J. Am. Chem. SOC. 1993,115, 5675-5683
5675
Ligand-to-Ligand Charge-Transfer Photochemistry Yingsheng Wang, Brian T. Hauser, Michael M. Rooney, Richard D. Burton, and Kirk S. Schanze. Contributionfrom the Department of Chemistry, University of Florida, Gainesville, Florida 3261 1 Received October 26. 1992
Abstract: The photochemistry and photophysics are reported for a unique series of complexes of the type fac-(bpy)Re1(CO)3-(a-AA), where bpy = 2,2’-bipyridine and a-AAis a “reactive donor ligand” that incorporates an a-amino alcohol functionality. Photoexcitation of these complexes into the dr(Re) r*(bpy) metal-to-ligand charge-transfer manifold is followed by intramolecular donor ligand (a-AA)-to-metal electron transfer to produce the ligand-to-ligand charge-transfer (LLCT) state (bpy*-)Rel(CO)~-(a-AA*+).The reactive donor ligand complexes are designed so that formation of the radical cation (a-AA’+)in the LLCT state triggers a rapid C-C bond fragmentation reaction within the CY-AAligand. This photochemical reaction is used to (1) confirm that the LLCT state is formed; (2) explore the kinetic parameters for formation and decay of the LLCT state; (3) to “clock” the rates for the photoinduced intraligand bond-fragmentation process.
-
Scheme I
Introduction The photophysics of d6 transition-metal complexesthat contain electron acceptor (A) and electron donor (D) ligands has been the topic of a number of recent investigations. In A-M(d6)-D complexes, the lowest excited state is often based on an electronic configuration which involves ligand-to-ligand charge transfer (LLCT) from the donor to the acceptor ligand, e.g., A*-M(d6)-D*+.1-11In many A-M(d6)-D complexes, although the LLCT state is the lowest energy configuration, due to poor electronic coupling between the donor and acceptor ligands, radiative transitions between the ground state and the LLCT state have low probability (e.g., the extinction coefficient c and radiative decay rate k, are small). As a result, the LLCT state is formed indirectly via initial metal-to-ligand charge-transfer (MLCT) excitation followed by rapid donor ligand-to-metal forward electron transfer (ET), and LLCT decay occurs primarily via nonradiative back-ET (see Scheme I). Since the LLCT state is often nonluminescent, its involvement in the photophysics of a complex must be inferred either indirectly from its effect on the properties of the MLCT state or, in favorable cases, from transient absorption studies.lg2.4s5,s Because of the difficulty of studying the kinetics and reactivity of the LLCT state, a new approach has been developed which is based on using “reactive donor ligands” (D-R) which undergo rapid bond-fragmentation upon single electron oxidation. In complexes which feature reactive donor ligands, A-M(d6)-(DR), the formation and dynamics of the LLCT state can, in principle, be deduced by examining the efficiency for formation of products which result from bond-fragmentation in the LLCT (1) Chen, P.; Westmoreland, T. D.; Danielson,E.; Schanze, K. S.;Anthon, D.; Neveaux, P. P.; Meyer, T. J. Znorg. Chem. 1987, 26, 1116. (2) Danielson, E.; Elliott, C. M.; Merkert, J. W.; Meyer, T. J. J . Am. Chem. Soc. 1987, 109, 2519. (3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (4) Perkins, T. A.; Pourreau, D. B.; Netzel, T. L.; Schanze, K. S.J . Phys. Chem. 1989, 93,451 1. (5) Perkins, T. A.; Humer, W.; Netzel, T. L.; Schanze, K. S . J. Phys. Chem. 1990.94, 2229. (6) Perkins, T. A,; Hauser, B. T.; Eyler, J. R.; Schanze, K. S.J. Phys. Chem. 1990, 94, 8745. (7) MacQueen, D. B.;Schanze, K. S.J. Am. Chem. Soc. 1991,113,6108. (8) MacQueen, D. B.;Schanze, K. S.J. Am. Chem. Soc. 1991,113,7470. (9) Schanze, K. S.;MacQueen, D. B.; Perkins, T. A.; Cabana, L. A. Coord. Chem. Rev. 1993, 122, 63. (10) Koester, V. J. Chem. Phys. Letf. 1975, 32, 575. (11) Truesdell, K. A,; Crosby, G. A. J. Am. Chem. Soc. 1985,107,1787.
