Conformational dynamics of charge-transfer states in donor-bridge-acceptorsystems

Donor-acceptor compounds containing a phenylenediamine electron donor and a naphthalene, a cyanobenzene, or a cyanonaphthalene acceptor were studied. The two chromophores are connected by three different bridging units, consi sting of CH2 groups linked to a semiflexible piperidine or piperazine ring or to a rigid 2,5-diazabicyclo[2.2.1]heptane group. All donor-acceptor comp ounds show photoinduced charge separation, resulting in the formation of a compact charge-transfer (CCT) state in nonpolar solvents. The conformationa l change needed to arrive at this species is impossible in a nonpolar polym er matrix and also in methylcyclohexane at low temperature. In the cases of the piperidine- and piperazine-bridged donor-acceptor compounds, evidence for the involvement of an extended charge-transfer (ECT) species as a precu rsor of the CCT species was obtained from time-resolved fluorescence measur ements. In contrast to harpooning systems studied previously, the energy di fferences between the species in the present case are so small that their i nterconversions are reversible within the excited state lifetimes.