Studies of Solvent Effects on Novel Molecular Photoswitches using Time-Resolved Infrared Spectroscopy
Tung T. To and Edwin J. Heilweil
Optical Technology Division, Physics Laboratory
National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8443
We are studying photo-induced chelation of organometallic compounds as a mechanism for ultrafast photoswitches. Our goal is to develop molecular photoswitches that undergo chelation mechanisms which minimize inefficient side processes such as solvation. Chelation dynamics of [η5-C5H4C(O)R]Mn(CO)3 complexes 1 (R = CH2(SCH3)), 2 (R = CH(SCH3)2), and 3 (R = C(SCH3)3) show promise for meeting these criteria and were investigated by UV-pump IR-probe picosecond transient absorption spectroscopy.
We find that the solvent environment strongly influences the reaction rates and chelation pathways for species 1 and 2 while there are minimal effects for 3. UV irradiation of 1 and 2 in heptane or hexane solutions induces CO loss to yield two initial products assigned to a alkane solvate of the unsaturated Mn fragment and a chelate product in which the pendant sulfide moiety coordinates to the metal center. The unstable alkane solvate eventually converts to the chelate product, but this secondary pathway hampers the response time of chelation (100 ns compare to <100 ps with direct chelation pathway) in its molecular photoswitch applications. In heptane solution, only 3 yields the chelate exclusively following UV irradiation, with no evidence of a competing solvation pathway. Comparing the response times and the chelation mechanisms in heptane and accetonitrile, we find that all three complexes yield the chelate exclusively within 13 ps upon UV irradiation in accetonitrile. Faster vibrational cooling in acetonitrile (<13 ps) versus alkane (50 ps to 100 ps) solutions suggests stronger solute-solvent interaction, perhaps via hydrogen bonding. Our results indicate that chelation can be used as mechanistic platform for development of high quantum yields and ultrafast molecular photoswitches.
Author: Tung T. To
Mentor: Edwin J. Heilweil
Room #: B107
Address: 100 Bureau Drive Stop 8443
Gaithersburg, Maryland 20899-8443
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