J. Phys. Chem. 1989, 93, 3885-3887
branch by our technique would then be more delicate but nevertheless possible, provided the attractors are not too close. In multidimensional systems, a more direct initial approach of the unstable branch (e.g., through sudden injections of reagents”) (17) Pifer, T.; Ganapathisubramanian, N.; Showalter, K. In Non-Equilibrium Dynamics in Chemical Systems; Vidal, C., Pacault, A., Eds.; Springer-Verlag: Berlin, 1984; pp 5C-54.
3885
might however be preferable to the initial “free fall” technique used in this paper. Acknowledgment. The author thanks Miss L. Chalykoff for her dedication to the project and her meticulous work. The author also acknowledges helpful discussions with Drs. Y.L’Heureux, G.Dewel, and p. kfckmans. This project s ~ n ~ r by e dDND CRAD grant FUHDS.
I ntrarnolecular Energy Transfer by an Electron/Energy Transfer “Cascade” Gilles Tapolsky, Rich Duesing, and Thomas J. Meyer* Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514 (Received: January 31, 1989)
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In t h e ligand-bridged complex [(bpy)(CO)3Re1(4,4’-bpy)Re’(CO)3(4,4f-(CO2Et)zbpy)]2+ (bpy is 2,2’-bipyridine; 4,4’-bpy is 4,4’-bipyridine; 4,4’-(C02Et)2bpy is 4,4’-bis(ethoxycarbonyl)-2,2’-bipyridine) Re’ bpy excitation is followed by rapid, k(CH$N,295 K) > 2 X lo8 s-l, intramolecular energy transfer to give [(bpy)(CO)3Re1(4,4’-bpy)Re*1(CO)3(4,4’(C02Et)2bpy’-)]2+*. Energy transfer appears t o occur by an electronfenergy transfer Ycascade”involving the bridging ligand.
A theme in our w o r k has been the construction of designed molecular structures in which the Occurrence and directionality of intramolecular light-induced electron or energy transfer can be controlled.’J We report here an example where intramolecular energy transfer can be controlled in a ligand-bridged complex by changing the acceptor properties of the bridging ligand. Our studies have been based on the series of complexes [(4,4’- (W2bpy 1(CO)3Re1(4,4’-bpy)Re1(C0)3(4,4’-(X’MPY) 12+ with (I) X = X’ = C02Et, (11) X = X’ = NHz, (111) X = X’ = H, and (IV) X = H and X’ = C02Et. The ligand abbreviations are b or bpy for 2,2’-bipyridine, 4,4‘-bpy for 4,4‘-bipyridine, and 4,4’-(X)2bpy for 4,4’-(X)*-2,2’-bipyridine (X = H, NH2, or C02Et).3 COzEt \
(b)
illustrates the facial geometry at the individual Re(1) sites in the ligand-bridged c~mplexes.~ 0 C
I
OC
X
-
/C02Et
(b(NHd2)
Shown below is a schematic representation of the structure which
2+
co
[xqN% O C h . Re‘,,‘l’
v X
Following dr(Re) r*(bpy;4,4’-bpy) metal to ligand charge transfer (MLCT) excitation of the ligand bridged complexes, the final electron acceptor site can be controlled by varying the substituents, X and Xf.s In [(b(C02Et)2)(CO)3Re1(4,4f-bpy)Re’(CO)3(b(C02Et)2)]2+ cyclic voltammetric measurements show that the first ligand-based reduction occurs at E112 = 4 . 8 2 V vs SSCE.6 Because of the electron-withdrawingester substituents, the reduction is localized at the (bpy(CO2Et)J ligands. In CH3CN at 295 f 2 K,laser flash excitation’ at 390 nm into the dr(Re) ~*(b(CO~Et)~;4,4’-bpy) MLCT manifold leads to an emission (A, = 650 nm) which decays with T = 118 3 ns. The transient absorbance spectrum8in Figure 1A, which was acquired
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*
~~
(1) Schanze, K.S.;Neyhart, G. A.; Meyer, T. J. J. Phys. Chem. 1986,90,
