Breaking of Kasha's Rule: Excitation Wavelength-Dependent Emission from Acenaphthylene-Based Hydrazones and their Biological Studies

Achieving multicolor emissions from a single molecule has been an active field of research, particularly in developing organic light-emitting diodes. Reported herein is acenaphthylenedione (AcD), which displays multiple colors as a function of excitation wavelengths. Experimental and theoretical data reveal that the latter emits strongly from S₃ and weakly from S₂, while S₁ remains as a dark state, thus violating Kasha's rule. The calculated large energy difference between S₂ and S₃ (i.e., 1.00 eV) promotes radiative decay from S₃ rather than internal conversion (IC) to S₁. Hydrazones derived from the same also possess excitation wavelength-dependent emission. Time-dependent density functional theory (TDDFT) calculations reveal that the longer wavelength emission can be assigned to enol-form, produced through excited-state intramolecular proton transfer (ESIPT) and locally excited (LE) keto-form, while that of the shorter wavelength to LE S₂, thus disobeying Kasha's rule. The calculated energy difference (ΔE_S1-S2) is found to be 0.64 eV, which reduces the rate of IC (i.e., S₂ → S₁), resulting in the emission from the higher excited state. N-methylated hydrazone, which blocks the ESIPT channel, also supports the hypothesis. Furthermore, all the compounds exhibit aggregation-induced emission behavior, and nitro- and cyano-substituted hydrazones are found as good candidates for antibacterial, antioxidant, and anticancer activities.