This dissertation presents my work towards the better understanding of the Excited-State Intramolecular Proton Transfer (ESIPT) photochemical process. ESIPT fluorescence is unique in a sense that it is environmentally sensitive and red-shifts a chromophore’s emission by upwards of 100 nm. This photochemical process has been studied and applied extensively but several key features remain unaccounted for in the literature. For instance an ESIPT dye’s sensitivity to a specific protein environment has not been explored which could inform probe development for a variety of applications. Additionally, the functional group tolerance and tunability, as well as governing factors in dictating the excited-state isomerization remain unmapped. First chapter of this dissertation is a brief overview of ESIPT fluorescence. The last chapter summarizes the previous four.

The second chapter is a study on the ESIPT properties of a fluorescent probe that binds specifically to a protein target. By engineering the binding site of a protein, it was demonstrated the emission wavelength of an ESIPT chromophore could be controlled by protein binding. This study highlighted the degree of sensitivity ESIPT dyes can provide by their dual emissive and ratiometric properties. The work highlighted in the chapter is a critical early step for the development of ESIPT chromophores for fluorescent-based medical diagnostics.

The third chapter characterizes the functional group tolerance of a class of ESIPT dyes. This study examined the photophysical properties of 25 unique monosubstituted ESIPT dyes in four different solvents. It was found that by functionalization at the 5’ position of HBO dyes can influence most aspects of ESIPT fluorescent. This includes tuning the absorption and emission properties, degree of dual emission, Stokes shift, and the fluorescence brightness. This study also exposed limitations to the general rules of ESIPT such as its solvent dependence.

The fourth chapter is an exploration on the role aromaticity plays in ESIPT. Using a combined experimental and theoretical approach, we demonstrate that aromaticity is a key driving force in excited-state isomerization by means of Baird’s rule. We showed that the degree of ground state aromaticity a molecule has, the more (anti)aromatic it becomes in the excited state and thus favors excited-state isomerization. This principle was then applied to example the puzzling photophysical behavior of naphthalene based ESIPT chromophores.

The fifth chapter presents a novel experimental method that is designed to disentangle the effects of a ring’s aromaticity and its acidity, which is a measure of electron density, on an experimental parameter. We measured the spectral properties of a series of ESIPT dyes, of which, controlled for varying ring aromaticity and electron density. In applying this method to ESIPT fluorescence, it was concluded that, although both important in determining an ESIPT dye’s photophysics, aromaticity and ring electron density function independently of one another.