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Ultrafast Dynamics of Photochemical Radical Formation from [Re(R)(CO) 3 (dmb)] (R=Me, Et; dmb=4,4′‐dimethyl‐2,2′‐bipyridine): A Femtosecond Time‐Resolved Visible Absorption Study
Author(s) -
Farrell Ian R.,
Matousek Pavel,
Kleverlaan Cornelis J.,
Vlček, Jr. Antonín
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/(sici)1521-3765(20000417)6:8<1386::aid-chem1386>3.0.co;2-g
Subject(s) - chemistry , excited state , photochemistry , ultrafast laser spectroscopy , singlet state , femtosecond , radical , triplet state , bipyridine , quantum yield , spectroscopy , fluorescence , atomic physics , crystallography , laser , crystal structure , physics , organic chemistry , quantum mechanics , optics
The excited‐state dynamics and photochemistry of [Re(R)(CO) 3 (dmb)] (R=Me, Et); dmb=4,4′‐dimethyl‐2,2′‐bipyridine) in CH 2 Cl 2 have been studied by time‐resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck‐Condon 1 MLCT (singlet metal‐to‐ligand charge transfer) excited state of [Re(R)(CO) 3 (dmb)] undergoes femtosecond branching between two pathways (≤400 fs for RMe; approximately 800 fs for REt). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re‐R bond via a 3 SBLCT (triplet σ ‐bond‐to‐ligand charge transfer) excited state, from which [Re(S)(CO) 3 (dmb)] . and R . radicals are formed. The other pathway leads to an inherently unreactive 3 MLCT state. For [Re(Me)(CO) 3 (dmb)], the 3 MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3 SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO) 3 (dmb)] behaves differently: branching of the Franck‐Condon state between two pathways still occurs, but the 3 MLCT excited state lies above the dissociative 3 SBLCT state and can decay into it. This shortens the 3 MLCT lifetime to 213 ps in CH 2 Cl 2 or 83 ps in CH 3 CN. Once populated, the 3 SBLCT state evolves toward radical photoproducts [Re(S)(CO) 3 (dmb)] . and Et . . Thus, population of the 3 MLCT excited state of [Re(Et)(CO) 3 (dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excited‐state dynamics of [Re(R)(CO) 3 (dmb)] (R=Me, Et) complexes are explained in terms of the relative ordering of the Franck‐Condon, 3 MLCT, and 3 SBLCT states in the region of vertical excitation and along the Re‐R reaction coordinate. A qualitative potential energy diagram is proposed.