First Direct Structural Information on a Reactive .sigma..pi.* Excited State: Time-Resolved UV-Vis and IR Spectroscopic Study of Re(benzyl)(CO)3(iPr-DAB)

Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts

“…59 Finally, it is instructive to note that findings reported here resemble results from detailed photochemical investigations of [Re(R)(CO)3(α-diimine)] complexes (R = Me, Et, Bz). 47,48,[60][61][62][63][64][65][66] For these complexes, it was found that the excited state Re-R bond homolysis is a result of the formation of a repulsive σπ* excited state. This dissociative state is not accessible via direct excitation, but is instead accessed from excitation of the 1 MLCT state (one electron on the αdiamine followed by Re-R LMCT).…”

FT-EPR study of alkyl radicals formed in the photochemical reaction of Re(alkyl)(α-diimine) and Ru(alkyl)(α-diimine) complexes

“…In this case flash photolysis measurements do show a transient absorption (lifetime 40 ns in toluene, 35 ns in CH2Cl2) attributed to an excited state with predominant 3MLCT character. However, time-resolved IR spectroscopy shows that the rate of 3 MLCT decay does not match the rate of radical formation. It appears, therefore, that excitation to the 'MLCT state of [Re(CH 3 ) (CO)3(DMB)] is followed by two independent deactivation pathways, one producing a nonreactive 3 MLCT state and another giving a short-lived excited state from which bond dissociation occurs [7].…”

“…1) and [Ru(E)(R)(CO) 2 (a-diimine)] (II) complexes, where R = alkyl or benzyl, E = I or SnPh3, and a-diimine = 4,4'-dimethyl-2,2'-bipyridine (DMB) or N,N'-diisopropyl-1,4-diazabutadiene (iPr-DAB) have been subject of detailed spectroscopic (UVNis, IR, Raman, NMR, EPR) studies [1][2][3][4][5][6][7]. It has been shown that photoexcitation of these complexes can trigger homolysis of the metal-alkyl (benzyl) bond.…”

“…The quantum yield of this process depends critically on the identity of the R ligand and is affected as well by variation of coligands. For example, in the case of the [Re(R)(CO) 3 (DMB)] (R = ethyl, iso-propyl, or benzyl) complexes, excitation in the longest-wavelength, metal-to-ligand charge transfer (MLCT), absorption band leads to a bond homolysis reaction with a near 100% quantum yield [7]. For these complexes, nanosecond transient absorption measurements do not show the presence of an excited state precursor, so that bond dissociation must occur within the time period (-7 ns) of the laser pulse.…”

“…It appears, therefore, that excitation to the 'MLCT state of [Re(CH 3 ) (CO)3(DMB)] is followed by two independent deactivation pathways, one producing a nonreactive 3 MLCT state and another giving a short-lived excited state from which bond dissociation occurs [7]. Spectroscopic and theoretical studies indicate that bond dissociation requires that photoexcitation populates a state with a(metal-alkyl)ic'(a-diimine) character [1][2][3][4][5][6][7]. According to this interpretation, variations in photoreactivity stems from changes in the quantum yield of formation of reactive ait excited states.…”