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 Original Research Article

The triplet–triplet (T1–T0) fluorescence and excitation spectra of the title compound have been recorded at 8 K in a Shpolʹskii matrix, n-decane. The ground-state vibronic frequencies and intensities were similar to those of a closed-shell model compound indicating that the four benzo rings have retained their aromatic character in the biradicaloid species. The broad feature in the excitation spectra up-shifted by 800 cm−1 with respect to the T1–T0 origin was assigned to the second purely electronic transition. The photochemical bond rupture in the cyclopropane precursor of the biradicaloid, 1,1-dimethyl-1a,11b-dihydro-1H-benzo[k]cyclopropa 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 solvent glasses. Because of intrinsic low-temperature photochemistry, the compound is well suited for probing intermolecular interactions in glassy solids. In the second part the influence of hydrostatic pressure on spectral holes is reported. A linear dependence of the pressure shift coefficient dν/dP on burning frequency ν was obtained. The slope of the plot was treated in terms of the isothermal compressibility of the matrix and the frequency at which there is no pressure shift was compared with the vacuum frequency of the nonsolvated chromophores. To account for the nontrivial relationships between these parameters a model was proposed basing on several microscopic solvent shift mechanisms with different interaction radii.