J. Phys. Chem. 1996, 100, 15145-15151
15145
Designed Intramolecular Competition in a Chromophore-Biquencher Complex Sandra L. Mecklenburg, Kimberly A. Opperman, Pinyung Chen, and Thomas J. Meyer* Department of Chemistry, UniVersity of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290 ReceiVed: April 2, 1996; In Final Form: June 3, 1996X
Competitive energy and electron transfer occur following excitation of the chromophore-biquencher [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 (MebpyCH2OCH2An is 4-[(9-anthrylmethoxy)methyl]-4′-methyl2,2′-bipyridine; MQ+ is N-methyl-4,4′,-bipyridinium ion). Following excitation with a 392-nm, 4-ns laser pulse in 1,2-dichloroethane (DCE), the initial excited state energy is distributed in a ∼2:1 ratio between 3An* and the lower lying ReII(MQ‚) metal-to-ligand charge transfer (MLCT) excited state. The appearance of the two states was monitored by transient absorption spectroscopy, which detected the triplet-triplet band for 3An* at λ 4 -1 • max ) 420 nm (τ ) 35 ( 2 µs, k ) (2.9 ( 0.2) × 10 s ) and a π f π* band of MQ at 610 nm 7 -1 (τ ) 37 ( 2 ns, k ) (2.7 ( 0.2) × 10 s ). These states result from a competitive partitioning in the initial excited state or states. They do not interconvert to any significant degree on the time scale of the shorter lived, higher energy ReII(MQ*) state.
Introduction ReI,
RuII,
OsII
Polypyridyl complexes of and have been utilized for studies of photoinduced intramolecular electron and energy transfer in solution and in molecular assemblies.1-4 These include oxidative and reductive electron transfer quenching of metal-to-ligand charge transfer (MLCT) excited states, e.g., eq 1-2,5,6 hν
98 *[ReII(bpy•-)(CO)3(MQ+)](PF6)2 f
states that result can undergo subsequent interconversion. With sufficient insight and properly designed assemblies, these molecules could provide the basis for observing and manipulating unusual phenomena such as intramolecular time delays and controlled, multiphotonic excitation. In this study, we have investigated a competition between intramolecular electron and energy transfer in [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+. This complex contains quenchers for both oxidation (MQ+) and energy transfer (-An).
[ReII(bpy)(CO)3(MQ•)](PF6)2 (1) 98 *[Re (bpy )(CO)3(py-PTZ)](PF6) f hν
II
•-
[ReI(bpy•-)(CO)3(py-PTZ•+)](PF6) (2)
and energy transfer, eq 3,7 hν
Experimental Section
98 *[Ru [(Mebpy )CH2OCH2An](MebpyCH2OCH2An)2](PF6)2 f [RuII[MebpyCH2OCH2(3An*)](MebpyCH2OCH2An)2](PF6)2 (3) III
•-
These reactions can occur rapidly and with high efficiency. When there are two different quenchers in the same molecule, an intramolecular competition for the initial excited state energy exists. The two quenching processes may compete, and the X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00977-X CCC: $12.00
General Methods. Uncorrected melting points were obtained with a Fisher-Johns apparatus. UV-vis spectra were recorded on a Hewlett-Packard 8452A photodiode-array spectrophotometer. Infrared spectra were recorded on a Nicolet DX20 Fouriertransform IR spectrophotometer. Mass spectra were recorded with a VG SEQ70 hybrid MS/MS spectrometer. 1H NMR spectra were recorded at 200 MHz on a Bruker AC 200 spectrometer or at 400 MHz on a Varian XL 400. Cationexchange HPLC was performed with an Aquapore CX-300 column (1.0 × 10 cm) of poly(DL-Asp)-silica (Brownlee) with a gradient of 0-400 mM KBr in 2:3 (v/v) CH3CN/0.6 mM phosphate buffer (pH 7.2). Thin-layer chromatograms were performed on Bakerflex silica-gel plates. Materials. The compounds 4-[(9-anthrylmethoxy)methyl]4′-methyl-2,2′-bipyridine (MebpyCH2OCH2An)7 and [ReI(dmb)(CO)3(4-Etpy)](PF6)8 were prepared and purified as described previously. © 1996 American Chemical Society
