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J. Phys. Chem. A 2001, 105, 5768-5777
Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile Jorge Peon, Xin Tan, J. David Hoerner, Chungeng Xia, Yin Fei Luk, and Bern Kohler* Department of Chemistry, The Ohio State UniVersity, 100 W. 18th AVe., Columbus, Ohio 43210, U.S.A. ReceiVed: December 31, 2000; In Final Form: March 30, 2001
The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1′dimethyl-4,4′-bipyridinium, MV2+) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest singlet excited state of MV2+. Transient absorption was measured between 270 and 740 nm as a function of delay time after excitation of the strong π-π* transition of MV2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical cation of methyl viologen (MV•+) appeared within our time resolution, indicating that forward electron transfer from a nearby donor quenches electronically excited MV2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV2+ and for the salt prepared with tetrafluoroborate counterions. This latter “superhalide” ion has a condensed-phase detachment threshold that is too high to permit oxidation by the excited state of MV2+. Thus, electron transfer does not take place within an associated MV2+-counterion complex in methanol but results instead from oxidation of a solvent molecule. Photoreduction of MV2+ in methanol is a novel example of ultrafast electron-transfer quenching of a photoexcited acceptor in an electron-donor solvent. This is the first demonstration that a hydrogen-bonding solvent can serve as the electron donor in an ultrafast intermolecular ET reaction. Decay of the initial MV•+ population and simultaneous recovery of ground-state MV2+ with a characteristic time constant of 430 ( 40 fs were observed immediately after the pump pulse and assigned to back electron transfer in the geminate radical pair. Despite the high rate of back electron transfer, a significant fraction of the initial radical pairs avoid recombination, and a finite yield (∼12%) of MV•+ ions is observed at delay times > 2 ps. There was no evidence of photoreduction when the solvent was acetonitrile or water. Both of these solvents have high gas-phase ionization potentials that prevent oxidation by excited MV2+. The transient absorption signals indicate, however, that very different excited-state decay channels exist in these two solvents. In aqueous solution, an unknown nonradiative decay process causes decay of excited MV2+ with a time constant of 3.1 ps in H2O and 5.3 ps in D2O. In acetonitrile, on the other hand, the transient absorption decays hundreds of times slower and fluorescence is observed. This is the first report of an efficient radiative decay pathway for MV2+ in fluid solution. The excited-state absorption spectrum (S1fSN spectrum) of MV2+ was measured in acetonitrile and the fluorescence was characterized by time-correlated singlephoton counting and steady-state measurements. The fluorescence quantum yield is 0.03 ( 0.01 and the lifetime in acetonitrile at room temperature is 1.00 ( 0.04 ns. The fluorescence is efficiently quenched by electron transfer from added quenchers with gas-phase ionization potentials lower than about 10.8 eV. Using the measured emission spectrum, the excited-state reduction potential is determined to be E° (MV2+*/MV•+) ) 3.65 V, confirming the highly oxidizing character of this photoexcited dication.
Introduction
SCHEME 1
Methyl viologen (1,1′-dimethyl-4,4′-bipyridinium, MV2+ in Scheme 1) has attracted a great deal of study by a diverse group of chemists due to its wide-ranging applications.1 The dichloride salt (trade name “paraquat”) is a commercial herbicide that has been widely used for weed control. MV2+ has been investigated for solar energy conversion.2-5 With a suitable catalyst MV2+ can produce hydrogen from water using visible light.5 Because it is an organic dication it adsorbs strongly to negatively charged assemblies such as zeolites,3 the sugar-phosphate backbone of DNA,6 and silica gel.7 MV2+ and its derivatives bind strongly to DNA and have been studied as potential photocleaving agents.8,9 Photodamage in DNA has been studied by photo* To whom correspondence should be addressed. E-mail: kohler@ chemistry.ohio-state.edu.
induced electron transfer (ET) from an intercalated donor to externally bound MV2+.10 It has also been used to study ET in zeolites.3,11-14 These diverse applications are made possible by the strong electron accepting character of MV2+ and the stability of the radical cation (MV•+ in Scheme 1) that is produced by oneelectron reduction. The absorption spectrum of MV•+ is characterized by two strong transitions, one in the near UV and one in the red (λmax ) 396 nm, and 606 nm in H2O,15 see Figure 2). The violet color due to the latter transition is the reason that
10.1021/jp010084d CCC: $20.00 © 2001 American Chemical Society Published on Web 05/17/2001
Excited State Dynamics of Methyl Viologen
Figure 1. Transient absorption at 600 nm (circles) and at 270 nm (triangles) following photoexcitation of a 2.3 mM solution of the tetrafluoroborate salt of methyl viologen in methanol by a 150 fs, 265 nm pump pulse. The solid curves are from a global nonlinear leastsquares fit (see text for details) to the data shown here plus additional transients at probe wavelengths between 270 and 700 nm.
