Time-resolved fluorescence study on the pressure-induced viscosity dependence of exciplex formation in liquid solution

The contribution of diffusion to the exciplex formation of the 1,2-benzanth racene (BZAN)/dimethylaniline (DMA) and pyrene (PY)/DMA systems in methycyc lohexane at pressures up to 400 MPa was investigated. The rate constant, ks , for the exciplex formation, which was determined by the analysis of the r ise and decay carve of the exciplex emission, decreased significantly with increasing pressure. The apparent activation volume for k(3), Delta V3(not equal), was 14.9 and 18.1 cm(3)/mol for BZAN/DMA and PY/DMA, respectively. For comparison, the rate constant, k(q), for the fluorescence quenching by carbon tetrabromide (CBr4) of BZAN and-PY that is believed to be fully diff usion-controlled was also measured as a function of pressure. The apparent activation volume for k(q), Delta V-q(not equal), was 17.9 and 20.1 cm(3)/m ol for BZAN/CBr4 and PY/CBr4, respectively. These results are interpreted b y assuming an encounter complex formed between the donor and acceptor pairs , and it was concluded that the exciplex formation reaction studied in this work is not fully diffusion controlled but competes with a diffusion proce ss of the donor and acceptor molecules that is expressed by a modified Deby e equation. It was also concluded that the quenching by CBr4 is not fully d iffusion controlled at the lower pressure range. The bimolecular rate const ant, k(bim) (= k(c)k(diff)/k(-diff)), for the exciplex formation was estima ted to be 1.5 x 10(10) M(-1)s(-1) for BZAN/DMA and 1.9 x 10(10) M(-1)s(-1) for PY/DMA. The activation volumes for the various rate processes also are discussed.