Fluorescence and phosphorescence emission and nonradiative relaxation of acridine in a crystalline matrix of 2,3-dimethylnaphthalene

The results of experimental studies of fluorescence and phosphorescence of acridine, selectively excited within its absorption band, in a crystalline host lattice of 2,3-dimethylnaphthalene, are reported. The energies of the first excited singlet and triplet states of acridine in this crystalline matrix were determined. The spectra of both emissions display a broadening caused by the orientational disorder of the host lattice. The intensity and the decay time of the fluorescence and phosphorescence show moderate temperature dependence (in the 4.2–300 K temperature range). It is concluded that over the whole temperature range the most important mode of the nonradiative relaxation of the excited acridine molecule is that of the (nearly) temperature-independent intersystem crossing from the S1(π, π∗) singlet to the T3(π, π∗) and/or T2(n, π∗) triplet states.