i-M(d5 ~ kd, Schemes I and 11). That k m is~faster in complexes Re-4 and Re-5 than in Re-1 and Re-2 is consistent with the fact that the free energy for this process ( A G ~ Tis) more exothermic for the aromatic amine donors (vide infra). Laser flash photolysis experiments were conducted on Re-4, Re-5,and Re-6 in order to gain spectroscopic and kinetic information on the MLCT and LLCT excited states. Transient spectra of the complexesobtained at various delay times following 355-nm laser excitation (6-11s fwhm, 10 mJ/pulse) are shown in Figure 4. Figure 4a shows the spectrum of Re-6 obtained at 204s delay after excitation. This spectrum is characteristic of theMLCTexcitedstatein (bpy)Re(CO)3-Lcomplexes; thestrong band at 370 nm is likely due to a T*,'R*transition of the bpy anion radical which is present in the MLCT state, e.g., *[(bpy*-)-
-
0
0.0
P 4
0.021
::I, 0.00
350
,
,
400
450
500
I
I
550
600
,
650
Wavelength, nm Figure 4. Transient absorption spectra obtained following 355-nmpulsed excitationfromaNdYAGlaser (6-nsfwhm,lOmJ/pulse). Allcomplexes were in degassed CH&N solution with [Re] a 5 X 10-5 M. (a) Re-6, 20-11s delay; (d) Re-5, 0-ns delay; (c) Re-4, 0-ns delay.
Re11(C0)3-L] .5.8,34 Figure 4b shows the transient absorption spectrum of R e 5 at 0-ns delay after 355-nm excitation. This spectrum differs significantly from that which is typical for the MLCT state; in particular, there is a strong absorption in the mid-visible (A, = 480 nm) in addition to moderate absorption in the near-UV region. This spectrum is attributed to the LLCT state, * [(bpy'-)Re1(CO)3-DMA.+)] which is formed from the MLCT state by rapid forward ET followingphotoexcitation. The LLCT assignment is supported by the presence of the 480-nm absorption band, which coincides with the reported absorption of theNJV-dimethylanilineradicalcation (DMA'+).35 The transient absorption which is assigned to the LLCT state for complex Re-5 decays with a lifetime of 15 ns. Figure 4c shows the transient absorption spectrum of reactive donor-substituted complex Re-4 obtained at 0-ns delay after 355-nm excitation. This spectrum is quite similar to the spectrum observed for DMA-substituted complex Re-5; in particular, a strong mid-visible absorption band is clearly apparent which can be assigned to the N-alkylanilinium radical cation. The mid-visible band in the spectrum of Re-4 exhibits two maxima (450 and 490 nm), which is interesting in view of the fact that the published absorption spectrum of the N-methylanilinium radical cation is dominated by a mid-visible band with two maxima.35 These facts strongly support the assignmentof the transient formed by nanosecond flash photolysis of Re-4 to the LLCT state. Analysis of the transient absorption decay shows that the LLCT state in reactive donor complex Re-4 has a lifetime of 12 ns. (c) Steady-State Photochemistry of Re-1. Extensive steadystate photochemical studies were carried out on the secondary aminoalcohol-substitutedcomplexRe-1. All photochemistrywas conducted on Ar-degassed CHpCN solutions using 366-nm monochromatic light from either medium- or high-pressure Hg lamps. This wavelength corresponds to the MLCT absorption band of the ( b ~ y ) R e ( C 0 ) chromophore. ~(34) Tapolsky, G.; Duesing, R.; Meyer, T. J. J . Phys. Chem. 1989, 93, 3885. (35) Shida, T. Electronic Absorptionof Spectra of Radical ions; Elsevier: Amsterdam, 1988.
