Knowledge Bank

“The detailed polarization of absorption, fluorescence, and phosphorescence, in four benzene derivatives containing conjugative substituents has been studied by polarized photoselection techniques. Solutions of aniline (A), N,N-dimethylanaline (DMA), paraphenylenediamine (PPD), and $N,N,N\prime ,N\prime$-tetramethylparaphenylendiamine examine (TMPD) in 3-methylpentaric or EPA were studied at $77\circ K$. By various combinations of polarized monochromatic or broad band viewing of the fluorescence and the phosphorescence, a resolution is achieved of all dipole transitions (absorption or emission) into three components along the molecular axes. Out-of-plane activity is taken to be absent in absorption. Vibronically induced mixed polarization is found both absorption and emission. The strong vibronic activity in the lowest absorption band, $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb$, originates largely through vibrational coupling with the neighboring $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}La$, band. These states are almost certainly not pure benzene analogue- Vibronic matrix elements have been computed employing a range of wave functions which are mixed benzene and charge transfer configurations. The matrix elements are not sufficiently sensitive to the choice of wave functions over a fair range so that only qualitative statements regarding details of state functions (apart from their symmetry) are possible. At the same time a, most striking disparity is observed in the strength of vibronic activity in $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb$, absorption and. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Lb$, emission. Vibronic activity in emission in from 1 to 2 1/2 times weaker than in absorption. To understand this within the framework of first order Herzberg-Teller theory it is necessary to expand electronic wavefunctions about the equilibrium position of the \emph{excited} electronic state for the vibronic properties in emission and about the equilibrium nuclear position of the ground electronic state for the vibronic properties in absorption. Finally. the mixed polarization in the phosphorescence, which is strongly wavelength dependent in two derivatives would seem to reflect both a configuration-Space allowed component and configuration-space forbidden component-the latter doubtlessely reminiscent of the pure vibronic activity in benzene phosphorescence itself. The polarization of phosphorescence is predominantly long axis (in plane) in all cases (with considerable variation) and can easily be reconciled with an anticipated $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}La$, assignment.”