Metal-to-ligand Charge Transfer Emission Research Articles

Photochemical and photophysical properties of [Mn(imidazole)(CO) 3(phen)](SO 3CF 3) complexes

Abstract The photoreactions resulting from metal-to-ligand charge transfer (MLCT) excitation of the manganese(I) tricarbonyl complexes fac-[Mn(CO)3(phen)(Im)](SO3CF3) and fac-[Mn(CO)3(phen)(Im-CH3)](SO3CF3) in several solvents at various wavelengths are reported. In each case, the reaction product is the corresponding mer-[Mn(CO)3(phen)(Im-L)]+ isomer. The high quantum yields, ∼0.66 mol einstein−1, reflect accurately primary photoprocesses from ligand-field states which are measured in a time scale during which thermal or secondary photoprocesses are not important. Further, in competition to ligand-field photoisomerization, these complexes are luminescent at room temperature. The emitting state can be tuned from a MLCT emission to intra-ligand (IL) fluorescence by varying the solvent polarity and excitation wavelength. Alterations in the character of the excited state result in large changes in the emission spectra suggesting the use of these complexes as probes.

Photophysical properties of Re(CO) 3L +3 (L=monoazine) complexes : Electronic delocalization effect in metal-to-ligand charge transfer excited states

The photophysical properties of the series of the [Re(CO)3L3]CF3SO3 (La4-phenylpyridine, 3-phenylpyridine, 4‐benzylpyridine, 4-(4 0 nitrobenzyl)-pyridine and 4,4 0 -bipyridine) have been studied. Emission spectral changes with temperature and biexponential decay of the luminescence both in dichloromethane fluid solution and frozen (77 K) media show two excited states, intraligand (IL) and metal-to-ligand charge transfer (MLCT) in character, emitting with intrinsic rates. Rates of MLCT emission badly correlate with the excited state-ground state energy gap. Departures from the energy gap law are rationalized in terms of excited state distortion and electronic delocalization. # 1998 Elsevier Science S.A. All rights reserved.

Mapping Electron Transfer Pathways in a Chromophore−Quencher Triad

A comparative study of the photophysics of the four positional isomers (one trans and three cis) of the chromophore−quencher triad [RuII(dmb)(bpyCH2PTZ)(bpyCH2MV2+)]4+ (dmb is 4,4‘-dimethyl-2,2‘-bipyridine, bpyCH2PTZ is (4‘-methyl-4-(2,2‘-bipyridin-4-yl)methyl)phenothiazine, and bpyCH2MV2+ is (4‘-methyl-4-(2,2‘-bipyridin-4-yl)methyl)-1‘-methyl-4,4‘-bipyridinium cation) has been undertaken. Following metal-to-ligand charge transfer (MLCT) excitation by laser flash photolysis at 460 or 532 nm, the redox-separated states [RuII(dmb)(bpyCH2PTZ•+)(bpyCH2MV•+)]4+ are formed rapidly (< 5 ns). Quenching of MLCT emission occurs with near unit efficiency for all four isomers. For the trans and cis3 isomers, formation of the redox-separated state is ∼25% efficient. For back electron transfer from −MV+ to −PTZ+, ΔG0 = −1.14 eV for all four isomers from electrochemical measurements and yet kET varies from 4.5 × 106 to 8.7 × 106 s-1 in acetonitrile at 25 °C.

Influence of Solvent on the Spectroscopic Properties of Cyano Complexes of Ruthenium(II)

UV−visible spectra, emission spectra, and RuIII/II reduction potentials have been measured for cis-[Ru(bpy)2(py)(CN)]+ (bpy is 2,2‘-bipyridine; py is pyridine), cis-Ru(bpy)2(CN)2, [Ru(tpy)(CN)3]- (tpy is 2,2‘:6‘,2‘‘-terpyridine), [Ru(bpy)(CN)4]2-, and [Ru(MQ+)(CN)5]2- (MQ+ is N-methyl-4,4‘-bipyridinium cation) in twelve solvents. The shifts in the metal-to-ligand charge transfer (MLCT) absorption (Eabs) or emission (Eem) band energies with solvent increase linearly with the number of cyano ligands and correlate well with the Gutmann “acceptor number” of the solvent. Intraligand π → π* band energies also correlate with acceptor number, but with only ∼30% of the shifts for the MLCT bands. The solvent dependence arises through mixing of the π → π* transitions with lower energy MLCT transitions. MLCT absorption and emission spectra are convolutions of overlapping vibronic components, and a Franck−Condon analysis of emission spectral profiles for cis-Ru(bpy)2(CN)2* has been used to evaluate the energy gap, E0,...

