Solvent-Dependent Dynamics of the MQ^•→Re^II Excited-State Electron Transfer in [Re(MQ⁺)(CO)₃(dmb)]²⁺

Abstract

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.

Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone

“…These observations are consistent with appearance of an additional intermediate, 3. 26 (dmb is 4,4 0 -dimethyl-2,2 0 -bipyridine) in CH 2 Cl 2 , λ max = 630 and 380 nm. The 400 nm absorption includes significant contributions from H 2 Q •+24 and HQ • , 5,27 the latter of which is formed by a following reaction, as discussed below.…”

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans‐[Re()(CO)2(L)2]n+ Complexes Containing N‐Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

“…Complex 2Re(BiPy‐Me) 2 containing viologen‐type N‐methyl‐4,4’‐bipyridinium ligand (MQ + ) demonstrates rich electrochemistry both in oxidation and reduction modes (Figure 8) due to availability of low lying filled and empty orbitals of the aromatic ligands and a variety of charge transfer processes, which may occur in these systems [65–74] …”

“…Complex 2Re(BiPy-Me) 2 containing viologen-type N-methyl-4,4'-bipyridinium ligand (MQ + ) demonstrates rich electrochemistry both in oxidation and reduction modes (Figure 8) due to availability of low lying filled and empty orbitals of the aromatic ligands and a variety of charge transfer processes, which may occur in these systems. [65][66][67][68][69][70][71][72][73][74] Application of several spectro-electrochemical techniques in the study of the [Re(N N)(CO) 3 MQ] 2 + complexes [67][68][69][70]75,76] showed that CV of this type of compounds commonly display four reversible one-electron reduction waves related to successive one and two electron reduction of the diimine and MQ…”

Probing the excited state nature of coordination complexes with blended organic and inorganic chromophores using vibrational spectroscopy

“…These systems have been extensively studied over the past few decades. Complexity in function and photophysics can be built into these systems by the use of differing ligands, such as in chromophore-quencher complexes [105][106][107] or heteroleptic complexes. [108][109][110][111][112][113][114][115][116] In addition even the apparently simplest MLCT systems may be affected by ancillary ligands that can play a significant role in the photophysics and spectroscopic properties.…”