Photophysics and Luminescence Spectroelectrochemistry of [Tc(dmpe

Oct 28, 2013 - Sayandev Chatterjee , Amie E. Norton , Matthew K. Edwards , James M. Peterson , Stephen D. Taylor , Samuel A. Bryan , Amity Andersen ...
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Photophysics and Luminescence Spectroelectrochemistry of [Tc(dmpe)3]+/2+ (dmpe = 1,2-bis(dimethylphosphino)ethane) Sayandev Chatterjee,† Andrew S. Del Negro,† Frances N. Smith,† Zheming Wang,‡ Sean E. Hightower,† B. Patrick Sullivan,§ William R. Heineman,∥ Carl J. Seliskar,*,∥ and Samuel A. Bryan*,† †

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, United States ∥ Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States ‡

S Supporting Information *

ABSTRACT: The ligand-to-metal charge transfer (LMCT) excited state luminescence of [Tc(dmpe)3]2+ (dmpe is 1,2-bis(dimethylphosphino)ethane) has been measured in solution at room temperature and is compared to its Re analogue. Surprisingly, both [M(dmpe)3]2+* (M = Re, Tc) species have extremely large excited-state potentials (ESPs) as oxidants, the highest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon of visible light (calculated for M = Tc; E°′* = +2.48 V versus SCE; λmax = 585 nm). Open shell time-dependent density functional theory (TDDFT) calculations support the assignment of the lowest energy transition in both the technetium and rhenium complexes to be a doublet−doublet process that involves predominantly LMCT (dmpe-to-metal) character and is in agreement with past assignments for the Re system. As expected for highly oxidizing excited state potentials, quenching is observed for the excited states of both the rhenium and technetium complexes. Stern−Volmer analysis resulted in quenching parameters for both the rhenium and technetium complexes under identical conditions and are compared using Rehm−Weller analysis. Of particular interest is the fact that both benzene and toluene are oxidized by both the Re and Tc systems.

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INTRODUCTION Highly oxidizing excited state potentials of transition metal complexes are gradually gaining prominence due to their versatility and wide applicability. These are excited states that are capable of oxidizing the most inert organic substrates and inorganic substrates by virtue of unusually large one-electron oxidation potentials. Upon photoexcitation, these complexes switch to an excited state that stores activation energy as shown in eq 1 below. E◦(M m +/M n +) + hυ = E◦(M m +*/M n +)

their ability to influence the half reactions below (eqs 2 and 3; potentials vs SCE). − Cl 2(g) + 2e− → 2Cl(aq)

C6H5CH3+ + e− → C6H5CH3

E°′ = +2.40 V

(2) (3)

As a perspective, the oxidation of chloride ions give rise to products that are considered highly oxidizing and therefore are desirable for chemical manufacture. The ability of highly oxidizing excited state potentials to oxidize Cl− ions to Cl2 can be used as an alternative to the electrochemical chlor-alkali

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This added energy can be used in the subsequent catalytic oxidation of even electrochemically inert organic or inorganic substrates, which are beyond the oxidizing potential range of the complex in the ground state (Mn+). Of particular interest is © 2013 American Chemical Society

E°′ = + 1.12 V

Received: June 27, 2013 Revised: October 28, 2013 Published: October 28, 2013 12749

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Figure 1. Photograph of the H-cell. Placement of auxiliary electrode (left side H-cell) and working and reference electrodes (right side H-cell) is shown. Laser light excitation and fluorescence emission were done at right angles.

[Tc(dmpe)3]X complex in acetonitrile, and 1 drop of hexafluorophosphoric acid or trifluoromethansulfonic acid (X = PF6 or OTf, respectively). Spectroscopic Measurements. Luminescence spectra were acquired using an Acton based system. The Acton Research InSpectrum 150 with controlling Spectrasense software was equipped with a back-thinned cooled CCD camera and fiber optic input. Excitation was performed using either a Lexel-95 Ar+ laser, with either 488 or 514 nm excitation (for the luminescence monitoring during the bulk electrolysis experiments) or 532 nm DPSS laser (Melles Griot, 20 mW). In the latter case, a 532 nm holographic notch filter (Kaiser) was used to reduce the amount of laser light backscattered into the InSpectrum 150 spectrometer. Signal integration times were typically 500 ms using a 2 mm slit width and a 600 gr/mm grating blazed at 500 nm. Step-index silica-on-silica optical fibers were purchased from Romack, Inc. Absorption spectra were acquired using the Ocean Optics system consisting of a USB-200FL spectrometer and Ocean Optics 00IBase32 Spectroscopy Software. Spectra reported were not corrected for instrumental responses. The instrumental setup and experimental procedures for emission spectroscopic measurements at near liquid He temperature (6 ± 1 K) have been described previously.10,11 In brief, the samples in 2 mm × 4 mm quartz cuvettes with airtight caps were mounted on the sample holder of a CRYO Industries RC152 cryostat with liquid helium vaporizing right beneath the sample. In the proximity of the sample holder, an electric heater was embedded allowing controlled heating of the sample to preset temperatures. The sample was excited with a Spectra-Physics Nd:YAG laser-pumped MOPO-730 laser at 415 nm, and the emitted light was collected at 85° to the excitation beam, dispersed through an Acton SpectroPro 300i double spectrograph and detected with a thermoelectrically cooled Princeton Instruments PIMAX intensified CCD camera. Data analysis was performed using the commercial software package IGOR. Luminescence lifetime measurements were carried out on a conventional time-correlated-single-photon-counting apparatus.12 Lifetimes were calculated by fitting the experimental decay curves with either IGOR or Globals Unlimited13 programs. Quantum yield measurements were performed using the method outlined by Parker and Rees.14