J. Phys. Chem. A, Vol. 105, No. 24, 2001 5769 years ago that photoexcitation of MV2+ in aqueous alcohol solution by ultraviolet (UV) light leads to the formation of MV•+ and aldehydes.23,24 Photoreduction was also observed in zeolites,12,25 and several authors have commented on the strongly oxidizing character of electronically excited bipyridinium salts.7,26-28 Despite previous investigation by laser flash photolysis,29-32 the elementary steps underlying the deactivation of excited MV2+ have remained unclear. Rodgers studied the photoreduction of MV2+ in neat methanol using nanosecond laser flash photolysis.29 A significant amount of MV•+ was generated faster than the experimental time resolution of 10 ns. Ebbesen et al. studied optically excited MV2+ under conditions that strongly favored association between this species and its counterions.30-32 They showed that ET occurs in a complex between MV2+ and one or more chloride counterions faster than their experimental time resolution of approximately 30 ps. Because of their limited time resolution, they were unable to observe the ET dynamics directly. Furthermore, their work does not provide any information about free MV2+ ions in solution. We therefore undertook a systematic study of the excited-state dynamics of MV2+ using femtosecond pump-probe spectroscopy. This report emphasizes the dynamics of the free ion in several polar solvents in the absence of association with counterions. We show that in neat methanol, the lowest singlet excited state of MV2+ is quenched by ultrafast electron transfer from a solvent molecule. The measured first-order rates of forward and back ET are greater than 1012 s-1, making MV2+ an important new model system for exploring ultrafast ET in hydrogen-bonding donor solvents. Experimental Section
Figure 2. Transient absorption spectra recorded 2 ps after a 265 nm pump pulse for solutions of MV2+(Cl-)2 in methanol (open circles) and for MV2+(PF6-)2 in acetonitrile (dashed curve). Both spectra have been corrected for group-delay dispersion (“chirp”) and scaled to have the same maximum amplitude. The solid curve shows the steady-state absorption spectrum of the methyl viologen radical cation (MV•+) generated by chemical reduction over Zn dust in deaerated H2O. This spectrum has been scaled for comparison.
this and other dialkylated bipyridinium compounds are known as “viologens”. MV•+ is stable for many hours in roomtemperature solution in the absence of oxygen,16 and its absorption spectrum is insensitive to pH between pH 0-7.17 These properties and the ease of spectroscopic detection of MV•+ in the visible spectral region have made MV2+ a versatile redox indicator. In addition, it has been used to study fundamental issues in photoinduced electron transfer.18,19 In most of these applications electron transfer occurs to the electronic ground state of MV2+. The favorable reduction potential of this state, E° (MV2+/MV•+) ) -0.45 V vs the normal hydrogen electrode (NHE),20 is a measure of its good electron-accepting character. Much less is known about the higher-lying electronic states of MV2+. Since electron accepting ability is strongly enhanced by electronic excitation,21,22 ET should be even more favorable for the lowest excited singlet state. Indeed, Ledwith and co-workers reported a little over 30
Materials. The dichloride salt of MV2+ was purchased from Aldrich and used as received. Some experiments were performed on counterion-exchanged salts prepared with either tetrafluoroborate (BF4-) or hexafluorophosphate (PF6-) counterions. The former salt is more soluble in water and short alcohols, whereas the latter salt is more soluble in acetonitrile. The hexafluorophosphate salt was prepared by addition of HPF6 to an aqueous solution of the chloride salt.33 The white precipitate formed immediately upon mixing was filtered and washed several times with water. The tetrafluoroborate salt was prepared in similar fashion by adding AgBF4 to an aqueous solution of MV2+(Cl-)2. Solvents of the highest available purity were purchased from Aldrich or Burdick and Jackson and used as received. Water was obtained from a laboratory ultrapurifier (Barnstead Nanopure). D2O (Aldrich) was 99.8% isotopically pure. For several experiments, acetonitrile was dried by refluxing over calcium hydride, which was followed by fractional distillation. The excellent agreement of transients obtained with and without this special drying procedure indicates that trace amounts of water have no significant effect on the dynamics. MV2+ salts are hygroscopic and readily form hydrates. Special measures to dry MV2+ were not taken in view of the fact that even ultrapure organic solvents contain trace quantities of water. No efforts were taken to deaerate the solutions under study since diffusionlimited quenching by the millimolar concentrations of oxygen present in organic solvents is much too slow to influence dynamics on the subnanosecond time scale of interest here. Femtosecond Transient Absorption. Transient absorption measurements were performed using the apparatus described previously.34 Briefly, a regeneratively amplified titanium sapphire laser system was used to generate 265 nm pump pulses,
