Development of Ultrafast Photochromic Organometallics and Photoinduced Linkage Isomerization of Arene Chromium Carbonyl Derivatives^†

2008 marked the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors.

NIST Guest Researcher/Postdoctoral Research Associate.

Tung T. To: Staff Scientist, Arizona Chemical Asia Ltd. Singapore. Regional Office 51, Goldhill Plaza, Savannah, Georgia 1201 W. Lathrop Ave, Savannah, GA 31415. Research Associate, National Institute of Standards and Technology, Physics Laboratory: Edwin Heilweil, advisor. Ph.D. (2005) The University of Memphis, Memphis, TN: Ted Burkey, Advisor. Research includes organometallic photochemistry, molecular switches, and organic syntheses.

Edwin J. Heilweil: Staff Research Chemist, Optical Technology Division, Physics Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8443 (1985−present). NRC Postdoctoral Research Fellow, National Bureau of Standards (1984−1985), John Stephenson, advisor; Ph.D. (1983) Chemistry Department, University of Pennsylvania, Philadelphia, PA; Robin Hochstrasser, advisor. B.A./M.A. (1978) Chemistry and Mathematics, Brandeis University, Waltham, MA. Research includes ultrafast spectroscopy of vibrational energy transfer and photochemistry; surface dynamics; solar energy materials; terahertz spectroscopy of biomolecules; and terahertz imaging for homeland security.

Charles B. Duke III: Ph.D. Candidate, The University of Memphis, Department of Chemistry, Memphis, TN: Ted Burkey, Advisor. Currently Postdoctoral Research Associate, University of Tennessee Health Sciences Center, Pharmaceutical Sciences Department, Memphis, TN: Duane Miller, advisor. Research interests include organometallic photochemistry, organic synthesis, and X-ray crystallography.

Kristie R. Ruddick: Graduate Student, The University of Memphis, Department of Chemistry, Charles Edwin Webster, Advisor. M.S., University of Arizona: Dennis L. Lichtenberger, Advisor. Research includes computational chemistry, inorganic chemistry, and organometallic synthesis.

Charles Edwin Webster: Assistant Professor, The University of Memphis, Department of Chemistry. Past positions include Visiting Professor, Zhengzhou University, Department of Chemistry; Research Scientist, Texas A&M University; Lecturer, Texas A&M University. Post doctoral fellow, Texas A&M University: Michael B. Hall, Advisor and Ph.D. (1999), Department of Chemistry, University of Florida: Russell S. Drago, Michael C. Zerner, and Michael J. Scott, Advisors. Research includes studies of inorganic, bioinorganic, and organometallic chemistry; structure and bonding; and homogeneous and heterogeneous catalysis.

Theodore J. Burkey: First Tennessee Professor, The University of Memphis, Department of Chemistry, Tennessee. Postdoctoral fellow, LSU Baton Rouge: William Pryor, Advisor Research Associate, National Research Council Canada, Ottawa, Ontario: David Griller, Advisor and Ph.D. (1982), Department of Chemistry, UC San Diego: Robert Fahey. Research includes organic synthesis, organometallics, ultrafast photochromic materials, and photochemistry.

We review recent studies of processes relevant to photoinduced linkage isomerization of organometallic systems with the goal of preparing organometallics with an efficient and ultrafast photochromic response. The organometallic system thus corresponds to two linkage isomers with different electronic environments that are responsible for different optical properties. Much of this work has focused on examining processes following irradiation of cyclopentadienyl manganese tricarbonyl derivatives (compounds 3−21) including solvent coordination, thermal relaxation, solvent displacement by tethered functional groups (chelation), dissociation of tethered functional groups, and linkage isomerization. A new platform is investigated for obtaining a photochromic response in new experiments with arene chromium dicarbonyl complexes. A photochromic response is observed for arene chromium dicarbonyl complexes with tethered pyridine and olefin functional groups based on light-driven linkage isomerization on the nanosecond time scale. Irradiation at 532 nm of 23 ([Cr{η⁶-C₆H₅CH(2-Py-κN)CH₂CHCH₂}(CO)₂]) (Py = pyridine) results in the isomerization to 22 ([Cr{η⁶-C₆H₅CH(2-Py)CH₂-η²-CHCH₂}(CO)₂]), and 355 nm irradiation isomerizes 22 to 23. The ultrafast linkage isomerization has been investigated at room temperature in n-heptane solution on the picosecond to microsecond time scale with UV- or visible-pump and IR-probe transient absorption spectroscopy by comparing the dynamics with model compounds containing only a tethered pyridine. Irradiation of 24 ([Cr{η⁶-C₆H₅(CH₂)₃(2-Py)}(CO)₃]) and 25 ([Cr{η⁶-C₆H₅(CH₂)₂(2-Py)}(CO)₃]) at 289 nm induces CO loss to immediately yield a Cr−heptane solvent coordinated intermediate of the unsaturated Cr fragment, which then converts to the κN¹-pyridine chelate within 200 and 100 ns, respectively. Irradiation of 26 ([Cr{η⁶-C₆H₅CH₂(2-Py)}(CO)₃]) also induces CO loss to immediately yield three species: the Cr−heptane solvent coordinated intermediate, a κN¹-Py nitrogen chelate, and an agostic η²-chelate in which the pyridine is coordinated to the metal center via a C−H agostic bond as opposed to the nitrogen lone pair. Both the transient Cr−heptane coordinated intermediate and the agostic pyridine chelate convert to the stable κN¹-pyridine chelate within 50 ns. Similar reaction dynamics and transient species are observed for the chelate 33 ([Cr{η⁶-C₆H₅CH₂(2-Py)-κN}(CO)₂]) where a Cr−Py bond, not a Cr−CO bond, initially cleaves.