Development of ultrafast photochromic organometallics and photoinduced linkage isomerization of arene chromium carbonyl derivatives


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{η 6-C 6H5CH(2-Py-κN)CH 2CH=CH 2}(CO) 2]) (Py = pyridine) results in the isomerization to 22 ([Cr{η 6- C 6H 5CH(2-Py) CH2-η 2-CH=CH 2}(CO) 2]), and 355 nm irradiation isomerizes 22 to 23. The ultrafast linkage isomerization has been investigated at room temperature in ra-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{η 6- C 6H 5(CH 2) 3(2-Py)}(CO) 3]) and 25 ([Cr{η 6-C 6H 5(CH 2) 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 kN 1 -pyridine chelate within 200 and 100 ns, respectively. Irradiation of 26 ([Cr{η 6-C 6H 5CH 3(2-Py)}(CO) 5]) also induces CO loss to immediately yield three species: the Cr-heptane solvent coordinated intermediate, a kN 1-Py nitrogen chelate, and an agostic η 2-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 kN 1 -pyridine chelate within 50 ns. Similar reaction dynamics and transient species are observed for the chelate 33 ([Cr{η 6-C 6H 5CH 2(2-Py)-/dV}(CO) 2]) where a Cr-Py bond, not a Cr-CO bond, initially cleaves. © 2009 American Chemical Society.

Publication Title

Journal of Physical Chemistry A