2182. (2) (a) Chen, P.; Danielson, E.; Meyer, T. J. J. Phys. Chem. 1988, 92, 3708. (b) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon, D.; Neveux, P. E., Jr. Inorg. Chem. 1987,26, 1116. (c) Danielson, E.; Elliot, C. M.; Merkert, J. W.; Meyer, T. J. J. Am. Chem. Sw.1987,109, 25 19. (3) The symmetrical complexes were obtained in refluxing THF by the reactions between 4,4’-bpy and an excess of the appropriate triflate complex [(4.4’-(X)2-bpy)(CO)3Re (CF3S03)]. The unsymmetrical complexes were synthesized from [(bpy)(CO)3Re’(L)]+(L = 4,4’-bpy or 3,3’-(CH3)2-4,4’-bpy) The product salts and an excess of [(4,4’-(C02Et)2-bpy)(CO)3Rei(CF3S03)]. precipitated spontaneously or were precipitated by the addition of ethyl ether. Purifications were achieved by chromatography on silica gel (75-230 mesh) by using a 1:2 (v:v) CH3CN/CH2C12solvent mixture. After the solids were collected by evaporation, they were rechromatographed, if necessary, by starting with the same solvent mixture and ending with 101 acetone/water M (NH4](PF6) as eluent. The product was dried, containing -5 X redissolved in CH,CN, and reprecipitated by the addition of ether. Satisfactory elemental analyses were obtained in all cases. ‘H NMR (CD$N, 200 MHz) spectra were consistent with the proposed formula. Complete experimental procedures will be described in a subsequent paper.
0022-3654/89/2093-3885$01.50/0
(4) (a) Chen, P.; Curry,M.;Meyer, T. J. Inorg. Chem., in preas. (b) Horn, E.; Snow, M.R. Aust. J. Chem. 1980.33, 2369. (5) Wacholtz, W. F.; Auerbach, R. A.; Schmehl, R. H. I w g . Chrm. 1986, 25, 227. (6) All cyclic voltamogramms were obtained in argon deaerated 0.1 M (N(n-C4H9)4](PF6)CHICN solutions vs SSCE by using a platinum disk as the working electrode at scan rates of 0.1 V/s. (7) Emission lifetime measurements were obtained by using a PRA LN 1000/LN 102 nitrogen laser/dye laser combination for sample excitation. Emission was monitored at a right angle to the excitation by using a PRA B204-3 monochromator and a cooled, 10-stage, Hamamatsu R928 PMT coupled to either a LeCroy 9400 or a LeCroy 8013 digital oscilloscope interfaced to an IBM PC. The absorbance (in 1-cm cuvettes) of the different solutions was -0.1 at the excitation wavelength. Solutions were deoxygenated by Ar bubbling. (8) Transient absorbance measurements were performed by using the third harmonic of a Quanta Ray DCR-2A Nd:YAG laser. The excitation beam was coincident to an Applied Photophysics laser kinetic spectrometerincluding a 250-W pulsed Xe arc probe source, a f/3.4 grating monochromator, and a 5-stage PMT. The output was coupled to a Tektronix 7912 digital osCilloscope interfaced to an IBM PC. Electronic control and synchronization of laser, probe, and digital oscilloscope was achieved by electronics of our own design.
0 1 9 8 9 American Chemical Society
3886 The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
Letters 60'Om*
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A
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Figure 2. Transient absorption spectra obtained in freeze-pumpthaw deoxygenated acetonitrile solutions at 295 k 2 K showing the difference that occurs in absorbancebefore and after laser flash photolysis of (-) [(b)(CO)3Re1(4,4'-bpy)Re1(CO)3(b(C02Et)2)]2+ and (- - -) [(b)(CO)3Re1(3,3'-(CH3)2-4,4'-bpy)Re1(CO)3(b(COzEt)2)J2'. The groundstate absorbances in both cases were 0.7 at the excitation wavelength, 355 nm. The laser pulse energy was 4 mJ/pulse, and the delay times were 15 and 50 ns, respectively.
67.SmA-.
