Investigation of solvent-mediated hole transfer in C-shaped donor-spacer-acceptor molecules

Solvents that enter the cleft between a donor and an acceptor in C-shaped donor-spacer-acceptor (DSA) molecules can facilitate electron transfer by making simultaneous orbital overlap with D and A. Such orbital interactions, referred to as electronic coupling, are requisite for electron transfer reactions. Solvent-mediated electronic coupling has been demonstrated for C-shaped DSA molecules with photoexcited electron donors. In such systems, electronic coupling has been shown to occur by way of the solvent's vacant molecular orbitals. Very little is known about the analogous role of solvent for DSA molecules employing photoexcited acceptors that undergo hole transfer (HT) reactions.; A novel C-shaped DSA molecule, pyrene-spacer-N,N-dimethylaniline (PSD), was synthesized for the study of solvent-mediated HT reactions. Pyrene serves as the photoexcited acceptor and is connected to N,N-dimethylaniline, the electron donor, by an 8-bond saturated spacer. Spectroscopic and kinetics studies were carried out in a number of solvents as a function of temperature. The intrinsic decay of pyrene's LE state, i.e., S₁ state, occurs with a lifetime of ∼300 ns, as determined from an acceptor only model of PSD (C-shaped pyrene-spacer-benzene). By contrast, PSD undergoes rapid HT (τ < 1 ns) to form a long-lived, emissive charge-transfer (CT) state (τ > 100 ns) in a variety of solvents. Evidence for the establishment of an LE CT state equilibrium was observed in toluene, benzene, and 2-methylfuran. By contrast, HT was irreversible in more polar solvents such as ethyl acetate, THF, anisole, and n-butyl acetate. Reported herein are qualitative and quantitative analyses of the CT emission band solvatochromism, temperature-dependent spectroscopic and kinetics studies, and an evaluation of the electronic coupling as a function of solvent.; Also described are two highly convergent synthetic routes to topologically distinct bichromophores, i.e., C- and L-shaped. Structural investigations of several C-shaped molecules are described, including NMR titration studies that unambiguously demonstrate that benzene occupies their clefts on the NMR time scale.