A Mechanistic Insight into the Emission Behavior of Boron Difluorohydrazones (BODIHYs): Suppression of Kasha's Rule

Anti-Kasha property of phenanthrene-based BODIHY compounds leads to excitation wavelength-dependent RGB emissions. Experimental and theoretical studies reveal that both molecules emit from three different excited states, namely S₁, S₂, and S₃. Importantly, the phenyl-substituted BODIHY exhibits thermal equilibrium between S₁ and S₂, while S₃ does not.

Abstract

Single component-based molecules and materials exhibiting excitation wavelength-dependent emission are of significant interest in both academic and industrial contexts. Luminophores capable of emitting multiple colors in response to different excitation wavelengths often deviate from Kasha's rule. The boron difluorohydrazone (BODIHY) derivatives reported herein exhibit excitation wavelength-dependent emissions, illuminating red, green, and blue (RGB) colors. Experimental and theoretical studies indicate that this multi-color luminescence originates from three bright excited states, namely, S₁, S₂, and S₃. Such anomalous fluorescence in small organic molecules that possess three bright excited states and emit RGB as a function of excitation wavelength is unique and unprecedented. Time-dependent density functional theory (TDDFT) calculations indicate that the energy gap (ΔE) between S₂ and S₃ exceeds 3000 cm⁻¹ (0.37 eV), facilitating radiative decay S₃→S₀ over the competitive nonradiative decay. For compound 1, the ΔE_(S1–S2) was found to be < 2000 cm⁻¹ (0.25 eV), leading to thermal equilibrium between the two states. In contrast, compound 2 possesses three distinct excited states that fluoresce independently due to large energy gaps between them. Furthermore, different excitation spectra, fluorescence lifetimes, and photoluminescence quantum yields (PLQYs) for these emission bands indeed support the presence of three bright excited electronic states.

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