Luminescence Spectroelectrochemistry

Jon R. Kirchhoff Luminescence spectroscopy is well recognized for improved selectivity Department of Chem- istry, University of and sensitivity relative to absorption spectroscopy. When luminescence Toledo, Toledo, Ohio spectroscopy is coupled to electrochemistry, luminescence 43606 spectroelectrochemistry provides the opportunity to selectively probe the Phone: (419) 530-2664 excited state properties of in situ generated chromophores. A versatile Fax: (419) 530-4033 E-mail: long optical path spectroelectrochemical cell suitable for luminescence [email protected] measurements is described.

Spectroelectrochemical methods use as a general spectroelectro- the development of a general spec- provide the opportunity to spectro- chemical method for investigating troelectrochemical cell for lumines- scopically probe unique chemical solution species even with the in- cence studies have tried to incorpo- species that are generated in situ herent sensitivity advantage of lu- rate as many of the advantages of during redox reactions at electrode minescence over absorption meth- the OTTLE cell while also fulfilling surfaces. In many cases electro- ods. The primary reason for this the 90° detection requirement of lu- chemistry yields synthetically inac- trend can be traced to the lack of a minescence spectroscopy (14). cessible oxidation states, and there- versatile spectroelectrochemical fore spectroelectrochemistry offers cell that satisfies the 90° detection Cell Design new windows for exploring novel requirement for luminescence chemical pathways. measurements and exhibits the The major challenge for the de- positive features of the OTTLE cell. The basic cell and electrode design for luminescence spec- velopment of a spectroelectro- Several reports have been published troelectrochemistry are shown in chemical method is to design an that use the OTTLE cell for lumi- F1 (14). This approach addresses electrochemical cell that is mutu- nescence spectroelectrochemical the two principal experimental con- ally compatible with the desired measurements (7-12). To accom- siderations for coupling lumines- spectroscopic technique. Numerous modate detection of the emitted cence spectroscopy with electro- cell designs and optically transpar- light and the short optical path, the chemistry: 1) reproducible excita- ent electrodes (OTEs) for a wide OTTLE cell was placed at 45° rela- tion and detection of the resultant tive to the excitation and emission range of spectroscopic techniques emission and 2) efficient electroly- are described in the literature, but in slits. These studies demonstrated sis within the optical channels. general have been developed for a the utility of luminescence spec- The cell body was developed specific application (1-4). An ex- troelectrochemistry, the sensitivity from a solid polyethylene block to ception to this is the optically trans- of luminescence over absorption resemble, and therefore replace, the parent thin-layer electrode (OT- spectroscopy, and the short elec- cuvette holder in a conventional lu- TLE), which has been routinely trolysis advantage provided by the minescence spectrophotometer. An used for transmission spectroelec- OTTLE. However, poor signal-to- upper compartment was first milled trochemistry (5). The OTTLE cell noise ratios resulting from scatter- into the top of the polyethylene has been implemented for numer- ing off the front face of the cell, block to house the reference and ous UV-visible spectroelectro- nonreproducible cell positioning auxiliary electrodes. A second, chemical studies under a variety of and the short optical path generally lower compartment was then milled experimental conditions due to ease offset the positive features. One al- into the center of the block for the of construction, the need for only ternative approach for lumines- working electrode. Rectangular small sample volumes, and the ca- cence spectroelectrochemistry used openings were cut into each face in pability for rapid electrolysis (6). a cuvette-based configuration with line with the lower compartment. A In contrast to absorption spec- a gold resinate film electrode that quartz cuvette was inserted into the troscopy, luminescence spectros- permitted detection of the emitted lower compartment to provide opti- copy has received relatively little light at 90° (13). Our efforts toward cal windows and a fixed 1 cm opti-

11 Current Separations 16:1 (1997) F1 Approximately 2-3 mL of solu- Platinum Contact Luminescence spec- to RVC Working Electrode tion is required to completely fill troelectrochemical Reference Electrode the cell for operation. The solution cell. Platinum Auxiliary volume occupied by the RVC work- Electrode ing electrode was determined by coulometry of a standard ferricy- anide solution to be approximately 0.4 mL. A platinum wire auxiliary electrode, which encircled the Inert Gas working electrode, and a Ag/AgCl Inlet Optical reference electrode (BAS MF- Channel 2021) were placed in the upper compartment to complete the elec- Quartz Cuvette RVC trochemical cell. Electrical contact to the working electrode was made with a platinum wire. A cell cover was also machined to support the Light electrodes and exclude oxygen. ation Excit Exit Results and Discussion

