Unraveling the T[formula omitted] formation in mono-arm styrylbenzene heteroanalogues.

We present a theoretical investigation of the nonradiative relaxation dynamics associated with the low-lying excited electronic states of mono-arm styrylbenzene heteroanalogues. These molecules possess near-degenerate S 1 (bright π π ∗ ) and S 2 (dark n π ∗ ) states, enabling intersystem crossing via both states upon photoexcitation to S 1. Singlet-triplet energy gaps and associated spin-orbit coupling parameters suggest favorable intersystem crossing via S 2 → T 5. The nuclear wavepacket initiated at the Franck-Condon point of the receiver triplet state (T 5) decays rapidly to lower triplet states via accessible multiple conical intersections, indicating the ultrafast formation of T 1 in these molecules. Our findings show that these molecules would transfer energy to molecular oxygen via T 1. • T 1 formation pathways of mono-arm styrylbenzene heteroanalogues are investigated. • TD-DFT calculations and SOC values reveal S 2 -T 5 as the triplet generating channel. • Conical intersections in the triplet manifold lead to ultrafast internal conversion. [ABSTRACT FROM AUTHOR]