Photoinduced Charge Separation Promoted by Ring Opening of a Piperazine Radical Cation

Metal complex dyads, M-D, comprise a transition metal chromophore (M) covalently linked to an organic electron donor (D).1-3 Photochemical excitation of these assemblies produces a charge-separated state, M•--D+•, which has a lifetime that is controlled by the dynamics of the highly exothermic (inverted region) charge recombination reaction.2,3 In previous studies it has been shown that the lifetime of the charge-separated state in M-D assemblies increases with the driving force for charge recombination2b,3c or the separation distance between M and D.3b Herein we report a new approach to increasing the lifetime of a charge-separated state in a metal complex dyad which relies on the reversible ring opening of the radical cation of a piperazine electron donor. Thus, metal complex dyad c-1 contains the (bpy)Re(CO)3(Re) chromophore7 covalently linked to a 2,3-diaryl-1,4-dimethylpiperazine electron donor. Intramolecular electron transfer from piperazine to photoexcited Re produces a charge-separated state in which Re is reduced and the piperazine is a cation radical. Carbon-carbon bond fragmentation in the piperazine cation radical produces a new charge-separated state in which the donor exists as an openchain distonic cation radical. The net result is the production of a charge-separated state having a lifetime which is 5-10fold longer than that observed in structurally related metal complex dyads. The isomeric complexes c-1 and t-1 were synthesized and fully characterized.8 The near-UV absorption of both compunds is dominated by the dπ (Re) f π* (bpy) metal-to-ligand charge transfer (MLCT) transition of the Re chromophore.9 Irradiation of a solution of c-1 in air-saturated or Ar-degassed CH3CN at 366 nm affords t-1 as the sole chemical product with high quantum efficiency (Φc-1ft-1 ) 0.46 ( 0.05 in Ar-degassed solution).10 A mechanism for c-1 f t-1 photoisomerization is provided in Scheme 1. Near-UV excitation produces MLCT excited state c-1*, which subsequently relaxes either by radiative and nonradiative decay to c-1 (step 1) or by forward electron transfer (step 2) to afford charge-separated state c-3 in which the piperazine donor is oxidized and the bpy acceptor ligand is reduced (i.e., bpy•-). The occurrence of step 2 in c-1* is indicated by the fact that the MLCT emission lifetime of c-1 (τem ) 55 ns) is suppressed compared to that of model complex 2 (τem ) 235 ns). The emission lifetimes of c-1 and 2 allow estimation of k2 (1.4 × 107 s-1) and the efficiency for formation of charge-separated state c-3 (φ2 ) 0.76).11 Piperazine isomerization is believed to occur via fragmentation of the 2,3-C-C bond (step 3) to form a new charge-separated state in which the piperazine exists as a distonic cation radical with two possible structures (4a or 4b). Carbon-carbon bond fragmentation in radical cations is well precedented in acyclic systems.12,13 Indeed, recent studies indicate that for acyclic 1,2-diaryl-1,2diaminoethanes C-C bond fragmentation is exoergonic and occurs with k > 108 s-1.13e Since C-C bond fragmentation in 4 is probably exoergonic,14 the reverse process (e.g., 4 f 3) cannot occur, and therefore isomerization occurs via charge recombination (step 4) followed by coupling of 1,6-diradical 5 (step 6).16 A significant question is why t-1 f c-1 photoisomerization does not occur. Quite remarkably, the MLCT emission lifetime of t-1 (τem ) 233 ns) is not suppressed substantially from that of model 2, which indicates that forward electron transfer is too slow to compete with normal decay of the MLCT excited state, t-1*. The lack of electron transfer quenching in t-1* precludes formation of charge-separated state t-3 (not shown in Scheme 1), thereby closing off the pathway to t-1 f c-1 isomerization via fragmentation of the piperazine cation radical.

