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 S 3 and weakly from S 2 , while S 1 remains as a dark state, thus violating Kasha's rule. The calculated large energy difference between S 2 and S 3 (i.e., 1.00 eV) promotes radiative decay from S 3 rather than internal conversion (IC) to S 1 . 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 S 2 , thus disobeying Kasha's rule. The calculated energy difference (Δ E S1‐S2 ) is found to be 0.64 eV, which reduces the rate of IC (i.e., S 2 → S 1 ), 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.