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

Abstract

Articles you may be interested inCharacterization of the minimum energy path for the reaction of singlet methylene with N2: The role of singlet methylene in prompt NOThe polarizations of the transitions to the lowest n1f* and the two lowest 1f1f* transitions of phthalaldehyde have been measured relative to the phosphorescence by the method of photoselection. The polarization of transitions to the lowest lO-methylene anthrone n1f* state and the 1 La and 1 Lv 1f1f* 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 T1-SO transition of lO-methylene anthrone is polarized along the carbonyl carbon-oxygen bond (Z axis). Application of Platt's spectroscopic moment theory to the lO-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 1f1f* region for a Z-polarized phosphorescence transition moment. The polarization curve in the 1f1f* region is explained on the basis of perturbed 1 La and 1 L" benzene states split by inter-ring exciton interaction. The n1f* 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 n1f* region is observed. Indirect evidence is presented which indicates that the T1-SO 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 1f1f* transition in phthalaldehyde is polarized parallel to that of the phosphorescence and is assigned to the 1 La~A benzene analog transition. The lower 1f1f* transition is perpendicular and is assigned to the 1 Lv~A transition on the basis of Platt's spectroscopic moment theory. The polarization in the phthalaldehyde n1f* 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.