C-C Bond Fragmentation as a Probe for Photoinduced Intramolecular Electron Transfer

Photochemical and photophysical studies are reported for the ~omplex,fac-(bpy)Re’(CO)3(DA)~+ (e-l), where bpy is 2,2’-bipyridine and DA is a “reactive donor ligand” that contains a vicinal diamine functionality. Photoexcitation of e-1 into the dn (Re) -. n* (bpy) metal-to-ligand charge transfer (MLCT) excited state leads to formation of the ligand-to-ligand charge transfer (LLCT) state, *(bpy’-)Re1(CO)3(DA’+)]+, via diamine ligand-to-Re electron transfer. In the LLCT state, the reactive donor ligand radical cation undergoes an exceedingly rapid heterolytic C-C bond fragmentation reaction to produce an iminium ion and an a-amino radical fragment. Steady-state photochemical studies reveal that in air-saturated solution the only products observed arise from C-C bond fragmentation, which implies that under these conditions bond fragmentation is irreversible. Laser flash photolysis studies indicate that the a-amino radical that is produced by bond fragmentation in the LLCT state absorbs strongly in the near-UV region. A kinetic analysis is carried out under the assumptions that: (1) the quantum yield for formation the LLCT state can be derived from MLCT emission lifetime data on the diamine complex and a suitable non-donor-substituted model complex and (2) C-C bond fragmentation is irreversible. This analysis affords lower limits for the rates of back electron transfer and bond fragmentation (1.5 x lo8 and 1.0 x lo8 s-l, respectively).

Intramolecular Energy Transfer in (diimine)ReI(CO)3-[CpMII(arene)] Dimers

This report presents the photochemistry and photophysics of the series of dimeric complexes fac-(b)Re[sup I](CO)[sub 3[minus]] [CpM[sub II](arene)][sup 2+], where b = a series of substituted 2,2[prime]-bipyridine ligands, Cp = [eta][sup 5]-cyclopentadienyl, and M = Fe and Ru. The two metals are linked by 4-benzylpyridine, which is coordinated [eta][sup 1] to Re(I) via the pyridyl nitrogen and [eta][sup 6] to M(II) via the benzyl group. Near-UV photoexcitation of these complexes produces the luminescent d[pi] (Re) [yields] [pi][sup *] (diimine) metal-to-ligand charge-transfer (MLCT) state. The MLCT state is quenched by intramolecular exchange energy transfer (E[sub N]T) to the reactive [sup 3]d-d state of CpM(arene). Evidence that E[sub N]T occurs is provided by the observation of the following reaction during Re [yields] diimine MLCT photoexcitation: (b)Re[sub I](CO)[sub 3[minus]][CpM[sup II]-(arene)][sup 2+] [yields][sup hv,CH[sub 3]CN] (b)Re[sub I](CO)[sub 3](4-benzylpyridine)[sup +] + CpM(CH[sub 3]CN)[sub 3[sup +]]. The driving force for Re [yields] M E[sub N]T ([Delta]E[sub E[sub N]]T) is varied by changing the energy of the Re [yields] diimine MLCT state and by varying M. For M = Ru, E[sub N]T is weakly endothermic, and for M = Fe, E[sub N]T is moderately exothermic. The rate of intramolecular E[sub N]T is estimated from the quantum efficiencymore » for CpM(arene) loss from the dimers and from MLCT emission lifetimes. The activation energy and frequency factor for E[sub N]T in the (b)Re-Ru series is determined from the temperature dependence of the MLCT emission decay rate.« less

Intramolecular Energy Transfer in Covalently Linked Polypyridine Ruthenium(II)/Osmium(II) Binuclear Complexes. Ru(II)(bpy)2Mebpy– (CH2)n–MebpyOs(II)(bpy)2 (n=2, 3, 5, and 7)

