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 ^TC_60* (and ^TC_70*) to 1 occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of 1.