5680 J . Am. Chem. SOC.,Vol. 115, No. 13, 1993
0 6 12 18 Irradiation Time I min
Figure 5. Rates of photochemical formation of benzaldehyde and consumption of Re-1. (a) Left bars, benzaldehyde formed (prompt analysis);right bars, Re-1 consumed. (b) Left bars, benzaldehydeformed (prompt analysis); right bars, benzaldehyde formed (analysis after hydrolysis of reaction mixture for 4 days).
Several product isolation experiments were carried out to determine the overall reaction@) which occur as a result of photolysis of Re-1. First, immediate postirradiation HPLC analysis indicates that benzaldehyde (PhCHO) is formed in 1:1 stoichiometry with respect to dissappearance of Re-1 (Figure 5a). Second, semipreparative-scalephotolysis of Re-1 followed by alumina chromatography led to isolation of the major metal complex product. Spectroscopiccharacterization (‘H and 13C NMR, high-resolution FAB mass spectrometry) indicates that the product is [(bpy)Re(C0)3(4-(aminomethyl)pyridine)]+ (Re8). Third, semipreparative photolysis of a CH3CN solution of Re-1 followed by overnight incubation with a 5-fold excess of NaBH3(CN) (a selective reducing agent for imines)36 led to isolation of secondary amine complex Re-2. Finally, since no evidence was found for the production of reduction products which are expected to accompany oxidativefragmentation of the a-AA ligand, we speculatedthat since reducing equivalents and protons are produced in the photoreaction, a possible ultimate reduction product could be Hz, Gas chromatography analysis of the headspace over a semipreparative-scale solution following exhaustive photolysis provided evidence that H2 is formed in 1:l stoichiometry with respect to dissappearance of Re-1. A reaction scheme which is consistent with the experimental observations is summarized in Scheme 111. Compounds which have not been isolated or whose presence has not been directly confirmed by at least two methods are shown in parentheses in this scheme. Formation of Re-8 is envisioned to occur via a pathway that involves two-electron oxidation of Re-1 to generate imine Re-7 and 1 equiv of PhCHO; subsequent hydrolysis of imine R e 7 duringsemipreparativechromatography produces R e 8 and a second equivalent of PhCHO. Isolation or independent synthesis of imine Re-7 proved impossible; however, its presence in the CH3CN reaction mixture was indirectly confirmed by the NaBH3(CN) reduction which yielded the expected reduction product Re-2. Since the stoichiometryfor formation of PhCHO is important in determining the kinetic parameters (vide infra) further analytical experimentsusing HPLC were carried out to establish the timedependenceofthe stoichiometry for formation of PhCHO relative to dissappearance of Re-1 (see Figure 5). These experiments indicate that “prompt” analysis (e.g., within 1-2 h of irradiation) of photoreaction mixtures gives 1:1 stoichiometry for PhCHO relative to disappearance of Re-1 (Figure 5a); however, if the photolysis reaction mixture is allowed to stand in the dark in a 1:l mixture of H20/CH$N for 4 days the (36) Lane, C. F. Synthesis 1975, 135.
