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Structurally well‐defined self‐assembled supramolecular multi‐modular donor‐acceptor conjugates play a significant role in furthering our understanding of photoinduced energy and electron transfer events occurring in nature, e. g. in the antenna‐reaction centers of photosynthesis and their applications in light energy harvesting. However, building such multi‐modular systems capable of mimicking the early events of photosynthesis has been synthetically challenging, causing a major hurdle for its growth. Often, multi‐modularity is brought in by combining both covalent and noncovalent approaches. In the present study, we have developed such an approach wherein a π‐extended conjugated molecular cleft, two zinc(II)porphyrin bearing bisstyrylBODIPY (dyad, 1), has been synthesized. The binding of 1 via a 'two‐point' metal‐ligand coordination of a bis‐pyridyl fulleropyrrolidine (2), forming a stable self‐assembled supramolecular complex (1 : 2), has been established. The self‐assembled supramolecular complex has been fully characterized by a suite of physico‐chemical methods, including TD‐DFT studies. From the established energy diagram, both energy and electron transfer events were envisioned. In dyad 1, selective excitation of zinc(II)porphyrin leads to efficient singlet‐singlet excitation transfer to (bisstyrly)BODIPY with an energy transfer rate constant, kEnT of 2.56×1012 s−1. In complex 1 : 2, photoexcitation of zinc(II)porphyrin results in ultrafast photoinduced electron transfer with a charge separation rate constant, kCS of 2.83×1011 s−1, and a charge recombination rate constant, kCR of 2.51×109 s−1. For excitation at 730 nm corresponding to bisstyrylBODIPY, similar results are obtained, where a biexponential decay yielded estimated values of kCS 3.44×1011 s−1 and 2.97×1010 s−1, and a kCR value of 2.10×1010 s−1. The newly built self‐assembled supramolecular complex has been shown to successfully mimic the early events of the photosynthetic antenna‐reaction center events. [ABSTRACT FROM AUTHOR]

Two water‐soluble zinc(II) phthalocyanines substituted with two or four permethylated β‐cyclodextrin (β‐CD) moieties at the α positions have been utilized as building blocks for the construction of artificial photosynthetic models in water. The hydrophilic and bulky β‐CD moieties not only can increase the water solubility of the phthalocyanine core and prevent its stacking in water but can also bind with a tetrasulfonated zinc(II) porphyrin (ZnTPPS) and/or sodium 2‐anthraquinonesulfonate (AQ) in water through host–guest interactions. The binding interactions of these species have been studied spectroscopically, while the photoinduced processes of the resulting complexes have been investigated using steady‐state and time‐resolved spectroscopic methods. In the ternary complexes, the ZnTPPS units serve as light‐harvesting antennas to capture the light energy and transfer it to the phthalocyanine core via efficient excitation energy transfer. The excited phthalocyanine is subsequently quenched by the electron‐deficient AQ units through electron transfer. Femtosecond transient absorption spectroscopy provides clear evidence for the singlet‐singlet energy transfer from the photo‐excited ZnTPPS to the phthalocyanine core with a rate constant (kENT) in the order of 109 s−1. The population of phthalocyanine radical cations indicates the occurrence of electron transfer from the excited phthalocyanine to the AQ moieties, forming a charge‐separated state. [ABSTRACT FROM AUTHOR]

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole‐fused porphyrin dimers and their mono‐ and bis−zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady‐state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free‐base and zinc porphyrin) reveals the occurrence of efficient singlet‐singlet energy transfer (>95 % efficiency and rate constant >1012 s−1) within the dyad. Further, donor‐acceptor conjugates were formed by metal‐ligand axial coordination of phenyl imidazole functionalized C60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge‐separated state was in the 30–40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π‐spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor. [ABSTRACT FROM AUTHOR]

