High resolution spectroscopy of a hydrocarbon with the triplet ground state, 2,2-dimethyl-2H-dibenzo [cd,k] fluoranthene and its closed-shell precursor in solid matrices

The triplet-triplet (T-1-T-0) fluorescence and excitation spectra of the ti tle compound have been recorded at 8 K in a Shpol'skii matrix, n-decane. Th e ground-state vibronic frequencies and intensities were similar to those o f a closed-shell model compound indicating that the four benzo rings have r etained their aromatic character in the biradicaloid species. The broad fea ture in the excitation spectra up-shifted by 800 cm with respect to the T-1 -T-0 origin was assigned to the second purely electronic transition. The ph otochemical bond rupture in the cyclopropane precursor of the biradicaloid, 1,1-dimethyl-la, 11b-dihydro-1H-benzo[k]cyclopropa [4,5]cyclopent[1,2,3-c, d]fluoranthene was carried out by a pulsed dye laser at 420 nm. This leads to spectral hole burning with a quantum yield of 1.5 +/-0.5% in frozen solv ent glasses. Because of intrinsic low-temperature photochemistry, the compo und is well suited for probing intermolecular interactions in glassy solids . In the second part the influence of hydrostatic pressure on spectral hole s is reported. A linear dependence of the pressure shift coefficient dv/dP on burning frequency v was obtained. The slope of the plot was treated in t erms of the isothermal compressibility of the matrix and the frequency at w hich there is no pressure shift was compared with the vacuum frequency of t he nonsolvated chromophores. To account for the nontrivial relationships be tween these parameters a model was proposed basing on several microscopic s olvent shift mechanisms with different interaction radii. (C) 2001 Elsevier Science B.V. All rights reserved.