A TIME-RESOLVED TRANSIENT INFRARED STUDY OF A BISTABLE PHOTOCHROMIC ORGANOMETALLIC COMPOUND BASED ON LINKAGE ISOMERIZATION
Kristy M. DeWitt, Tung T. To, and Edwin J. Heilweil
Laser Applications Group, Optical Technology Division, Physics Laboratory
National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8443
Previous work has shown that the cyclopentadienylmanganese complex (h5-C3H4CH3)Mn(CO)2(3-cyanomethylpyridine) can act as a bistable photochromic organometallic complex via linkage isomerization. Steady-state room temperature experiments in solution show that the red-orange pyridine nitrogen bound complex, Mn-Py, isomerizes to the yellow cyano bound isomer, Mn-NC, upon visible irradiation. Further irradiation with UV light reverses ligand coordination back to the pyridine ring, restoring the red-orange solution color.
A time-resolved transient infrared study was performed to investigate the mechanism and time-scale of this linkage isomerization. To date, only the visible pathway has been studied in isooctane. Strong bleach features are observed at 1932 and 1868 cm-1, corresponding to loss of the pyridine-coordinated complex. An absorption feature at 1985 cm-1 appears immediately and decays completely within 400 ps, while absorption features at 1952 and 1888 cm-1 grow in at a similar rate to the loss of the 1985 cm-1 peak. Previous FTIR experiments have shown that absorbances corresponding to the final cyano-coordinated complex appear at 2025, 1943, and 1885 cm-1.1
Based on these results we propose the following mechanism. Following absorption of a 400 nm photon, the cyanomethylpyridine ligand dissociates from the Mn center. The peak at 1985 cm-1 corresponds to a “bare radical”, triplet state, or some other form of ring-slipped intermediate that relaxes to form an isooctane-solvated complex. The unstable alkane solvate then converts to the chelate product on the >nanosecond timescale. Additional confirmation of this mechanism was obtained by comparing to transient infrared studies of methylcyclopentadienyl manganese tricarbonyl (MMT) in isooctane where both MMT and (h5-C3H4CH3)Mn(CO)2(3-cyanomethylpyridine) form an identical transient Mn-alkane solvate (and CO-stretch absorption features) upon UV and visible irradiation, respectively.
In summary, our results indicate that the linkage isomerization pathway for (h5-C3H4CH3)Mn(CO)2(3-cyanomentylpyridine) from the pyridine-coordinated to the nitrile-coordinated isomer proceeds through a solvated intermediate. Future work will include studying the reverse isomerization, and exploring the photochemistry of tethered bifunctional ligands to potentially eliminate the solvated intermediate.
Authors: Kristy M. DeWitt and Tung T. To
Mentor: Edwin J. Heilweil
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 To, T. T.; Barnes, C. E.; Burkey, T. J. Organometallics 2004, 23, 2708.