Polarizations of Low Energy Sn← S Transitions in Phthalaldehyde and 10-Methylene Anthrone

Abstract

The polarizations of the transitions to the lowest nπ* and the two lowest ππ* transitions of phthalaldehyde have been measured relative to the phosphorescence by the method of photoselection. The polarization of transitions to the lowest 10‐methylene anthrone nπ* state and the ¹L_a and ¹L_b ππ* states, which are split by exciton interaction, has also been determined. Molecular orbital calculations, spin‐orbit coupling, and considerations of the experimental results indicate that the $T_1{\mathrm{\leftarrow \; S}}_0$ transition of 10‐methylene anthrone is polarized along the carbonyl carbon‐oxygen bond (Z axis). Application of Platt's spectroscopic moment theory to the 10‐methylene anthrone system, considered as two weakly interacting benzene units perturbed by formyl and vinyl substituents, does not account for the observed polarization in the ππ* region for a Z‐polarized phosphorescence transition moment. The polarization curve in the ππ* region is explained on the basis of perturbed ¹L_a and ¹L_b benzene states split by inter‐ring exciton interaction. The nπ* transition is believed to be polarized in the molecular plane and perpendicular to the carbonyl axis; that is, along the Y axis. An alternation in polarization in the nπ* region is observed. Indirect evidence is presented which indicates that the $T_1{\mathrm{\leftarrow \; S}}_0$ transition in phthalaldehyde is primarily polarized along the Z axis. If this transition is localized in the formyl groups, a small out‐of‐plane component of the transition moment may be present since the formyl groups are not expected to be in the molecular plane. The higher ππ* transition in phthalaldehyde is polarized parallel to that of the phosphorescence and is assigned to the ${}^1{L}_a{\leftarrow }^{1}A$ benzene analog transition. The lower ππ* transition is perpendicular and is assigned to the ${}^1{L}_b{\leftarrow }^{1}A$ transition on the basis of Platt's spectroscopic moment theory. The polarization in the phthalaldehyde nπ* region is uncertain but appears to be polarized in the molecular plane and approximately parallel to the carbonyl carbon‐oxygen bond; that is, along the Z axis. The lifetime of the phthalaldehyde phosphorescence was found to be 3 msec.