Volume change associated with large photoinduced dipole formation in a rigid donor-acceptor compound: New approach to optoacoustic volume determination

Time-resolved laser-induced optoacoustic spectroscopy was used to study the charge separation process taking place in the excited state of a donor-bridge-acceptor (D-br-A) compound in alkane solvents. This molecule contains a D-A pair separated by a rigid saturated hydrocarbon bridge 6 sigma bonds long. Excitation at 308 nm results in very efficient long-range charge separation, leading to formation of a charge-transfer (CT) state with a dipole moment of 38 D. By monitoring the pressure waves generated during the decay of the excited species, we could discern three consecutive relaxation processes. To separate the contributions of enthalpy and structural volume changes (Delta V-str), experiments were carried out with a series of n-alkanes having different photothermal properties. However, the usual separation method clearly failed for this system, and a new approach was developed to ensure that Delta V-str remained constant across the alkane series. This consisted of selecting conditions where the solvent compressibility could be considered constant over the experimental range. Ln this way, Delta V-str values between -110 and -200 mL/mol were obtained, depending on alkane length and temperature. These are remarkably large contractions, which can be attributed only to electrostriction of the alkane solvent around the dipolar CT species. Interestingly, the contractions are as large as predicted by classical electrostatic theory. The quantum yield of triplet formation, Phi(T), determined from the optoacoustic data, revealed a strong dependence on the oxygen content of the solution. A value of Phi(T) = 0.04 was obtained for the intrinsic quantum yield in the absence of oxygen.