Studies of excited electronic states of anthracene and some of its derivatives by photoselection and PPP-SCF calculations
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 -1b1 g molecular vibration and a weak 60 cm -1bg 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 1La 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 1La absorption which could be evidence for the hidden 1Lb 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 1Lb band appears at 365 nm as a new transition. In 2-methoxyanthracene the 1La and the 1Lb are strongly mixed. The anomalous short-axis polarization of the 1Lb band, predicted by the theory for 2-methoxyanthracene, is confirmed by the polarization data. Methoxy substitution also reveals new weak bands, located between the 1Lb and 1Bb bands, of B1 g and A1 g parentage, which are successfully predicted by the PPP-SCF calculations.