Supramolecular tetrad featuring covalently linked bis(porphyrin)-phthalocyanine coordinated to fullerene: construction and photochemical studies.

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.