5770 J. Phys. Chem. A, Vol. 105, No. 24, 2001 approximately 150 fs in duration. Continuum generation in a 1 cm water cell provided tunable probe pulses from 390 to 1000 nm. Probe pulses at 270 nm were obtained from an ultraviolet continuum generated by focusing several hundred nJ of the laser second harmonic (λ ) 398 nm) in a 1 mm thick CaF2 plate. Pump and probe pulses were overlapped in a 500 µm thick, free-standing jet of the solution under study. The pulses were linearly polarized and the angle between their planes of polarization was set to the magic angle (54.7°) to eliminate dynamics due to molecular reorientation. The instrumental response time was approximately 180 fs at most probe wavelengths, as determined by cross-correlation of pump and probe pulses in a BBO crystal. Transient absorption signals were monitored at a fixed probe wavelength as a function of pump-probe delay time. After the sample, the probe pulse was directed through an f ) 0.25 m monochromator and detected by an amplified Si photodiode or a photomultiplier tube positioned at the exit slit. The slit width was adjusted to optimize the time resolution. The signal from the photodetector was sent to a lock-in amplifier (Stanford Research Systems) together with a reference signal from an optical chopper positioned in the pump beam. The output from the lock-in amplifier is directly proportional to the absorbance change induced in the probe pulse by the pump pulse. With this phase-sensitive detection technique our instrument is able to measure absorbance changes of less than 10-4. Transient spectra were recorded by passing the entire continuum probe beam through the sample jet and into a monochromator. A lock-in amplifier detected the signal as the wavelength was scanned under computer control. During wavelength scanning the position of the optical delay line was adjusted to compensate for the group-delay dispersion of the probe pulse.35 All transient absorption signals in methanol and acetonitrile solutions were independent of laser power. It was verified that the signals changed linearly as the pump intensity was varied between 5 and 50 GW cm-2. In water, however, a weak offset due to two-photon ionization of the solvent was sometimes seen at long delay times after the solute signal had decayed away. This is described more fully in the results section. Time-Resolved Fluorescence. Fluorescence decays were measured by time-correlated single-photon counting (TCSPC). By cavity dumping a synchronously pumped dye laser, picosecond pulses at 287 nm were generated at a repetition rate of 1 MHz and used to excite solutions held in a 1.0 cm cuvette. Fluorescence was collected at 90°, directed through a double monochromator (American Holographic), and detected by a microchannel-plate photomultiplier tube (Hammamatsu). The instrument response time was approximately 40 ps (fwhm).36 All fluorescence transients were recorded at an emission wavelength of 350 nm. Lifetimes were determined by iterative reconvolution of a double-exponential model function with the instrument response. Steady-State Spectroscopy. Steady-state fluorescence emission and excitation spectra were recorded with a commercial fluorimeter (Fluoromax-2, Spex Instruments). All spectra were corrected for the instrument’s spectral response by calibration of the excitation monochromator using rhodamine 6G in ethanol as a quantum counter.37 The optical density of all solutions was kept as low as possible to avoid the reabsorption of fluorescence. Emission measurements in acetonitrile were performed primarily with MV2+(PF6-)2 which is considerably more soluble in this solvent than the dichloride salt. Measurements carried out on saturated solutions of MV2+(Cl-)2 gave identical results.
Peon et al. Absorption spectra were recorded with a conventional UVvis spectrophotometer (Lambda 6, Perkin-Elmer). All measurements were carried out on room-temperature solutions (T ) 22 ( 1 °C). Least-Squares Curve Fitting. Transient absorption signals were fit to a sum of exponential functions using global analysis software written in our laboratory. Our software allows multiple transients to be fit simultaneously using a common model function. An arbitrary combination of the parameters of this function can be linked for any combination of the transients in the set. All remaining parameters are independently optimized for each transient. If n transients are simultaneously fit to a model function with m parameters, then the full parameter set is described by an n × m matrix. In global fitting, an arbitrary set of these matrix elements can be linked (i.e., constrained to take on identical values), thus reducing the dimensionality of the full parameter space. For example, assume that the three parameter function a + be-kt is used to simultaneously fit four transients. If the rate constant, k, is globally linked for all four, but the parameters a and b are allowed to take on unique values for each transient, then the total number of independent parameters is 9. Developed originally for analyzing time- and wavelength-resolved emission data,38,39 global fitting is invaluable for analyzing two-dimensional data sets from transient absorption experiments. In particular, global fitting provides highly robust estimates of parameters such as exponential time constants that are common to a set of independently measured signals.40 This situation occurs frequently in transient absorption experiments, where one or more transient species contribute to the probe absorption at multiple probe wavelengths. Our program uses the Marquardt-Levenberg algorithm for parameter estimation.41 All