Wang et al. stoichiometryapproaches 2: 1 for PhCHO relative to disappearance of Re-1 (Figure 5b). Presumably the second equivalent of PhCHO is formed by slow thermal hydrolysis of the intermediate metal complex imine Re-7, which is formed in the primary photochemical step. Quantum yields for the disappearance of Re-1 (aRe) and for prompt formation of PhCHO (aPhcHo) were determined using the Aberchrome 540actinometer,and the results are summarized in Table 111. As can be seen from this data, aPhCHO and @Re are the same within experimental error (1 :1 stoichiometry), and (PPhCHo is the same for [Re-l],,~ = 1 X 10-4 M and [Re-l],,o = 5 X 10-4 M. The lack of a concentration dependence of W C H O strongly supports the hypothesis that the photoreaction is unimolecular. (d) Steadystate Photochemistry of Re-4. Less extensive characterization of the photoreactionwas carried out for complex Re-4. Steady-state experiments were limited to quantitative prompt HPLC analysis of reaction mixtures to determine the amount of PhCHO produced and the amount of Re-4 consumed by 366-nm irradiation. Quantum yield data for this reaction are summarized in Table 111. Several points are of interest with respect to this data. First, it is clear that aPhCHo and QRe are the same within experimentalerror, indicatingthat the stoichiometry for prompt formation of PhCHO is 1:1 relative to disappearance of Re-4. Second, note that the overall quantum yield for photoreactionof R e 4 is lower than for Re-1. This is interesting in view of the fact that MLCT quenching (formation of the LLCT state) is substantially more efficient for Re-4; the significance of this point will be discussed in more detail below.
Discussion
(a) Excited-State Kinetic Model. In order to relate the photophysical and photochemical results to the rate of the photochemically induced bond-fragmentation reaction which is observed for Re-1 and Re-4, a kinetic and thermodynamicmodel must be developed (Scheme IV). Using the model presented in this scheme, the photochemical reaction is proposed to occur via the following sequence: (1) initial photoexcitation to the luminescent MLCT excited state; (2) irreversibleintramoleculardonor ligand-to-metal ET (with rate k m ) to produce the LLCT state; and (3) irreversible C-C bond-fragmentationin the LLCT state to produce an cr-aminoradical (with rate kBF). Note that forward ET competes with decay of the MLCT excited state via normal radiative and nonradiative pathways (with rate kd), and bondfragmentation competes with decay of the LLCT state via backET (with rate kern). Under theassumptionsimplied by themodel presented in Scheme IV, the overall quantum efficiency for prompt formation of benzaldehyde (@WcHO) is given by the product of the quantum yield for formation of the LLCT state ( @ L L ~ )and the efficiency for bond-fragmentation within the LLCT state
(BBF): ipPhCHO
=
@LLC@FB
where
In order to more easily relate k g F to experimentally accessible parameters, eqs 2 and 3 can be rearranged to express this parameter in terms of the MLCT emission lifetimes of the donorsubstituted complex, the corresponding non-donor-substituted model complex ( 7 and ‘ T ~ o ~ respectively), ~I, and the LLCT excited state (TLLCT),
Ligand-to-Ligand Charge- Transfer Photochemistry
J. Am. Chem. SOC.,Vol. 115, No. 13, 1993 5681
Scheme III
Re-1
NaBH3(CY (bpy)Re'-NDCHZ-NH-CHz-Ph I
(Cob Re-8
Re-2
Table III. Steady-State Quantum Yieldsa
complex
[Re]/Mb
Re-1 Re-1
1.0 X 10-4
Wd
cpPhCHO e
0.014
*
0.004 0.002
0.016 0.004
5.0 X 10-4 0.013 Re-4 5.0 X lo-" 0.006 & 0.001 0.007 & 0.001 a All experiments in Ar-degassed CH&N solution with 366-nm irradiation. Va1ue.srepresent the averageof 1 5 individualmeasurements; errors are standard deviations in the averages. Aberchrome 540 was used for actinometry. Concentrationof Re complexat t = 0. e Quantum yield for "prompt" formation of benzaldehyde. Quantum yield for disappearance of starting Re complex.