process. However, the oxidation of toluene to its radical cation in eq 3 is a process that occurs at potentials that are past the limits of redox stability of many electrochemical solvents and therefore meets another criterion for highly oxidizing.1 Several materials possessing excited state oxidation potentials in the range 1.6−2.6 V have been reported including complexes of Re,2−4 Pt,5 Ru, and Os6 as well as polyoxometalates such as α-HP3M12O40·6H2O (where M = W and Mo)7 and the uranyl ion.8 Our preliminary results have demonstrated that [Re(dmpe) 3 ] 2 + and [Tc(dmpe) 3 ] 2 + (dmpe = 1,2-bis(dimethylphosphino)ethane) complexes possess highly oxidizing excited state potentials, which can readily oxidize aromatic hydrocarbons (including benzene and toluene) via ligand-tometal charge transfer.2 In this article, we report the properties of the analogous Tc(II) complex, [Tc(dmpe)3]2+, and discuss the unusual fundamental photophysical and spectroelectrochemical properties of the couple [Tc(dmpe) 3 ] +/[Tc(dmpe)3]2+ in aqueous solution. The electronic and redox properties of [Tc(dmpe)3]2+ are also compared with the Re congener [Re(dmpe)3]2+.

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EXPERIMENTAL SECTION Radiation Safety Disclaimer. Technetium-99 has a halflife of 2.12 × 105 yrs and emits a low-energy (0.292 MeV) β particle; common laboratory materials provide adequate shielding. Normal radiation safety procedures must be used at all times to prevent contamination. General Considerations and Synthesis. All commercially available chemicals were obtained from Aldrich. KTcO4 was obtained from the Radiochemical Processing Laboratory at Pacific Northwest National Laboratory. Preparation of [Tc(dmpe)3]+/2+. The Tc(I) complex, [Tc(dmpe)3]+, was prepared according to the published procedure in high yield by reduction of the pertechnetate (TcO4−) anion in a single-step reaction with dmpe, where the diphosphine is both the reducing agent and the complexing ligand.9 The colorless Tc(I) complex was then isolated as the white solid [Tc(dmpe)3]PF6 by adding saturated NH4PF6 or as [Tc(dmpe)3]OTf by adding LiOTf solution to the reaction mixture (OTf = trifluoromethanesulfonate). The reddishpurple [Tc(dmpe)3]X2 complex can be prepared by adding 1 drop of 30% H2O2 to a solution containing ∼50 mg of 12750

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Figure 2. Emission spectra of crystalline [Tc(dmpe)3](PF6)2 at (a) near 6 K (λexcitation = 532 nm) and (b) room temperature (∼298 K) (λexcitation = 532 nm).

Electrochemical Measurements. Electrochemical measurements were performed using a PARR model 273A potentiostat/galvanostat (EG&G Princeton Applied Research) computer controlled by electrochemical software from Scribner Associates (Corrware Electrochemical Research Software, Version 2.9c). For bulk electrolysis experiments, a custom Hcell suitable for use in a radiological hood (Figure 1) was fabricated on-site from round borosilicate glass tubing and a fritted disc, such that either side arm fit snugly into a standard 1 cm cuvette holder, thus permitting simultaneous electrochemical and spectroscopic/photophysical experiments. The working solution was separated from the auxiliary cell via a fritted disk, which served as the salt bridge between the working and auxiliary compartments. For all electrochemical experiments, a standard three-electrode configuration was used employing a 10 × 40 mm ITO-glass working electrode (Thin Film Devices, ∼135 nm thick ITO layer, 11−50 Ω/square), a platinum auxiliary electrode, and an Ag/AgCl reference electrode (3 M NaCl, Bioanalytical Systems, Inc.). DFT Calculations. Gas-phase, electronic ground-state calculations, and geometry optimizations were carried out in Gaussian0315 using the B3LYP approximation.16,17 Timedependent density functional theory (TDDFT) calculations of the lowest excited states18,19 employed the same B3LYP functional.2 Calculations utilized the 6-31G* basis set for the ligands15,20 (H, C, N, and O)21−23 and the LANL2 relativistic effective core potential (RECP)24 for the transition metal centers. Symmetry constraints were not imposed on the molecules during geometry optimization. The program AOMix (revision 6.46)25 was used to analyze molecular orbital occupancy, based on Mulliken population analysis.26

Figure 3. Absorption (dashed line) and luminescence (solid line) spectra of [Tc(dmpe)3]2+ (red) in aqueous solution compared with [Re(dmpe)3]2+(blue) in aqueous solution. Adapted from ref 2. Copyright 2006 American Chemical Society.