15146 J. Phys. Chem., Vol. 100, No. 37, 1996 Preparation of [ReI(dmb)(CO)3(MQ+)](PF6)2. [Re(dmb)(CO)3(OTf)] (166 mg, 0.275 mmol; OTf is trifluoromethane sulfonate) and [MQ+](PF6) (96 mg, 0.32 mmol) were dissolved in ∼40 mL of CH3OH and heated at reflux for 3 h. After cooling, the solvent was removed under reduced pressure. The resulting solid was dissolved in 20 mL of 1:1 CH3OH/H2O, and NH4PF6 (0.78 g) dissolved in 5 mL of H2O was added with stirring. The yellow precipitate was collected by filtration and then recrystallized from 20 mL of CH3OH. Anal. Calcd for C26H20N4O3F12P2Re: C 34.22, H 2.21, N 6.14. Found: C 34.47, H 2.67, N 5.81. 1H NMR (CD3CN): 2.54 (3 H, s); 4.29 (3 H, s); 7.62 (2 H, d); 7.67 (4 H, dd); 8.14 (2 H, d); 8.24 (1 H, s); 8.45 (2 H, d); 8.67 (2 H, d); 9.05 (2 H, d). Electrochemistry (0.1 M [(n-C4H9)4N](PF6)/CH3CN at 100 mV/s, vs SSCE reference electrode): E1/2 ) -0.68 (MQ+/0), -1.20, -1.40 V (all reversible, one-electron waves). Oxidation (ReII/I) is irreversible with Ep,a ≈ 1.8 V. Preparation of [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)](PF6). [ReI(CO)5(OTf)] was prepared by dissolving 0.500 g (1.38 mmol) of [ReI(CO)5Cl] and 1.1 molar equiv of AgOTf in 30 mL of CH2Cl2. The mixture was allowed to stir under a nitrogen atmosphere in the dark for 1.5 h and then filtered to remove the AgCl precipitate. Petroleum ether was added to the filtrate, and the resulting precipitate, [ReI(CO)5(OTf)], was collected by filtration and dried. [ReI(CO)5(4-Etpy)](OTf) was then prepared by combining 237 mg (0.498 mmol) of [ReI(CO)5(OTf)] and 0.057 mL (53 mg, 0.5 mmol) of 4-ethylpyridine in 15 mL of toluene. The solution was heated at reflux under a nitrogen atmosphere for 2.0 h. The resulting precipitate was collected by filtration and washed with toluene and diethyl ether. Then, 86 mg (0.149 mmol) of [ReI(CO)5(4-Etpy)](OTf) was placed into a reaction vessel with 58 mg (0.149 mmol) of MebpyCH2OCH2An and 10 mL of toluene. The stirred solution was heated to reflux under Ar and allowed to reflux for 2 h. The reaction mixture was allowed to cool to room temperature, and the precipitate was collected by filtration. The solid was dissolved in acetonitrile, and a saturated aqueous solution of NH4PF6 was added. The crude product was collected by filtration and then purified chromatographically on alumina with 1:1 toluene/acetonitrile as the eluent, followed by ion-exchange HPLC. Anal. Calcd for C37H31N3O4F6PRe: C 48.68, H 3.42, N 4.60. Found: C 48.39, H 3.63, N 4.68. 1H NMR (CD3CN): 1.10 (3 H, t); 2.49 (3 H, s); 2.54 (2 H, q); 4.88 (2 H, s); 5.69 (2 H, s); 7.10 (2 H, d); 7.43-7.69 (6 H, m); 7.81 (1 H, s); 8.02 (5 H, m); 8.45 (3 H, m); 8.90 (1 H, d); 9.05 (1 H, d). Preparation of [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2. [ReI(CO)5(MQ+)](OTf)(PF6) was prepared by dissolving 0.55 g (1.16 mmol) of [ReI(CO)5(OTf)] and 370 mg (1.16 mmol) of MQ+PF6 in 30 mL of toluene. The mixture was heated at reflux for 20 h in the dark under a nitrogen atmosphere. The resulting yellow precipitate was collected by filtration and washed with toluene and diethyl ether. It was used to prepare [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 as follows. To a refluxing solution of 0.292 g of [ReI(CO)5(MQ+)](OTf)(PF6) in 25 mL of toluene under Ar was added 0.145 g of MebpyCH2OCH2An. The mixture was allowed to reflux for 26 h, after which the precipitate was filtered off. The solid was dissolved in a minimum amount of MeOH, and a saturated solution of NH4PF6 in MeOH was added. The mixture was allowed to cool in the freezer for 2 h, and the solid was collected by filtration. The crude product was purified on alumina with 1:1 toluene/ acetonitrile as the eluent and purified by ion-exchange HPLC. 1H NMR (CD CN): 2.51 (3 H, s); 4.28 (3 H, s); 4.91 (2 H, s); 3 5.72 (2 H, s); 7.44-7.62 (8 H, m); 7.86 (1 H, s); 8.01 (2 H, d);
Mecklenburg et al. 8.03-8.10 (3 H, m); 8.35-8.41 (2 H, m); 8.42-8.51 (3 H, m); 8.62 (2 H, d); 8.93-9.50 (2 H, m) ppm. Electrochemistry. Tetra(1-butyl)ammonium hexafluorophosphate, [N(n-C4H9)4](PF6) (Fluka), was twice recrystallized from ethanol and vacuum dried for 10 h. UV-grade CH3CN (Burdick and Jackson) was used as received. Cyclic voltammograms were obtained in 0.1 M [N(n-C4H9)4](PF6)/CH3CN solutions with a computer-interfaced Princeton Applied Research 273 potentiostat/galvanostat, a silver/silver nitrate (0.1 M) reference electrode, a platinum-wire auxiliary electrode, and a BAS MF-2013 platinum-disk working electrode (0.31 cm2 electrode area) at a scan rate of 100 