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Probing Organometallic Reactions by Time-Resolved Infrared Spectroscopy in Solution and in the Solid State Using Quantum Cascade Lasers James A. Calladine,a Raphael Horvath,a Andrew J. Davies,a Alisdair Wriglesworth,a Xue-Zhong Sun,a Michael W. George a,b* a
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD United Kingdom Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Talking East Road, Ningbo 315100, China b
The photochemistry and photophysics of metal carbonyl compounds (W(CO) 6 , Cp*Rh(CO) 2 (Cp* ¼ g5 -C 5 Me 5 ), and fac-[Re(CO)3(4,4 0 -bpy)2Br] [bpy ¼ bipyridine]) have been examined on the nanosecond timescale using a time-resolved infrared spectrometer with an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning it can monitor small transients with absorbance changes less than 106 DOD. The C–H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we demonstrate that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements. Index Headings: Quantum cascade laser; QCL; Photochemistry; Time-resolved; Infrared; TRIR.
INTRODUCTION Since the early 1980s, time-resolved infrared spectroscopy, a combination of ultraviolet (UV) or visible flash photolysis and infrared (IR) detection, has proved to be an increasingly important tool for studying the photophysics and photochemistry of a wide range of photochemical processes, particularly of organometallic and coordination compounds.1–8 The use of IR detection in flash photolysis experiments has a long history. In 1958, Tanner and King developed a sophisticated IR rapid-scan technique using a dispersive IR spectrometer equipped with a continuously rotating Littrow mirror to obtain spectra on the millisecond timescale.9 This approach was further developed by Received 4 September 2014; accepted 22 October 2014. * Author to whom correspondence should be sent. E-mail: mike. [email protected]. DOI: 10.1366/14-07708
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Pimentel and co-workers over the next 20 years. Siebert and co-workers combined UV flash photolysis and a dispersive IR grating spectrometer with a fast-rise-time mercury cadmium telluride (MCT) IR detector.10 This enabled them to measure transient IR spectra in a ‘‘point-by-point’’ fashion, whereby a single IR frequency is monitored following photoexcitation and the measurement then repeated across the desired spectral region. Time-resolved infrared (TRIR) spectra are built up by plotting the change in IR intensity against frequency. Nowadays, a variety of different methods are available to perform TRIR measurements. Processes on a nanosecond-to-millisecond timescale can be monitored using step-scan FT-IR,3,11 while ultrafast laser-based techniques can be used for those occurring on a femtosecond to microsecond timescale. 5–7,12–16 The recent development of commercially available quantum cascade lasers (QCLs), which operate at room temperature, has enabled these versatile, high-powered sources to find uses in a wide variety of applications. External cavity quantum cascade lasers are useful for TRIR measurements, particularly for monitoring reactions in the condensed phase where broadband tunability is of greater importance than narrow line widths, which are more applicable to gas phase measurements. As such, QCLs have recently found extensive use in nanosecond time-resolved and pulse-radiolysis experiments.17–23
MATERIALS AND INSTRUMENTATION In this paper we use two external cavity quantum cascade lasers (Daylight Solutions) to construct a sensitive point-by-point nanosecond TRIR spectrometer. These lasers are able to provide coverage in the metal carbonyl region, between 1850 and 2100 cm1. The spectrometer is constructed around our previous leadsalt diode laser spectrometer,1 with only minor changes to the optical layout; this allows us to easily change between the two configurations.
0003-7028/15/6905-0519/0 Q 2015 Society for Applied Spectroscopy
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FIG. 1. (a) Traces showing the spectral region and output power of the QCLs at Nottingham (courtesy of Daylight Solutions). (b) FT-IR spectra of the lasers’ output recorded at 10 cm1 intervals.
