Non-linear photo-switching in molecular actuators through intra-molecular energy transfer from an electron donating core

Abstract

We present a new design for multi-chromophoric molecules where the non-linear absorption properties of a central electron-donating core are used to confer indirect biphotonic absorption features to molecular actuators. This design allows for a general way to provide two-photon reactivity to molecular systems which, by themselves, could not be controlled by a non-linear optical input. The compounds are based on a two-photon-active N,N′-1,4-dihydropyrrolo[3,2-b]pyrrole central unit. This core is symmetrically substituted at the ends of its long axis by nitrophenyl groups defining a pseudo-centrosymmetric acceptor–donor–acceptor (A–D–A) antenna-unit. For this first proof-of-concept study, azo-isomerizable actuators were bonded to the central unit through the nitrogen atoms of the heterocyclic core. This design allows for the pyrrolo-pyrrole chromophore to act as a non-linear absorbing unit, so that E–Z isomerization of the actuators can be induced after excitation into higher states localized at the central unit, followed by energy transfer into acceptor-localized excited states. The non-linear isomerization of the actuators was observed using pulsed NIR light from a Ti:Sapphire laser in a setup that allows for significant accumulation of the actuator-isomerized product in macroscopic samples. We demonstrate that the two-photon photochemistry of the actuators is due to the coupling with the central core unit since models of the azo-actuator compounds alone do not undergo these transformations at the same irradiation levels. Time-resolved emission studies indicate that the population associated with higher core-centered ¹ππ* states evolve towards actuator-localized in a time-scale of picoseconds. These signals and the overall photochemistry are consistent with a mechanism where the formation of the actuator-localized ¹nπ* states is directly followed by the azo-isomerization reactions. Computational studies at the TD-DFT level of theory, together with the use of a quadratic response method were used to characterize the states responsible for the two-photon absorption properties. This kind of molecular assembly can be extended to induce non-linear photochemistry in other types of actuators, including dissociative systems or uncaging ligands, or to induce catalytic activity in highly localized environments.