mV/s. Half-wave potentials (E1/2) were calculated by averaging the oxidative (Ep,a) and reductive (Ep,c) peak potentials and were not corrected for junction potentials. All half-wave potentials are reported vs the SSCE reference electrode. Photophysical Measurements. Emission and excitation spectra were obtained with a SPEX Fluorolog 212 photoncounting spectrofluorimeter. Emission spectra were collected with 390-nm excitation and a 1-mm slit width and were corrected for instrument response. Emission quantum yields, Φem, were measured in optically dilute CH3CN solutions (A390 ) 0.09-0.12, ∼1 × 10-5 M) relative to [RuII(bpy)3](PF6)2, for which Φem ) 0.062 in CH3CN at 295 K.9 The quantum yields were calculated as reported previously.10 Excitation spectra were obtained by monitoring at the emission maximum for each complex. Emission lifetimes and nanosecond transient absorption spectra and kinetics were measured as described previously11 at 295 K by using 392-nm, 4-ns laser pulses (2.5 mJ/pulse) obtained when the third harmonic (354.7 nm) of a Quanta Ray DCR-2A Nd:YAG laser was used to pump a Quanta Ray PDL-2 dye laser containing the appropriate dye. For transient absorption studies, the excitation beam was coincident at the sample with the white-light monitoring beam provided by a modified Applied Photophysics laser kinetic spectrometer, which utilized a 300-W pulsed Xe arc lamp probe source, a f/3.4 grating monochromator, and a five-stage PMT. The resulting output was collected with the use of the LeCroy 6880/6010 transient digitizer and an IBM PC. Electronic control and synchronization of the laser, probe, and transient digitizer were provided by electronics of our own design. Samples dissolved in UVgrade 1,2-dichloroethane (DCE) or CH3CN (Burdick and Jackson) had an absorbance of ∼0.15-0.25 at 392 nm (∼1 × 10-5 M) in a 1-cm quartz cuvette and were bubble-deoxygenated with high-purity argon for at least 10 min. Additional transient absorption measurements were performed with 355- and 420nm excitation; for each excitation wavelength, the sample concentration was adjusted so that the absorbance was ∼0.150.25 at the excitation wavelength. The laser power was 1.2 mJ/pulse at 420 nm (collinear excitation geometry), 2.5 mJ/ pulse at 392 nm, and 3-5 mJ/pulse at 532 nm (both with perpendicular excitation geometry). The same instrumentation and excitation source, without the probe beam, were utilized for transient emission lifetime measurements. Results UV-Visible Spectroscopy and Electrochemistry. UVvisible data are summarized in Table 1, and the spectra of [ReI(dmb)(CO)3(4-Etpy)]+, [ReI(dmb)(CO)3(MQ+)]2+, and [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+ in DCE are shown in Figure 1. A dπ(Re) f π*(dmb) metal-to-ligand charge transfer (MLCT) band is observed in the 350-nm region, while the near UV is dominated by ligand-based π f π* bands. In compounds
Chromophore-Biquencher Complex
J. Phys. Chem., Vol. 100, No. 37, 1996 15147 TABLE 2: Electrochemical Data in CH3CNa complexb [ReI(dmb)(CO)3(4-Etpy)]+,c [ReI(dmb)(CO)3(MQ+)]2+ [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)]+ [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+
E1/2 (ReII/I)
E1/2 (dmb0/-)
E1/2 (MQ+/0)
1.69 1.80 1.79
-1.25 -1.20 -1.21
-0.68
1.82
-1.19
-0.69
a
Values obtained in 0.1 M [N(n-C4H9)4](PF6)/CH3CN solutions at 22 ( 1 °C at a scan rate of 100 mV/s. Details given in the Experimental Section. b As PF6- salts. c As the TFMS salt.8
TABLE 3: Photophysical Data in DCE at 295 K complexa [ReI(dmb)(CO)3(4-Etpy)]+ [ReI(dmb)(CO)3(MQ+)]2+ [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)]+ [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+
Figure 1. (A) UV-visible spectra of [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 (s, i), [ReI(dmb)(CO)3(MQ+)](PF6)2 (- - -, ii), and [ReI(dmb)(CO)3(4-Etpy)](PF6) (‚‚‚, iii) in 1,2-dichloroethane. (B) Difference spectra showing the contributions to the ground state absorption spectrum of [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 in DCE by the [ReI f (Mebpy)-] chromophore (s, same as iii in A), [ReI f MQ+] (- - -, ii - iii), and π f π* (An) (‚‚‚, i - ii) absorptions.
