Knowledge Bank

The polarized absorption and emission spectra of biphenyl have been investigated at $4.2\circ K$ in various media. The first multiplicity allowed transitions of biphenyl-$h10$ (at 33,357 $cm-1$) and biphenyl-$d10$ (at 33,501 $cm-1$) have been assigned as arising from the out of phase combination of the benzene $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B20$ state (ca. $2600\backslash AA$). The $b2u$ vibration which perturbs this $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3g\to \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A1g$ transition (610 $cm-1$ in the ground state of biphenyl-$h10$) arises from the benzene $e2g$ vibration ($606cm-1$ in the ground state) which mixes the benzene $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}E10$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2u$ states. The vibronic coupling mechanism in the first and second states of biphenyl is discussed.