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64 f 3 ns in CH3CN. In the transient absorbance difference spectrum, acquired 65 ns after excitation at 355 nm, Figure lB, a broad, intense band appears a t 570-590 nm and an additional absorption at 390-400 nm. The appearance of the low-energy band is a characteristic feature for 4,4'-bipyridine derivatives in which single electron occupation leads to a coplanar, delocalized structure between the two ring^.'.^^*^*^ The spectrum of the intermediate is consistent with the occupation of r*(4,4'-bpy) by the excited electron following MLCT excitation.
hu
[(~(NH~)~)(CO)R~'(~,~'-~PY)R~'(CO)~((NH,)~)~~+ 1(b(NH2) 2) (CO) 3Re11(4,4'-bpy Re1(CO)3(b(NH2)2) I *+*
25. OmA-.
For [ (b)(CO)3Re1(4,4'-bpy)Re*(CO)3(b)]2+ electrochemical studies show that an initial bridge-based reduction occurs at -1 -06 V. It is followed by reduction of bpy at -1.20 V. In the absorption spectrum the lowest lying dn(Re) u*(2,2'-bpy) and du(Re) MOA, : : : ; : : ; ; : 4 r*(4,4'-bpy) transitions are completely overlapped and appear a t A, = 340 nm in CH3CN. The excited-state electronic distribution that results following MLCT excitation is solvent deFigure 1. Transient absorption spectra obtained in freeze-pumpthaw pendent. In a low-polarity solvent such as 1,2-dichloroethane deoxygenated acetonitrile solutions at 295 & 2 K. The spectra show the (DCE), 355-nm excitation leads to a maximum a t 360-380 nm difference that exists in absorbance before and following laser flash and a spectrum which is superimposable with that of the monomer excitation at 355 nm: (A) [(b(C02Et)2)(CO)3Re1(4,4'-bpy)Re1(CO)3- [(b)(CO)3Re1(4-Etpy)]+.2b In CH3CN, Figure IC, 355-nm ex(b(C02Et)dl2+; (B) [(~(NHz)z)(CO)~R~'(~,~'-~PY)R~'(CO)~(~citation leads to the appearance of the low-energy feature at (NHJ2)l2+;(C) [(b)(CO)3Re1(4,4'-bpy)Re1(CO)3(b)]2+. The ground570-590 nm. A careful inspection of relative intensities suggests state absorbance was 0.7 at the excitation wavelength, 355 nm. The laser that in CH3CN significant populations of both du(Re)-u*pulse energy was 4 mJ/pulse. The delay times were 50 ns for (A)and (2,2'-bpy) and ds(Re)-u*(4,4'-bpy) MLCT excited states must 65 ns for (B)and (C). exist. Excited-state decay in CH,CN follows wavelength-independent, first-order kinetics with 7 = 370 f 10 ns showing that 50 ns after laser excitation at 355 nm, shows that an intermediate the time scale for equilibration between states is rapid or that they is formed. The intermediate has absorption maxima at 380-390 decay at identical rates. Excited-state decay in the monomer, nm and at 460-480 nm. Excitation of the model complex [(b[(b'-)(C0),Ren(4-Etpy)]+*, occurs with 7 = 220 f 5 ns under (C02Et)2)(CO)3Re1(4-Etpy)]+ (4-Etpy is 4-ethylpyridine), under the same conditions. the same conditions gives essentially the same intermediate The ability of 4,4'-bipyridine to act as an electron acceptor spectrum. The comparison shows that following MLCT excitation provides a basis for an intramolecular electron/energy transfer and excited-state equilibration, the excited electron resides on the "cascaden pathway for energy transfer between MLCT chromoester substituted 2,2'-bpy ligand. phores in the ligand-bridged complexes."'-12 From the excitedhv [(b(COzEt)2)(CO)BRe1(4,4'-bpy)Re'(C0)3(b(CO2Et)2)]2+ state energies of -2.1 eV in [(b'-)(C0)3Re11(4,4'-bpy)Re'12. 5mA..
