HIGH-PRESSURE STUDY OF FLEXIBLE FLUORESCENT DYE MOLECULES IN SOLID POLYMERIC MEDIA .1. JULOLIDINEMALONONITRILE

The pressure effect on the fluorescence of julolidinemalononitrile (JDMN) in several solid polymers has been studied. An increase of fluorescence intensity by a factor of 8 to 15 has been observed, in different polymers, within 120 kbar. Two general regions of pressure behavior have been distinguished, in all polymers. In the first pressure region (0-20 kbar), the amplification of fluorescence intensity is explained by a retardation of motion (twist) in the excited state by increasing pressure. This process is shown to be controlled by an energy barrier between two conformations of molecule (planar ''P'' and twisted ''T'') in its excited state. The relaxation from one form to the other is a single process reflecting thus one possible twist of the malonitrile group. The exponential relation obtained between nonradiative rate for intramolecular twist (k(nr)') and emitting state energy (E(fP)) implies the pressure insensitivity of the energy between intersection of the potential wells for ''P'' and ''T'' excited states relative to the ''P'' ground state. In the second pressure region (> 40 kbar), the intensity decrease is controlled by the nonradiative rate k(p)nr for the originally excited state. It has been shown that the energy gap law is applicable to explain experimental data in this region.