THE EFFECT OF PRESSURE ON THE LUMINESCENCE OF THE FLEXIBLE DYE MOLECULE 6-AMINO-COUMARIN IN 4 SOLID POLYMERIC MEDIA

The luminescence energy and efficiency of 6-amino-coumarin (C-6) and C-6.HCl were measured to 100 kbar pressure at 295 K in four solid polymeric media. Data were also obtained at 0 kbar and 80 kbar at 85 K. The large Stokes shift (approximately 10 000 cm-1) in some media has been taken as an indication that this molecule may exhibit two different geometric conformations in the excited state (a twisted intramolecular charge transfer (TICT) molecule). In general, the emission shifts to lower energy and decreases in intensity with increasing pressure. From the combined pressure and temperature data, it is possible to separate the change in barrier height with pressure from the ordinary nonradiative processes. The increase in barrier height with pressure is larger in the more polarizable media, which indicates that the initial excited state is more polarizable than the emitting state.