Three types of phenothiazines dimers (PTZ‐PTZ, 1–3), covalently linked with one or two acetylene linkers, were synthesized by copper‐mediated Eglinton and Pd‐catalyzed Sonogashira coupling reactions in excellent yields. The dimers 1–3 were further engaged in [2+2] cycloaddition‐retroelectrocyclization reactions with strong electron acceptors, tetracyanoethylene (TCNE) and 7,7,8,8‐tetracyanoquinodimethane (TCNQ) to yield tetracyanobutadiene (TCBD, 1 a–3 a), and dicyanoquinodimethane (DCNQ, 1 b–3 b) functionalized donor‐acceptor (D‐A) conjugates, respectively. The conjugates were examined by a series of spectral, computational, and electrochemical studies. Strong ground state polarization leading to new optical transitions was witnessed in both series of D‐A conjugates. In the case of DCNQ derived D‐A system 1 b, the optical coverage extended until 1200 nm in benzonitrile, making this a rare class of D‐A ICT system. Multiple redox processes were witnessed in these D‐A systems, and the frontier orbitals generated on DFT optimized structures further supported the ICT phenomenon. Photochemical studies performed using femtosecond pump‐probe studies confirmed solvent polarity dependent excited state charge transfer and separation in these novel multi‐modular D‐A conjugates. The charge‐separated states lasted up to 70 ps in benzonitrile while in toluene slightly prolonged lifetime of up to 100 ps was witnessed. The significance of phenothiazine dimer in wide‐band optical capture all the way into the near‐IR region and promoting ultrafast photoinduced charge transfer in the D‐A‐D configured multi‐modular systems, and the effect of donor‐acceptor distance and the solvent polarity was the direct outcome of the present study. [ABSTRACT FROM AUTHOR]

Graphene doped with heteroatoms such as nitrogen, boron, and phosphorous by replacing some of the skeletal carbon atoms is emerging as an important class of two‐dimensional materials as it offers the much‐needed bandgap for optoelectronic applications and provides better access for chemical functionalization at the heteroatom sites. Covalent grafting of photosensitizers onto such doped graphenes makes them extremely useful for light‐induced applications. Herein, we report the covalent functionalization of N‐doped graphene (NG) with two well‐known electron donor photosensitizers, namely, zinc porphyrin (ZnP) and zinc phthalocyanine (ZnPc), using the simple click chemistry approach. Covalent attachment of ZnP and ZnPc at the N‐sites of NG in NG−ZnP and NG−ZnPc hybrids was confirmed by using a range of spectroscopic, thermogravimetric and imaging techniques. Ground‐ and excited‐state interactions in NG−ZnP and NG−ZnPc were monitored by using spectral and electrochemical techniques. Efficient quenching of photosensitizer fluorescence in these hybrids was observed, and the relatively easier oxidations of ZnP and ZnPc supported excited‐state charge‐separation events. Photoinduced charge separation in NG−ZnP and NG−ZnPc hybrids was confirmed by using the ultrafast pump‐probe technique. The measured rate constants were of the order of 1010 s,−1 thus indicating ultrafast electron transfer phenomena. [ABSTRACT FROM AUTHOR]

Two covalently linked tetrads comprised of charge stabilizing triphenylamine (TPA)-oligothiophene (bi- or terthiophene, BT or TT) linked at the meso-position of a well-known photosensitizer, BF2-chelated boron dipyrromethene (BODIPY), was further functionalized to carry an electron acceptor, fulleropyrrolidine (C60) by modification of BODIPY at the boron center. Both tetrads were characterized by a range of spectroscopic techniques including single-crystal X-ray analysis of some of the intermediates. Electrochemical studies were performed to evaluate the redox potentials of different entities of the tetrads. Electronic structures deduced from computational studies revealed the HOMO to be on the triphenylamine-oligothiophene segment while the HOMO-1 to be on the BODIPY entity while the LUMO to be fully localized on the fullerene entity. These results coupled with electrochemical redox potentials indicated the formation of (TPA-TT).+-BODIPY-C60.- and (TPA-BT).+-BODIPY-C60.- as the final charge-separated states upon photoexcitation of BODIPY in these tetrads. Expectedly, steady-state emission studies revealed fluorescence quenching in the tetrads indicating the occurrence of excited state events. Further, femtosecond transient absorption studies coupled with data analysis using Glotaran, performed in polar benzonitrile and nonpolar toluene, confirmed the occurrence of photoinduced charge separation in both dyads. A long-lived charge-separated state (~20 ns) was possible to observe in the case of TPA-TT-BODIPY-C60 as a result of hole shift mechanism while for TPA-BT-BODIPY-C60 lifetime of the charge separated state was ~ 1 ns. Sequential photoinduced electron transfer followed by hole transfer leading to long-lived charge-separated states in newly synthesized covalently linked tetrads is demonstrated. [ABSTRACT FROM AUTHOR]

