Role of Intramolecular Charge‐Transfer Transitions in Spin‐Orbit Coupling of Aniline and Its N‐alkyl Derivatives

Abstract

The lone-pair electrons in aniline and related molecules can give rise to intramolecular charge-transfer (CT) transitions corresponding to the promotions of an electron from the lone-pair orbital to antibonding π orbitals of the benzene ring. The CT transitions lead to appearance of two-center spin-orbit interaction elements involving the lone-pair orbital (Θ) and the 2 p π orbital (χ1) of the adjacent carbon atom. The matrix elements 〈χ1∣ HSO ∣ Θ〉 is non-vanishing if the lone-pair orbital is not strictly parallel to the 2 p π orbitals of the benzene. This condition is indeed met in aniline and its N-alkyl derivatives, and intramolecular CT transitions are expected to be important sources of intensity for phosphorescence of these molecules. The conclusion is supported by: (1) shortening of phosphorescence lifetime and increase in phosphorescence intensity polarized parallel to the transition vector of CT transition (1La band) with decrease in the energies of CT states, (2) shortening of lifetime and increase in phosphorescence intensity polarized parallel to the transition vector of CT transition accompanying steric hindrance, and (3) short radiative lifetimes of the lowest triplet states as compared to radiative lifetimes in benzene and its alkyl derivatives.