Light Emissio tion to Detectorn Excita From an electrochemical point of view, RVC is inert and displays a wide potential range for electro- 58 mm chemical measurements in aqueous and nonaqueous solutions (15). The porous structure also maintains good communication between the 35 mm Polyethylene working electrode in the lower sample compartment and the auxiliary F2 man Series 2 spectrophotometer and reference electrodes in the up- Cyclic voltammogram (SLM Instruments, Inc.). per sample compartment. The po- of 1 mM o-tolidine in Reticulated vitreous carbon rosity of 100 pores per inch RVC is 0.5 M CH3COOH, 1 400 µA M HClO4 in the lumi- (RVC, 100 pores per inch, Electro- small enough that diffusional mix- nescence spec- ing from the upper compartment troelectrochemical synthesis Co., Inc.) was used as the cell with 2 mm diame- working electrode. The three-di- does not occur in the optical chan- ter optical channels. nels. Thus, exhaustive electrolysis Initial potential: +0.4 mensional structure of RVC permits is achieved in the optical channels V vs. Ag/AgCl. Scan facile fabrication of a working elec- rate: 2 mV/s. (Re- within 10-25 mins depending on the printed with permis- trode that fits into the lower com- sion from reference optical channel diameter, solvent, 14.) partment and extends into the upper electroactive species, analyte con- compartment. The challenge of in- centration and electrolyte. An addi- corporating the 90° detection re- tional advantage of RVC is that if quirement of luminescence spec- fouling occurs, the electrode can be troscopy within an electrochemical easily removed and replaced. cell is also addressed by drilling op- The electrochemical and lumi- tical channels of a tee configuration nescence capabilities of the spec- in line with the windows of the troelectrochemical cell are easily demonstrated by the o-tolidine re- lower compartment. This configu- 0.8 0.4 dox couple. F2 depicts the cyclic ration provides long optical paths voltammogram for the two-electron E, V vs. Ag/AgCl for both reproducible excitation and oxidation of a 1 mM aqueous solu- detection of the emission. A diame- tion of o-tolidine with 0.5 M cal path. The windows were posi- ter of 2 mm was found to afford the CH3COOH and 1 M HClO4 in the tioned to take advantage of the ex- optimum tradeoff between elec- luminescence spectroelectrochemi- trolysis time and signal-to-noise ra- cal cell. The electrochemical pa- isting optics in an Aminco-Bow- tio. rameters were determined to be Eo’

Current Separations 16:1 (1997) 12 F3 0.0591/n and the y-intercept equal A Spectra recorded dur- to Eo’. The Nernst plot for the data ing a luminescence B o’ spectropotentiostatic at 405 nm (F3) yields E = +0.638 experiment of 5 µM V and n = 1.91, which is consistent C o-tolidine in 0.5 M with the cyclic voltammetry data. CH3COOH, 1 M HClO4 in the luminescence One area where our group has spectroelectrochemical D cell with 2 mm diame- applied luminescence spectroelec- ter optical channels. Ap- trochemistry is in the investigation plied potentials in V vs. Ag/AgCl are as follows: E of the excited state properties of (A) +0.405; (B) +0.608; transition metal complexes. Typical (C) +0.628; (D) +0.637; (E) +0.648; (F) +0.658; F photochemical and photophysical (G) +0.670; (H) +0.806. studies rely on the synthesis and λex is 270 nm. (Re-

e Intensity / arbitrary units G printed with permission purification of the complex of inter- from reference 14.) est. Therefore, most photolumines- H cence studies of transition metal

Relativ complexes center around easily synthesized electron configurations such as d3,d6 or d10. In contrast, 360400 450 500 the luminescence spectroelectro- Wavelength / nm chemical cell permits the investigation and characterization of syn- F4 thetically inaccessible oxidation Uncorrected emission states provided the target oxidation spectra of 0.14 mM (A) 2+ state is electrochemically accessible [Re(dmpe)3] , λex = A 530 nm; and B) from a parent complex and is stable + [Re(dmpe)3] , λex = 220 nm in acetonitrile in solution. with 0.1 M (n-Bu)4NPF6 One example of this is the in the luminescence 2+ spectroelectrochemical Re(II) complex, [Re(dmpe)3] , cell. (Reprinted with where dmpe is 1,2-bis(dimethyl- permission from reference 16.) phosphino)ethane (16). The parent + Re(I) complex, [Re(dmpe)3] ,is easily synthesized and exhibits a re- versible one-electron oxidation to e Intensity / arbitrary units Re(II) in acetonitrile. The d6 Re(I) form is colorless and upon excita-

