Excited State Dynamics in 1,8-Naphthalimide Derivatives Mediated by Vibronic Coupling between Locally Excited and Charge Transfer States

Alkyl-1,8-naphthalimide (NI) derivatives are emerging candidates for organic light-emitting diodes (OLEDs), owing to their potential for thermally activated delayed fluorescence (TADF). The nature of the first excited singlet state (S1) in these molecules remains under debate, with authors presenting conflicting claims. Here, we resolve this ambiguity by demonstrating that S1 possesses charge transfer (CT) character. In solution, UV/vis absorption spectra show a strong transition below 375 nm, indicative of a locally excited (LE) state, while the CT (S1) transition remains optically quasi-dark due to its low oscillator strength. Direct excitation into S1 is inefficient; however, S1 becomes emissive following population from the higher-lying S2 (LE) state. By contrast, in the solid state, a pronounced absorption band appears beyond 400 nm is present, concurrent with a single emission band. Solution-phase dual emission is contrasted by the solid state’s single emission. Time-resolved measurements reveal complex emission dynamics, including nanosecond prompt fluorescence and microsecond delayed fluorescence via TADF and triplet-triplet annihilation. Ultrafast spectroscopy (35-fs pump) uncovers coherent oscillations, attributing these to vibronic coupling between S2 (LE) and S1 (CT) states. The transition dipole moment analysis for vibronically coupled states indicates that S1 and S2 possess opposing dipole orientations, causing destructive interference and “quasi-dark” S1 absorption in solution; this cancellation is suppressed in the solid, enabling bright CT absorption. These insights clarify the excited-state dynamics of NI derivatives and expand their applicability in next-generation OLEDs with tailored TADF properties