Ultrafast nonadiabatic dynamics of tetraphenylsubstituted nitrogen-based heterocycles

TL;DR

This study investigates the ultrafast nonadiabatic dynamics of two related heterocycles, TPP and TePP, using mixed quantum-classical simulations to understand their differing excited-state relaxation and emission behaviors.

Contribution

It provides detailed mechanistic insights into the excited-state deactivation pathways of TPP and TePP through advanced trajectory simulations and compares gas-phase and solid-state behaviors.

Findings

Simulated observables reveal distinct deactivation pathways in TPP and TePP.

Intramolecular rotations influence emission properties differently in the two heterocycles.

Mechanistic insights help explain solid-state luminescence enhancement and dual-state emission.

Abstract

Tetraphenylpyrazine (TPP) and 2,3,4,5-tetraphenyl-1H-pyrrole (TePP) are closely related heterocycles bearing four phenyl substituents, whose structural similarity makes them a useful pair for comparing how intramolecular flexibility influences excited-state relaxation and emission in the gas phase and in the solid state. TPP is a prototypical solid-state luminescence enhancement (SLE) emitter, exhibiting a markedly increased quantum yield upon molecular aggregation. In contrast, TePP displays similar quantum yields in solution and solid state, characteristic of dual-state emission (DSE). This behaviour indicates that intramolecular rotations are already significantly hindered in the isolated-molecule regime, consistent with our previous observations for TPP and other solid-state emitters (Hern\'andez-Rodr\'iguez et al., ChemPhysChem, 2024, 25, e202400563). To unravel the excited-state dynamics underlying this contrasting behaviour, we performed mixed quantum-classical trajectory simulations on a single molecule of TPP and TePP employing the surface-hopping method. Twelve singlet states were included at the TD-B3LYP-D3/def2-SVP level, which were previously benchmarked against coupled cluster methods. Simulated observables such as gas phase ultrafast electron diffraction (GUED) and time-resolved fluorescence (TR-FL) signals allow us to dissect the distinct deactivation pathways operating in both systems in the gas phase, while also providing mechanistic insight into how these pathways are expected to evolve in solution and solid-state environments.