Photoinduced electron transfer (PET) in newly assembled dyads formed via metal‐ligand axial coordination of phenylimidazole‐functionalized bis(styryl)BODIPY (BODIPY(Im)2) and zinc tetrapyrroles, that is, zinc tetratolylporphyrin (ZnP), zinc tetra‐t‐butyl phthalocyanine (ZnPc) and zinc tetra‐t‐butyl naphthalocyanine (ZnNc), in non‐coordinating o‐dichlorobenzene (DCB) is investigated using both steady‐state and time‐resolved transient absorption techniques. The structure of the BODIPY(Im)2 was identified by using single crystal X‐ray structural analysis. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral studies were in the range of ∼104 M−1 for the first zinc tetrapyrrole binding and suggested that the two imidazole entities of bis(styryl)BODIPY behave independently in the binding process. The energy level diagram established using spectral and electrochemical studies suggested PET to be thermodynamically unfavorable in the ZnP‐bearing complex while for ZnPc‐ and ZnNc‐bearing complexes such a process is possible when zinc tetrapyrrole is selectively excited. Consequently, occurrence of efficient PET in the latter two dyads was possible to establish from femtosecond transient absorption studies wherein the electron transfer products, that is, the radical cation of zinc tetrapyrrole and the radical anion of BODIPY(Im)2, was possible to spectrally identify. From target analysis of the transient data, time constants of circa 3 ns for ZnPc⋅+:BODIPY⋅− and circa 0.5 ns for ZnNc⋅+:BODIPY⋅− were obtained indicating persistence of the radical ion‐pair to some extent. The electron acceptor property of bis(styryl)BODIPY in donor‐acceptor conjugates is borne out from the present study. [ABSTRACT FROM AUTHOR]

A panchromatic triad, consisting of benzothiazole (BTZ) and BF2‐chelated boron‐dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using 1H NMR spectroscopy, ESI‐MS, UV‐visible, steady‐state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ‐BODIPY‐ZnP, and dyads, BTZ‐BODIPY and BODIPY‐ZnP, along with the reference compounds BTZ‐OMe, BODIPY‐OMe, and ZnP‐OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ‐BODIPY‐ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ‐BODIPY‐ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from 1BTZ* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the 1BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two‐step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ‐BODIPY‐ZnP:ImC60, was formed by axially coordinating the triad with imidazole‐appended fulleropyrrolidine (ImC60), and parallel steady‐state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from 1ZnP* to ImC60. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, 1BTZ*‐BODIPY‐ZnP:ImC60→BTZ‐1BODIPY*‐ZnP:ImC60→BTZ‐BODIPY‐1ZnP*:ImC60→BTZ‐BODIPY‐ZnP.+:ImC60.− in the supramolecular tetrad. The evaluated rate of energy transfer, kEnT, was found to be 3–5×1010 s−1, which was slightly faster than that observed in the case of BODIPY‐ZnP and BTZ‐BODIPY‐ZnP, lacking the coordinated ImC60. The rate constants for charge separation and recombination, kCS and kCR, respectively, calculated by monitoring the rise and decay of C60.− were found to be 5.5×1010 and 4.4×108 s−1, respectively, for the BODIPY‐ZnP:ImC60 triad, and 3.1×1010 and 4.9×108 s−1, respectively, for the BTZ‐BODIPY‐ZnP:ImC60 tetrad. Initial excitation of the tetrad, promoting two‐step energy transfer and a final electron‐transfer event, has been successfully demonstrated in the present study. [ABSTRACT FROM AUTHOR]

A directly linked BODIPY–oxoporphyrinogen dyad has been newly synthesized and occurrence of sequential photoinduced energy and electron transfer upon fluoride anion binding to oxoporphyrinogen has been demonstrated by spectral, electrochemical and femtosecond transient absorption studies. [ABSTRACT FROM AUTHOR]

The bis‐iodo β‐pyrrole‐substituted BF2‐chelated dipyrromethene, I2BODIPY, and its structural analogue BF2‐chelated aza dipyrromethene, I2azaBODIPY, carrying a nitrogen at the meso‐position instead of carbon, were synthesized and characterized as new set of triplet sensitizers using different techniques. These sensitizers were further functionalized with fullerene, C60, at the central boron atom to build donor‐acceptor conjugates. Using spectral, electrochemical, and computational methods, these conjugates were characterized, and the energy levels were established. Intersystem crossing to populate the triplet state was observed upon excitation of I2BODIPY and I2azaBODIPY, however, the measured rates of kISC were found to be nearly two orders of magnitude higher for I2azaBODIPY (kISC∼1011 s−1) compared to I2BODIPY (kISC∼109 s−1). The energetics, kISC, and position of HOMO and LUMO levels was found to control the ability of the dyad to undergo electron transfer, although the donor‐acceptor distances were virtually the same in both I2BODIPY‐C60 and I2azaBODIPY‐C60 conjugates. Free‐energy calculations revealed that the photoinduced electron transfer process was thermodynamically feasible from only the singlet excited states in both conjugates. Consequently, electron transfer from the 1I2BODIPY* in competition with intersystem crossing was witnessed in the case of I2BODIPY‐C60 dyad while in the case of I2azaBODIPY‐C60 dyad, excitation of azaBODIPY led to a short‐lived charge transfer state involving the catechol bridge followed by populating the low‐lying 3I2azaBODIPY* state without promoting the process of charge separation involving C60. The lifetime of the charge‐separated states was in the ns range in the I2BODIPY‐C60 conjugate both in polar and nonpolar solvents. [ABSTRACT FROM AUTHOR]