The output spectrum of the QCLs is shown in Fig. 1a. An FT-IR spectrometer was used to measure the output of the two QCLs, to ensure that single-mode operation was achieved over the range 1850–2100 cm1 (see Fig. 1b). This is important, since the single mode output of the QCL means we do not require a monochromator for separating multi-mode outputs, which is a necessity for IR diode laser spectrometers. The QCLs are particularly attractive for use in TRIR applications where sensitivity or high photon flux are required because they should allow us to detect much smaller changes in absorbance. Cp*Rh(CO)2 (Strem), W(CO)6, (Sigma Aldrich), and CO (BOC, commercially pure grade) were used as received. Fac-[Re(CO)3(4,4 0 -bpy)2Br] was synthesized according to the literature procedure.24 Cyclohexane and cyclopentane (Sigma-Aldrich) were dried by refluxing over CaH2 under an argon atmosphere and degassed prior to use. All experiments were performed in either CO- or Arsaturated solutions at room temperature (22 6 2 8C). Solutions were made using standard Schlenk line techniques in dried, degassed solvents. For the experiments on the point by point QCL and ultrafast ns-TRIR spectrometers, a recirculating flow system was used to flow the solution while data were acquired. This consisted of a sample reservoir, IR cell (Harrick Scientific Corp.) with CaF2 windows (25 3 2 mm, Crystran) spaced at path lengths between 0.3 and 0.5 mm and a peristaltic pump (Cole-Palmer).
RESULTS AND DISCUSSION The increase in photon flux provided by QCLs for the point-by-point TRIR measurement potentially offers a much more simple and sensitive approach compared to other IR sources such as diode lasers or globars. The photochemistry of a typical metal carbonyl complex such as W(CO)6 has been studied previously25 and continues to produce interesting results.26,27 Figure 2 shows the point-by-point TRIR difference spectra 5 ls after photolysis (355 nm) of solutions of W(CO)6 in cyclopentane (5 mM, Fig. 2a and 50 nM, Fig. 2b), saturated with CO (2 atm). Points were recorded every 2 cm1, and
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FIG. 2. Time-resolved infrared difference spectra of W(CO)6 in cyclopentane, recorded using the point-by-point QCL spectrometer: (a) 5 mM and (b) 50 nM solutions. The solid line is a Lorentzian fit to the data.
FIG. 3. Kinetic traces for the reaction of W(CO)5(C5H10) with CO, recorded using the point-by-point QCL spectrometer with a 16-bit oscilloscope. (a) 5 mM (16 laser shots, 1954 cm1) and (b) 1 lM (4000 laser shots, 1958 cm1).
averages of 16 and 256 laser shots were used for traces shown in Fig. 2a and Fig. 2b, respectively. It is clear that upon irradiation the parent m(CO) band at 1982 cm1 is bleached, and two new transient bands, due to the formation of the alkane complex, W(CO)5(C5H10), are produced at 1954 and 1928 cm1. In the presence of excess CO, these bands are found to decay on the microsecond timescale. Original measurements were made with an 8-bit oscilloscope (Tektronix, TDS5032B), and as such the digitization will ultimately limit the signal-to-noise ratio (SNR) of the measurements. To investigate this further we have also monitored this reaction using a 16-bit oscilloscope (Picoscope 5444B), which has the advantage of greatly increased resolution in both the time and intensity domains, allowing for averaging of very small TRIR signals. Figure 3 shows kinetic traces of the decay of W(CO)5(C5H10) for
Root mean square noise was used to calculate the SNR.
FIG. 4. (a) Point-by-point QCL TRIR spectra and (b) ns-TRIR spectra recorded on the Nottingham ultrafast spectrometer, after 266 nm photolysis of a solution of Cp*Rh(CO)2 in cyclohexane in the presence of 2 atm CO shown at 30 and 200 ns after the laser pulse.
SCHEME 1. Photochemical reaction of Cp*Rh(CO)2 in alkane solvents.
two solutions of different concentrations. The trace in Fig. 3a shows a 16-scan average of the band at 1954 cm1 of a 5 mM solution; a SNR greater than 140:1 is obtained. Figure 3b shows the trace acquired on the shoulderà of the same band of a solution with 1 lM concentration. Using an amplification factor of 500 and a 4000-scan average, a change in absorbance of less than 1 3 106 was measured with an SNR of 10:1. We have also investigated the photochemistry of Cp*Rh(CO)2 (Cp*=g5-C5Me5) in alkane solution, assessing the ability of the system to record data on very short, nanosecond timescales, which is limited only by the rise time of the MCT detector (about 10 ns). The reactions of the photochemically generated CO-loss product, Cp*Rh(CO) with alkanes have been studied in detail and are summarized in Scheme 1.28–34 Upon UV photolysis, loss of CO from the parent occurs within a few picoseconds, and the fragment, Cp*Rh(CO), forms a r-complex with the alkane solvent. This species is characterized by a m(CO) band to lower energy of the parent vibrations. The initially formed alkane complex is then observed to undergo rapid C–H activation (within a few tens of nanoseconds) to form an alkyl hydride. The m(CO) band of the alkyl hydride species is found to higher energy of the r-complex, due to the oxidation of the metal from Rh(I) to Rh(III). The TRIR spectra obtained after photolysis (266 nm) of a 0.5 mM solution of Cp*Rh(CO)2 in cyclohexane in the presence of 2 atm of CO is shown in Fig. 4a, and the same experiment carried out on the Nottingham ultrafast instrument is shown in Fig. 4b. Upon photolysis, the parent bands at 2025 and 1962 cm1 are bleached within à
The shoulder was measured to demonstrate that signals with an absorbance less than 106 can easily be obtained.
the time resolution of the apparatus (about 10 ns as determined by the rise time of the Kolmar MCT detector and the pulse width of the Nd:YAG laser), and a new band at 1938 cm1 is observed to form. This new band can be readily assigned to the formation of the Rh-cyclohexane r-complex, Cp*Rh(CO)(C6H12), by comparison with previous work.28–34 The r-alkane complex reacts to form the C–H-activated product, Cp*Rh(CO)(C6H11)H, with a lifetime of 26 (63) ns, which is in good agreement to the previously published literature value of 26 (62) ns.34 The lifetime for this process was determined by deconvoluting the detector response function from the measured kinetic traces. In summary, we have shown that the QCL-based system is able to probe very small changes in absorbance, while providing comparable kinetics to the ultrafast system, even on a timescale on tens of nanoseconds. Time-Resolved Diffuse Reflection Spectroscopy. The high photon flux afforded by QCLs has allowed us to combine the time-resolved capabilities of our instrument with a commercially available diffuse reflection (DR) accessory, which is often used for studying solids, but has the potential to be useful in the investigation of photochemical processes in the solid state. In the past, TRIR measurements have been performed by making thin disks from a dilute solution of the molecule of interest in KBr, using a pellet press.35 The DR technique can be used to quickly probe powders (without the need for a hydraulic pellet press) and at a range of temperatures (from 150 to 900 8C), which is not readily achievable using standard transmission cells. The photophysics and photochemistry of mixed-ligand transition metal complexes are an area of intense interest.2,3,36,37 Transition metal a-diimine complexes usually offer a rich manifold of excited states, the relative energies of which can be tuned by donor– acceptor substituents and environmental properties.38,39 Time-resolved infrared spectroscopy is particularly useful for the elucidation of the nature of the lowest excited states of metal carbonyls and cyanides due to the
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FIG. 5.