-
[(b+( CO,Et),) (CO) 3Re11(4,4'-bpy)Rel(CO) 3(b(C0,Et ),)I
-,+*
-
(9) (a) Wanatabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617. (b) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1684, 86, 5524. In the complex [(b(NH2)2)(CO)3Re1(4,4'-bpy)Re1(CO)3(b- (IO) Fuchs, Y.;Lofters, S.; Dieter, T.; Shi, W.;Morgan, R.; Strekas, T. (NH2),)I2+ an inversion in the u* levels occurs. The first reduction C.; Gafney, H. D.; Baker, A. D. J. Am. Chem. Soc. 1987, 109, 2691. (11) (a) Schmehl, R. H.;Auerbach, R. A.; Wacholtz, W . F. J . Phys. is at -1.09 V. It is a bridge-based reduction. Because of the Chem. 1988, 92,6202. (b) Moore, K. J.; Lee, L.;Figard, J. E.; Gelroth, A.; electron-donating amino substituents, the first reduction at bpyWohlers, D. H.; Petersen, J. D. J. Am. Chem. SOC.1983, 105, 2214. (NH2), does not occur until -1.60 V vs SSCE. A broad, weak (12) Curtis, J. C.; Bernstein, J. S.; Meyer, T. J. Inorg. Chem. 1985, 24, = 650 nm and T = emission occurs from the complex with A, 385.
The Journal of Physical Chemistry, Vola93, No. 10, 1989 3887
Letters
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SCHEME I
k>2xlds-'
[(b'~(CO)3ReTI(L)Re1(C0)~(b(X)2)]2+* hv
-
II
[(b)(CO)3Ren(L'~Re1(CO)3(b(X)~]*'* (-2.1 ev)
1lr
-
(C0),(b)l2+* and 1.9 eV in [(b'-(CO2Et),)(C0),Re"(4,4'bpy)Re1(CO)3(b(C02Et)2)]2+*, dr(Re)-r*(b) dr(Re)-r*(b(C02Et)2) energy transfer in [(b)(C0),Re1(4,4'-bpy)Re1-
greater contribution from the lower energy d~(Re)-?r*(b(CO,Et)~) based MLCT state. The decay of the transient absorbance of the excited state is nonexponential and can be fit to a biexponential function with T~ = 122 f 18 ns and T~ = 263 f 30 ns. Within (CO)3(b(C02Et)2)]2+ is favored by -0.2 eV. Optical excitation throughout the d r A* region of either chromophore (340420 experimental error, the same decay constants were obtained by nm) gives the same result. An excited state is reached which emits fitting the emission lifetime decay data. This shows that both emissive components arise from the complex. In the transient at A, = 650 nm in CH3CN (7 = 116 f 3 ns). From the transient absorbance experiment a direct observation is made of the d r absorbance difference spectrum in Figure 2, the excited state is (Re)-r*(b) and dr(Re)-r*(b(CO,Et),) excited states. [ (b)(CO),Rel( 4,4'- bp y) Re"( CO) ,( bo-(CO,Et),)] 2+*. The d r (Re)-~*(b(cO,Et)~)excited state is formed with the same efFrom our results, dr(Re)-r*(b) dr(Re)-~*(b(CO~Et)~) energy transfer occurs rapidly and with high efficiency across ficiency within the laser pulse ( k > 2 X lo8 s-l) either by direct 4,4'-bipyridine as a bridging ligand. It is a relatively unimportant excitation, dr(Re) ~ * ( b ( C o ~ E t )or ~ ) by , energy transfer event on the lifetime of the d~(Re)-?r*(b;b(CO~Et)~) following dr(Re) r*(b) excitation. excited states in the complex [ (b) (CO),Re1( 3,3'- (CH3),-4,4'-bpy) Re1(CO) (bIn the equivalent complex but with 3,3'-dimethyL4,4'-bipyridine (C02Et),)12+. Our results point toward the existence of intraas the bridging ligand, the methyl groups prevent the pyridyl molecular energy transfer by an electron/energy transfer "cascade" in the 4,4'-bpy bridged complex, Scheme I, although we cannot rule out enhanced, direct energy transfer promoted by greater * N electronic coupling through 4,4'-bipyridine as a bridge. In the cascade mechanism, the first step is an 'internal conversion" in which bpy to 4,4'-bpy intramolecular electron 3,3'-(CH3)2-4.4'-bpy transfer leads to a nearly iscenergetic, ligand-bridged excited state. The following step, which leads to [-Re11(CO)3(b'-(C02Et)2)]+*, groups of the bridging ligand from becoming cop~lanar.