and 6.3×1011 s−1 in o-dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy-transfer product, that is, singlet-excited zinc phthalocyanine to fullerene was verified from the femtosecond-transient spectral studies, both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc ⋅+ in the 850 nm range and C60 ⋅− in the 1020 nm range were clearly observed. The rate of charge separation, kCS, and rate of charge recombination, kCR, for the (ZnP-ZnP)-ZnPc ⋅+:Py2C60 ⋅− radical ion pair (from the time profile of 849 nm peak) were found to be 2.20×1011 and 6.10×108 s−1 in toluene, and 6.82×1011 and 1.20×109 s−1 in o-dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Donor–acceptor conjugates based on zinc porphyrin as the electron donor and copper(III) corrole as the electron acceptor have been successfully prepared with outstanding yields. The cover shows a “smart polymer superhero” (one of our colleagues in a red costume, which we use in our institute for educational purposes), holding one of these fascinating molecules. More information can be found in the Full Paper by T. H. Ngo et al. on page 1301 ff. [ABSTRACT FROM AUTHOR]

Donor-acceptor systems in which a donor phenanthroimidazole (PhI) is directly connected to a BODIPY acceptor ( Dyad1 ) and separated by an ethynyl bridge between PhI and BODIPY ( Dyad2 ) have been designed, synthesized, and characterized by various spectroscopic and electrochemical techniques. Optical absorption and 1 H NMR characteristics of both dyads with those of constituent individuals suggest that there exists a minimum π-π interaction between phenanthroimidazole and BODIPY. Quenched emission of both the dyads was observed when excited either at phenthaoimidazole absorption maxima or at BODIPY absorption maxima in all three investigated solvents. The detailed spectral analysis provided evidence for an intramolecular photoinduced excitation energy transfer (PEnT) from the singlet excited state of phenanthroimidazole to BODIPY and photoinduced electron transfer (PET) from the ground state of phenanthroimidazole to BODIPY. Transient absorption studies suggest that charge-separated species (PhI •+ - BODIPY •- ) are generated at a rate constant of (1.16 ± 0.01) × 10 8 s -1 for the dyads Dyad1 and (5.15 ± 0.03) × 10 8 s -1 and for Dyad2 whereas energy transfer rate constants were much higher and were on the order of (1.1 ± 0.02) × 10 10 s -1 and (1.6 ± 0.02) × 10 10 s -1 for Dyad1 and Dyad2 , respectively, signifying their usefulness in light energy harvesting applications.

A series of vertically assembled photoanodes, consisting of 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF 3 ), a pyridine appended electron donor (PTZ-Py, PTZ = phenothiazine; TTF-Py, TTF = tetrathiafulvalene), and semiconductor TiO 2 , have been fabricated by exploiting the unique axial properties of AlPorF 3 . The new photoanodes were characterized by steady-state and transient spectroscopic techniques. Transient-absorption studies show that in the absence of a donor, both the photoanodes (AlPorF 3 -TiO 2 and AlPorF 3 -Ph-TiO 2 ) exhibit electron injection from AlPorF 3 into the conduction band of TiO 2 and the injection efficiencies are strongly dependent on the linker. Faster electron injection and recombination is revealed when AlPorF 3 is directly bound to TiO 2 . Although a secondary electron donor is coordinated to AlPorF 3 (viz., Donor-Py-AlPorF 3 -TiO 2 and Donor-Py-AlPorF 3 -Ph-TiO 2 ), the primary charge separation occurs in the form of electron injection from AlPorF 3 to TiO 2 followed by a secondary process involving photooxidation of the donor (PTZ and TTF) with AlPorF 3 ˙ + . The estimated electron injection lifetimes and the AlPorF 3 ˙ + decay lifetimes strongly depend on the electron richness of the donor; the higher the electron density of the donor, the faster the electron injection and photooxidation witnessed. The photoanodes with TTF (TTF-Py-AlPorF 3 -TiO 2 and TTF-Py-AlPorF 3 -Ph-TiO 2 ) show faster injection and shorter decay lifetimes of AlPorF 3 ˙ + over their PTZ counterparts (PTZ-Py-AlPorF 3 -TiO 2 and PTZ-Py-AlPorF 3 -Ph-TiO 2 ). The observed trends suggest that the strong secondary electron donor enhances the injection and the subsequent photooxidation processes in the investigated photoanodes. The successful mimicking of a sequential charge-separation process makes aluminum(iii) porphyrins potential sensitizers for the construction of photoanodes, especially for photocatalytic and dye-sensitized solar cells for conversion and storage of solar energy.

A multi-modular donor-acceptor triad composed of zinc porphyrin, BF 2 -chelated dipyrromethene (BODIPY), and C 60 was newly synthesized, with the BODIPY entity at the central position. Using absorbance and emission spectral, electrochemical redox, and computational optimization results, energy level diagrams for the ZnP-BODIPY dyad and ZnP-BODIPY-C 60 triad were constructed to envision the different photochemical events upon selective excitation of the BODIPY and ZnP entities. By transient absorption spectral studies covering a wide femtosecond-to-millisecond time scale, evidence for the different photochemical events and their kinetic information was secured. Efficient singlet-singlet energy transfer from 1 BODIPY* to ZnP with a rate constant k ENT = 1.7 × 10 10 s -1 in toluene was observed in the case of the ZnP-BODIPY dyad. Interestingly, in the case of the ZnP-BODIPY-C 60 triad, the selective excitation of ZnP resulted in electron transfer leading to the formation of the ZnP˙ + -BODIPY-C 60 ˙ - charge-separated state. Owing to the distal separation of the radical cation and radical anion species (edge-to-edge distance of 18.7 Å), the radical ion-pair persisted for microseconds. By contrast, the selective excitation of BODIPY resulted in an ultrafast energy transfer to yield ZnP-BODIPY- 1 C 60 * as the major product. The 1 C 60 * populated the low-lying 3 C 60 * via intersystem crossing prior to returning to the ground state. The present study successfully demonstrates the importance of supramolecular geometry and selection of excitation wavelength in regulating the different photoprocesses.

