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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]

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]

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]

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-free-base porphyrin (AlPor-Ph-H2Por) dyad, the AlPor occupies the basal plane, while the free-base porphyrin (H2Por) with electron withdrawing groups resides in the axial position through a benzoate spacer. The NMR, UV-visible absorption, and steady-state fluorescence studies confirm that the coordination of pyridine appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the dyad in noncoordinating solvents afford vertically arranged supramolecular self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por). Time-resolved studies revealed that the AlPor in dyad and triads undergoes photoinduced energy and/or electron transfer processes. Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-dichlorobenzene), excitation of AlPor leads singlet-singlet energy transfer from the excited singlet state of AlPor (¹AlPor*) to H2Por with a moderate rate constant (kEnT) of 1.78 × 108 s-1. In contrast, excitation of AlPor in the triad results in ultrafast electron transfer from TTF to ¹AlPor* with a rate constant (kET) of 8.33 × 109-1.25 × 1010 s-1, which outcompetes the energy transfer from ¹AlPor* to H2Por and yields the primary radical pair TTF+•-AlPor-•-H2Por. A subsequent electron shift to H2Por generates a spatially well-separated TTF+•-AlPor-H2Por-• radical pair. [ABSTRACT FROM AUTHOR]