Knowledge Bank

The polarization of the 0, 0-band of the phosphoresence of benzaldehyde, acetophenone, benzophenone and anthrone were measured relative to their near ultraviolet absorption. For these molecules it is accepted that the lowest triplet and the lowest singlet states are $(n,\pi \ast )$ in assignment. Positive polarization of the most intense progression in the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}U(n,\pi \ast )\leftarrow A1$ system, except for anthrone, indicates that this transition is allowed through perturbation by an $a2$ vibration and hence intensity is stolen from an $A1$ state. A second weaker progression having negative polarization indicates perturbation by a $b2$ vibration with intensity stolen from a $B1$ state. In anthrone the situation is reversed. Negative polarization of the principal progression indicates that the transition in this molecule is mainly allowed through a $b2$ vibration. These vibronic effects are discussed in terms of $Pople-Sidman\prime s1$ and $Callomon-Innes2$ scheme for formaldehyde. In benzophenone and anthrone, the coupling of the dipole moment vectors of two benzene rings causes the $Lb$ state to split into $A1(Lb+Lb)$ and $B2(Lb-Lb)$ states. The polarization of the second absorption band of these molecules indicates that this band actually consists of two transitions $A1\leftarrow A1$ and $B2\leftarrow A1$.