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, UnitedStates§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, thehighest for any simple coordination complex of a transition metal. Furthermore, this potential is available using a photon ofvisible light (calculated for M = Tc; E°′* = +2.48 V versus SCE; λmax = 585 nm). Open shell time-dependent density functionaltheory (TDDFT) calculations support the assignment of the lowest energy transition in both the technetium and rheniumcomplexes to be a doublet−doublet process that involves predominantly LMCT (dmpe-to-metal) character and is in agreementwith past assignments for the Re system. As expected for highly oxidizing excited state potentials, quenching is observed for theexcited states of both the rhenium and technetium complexes. Stern−Volmer analysis resulted in quenching parameters for boththe rhenium and technetium complexes under identical conditions and are compared using Rehm−Weller analysis. Of particularinterest is the fact that both benzene and toluene are oxidized by both the Re and Tc systems.

■ INTRODUCTION

Highly oxidizing excited state potentials of transition metalcomplexes are gradually gaining prominence due to theirversatility and wide applicability. These are excited states thatare capable of oxidizing the most inert organic substrates andinorganic substrates by virtue of unusually large one-electronoxidation potentials. Upon photoexcitation, these complexesswitch to an excited state that stores activation energy as shownin eq 1 below.

υ+ =◦ + + ◦ +* +E M M h E M M( / ) ( / )m n m n(1)

This added energy can be used in the subsequent catalyticoxidation of even electrochemically inert organic or inorganicsubstrates, which are beyond the oxidizing potential range ofthe complex in the ground state (Mn+). Of particular interest is

their ability to influence the half reactions below (eqs 2 and 3;potentials vs SCE).

+ → °′ = +− − ECl 2e 2Cl 1.12 V2(g) (aq) (2)

+ → °′ = ++ − EC H CH e C H CH 2.40 V6 5 3 6 5 3 (3)

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

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

Article

pubs.acs.org/JPCA

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process. However, the oxidation of toluene to its radical cationin eq 3 is a process that occurs at potentials that are past thelimits of redox stability of many electrochemical solvents andtherefore meets another criterion for highly oxidizing.1

Several materials possessing excited state oxidation potentialsin the range 1.6−2.6 V have been reported including complexesof Re,2−4 Pt,5 Ru, and Os6 as well as polyoxometalates such asα-HP3M12O40·6H2O (where M = W and Mo)7 and the uranylion.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 oxidiz-ing excited state potentials, which can readily oxidize aromatichydrocarbons (including benzene and toluene) via ligand-to-metal charge transfer.2 In this article, we report the propertiesof the analogous Tc(II) complex, [Tc(dmpe)3]

2+, and discussthe unusual fundamental photophysical and spectroelectro-chemical properties of the couple [Tc(dmpe)3]

+/[Tc-(dmpe)3]

2+ in aqueous solution. The electronic and redoxproperties of [Tc(dmpe)3]

2+ are also compared with the Recongener [Re(dmpe)3]

2+.

■ EXPERIMENTAL SECTION

Radiation Safety Disclaimer. Technetium-99 has a half-life of 2.12 × 105 yrs and emits a low-energy (0.292 MeV) βparticle; common laboratory materials provide adequateshielding. Normal radiation safety procedures must be used atall times to prevent contamination.General Considerations and Synthesis. All commercially

available chemicals were obtained from Aldrich. KTcO4 wasobtained from the Radiochemical Processing Laboratory atPacific Northwest National Laboratory.Preparation of [Tc(dmpe)3]

+/2+. The Tc(I) complex,[Tc(dmpe)3]

+, was prepared according to the publishedprocedure in high yield by reduction of the pertechnetate(TcO4

−) anion in a single-step reaction with dmpe, where thediphosphine is both the reducing agent and the complexingligand.9 The colorless Tc(I) complex was then isolated as thewhite solid [Tc(dmpe)3]PF6 by adding saturated NH4PF6 or as[Tc(dmpe)3]OTf by adding LiOTf solution to the reactionmixture (OTf = trifluoromethanesulfonate). The reddish-purple [Tc(dmpe)3]X2 complex can be prepared by adding 1drop of 30% H2O2 to a solution containing ∼50 mg of

[Tc(dmpe)3]X complex in acetonitrile, and 1 drop ofhexafluorophosphoric acid or trifluoromethansulfonic acid (X= PF6 or OTf, respectively).

