Volume changes associated with intramolecular exciplex formation in a semiflexible donor-bridge-acceptor compound

The photophysical processes taking place in the excited state of a donor-bridge-acceptor compound (1) in alkane solvents were studied with time-resolved laser-induced optoacoustic spectroscopy. 1 contains an aniline/cyanonaphthalene D/A pair, separated by a semiflexible saturated hydrocarbon bridge. Excitation at 308 nm leads to efficient long-range charge separation, followed by rapid Caulomb-induced intramolecular exciplex formation. By monitoring the pressure waves generated by the decay of the excited species, three consecutive heat release processes could be discerned. In order to separate the contributions to the observed acoustic signals of structural volume changes (Delta V-str) and enthalpy changes, experiments were carried out in a series of normal alkanes, differing in their photothermal properties. A value of Delta V-str = -40 +/- 5 mL/mol was obtained for the difference in reaction volume between the ground state and the exciplex state of 1. This large contraction should be attributed in part to electrostriction of the solvent around the dipolar species, according to classical electrostatic theory. Furthermore, then seems to be an additional contraction due to the internal volume change of 1, i.e., the conformational change involved in the intramolecular exciplex formation.