Substitution-dependent excited-state relaxation pathways in the TADF structural isomers MA-TA and mMA-TA

The photophysical properties of the para-connected MA-TA and meta-connected mMA-TA donor-acceptor thermally activated delayed fluoresence (TADF) structural isomers were investigated to clarify how substitution patterns affect excited-state relaxation dynamics in TADF molecules. Time-resolved photoluminescence measurements showed that MA-TA exhibits relatively simple emissive-state relaxation, whereas mMA-TA shows a time-dependent red shift of the emission band and biexponential photoluminescence decay. Using reference spectra obtained from steady-state absorption and fragment transient absorption measurements, femtosecond transient absorption spectroscopy revealed that MA-TA undergoes rapid formation of a charge-transfer-like excited state within several picoseconds, as indicated by the decay of a broad MA-like near-infrared excited-state absorption component monitored around 850 nm and the concomitant rise of a CT-like excited-state absorption around 600 nm. In contrast, mMA-TA exhibited a spectroscopic signature of an additional intermediate excited-state species, observed as a transient absorption band around 700 nm, that was not clearly observed for MA-TA. The delayed appearance and relatively fast decay of this signal indicate that mMA-TA forms an intermediate excited-state species during early relaxation. These results demonstrate that the substitution pattern strongly influences the early excited-state relaxation pathway in these TADF molecules. The formation of an additional intermediate excited-state species in mMA-TA may provide a plausible origin for its more complex photoluminescence dynamics and lower emission efficiency. This study highlights the importance of controlling substitution-dependent intermediate excited-state formation in the molecular design of efficient TADF emitters.