Hydroxyaromatic Fluorophores

Many fluorophores that are widely used in analytical biochemistry and in biological microscopy contain a hydroxyaromatic component. One could also find fascinating chemistries of hydroxyaromatic dyes, especially those capable of excited state proton transfer (ESPT) to produce dual emission, in the literature of materials and physical chemistry. The ESPT-capable compounds have attracted interest based on their fundamental intellectual values in molecular photophysics and their potential utilities as light emitters in organic light-emitting diodes (LEDs) or fluorescent sensors. The hydroxyaromatic dyes could undergo either intra- or intermolecular proton transfer in either electronic ground or excited states. Although having long been applied for various purposes, some of their absorption and emission properties have not always been clearly described because of the insufficient attention given to proton transfer equilibria in either the ground or excited state and the challenges in computationally modeling the true emitters of these dyes under any given conditions. In this article, an attempt is made to summarize the spectroscopic properties of a few common hydroxyaromatic dyes that have been studied for both fundamental and practical purposes, with the help from quantum chemical calculations of the absorption and emission energies of these dyes in neutral and anion forms. The goal of this article is to provide readers some clarity in the optical properties of these compounds and the tools to understand and to predict the photon-initiated behaviors of hydroxyaromatic fluorophores.