Benchmarking electronic-structure methods for nonadiabatic dynamics in a propeller-shaped molecular rotor: insights into conical intersections

Abstract

Understanding light-activated processes in molecular rotors is essential for applications ranging from energy storage to light-emitting materials. Rotation around single bonds provides an efficient nonradiative decay pathway, leading many molecular rotors to undergo quenching across environments. Nonadiabatic dynamics simulations offer a route to disentangling competing decay channels, but the predicted mechanisms depend sensitively on the level of theory. Here, we examine how different electronic-structure methods shape the topology of conical intersections and, in turn, the photochemical behaviour of sym-triphenylcyclopropene (Ph3C3H), a propeller-shaped rotor exhibiting low-quantum-yield emission from a locally excited state. Using exploratory-based surface-hopping dynamics with TD-CAM-B3LYP, ADC(2), and MRSF-TD-CAM-B3LYP, we characterise the S2–S1 and S1–S0 conical intersections and relate their topographies to the observed decay pathways. Consistent with the underlying potential-energy surfaces, the trajectories computed with ADC(2) evolve to S0 significantly faster than those obtained with TD-CAM-B3LYP and MRSF-TD-CAM-B3LYP. The S2–S1 intersection is peaked and bifurcating, promoting extensive population exchange, particularly for MRSF-TD-CAM-B3LYP. Two distinct S1–S0 intersections are identified: CI1, consistently peaked across CASSCF and all MRSF-TDDFT functionals, and CI2, which is sloped and single-path for all MRSF-TDDFT functionals but appears sloped and bifurcating at the MS-CASPT2 level. Across all methods, CI1 emerges as the dominant decay channel. MRSF-TD-CAM-B3LYP dynamics yield three outcomes: reformation of the initial geometry, formation of a diradical intermediate, and production of the bicyclic photoproduct, reflecting its bifurcating topology and more reliable ground-state description relative to LR-TDDFT. A systematic analysis of exchange–correlation functional dependence within MRSF-TDDFT reveals strong consistency across functionals, with only minor deviations such as the borderline single-path behaviour of BHHLYP and DTCAM-AEE. These findings raise important questions about the reliability, limitations, and future development of black-box excited-state methods for modelling photochemical decay in molecular rotors.