Photophysical and Theoretical Insights on Fullerene/Zincphthalocyanine Supramolecular Interaction in Solution

The present article reports photophysical studies on supramolecular interaction of a zinc phthalocyanine derivative, namely, zinc-2,9,16,23-tetra-tert-butyl phthalocyanine (1) with C60 and C70 in solvents having varying polarity, i.e., toluene and 1,2-dichlorobenzene (DCB). The interesting feature of the present work is the observation of charge transfer (CT) absorption bands of the fullerene/1 complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of 1, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of 1 on the UV–vis spectral characteristics of C60 and C70 have been explained using a theoretical model that takes into account the interaction between electronic subsystems of 1 with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of 1 in the presence of both C60 and C70. The average binding constants of the C60 and C70 complexes of 1 (estimated by UV–vis and steady state fluorescence measurements) are determined to be 18 330 dm3·mol–1 (12 595 dm3·mol–1) and 19 160 dm3·mol–1 (15 292 dm3·mol–1) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C70/1 species. The faster charge recombination of the fullerene/1 systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the ab initio method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and 1 during fullerene/1 interaction. A variable temperature 13C NMR study nicely demonstrates that the end-on orientation of C70 is very much responsible for the low selectivity in binding between C60/1 and C70/1 systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited 1 to C60 and C70 in the C60/1 and C70/1 complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from TC60* (and TC70*) to 1 occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of 1.