Regioisomeric Engineering of Carbazole-Naphthalimide Donor-Acceptor Systems Enables Efficient Triplet Generation and Room-Temperature Phosphorescence in Molecular and Nanoaggregated States

Abstract

Regioisomeric engineering provides an effective strategy for controlling excited-state dynamics in donor–acceptor systems, yet its influence on triplet generation and room-temperature phosphorescence (RTP) remains insufficiently understood. Herein, we report two series of carbazole–phenyl–naphthalimide donor–acceptor regioisomers in which carbazole is connected to either the C3 or C4 position of the naphthalimide acceptor through ortho-, meta-, and para-substituted phenyl linkers. Systematic photophysical investigations reveal that subtle changes in molecular connectivity profoundly influence charge-transfer character, fluorescence and intersystem crossing efficiency. Para-linked and C4-substituted derivatives exhibit enhanced fluorescence owing to improved conjugation and reduced structural relaxation, whereas ortho- and meta-isomers as well as C3-substitution promote more efficient triplet generation through spin-orbit charge-transfer-assisted intersystem crossing, yielding triplet quantum yields approaching unity. Femtosecond and nanosecond transient absorption spectroscopy directly elucidate the singlet-to-triplet conversion dynamics, while TD-DFT calculations and single-crystal X-ray diffraction establish clear relationships between molecular geometry, electronic coupling, and excited-state behaviour. Efficient orange-to-red RTP is observed in benzophenone host–guest matrices as well as in water-dispersed nanoaggregates, suggesting that regioisomeric control can activate persistent phosphorescence in both solid-state and colloidal environments. These findings provide valuable molecular design principles for developing purely organic RTP materials for optoelectronics, time-resolved bioimaging, and photonic applications.