Impact of π‑Conjugation Length on the Excited-State Dynamics of Star-Shaped Carbazole-π-Triazine Organic Chromophores

Correlating star-shaped donor-bridge–acceptor (DBA) molecular structures with intramolecular charge transfer (ICT) and intersystem crossing (ISC) is essential to their application in photocatalysis, photovoltaics, and organic light-emitting diodes (OLEDs). In this work, we report a systematic photophysical study on a series of star-shaped triazine–phenylene–carbazole DBA molecules with 0, 1, and 2 bridging phenylene units (pTCT-0P, pTCT-1P, pTCT-2P). Using a combination of steady-state and time-resolved spectroscopy with time-dependent density functional theory (TDDFT), we find that the bridge length can strongly impact the structural conformation, ICT, and ISC. Global target analysis of the time-resolved spectroscopy reveals that pTCT-0P has the most favorable ISC rate of 1.96 × 10^–4 ps^–1, which is competitive with a singlet relaxation rate of 1.92 × 10^–4 ps^–1. TDDFT aligns with spectroscopic results within an order of magnitude, predicting an ISC rate of 2.1 × 10^–5 ps^–1 and revealing that the donor/acceptor orthogonalization concomitantly suppresses singlet exciton recombination and lowers the singlet–triplet energy gap. The new fundamental insights gained from this work will help design the next generation of star-shaped DBA-type molecules for photocatalytic and photoelectronic applications.