Metal-organic complexes are widely used across disciplines for energy and biological applications, however, their photophysical and photochemical reaction coordinates remain unclear in solution due to pertaining molecular motions on ultrafast timescales. In this study, we apply transient absorption and tunable femtosecond stimulated Raman spectroscopy (FSRS) to investigate the UV photolysis of tungsten hexacarbonyl and subsequent solvent binding events. On the macroscopic timescale with UV lamp irradiation, no equilibrated intermediate is observed from W(CO)6 to W(CO)5(solvent), corroborated by vibrational normal mode calculations. Upon 267 nm femtosecond laser irradiation, the excited-state absorption band within ~400—500 nm exhibits distinct dynamics in methanol, tetrahydrofuran, and acetonitrile on molecular timescales. In methanol, solvation of the nascent pentacarbonyl-solvent complex occurs in ~8 ps and in tetrahydrofuran, 13 ps which potentially involves the associative oxygen-donating ligand rearrangement reaction. In contrast, a stimulated emission feature above 480 nm emerges after ~1 ps in acetonitrile with a nitrogen-donating ligand. These structural dynamics insights demonstrate the combined resolving power of ultrafast electronic and stimulated Raman spectroscopy to elucidate the photochemistry of functional organometallic complexes in solution. The delineated reaction pathways in relation to ligand nucleophilicity and solvent reorientation time provide the rational design principles for solution precursors in nanowrite applications.
Zhu, L.; Saha, S.; Wang, Y.; Keszler, D. A.; Fang, C. Monitoring Photochemical Reaction Pathways of Tungsten Hexacarbonyl in Solution from Femtoseconds to Minutes. J. Phys. Chem. B 2016, acs.jpcb.6b11773