Emission of acridine and its aggregates isolated in the neon matrix

The results of experimental studies of fluorescence and phosphorescence of acridine in the low-temperature inert neon matrix, at 7 K, are reported. It is found that the low-temperature inert matrix of neon affects the energy levels of acridine molecules very weakly even as compared with nonpolar (ap rotic) and non-reactive solvent (e.g. hexane) and that there are different sites for acridine molecules in the neon matrix. However, the observed fluo rescence spectra are strongly dependent on the excitation wavelength and be sides the different (monomer) sites other emitting species are also contrib uting to the observed fluorescence emission of acridine in the neon matrix. Clear-cut evidence of the formation of singlet excimers of acridine in the neon matrix demonstrates itself as a very broad and structureless fluoresc ence spectrum with a relatively large shift from the origin of monomer (sit e) fluorescence which is characterized by a very distinct vibrational struc ture. The phosphorescence emission was observed only for the monomers. The observed differences in the low-energy part of excitation spectra of phosph orescence and fluorescence are discussed in terms of the close-lying excite d singlet states of n, pi* and pi, pi* character (mixed by the vibronic cou pling) and tentatively interpreted as due to the formation of resonance acr idine dimers. Their fluorescence spectrum is slightly shifted toward lower energies from the origin of monomer (site) fluorescence.