(-)(
aPhCHO
kBF
=
*LLCT
L ~ L L C T,
(4)
where
(5) *LLCT = 1 - ( 7 / 7 m d c J The significance of eqs 4 and 5 is that k g can ~ be determined if T , Tmdcl, T L L ~ and , @mCHo are known. (b) Thermodynamicsand Kinetics for Formation of the LLCT State. Thekineticmodel presentedaboveindicatesthat theoverall quantum efficiency for the photochemistry depends on both the efficiency for formation of the LLCT state, @ L L ~ and , on the branching ratio for reaction within the LLCT state, BBF. Because of the significance of @ L L in ~ determining the overall reaction efficiency, it is important to consider how structural and thermodynamic terms influencethis parameter in the two systems which have been examined in the present study. In previous investigations it has been demonstrated that the free energy for forward ET ( A G ~ T is) given approximately by the following expression,8.9
(6) A G m = El,Z(D/D'+) - E,,z(bPY/bPY'-) - J%,o whereElp(D/D'+) and El/Z(bpy/bpy+) are electrochemical halfwave potentials for oxidation of the donor ligand and reduction of the bpy ligand, respectively, and E0.0 is the 0-0 energy of the relaxed MLCT state which is obtained from emission spectra. Using the relevant electrochemicaland spectroscopicinformation collected in Tables I and 11,A G ~ isTestimated for Re-1 and Re-4 (Table IV). Because the D/D'+ couples are irreversible, these A G ~ values T are only approximations; however, in spite of this limitation two points are qualitatively clear: (1) A G ~ isT more exothermic for aromatic amine-substituted complex Re-4 than for complex Re-1, which features a 2' aliphatic amine donor and (2) A G ~ for T Re-1 is at best only weakly exothermic. The rate for forward ET in a donor-substituted complex can be estimated from MLCT emission lifetimes by the following equation? k,T = (l/7)- (1/7mdcJ (7) where 7 and rndelare emission lifetimes of the donor-substituted complex and a suitable non-donor-substituted model complex,
respectively. As noted above, eq 5 allows calculation of @ L L ~ , also by using the emission lifetime data. Values of k m and ~ @LLCT for Re-1 and Re-4, calculated using the emission lifetime data and eqs 5 and 7, are collected in Table IV. Several comments regarding the accuracy of these parameters are in order. First, for Re-4, it is clear that forward ET from the MLCT state is very fast; the MLCT emission decays with k = 2 X 109 s-1, which is nearly IO3 times faster than the normal MLCT decay rate (kd 4.8 X 106 s-l). Because forward ET is so rapid in Re-4, @ L L = ~ 1.0. On the other hand, the situation regarding k m and ~ @ L L for ~ Re-1 is somewhat less clear. Timeresolved studies indicate that 7cmfor this complex is depressed only slightly compared to model complexes Re-3 and Re-6. However, based on the fact that model complexes (e.g., Re-3, Re-6, and another model complex, (bpy)Re'(C0)3(4-[(Wbenzoylamin~)methyl]pyridine))~consistently give re,,,in the 21CL 2124s range, we believe that the reproducible but modest suppression of rem for complex R e 1 is due to formation of the LLCT state via forward ET. Thus, using emission lifetime data for Re-1, Re-3, and Re-6, it is estimated that k m ~ 2 X 105 s-1 and @ L L ~ 0.05. It is important to note that the significantly lower k m observed ~ for Re-1 compared to R e 4 is in accord with the fact that A G m is less exothermic for Re-1 than for Re-4. In addition, the absolute values of k m and ~ their relationship to A G ~ for T these two complexes are consistent with expectations based on a previous study which examined the dependence of k m on ~ A G ~ in T a series of nonreactive donor-substituted Re(1) complexes.* (c) C-C Bond Fragmentation in the LLCT State: Mechanism and Estimated Rates. The primary mechanistic step which is proposed to lead to decomposition of the donor ligand is C-C bond-fragmentation in the LLCT excited state (Scheme IV). A question which is of central interest in this study concerns the rate of this bond-fragmentation reaction. The a-AAreactivedonor ligands were selected because previous bimolecular photochemical studies indicated that in a-AA'+the a-bond between the amine and alcohol carbon atoms undergoes heterolyticfragmentation (seeeq 1) with a rate that is sufficiently fast that the process can compete with annihilation of a-AA'+ by bimolecular back-ET.15 Although studies of this process in bimolecular systems do not allow direct measure of the rate for bond-fragmentation ( k g ~ )results , suggest that in a-AAs which feature 2O or 3 O amines, k g ~ IO5s-l.15a In addition, comparison of fragmentation efficiencies for a-AAs that feature either aromatic or 3 O aliphatic amines provides qualitative evidence that k g is ~ slower in the aromatic a-AA systems.15a As noted above, under the assumption that the mechanism in Scheme IV is correct, keF can be calculated for Re-1 and R e 4 from the values of @PhCHO, @LUST, and T L L by ~ using eq 4. All of these parameters are available for Re-4: (PphcHo and @ L L ~ were obtained from steady-state photochemical measurements and emission studies, respectively, and T L was~ obtained by
-
-
-
-
Wang et al.
