Photoreactivity of E-N,N-bis(2-hydroxyethyl)cinnamamide in Methanol and Solid State: A Complementary Experimental and Theoretical Investigation.

N,N-bis(2-hydroxyethyl)cinnamamide (BHECA) is a photoreactive molecule exclusively known for its selective photodimerization via reversible [2π + 2π] cycloaddition reaction. However, this molecule presents a cinnamoyl core within its structure capable of undergoing reversible [2π + 2π] cycloaddition and/or E → Z photoisomerization reactions. The present work exploits this hypothesis and provides comprehensive insights into the photoreactivity of E-BHECA through complementary experimental and theoretical approaches. Firstly, the impact of the physical state of the reaction medium (methanol vs. solid state (SS)) and irradiation time (six and ten hours) on photochemical outcomes and photoproduct distributions were studied using 1H NMR, HPLC-ESI-MS and UV-Vis spectroscopy. Experimental data reveals that in methanol a ten-hour UV irradiation induces selective E → Z photoisomerization, with a conversion of 49 ± 0.4%. In contrast, irradiation in SS promotes predominant E → Z photoisomerization (41 ± 0.7%) over minor photodimerization yielding four distinct dimers with unresolved stereochemistry at a very low yield (2 ± 0.3%). Single Crystal X-Ray Diffraction (SC-XRD) reveals an unfavorable γ-crystal form (5.025 Å) of E-BHECA that limits photodimerization in the SS. Notably, the major photoproduct, Z-BHECA, was isolated and characterized, enabling unambiguous structural and spectroscopic assignments. Secondly, Time-Dependent Density Functional Theory (TD-DFT) simulations of the absorption spectra of BHECA monomers and its possible photodimers confirmed the experimental results, further supporting the predominance of the E → Z pathway under defined conditions. This study provides additional insights into the photoreactivity of E-BHECA, crucial for developing potential applications in pharmaceuticals and smart materials.