Schematic diagram showing the setup for time-resolved diffuse reflection, using a Praying Mantis DR accessory.
high oscillator strengths of m(CO) and m(CN) vibrations and the sensitivity of their position and bandwidth to the electron density distribution in the molecule.2,37,40,41 Excitation of a metal complex to produce a triplet metal-to-ligand charge transfer (3MLCT) excited state results in a reduction of electron density at the metal center and, in the case of metal carbonyl complexes, a shift of the m(CO) bands to higher energy by about 50– 60 cm1 relative to the ground state absorptions. For complexes with triplet intraligand (3IL) 3p–p* lowest excited states, both the HOMO and LUMO are localized on the diimine ligand, and the m(CO) bands shift slightly to lower energy (about 5–10 cm1) upon formation of the excited state.42,43 The point-by-point QCL spectrometer was adapted to incorporate a DR module (Praying Mantis, Harrick Scientific Corp.; see Fig. 5). Additional mirrors and lenses were required, and the UV laser was focused onto the sample through the third window of the
sample chamber. To demonstrate the operation of the time-resolved diffuse reflection setup, the photophysics of powdered fac-[Re(CO)3(4,4 0 -bpy)2Br] (diluted with NaCl) was investigated. Figure 6 shows the TRIR difference spectrum 1.4 ls after 355 nm photolysis. A prominent transient band appears at 2012 cm1, which is red-shifted from the parent bleach at 2022 cm1 and is characteristic of an intraligand ( 3p-p*) excited state.42 This is markedly different from the photophysics usually observed in solution, where a 3MLCT state is formed, with m(CO) bands to higher energy.43,44 There is an accompanying transient at lower energy, but this strongly overlaps with the parent bleach, giving rise to the skewed band shape around 1910 cm1. A similar switch in the photophysics of metal-diimine complexes between solution and the solid state has been observed before.35 It was found that the environment of the fac-[Re(CO)3(4,4 0 -bpy)2Br] lowers the energy of the 3 IL state at the expense of the 3MLCT state; this is an interesting way of accessing excited states that are usually unobtainable in solution. This preliminary experiment, utilizing a diffuse reflection accessory for time-resolved infrared measurements, is promising and could be readily extended to investigate a whole range of organometallic complexes and their excited states.
CONCLUSIONS
FIG. 6. (a) Time-resolved spectrum of fac-[Re(CO)3(4,4 0 -bpy)Br] in NaCl 1.4 ls after 355 nm photolysis recorded using the DR accessory, showing the presence of the p–p* IL excited state. (b) DR spectrum of the same sample.
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In this article we have described some TRIR spectroscopic investigations using a QCL TRIR spectrometer to probe excited states and reaction intermediates in organometallic and coordination chemistry. Additionally, the sensitivity of the spectrometer has been demonstrated to be very good (DAbs , 106), and data with a high SNR are achievable with only a few seconds of averaging. The time resolution of the instrument is limited by the rise time of the MCT detector, but this is still sufficiently fast ( 10 ns) to allow some important processes, such as C–H activation reactions, to be measured. Finally, we have shown that time-resolved diffuse reflection TRIR measurements are relatively
