Ultrafast Excited-State Dynamics of Dithienyltetrazine-Based Donor-Acceptor Copolymers

Tetrazine-based donor-acceptor (D-A)-conjugated polymers exhibit strong optical absorption and tunable electronic structures, yet they often suffer from weak photoluminescence and limited optoelectronic performance. The microscopic origins of this fluorescence quenching, particularly on ultrafast time scales, remain insufficiently understood. Here, we investigate the excited-state dynamics of dithienyltetrazine (TTz) and two D-A copolymers, poly(carbazole-TTz) (PCTTz) and poly(indolocarbazole-TTz) (PICTTz), using a combination of steady-state spectroscopy, time-resolved fluorescence upconversion, time-correlated single photon counting, femtosecond transient absorption spectroscopy, and complementary density functional theory calculations. We show that photoexcitation of the copolymers initially populates a strongly allowed intramolecular charge-transfer (ICT) state, which undergoes ultrafast (<100 fs) internal conversion to a weakly emissive tetrazine-localized singlet n-π* state, consistent with partial population of long-lived states likely involving the triplet manifold. Steady-state and time-resolved measurements reveal pronounced conformational heterogeneity, with a minor subpopulation of highly twisted polymer segments suppressing ICT formation and enabling brighter, TTz-like emission at higher excitation energies. Comparison between PCTTz and PICTTz further demonstrates that subtle variations in donor structure critically influence the lifetime and deactivation pathways of the dark singlet excited state, leading to either nanosecond singlet lifetime or rapid internal conversion. These findings establish a unified mechanistic framework for fluorescence quenching in tetrazine-based D-A polymers and provide clear design guidelines for mitigating ultrafast internal conversion in future organic optoelectronic materials.