Deciphering Photochemical Reactivity of Maleimides by Ultrafast Spectroscopy: How Minor Pathways Have Major Implications in Photochemical Reactions

Excited-state relaxation dynamics of hydroxy substituted <i>N</i>-phenyl maleimide are investigated following 355 nm excitation by ultrafast femtosecond transient absorption spectroscopy with dispersed, broadband probing. The dynamics of hydroxy <i>N</i>-phenyl maleimide is compared to that of <i>N</i>-phenyl maleimide lacking the hydroxy group using the same technique after 350 nm excitation. We found that the hydroxy-substituted maleimide and the maleimide lacking the hydroxyl substituent feature very short excited-state lifetimes due to radiationless vibronic deactivation which is the major relaxation pathway in both the maleimide chromophores. In addition, the hydroxy-substituted maleimide, which was previously shown not to undergo the photocycloaddition reaction with alkenes upon direct irradiation consitions displays a very minor pathway with spectral and kinetic signatures consistent with an excited-state proton transfer reaction. Removal of the -OH group in the maleimide lacking the hydroxyl substituent opens an intersystem crossing channel, which explains the photocycloaddition under direct irradiation in this class of maleimides lacking hydroxyl substituents.