Abstract A novel series of polymethylene-linked heterobinuclear complexes of polypyridine ruthenium(II)/osmium(II) complex Ru(II)(bpy)2Mebpy–(CH2)n–MebpyOs(II)(bpy)2(bpy=2,2′-bipyridine and n=2, 3, 5, and 7), 1, was prepared. The photophysical behavior was examined in various solvents. The emission spectra of 1 (excitation wavelength: 455nm) showed a nearly complete quenching of RuII→π*(bpy) metal-to-ligand charge transfer (MLCT) emission and the enhancement of OsII→π*(bpy) MLCT emission. The luminescence lifetime measurements by a time-correlated single photon-counting method provided evidence that intramolecular energy transfer is a significant pathway for the observed emission quenching. The rate constants of the intramolecular energy transfer in ethanol are 5.3×108, 3.3×108, 1.3×108, and 1.0×108s−1 for 1(n=2, 3, 5, and 7), respectively. They were found to be proportional to the inverse sixth power on the center-to-center distance of the two complexes. The mechanism is discussed in terms of the Förster (a dipole-dipole interaction) mechanism.

Production and storage of multiple, photochemical redox equivalents on a soluble polymer

Soluble polymers have been prepared that contain high loadings of metal-to-ligand charge-transfer (MLCT) visible-light absorbers based on polypyridyl complexes of Ru{sup II} or Os{sup II}. The complexes have been attached to a styrene/chloromethylstyrene copolymer by nucleophilic displacement of the chloro groups. By using excesses of the complexes, complete substitution at the available {approximately} 30 chloro-methylated sites was achieved. A series of mixed-valence polymers with controlled M{sup III}/M{sup II} ratios has been prepared in acetonitrile or acidic aqueous solution by oxidation with Ce(IV). For the Os{sup II}-containing polymers, Os{sup II}*-based MLCT excited-state lifetimes and emission quantum yields are relatively unaffected by the Os{sup III}/Os{sup II} ratio. This shows that intramolecular oxidative quenching of Os{sup II*} by Os{sup III} is slow. The slow rate of electron-transfer quenching is a manifestation of the inverted region. Because excited-state properties are relatively unaffected in the mixed-valence polymers, it is possible to build up and store {approximately} 30 oxidative equivalents on individual polymeric strands by photochemical oxidation.

Photoinduced intramolecular electron transfer in peptide-bridged molecules

Two series of molecules have been prepared and characterized in which a polypyridyl Ru(II) complex is linked to p-dimethoxybenzene (DMB) and p-benzoquinone (Q) moieties by peptide bridges containing the amino acid L-proline (Pro). The photophysical properties of the metal-to-ligand charge transfer (MLCT) excited state of the Ru(II) chromophore have been examined for the complexes with 0, 1, 2, 3, and 4 intervening Pro residues. Steady state and time resolved luminescence experiments on the Pro-bridged DMB system show that the properties of the Ru MLCT excited state are only slightly modified from those of an unsubstituted model complex by the presence of covalently attached DMB peptides. Experiments on the Pro-bridged Ru-Q complexes show that the Q site quenches the yield and lifetime of the Ru MLCT emission. Furthermore, the quenching efficiency is diminished as the number of peptide spacers is increased. The quenching process is ascribed to long-range intramolecular Ru-to-Q electron transfer. This hypothesis is supported by time-resolved luminescence data which suggest that the average electron transfer rate falls sharply with an increase in the peptide bridge length.

Excited state relaxation of Ru(bpy) 32+ at low temperature. Time evolution of the emission quantum yield

Abstract Decay of emission from the metal-to-ligand charge transfer (MLCT) excited state of Ru(bpy)32+ (bpy=2,2′-bipyridine) was studied by single photon counting as well as by nanosecond time-resolved emission spectroscopy in 4:1 ethanol-methanol. While the MLCT emission decay of *Ru (bpy)32+ could be fitted by a double exponential function at 125 K, the decay profile was strongly dependent on the monitoring wavelength. This dependence, and a rise component detected for the emission at the lower energy edge of the spectrum, were interpreted on the basis of a time-dependent shift of the emission spectrum.