Influence of solvent viscosity and permittivity on the dynamics of large amplitude motions in semiflexibly bridged electron donor-acceptor systems

In electron donor-acceptor systems containing a nonrigid bridge, geometrical relaxations can occur after photoinduced charge separation under the influence of attractive Coulomb forces (harpooning mechanism). The investigated systems contained an aniline moiety as donor and a substituted naphthalene as acceptor, either one or both being substituted with large aliphatic wings. Two different types of large amplitude motions were identified: a rotation around a single bond leading from the fully extended charge transfer (ECT) conformer to an intermediate charge transfer (ICT) conformer, and the folding of the bridging piperidyl ring forming a compact charge transfer (CCT) state. To elucidate the effects of solvent viscosity and permittivity on these two relaxation processes, the time profiles of the fluorescence from the CCT state were recorded as functions of temperature (283 < T/K < 323) and pressure (0.1 < p/MPa < 320). The reaction constants for both relaxation processes were analysed assuming the functional form: k = Ceta(-a) exp[-DeltaG(EX)(double dagger)(epsilon(r))/RT]. It was found that the exponent a ranges between 0.75 and 0.85. In contrast to estimates for the gas phase, the deduced Gibbs activation energies DeltaG(EX)(double dagger)(epsilon(r)) range between 8 and 15 kJ mol(-1) for single bond rotation and between 10 and 18 kJ mol(-1) for folding of the piperidyl ring. The results confirm the expectation that consideration of permittivity effects is crucial for an adequate interpretation when educt and transition state exhibit different dipole moments.