Substituent-sensitive fluorescence of sequentially N-alkylated tetrabenzotetraaza[8]circulenes

Abstract

We explore the use of substituent-sensitive balance between fluorescence and non-radiative decay as a tool for optical tuning of promising materials for organic light emitting diode applications. A series of N-butylated tetrabenzotetraaza[8]circulenes is studied computationally in order to explain the gradual decrease of fluorescence intensity with the increase of the substituent number. The inter-system crossing probability is found to increase upon the gradual substitution of the circulene macrocycle as a result of the decrease of the S₁–T₁ energy gap due to the deformation of the tetrabenzotetraaza[8]circulenes and therefore the distortion of the π-conjugation within the macrocycles. In contrast, the S₁–T₁ spin–orbit coupling matrix elements are quite insensitive to the number of outer substituents. As a result, the fluorescence-responsible ππ* transition becomes less intense and the fluorescence rate constant decreases.