Temperature dependence of local density augmentation around exciplex in supercritical carbon dioxide

To clarify the temperature dependence of local density augmentation around short-lived species, the pressure dependence of the formation and decay processes of exciplex between two neutral species, acetophenone (AP) and N,N,N′,N′-tetramethylbenzidine (TMB), in supercritical carbon dioxide was investigated by a transient absorption technique at 40, 55, and 70°C. In the high-density (>0.6gcm^-3) region, the decay rate constant could be described by the Kirkwood equation. However, in the low-density (0.35-0.6gcm^-3) region, the exciplex was more stable than that predicted by Kirkwood analysis, which means that strong influence of local density augmentation around the exciplex occurred. The local density augmentation can be described in terms of an excess density which is defined as a difference between local and bulk density. The excess density was derived from the difference between experimental results and Kirkwood analysis and exhibited a maximum at near-critical density. The excess density decreased with increasing temperature and became negligible at high temperatures and high bulk densities.