U-Shaped Donor [Bridge] Acceptor Systems with Remarkable Charge Transfer Fluorescent Properties: An Experimental and Computational Investigation

Abstract

The photoinduced intramolecular electron transfer in two donor−bridge−acceptor systems was studied using (time-resolved) fluorescence and transient absorption techniques. DPN[8cy]DCV and DPMN[8cy]DCV consist of a 1,4-diphenylnaphthalene (DPN) and a 1,4-diphenyl-5,8-dimethoxynaphthalene (DPMN) electron donor, respectively, and the 1,1-dicyanovinyl acceptor (DCV) in both systems. The overall geometry of the saturated hydrocarbon bridge is U-shaped, separating the donor and acceptor by an 8-σ-bond through-bond distance and a 5.8 Å (center-to-center) through-space distance in the ground state. In all solvents fast electron transfer is observed in both systems resulting in a fluorescent charge transfer (CT) state. Especially for DPN[8cy]DCV CT fluorescence can be detected over a wide range of solvent polarity. The solvent dependence of the CT fluorescence position, lifetime, and quantum yield could thus be employed to estimate the solvent effect on the dipole moment of the CT state, the rate of charge recombination, and the electronic coupling (V) between donor and acceptor. It is concluded that in the (luminescent) CT state both the distance between donor and acceptor and their electronic coupling are virtually solvent independent, which excludes a solvent-mediated electron-transfer pathway. Gas phase (U)HF ab initio MO calculations carried out on the model molecule DMN[8cy]DCV (which contains a computationally less demanding 1,4-dimethoxynaphthalene donor) predict that the center-to-center distance between the two chromophores in the CT state is about 4.4 Å which amounts to a 1.4 Å contraction with respect to the ground state geometry. The degree of contraction is almost entirely due to pyramidalization at the DCV radical anion site and occurs in the direction of the dimethoxynaphthalene radical cation for electrostatic reasons. The calculated weak out-of-plane bending potential associated with this pyramidalization implies that the degree and direction of pyramidalization in the CT state of the DCV moiety can be preserved in solution and that it is fairly insensitive toward solvent polarity as shown by the results of UHF/6-31G(d) continuum solvation calculations and as supported by the experimental results for DPN[8cy]DCV. The small and constant D/A separation in the CT state also explains the experimentally found constancy of the electronic coupling, which must be of a direct through-space (TS) nature because no solvent molecules can be accommodated between D and A. Remarkably, while the charge recombination in DPMN[8cy]DCV displays the strong rate enhancement with increasing solvent polarity typical for charge recombination occurring under “inverted region conditions”, the rate of charge recombination in DPN[8cy]DCV is virtually constant over a wide range of solvent polarities. This very unusual behavior appears to be related to the presence of parallel charge recombination pathways to respectively the ground state and to a local triplet state with an opposite solvent dependence of their rate.