Transition metal photosensitizers with metal-metal bonds

Project Summary/Abstract Many of the efficient photosensitizers utilized today are monometallic and consist of noble metals, such as ruthenium and iridium. These noble metal photosensitizers utilize high-energy radiation which leads to substrate degradation & unwanted side products in organic synthesis. Low permeability of high-energy radiation also limits photodynamic therapy applications only to the skin. These noble metals are also high in toxicity and create adverse side effects. The proposed research focuses on achieving complementary photoreactivity using inexpensive, environmentally benign and cheap earth-abundant metals by switching to a multimetallic paradigm. The overall goal is to invent new photosensitizers using earth-abundant metals to complement traditional noble metal reactivity and develop sustainable organic reactions and photodynamic therapies. Metal-metal bonding in the proposed photosensitizers will also facilitate a red-shift in the operational wavelength eliminating all the drawbacks of high-energy radiation in the above applications. Upon successful completion, aim 1 will yield a novel class of multimetallic earth-abundant metal photosensitizers which can perform photoredox catalysis. These photosensitizers will feature high excited state lifetimes and perform organic transformations in target substrates identified utilizing their redox potentials. Successful completion of aim 2 will yield a class of multimetallic carbon-monoxide-releasing-molecules that feature metal-metal bonds which facilitate red-shifted absorptions and photo-dissociation of CO in wavelengths in the therapeutic window laying out the groundwork for photodynamic therapies in internal tissues. This project will also yield working predictive computational models for optimization of the photosensitizers. The proposed research is significant because it applies an underdeveloped photosensitizer paradigm— metal–metal cooperativity—to develop photosensitizers that overcome the limitations of traditional noble metal monometallic photosensitizers in organic synthesis and photodynamic therapies. The proposed research will also provide the intellectual foundation for potential long-term applications in LEDs and solar cells using earth- abundant metal photosensitizers, thereby advancing knowledge across different fields. The rationale for pursing the proposed studies is derived from the careful analysis of the independent literature on precious-metal bimetallic photosensitizers, which show a relationship between the metal–metal cooperativity and photosensitizer parameters, such as absorption intensity and wavelength. Preliminary experimental results in our lab also support our hypothesis. All the resources and the expertise required to pursue the proposed research are available at Saint Louis University or in nearby institutions with which the PI has already established relevant connections.