Determining Static Dielectric Constants from Fluorescence Spectra

Quantifying the static dielectric constant (ϵ) of complex materials and biological environments is essential for understanding their properties, yet capacitance-based measurements remain difficult to implement in thin films and heterogeneous media. We present an entirely optical approach for determining ϵ from the fluorescence spectra of donor-acceptor (DA) probe molecules. DA fluorophores exhibit highly polar charge-transfer (CT) states, whose energies are strongly stabilized by solvent interaction, producing solvatochromic shifts. By measuring CT-state emission across solvents of known polarity, we show that the fluorescence energy can be parametrized with two quantities: the vacuum emission energy and the solvatochromic susceptibility χ. With these parameters established, the same DA molecules function as quantitative polarity probes, enabling the determination of ϵ in organic thin films, complex materials, and live-cell environments without capacitive measurements. This work provides a noninvasive methodology for extracting static dielectric constants from optical spectra and introduces a software package to facilitate its application.