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By considering the character of n-conjugated systems of polycyclic aromatic hydrocarbons (PAHs), it has become apparent that a new two centre electron repulsion integral, new-y, which takes the chemical softness of a n-conjugated system into account is useful for the Pariser-Parr-Pople molecular orbital (PPP MO) calculations of the excitation energy of the p-band of PAHs. Employing the correlation between the chemical softness and the absolute hardness, the method to evaluate the chemical softness parameter k was established. The calculated excitation energies for 38 PAHs were greatly improved compared with the calculated ones using the conventional NishimotoeMataga y (NeM-y) function, and were well correlated with the observed ones. This is the first example of precise PPP MO calculations of excitation energies of various PAHs using new-y.Paper 4/07756C

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Fluorescence emission spectra are reported for 1-azabenz[a]anthracene, 2-azabenz[a]anthracene, 9-azabenz[a]anthracene, 4-azachrysene, 1,2,7,8-tetraazacoronene, dibenzo[2,3:10,11]perylo[1,12bcd]furan, dibenzo[2,3:10,11]perylo[1,12bcd]thiophene, and dibenzo[2,3:10,11]perylo[1,12bcd] selenophene dissolved in various organic solvents. Results of these measurements are used to classify the solutes into probe or nonprobe molecules depending upon whether emission intensity ratios vary systematically with solvent polarity. Of the eight aromatic solutes examined, only the three azabenzanthracenes showed slight probe character.