Spectroscopic Measurements. Luminescence spectrawere acquired using an Acton based system. The ActonResearch InSpectrum 150 with controlling Spectrasensesoftware was equipped with a back-thinned cooled CCDcamera and fiber optic input. Excitation was performed usingeither a Lexel-95 Ar+ laser, with either 488 or 514 nm excitation(for the luminescence monitoring during the bulk electrolysisexperiments) or 532 nm DPSS laser (Melles Griot, 20 mW). Inthe latter case, a 532 nm holographic notch filter (Kaiser) wasused to reduce the amount of laser light backscattered into theInSpectrum 150 spectrometer. Signal integration times weretypically 500 ms using a 2 mm slit width and a 600 gr/mmgrating blazed at 500 nm. Step-index silica-on-silica opticalfibers were purchased from Romack, Inc. Absorption spectrawere acquired using the Ocean Optics system consisting of aUSB-200FL spectrometer and Ocean Optics 00IBase32Spectroscopy Software. Spectra reported were not correctedfor instrumental responses.The instrumental setup and experimental procedures for

emission spectroscopic measurements at near liquid Hetemperature (6 ± 1 K) have been described previously.10,11

In brief, the samples in 2 mm × 4 mm quartz cuvettes withairtight caps were mounted on the sample holder of a CRYOIndustries RC152 cryostat with liquid helium vaporizing rightbeneath the sample. In the proximity of the sample holder, anelectric heater was embedded allowing controlled heating of thesample to preset temperatures. The sample was excited with aSpectra-Physics Nd:YAG laser-pumped MOPO-730 laser at415 nm, and the emitted light was collected at 85° to theexcitation beam, dispersed through an Acton SpectroPro 300idouble spectrograph and detected with a thermoelectricallycooled Princeton Instruments PIMAX intensified CCD camera.Data analysis was performed using the commercial softwarepackage IGOR.Luminescence lifetime measurements were carried out on a

conventional time-correlated-single-photon-counting appara-tus.12 Lifetimes were calculated by fitting the experimentaldecay curves with either IGOR or Globals Unlimited13

programs. Quantum yield measurements were performedusing the method outlined by Parker and Rees.14

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

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Electrochemical Measurements. Electrochemical meas-urements were performed using a PARR model 273Apotentiostat/galvanostat (EG&G Princeton Applied Research)computer controlled by electrochemical software from ScribnerAssociates (Corrware Electrochemical Research Software,Version 2.9c). For bulk electrolysis experiments, a custom H-cell suitable for use in a radiological hood (Figure 1) wasfabricated on-site from round borosilicate glass tubing and afritted disc, such that either side arm fit snugly into a standard 1cm cuvette holder, thus permitting simultaneous electro-chemical and spectroscopic/photophysical experiments. Theworking solution was separated from the auxiliary cell via afritted disk, which served as the salt bridge between the workingand auxiliary compartments. For all electrochemical experi-ments, a standard three-electrode configuration was usedemploying a 10 × 40 mm ITO-glass working electrode (ThinFilm Devices, ∼135 nm thick ITO layer, 11−50 Ω/square), aplatinum auxiliary electrode, and an Ag/AgCl referenceelectrode (3 M NaCl, Bioanalytical Systems, Inc.).DFT Calculations. Gas-phase, electronic ground-state

calculations, and geometry optimizations were carried out inGaussian0315 using the B3LYP approximation.16,17 Time-dependent density functional theory (TDDFT) calculationsof the lowest excited states18,19 employed the same B3LYPfunctional.2 Calculations utilized the 6-31G* basis set for theligands15,20 (H, C, N, and O)21−23 and the LANL2 relativisticeffective core potential (RECP)24 for the transition metalcenters. Symmetry constraints were not imposed on themolecules during geometry optimization. The programAOMix (revision 6.46)25 was used to analyze molecular orbitaloccupancy, based on Mulliken population analysis.26

