| 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 consistof 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 saturatedhydrocarbon bridge is U-shaped, separating the donor and acceptor by an 8-σ-bond through-bond distanceand a 5.8 Å (center-to-center) through-space distance in the ground state. In all solvents fast electron transferis 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 theCT fluorescence position, lifetime, and quantum yield could thus be employed to estimate the solvent effecton 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 betweendonor 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 modelmolecule 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 whichamounts to a 1.4 Å contraction with respect to the ground state geometry. The degree of contraction is almostentirely due to pyramidalization at the DCV radical anion site and occurs in the direction of thedimethoxynaphthalene radical cation for electrostatic reasons. The calculated weak out-of-plane bendingpotential associated with this pyramidalization implies that the degree and direction of pyramidalization inthe CT state of the DCV moiety can be preserved in solution and that it is fairly insensitive toward solventpolarity as shown by the results of UHF/6-31G(d) continuum solvation calculations and as supported by theexperimental results for DPN[8cy]DCV. The small and constant D/A separation in the CT state also explainsthe 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 chargerecombination in DPMN[8cy]DCV displays the strong rate enhancement with increasing solvent polaritytypical for charge recombination occurring under “inverted region conditions”, the rate of charge recombinationin DPN[8cy]DCV is virtually constant over a wide range of solvent polarities. This very unusual behaviorappears to be related to the presence of parallel charge recombination pathways to respectively the groundstate and to a local triplet state with an opposite solvent dependence of their rate.
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