Overcoming Subjectivity in Bandgap Evaluation through On-Resonance Fluorescence: Establishing a Robust and Objective Optical Metric for Fluorescent Materials.

Accurate determination of optical bandgaps is essential for understanding and designing materials used in photovoltaics, photocatalysis, light-emitting devices, and quantum technologies. Yet, the most widely employed methods, such as Tauc-plot and photoelectron spectroscopy extrapolation, remain inherently subjective, relying on user-defined fitting regions and/or assumptions about transition types. Here we demonstrate that on-resonance fluorescence (ORF), a universal photophysical process occurring where absorption and emission spectra overlap, provides a direct, objective, and experimentally accessible measure of bandgap energy in fluorescent materials. We show both theoretically and experimentally that the ORF peak wavelength corresponds to the electronic bandgap, independent of scattering, absorption tails, or operator bias. In contrast to traditional methods that are valid only for single-bandgap systems and yield erroneous results for materials containing multiple emissive components, the ORF approach uniquely resolves the individual bandgaps of multicomponent or heterojunction samples without sample separation or prior assumptions. Using small-molecule fluorophores, quantum dots, and OLED dopants as model systems, we establish ORF as a broadly applicable, high-throughput, and reproducible approach for bandgap evaluation with experimental accuracy within ± 0.01 eV. Another key breakthrough capability is the determination of the individual bandgaps in samples containing two fluorescence materials. Beyond its simplicity, this method reframes ORF as a quantitative spectroscopic marker of bandgaps, offering a paradigm-shifting, broadly accessible alternative to Tauc-based analyses for both research and chemical education.