Murray State University

Poster Title

Two-Photon-Induced Electron Transfer between β-Carotene and Carbon Tetrachloride.

Institution

Murray State University

Abstract

β-carotene (βC) is the plant pigment responsible for the orange color of carrots, oranges and other “yellow” fruits and vegetables. It is also present in green leaves, where it serves to protect plants from light-induced damage during photosynthesis. Because plants lacking βC die upon exposure to light, some “light-activated herbicides” are designed to mediate their toxicity by destroying this pigment via a βC-to-herbicide “photoinduced electron transfer” (PET) process. In similar fashion, we find that solutions of βC in chloromethane solvents are stable upon exposure to diffuse visible light, but rapidly turn colorless upon exposure to intense, green laser pulses. The rate of color loss depends on the square of the laser intensity, suggesting that either βC or solvent molecules absorb two photons and subsequently generate free radicals which degrade βC. To specify whether βC or solvent molecules absorb two photons, we have characterized the yield of chlorine radicals (•Cl) and chloride (Cl–) ions by placing βC-chloromethane solutions in contact with pure water and aqueous potassium iodide and silver nitrate. We find that two photons are absorbed and Cl and •Cl are generated only when βC is present. Accordingly, we propose a “two-photon βC to-solvent” PET mechanism which is consistent with our results and discuss the implications of this mechanism for herbicide design and development.

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Two-Photon-Induced Electron Transfer between β-Carotene and Carbon Tetrachloride.

β-carotene (βC) is the plant pigment responsible for the orange color of carrots, oranges and other “yellow” fruits and vegetables. It is also present in green leaves, where it serves to protect plants from light-induced damage during photosynthesis. Because plants lacking βC die upon exposure to light, some “light-activated herbicides” are designed to mediate their toxicity by destroying this pigment via a βC-to-herbicide “photoinduced electron transfer” (PET) process. In similar fashion, we find that solutions of βC in chloromethane solvents are stable upon exposure to diffuse visible light, but rapidly turn colorless upon exposure to intense, green laser pulses. The rate of color loss depends on the square of the laser intensity, suggesting that either βC or solvent molecules absorb two photons and subsequently generate free radicals which degrade βC. To specify whether βC or solvent molecules absorb two photons, we have characterized the yield of chlorine radicals (•Cl) and chloride (Cl–) ions by placing βC-chloromethane solutions in contact with pure water and aqueous potassium iodide and silver nitrate. We find that two photons are absorbed and Cl and •Cl are generated only when βC is present. Accordingly, we propose a “two-photon βC to-solvent” PET mechanism which is consistent with our results and discuss the implications of this mechanism for herbicide design and development.