Peripheral Engineering of Multiple-Resonance Framework Targeting Efficient Organic Lasers.

Multiple-resonance thermally activated delayed fluorescent (MR-TADF) emitters have emerged as promising candidates for organic laser applications due to the potential for simultaneously achieving large oscillator strength and triplet utilization. In this study, we investigate the impact of peripheral tert-butyl (t-Bu)- and phenyl (Ph)-substituents on the typical 9-(phenylcarbazol-3-yl)-9H-carbazole-3-carbonitrile (CzBN) MR framework. Although these modifications preserve the frontier molecular orbital distribution with large oscillator strengths, they significantly influence excited-state dynamics and molecular aggregation even at low doping concentrations. Introducing Ph substituents extends the π-conjugation extension of CzBN, promoting closer molecular packing, detrimental molecular aggregation, and significantly broadening the excited-state absorption (ESA) band, which negatively impacts lasing performance. In contrast, CzBN-tBu, incorporating t-Bu groups as nonconjugated substituents, demonstrated reduced molecular aggregation and a distinct separation between the ESA band and stimulated emission region. Consequently, the optimal distributed feedback lasing performance is achieved by CzBN-tBu across various doping concentrations, resulting in the lowest lasing threshold of 3.4 µJ cm-2. These findings underscore the impact of inherent aggregation at low doping ratios on lasing activities, highlighting the crucial role of rational peripheral engineering in modulating molecular interactions and excited-state dynamics, offering design strategies for developing MR lasing molecules.