Aggregation Induced Emission in the Tetraphenylthiophene Crystal: The Role of Triplet States

Propeller-shaped molecules have received much attention due to their enhanced emission in the condensed phase (Aggregation Induced Emission, AIE) and their potential use in optoelectronic devices. In this contribution, we examine the excited state mechanisms of tetraphenyl-thiophene (TPT), one member of the family which features weaker AIE. We perform a detailed analysis of the potential energy surfaces with special focus on the role of triplet states considering the crystal structure, intermolecular interactions, exciton couplings and reorganisation energies in the vacuum and solid state. In contrast with other members of the propeller-shaped family, nonradiative decay in TPT is driven by bond breaking. Because of the significant spin-orbit couplings along the reaction coordinate, intersystem crossing plays an important role in the mechanism. Our calculations show that aggregation in the solid state hampers the access to internal conversion pathways, however, intersystem crossing is active in the crystal phase, which explains the weak AIE of this molecule. This new understanding of the role of triplet states on the relaxation mechanisms of AIEgens has implications for the design of solid state highly-emissive materials based on TPT.