Tuning the competition between photoisomerization and photothermy in biomimetic cyclocurcumin analogues

We report the synthesis and photophysical characterization of biomimetic D-A-D’ cyclocurcumin derivatives, which can potentially be used in light activated chemotherapy. Particularly we highlight that both the donor (D) and acceptor (A) groups significantly influence the photophysical response of the chromophore inducing strong bathochromic shift with D/A strength increase and notable fluorescence quantum yield enhancement with donor strength increase. More important, the nature of the acceptor group (oxo or malonitrile) dramatically modifies the outcome in non-radiative deactivation channels. Indeed, while compounds functionalized with an oxo-moiety undergo ethylenic E→Z photoisomerization, the one bearing a malonitrile group leads exclusively to other non-adiabatic internal conversion channels which much favor photothermal conversion as no isomerization of the ethylenic double bond being observed. The tuning of the photophysical properties and the alteration of isomerization vs. photothermal conversion is rationalized through the analysis of the potential energy surfaces along the most relevant degrees of freedom and shows a competitive pathway over malonitrile rotation. Our results offer novel perspective in oxygen-independent light activated chemotherapy and in the control of photochemical processes in biomimetic chromophores.