BF2-chelated dipyrromethene, BODIPY, was functionalized to carry two styryl crown ether tails and a secondary electron donor at the meso position. By using a 'two-point' self-assembly strategy, a bis-alkylammonium-functionalized fullerene (C60) was allowed to self-assemble the crown ether voids of BODIPY to obtain multimodular donor-acceptor conjugates. As a consequence of the two-point binding, the 1:1 stoichiometric complexes formed yielded complexes of higher stability in which fluorescence of BODIPY was found to be quenched; this suggested the occurrence of excited-state processes. The geometry and electronic structure of the self-assembled complexes were derived from B3LYP/3-21G(*) methods in which no steric constraints between the entities was observed. An energy-level diagram was established by using spectral, electrochemical, and computational results to help understand the mechanistic details of excited-state processes originating from 1bis-styryl-BODIPY*. Femtosecond transient absorbance studies were indicative of the formation of an exciplex state prior to the charge-separation process to yield a bis-styryl-BODIPY .+-C60 .− radical ion pair. The time constants for charge separation were generally lower than charge-recombination processes. The present studies bring out the importance of multimode binding strategies to obtain stable self-assembled donor-acceptor conjugates capable of undergoing photoinduced charge separation needed in artificial photosynthetic applications. [ABSTRACT FROM AUTHOR]

Sequential photoinduced energy transfer followed by electron transfer and the formation of charge-separated states, which are primary events of natural photosynthesis, have been demonstrated in a newly synthesized multichromophoric covalently linked triad, PDI-SiPc-C60. The triad comprises a perylenediimide (PDI), which primarily fulfils antenna and electron-acceptor functionalities, silicon phthalocyanine (SiPc) as an electron donor, and fulleropyrrolidine (C60) as a second electron acceptor. The multi-step convergent synthetic procedure developed here produced good yields of the triad and control dyads, PDI-SiPc and SiPc-C60. The structures and geometries of the newly synthesized donor-acceptor systems have been established from spectral, computational, and electrochemical studies with reference to appropriate control compounds. Ultrafast energy transfer from 1PDI* to SiPc in the case of PDI-SiPc and PDI-SiPc-C60 was witnessed. An energy-level diagram established from spectral and electrochemical data suggested the formation of two types of charge-separated states, that is, PDI-SiPc.+-C60.− and PDI.−-SiPc.+-C60 from the 1SiPc* in the triad, with generation of the latter being energetically more favorable. However, photochemical studies involving femtosecond transient spectroscopy revealed the formation of PDI-SiPc.+-C60.− as a major charge-separated product. This observation may be rationalized in terms of the closer spatial proximity to SiPc of C60 compared to PDI in the triad. The charge-separated state persisted for a few nanoseconds prior to populating the 3SiPc* state during charge recombination. [ABSTRACT FROM AUTHOR]

