Studies on photophysical properties of some 2,7-dimethoxycarbazoles in various environments by steady state and time-resolved spectroscopic methods Part 2. INDO/S molecular orbital calculations, fluorescence lifetime measurements and polarization studies at 77 K

Abstract

Fluorescence excitation and polarization spectra and fluorescence and phosphorescence polarization spectra are presented for 2,7-dimethoxycarbazole (DMC) and 2,7-dimethoxy- N-methylcarbazole (DMNMC) at 77 K in 3-methylpentane (3MP) and ethanol (EtOH) glassy matrices. Nanosecond time-resolved fluorescence spectra have also been recorded for these systems. Semiempirical INDO/S-CI molecular orbital calculations are presented to correlate with the observed spectral changes caused by the methoxy substitution. The results are interpreted in terms of overlapping ππ* electronic transitions 1L a ← 1A and 1L b ← 1A. Both molecules exhibit fluorescence polarization and time-resolved fluorescence consistent with 1L b as the lowest excited singlet state. Both molecules exhibit dual fluorescence emission at 77 K whose average lifetime approximately equals the room temperature fluorescence lifetime in any solvent. A reversible two-state kinetic model has been used to interpret the double exponential decay of DMC and DMNMC in low temperature matrices. At any given temperature, the fluorescence lifetimes observed in these systems do not correspond to any of the “pure” electronic state lifetimes. The dual fluorescence observed in these dimethoxy molecules is mainly due to the fact that the ▪ radiationless process ( k ab ≈ 10 8 s −1) becomes relatively forbidden. A low density of states isoenergetic with the 1L a state is presumably the reason for this forbidden reaction.