Local Density Augmentation around Exciplex in Supercritical Carbon Dioxide as Revealed by Transient Absorption and Fluorescence Lifetime Measurements

The pressure dependence of the decay of exciplex formed between acetophenone (AP) and N,N,N',N'-tetramethylbenzidine (TMB), and anthracene (A) and N,N-dimethylaniline (DMA) was studied in supercritical carbon dioxide by transient absorption and fluorescence techniques. Both the exciplexes showed similar trend in decay rate constant measured at various pressures of supercritical carbon dioxide. The results showed local density augmentation around exciplex in low-pressure (8.8-11 MPa) region. In the high-pressure (1117.5 MPa) region, decay rate constant could be well explained by Kirkwood equation. However, in the low-pressure region, exciplex was more stable than predicted by Kirkwood analysis, which has been interpreted as a result of local density augmentation around the exciplex. At a measured pressure and temperature, solvent aggregation around the A-DMA exciplex was greater than that around the AP-TMB exciplex. The difference in the solvent aggregation also known as local density augmentation around the exciplex has been attributed to the nature of exciplex species.