Utilization of polyethyleneglycol (PEG) functionalized zinc phthalocyanine, (PEG)4ZnPc in (i) singlet oxygen generation, and (ii) in building energy harvesting donor-acceptor systems using fullerene, C60 as an acceptor via the well-known metal-ligand axial coordination approach is reported. The (PEG)4ZnPc was found to be capable of producing singlet oxygen with a quantum yield, ΦΔ of 0.77 in toluene, a value higher than that obtained for pristine ( t-bu)4ZnPc ( ΦΔ=0.54) carrying no PEG groups, revealing its usefulness in photodynamic therapy applications. Spectroscopic studies revealed efficient binding of phenylimidazole functionalized fullerene, C60Im with 1:1 stoichiometry to (PEG)4ZnPc. Binding constant K for the formation of (PEG)4ZnPc:ImC60 dyad was found to be 6×103 M−1 revealing moderate stability. Geometric and electronic studies of the dyad was arrived by B3LYP/3-21G(∗︁) method. The HOMO level was found to be on zinc phthalocyanine entity while the LUMO level was found to be on the C60 entity suggesting formation of (PEG)4ZnPc·+:ImC60· − charge separated state during the process of electron transfer reaction. Redox studies on the (PEG)4ZnPc:ImC60 dyad enabled accurate determination of the oxidation and reduction potentials of the donor-acceptor system, and to evaluate free-energy changes associated for the charge separation process. Kinetics of photoinduced charge separation and recombination in the (PEG)4ZnPc:ImC60 dyad was investigated using femtosecond transient absorption studies. Relatively long-lived charge separated states were confirmed for the dyad suggesting their potential usefulness in energy harvesting applications. [ABSTRACT FROM AUTHOR]

The natural photosynthetic light energy harvesting functional nano-architectures, organized exclusively by non-covalent interactions, have inspired the design of artificial supramolecular donor–acceptor systems held solely by self-assembly methods. Various non-covalent interactions have been employed to design such supramolecular conjugates. The present review highlights recent progress made toward the design and photochemical study of energy-harvesting systems assembled by metal–ligand axial coordination. Due to the remarkable photophysical and photochemical properties of porphyrins and phthalocyanines, and importantly, their ability to form metal–ligand coordination bonds, they have been widely used as sensitizers in the construction of donor–acceptor conjugates of this class. Both fullerene and non-fullerene molecules, functionalized with nitrogenous ligands that are capable of donating a pair of electrons to the central metal atom of tetrapyrrolic macrocycles, have commonly been used as electron acceptors. The significance of endohedral fullerenes and carbon nanotubes in mimicking natural photosynthesis by stabilizing the light induced charge-separated states in supramolecular multicomponent donor–acceptor assemblies held via metal–ligand axial coordination interaction is a burgeoning area of research. Particular attention has also been given to the generation of long-lived charge-separated states, in addition to the design principles and characterization of donor–acceptor systems assembled by metal–ligand axial coordination. [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]

To harvest energy from the near-infrared (near-IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near-IR and IR light-absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF2-chelated azadipyrromethene (ADP; to extend absorption and emission into the near-IR region) and fullerene as electron-donor and electron-acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near-IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated-ADP and benzanulated thiophene-ADP in the respective dyads, and triplet state of C60 in the case of BF2-chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near-IR region of the solar spectrum and for building fast-responding optoelectronic devices operating under near-IR radiation input. [ABSTRACT FROM AUTHOR]

To solve the energy demands for a sustainable society, it is crucial to use clean solar energy. Thus, finding methods for efficient light energy conversion is an important scientific goal. For this aim, molecular devices employing composites of nanocarbons such as fullerenes, single wall carbon nanotubes (SWCNTs), and graphene are promising. In this review, we survey light-induced electron-transfer processes of these nanocarbons hybridized with photosensitizers, which have proved to be more favorable than ordinary molecules. Fullerenes connected with suitable electron donors yield characteristic long-lived radical ion pairs, suitable for constructing artificial photosynthetic systems. For SWCNTs, a combination of photosensitizers is also crucial to achieve efficient photo-induced electron transfer processes. Furthermore, in the case of diameter-sorted semi-conductive SWCNTs, each tube can be treated as a 'molecule', thus, allowing molecular orbital calculations to probe the geometry as well as the electronic structures of the photosensitizer-nanotube hybrids. The two-dimensional graphene has afforded a new reaction field with a wide π-system for the attached molecules. For the confirmation of electron transfer processes, transient absorption methods in the wide wavelength regions have been used effectively. The kinetic data obtained in solution are quite useful to understand the electron transfer mechanisms, which afford useful guiding principles to construct efficient light-energy harvesting devices such as photoelectrochemical and photovoltaic solar cells. [ABSTRACT FROM AUTHOR]

Keywords: Keywords: click chemistry; N-doped graphene; photoinduced electron-transfer; zinc phthalocyanines; zinc porphyrins EN Keywords: click chemistry N-doped graphene photoinduced electron-transfer zinc phthalocyanines zinc porphyrins 1 1 1 04/21/22 20220419 NES 220419 B Heteroatom-doped graphene b is emerging as an outstanding material for tailoring the properties of graphene by chemical functionalization. Cover Feature: Formation and Photoinduced Electron Transfer in Porphyrin- and Phthalocyanine-Bearing N-Doped Graphene Hybrids Synthesized by Click Chemistry (Chem. Eur. J. 22/2022) With the use of classical click chemistry, N-doped graphene has been functionalized to carry well-known photosensitizer electron donors, viz., zinc porphyrin and zinc phthalocyanine. [Extracted from the article]