derivatives are evaluated by finite differences. The fitting function used in this study was a sum of exponentials analytically convoluted with a Gaussian function to represent the instrument response. The Gaussian fwhm was fixed at 180 fs based on cross-correlation measurements between the 265 nm pump pulse and the 600 nm probe pulse. Parameter uncertainties stated below are equal to twice the standard deviations determined by the fitting program. Results Figure 1 shows the absorbance change (∆A, equivalent to ∆OD) at 600 and 270 nm as a function of time after photoexcitation of a 2.3 mM solution of MV2+(Cl-)2 in methanol. The signal at 600 nm (open circles) shows an instrument-limited rise, followed by a rapid decay to an offset or plateau. The signal offset shows little decay up to 800 ps after the pump pulse, the longest delay studied. Similar dynamics were observed at probe wavelengths between 380 and 740 nm. After dividing each trace by its maximum amplitude, all agree within experimental error. Because the dynamics are independent of probe wavelength throughout this broad spectral window, scans at 7 separate probe wavelengths were globally fit to a single-exponential plus a constant term to model the offset. Fits were made to just the first 8 ps of data at each wavelength. The time constant was linked for all scans but the amplitude of the exponential term and the magnitude of the long-time offset were optimized independently. The description of the full data set by a common exponential time constant (τ ) 430 ( 40 fs) is excellent. Identical dynamics were observed in methanol for the MV2+(BF4-)2 salt. At a probe wavelength of 270 nm a bleach signal (negative absorbance change) is observed in methanol (triangles in Figure
Excited State Dynamics of Methyl Viologen
Figure 3. Transient absorption at 600 nm for MV2+(Cl-)2 in H2O (closed circles) and D2O (open circles) after excitation by a 150 fs, 265 nm pump pulse. Lifetimes obtained from fits to a single-exponential function are indicated in the figure (solid curves).
Figure 4. (a) Evolution of the transient spectra for MV2+(Cl-)2 in H2O at various delay times between 100 fs (uppermost curve) and 12 ps (lowest curve) after the pump pulse. (b) Transient spectra recorded in methanol from 200 fs (uppermost curve) to 3 ps (lowest curve) after the pump pulse. All spectra have been corrected for group-delay dispersion.
1). The 270 nm probe wavelength lies within the ground-state absorption band of MV2+ (see the steady-state absorption spectrum in Figure 6). The observed bleach is consistent with depopulation of the electronic ground state by the pump pulse. The maximum amount of bleaching appears within our time resolution and then recovers rapidly to a residual bleach that remains nearly constant for hundreds of picoseconds. The time constant that best fits the bleach recovery is the same as the time constant that characterizes the decay of the transient absorption at visible wavelengths. Thus, performing a global fit on the set of visible transients plus the bleach signal at 270 nm again gives a time constant of 430 fs. The fit is excellent at all wavelengths, indicating that the decaying signals in the visible reflect repopulation of the electronic ground state of MV2+. The open circles in Figure 2 show the transient spectrum recorded for MV2+ in methanol 2 ps after the pump pulse. This spectrum was corrected for group delay dispersion (“chirp”) of the spectrally broad continuum pulse. The solid curve in Figure 2 shows the spectrum of one-electron reduced methyl viologen, MV•+, in H2O, which has been scaled for comparison with the transient spectrum. To record the spectrum shown by the solid
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Figure 5. Transient absorption signal at 600 nm following 265 nm photoexcitation of MV2+(PF6-)2 in acetonitrile (open circles) and in acetonitrile with 0.66 M of added H2O (solid circles). Fits to timeresolved fluorescence decays at 350 nm (solid curves) are shown for the same two samples. The excellent agreement indicates that the 600 nm signal is due to excited-state absorption by the fluorescent singlet state of methyl viologen.
Figure 6. Steady-state emission spectrum of a 5 × 10-4 M solution of MV2+(PF6-)2 in acetonitrile recorded at an excitation wavelength of 265 nm (dashed curve). The fluorescence excitation spectrum recorded at an emission wavelength of 350 nm (solid curve) is compared with the absorption spectrum of MV2+(PF6-) in the same solvent (dotdashed curve).
curve in Figure 2, MV•+ was generated by chemical reduction of an aqueous solution of MV2+ using Zn dust in a sealed, oxygen-free cuvette. The spectrum of this highly stable radical cation was then recorded with a conventional UV-vis spectrometer. The band maxima (396 and 606 nm) and characteristic vibrational structure are in complete agreement with literature findings.15 The excellent agreement of this reference spectrum with the one from the pump-probe experiment in methanol leaves no doubt that the absorbing species in the plateau region of the signal is MV•+. Transient absorption at 600 nm is shown in Figure 3 for MV2+ in both H2O and D2O. The dynamics are considerably different than in methanol. In particular, the significant offset seen in methanol is absent, and the signal decays virtually to the baseline. Close inspection of the signals in Figure 3 reveals the presence of a weak (
