Graduate Student Christian Guzman

"Modifying the Photophysics of Heterocyclic Azo Dyes using Steric and Electronic Substitution"

Worldwide population growth necessitates the development of renewable energy sources. A promising avenue is through the conversion of solar energy to thermal energy using organic photoswitches, or compounds which undergo chemical/physical transformations when irradiated. One class of compounds are azo dyes which are characterized by an N=N double bond connecting two aryl groups. Most azo compounds exhibit a thermally reversible photoisomerization, or light-induced conformational change, from the lowest-energy trans- to the meta-stable cis-isomer. Efficient energy storage requires the cis-isomer to persist for long periods of time. Many studies have investigated solvent and substituent effects on the photophysics of aryl azo compounds. Addition of heterocyclic rings offers access to unique electronic structures while adding additional functionality such as metal binding and hydrogen bonding sites. Despite the growing number of studies on the structure-function relations of heterocyclic azo photophysics, many systems remain underexplored. In this work, I will discuss the effect that steric and electronic influences have on the cis→trans thermal reversion of phenylazo indoles. My results show that in substituted dyes increasing steric hinderance near the azo bond results in longer thermal reversion times. Counterintuitively, the unsubstituted dye takes roughly four times longer to revert than the largest substituted dye. In addition, changing the electronic structure of the dyes significantly impacts the timescale for both the photoisomerization and thermal reversion events. These findings indicate that substituent effects influence multiple structural factors, such as the orientation of the cis-isomer and thermal reversion mechanism, that are not readily apparent. This work provides insight into the rational design of heterocyclic azo compounds for applications like optical data storage and energy storage that require extended cis-lifetimes.