Orientational Dependence of Cofacial Porphyrin-Quinone Electronic Interactions within the Strong Coupling Regime.

Abstract

Understanding how donor (D)-acceptor (A) relative orientation impacts electronic coupling (HDA) in face-to-face D-A systems is complicated by the fact that structural relaxation events generally occur on the time scales of photoinduced charge separation (CS) and thermal charge recombination (CR) in solution. We examine the relative magnitudes of electronic coupling HDA in two strongly coupled face-to-face and rigid (porphinato)zinc(II)-quinone (PZn-Q) assemblies, 1β-ZnA, and 1β-ZnB: these structures are diastereomers in which the six quinonyl carbon atoms lie in virtually an identical arrangement relative to the PZn plane at indistinguishable sub-van der Waals D-A interplanar separations. Steady-state and time-resolved experimental and computational data acquired for these systems underscore that these minor differences in relative D-A cofacial orientation give rise to disparate HDA magnitudes for both photoinduced CS and thermal CR [HDA(1β-ZnA) ~2-5 fold greater than HDA(1β-ZnB)]. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) computations illuminate the nature of direct charge transfer states, and the electronic structural factors that give rise to these differential HDAs. These data show that these HDA differences derive from more extensive mixing of locally excited (LE) and CS states 1β-ZnA relative to 1β-ZnB, and that these differences track experimental excitation energies and oscillator strengths, as well as electronic coupling matrix elements determined from steady-state electronic spectral data and the magnitudes of thermal CR rate constants measured via pump-probe spectroscopy. Collectively, this body of work shows that electron transfer dynamics may be manipulated in cofacial D-A systems, even at sub-van der Waals contact, provided that conformational rigidity precludes structural fluctuations that modulate D-A interactions on the charge transfer time scale.