TABLE 1: UV-Visible Absorption Spectral Features in DCE at 295 Ka complex [ReI(dmb)(CO)3(4-Etpy)](PF6) [ReI(dmb)(CO)3(MQ+)](PF6)2 [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)](PF6)
[ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2
a
λ, nm (, M-1 cm-1)
assignments
366 (4100) 322 (11500) 270 (21900) 252 (21500) 350 (5600) 320 (8500) 308 (7850) 254 (22600) 388 (3640) 368 (4500) 352 (4050) 320 (7700) 258 (44160) 388 (4500) 368 (6000) 350 (6150) 320 (8200) 306 (7800) 258 (20400)
dπ f π* π f π* π f π* π f π* dπ f π* π f π* π f π* π f π* π f π* (An) π f π* (An) π f π* (An); dπ f π* π f π* π f π* π f π* (An) π f π* (An) π f π* (An); dπ f π* π f π* π f π* π f π*
Error, (5%.
containing the anthracene derivative, the characteristic vibronic progression for the S0 f S1 transition of anthracene appears superimposed on the MLCT band. In Figure 1B are shown difference spectra that illustrate the approximate contributions of the individual chromophores to the spectrum of [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+.
em λmax (nm)b
Φemc
τem (ns)d
τMQ• (ns)e
570 695 589g
0.067 0.0006 0.032
473 44 440g
44
629h
0.0013
70
37
τ3An* (µs)f
37 35
a As PF - salts. b Emission band maximum in nm ((2). c Emission 6 quantum yield ((10%) with [RuII(bpy)3](PF6) in CH3CN at 295 K as the standard.9 d Lifetime ((2 ns) obtained by analyzing emission decay curves at the emission maxima following laser excitation at 420 nm. e Redox-separated state lifetime ((2 ns) obtained by analyzing transient absorption decay curves at 610 and/or 390 nm following laser flash excitation at 420 nm. f Triplet anthracene (3An*) lifetimes ((2 µs) obtained by analyzing transient absorption decay curves at 430 nm following laser excitation at 420 nm. g Incomplete quenching apparently because of endoperoxide formation (see text). h Impurity emission (see text).
Electrochemical data acquired in CH3CN are compiled in Table 2. The ReII/I couples fall in the range 1.69-1.83 V vs SSCE and E1/2 values for the dmb0/- couples from -1.19 to -1.25 V. The MQ+/0 couple appears at E1/2 ) -0.68 V. The triplet energy of anthracene is 1.85 eV.12 The energy of the ReII(dmb•-) excited state in DCE is 2.60 eV, as calculated from emission spectral fitting parameters.13 Photophysical Properties. Photophysical data acquired in DCE, including emission maxima (λmax), quantum yields (Φem), emission lifetimes (τem), and the lifetimes for MQ• (τMQ•) and triplet anthracene (τ3An*) from transient absorption measurements, are summarized in Table 3. MLCT emission from [ReI(dmb)(CO)3(4-Etpy)]+ in DCE occurs at 570 nm. The excitation spectrum, acquired at 570 nm, follows the absorption spectrum in Figure 1 until ∼325 nm; at shorter wavelengths the relative emitted intensity falls by more than a factor of 5. The ratio of the molar extinction coefficients () at 322 and 366 nm is 322/366 ) 2.6. The ratio of total integrated emission intensities (Iem) at these excitation wavelengths is Iem,322/Iem,366 ) 0.9. In the transient absorption difference spectrum, following 392-nm excitation in DCE, a band appears at 380 nm due to a π f π* transition at dmb•-. In an earlier investigation on [ReI(bpy)(CO)3(MQ+)]2+,5 it was shown that Re f bpy excitation is followed by rapid bpy•f MQ+ intramolecular electron transfer to give [ReII(bpy)(CO)3(MQ.)]2+*. This lower energy MLCT state decays with τMQ• ) 20 ns (k ) 5.0 × 107 s-1) in CH3CN5b and τMQ• ) 52 ns (k ) 1.9 × 107 s-1) in DCE.5e Transient absorption and emission measurements on [ReI(dmb)(CO)3(MQ+)]2+ show that the same sequence of events occurs, Scheme 1. The lowest lying ReII(MQ•) excited state is also accessible by direct ReI f MQ+ excitation, hν′ in Scheme 1. In the transient absorption difference spectrum of [ReI(dmb)(CO)3(MQ+)]2+ in DCE following 392-nm excitation, absorp-