solution are not sensitive to the presence of dissolved oxygen. The molar extinction coefficient (ε = 1850 M−1 cm−1 at 585 nm, acetonitrile)2 of [Tc(dmpe)3]2+ is very similar to that of [Re(dmpe)3]2+ (ε = 2110 M−1 cm−1 at 530 nm, acetonitrile)27 at the peak of its longest wavelength absorption band, which was assigned to a σ(P) → dπ(Re) transition. The corresponding Stokes shifts are similar, leading to a red-shift of the technetium fluorescence of about 60 nm. The excited state lifetimes (τ) and luminescence quantum yields (Φlu) under room temperature conditions for [Tc(dmpe)3]2+ were measured in anhydrous acetonitrile and were unaffected by the presence of dissolved O2. From the observed single-exponential decay, an excited state lifetime of 8 ns was determined by observing the fluorescence decay at 660 nm, following excitation at 415 nm. A quantum yield (Φlu = 0.021) was determined by the Parker and Rees (1960) method14 using aqueous [Ru(bpy)3]2+ (Φlu = 0.041) as a reference and was confirmed by comparison to [Re(dmpe)3]2+ in anhydrous acetonitrile (Φlu = 0.066, 16 ns). The principal cause of the difference in Φlu and τ between Tc and Re is expressed principally in a larger nonradiative decay rate constant (1.2 × 108 s−1 vs 5.2 × 107 s−1), which is consistent with metal− phosphorus vibrations contributing as acceptor modes since they are the only modes that are substantially perturbed by the change of metal ion. Attempts to resolve vibronic structure in solid solutions at 77 K (in 3:1 EtOH:MeOH) resulted in only an increase in intensity and a decrease in fluorescence bandwidth.

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RESULTS AND DISCUSSION Photophysics and Calculations. The emission spectrum of crystalline [Tc(dmpe)3](PF6)2 at near liquid helium temperature shows a hint of vibronic definition (Figure 2). From the spectrum a rough estimation of the coupled ground state mode (∼1100 cm−1) can be made, and this is consistent with a ligand vibration active in the transition. Also shown in this figure is the corresponding emission spectrum of crystalline [Tc(dmpe)3](PF6)2 at room temperature (∼298 K), which does not include the vibronic features observed at liquid helium temperature and is similar to the measurement performed in aqueous solution. Absorption and emission spectra in fluid solution were determined using the H-cell and are shown (Figure 3) compared with those of the analogous [Re(dmpe)3]2+ complex. The absorption and luminescence spectra of [Tc(dmpe)3]2+ in 12751

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Assignment of the lowest energy transitions in both [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ complexes to doublet− doublet processes that involve predominantly LMCT (dmpeto-metal) character is supported by unrestricted-open shell TDDFT calculations.15 Detailed analyses indicate that these transitions are nearly a 100% single electron promotion from a predominantly ligand MO (MO 127) to a dominantly metal MO (MO 130), as indicated in Figure 4 for the Tc complex. Details of the DFT/TDDFT calculations are provided in the Supporting Information.

Figure 5. Cyclic voltammetry showing the reversible Tc(I)/Tc(II) couple for the [Tc(dmpe)3]+/2+ triflate salt in aqueous 0.1 M KNO3 at ITO vs Ag/AgCl. This plot is a compilation of multiple scans at variable scan rates from 4 to 196 mV/s.

distinct changes in luminescence in the visible wavelength region.2,32 Solution spectroelectrochemistry was performed using the H-cell (see Figure 1) configured with two optical fibers, one to direct excitation light and one to collect fluorescence. The initial state of the experiment consisted of a solution of the colorless [Tc(dmpe)3]+ (1 mM) in the working electrode compartment, and supporting electrolyte in the reference arm. Upon electrochemical oxidation (i.e., electrode potential stepped to +0.75 V vs Ag/AgCl), the oxidation of the Tc(I) complex produced the [Tc(dmpe)3]2+ complex, which was accompanied by the formation of the purple color, which can be seen in the working electrode compartment in the photograph in Figure 1. Upon stepping the potential to −0.3 V, reduction of the Tc(II) complex proceeded reversibly back to the [Tc(dmpe)3]+ complex, as was verified by the loss of color during the electrolysis. Concurrent with the step oxidation (at +0.75 V) and reduction (at −0.2 V) described in the previous paragraph, the luminescence intensity of the solution was measured while illuminating at 404 nm (laser excitation, 5 mW). Figure 6 shows the consecutive emission spectra starting at time-zero with the initial +0.75 V (oxidizing) potential applied to the working electrode. Consecutive scans were recorded every 2−3 min until ∼30 min, when the working electrode potential was stepped to −0.2 V (reducing); consecutive spectral scans continued every 5 min. The luminescent intensity increased during the oxidation of the [Tc(dmpe)3]+ complex to [Tc(dmpe)3]+2 and peaked just prior to reversing the electrode potential (at ∼30 min). The reduction of [Tc(dmpe)3]+2 was coincident with the loss of emission intensity and continued to decrease until the electrolysis was stopped at ∼90 min. Also shown in this figure is a photograph of the emission from an aqueous solution of fully oxidized [Tc(dmpe)3]2+ complex under 514 nm laser light (Ar+) excitation. Determination of Excited State Potentials and Quenching Measurements. The excited state potential (ESP) of a compound is approximately the ground state potential, either as an oxidant or as a reductant, pumped-up by the free-energy difference E00 between the thermally equilibrated ground and excited state surfaces. For an oxidant, it is given by eq 5 and depicted in Scheme 1.

Figure 4. Calculated molecular orbitals involved in the LMCT transition of [Tc(dmpe)3]2+.

This result is in agreement with the conclusions of Lee and Kirchhoff in their original work on [Re(dmpe)3]2+.27 Our calculations for the Tc(Re) complexes predict the transition energies to be 2.46 eV(2.63 eV) compared with the observed absorption energies of 2.12 eV (2.35 eV). As in our previous study on trans-[MO2(L)4]+ complexes,28,29 the correct relative ordering of energies is predicted. The corresponding oscillator strengths are 0.013(0.014) and are consistent with the near equal molar extinction coefficients for these transitions. The compositions of the lowest energy transition for the Tc(Re) complexes are nearly identical, i.e., 86% dmpe to 79.9% M for Tc (83.5% dmpe to 77.7% M for Re). The picture of the excited state that emerges is one in which the metal is near +1 oxidation state due to a hole on a dπ orbital, and where electron density has been removed from all dmpe ligands (and all phosphorus atoms). An additional, intriguing feature of the calculations is the prediction of two extremely low-lying transitions at ca. 0.3 eV above the ground state that are intradπ in nature that carry no oscillator strength, suggesting the possibility of a competitive transition that terminates at these higher states. Attempted multiple-exponential fitting of the 293 K emission decays was not successful in resolving this issue, however. Elecetrochemistry and Spectroelectrochemistry in Aqueous Solution. Cyclic voltammograms for [Tc(dmpe)3]+ in 0.1M KNO3 at ITO vs Ag/AgCl as a function of scan rates are shown in Figure 5. The voltammograms exhibit a welldefined, reversible wave for the one electron Tc(II)/Tc(I) couple: [Tc(dmpe)3 ]2 + + e− ⇄ [Tc(dmpe)3 ]+

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The formal reduction potential E°′ obtained from the midpoint between the anodic and cathodic peaks is ca. 50 mV vs Ag/ AgCl. This value is to be compared with 93 mV vs Ag/AgCl, 3 M Cl− (reported as 290 mV vs NHE) in aqueous 0.1 M LiCl,30 and 329 mV in propylene carbonate solvent.31 We have recently reported that in aqueous and nonaqueous solutions, this reversible electrochemistry is accompanied by

E°′(ES/GS−) = E°′(GS/GS−) + E00 12752

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Figure 6. (A) Consecutive luminescence measurement scans during electrochemical oxidation and reduction experiment. Oxidation (time zero to ∼30 min); time during reducing 30 to ∼90 min. (B) Photograph of emission from fully oxidized [Tc(dmpe)3]2+ species under 514 nm laser light (Ar+) excitation.

Scheme 1. Free Energy Relationships between Ground and Excited State Potentials1,33

Scheme 2. Schematic Energy-Coordinate Diagram Illustrating the Relationship between Absorption and Emissiona

In Scheme 1, the reduced ground state complex is shown as GS−. Upon electrochemical oxidation, it is transformed to the ground state oxidized species, GS, by the change in electrochemical potential energy equal to −E°′(GS/GS−). The ground state complex (GS) is photoexcited to form the excited state complex (ES) by the transfer of energy, E00. The measure of the highly oxidizing excited state potential, E°′(ES/GS−), is the difference in the potential between the excited state complex (ES) and the ground state reduced species (GS−). In the classical limit, the free-energy difference E00 is related to the spectroscopic quantities Eabs and Eem by the relationships as reflected by eqs 6a and 6b, and depicted in Scheme 2:34

Eem = E00 − δ

(6a)

Eabs − Eem = 2δ

(6b)

a

Adapted from ref 34. Copyright 2005 American Chemical Society.

quantum mechanical, harmonic oscillator analysis in the limit of small frequency changes. Because emission typically occurs from a single state, analysis of emission spectral profiles is far simpler than for absorption, where there are usually complications from overlapping bands. A physical picture of E00 is represented in Scheme 2, where the ground (ψ°) and lowest excited (ψ*) state potential energy surfaces are similar and vertically displaced. In keeping with the Franck−Condon principle, the most probable transitions will be those that occur vertically. As a result, on the basis of the qualitative positioning of the surfaces in the figure, the 0−2

Here, the δ values represent the difference in energy caused by having the nuclear coordinates of the ground state in the excited state electronic configuration during absorption, and the relaxed nuclear coordinates of the excited state in the ground state electronic configuration during emission. The δ values calculated for [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ using eqs 6a and 6b are shown in Table 1. This has been shown to be the case based on both a classical free-energy surface analysis and a 12753

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Table 1. Ground State and Excited State Potential Energy Values Used in the Calculation of the Excited State Oxidation Potentials for [Tc(dmpe)3]2+ (This Work) and [Re(dmpe)3]2+ (Previous Work by Del Negro et al.2) compound 2+

[Tc(dmpe)3] [Re(dmpe)3]2+

λabs (nm) (V)

λem (nm) (V)

δ (V)

E00 (V)

E°′ (GS/GS‑) (V)

E°′ (ES/GS‑) (V)

585 (2.12) 528 (2.35)

660 (1.88) 600 (2.07)

0.12 0.14

2.00 2.21

0.48 0.40

2.48 2.61

Scheme 3. Reaction Scheme for the Formation and Reaction of the Encounter Complex ([Tc(dmpe)3]2+*···Q), by Electron Transfer

1,10-phenanthroline, dppe = bis-1,2-diphenylphosphinoethane).5 These complexes exhibit excited state potentials in the range of 1.40−1.85 V. For [Ru(TAP)3]2+, the excited state emission is attributed as having a MLCT character, while for [Pt(5,6-Me2phen)(dppe)]2+ the excited state is assigned to be localized on the 5,6-Me2phen ligand. For [Re(bpm)(CO)4]+, absorption apparently occurs to create a MLCT excited state, but the emission may be ligand-localized.3 Tungsten and molybdenum based polyoxometalates (POM) have also been reported to possess highly oxidizing excited state potentials, thereby generating interest in their applications in photocatalysis, HP3W12O40·6H2O being a representative example.7 Excitation of the LMCT (oxygen to metal) charge transfer manifold (near visible and ultraviolet photons) renders an excited state that is a potent oxidizing agent, capable of oxidizing a variety of organic small molecule substrates. The net photochemical reaction results in the oxidation of the organic substrate, typically the oxidation of alcohols to the corresponding aldehydes and ketones and gives the reduced POM. The reduced POM can then be reoxidized by air or by proton reduction to complete the catalytic cycle.1,35 An absorption threshold of about 400 nm, which is common for POMs, results in an ESP that is about 3 V more oxidizing than the GS reduction potential, which for most POMs shifts the oxidizing potential more positive than the 2.8 V (vs NHE) necessary to form OH radical. Because of their ideal GS and ES properties, POMs have successfully photocatalyzed the oxidation of a number of organic pollutants.1,36,37 Uranyl ion (UO22+) is another example of an oxygen containing transition metal ion that has a highly oxidizing excited state potential. The lowest energy transition in the uranyl ion is thought to involve excitation of an electron from the highest filled O π orbital to a nonbonding orbital on the uranium. 38 The thermally equilibrated excited state is reached via a singlet−triplet transition (λmax = 414 nm in 0.1 M HClO4).39,40 The UO22+* is highly oxidizing, with an estimated ESP of +2.6 V.8 The [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ complexes appear to have a significantly higher excited potential than [Ru(TAP)3]2+, [Re(bpm)(CO)4]+, or [Pt(5,6-Me2phen)(dppe)]2+ while having potentials similar to the more oxidizing polyoxometalates or uranyl anion. These complexes also absorb

absorption/excitation transitions are expected to be relatively intense compared to the 0−0 and 0−1 transitions. In emissions, in analogy with absorption, the most probable transitions are the ones occurring vertically as well. However, for emission, there is the added consideration that the rate of vibrational and electronic energy relaxation is very rapid compared to the rate of emission. As a result, emission always occurs from the v = 0 vibrational level of the lowest excited state ψ*. The overlap between the excitation (or absorption) and emission spectra denotes the E00 state, and the greater the overlap, the greater the similarity between the ground and excited state potential energy surfaces. The spectroscopy of [Tc(dmpe)3]2+ (and [Re(dmpe)3]2+) show a similar overlap between absorption and emission profiles (Figure 3). Emission from the [Tc(dmpe)3]2+ complex has been assigned as LMCT in nature in analogy with a similar emission from [Re(dmpe)3]2+ and as such is an unusual example of a doublet−doublet process.27 The narrow bandwidths of absorption and emission coupled with the spin-allowed nature of the transition make the estimation of the ESP an easier task than for many transition metal complexes. Using the mirror image relationship between absorption and emission, the determination of E00 can be made more accurately than in the case of MLCT triplet emitters since, for the latter, the lowest energy singlet−triplet absorption is rarely observed without interference from the singlet manifold.1,33 Thus, by using the E00 value for the [Tc(dmpe)3]2+ complex (2.00 V), derived from eqs 6a and 6b, and the ground state reduction potential (0.48 V for vs Ag/AgCl;), the excited state potential value of E°′([Tc(dmpe)3]2+*/[Tc(dmpe)3]+) is calculated to be 2.48 V (versus Ag/AgCl), according to eq 5. This value is comparable to the excited state potential value of 2.61 V (= E1/2([Re(dmpe)3]2+*/[Re(dmpe)3]+)). These values are tabulated in Table 1. Previous work has identified only a few highly oxidizing excited states, with some of the notable representative examples being found in metal polypyridine chemistry. Two of them are octahedral d6 complexes [Ru(TAP)3]2+ (TAP = 1,4,5,8, tetraazaphenanthrene) and [Re(bpm)(CO)4]+ (bpm = 2, 2′bipyrimidine), and the third is a d8 square-planar complex [Pt(5,6-Me2phen)(dppe)]2+ (5,6-Me2phen = 5,6-dimethyl12754

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strongly in the visible region, making them suitable candidates for solar energy conversion to store oxidizing equivalents. As expected from such high excited-state potentials, quenching by aromatic hydrocarbons is observed for the excited states of both rhenium and technetium complexes. Following the generalized mechanism for the luminescence quenching process by electron transfer in polar solvents based on the classic work of Rehm and Weller,41 Scheme 3 can be drawn for the excited state complex [Tc(dmpe)3]2+* and the nonexcited quencher molecule, Q. An identical scheme can be drawn for the luminescence quenching process for [Re(dmpe)3]2+*, by substituting Re for Tc in Scheme 3. In this scheme, [Tc(dmpe)3]2+* has a lifetime of τ0 and can convert to the ground state by radiative decay or can form an encounter complex with a quencher, Q, through diffusion. The encounter complex ([Tc(dmpe)3]2+*···Q) can undergo electron transfer through k23, leading to the ion-pair ([Tc(dmpe)3]+···Q+). The reaction denoted by k30 comprises all possible modes by which the ion-pair can disappear, including the back electron transfer leading to excited state or ground state molecules. The free energy change, ΔG23, involved in the electron transfer process between the encounter complex and ion-pair (Scheme 3), can be calculated according to Rehm and Weller (1970)41 with the following equation: ΔG23 = E(D+ /D) − E(A /A−) − E00 +

e0 2 εd

Figure 7. Stern−Volmer quenching of [Tc(dmpe)3]2+* (green squares) and [Re(dmpe)3]2+* (red circles) by toluene in anhydrous acetonitrile.

quenching parameters from Figure 7 are summarized in Table 2, along with other organic substrates. Table 2. Luminescence Quenching Rate Constants, kq, and ΔG23 Values in Acetonitrile

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In this equation, E(D+/D) describes the reduction potentials for the donor complex ([Tc(dmpe)3]2+ or [Re(dmpe)3]2+) and E(A/A−) describes the reduction potential for the acceptor molecules (organic quenchers, Q) as shown in eqs 8a and 8b, respectively. D+ + e → D

(8a)

−

(8b)

A+e→A

[Re(dmpe)3]2+

The reduction potentials for the quencher hydrocarbons were taken from Howell et al. (1984).42 E00 (in eq 7) is the free-energy difference between the thermally equilibrated ground and excited state surfaces, described in the text previously; values for both the [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ complexes are given in Table 1. The last term, e02/εd is the Coulombic interaction energy experienced by the ion pair produced following the electron transfer reaction and takes into account the free energy of bringing two radical ions to the encounter distance, d, within solvent of dielectric constant, ε, and is estimated as 0.06 eV in acetonitrile solvent.43,44 The form of eq 7 can be understood intuitively; the E(D+/D) and E(A/A−) terms appear with opposite signs because these are both written as reduction potentials, where D is oxidized to D+ and A is reduced to A−. For the same reaction, the oxidation potential is the negative of the reduction potential. The E00 term has a negative sign because energy is lost when the light energy is dissipated during the electron transfer reaction. The e02/εd term has a positive sign because of the repulsive Coulombic interaction created between the two like charges produced in the ion pair complex ([Tc(dmpe)3]+···Q+), after the electron transfer reaction. Figure 7 shows an example of Stern−Volmer quenching data using toluene as the quencher for [Tc(dmpe)3]2+* and [Re(dmpe)3]2+* in anhydrous acetonitrile. The resulting

[Tc(dmpe)3]2+

quencher

kq × 109 (M−1s−1)

ΔG23 (V)

kq × 109 (M−1 s−1)

ΔG23 (V)

benzene toluene p-xylene mesitylene anisole p-dimethoxybenzene 10-methylphenothiazine N,N-dimethylaniline

0.0258 1.34 8.93 8.67 13.9 17.5 19.6 23.80

0.0724 −0.298 −0.4876 −0.5276 −0.8164 −1.2064 −1.7164 −1.7664

0.0025 0.288

0.20 −0.1693

7.5 18.9

−0.3993 −0.6881

The Stern−Volmer derived quenching rate constants for [Tc(dmpe)3]2+* with benzene, toluene, mesitylene, and anisol are 2.5 × 106, 2.8 × 108, 7.5 × 109, and 1.89 × 1010 M−1s−1, respectively, which compares with 2.58 × 107, 1.34 × 109, 8.67 × 109, and 1.39 × 1010 M−1s −1 for [Re(dmpe)3]2+*. The quenching rate constants for the former three are consistent with the slightly lower excited state potential of the Tc complex. Of particular note is the fact that benzene and toluene are oxidized. It is somewhat surprising that no direct evidence of the Marcus inverted region is found given the presumed lack of large reorganizational energies for the quenchers and the Re(II/ I) and Tc(II/I) redox pairs.45 The electron-transfer quenchingrate constant of [Ru(bpy)3]2+* by [Re(dmpe)3]+ is 1.5 × 109 M−1 s−1, consistent with a small reorganizational energy. The quenching rate constants for [Tc(dmpe)3]2+* and [Re(dmpe)3]2+* measured under identical conditions leads to a Rehm−Weller analysis for both congeners. On the basis of Scheme 3, the overall rate constant for fluorescent quenching can be written:41 kq =

12755

⎛ k 1 + ΔV 12k ⎜exp 12 30 ⎝

k12

( ) + exp( )⎞⎠ ‡ ΔG23 RT

ΔG23 ⎟ RT

(9)

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where ΔG‡23 is assumed to be a monotonous function of ΔG23 and has been given by Rehm and Weller (1970)41 as ⎞1/2 ⎛⎛ ΔG ⎞2 ΔG23 23 ‡ ‡ ⎟ + (ΔG23 (0))2 ⎟⎟ + ΔG23 = ⎜⎜⎜ ⎝ ⎠ 2 2 ⎠ ⎝

visible spectroscopy shows the progressive production of the MePTZ radical cation; electronic spectral data for MePTZ+ is from Wagner (1988).46 After 40 min, a ratio of 0.60/1 MePTZ+•/Re is found. This irreversible behavior is rationalized by the photooxidation of MePTZ by [Re(dmpe)3]2+* followed by oxidation of [Re(dmpe)3]+ by oxygen to regenerate the starting metal complex and is diagrammed in Scheme 4.

(10)

(ΔG‡23(0)) 41

The activation free energy at ΔG23 = 0 was determined to equal 2.4 kcal/mol. Subsequent determination of other constants in eq 9 has led to the general formula in eq 11 kq =

2 × 1010 M−1 s−1 ⎛ ΔG ‡ 1 + 0.25⎜exp RT23 + exp ⎝

( )

ΔG23 ⎞ ⎟ RT

( )⎠

Scheme 4. Schematic Representation Showing Interconversion between M(I) and M(II)/M(II)* Resulting in Oxidation of Small Molecule Substrate (sub) to (sub+•); (M = Re, Tc) Reaction of [Re(dmpe)3]2+* with MePTZ Results in Oxidation to MePTZ+• That Can Be Subsequently Isolated along with [Re(dmpe)3]+

(11)

which can be used to calculate rate constants of luminescence quenching by electron transfer in acetonitrile from ΔG23 values, which, according to eq 7, can be obtained from electrochemical and spectroscopic data. Our experimental quenching rate data for [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ from Table 2 are plotted against ΔG23 derived above in Figure 8, along with the calculated quenching

Highly oxidizing excited states of transition-metal complexes that absorb visible photons are rare species but have great potential in solar energy conversion applications. Examples include photogeneration of molecular chlorine or use in regenerative photoelectrochemical cells where the valence band oxidation of the semiconductor is utilized. Other lowspin d5 complexes are likely to exhibit similar behavior if the intervening low-lying excited states are close in energy to the ground state so that radiationless decay rate constants are minimized.

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CONCLUSIONS The agreement of experimental results with TDDFT calculations shows that the strong, longest-wavelength absorptions in [Tc(dmpe)3]2+ LMCT transitions is similar to the [Re(dmpe)3]2+ system. DFT calculations further indicate that the LMCT transition is essentially a one-electron promotion from a ligand-based MO to a metal-based MO. The corresponding luminescences have quantum yields and lifetimes consistent with this conclusion. Luminescence quenching followed by Stern−Volmer and Rehm−Weller analyses for both complexes suggests that the LMCT excited states have unusually high excited state potentials, the highest for any simple coordination complex of a transition metal and sufficiently high to oxidize simple aromatics as benzene and toluene. Furthermore, this potential is available using a photon of visible light (M = Tc(Re); E°′* = +2.48 (+2.61) V calculated vs SCE; 585 (528) nm). Examination of the spectroelectrochemistry of the LMCT transitions in both complexes in aqueous solution shows a reversible one-electron oxidation−reduction reaction in the ground state with corresponding modulation of the fluorescence of the LMCT state. This well-behaved spectroelectrochemistry is particularly attractive and has been useful in devising sensing schemes for environmental monitoring of technetium contamination as well.32

Figure 8. Rehm−Weller plot of quenching data for [Tc(dmpe)3]2+ (green squares) and [Re(dmpe)3]2+ (red circles) in anhydrous acetonitrile.

rate data based on the Rehm−Weller treatment (eq 11). The agreement between the experimentally determined quenching rate constants and the Rehm−Weller derived quenching rate constants is evidence in favor of the electron transfer mechanism on which eq 11 is based. This agreement also provides secondary confirmation that the excited state oxidation potentials determined for [Tc(dmpe)3]2+ and [Re(dmpe)3]2+ in Table 1 are valid. Since none of the substrates in Table 2 and Figure 8 possesses an excited state below that of [Re(dmpe)3]2+* or [Tc(dmpe)3]2+*, it is reasonable to assume that there is no energy-transfer quenching but that single electron-transfer quenching via oxidation of the hydrocarbon substrate occurs. Beside the prima facie evidence from the Rehm−Weller analysis, support for this mechanism comes from a steadystate experiment where redox products are detected directly. When a CH3CN solution of 1.42 × 10−4 M [Re(dmpe)3]2+ containing 0.02 M 10-methylphenothiazine (MePTZ) and 0.3 M HClO4 is irradiated with a 200 W quartz halogen lamp, UV− 12756

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(9) Vanderheyden, J. L.; Ketring, A. R.; Libson, K.; Heeg, M. J.; Roecker, L.; Motz, P.; Whittle, R.; Elder, R. C.; Deutsch, E. Synthesis and Characterization of Cationic Technetium Complexes of 1,2Bis(dimethylphosphino)ethane (dmpe): Structure Determinations of trans-[Tc-V(dmpe)2(OH)(O)](F3CSO3)2, trans-[Tc-III(dmpe)2Cl2]F3CSO3, and [Tc-I(dmpe)3]+ Using X-ray-Diffraction, EXAFS, and Tc-99 NMR. Inorg. Chem. 1984, 23 (20), 3184−3191. (10) Wang, Z.; Zachara, J. M.; Gassman, P. L.; Liu, C.; Qafoku, O.; Catalano, J. G. Fluorescence Spectroscopy of U(VI)-Silicate and U(VI)-Contaminated Hanford Sediment. Geochim. Cosmochim. Acta 2005, 69 (6), 1391−1403. (11) Wang, Z. M.; Zachara, J. M.; McKinley, J. P.; Smith, S. C. Cryogenic Laser Induced U(VI) Fluorescence Studies of a U(VI) Substituted Natural Calcite: Implications to U(VI) Speciation in Contaminated Hanford Sediments. Environ. Sci. Technol. 2005, 39 (8), 2651−2659. (12) Wang, Z.; Hemmer, S. L.; Friedrich, D. M.; Joly, A. G. Anthracene as the Origin of the Red-Shifted Emission from Commercial Zone-Refined Phenanthrene Sorbed on Mineral Surfaces. J. Phys. Chem. A 2001, 105 (25), 6020−6023. (13) Beechem, J. M.; Gratton, E. An Expert System for Global Analysis of Time-Resolved Fluorescence Data in Terms of Discrete and Distributed Physical Models. Biophys. J. 1988, 53 (2), A403− A403. (14) Parker, C. A.; Rees, W. T. Correction of Fluorescence Spectra and Measurement of Fluorescence Quantum Efficiency. Analyst 1960, 85 (1013), 587−600. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (16) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle− Salvetti Correlation-Energy Formula into a Functional of the ElectronDensity. Phys. Rev. B 1988, 37 (2), 785−789. (17) Becke, A. D. Density-Functional Thermochemistry 0.3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98 (7), 5648−5652. (18) Runge, E.; Gross, E. K. U. Density-Functional Theory for TimeDependent Systems. Phys. Rev. Lett. 1984, 52 (12), 997−1000. (19) Gross, E. K. U.; Kohn, W. Local Density-Functional Theory of Frequency-Dependent Linear Response. Phys. Rev. Lett. 1985, 55 (26), 2850−2852. (20) Frisch, M. J.; Pople, J. A.; Binkley, J. S. Self-Consistent Molecular-Orbital Methods 25. Supplementary Functions for Gaussian-Basis Sets. J. Chem. Phys. 1984, 80 (7), 3265−3269. (21) Harihara, P.; Pople, J. A. Influence of Polarization Functions on Molecular-Orbital Hydrogenation Energies. Theor. Chim. Acta 1973, 28 (3), 213−222. (22) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; Defrees, D. J.; Pople, J. A. Self-Consistent Molecular-Orbital Methods 0.23. A Polarization-Type Basis Set for 2nd-Row Elements. J. Chem. Phys. 1982, 77 (7), 3654−3665. (23) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. Gaussian-3 (G3) Theory for Molecules Containing First and Second-Row Atoms. J. Chem. Phys. 1998, 109 (18), 7764−7776. (24) Hay, P. J.; Wadt, W. R. Abinitio Effective Core Potentials for Molecular Calculations: Potentials for K to Au Including the Outermost Core Orbitals. J. Chem. Phys. 1985, 82 (1), 299−310. (25) Gorelsky, S. I.; Lever, A. B. P. Electronic Structure and Spectral, of Ruthenium Diimine Complexes by Density Functional Theory And INDO/S. Comparison of the Two Methods. J. Organomet. Chem. 2001, 635 (1−2), 187−196. (26) Mulliken, R. S. Electronic Population Analysis on LCAO-MO Molecular Wave Functions 0.1. J. Chem. Phys. 1955, 23 (10), 1833− 1840. (27) Lee, Y. F.; Kirchhoff, J. R. Absorption and Luminescence Spectroelectrochemical Characterization of a Highly Luminescent Rhenium(II) Complex. J. Am. Chem. Soc. 1994, 116 (8), 3599−3600. (28) Del Negro, A. S.; Wang, Z. M.; Seliskar, W. R.; Heineman, W. R.; Sullivan, B. P.; Hightower, S. E.; Hubler, T. L.; Bryan, S. A.

Highly oxidizing excited states of transition-metal complexes that absorb visible photons are rare but have great potential in solar energy conversion applications. The implications for solar electricity and artificial photosynthesis are manifold, including the ability of such highly oxidizing excited states to directly store their energy in potent oxidants. This work also gives insight as to how the highly oxidizing excited state potentials compare down a group. The work presented here also provides the motivation to study other low-spin d5 complexes that are likely to exhibit similar behavior if the intervening low-lying excited states are close enough in energy to the ground state so that radiationless decay rate constants are minimized.

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ASSOCIATED CONTENT

S Supporting Information *

DFT/TDDFT calculations for the complexes are provided. The complete author listing for refs 15 and 29 are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Office of Biological and Environmental Research (OBER) of the U.S. Department of Energy (Grant DE-FG0799ER62331) is greatly acknowledged. Part of this research was performed at EMSL, a national scientific user facility at PNNL managed by the Department of Energy’s Office of Biological and Environmental Research. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC0676RLO 1830.

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REFERENCES

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