A major challenge in designing artificial photosynthetic systems is to find a suitable mimic of the highly oxidizing photoactive species P 680 in photosystem II. High-potential phosphorus(V) porphyrins have many attractive properties for such a mimic but have not been widely studied. Here, we report the synthesis and photophysical characterization of a novel phosphorus(V) octaethylporphyrin-oxyphenyl-terpyridine conjugate (PPor-OPh-tpy, 1) and its corresponding manganese(II) complex (PPor-OPh-Mn(tpy)Cl 2 , 2). The X-ray structure of 2 shows that the Mn(II) and P(V) centers are 11.783 Å apart and that the phenoxy linker is not fully conjugated with the terpyridine ligand. The porphyrin fluorescence in 1 and 2 is strongly quenched and has a shorter lifetime compared to a reference compound without the terpyridine ligand. This suggests that electron transfer from tpy or Mn(tpy) to the excited singlet state of the PPor may be occurring. However, femtosecond transient absorbance data show that the rate of relaxation to the ground state in 1 and 2 is comparable to the fluorescence lifetimes. Thus, if charge separation is occurring, its lifetime is short. Because both 1 and 2 are positively charged, they can be electrostatically deposited onto the surface of negatively charged SnO 2 nanoparticles. Freeze-trapping EPR studies of 2 electrostatically bound to SnO 2 suggest that excitation of the porphyrin results in electron injection from 1 PPor* into the conduction band of SnO 2 and that the resulting PPor •+ species acquires enough potential to photo-oxidize the axially bound Mn(II) (tpy) moiety to Mn(III) (tpy).

This study demonstrates molecular engineering of a series of donor-acceptor systems to allow control of the lifetime and initial spin multiplicity of the charge-separated state. By tuning the rate of intersystem crossing (ISC) and the donor-acceptor distance, electron transfer can be made to occur exclusively from the triplet excited state of the electron donor resulting in long-lived charge separation. To achieve this, three new palladium porphyrin-fullerene donor-acceptor systems were synthesized. The heavy Pd atom enhances the rate of ISC in the porphyrin and the rates of electron and energy transfer are modulated by varying the redox potential of the porphyrin and the porphyrin-fullerene distance. In the case of the meso-tris(tolyl)porphyrinato palladium(ii)-fulleropyrrolidine, the donor-acceptor distance is relatively long (13.1 Å) and the driving force for electron transfer is low. As a result, excitation of the porphyrin leads to rapid ISC followed by triplet-triplet energy transfer to fullerene. When the fullerene is bound directly to the porphyrin shortening the donor-acceptor distance to 2.6 Å electron transfer from the singlet excited palladium porphyrin leading to the generation of a short-lived charge separated state is the main process. Finally, when the palladium porphyrin is substituted with three electron rich triphenylamine entities, the lower oxidation potential of the porphyrin and appropriate donor-acceptor distance (∼13 Å), lead to electron transfer exclusively from the triplet excited state of palladium porphyrin with high quantum yield. The results show that when electron transfer occurs from the triplet state, its increased lifetime allows the distance between the donor and acceptor to be increased which results in a longer lifetime for the charge separated state.

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10(10)  s(-1) and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60 , possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the β-pyrrole position was newly synthesized, as were the component dyads TPA-Cor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1) TPA* to the corrole and subsequent electron transfer from (1) corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(⋅+) in Cor-C60 and TPA-Cor(⋅+) in TPACor-C60) and fullerene radical anion (C60(⋅-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS , was found to be about 10(11)  s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10)  s(-1)), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A multimodular donor-acceptor tetrad featuring a bis(zinc porphyrin)-(zinc phthalocyanine) ((ZnP-ZnP)-ZnPc) triad and bis-pyridine-functionalized fullerene was assembled by a "two-point" binding strategy, and investigated as a charge-separating photosynthetic antenna-reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis-pyridine-functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi-Hildebrand plot by using the optical data was found to be 1.17×10(5) M(-1), whereas a plot of "mole-ratio" method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6-31G* optimization, whereas the energy levels for different photochemical events was established by using data from the optical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet-singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer (k(ENT)) in the case of the triad was found to be 7.5×10(11) s(-1) in toluene and 6.3×10(11) s(-1) in o-dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy-transfer product, that is, singlet-excited zinc phthalocyanine to fullerene was verified from the femtosecond-transient spectral studies, both in o-dichlorobenzene and toluene. Transient bands corresponding to ZnPc(⋅+) in the 850 nm range and C60(⋅-) in the 1020 nm range were clearly observed. The rate of charge separation, k(CS), and rate of charge recombination, k(CR), for the (ZnP-ZnP)-ZnPc(⋅+):Py2C60(⋅-) radical ion pair (from the time profile of 849 nm peak) were found to be 2.20×10(11) and 6.10×10(8) s(-1) in toluene, and 6.82×10(11) and 1.20×10(9) s(-1) in o-dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)