Empirical Quantitative Relationship between Molecular Structure and Phosphorescence Transition Energy of Polycyclic Aromatic Thiophenes

Polycyclic aromatic thiophenes (thiaarenes) ex­ hibit vibronically well-resolved phosphorescence spectra in rigid matrices (e.g. ethanol at 77 K). Al­ though several papers on the properties of the lowest triplet (T,) states o f thiophene benzologs have appeared in the literature [1 -5 ] , no systematic study on the relationship between the T } state ener­ gies (phosphorescence 0 . 0 bands) of polycyclic aro­ matic hydrocarbons and their thiophene analogs seems to be available. Such relationships, however, could be of some value in connection with the struc­ ture elucidation of unknown thiaarenes. The present study concerns the thiaarenes 1 20. Phospho­ rescence data o f compounds 1, 4. 7, and 1 2 1 6 were taken from the literature, while to our knowl­ edge, data of the remaining compounds hitherto have not been reported (with the exception of the phosphorescence 0.0 band energies of 5, 9, and 18 measured in a Shpol'skii matrix at 63 K [4]). Three different linear models A, B, and C were tested by correlation analysis. The models differ by the variables k _ __ o which are defined as fol­ lows: