J. Phys. Chem. 1994,98, 5095-5099
5095
Determination of Excited-State Redox Potentials by Phase-Modulated Voltammetry Wayne E. Jones, Jr., and Marye Anne Fox’ Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712 Received: December 6, 1993; In Final Form: March 17, 19948
Phase-modulated voltammetry (PMV) permits direct electrochemical interrogation of excited-state oxidation and reduction potentials. The excited states examined include two organic triplet states (anthracene and triphenylene) and several inorganic metal-to-ligand charge-transfer states ( R u L ~ where ~ + L = 2,2’-bipyridine (bpy), 4,4’-dimethyl-2,2’-bipyridine (dmb), phenanthroline (phen), or bipyrazine (bpz)). The redox potentials obtained in this analysis a r e roughly consistent with the excited-state potentials of these compounds calculated from the Rehm-Weller equation. P M V can also be used for the determination of excited-state energies for species that a r e spectroscopically inaccessible in room temperature solution.
Introduction Photoinduced electron transfer (ET) is a key process in solar energy conversion, optical device applications, and catalysis.’-3 Crucial to the development of these technologies is an understanding of the thermodynamic factors that control electron transfer. By comparing the observed electron-transfer rate with reaction exothermicity, theoretical models for electron transfer can be proposed or ~ o n f i r m e d . ~Photoexcited .~ reactants have been used extensively in the study of electron transfer because excited states, created upon absorption of a photon, are simultaneously better electron donors and acceptors than their groundstate precursors. Although the oxidation and reduction potentials (and hence the driving force for thermal ET) of ground-state species can be conveniently obtained through electrochemical techniques such as cyclic voltammetry,6 the typically short lifetimes of excited states preclude the use of such methods for a direct determination of thedriving force for excited-stateelectron exchange. Excited-state redox potentials have been estimated qualitatively in two ways. One method estimates the potential of the redox couple from a comparison of the rates of excited-state electron transfer to a series of stable reactants with known ground-state potentials.’ The second, and more common, estimation, first employed by Rehm and Weller in 1963,* uses the excited-state energy Eo,oto correct the ground-state redox potentials, eqs 1 and 2. E*1l;* E*,/?
= = El/?
+ E, + w, - E, + W,
where E l p * is the excited-state reduction (red) or oxidation (ox) potential, E I p is the potential of the ground-state redox couple, Em is the energy gap between the zeroth vibrational levels of the ground and excited states (often estimated as the 0-0 vibrational transition in the fluorescence spectrum of the lowest-lying singlet stateg), and w, is an electrostatic work term describing charge generation and separation within the electron-transfer complex. With either method, the calculatedvalues remain only estimates and typically contain uncertainties of 100 mV or more. Because of the uncertainty in these measurements and the inherent difficulty in using ground-state parameters to estimate excitedstate characteristics,IO a direct measurement of the excited-state electrochemical characteristics would be invaluable, particularly for excited states for which spectroscopic measurements fail to provide accurate estimates of excited-state energy. Abstract published in Advance ACS Abstracts. April 15, 1994.
0022-3654/94/2098-5095%04.50/0
Wayner and Griller have recently shown that thermodynamically meaningful redox potentials can be obtained for metastable species (e& radicals) with phase-modulatedvoltammetry (PMV) measurements.” PMV uses a continuous wave (CW) light source (1000-W Xe/Hg arc lamp) to create a transient population of the unstable species to be interrogated within an electrochemical cell (Figure 1). By modulating thelight source sinusoidally (using a modified light chopper), theconcentration of the photogenerated transient species of interest is modulated in parallel. If theapplied potential is sufficient to induce electron transfer, the resulting change in concentration can be observed electrochemically as a modulated or alternating current (ac) component to the current output. By using a phase- and frequency-sensitive detector, it is possible to “lock in” on the resulting ac signal and discriminate against high-frequency noise and low-frequency background currents. From the observed phase shifts of the electrochemical signal, it is also possible to determine the lifetimes of the analyte. In this report, we describe a modification of PMV that permits characterization of transient species with lifetimes as short as 500x1s. This technique is then utilized for thedirect determination of electrochemical redox potentials of two organic triplets (anthracene and triphenylene) and several ruthenium complexes with metal-to-ligand charge transfer (MLCT) excited states, R u L ~ ~where + L = 2,2’-bipyridine (bpy), 4,4’-dimethyl-2,2’bipyridine (dmb), phenanthroline (phen), or bipyrazine (bpz). By comparing these values with those estimated from eqs 1 and 2, the utility of this technique in determining excited-state energies in room temperature solutions can be established and the applicability of the Rehm-Weller equation evaluated.
Experimental Section Anthracene (Aldrich, gold label), tetrabutylammonium hexafluorophosphate (TBAH) (Sachem Chemicals), and spectral grade acetonitrile (Burdick and Jackson) were used without further purification. [Ru(bpy)31(PF&, [Ru(dmb)3l(PFd~,[Ru(phen)31(PF&and [Ru(bpz)3](PF& were prepared from [Ru(DMS0)4]Clz, as described previously.12 The purity of all complexes was confirmed by elemental analysis, solution-phase emission lifetimes, and the overlap between the absorption and excitation spectra of the observed luminescence. Initial experiments were conductedon a PMV system analogous to that described previously11 (Figure l ) , containing a 1000-W Hg arc lamp (Oriel Model 66020 with power supply 8540) modulated by a mechanical chopper (Princeton Applied Research Model 194 A) at frequencies to 500 Hz. This excitation source is incident upon an electrochemical cell which is a modification of the design previously described by Evans et al. (Figure 2).*3 The electrochemical cell contains a AgCl quasi-reference elec0 1994 American Chemical Society
5096 The Journal of Physical Chemistry. Vol. 98, No. 19, 1994
Jones and Fox
Light
Amplifier
AC Reference Signal
Amplifier
Polarizer
AC Electmshsmical
.... ,. Potendostat/ C m n l Follower
Figurr 1. Aschematicreprcscntationofaphase-modulatedvoltammetry (PMV) experimental apparatus.
Photodiode
u Cell
apparatus including a high-frequency e l w trooptic modulator and an Ar ion laser excitation source. Figure 3. Modified PMV
,,,,
-,Illlr
ICtl,"
"1l""j"
.111,1
,,,, 10 kHz. This provides for direct observation of transient photocurrents on a time scale appropriate for the characterization of the MLCT excited states. A typical phase-modulated voltammogram for one such complex, R ~ ( b p y ) , ~ +is * ,shown in Figure 5a, with the
observed background photocurrent overlaid. The two observable transient electrochemical signals correspond to the excited-state reduction and oxidation. These results are listed in Table 1, along with those of several other MLCT excited states. Also included are the excited-state redox potentials calculated from eqs 1 and 2, the ground-state redox potentials, and the Ewemission maxima observed at 77 K in a 4:l ethano1:methanol glass (Table 2). Each entry represents an average of two to four experiments on freshly prepared analyte solutions and freshly cleaned electrodes. The integrated area of the anodic-to-cathodic peak currents a t a scan rate of 10 mV/s were 3.5 and 1.2 for the Ru(bpy)32+ and R ~ ( d m b ) MLCT ~ ~ + states, respectively, suggesting that it is inherently more difficult to observe the oxidation than the reduction potential ot metastable states by this technique. For the MLCT states with shorter lifetimes (
