Photolabile coumarins with improved efficiency through azetidinyl substitution

Azetidinyl substituents have been recently used to improve the fluorescence quantum yield of several classes of fluorophores. Herein, we demonstrate that other useful photochemical processes can be modulated using this strategy. In particular, we prepared and measured the quantum yield of photorelease of a series of 7-azetidinyl-4-methyl coumarin esters and compared it to their 7-diethylamino and julolidine-fused analogues. The efficiency of the photorelease reactions of the azetidinyl-substituted compounds was 2- to 5-fold higher than the corresponding diethylamino coumarins. We investigated the origin of this effect in model fluorophores and in the photoactivatable esters, and found that H-bonding with the solvent seems to be the prominent deactivation channel inhibited upon substitution with an azetidinyl ring. We anticipate that this substitution strategy could be used to modulate other photochemical processes with applications in chemical biology, catalysis and materials science.