U-shaped donor [bridge] acceptor systems with remarkable charge transfer fluorescent properties: An experimental and computational investigation

The photoinduced intramolecular electron transfer in two donor-bridge-accep tor systems was studied using (time-resolved) fluorescence and transient ab sorption techniques. DPN[8cy]DCV and DPMN[8cy]DCV consist of a 1,4-diphenyl naphthalene (DPN) and a 1,4-diphenyl-5,8-dimethoxynaphthalene (DPMN) electr on donor, respectively, and the 1,1-dicyanovinyl acceptor (DCV) in both sys tems. The overall geometry of the saturated hydrocarbon bridge is U-shaped, separating the donor and acceptor by an 8-sigma -bond through-bond distanc e and a 5.8 Angstrom (center-to-center) through-space distance in the groun d 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 polar ity. 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 el ectronic coupling (V) between donor and acceptor. It is concluded that in t he (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 t hat the center-to-center distance between the two chromophores in the CT st ate is about 4.4 Angstrom which amounts to a 1.4 Angstrom contraction with respect to the ground state geometry. The degree of contraction is almost e ntirely 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 wit h this pyramidalization implies that the degree and direction of pyramidali zation in the CT state of the DCV moiety can be preserved in solution and t hat it is fairly insensitive toward solvent polarity as shown by the result s 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 el ectronic coupling, which must be of a direct through-space (TS) nature beca use no solvent molecules can be accommodated between D and A. Remarkably, w hile the charge recombination in DPMN[8cy]DCV displays;the strong rate enha ncement with increasing solvent polarity typical for charge recombination o ccurring under "inverted region conditions", the rate of charge recombinati on in DPN[8cy]DCV is virtually constant over a wide range of solvent polari ties. This very unusual behavior appears to be related to the presence bf p arallel charge recombination pathways to respectively the ground state and to a local triplet state with an opposite solvent dependence of their rate.