15148 J. Phys. Chem., Vol. 100, No. 37, 1996
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SCHEME 1
tions due to MQ• appear at 390 and 610 nm.14 These transients decayed to the ground state with τ ) 44 ns (k ) 2.3 × 107 s-1). In DCE, the ReII(MQ•) state emitted at 695 nm and decayed with τ ) 44 ns (k ) 2.3 × 107 s-1). In CH3CN, we measured a much shorter excited state lifetime, τ < 4 ns (k > 2.5 × 109 s-1). Excitation of the model [ReI(MebpyCH2OCH2An)(CO)3(4Etpy)]+ (shown below) results in the appearance of the characteristic triplet-triplet absorption of anthracene at λmax ) 420 nm15 in DCE in the transient absorption difference spectrum. 3An* is formed during the laser pulse and decays with τ ) 35 ( 2 µs (k ) 2.85 × 104 s-1). When 420-nm excitation was used, the 3An* absorption was monitored at 430-435 nm. From the data in Table 3 there is a residual MLCT emission at λmax ) 589 nm. It decays with a lifetime of 440 ns (k ) 2.3 × 106 s-1), comparable to the lifetime of the ReII(dmb•-) MLCT excited state in [ReII(dmb•-)(CO)3(4-Etpy)]+*, τ ) 473 ns (k ) 2.1 × 106 s-1). Excitation spectra for the two monitored at 589 nm were similar. In the transient absorption difference spectrum, a π f π* (dmb•-) absorption feature for ReII(dmb•-) appears at 380 nm and decays with τ ) 440 ns. The observed emission is attributed to an impurity in the sample of [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)]+. Given the observations of DeCola et al,2a it is most likely the endoperoxide formed during workup of the sample in the presence of light in nondeaerated sample solutions.
Figure 2. Transient absorption decay traces following laser flash photolysis of [ReI(MebpyCH2OCH2An)(CO)3(MQ+)](PF6)2 in DCE with a 392-nm, 4-ns pulse (2.5 mJ/pulse) at (A) 610 nm (MQ•.); (B) 430 nm (3An*); (C) 430 nm (3An*, longer time scale). The smooth curves are the best fits to a single-exponential function with rate constants cited in the text.
TABLE 4: Transient Absorption Spectral Changes Following Laser Flash Excitation in DCE at 295 K complexa
Both electron and energy transfer quenchers are present in [ReI(MebpyCH2OCH2An)(CO)3(MQ+)]2+. There is a weak emission in DCE at 629 nm (τem ≈ 70 ns). On the basis of the low quantum yield (Φem ) 0.0013) and the results of reverse phase TLC, the observed emission is assigned to an impurity which is 20 µs (k < 5 × 104 s-1). It was not possible to study the competition between energy and electron transfer in this solvent with our apparatus because the ReII(MQ•) excited state is apparently too short-lived (τ < 4 ns), as is the model, [ReI(dmb)(CO)3(MQ+)]2+ (τ < 4 ns), in CH3CN. Quenching Mechanisms. Our time resolution was insufficient to probe the time evolution of the intramolecular competition directly in DCE because both -3An* and ReII(MQ•) were formed during the 4-ns laser pulse. Inferences can be drawn about quenching mechanisms based on the behavior of [ReI(dmb)(CO)3(MQ+)]2+ and [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)]+. In these complexes absorption is dominated by transitions to MLCT excited states, ReII(dmb•-), ReII(MQ•) or [ReII(Mebpy•-)-], that are largely singlet in character. These overlap with transitions to the corresponding triplet MLCT states, which are at lower energy and which are the emitting states. Because of the magnitude of the spin-orbit coupling constant at Re (ζ ≈ 2200 cm-1), the “singlet” and “triplet” states are highly mixed and interconversions between them are rapid.15-17 In [ReII(dmb)(CO)3(MQ•)]2+*, there is a significant decrease in energy content (∼0.3 eV) between the vertical FranckCondon state and the thermally equilibrated state.5e