Intramolecular subtleties in indole azo dyes revealed by multidimensional potential energy surfaces

Despite their wide use as molecular photoswitches, the mechanistic photophysics of azo dyes are complex and nuanced, and therefore under-explored. To understand the complex electronic interactions that govern the photoisomerization and thermal reversion of two phenyl-azo-indole dyes that differ by R-sterics near the azo bond, potential energy surfaces that combine the dihedral rotation of the azo bond and the aryl inversion on each side of the azo bond were calculated with density functional theory and time-dependent density functional theory. These multidimensional singlet surfaces provide insights into the correlated rotation and inversion pathways allowing for detailed understanding of both photoisomerization, governed by the excited-state surfaces, and thermal reversion, governed by the ground-state surface, mechanisms to be developed. Large plateaus on the S₁ surface arise from strong intramolecular interactions between a phenyl substituent and one of the aryl groups, extending the experimental photoisomerization lifetime of the dye with a phenyl R-group by two times over the unsubstituted dye. While one might expect the sterics of the larger phenyl substituent to lead to a slower thermal reversion rate, this was not the case. The thermally accessible meta-stable rotamers of the cis-isomer leads to more reversion pathways and a longer cis-lifetime for the unsubstituted dye, by a factor of four in the experiment. Careful computational mapping of multidimensional potential energy surfaces allows accurate mechanistic understanding for systems with interdependent degrees of freedom between meta-stable states.