Knowledge Bank

Aniline vapor has a well resolved vibronic absorption spectrum in the region from 3000 to 2750 \AA. By means of high resolution phase fluorimetry we have determined the spectrum of fluorescence life times and quantum yields over this region with a band width of 2.5 \AA. The emission spectrum was not resolved. Relative values of quantum yields for different wavelengths were obtained directly, while absolute values were estimated from literature data. Quantum yields fluctuate between 0.25 and 0.5 with a downward tendency as the energy increases. The reciprocal life time, $1/\tau =kisc+kj$ (the sum of intersystems crossing and radiative transition probabilities) varies in the range (1.3 to 2.3) $\times 103sec-1$ with an upward trend toward higher energies. Moreover, the reciprocal life times show sharply pointed minima whereas the quantum yields have maxima at the wavelengths of all major absorption peaks in the aniline spectrum. The larger portion of this effect can be attributed to changes in $kisr$ which are interpreted as vibronic fine structure, superimposed on spin-orbit coupling. The latter mixes the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A1$ states involved here. Vibronic coupling between these states is symmetry-forbidden but can be induced by certain vibrational distortions. In aniline, the major absorption peaks are due to totally symmetric ($a1$) or to ($b1$) modes. For molecules selectively excited to these modes, the selection rule is not likely to be lifted and hence, vibronic coupling and the value of $kisr$ will be at a minimum.