Excited-State Dynamics and Electron-Withdrawing Group Effects in Two-Photon Switchable Donor-Acceptor Stenhouse Adducts.

Non-linear optical reactive systems have important applications which require highly localized effects. Recently, we reported for the first time that donor-acceptor Stenhouse adducts (DASAs) have important two-photon switching properties. Here, we report on the nature of the non-linear excitation event and the resulting dynamics through femtosecond-resolved measurements. From these experiments we directly show the central role of specific higher singlet states in the initial two-photon transition step. To test the generality of the involvement of such higher states, we synthesized new two-photon switchable DASAs with varying electron accepting groups. Specifically, we detected a rapid decaying emission from the initially formed higher singlets (S, n>1) which is followed by the indirect formation of the S state from S→S internal conversion which in turn leads to the switching reaction. Importantly, the new DASAs show up to a factor of three larger two-photon cross sections in comparison with our previously reported molecules. Computational results are consistent with the central role of specific higher singlets in the non-linear switching of DASAs at approximately 3 eV above the electronic ground state. The present results identify the key variables with respect to the non-linear photo-switching reactions of these compounds.