Approximate estimate of the form of molecular vibrations

Detailed polarized absorption and emission spectra are presented for anthracene and some of its methyl and methoxy derivatives in rigid solution at 77°K. PPP-SCF calculations show that no new states are introduced below 5 eV by the substitution. The observed effects of chemical substitution on state ordering, polarizations, and oscillator strength correlate quite well with the predictions of the PPP-SCF calculations. A vibrational analysis of the anthracene polarization spectra is carried out. In the near uv long-axis polarized absorption and fluorescence of anthracene, two vibronic origins are identified as a 1630 cm^?1b_1g molecular vibration and a weak 60 cm^?1b_g type of lattice vibration. All long-axis polarized structure in the near uv absorption can be assigned to Franck-Condon progressions built upon these vibronic origins in the ¹L_a band (which is electronically short-axis polarized). Differences in the spectral integrated transition intensity (oscillator strength) between the absorption and the emission reveal an additional long-axis component in the high energy region of the ¹L_a absorption which could be evidence for the hidden ¹L_b band in anthracene. However, there is no clear cut structural evidence for this state in the polarization spectrum. This hidden transition is made allowed by methoxy substitution. In the symmetric 2,3-dimethoxyanthracene, the ¹L_b band appears at 365 nm as a new transition. In 2-methoxyanthracene the ¹L_a and the ¹L_b are strongly mixed. The anomalous short-axis polarization of the ¹L_b band, predicted by the theory for 2-methoxyanthracene, is confirmed by the polarization data. Methoxy substitution also reveals new weak bands, located between the ¹L_b and ¹B_b bands, of B_1g and A_1g parentage, which are successfully predicted by the PPP-SCF calculations.