Enhancement of the Two-Photon Initiating Efficiency of a Thioxanthone Derivative through a Chevron-Shaped Architecture

We investigated the one- and two-photon excitation properties of a new hybrid anthracene thioxanthone (ANTX) system, which was assembled into a chevron-shaped molecular architecture. Such a geometrical association first preserves the high efficiency of the free radical photoinitiator and leads, in the same time, to a strong increase in the two-photon absorption (2PA) cross section by more than a factor of 30 as compared to thioxanthone (TX). A four-state model was required to fit the 2PA spectrum of ANTX, which clearly undergoes a strong enhancement in the low-frequency region. As a consequence, ANTX constitutes a suitable two-photon initiating chromophore with a much higher efficiency as TX used as reference. At lambda(exc) = 710 nm for instance, we showed that the two-photon polymerization threshold of ANTX is five times lower with respect to that of TX. Additionally, we demonstrated by the fabrication of three-dimensional microstructures that this new hybrid system remains two-photon activable when excited at wavelengths near its one-photon absorption region as suggested by the sum overstates model. We also show that this new two-photon initiator exhibits a higher reactivity than a diaminofluorene derivative, which is commonly used in two-photon polymerization.