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-<i>tert</i>-butyl phthalocyanine (<b>1</b>) with C<sub>60</sub> and C<sub>70</sub> 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/<b>1</b> complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of <b>1</b>, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of <b>1</b> on the UV–vis spectral characteristics of C<sub>60</sub> and C<sub>70</sub> have been explained using a theoretical model that takes into account the interaction between electronic subsystems of <b>1</b> with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of <b>1</b> in the presence of both C<sub>60</sub> and C<sub>70</sub>. The average binding constants of the C<sub>60</sub> and C<sub>70</sub> complexes of <b>1</b> (estimated by UV–vis and steady state fluorescence measurements) are determined to be 18 330 dm<sup>3</sup>·mol<sup>–1</sup> (12 595 dm<sup>3</sup>·mol<sup>–1</sup>) and 19 160 dm<sup>3</sup>·mol<sup>–1</sup> (15 292 dm<sup>3</sup>·mol<sup>–1</sup>) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C<sub>70</sub>/<b>1</b> species. The faster charge recombination of the fullerene/<b>1</b> systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the <i>ab initio</i> method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and <b>1</b> during fullerene/<b>1</b> interaction. A variable temperature <sup>13</sup>C NMR study nicely demonstrates that the end-on orientation of C<sub>70</sub> is very much responsible for the low selectivity in binding between C<sub>60</sub>/<b>1</b> and C<sub>70</sub>/<b>1</b> systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited <b>1</b> to C<sub>60</sub> and C<sub>70</sub> in the C<sub>60</sub>/<b>1</b> and C<sub>70</sub>/<b>1</b> complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from <sup>T</sup>C<sub>60*</sub> (and <sup>T</sup>C<sub>70*</sub>) to <b>1</b> occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of <b>1</b>.