5682 J. Am. Chem. Soc., Vol. 115, No.13, 1993
Scheme IV
II
CHOH ph'
-hV,-A
+hv
1
@py)Re'-ND@)-I&-C€!
.F'h
I
FHOH
(COh
Ph
Table IV. Estimated ET Free Energies, ET Rates, and Bond
Fragmentation Ratesa complex AGm/eV AG&eV
~ e - 1 o + 0.2 R e 4 -0.27
~PET/S-~
O
L k~s e r / d
-2.4+ 0.2 2.2 x 105 0.05 -2.11 2.0X lo9 1.0
kep/fr'
=I x 107 =4 x 106 8.3 X lo7 5.OX 10'
CHlCN solution, 25 "C. I
'U1
L.1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
[DMA], m M
Figure6. Stern-Volmer quenching of Re1 MLCT emission lifetime ( 7 ) and quantum yield for benzaldehyde formation (a) by DMA. (A) emission lifetime quenching ( T " / T ) ; (+) quantum yield quenching (ao/ 9. laser flash photolysis. Using these data for Re-4 we estimate kgp = 5 x 105 s-1. By contrast, the kinetics are less well-defined for complex Re1. Although @PhCHO and @LLC'I. are available for this complex, direct measurement of T L L by ~ laser flash photolysis was not possible. Detection of the LLCT state in Re-1 by flash photolysis is precluded because (1) @ L L is~ small and (2) the 2 O aliphatic amine radical cation does not absorb strongly in the near-UV or visible region.35 Thus, in an effort to provide an estimate for T L L for ~ Re-1, a Stern-Volmer (S-V) experiment was carried out in which N,N-dimethylaniline (DMA) was used to quench the bond-fragmentation reaction by reductive ET from DMA to the LLCT state, (bpy*-)Re1(CO)3-(D-R'+). Because DMA also quenches the MLCT state, a parallel experiment was conducted to determine the efficiency of S-V quenching of the MLCT emission by DMA. The result of this experiment is presented in Figure 6,where (a" PhCHo/@PhCHo) and (Tcmo/T,m) are plotted vs [DMA]. This experiment indicates that, as expected, DMA quenchesthe MLCT state at the diffusionantrolled rate (kqMLCT = 1.1 X 101OM-ls-l). However,S-Vquenchingof@PhCHOexactly parallels quenching of the MLCT emission. The parallel quenching of T , (MLCT) and P C H O indicates two important features: (1) quenching of the LLCT state, if it occurs at all, is
too inefficient to detect and (2) the MLCT state is clearly a precursor in the mechanism leading to bond-fragmentation, consistent with the proposed mechanism in Scheme IV. Two possible reasons can be suggested to explain the lack of LLCT quenching in the S-V experiment. The first is that the bimolecular rate constant for interception of the LLCT state by DMA is large (e.g., kpLLm 1 lo9M-l s-l), but TLLCT is short (e.g., TLLCT 5 100 ns), so that the overall quenching efficiency is low. The second is that bimolecular quenching is slow (e.g., kqLLm
