THE EFFECT OF PRESSURE ON FLUORESCENCE OF CRYSTALLINE FLEXIBLE DYE MOLECULES

The effect of high pressure on the fluorescence of flexible dye molecules of a series of para-substituted benzylidenemalononitriles in the crystalline state has been studied. Contrary to their behavior in polymeric environments where over a considerable pressure range only an increase of the fluorescence intensity was observed, in crystalline environments two different types of intensity behavior are exhibited. Polyester yellow 6G-LSW (PY) (up to 20 kbars), as in the polymeric environment, exhibits an increase of fluorescence intensity which is considered to be due to an increase of the barrier height for the intramolecular twist. The relation between emitting state energy and fluorescence intensity is derived. Above 20 kbars for PY and over a whole pressure range for p-N,N-dimethylaminobenzylidenemalononitrile (DMABMN) and julolidinemalononitrile (JDMN) the fluorescence intensity decreases and is primarily controlled by the nonradiative rate k(p)nr from the originally excited state. For this case, it has been shown that energy gap law is applicable to interpret the behavior of the intensity and emission energy with pressure.