Photoinduced Intra- and Intermolecular Charge-Transfer Processes

In some systems, as for example the photosynthesis center, after light absorption a photoinduced electron transfer occurs. Much more sytems, however, form after absorption of a photon charge-transfer (CT) states with only partial electron transfer. This can be an intramolecular reaction as for example with 9,9'-bianthryl or a bimolecular process in which the charge shift occurs only upon the formation of excited complexes as for example the pyrene/N,N-diethylaniline exciplex. The fast formation of these strongly polar excited species is accompanied by considerable reorganization of the surrounding solvent shell, leading to significant volume effects. The corresponding reaction volumes can be determined by photoacoustic calorimetry (PAC). PAC is a method which is complementary to emission spectroscopy. PAC measures the laser pulse induced volumechanges via ultra-sonic detection. The volume changes can directly result from chemical reactions and indirectly be caused via the thermal expansion caused by radiationless deactivation processes. The quantitative separation of these contributions is possible. Thus reaction enthalpies and reaction volumes of photoinduced reactions can be determined by PAC.

A complete thermodynamic picture of the formation and decay of CT states, exciplexes and even excimers can be obtained by combining photoacoustic and emission measurements. We have performed such investigations with 9,9'-bianthryl and the pyrene/N,N-diethylaniline exciplex. Refs. 93,98

These studies have been financially supported by the Deutschen Forschungsgemeinschaft.

Procedures and Instruments:

Time-resolved PAC (ns-ms), iterative deconvolution,

stationary and time-resolved emission in the UV/VIS (ns-ms)region,

N2 laser, avalanche diode.