Derivatized Benzothiazoles as Two-Photon-Absorbing Organic Photosensitizers Active under Near Infrared Light Irradiation.

Abstract

Homogeneous organic photocatalysis typically requires molecular photosensitizers absorbing in the ultraviolet-visible (UV/vis) region, because UV/vis photons possess the sufficient energy to excite those one-photon-absorbing photosensitizers to the desired excited states. However, UV/vis light irradiation has many potential limitations, especially for large-scale applications, such as low penetration through reaction media, competing absorption by substrates and co-catalysts, and incompatibility with substrates bearing light-sensitive functionalities. In fact, these drawbacks can be effectively avoided if near infrared (NIR) photons can be utilized to drive the target reactions. Herein, we report two benzothiazole-derived compounds as novel two-photon-absorbing (TPA) organic photosensitizers, which can function under NIR light irradiation using inexpensive LED as the light source. We demonstrate that by judicially modulating the donor-π-acceptor-π-donor-conjugated structure containing a bibenzothiazole core and imine bridges, excellent two-photon absorption capability in the NIR region can be achieved, approaching 2000 GM at 850 nm. Together with large quantum yields (∼0.5), these benzothiazole-derived TPA organic photosensitizers exhibit excellent performance in driving various O2-involved organic reactions upon irradiation at 850 nm, showing great penetration depth, superior to that upon blue light irradiation. A suite of photophysical and computational studies were performed to shed light on the underlying electronic states responsible for the observed TPA capability. Overall, this work highlights the promise of developing Ru/Ir-free organic photosensitizers operative in the NIR region by taking advantage of the two-photon absorption mechanism.