Photoinduced Radical Formation from the Complexes [Re(R)(CO)₃(4,4‘-Me₂-bpy)] (R = CH₃, CD₃, Et, ⁱPr, Bz):  A Nanosecond Time-Resolved Emission, UV−Vis and IR Absorption, and FT-EPR Study

Abstract

Irradiation of the complexes [Re(R)(CO)3(dmb)] (R = CH3, CD3, Et, i Pr, or Bz; dmb = 4,4‘-dimethyl-2,2‘-bipyridine) into their visible absorption band gives rise to a homolytic cleavage of the Re−R bond with formation of the radicals [Re(CO)3(dmb)]• and R•. In the case of R = Et, i Pr, or Bz this reaction proceeds with unit efficiency. The nanosecond time-resolved absorption (TA) spectra show that the long-lived (τ = 7 μs) [Re(CO)3(dmb)]• radicals are formed within the 7 ns laser pulse. The CH3 complex photodecomposes with a quantum yield of only 0.4. The time-resolved UV−vis and IR absorption spectra reveal that this complex, after excitation into a 1MLCT state, may either pass a barrier of 1560 cm-1 to the dissociative 3σπ* state and decompose into radicals or decay to the ground state via an excited-state having predominant 3MLCT state character. Qualitative potential energy diagrams are presented for the two types of complexes. In a glass at 80 K and in a low-temperature solution (T < 195 K), the methyl complex is photostable, and this allowed us to study its excited-state properties with time-resolved absorption, emission, and IR spectroscopies. According to these spectra the lowest-excited 3MLCT state has a significant admixture of a σπ* character. Finally, nanosecond time-resolved FT-EPR spectra were recorded of the CH3 and CD3 radicals produced by irradiation of [Re(CH3/CD3)(CO)3(dmb)]. The spectra of the CH3 radical exhibit a pronounced low-field emission/high-field absorption pattern due to an ST0 radical pair mechanism (RPM) CIDEP effect. The occurrence of this ST0 RPM confirms that the radicals are formed from an excited state, having spin-triplet character, in agreement with the proposed mechanism involving a reactive 3σπ* state.

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

“…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. By contrast, in the case of [Re(R)(CO)3(DMB)] (R = methyl) the quantum yields of the reaction in toluene and dichloromethane are 0.4 and 0.7, respectively [7]. 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.…”

“…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.…”

“…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]. 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].…”

Co‐Ligand Involvement in Ground and Excited States of Electron‐Rich (Polypyridyl)PtII Complexes

“…Time-resolved emission spectra of samples in 2-MeTHF glasses (90 K) were recorded in an Oxford Instruments cryostat, using Spectra Physics GCR3 Nd:YAG or Coherent Infinity XPO lasers as excitation sources and an OMA detection system described elsewhere. [62] The solutions were prepared under nitrogen and freezepump-thaw-degassed three times before measurements.…”