HIGH-PRESSURE LUMINESCENCE STUDIES ON THE TWISTED INTRAMOLECULAR CHARGE-TRANSFER MOLECULE 4-(N,N-DIMETHYLAMINO)BENZONITRILE IN POLYMER MATRICES

We present steady state and time dependent luminescence date from 4-(N,N-dimethylamino)benzonitrile (DMABN) in solid (PMMA) and poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA) and steady state date for poly(n-butyl methacrylate) PBMA to 100 kbar pressure. The results for PMMA and PEMA were essentially identical. At 1 atm the spectra consist of a strong peak at similar to 27 000 cm(-1) and a much wealer one at similar to 24 000 cm(-1) assigned to emission from the planar and twisted (twisted intramolecular charge transfer, TICT) singlet states, respectively. A new (phosphorescent) peak grew in rapidly with increasing pressure at similar to 22 500 cm(-1) (lower energy peak, LEP). By similar to 20 kbar it was 1.5-2.0 times as intense as the higher energy peak (HEP). Therefore it decreased in relative intensity by a factor of 2 by 100 kbar. Time dependence measurements gave a rate constant for the HEP of similar to 0.35 ns(-1), which increased by a factor of 2 at 100 kbar. In the 27 700 cm(-1) region two decays were obtained, one in the inverse nanosecond range and one on the order of inverse seconds. The data could be explained in terms of a model in which the slow steps involved transfer via the TICT singlet from the TICT triplet to the planar triplet, from which state the phosphorescent emission occurred. It was possible to relate in a very consistent way the trends displayed by the steady state and time dependent data even where the behavior was nonmonotonic with pressure. The data for PBMA were qualitatively similar, but the 22 500 cm(-1) peak did not appear until between 5 and 10 kbar. The maximum ratio of LEP/HEP at 20 kbar was significantly lower, and the decrease in the ratio at high pressure was less.