Probable location of a 1E2g state of the benzene molecule

Spectroscopic investigations of intermolecular interactions in organic molecular crystals

Besides the usual nπ* phosphorescence spectra, broad, long-wavelength nπ* phosphorescence spectra are observed in normal paraffine solutions, such as a heptane solution, of 1,4-naphthoquinone, and in a benzene solution of 5,14 : 7,12-pentacenediquinone at 77 K. The energy difference between the two spectra are ca. 750 and 980 cm−1 respectively. The intensity of the long-wavelength spectra relative to the short-wavelength spectra becomes weaker with decreases in the concentrations of the quinones, and the lifetimes of the long-wavelength spectra are shorter than those of the short-wavelength spectra. These long-wavelength spectra are assigned to those of the quinone aggregates. The excitation spectra of these phosphorescences have also been obtained. An explanation of the above red-shift of the nπ* phosphorescence spectra of the aggregates is presented; in it, the phosphorescence state of the aggregates corresponds to the nπ* triplet excimer state.

The long-wavelength nπ* phosphorescence spectra of p-quinones in polycrystalline solutions at 77 K

Electronic and vibrational energy transfer and relaxation mechanisms have been studied in crystalline ϱ-dibromobenzene. Variations in linewidths in the phosphorescence spectrum have been related to the vibrational exciton density-of-states functions. Migration by ground state vibrational excitons is found to be generally more probable than by triplet excitons in these crystals. Broadening in the absorption spectrum has been attributed to fast vibrational relaxation (≈10−13 s) enhanced by exciton—phonon coupling. The phosphorescence quantum yield is found to be constant throughout the SO → T1 absorption spectrum. External spin—orbit coupling has been found to inhance the activity of the totally symmetric fundamentals, whereas Herzberg—Teller vibronic coupling dominates in the forbidden part of the spectrum. The Herzberg—Teller active b2g mode is assigned as shifting in frequency from 277 cm−1 in the ground state to 104 cm−1 in the excited state.

Electronic and vibrational energy transfer and relaxation in crystalline ϱ-dibromodenzene

Evidence is presented to confirm triplet excimer emission from halogenated benzene crystals. The emission is not due to photochemical imurities or other effects provided the samples are in vacuum. It is shown that the excimer phosphorescence maxima shifts substantially to lower energy from crystal to solid solution, probably due to a reduction in the repulsive potential in the dimer ground state from solution to crystal. The triplet excimer energy levels in solution and in single crystals are presented for several symmetrically substituted halogenated benzenes and some qualitative data presented for the distribution of the excitaton energy in the monomer and dimer triplet manifold

G. C. Morris

Papers

Triplet Excimer Emission from Halogenated Benzenes