Regioisomeric engineering of carbazole-naphthalimide donor-acceptor systems enables efficient triplet generation and room temperature phosphorescence in molecular and nanoaggregated states.

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.