■ RESULTS AND DISCUSSION

Photophysics and Calculations. The emission spectrumof crystalline [Tc(dmpe)3](PF6)2 at near liquid heliumtemperature shows a hint of vibronic definition (Figure 2).From the spectrum a rough estimation of the coupled groundstate mode (∼1100 cm−1) can be made, and this is consistentwith a ligand vibration active in the transition. Also shown inthis figure is the corresponding emission spectrum of crystalline[Tc(dmpe)3](PF6)2 at room temperature (∼298 K), whichdoes not include the vibronic features observed at liquid heliumtemperature and is similar to the measurement performed inaqueous 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

solution are not sensitive to the presence of dissolved oxygen.The molar extinction coefficient (ε = 1850 M−1 cm−1 at 585nm, 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, whichwas assigned to a σ(P) → dπ(Re) transition. Thecorresponding Stokes shifts are similar, leading to a red-shiftof 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 wereunaffected by the presence of dissolved O2. From the observedsingle-exponential decay, an excited state lifetime of 8 ns wasdetermined 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 usingaqueous [Ru(bpy)3]

2+ (Φlu = 0.041) as a reference and wasconfirmed by comparison to [Re(dmpe)3]

2+ in anhydrousacetonitrile (Φlu = 0.066, 16 ns). The principal cause of thedifference in Φlu and τ between Tc and Re is expressedprincipally 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 sincethey are the only modes that are substantially perturbed by thechange of metal ion. Attempts to resolve vibronic structure insolid solutions at 77 K (in 3:1 EtOH:MeOH) resulted in onlyan increase in intensity and a decrease in fluorescencebandwidth.

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).

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.

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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 (dmpe-to-metal) character is supported by unrestricted-open shellTDDFT calculations.15 Detailed analyses indicate that thesetransitions are nearly a 100% single electron promotion from apredominantly ligand MO (MO 127) to a dominantly metalMO (MO 130), as indicated in Figure 4 for the Tc complex.Details of the DFT/TDDFT calculations are provided in theSupporting Information.

This result is in agreement with the conclusions of Lee andKirchhoff in their original work on [Re(dmpe)3]

2+.27 Ourcalculations for the Tc(Re) complexes predict the transitionenergies to be 2.46 eV(2.63 eV) compared with the observedabsorption energies of 2.12 eV (2.35 eV). As in our previousstudy on trans-[MO2(L)4]

+ complexes,28,29 the correct relativeordering of energies is predicted. The corresponding oscillatorstrengths are 0.013(0.014) and are consistent with the nearequal molar extinction coefficients for these transitions. Thecompositions of the lowest energy transition for the Tc(Re)complexes are nearly identical, i.e., 86% dmpe to 79.9% M forTc (83.5% dmpe to 77.7% M for Re). The picture of theexcited state that emerges is one in which the metal is near +1oxidation state due to a hole on a dπ orbital, and where electrondensity has been removed from all dmpe ligands (and allphosphorus atoms). An additional, intriguing feature of thecalculations is the prediction of two extremely low-lyingtransitions at ca. 0.3 eV above the ground state that are intra-dπ in nature that carry no oscillator strength, suggesting thepossibility of a competitive transition that terminates at thesehigher states. Attempted multiple-exponential fitting of the 293K 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 ratesare shown in Figure 5. The voltammograms exhibit a well-defined, reversible wave for the one electron Tc(II)/Tc(I)couple:

+ ⇄+ − +[Tc(dmpe) ] e [Tc(dmpe) ]32

3 (4)

The formal reduction potential E°′ obtained from the midpointbetween the anodic and cathodic peaks is ca. 50 mV vs Ag/AgCl. This value is to be compared with 93 mV vs Ag/AgCl, 3M 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 nonaqueoussolutions, this reversible electrochemistry is accompanied by

distinct changes in luminescence in the visible wavelengthregion.2,32 Solution spectroelectrochemistry was performedusing the H-cell (see Figure 1) configured with two opticalfibers, one to direct excitation light and one to collectfluorescence. The initial state of the experiment consisted ofa solution of the colorless [Tc(dmpe)3]

+ (1 mM) in theworking electrode compartment, and supporting electrolyte inthe reference arm. Upon electrochemical oxidation (i.e.,electrode potential stepped to +0.75 V vs Ag/AgCl), theoxidation of the Tc(I) complex produced the [Tc(dmpe)3]

2+

complex, which was accompanied by the formation of thepurple color, which can be seen in the working electrodecompartment in the photograph in Figure 1. Upon stepping thepotential to −0.3 V, reduction of the Tc(II) complex proceededreversibly back to the [Tc(dmpe)3]

+ complex, as was verified bythe 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, theluminescence intensity of the solution was measured whileilluminating at 404 nm (laser excitation, 5 mW). Figure 6 showsthe consecutive emission spectra starting at time-zero with theinitial +0.75 V (oxidizing) potential applied to the workingelectrode. Consecutive scans were recorded every 2−3 minuntil ∼30 min, when the working electrode potential wasstepped to −0.2 V (reducing); consecutive spectral scanscontinued every 5 min. The luminescent intensity increasedduring the oxidation of the [Tc(dmpe)3]

+ complex to[Tc(dmpe)3]

+2 and peaked just prior to reversing the electrodepotential (at ∼30 min). The reduction of [Tc(dmpe)3]

+2 wascoincident with the loss of emission intensity and continued todecrease until the electrolysis was stopped at ∼90 min. Alsoshown in this figure is a photograph of the emission from anaqueous solution of fully oxidized [Tc(dmpe)3]

2+ complexunder 514 nm laser light (Ar+) excitation.

Determination of Excited State Potentials andQuenching Measurements. The excited state potential(ESP) of a compound is approximately the ground statepotential, either as an oxidant or as a reductant, pumped-up bythe free-energy difference E00 between the thermally equili-brated ground and excited state surfaces. For an oxidant, it isgiven by eq 5 and depicted in Scheme 1.

°′ = °′ +− −E E E(ES/GS ) (GS/GS ) 00 (5)

Figure 4. Calculated molecular orbitals involved in the LMCTtransition of [Tc(dmpe)3]

2+.

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 atITO vs Ag/AgCl. This plot is a compilation of multiple scans atvariable scan rates from 4 to 196 mV/s.

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In Scheme 1, the reduced ground state complex is shown asGS−. Upon electrochemical oxidation, it is transformed to theground state oxidized species, GS, by the change in electro-chemical potential energy equal to −E°′(GS/GS−). The groundstate complex (GS) is photoexcited to form the excited statecomplex (ES) by the transfer of energy, E00. The measure of thehighly oxidizing excited state potential, E°′(ES/GS−), is thedifference 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 relationshipsas reflected by eqs 6a and 6b, and depicted in Scheme 2:34

δ= −E Eem 00 (6a)

δ− =E E 2abs em (6b)

Here, the δ values represent the difference in energy caused byhaving the nuclear coordinates of the ground state in theexcited state electronic configuration during absorption, and therelaxed nuclear coordinates of the excited state in the groundstate electronic configuration during emission. The δ valuescalculated 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 thecase based on both a classical free-energy surface analysis and a

quantum mechanical, harmonic oscillator analysis in the limit ofsmall frequency changes. Because emission typically occursfrom a single state, analysis of emission spectral profiles is farsimpler than for absorption, where there are usuallycomplications from overlapping bands.A physical picture of E00 is represented in Scheme 2, where

the ground (ψ°) and lowest excited (ψ*) state potential energysurfaces are similar and vertically displaced. In keeping with theFranck−Condon principle, the most probable transitions willbe those that occur vertically. As a result, on the basis of thequalitative positioning of the surfaces in the figure, the 0−2

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 andExcited State Potentials1,33

Scheme 2. Schematic Energy-Coordinate DiagramIllustrating the Relationship between Absorption andEmissiona

aAdapted from ref 34. Copyright 2005 American Chemical Society.

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absorption/excitation transitions are expected to be relativelyintense compared to the 0−0 and 0−1 transitions. In emissions,in analogy with absorption, the most probable transitions arethe ones occurring vertically as well. However, for emission,there is the added consideration that the rate of vibrational andelectronic energy relaxation is very rapid compared to the rateof emission. As a result, emission always occurs from the v = 0vibrational level of the lowest excited state ψ*. The overlapbetween the excitation (or absorption) and emission spectradenotes the E00 state, and the greater the overlap, the greaterthe similarity between the ground and excited state potentialenergy surfaces.The spectroscopy of [Tc(dmpe)3]

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

show a similar overlap between absorption and emissionprofiles (Figure 3). Emission from the [Tc(dmpe)3]

2+ complexhas been assigned as LMCT in nature in analogy with a similaremission from [Re(dmpe)3]

2+ and as such is an unusualexample of a doublet−doublet process.27 The narrowbandwidths of absorption and emission coupled with thespin-allowed nature of the transition make the estimation of theESP an easier task than for many transition metal complexes.Using the mirror image relationship between absorption andemission, the determination of E00 can be made moreaccurately than in the case of MLCT triplet emitters since,for the latter, the lowest energy singlet−triplet absorption israrely observed without interference from the singletmanifold.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 statereduction potential (0.48 V for vs Ag/AgCl;), the excited statepotential 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 of2.61 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 examplesbeing found in metal polypyridine chemistry. Two of them areoctahedral 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-dimethyl-

1,10-phenanthroline, dppe = bis-1,2-diphenylphosphino-ethane).5 These complexes exhibit excited state potentials inthe range of 1.40−1.85 V. For [Ru(TAP)3]

2+, the excited stateemission is attributed as having a MLCT character, while for[Pt(5,6-Me2phen)(dppe)]

2+ the excited state is assigned to belocalized 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 statepotentials, thereby generating interest in their applications inphotocatalysis, HP3W12O40·6H2O being a representativeexample.7 Excitation of the LMCT (oxygen to metal) chargetransfer manifold (near visible and ultraviolet photons) rendersan excited state that is a potent oxidizing agent, capable ofoxidizing a variety of organic small molecule substrates. The netphotochemical reaction results in the oxidation of the organicsubstrate, typically the oxidation of alcohols to the correspond-ing aldehydes and ketones and gives the reduced POM. Thereduced POM can then be reoxidized by air or by protonreduction to complete the catalytic cycle.1,35 An absorptionthreshold of about 400 nm, which is common for POMs, resultsin an ESP that is about 3 V more oxidizing than the GSreduction potential, which for most POMs shifts the oxidizingpotential more positive than the 2.8 V (vs NHE) necessary toform OH radical. Because of their ideal GS and ES properties,POMs have successfully photocatalyzed the oxidation of anumber of organic pollutants.1,36,37 Uranyl ion (UO2

2+) isanother example of an oxygen containing transition metal ionthat has a highly oxidizing excited state potential. The lowestenergy transition in the uranyl ion is thought to involveexcitation of an electron from the highest filled O π orbital to anonbonding orbital on the uranium.38 The thermallyequilibrated excited state is reached via a singlet−triplettransition (λmax = 414 nm in 0.1 M HClO4).

39,40 TheUO2

2+* is highly oxidizing, with an estimated ESP of +2.6V.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 oxidizingpolyoxometalates or uranyl anion. These complexes also absorb

Table 1. Ground State and Excited State Potential Energy Values Used in the Calculation of the Excited State OxidationPotentials for [Tc(dmpe)3]

2+ (This Work) and [Re(dmpe)3]2+ (Previous Work by Del Negro et al.2)

compound λabs (nm) (V) λem (nm) (V) δ (V) E00 (V) E°′ (GS/GS‑) (V) E°′ (ES/GS‑) (V)[Tc(dmpe)3]

2+ 585 (2.12) 660 (1.88) 0.12 2.00 0.48 2.48[Re(dmpe)3]

2+ 528 (2.35) 600 (2.07) 0.14 2.21 0.40 2.61

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

Transfer

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strongly in the visible region, making them suitable candidatesfor solar energy conversion to store oxidizing equivalents.As expected from such high excited-state potentials,

quenching by aromatic hydrocarbons is observed for theexcited states of both rhenium and technetium complexes.Following the generalized mechanism for the luminescencequenching process by electron transfer in polar solvents basedon the classic work of Rehm and Weller,41 Scheme 3 can bedrawn for the excited state complex [Tc(dmpe)3]

2+* and thenonexcited quencher molecule, Q. An identical scheme can bedrawn 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 canconvert to the ground state by radiative decay or can form anencounter complex with a quencher, Q, through diffusion. Theencounter complex ([Tc(dmpe)3]

2+*···Q) can undergo elec-tron transfer through k23, leading to the ion-pair ([Tc-(dmpe)3]

+···Q+). The reaction denoted by k30 comprises allpossible modes by which the ion-pair can disappear, includingthe back electron transfer leading to excited state or groundstate molecules. The free energy change, ΔG23, involved in theelectron transfer process between the encounter complex andion-pair (Scheme 3), can be calculated according to Rehm andWeller (1970)41 with the following equation:

εΔ = − − ++ −G E D D E A A E

ed

( / ) ( / )23 000

2

(7)

In this equation, E(D+/D) describes the reduction potentialsfor the donor complex ([Tc(dmpe)3]

2+ or [Re(dmpe)3]2+) and

E(A/A−) describes the reduction potential for the acceptormolecules (organic quenchers, Q) as shown in eqs 8a and 8b,respectively.

+ →+D e D (8a)

+ → −A e A (8b)

The reduction potentials for the quencher hydrocarbonswere taken from Howell et al. (1984).42 E00 (in eq 7) is thefree-energy difference between the thermally equilibratedground and excited state surfaces, described in the textpreviously; 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 andtakes into account the free energy of bringing two radical ionsto the encounter distance, d, within solvent of dielectricconstant, ε, and is estimated as 0.06 eV in acetonitrilesolvent.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 theseare both written as reduction potentials, where D is oxidized toD+ and A is reduced to A−. For the same reaction, the oxidationpotential is the negative of the reduction potential. The E00term has a negative sign because energy is lost when the lightenergy is dissipated during the electron transfer reaction. Thee02/εd term has a positive sign because of the repulsive

Coulombic interaction created between the two like chargesproduced in the ion pair complex ([Tc(dmpe)3]

+···Q+), afterthe 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

quenching parameters from Figure 7 are summarized inTable 2, along with other organic substrates.

The Stern−Volmer derived quenching rate constants for[Tc(dmpe)3]

2+* with benzene, toluene, mesitylene, and anisolare 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+*. Thequenching rate constants for the former three are consistentwith the slightly lower excited state potential of the Tccomplex. Of particular note is the fact that benzene and tolueneare oxidized. It is somewhat surprising that no direct evidence ofthe Marcus inverted region is found given the presumed lack oflarge reorganizational energies for the quenchers and the Re(II/I) and Tc(II/I) redox pairs.45 The electron-transfer quenching-rate 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 aRehm−Weller analysis for both congeners. On the basis ofScheme 3, the overall rate constant for fluorescent quenchingcan be written:41

=+ +Δ

Δ Δ‡⎜ ⎟⎛⎝

⎞⎠( ) ( )

kk

1 exp expkV k

GRT

GRT

q12

12

12 30

23 23

(9)

Figure 7. Stern−Volmer quenching of [Tc(dmpe)3]2+* (green

squares) and [Re(dmpe)3]2+* (red circles) by toluene in anhydrous

acetonitrile.

Table 2. Luminescence Quenching Rate Constants, kq, andΔG23 Values in Acetonitrile

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

2+

quencherkq × 109

(M−1s−1)ΔG23(V)

kq × 109

(M−1 s−1)ΔG23(V)

benzene 0.0258 0.0724 0.0025 0.20toluene 1.34 −0.298 0.288 −0.1693p-xylene 8.93 −0.4876mesitylene 8.67 −0.5276 7.5 −0.3993anisole 13.9 −0.8164 18.9 −0.6881p-dimethoxybenzene 17.5 −1.206410-methylphenothiazine 19.6 −1.7164N,N-dimethylaniline 23.80 −1.7664

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where ΔG23‡ is assumed to be a monotonous function of ΔG23

and has been given by Rehm and Weller (1970)41 as

Δ =Δ

+ Δ +Δ‡ ‡

⎛⎝⎜⎜⎛⎝⎜

⎞⎠⎟

⎞⎠⎟⎟G

GG

G2

( (0))223

232

232

1/223

(10)

The activation free energy (ΔG23‡ (0)) at ΔG23 = 0 was

determined to equal 2.4 kcal/mol.41 Subsequent determinationof other constants in eq 9 has led to the general formula in eq11