A major contributor is an increase in delocalization energy in the equilibrated state. The angle between the planes of the pyridyl rings of the MQ+ ligand change from ∼47° in the FranckCondon state to ∼0° in the thermally equilibrated state to maximize delocalization of the excited electron.5c ReI f dmb excitation of [ReI(dmb)(CO)3(MQ+)]2+ and relaxation to give 3[ReII(dmb•-)(CO)3(MQ+)]2+* are followed by rapid dmb•- f MQ+ electron transfer in fluid solution to give 3[ReII(dmb)(CO)3(MQ.)]2+*. From the difference in emission energies between 3[ReII(dmb)(CO)3(MQ•)]2+* and 3[ReII(dmb•-)(CO)3(4-Etpy)]+* in Table 3, ReII(MQ•) is favored by ∼0.4 eV. The photophysical properties of [ReI(MebpyCH2OCH2An)(CO)3(4-Etpy)]+ are complicated by an emitting impurity (see the Results section), but intramolecular energy transfer following ReII(dmb•-) excitation is also rapid. This is demonstrated by the appearance of -3An* during the laser pulse (τ < 4 ns, k > 2.5 × 108 s-1). Its subsequent decay (τ ) 37 µs, k ) 2.7 × 104 s-1) is within experimental error of the decay of [ReI(MebpyCH2OCH2-3An*)(CO)3(MQ+)]2+.
15150 J. Phys. Chem., Vol. 100, No. 37, 1996
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SCHEME 2
Intramolecular Competition. On the basis of our observations, it is possible to propose a mechanism for the intramolecular competition. The relative amounts of ReII(MQ•) and -3An* are slightly affected by excitation wavelength and, therefore, by the relative amounts of [ReI(MebpyCH2OCH21An*)(CO) (MQ+)]2+*, 1[ReII(MebpyCH OCH An)(CO) (M3 2 2 3 Q•)]2+*, and 1[ReII((Mebpy•-)CH2OCH2An)(CO)3(MQ+)]2+* formed in the initial excitation. (The slight decrease in relative -3An* production at higher wavelengths, paradoxically, points to decreased sensitization upon -An f -1An* excitation.) The relatively excitation independent product distribution suggests that competitive electron and energy transfer occur from a common intermediate state, or rapidly equilibrated states. The various possible reactions are summarized in Scheme 2 along with estimates for the energies of the states involved. In Scheme 2, the ligand MebpyCH2OCH2An is abbreviated as An-bpy and the chromophore biquencher as [(An-bpy)Re(CO)3(MQ+)]2+. It is assumed that ReI f (Mebpy-), ReI f MQ+ excitation is followed by rapid equilibration to states of dominantly triplet character. The energy of -3An*, 1.85 eV, was taken from the literature12 and 3.2 eV for -1An* from the energy of the lowest energy vibronic component in the absorption spectrum at 388 nm. The energy of 3[ReII((Me-bpy•-)CH2OCH2An)(CO)3(MQ+)]2+* (which is represented as 3[(Anbpy•-)ReII(CO)3(MQ+)]2+* in Scheme 2) was taken as the energy of 3[ReII(dmb•-)(CO)3(4-Etpy)]+*, 2.6 eV.13 There are two ReII(MQ•) states, one the initial Franck-Condon state, 3[(An-bpy)ReII(CO)3(MQ•)]2+*, and the other the equilibrated form with the pyridyl rings coplanar, 3[(An-bpy)ReII(CO)3(MQe•)]2+*. The energy of the equilibrated state, 2.2 eV, was taken as the difference in emission energies between 3[ReII(dmb•-)(CO)3(4-Etpy)]+* and 3[ReII(dmb)(CO)3(MQ•)]+* (0.4 eV, Table 3). The Franck-Condon state, 3[(An-bpy)ReII(CO)3(MQ•)]2+*, is assumed to lie 0.3 eV higher in energy, at 2.5 eV. It is not possible to account for the partitioning of the product distribution among the various reaction channels in any quantitative way, but some qualitative observations can be made based on the available data: (1) In anthracene, 1An* f 3An* intersystem crossing is inefficient. Thus in the chromophore biquencher, k1 is not an important pathway for formation of -3An* unless a heavy atom effect exerted by Re facilitates intersystem crossing. The -3An* state may arise indirectly, through the intermediacy of the [ReII(Mebpy•-)-] MLCT excited state and sequential energy transfer (k0 followed by k2 in Scheme 2). (2) The conversion between Franck-Condon and equilibrated forms of ReII(MQ•) (k4) involves simple rotation around the py-py axis of MQ• to coplanarity (coupled to a change in internal electronic structure)5e and is presumably rapid. (3) From related results on the models, both energy transfer to
