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Publication costs assisted by Consigtio Nazionale delle Ricerche (Roma) Intramolecular reasons were sought for the deviation of absorption and emission spectra of polyphenyls from the mirror image relation. As a working hypothesis it was supposed that the differences between absorption and fluorescence spectra originate in the excitation of the twisting mode of interannular bonds. Curves of the potential energy as a function of the dihedral angle between phenyl rings were obtained for the ground state and the lowest singlet excited states by using semiempirical calculation procedures. A polynomial approximation was adopted for the potential functions so that vibrational energies and wave functions could be calculated simply by expanding the wave functions in a harmonic oscillator basis set. By this technique, Franck-Condon distributions were obtained for both absorption and emission. The contours of the distributions enabled us to explain both the fine structure of fluorescence spectra and the structureless long-wavelength uv bands in polyphenyls.

The photophysical and photochemical properties and the ground-state conformational equilibrium of fz-azzi-rz-styrylphenanthrene (n-StPh, with zi = 1, 2, 3, 4, 9) have been studied in inert solvents. The kinetic parameters of the competitive radiative and reactive decay processes have been obtained. A detailed analysis of the fluorimetric behavior as a function of the excitation wavelength and temperature has allowed the distinct decay parameters and the ground-state energy difference of the two rotamers of zrazu-3-StPh to be obtained. Parallel theoretical calculations of the potential energy curves for the internal rotation of the phenanthryl group in the ground state, of the energies and oscillator strengths of the lowest transitions, and of the activation energies for trans -*• cis isomerization in the ground and lowest excited singlet states have been carried out with a modified (CS) INDO method. The results of the experimental and theoretical studies are in satisfactory agreement and provide a general description of the photophysical and photochemical behavior of this class of compounds.

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