STUDIES ON THE PHOTOPHYSICAL PROPERTIES OF SOME 2,7-DIMETHOXYCARBAZOLES IN VARIOUS ENVIRONMENTS BY STEADY-STATE AND TIME RESOLVED SPECTROSCOPIC METHODS .1. SYNTHESIS, ABSORPTION AND FLUORESCENCE-SPECTRA AT ROOM-TEMPERATURE

The electronic spectroscopic and photophysical properties of 2,7-dimethoxy-N-methyl carbazole (DMNMC) and 2,7-dimethoxycarbazole (DMC) in various polar and non-polar environments at room temperature were compared with those of N-methylcarbazole (NMC). The synthesis and characterization of these compounds are described. The first two electronic transitions L-1b <-- A-1 and L-1a <-- A-1 which are well separated and characterized in NMC and carbazole are mixed in the absorption and fluorescence spectra of DMNMC and DMC. This "proximity effect" is responsible for the unidentified first absorption band system observed in solvents at room temperature, for a marked decrease in both the fluorescence quantum yields and lifetimes (NMC, tau-F = 15.1 ns; DMNMC, tau-F = 8.4 ns; DMC, tau-F = 3.9 ns in 3-methylpentane) and for a loss of the mirror-image between the absorption and the fluorescence spectra of these dimethoxy derivatives. The importance of the vibronic interactions between these two states is shown to be controlled (reduced) by N-methylation and/or the intermolecular hydrogen bonding formation in a proton-acceptor solvent such as ethanol.