-An (k2) and electron transfer from [(Mebpy•-)-] to MQ+ (k3) are rapid. (4) From the absence of a significant excitation dependence, there may be a common state or states through which the competition flows, in this case possibly 3[ReII((Mebpy•-)CH2OCH2An)(CO)3(MQ+)]2+*. Once formed, the separate -3An* and 3[ReII(MQ•)] states decay independently with only slight evidence that they are interconverted or kinetically coupled. The lifetime of [ReI(MebpyCH2OCH2-3An*)(CO)3(MQ+)]2+ (τ ) 35 µs, k ) 2.9 × 104 s-1) is the same as [ReI(MebpyCH2OCH2-3An*)(CO)3(4-Etpy)]+ (τ ) 37 µs, k ) 2.7 × 104 s-1) within experimental error. The lifetime of 3[ReII(MebpyCH2OCH2An)(CO)3(MQ•)]2+* (τ ) 37 ns, k ) 2.7 × 107 s-1) is decreased slightly compared to the model 3[ReII(dmb)(CO)3(MQ•)]2+* (τ ) 44 ns, k ) 2.3 × 107 s-1). This could arise because of ReII(MQ•) f -An energy transfer (k5 in Scheme 2), which is favored by ∼0.35 eV. However, there is no evidence for a growth in -3An* in the transient absorption measurements on the time scale of ReII(MQ•) decay (37 ns). The absence of efficient energy transfer between ReII(MQ•) and -An illustrates the importance of spatial proximity effects on intramolecular energy transfer. Acknowledgment. We acknowledge financial support from the National Science Foundation under Grant CHE-8806664 and thank T. B. for recrystallizing 9-anthracenemethanol. References and Notes (1) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (c) Meyer, T. J. In Photochemical Processes in Organized Molecular Systems; Honda, K., Ed.; Elsevier: Amsterdam, 1991; p 133. (d) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: Chiehester, 1991. (e) Photoinduced Electron Transfer, Vols. 1-3; Mattey, J., Ed. Top. Curr. Chem. 1991, 159. (f) Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: New York, 1988. (g) Petersen, J. D. Coord. Chem. ReV. 1985, 64, 261. (h) Meyer, T. J. Progress in Inorganic Chemistry; John Wiley and Sons: New York, 1983; Vol. 30. (i) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (j) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic Press: New York, 1979. (2) (a) De Cola, L.; Balzani, V.; Barigellitti, F.; Flamigni, L.; Belser, P.; von Zelewsky, A.; Frank, M.; Voegtle, F. Inorg. Chem. 1993, 32, 5228. (b) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (c) Leasure, R. M.; Sacksteader, L. A.; Nesselrodt, D.; Reitz, G. A.; Demas, J. N.; Degraff, B. A. Inorg. Chem. 1991, 30, 1330. (d) Lee, E. J.; Wrighton, M. S. J. Am. Chem. Soc. 1991, 113, 8562. (e) Cooley, L. F.; Lareson, S. C.; Elliott, C. M.; Kelley, D. F. J. Phys. Chem. 1991, 95, 10694. (f) Resch, U.; Fox, M. A. J. Phys. Chem. 1991, 95, 6169. (g) Haga, M.; Kiyoshi, I.; Boone, S. R.; Pierpont, C. G. Inorg. Chem. 1990, 29, 3795. (h) Perkins, T. A.; Humer, W.; Netzel, T. L.; Schanze, K. S. J. Phys. Chem. 1990, 94, 2229. (i) Ohno, T.; Yoshimura, A.; Prasad, D. R.; Hoffman, M. Z. J. Phys. Chem. 1991, 95, 4723. (3) (a) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952. (b) Reitz, G. A.; Demas, J. N.; Degraff, B. A. J. Am. Chem. Soc. 1988, 110, 5051. (c) Midler, J. S.; Gold, J. S.; Kliger, D. S. J. Phys. Chem. 1986, 90,