Photosensitized electron-transfer processes of fullerenes hybridized with electron donating or other electron accepting molecules have been surveyed in this review on the basis of the recent results reported mainly from our laboratories. Fullerenes act as photo-sensitizing electron acceptors with respect to a wide variety of electron donors; in addition, fullerenes in the ground state also act as good electron acceptors in the presence of light-absorbing electron donors such as porphyrins. With single-wall carbon nanotubes (SWCNTs), the photoexcited fullerenes act as electron acceptor. In the case of triple fullerene/porphyrin/SWCNT architectures, the photoexcited porphyrins act as electron donors toward the fullerene and SWCNT. These mechanisms are rationalized with the molecular orbital considerations performed for these huge supramolecules. For the confirmation of the electron transfer processes, transient absorption methods have been used, in addition to time-resolved fluorescence spectral measurements. The kinetic data obtained in solution are found to be quite useful to predict the efficiencies of photovoltaic cells. [ABSTRACT FROM AUTHOR]

A 'molecular clip' featuring a near-IR emitting fluorophore, BF2-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 ¹ZnP* 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 C60 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/SnO2/polyad electrodes. The presence of aza-BODIPY 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. [ABSTRACT FROM AUTHOR]

Abstract: This review describes light induced energy or electron transfer reactions in self-assembled supramolecular zinc porphyrin/zinc phthalocyanine and fullerene bearing donor–acceptor systems. The self-assembled supramolecular dyads and triads are formed by using one or two types of the binding mechanisms including metal–ligand axial coordination. The photochemical properties of the metal porphyrin/phthalocyanine and fullerene moieties are shown to be tuned in a controlled manner upon coordination of metal center. The nature of the linker between the donor and acceptor entities influences the overall self-assembly process followed by the photochemical reactivity. In these self-assembled supramolecular systems, the photoinduced charge separation occurs mainly from the excited singlet state of the donor; and the back electron transfer rates generally occurs giving reversible systems. In some of the reported donor–acceptor conjugates, the predicted acceleration of the charge separation process and deceleration of the charge recombination process have been clearly observed, mainly due to the small reorganization energies of fullerenes in electron transfer reactions. Elegantly designed supramolecular triads to achieve sequential electron transfer to obtain the charge-separated states, and sequential energy transfer followed by electron transfer to mimic the photosynthetic ‘antenna-reaction center’ have also been developed and studied. The relations between structures and photochemical reactivities of these novel supramolecular systems are discussed in relation to the efficiency of forward electron transfer and slowing down the charge recombination process. [Copyright &y& Elsevier]

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]

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 phtlsalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor-acceptor poiyads (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 unfevorable 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 (kENT) was monitored by performing transient absorption and emission measurements using pump-probe and upconversion techniques in toluene, respectively, and modeled using a Forster-type energy transfer mechanism. Such studies revealed the experimental ken-c values on the order of 1010 — 1011 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. [ABSTRACT FROM AUTHOR]

The attainment of a better understanding of the dependence of photoinduced electron transfer reaction rates on the molecular structures of the donor and acceptor entities results in improving the capture and storage of solar energy. Here, the intermolecular and supramolecular electron transfer processes from electron donors (porphyrins (P), chlorophylls (Chl), phthalocyanines (Pc) and naphthalocyanines (Nc)) and their metal derivatives to electron acceptors (fullerenes such as C60 and C70) studied by nanosecond and picosecond laser flash photolysis techniques in polar and nonpolar solvents are reviewed. For intermolecular systems in polar solvents, photoinduced electron transfer takes place via the excited triplet states of C60/C70 or via the excited triplet states of P/Pc/Nc, yielding solvated radical ions in polar solvents; thus, the back electron transfer rates are generally slow. In the case of the supramolecular dyads and triads formed by axial coordination, hydrogen bonding, crown ether complexation, or rotaxane formation, the photoinduced charge separation takes place mainly from the excited singlet state of the donor; however, the back electron transfer rates are generally quite fast. The relations between structures and photochemical reactivities of these novel supramolecular systems are discussed in relation to the efficiency of charge separation and charge recombination. [Copyright &y& Elsevier]