Property Tuning in N-Methylpyrrole Azo-Photoswitches via Modification of the Peripheral Substituents

Differently substituted pyrrole-azo-benzene molecular photoswitches were prepared in a straightforward synthetic way. Their fundamental properties were investigated by XRD analysis, differential scanning calorimetry, thermogravimetric analysis, cyclic voltammetry, UV-Vis absorption spectroscopy, Hyper-Rayleigh Scattering, and NMR spectroscopy; the experimental results were further corroborated by DFT calculations. Thermal robustness, the HOMO/LUMO levels, and the absorption properties were altered mostly by substituting the N-methylpyrrole moiety and further fine-tuned by modifying the benzene substituents. The pyrrole substituent also proved crucial for the second-order non-linear optical (NLO) response as well as the photoswitching performance. Both fast and slow molecular switches can be designed with the half-life of the (Z) -isomer ranging from 48 seconds to 23.28 hours and the E/Z molar ratio up to 12/88. This comprehensive study allowed elucidation of the fundamental structure-property relationships and subsequently addresses the key aspects of the property tuning via substitution in molecular azo-photoswitches.