Electronic and Steric Effects on the Photoswitching and Thermal Relaxation of Ortho ‐Substituted Pyrazole Azo Phthalimides

ABSTRACT

Ortho ‐substituted phthalimide–azopyrazole photoswitches bearing CH ₃ , CF ₃ , tert ‐Bu, and Ph groups were developed to determine how local steric and electronic perturbations control photochemical addressability and thermal Z→E relaxation. The substituents induced substantial shifts in absorption maxima, enabling different irradiation wavelengths while producing modest differences in the attainable Z ‐rich photostationary states. The CF ₃ derivative underwent E→Z photoisomerization at 530 nm, whereas the other derivatives were switched at 370 nm. TD‐DFT analysis attributes this spectral differentiation to orbital reordering, configuration mixing, and redistribution of oscillator strength within the low‐energy excited‐state manifold. Thermal Z→E half‐lives span more than three orders of magnitude, from 112 s for the CH ₃ derivative to 73 h for the CF ₃ derivative. Eyring analysis reveals predominantly entropic differentiation among the CH ₃ , tert ‐Bu, and Ph derivatives, whereas CF ₃ persistence is primarily enthalpic. Adiabatic S ₀ calculations reproduce the experimental stability order but do not account for the substantially negative experimental activation entropies. Relaxed T ₁ scans and optimized S ₀ /T ₁ crossing points identify accessible twisted crossing regions. The combined results support a multidimensional relaxation landscape in which CNNC torsion, azo‐nitrogen bending, and changes in N═N bond character constitute coupled coordinates whose energetic and configurational accessibility is reshaped by ortho substitution.