= ×

+ +

− −

Δ Δ‡⎜ ⎟⎛⎝

⎞⎠( ) ( )

k2 10 M s

1 0.25 exp expGRT

GRT

q

10 1 1

23 23

(11)

which can be used to calculate rate constants of luminescencequenching by electron transfer in acetonitrile from ΔG23 values,which, according to eq 7, can be obtained from electrochemicaland 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

rate data based on the Rehm−Weller treatment (eq 11). Theagreement between the experimentally determined quenchingrate constants and the Rehm−Weller derived quenching rateconstants is evidence in favor of the electron transfermechanism on which eq 11 is based. This agreement alsoprovides secondary confirmation that the excited stateoxidation 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-transferquenching via oxidation of the hydrocarbon substrate occurs.Beside the prima facie evidence from the Rehm−Welleranalysis, support for this mechanism comes from a steady-state 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 0.3M HClO4 is irradiated with a 200 W quartz halogen lamp, UV−

visible spectroscopy shows the progressive production of theMePTZ radical cation; electronic spectral data for MePTZ+ isfrom Wagner (1988).46 After 40 min, a ratio of 0.60/1MePTZ+•/Re is found. This irreversible behavior is rationalizedby the photooxidation of MePTZ by [Re(dmpe)3]

2+* followedby oxidation of [Re(dmpe)3]

+ by oxygen to regenerate thestarting metal complex and is diagrammed in Scheme 4.

Highly oxidizing excited states of transition-metal complexesthat absorb visible photons are rare species but have greatpotential in solar energy conversion applications. Examplesinclude photogeneration of molecular chlorine or use inregenerative photoelectrochemical cells where the valenceband oxidation of the semiconductor is utilized. Other low-spin d5 complexes are likely to exhibit similar behavior if theintervening low-lying excited states are close in energy to theground state so that radiationless decay rate constants areminimized.

■ CONCLUSIONSThe agreement of experimental results with TDDFTcalculations shows that the strong, longest-wavelengthabsorptions in [Tc(dmpe)3]

2+ LMCT transitions is similar tothe [Re(dmpe)3]

2+ system. DFT calculations further indicatethat the LMCT transition is essentially a one-electronpromotion from a ligand-based MO to a metal-based MO.The corresponding luminescences have quantum yields andlifetimes consistent with this conclusion. Luminescencequenching followed by Stern−Volmer and Rehm−Welleranalyses for both complexes suggests that the LMCT excitedstates have unusually high excited state potentials, the highestfor any simple coordination complex of a transition metal andsufficiently high to oxidize simple aromatics as benzene andtoluene. Furthermore, this potential is available using a photonof visible light (M = Tc(Re); E°′* = +2.48 (+2.61) V calculatedvs SCE; 585 (528) nm).Examination of the spectroelectrochemistry of the LMCT

transitions in both complexes in aqueous solution shows areversible one-electron oxidation−reduction reaction in theground state with corresponding modulation of the fluores-cence of the LMCT state. This well-behaved spectroelec-trochemistry is particularly attractive and has been useful indevising sensing schemes for environmental monitoring oftechnetium 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.

Scheme 4. Schematic Representation ShowingInterconversion between M(I) and M(II)/M(II)* Resultingin Oxidation of Small Molecule Substrate (sub) to (sub+•);(M = Re, Tc) Reaction of [Re(dmpe)3]

2+* with MePTZResults in Oxidation to MePTZ+• That Can Be SubsequentlyIsolated along with [Re(dmpe)3]

+

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Highly oxidizing excited states of transition-metal complexesthat absorb visible photons are rare but have great potential insolar energy conversion applications. The implications for solarelectricity and artificial photosynthesis are manifold, includingthe ability of such highly oxidizing excited states to directlystore their energy in potent oxidants. This work also givesinsight as to how the highly oxidizing excited state potentialscompare down a group. The work presented here also providesthe motivation to study other low-spin d5 complexes that arelikely to exhibit similar behavior if the intervening low-lyingexcited states are close enough in energy to the ground state sothat radiationless decay rate constants are minimized.

■ ASSOCIATED CONTENT*S Supporting InformationDFT/TDDFT calculations for the complexes are provided. Thecomplete author listing for refs 15 and 29 are also provided.This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: Sam.Bryan@pnnl.gov.*E-mail: seliskcj@ucmail.uc.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSSupport from the Office of Biological and EnvironmentalResearch (OBER) of the U.S. Department of Energy (GrantDE-FG0799ER62331) is greatly acknowledged. Part of thisresearch was performed at EMSL, a national scientific userfacility at PNNL managed by the Department of Energy’sOffice of Biological and Environmental Research. PacificNorthwest National Laboratory is operated for the U.S.Department of Energy by Battelle under Contract DE-AC06-76RLO 1830.

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