Chromophore-Biquencher Complex 548. (d) Reitz, G. A.; Dressick, W. J.; Demas, J. N.; Degraff, B. A. J. Am. Chem. Soc. 1986, 108, 5344. (e) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1985, 24, 106. (f) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1984, 23, 3877. (g) Drickamer, G. G.; Salman, O. A. J. Phys. Chem. 1982, 77, 3337. (h) Bradley, P. G.; Kress, N.; Hornberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1981, 103, 7441. (4) (a) Worl, L. A.; Strouse, G. F.; Younathan, J. N.; Baxter, S. M.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 7571. (b) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (c) Kober, E. M.; Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1988, 27, 4587. (d) Barqawi, K. R.; Llobet, A.; Meyer, T. J. J. Am. Chem. Soc. 1988, 110, 7751. (e) Johnson, S. R.; Westmoreland, T. D.; Caspar, J. V.; Barqawi, K. R.; Meyer, T. J. Inorg. Chem. 1988, 27, 3195. (f) Olmsted, J., III; McClanahan, S. F.; Danielson, E.; Younathan, J. N.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 3297. (g) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (5) (a) Westmoreland, T. D.; Le Bozec, H.; Murray, R. W.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5952. (b) Chen, P.; Danielson, E.; Meyer, T. J. J. Phys. Chem.. 1988, 92, 3708. (c) Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271. (d) Jones, W. E., Jr.; Chen, P.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 387. (e) Schoonover, J. R.; Chen, P.; Bates, W. D.; Dyer, B. R.; Meyer, T. J. Inorg. Chem. 1994, 33, 793. (6) (a) Westmoreland, T. D.; Schanze, K. S.; Neveux, P. E., Jr.; Danielson, E.; Sullivan, B. P.; Chen, P.; Meyer, T. J. Inorg. Chem. 1985, 24, 2596. (b) Chen, P.; Westmoreland, T. D.; Danielson, E. Inorg. Chem. 1987, 26, 1116. (c) Chen, P.; Duesing, R.; Tapolsky, G.; Meyer, T. J. J. Am. Chem. Soc. 1989, 111, 8305. (d) Chen, P.; Deusing, R.; Graff, D. K.; Meyer, T. J. J. Phys. Chem. 1991, 95, 5850. (7) Boyde, S.; Strouse, G. F.; Jones, W. E., Jr.; Meyer, T. J. J. Am. Chem. Soc. 1989, 111, 7448. (8) Hino, J. K.; Della Ciana, L.; Dressick, W. J.; Sullivan, B. P. Inorg. Chem. 1992, 31, 1072.
J. Phys. Chem., Vol. 100, No. 37, 1996 15151 (9) Strouse, G. F.; Anderson, P. A.; Schoonover, J. R.; Meyer, T. J.; Keene, R. F. Inorg. Chem. 1992, 31, 3004. (10) Duesing, R.; Tapolsky, G.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 5378 (11) Mecklenburg, S. L.; Peek, B. M.; Schoonover, J. R.; McCafferty, D. G.; Wall, C. G.; Erickson, B. W.; Meyer, T. J. J. Am. Chem. Soc. 1993, 115, 5479. (12) (a) Porter, G.; Windsor, M. W. Proc. R. Soc. A 1958, 245, 238. (b) McGlynn, S. P.; Boggus, J. D.; Elder, E. J. Chem. Phys. 1960, 32, 357. (c) For 3An* in benzene or toluene, λmax ) 428 nm and 428 nm ) 42 000 M-1 cm-1.12b In DCE complexes containing the ligand MebpyCH2OCH2An, we measure λmax ) 420 nm for -(3An*), and estimate 430 nm ) 33 000 M-1 cm-1. (13) (a) Emission spectral fitting for [ReII(dmb.-)(CO)3(4-Etpy)]+* in DCE at 25 °C was carried out with the program GOODFIT according to previously described procedures.13b The parameters obtained for a onemode fit with pω ) 1450 cm-1 were E0 ) 18 394 cm-1; SM ) 1.41; ∆νj0,1/2) 2474 cm-1. The excited state energy was calculated from ∆Ges ) E0 + [(∆νj0,1/2)2](16KBT ln 2)-1, which gave ∆Ges ) 2.60 eV. (b) Claude, J. P. Ph.D. Dissertation, University of North Carolina, 1994. (14) Sullivan, B. P.; Abruna, H.; Finklea, H. O.; Salmon, D. J.; Nagle, J. K.; Meyer, T. J.; Sprintschnik, H. Chem. Phys. Lett. 1978, 58, 389. (15) (a) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (b) Turro, N. J. Molecular Photochemistry; W. A. Benjamin: New York, 1967. (16) Figgis, B. N. Introduction to Ligand Fields; Robert E. Krieger Publishing Co.: Malabar, FL, 1986; p 57. (17) (a) Striplin, D. R.; Crosby, G. A. Chem. Phys. Lett. 1994, 221, 426. (b) Striplin, D. R.; Crosby, G. A. Submitted.
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