Spectral features of substituted 9-(phenoxycarbonyl)-acridines and their protonated and methylated cation derivatives

The long-wavelength absorption of eight 9-(phenoxycarbonyl)-acridines and the 10-H-9-(phenoxycarbonyl)-acridinium and 10-methyl-9-(phenoxycarbonyl)-acridinium cations derived from them, substituted with an alkyl or trifluoroalkyl group at the benzene ring, occurs above 300 nm as the superposition of four bands. Three of these bands occupy comparable positions (expressed in nm) in all the compounds; the fourth one, however, changes position, appearing in neutral molecules as a long-wavelength shoulder below 400 nm, but in cations as an almost separate band above 400 nm. The weak fluorescence resulting from excitation within the long-wavelength absorption band is red-shifted relative to absorption, such that Stokes shifts are similar for both neutral molecules and cations. Stokes shifts tend to increase with the orientational polarisability of a medium. Computations predict that long-wavelength electronic transitions are accompanied by structural changes in molecules. They also indicate that such transitions are followed by roughly uniform electron density changes in whole molecules accompanied by small changes in their dipole moments, which accounts for the weak absorption in the long-wavelength region. The predicted radiative and non-radiative deactivation rate constants suggest the occurrence of efficient spin-orbital coupling in the molecules investigated, which is the cause of the relatively low fluorescence quantum yields. Apart from the cognitive significance of these investigations, the results demonstrate that absorption of radiation by 10-methyl-9-(phenoxycarbonyl)-acridinium cations above 400 nm may influence their chemiluminescence output.