~."The could occur via direct d?r(Re)-r*(4,4'-bpy) dr(Re)-r*(bbridge-based reduction occurs at E I l 2> -1.6 V vs SSCE. Following laser excitation of [(b)(CO)3Re1(3,3'-(CH3)z-4,4'-bpy)-(C02Et)2)energy transfer. It could also occur via two different le--transfer sequences. In one, Re" Re1 electron transfer in Re1(CO)3(b(C02Et)2)]2+ at 355 nm in CH3CN, there is no evthe optically prepared mixed-valence complex idence in the transient absorbance difference spectrum, Figure 2, for a bridged-based MLCT excited state. A careful inspection [(b)(CO)3Re1*(4,4'-bpy'-)Re1(CO)3(b(C02Et),)]2+* of the data shows that two separate components exist in the [(b)Rel(4,4'-bpy'-)Re11(CO)3(b(C02Et)2)] 2+* 360-390-nm spectra region. They appear at 370 2 nm and at 383 f 4 nm and arise from the simultaneous appearance of d r would be followed by 4,4'-bpy b(C02Et), electron transfer (Re)-r*(b) and dr(Re)-?r*(b(CO,Et),) excited states. A tran[ (b)(CO)3Re1(4,4'-bpy'-)Re11(CO)3(b(C02Et)2)]2+* sient absorbance difference spectrum of a solution containing an [(b)( CO)3Re'(4,4'-bpy)Re11(CO)3(b+(C02Et)2)]2+* equimolar mixture of the monomers [(b)(C0),Re1(4-Etpy)]+ and [(b(CO2Et),)(C0),Re1(4-Etpy)]+ at the same concentration (From electrochemical data and estimated excited-state energies, 1.2 X 10-4 M) showed the same features. The ratio of absorbances the initial electron-transfer step in this sequence is disfavored by in the region 365-375 nm to the region 380-390 nm was, within -0.15 eV. In the second case, initial 4,4'-bpy b(CO,Et), experimental error, the same for the two solutions (1.24 and 1.28). electron transfer This result shows that the same relative amounts of the d r (Re)-?r*(b) and d~(Re)-?r*(b(CO~Et)~) excited states are formed [(b)(CO)3Re11(4,4'-bpy'-)Re1(CO)3(b(C02Et)z)]2+* in the ligand-bridged complex as in the separated complexes [ (b)(C0),ReI1( 4,4'-bpy) Re1(CO) ,( b*-(CO,Et),)] ,+* following 355-nm excitation. Emission maxima and quantum yields for [ (b)(CO),Re'( 3,3'-(CH3),-4,4'-bpy)Re1(C0),(b- is favored by - 4 . 2 eV. It would be followed by Re" Re' (CO,Et),)l2+ are wavelength dependent. High-energy excitation electron transfer results in a greater contribution to emission from the higher energy [(b)(CO)3Re11(4,4'-bpy)Re1(CO)3(b'-(EtC02)2)]2+* dr(Re)-r*(b) excited state. Low-energy excitation leads to a
-
-
--
-
*
-
-
-
-
-
-
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[(b)(C0)3Re1(4,4'-bpy)Re11(C0)3(b'-(EtC02)2)12+*
(13) fa) Sullivan, P. D.; Fong, J. Y. Chem. Phys. Lett. 1976.38.555. (b) Suzuki, H. Electronic Absorption Spectra and Geometry of Organic Molecules; Academic: New York, 1967. (c) Bastiansen, 0.;Samdal, S.J . Mol. Siruct. 1985. 128, 115.
Acknowledgment is made to the National Science Foundation under Grant CHE-8806664 for support for this research and to RHONE-POULENC GROUP for support for G.T.
