Photophysics of donor-naphthalimide systems: hidden charge transfer states and emissive pathways governed by vibronic coupling

Abstract

Alkyl-substituted 1,8-naphthalimide (NI) derivatives are promising luminophores for organic light-emitting diodes (OLEDs), photopolymerization, photoinitiation, and thermally activated delayed fluorescence (TADF). In solution, these compounds exhibit a dominant absorption band at 320-375 nm attributed to a locally excited (LE) state, with no absorption seen beyond 400 nm. This CT absorption beyond 400 nm has been debated, with conflicting claims of its presence and absence by authors without any definitive proof. We demonstrate the presence of a CT absorption beyond 400 nm, which, however, remains “quasi-dark” in solution, even at concentrations up to 10-4 M. This hidden state becomes bright during emission by gaining oscillator strength via molecular planarization and intensity borrowings from the neighboring state. DFT ground-state and coupled-cluster (CC2) excited-state calculations confirm the vanishing oscillator strength of the CT transition in absorption and emission brightening via excited-state geometry changes. Notably, decay kinetics of ultrafast transient absorption with 35-fs excitation reveal coherent Rabi oscillations assigned to vibronic coupling between LE and CT states. Time-resolved emission shows nanosecond prompt fluorescence and microsecond delayed fluorescence from triplet-triplet annihilation (TTA), with one compound exhibiting TADF due to a small ΔEST (0.16 eV) in the solid state. These findings clarify the excited-state dynamics of NI derivatives and highlight the critical role of structural relaxations and LE/CT vibronic coupling in absorption/emission of these molecules, providing design principles for more efficient photocatalysts, photoinitiators, and OLEDs.