Relativ tion into the UV absorption band B shows no luminescence (F4). How- ever, oxidation in the luminescence 400500 600 700 spectroelectrochemical cell to the d5 Re(II) electron configuration Wavelength / nm gives a reddish-pink solution with ∆ = +0.635 V, Ep =90mVand lution under potential conditions an absorption maximum at 530 nm, which has been assigned as a li- ipa/ipc = 1.0, which are in good that yield the completely reduced agreement with literature values form, the completely oxidized form gand-to-metal charge-transfer (LMCT) band. Excitation at 530 (6,13). Furthermore, the spectropo- and a mixture of the oxidized and nm produces an intense emission at tentiostatic oxidation of o-tolidine reduced forms, respectively, φ is the 593 nm (F4) with a quantum effi- illustrates the stepwise decrease in luminescence quantum efficiency, ciency of 0.066; for comparison, luminescence intensity following and b is the optical path length (13). the quantum efficiency of excitation at 270 nm (F3). In a [Ru(bpy) ]2+ in water is 0.042. manner similar to absorption spec- 3 [Ox] (I - I) /φ b I-IConsequently, luminescence in tropotentiostatic experiments, Eo’ = red = red [Red] (I - I ) /φ b I-I conjunction with absorption spec- and n values can be calculated from ox ox troelectrochemistry has enabled the the individual spectra with the discovery of a rare example of a Nernst equation and equation be- A Nernst plot of the applied poten- highly luminescent transition metal 5 low, where Ired,Iox, and I are the tial versus log [Ox]/[Red] yields a complex with a d electron configu- luminescence intensities of the so- straight line with the slope equal to ration.

13 Current Separations 16:1 (1997) Conclusion References 9. C.W. McLeod and T.S. West, Analyst 107 (1982) 1-11. Luminescence spectroelectro- 1. T. Kuwana and N. Winograd in “Elec- 10. B.L. Cousins, J.L. Fausnaugh and T.L. chemistry is easily accomplished troanalytical Chemistry” Vol. 7 Miller, Analyst 109 (1984) 723-726. with the versatile cell design de- (A.J. Bard, Ed.), Dekker, New 11. E.T. Turner-Jones and L.R. Faulk- York, 1974, pp 1-78. scribed above. In situ electrochemi- ner, J. Electroanal. Chem. 179 2. W.R. Heineman, J. Chem. Educ. 60 (1984) 53-64. cal generation of stable species (1983) 305-308. 12. R.G. Compton, A.C. Fisher and coupled to spectroscopic charac- 3. W.R. Heineman, F.M. Hawkridge and R.G. Wellington, Electroanalysis 3 terization by luminescence and ab- H.N. Blount in “Electroanalytical (1991) 27-29. sorption methods clearly opens Chemistry” Vol. 13 (A.J. Bard, Ed.), 13. M.J. Simone, W.R. Heineman and possibilities for the investigation of Dekker, New York, 1984, pp 1-113. G.P. Kreishman, Anal. Chem. 54 the properties of novel excited state 4. “Spectroelectrochemistry” (R.J. Gale, (1982) 2382-2384. species and their reactivity. Ed.) Plenum, New York, 1988. 14. Y.F. Lee and J.R. Kirchhoff, Anal. 5. R.W. Murray, W.R. Heineman and Chem. 65 (1993) 3430-3434. Acknowledgments G.W. O’Dom, 39 (1967) 1666-1668. 15. J. Wang, Electrochim. Acta 26 6. T.P. DeAngelis and W.R. Heineman, (1981) 1721-1726. J. Chem. Educ. 53 (1976) 594-597. 16. Y.F. Lee and J.R. Kirchhoff, J. Am. Financial support from the do- Chem. Soc. 116 (1994) 3599-3600. nors to The Petroleum Research 7. A. Yildiz, P.T. Kissinger and C.N. Reilley, Anal. Chem. 40 (1968) Fund, administered by the Ameri- 1018-1024. can Chemical Society (Grant 8. M.J. Simone, W.R. Heineman and 23968-G3), and The University of G.P. Kreishman, J. Coll. Inter. Sci- Toledo is gratefully acknowledged. ence 86 (1982) 295-298.

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