straightforward using a QCL as the IR source, and that this approach is applicable to the study of solid-state photophysics and photochemical reactions. The application of fast TRIR spectroscopy is likely to continue to be a powerful tool to probe photochemical and photophysical processes, and QCL technology will be increasingly used for this purpose. Increased activity in the QCL field is likely to drive down the cost of such instrumentation. ACKNOWLEDGMENTS We would like to thank Peter Fields, Richard Wilson, David Litchfield, and Mark Guyler for their technical assistance. We thank Jeff Christenson of Harrick Scientific Corp. for very helpful discussions and the generous loan of a Praying Mantis DR accessory. This work was supported by the EPSRC. MWG gratefully acknowledges receipt of a Royal Society Wolfson Merit Award. 1. M.W. George, M. Poliakoff, J.J. Turner. ‘‘Nanosecond TimeResolved Infrared Spectroscopy: A Comparative View of Spectrometers and Their Applications in Organometallic Chemistry’’. Analyst. 1994. 119(4): 551-560. 2. J.R. Schoonover, G.F. Strouse. ‘‘Time-Resolved Vibrational Spectroscopy of Electronically Excited Inorganic Complexes in Solution’’. Chem. Rev. 1998. 98(4): 1335-1356. 3. J.M. Butler, M.W. George, J.R. Schoonover, D.M. Dattelbaum, T.J. Meyer. ‘‘Application of Transient Infrared and Near Infrared Spectroscopy to Transition Metal Complex Excited States and Intermediates’’. Coord. Chem. Rev. 2007. 251(3-4): 492-514. 4. J.A. Smith, M.W. George, J.M. Kelly. ‘‘Transient Spectroscopy of Dipyridophenazine Metal Complexes Which Undergo Photo-Induced Electron Transfer with DNA’’. Coord. Chem. Rev. 2011. 255(21-22): 2666-2675. 5. S. Garrett-Roe, P. Hamm. ‘‘What Can We Learn from ThreeDimensional Infrared Spectroscopy?’’ Acc. Chem. Res. 2009. 42(9): 1412-1422. 6. Z. Ganim, H.S. Chung, A.W. Smith, L.P. DeFlores, K.C. Jones, A. Tokmakoff. ‘‘Amide I Two-Dimensional Infrared Spectroscopy of Proteins’’. Acc. Chem. Res. 2008. 41(3): 432-441. 7. J. Zheng, K. Kwak, M.D. Fayer. ‘‘Ultrafast 2D IR Vibrational Echo Spectroscopy’’. Acc. Chem. Res. 2007. 40: 75-83. 8. J.P. Toscano. ‘‘Structure and Reactivity of Organic Intermediates as Revealed by Time-Resolved Infrared Spectroscopy’’. In: D.C. Neckers, G. Von Bu¨neau, editors. Advanced Photochemistry. Hoboken, NJ: John Wiley and Sons, 2001. Vol. 26. Pp. 41-91. 9. K.N. Tanner, R.L. King. ‘‘Infra-red Spectra of Free Radicals’’. Nature. 1958. 181: 963-965. 10. F. Siebert, W. Ma¨ntele, W. Kreutz. ‘‘Flash-Induced Kinetic Infrared Spectroscopy Applied to Biochemical Systems’’. Biophys. Struct. Mech. 1980. 6(2): 139-146. 11. G.D. Smith, R.A. Palmer. ‘‘Fast Time-Resolved Mid-Infrared Spectroscopy Using an Interferometer’’. In: J.M. Chalmers, P.R. Griffiths, editors. Handbook of Vibrational Spectroscopy. Chichester, UK: John Wiley and Sons, 2006. Vol. 1. Pp. 625-640. 12. M. Fayer. Ultrafast Infrared and Raman Spectroscopy. New York, NY: Marcel Dekker, 2001. Vol. 2. 13. A. Remorino, R.M. Hochstrasser. ‘‘Three-Dimensional Structures by Two-Dimensional Vibrational Spectroscopy’’. Acc. Chem. Res. 2012. 45(11): 1896-1905. 14. E.T.J. Nibbering, T. Elsaesser. ‘‘Ultrafast Vibrational Dynamics of Hydrogen Bonds in the Condensed Phase’’. Chem. Rev. 2004. 104(4): 1887-1914. 15. G.M. Greetham, D. Sole, I.P. Clark, A.W. Parker, M.R. Pollard, M. Towrie. ‘‘Time-Resolved Multiple Probe Spectroscopy’’. Rev. Sci. Instrum. 2012. 83: 103107-5. 16. G.M. Greetham, P. Burgos, Q. Cao, I.P. Clark, P.S. Codd, R.C. Farrow, M.W. George, M. Kogimtzis, P. Matousek, A.W. Parker, M.R. Pollard, D.A. Robinson, Z.-J. Xin, M. Towrie. ‘‘ULTRA: A Unique Instrument for Time-Resolved Spectroscopy’’. Appl. Spectrosc. 2010. 64(12): 1311-1319. 17. G. Hancock, S.J. Horrocks, G.A.D. Ritchie, J.H. v. Helden, R.J. Walker. ‘‘Time-Resolved Detection of the CF3 Photofragment Using Chirped QCL Radiation’’. J. Phys. Chem. A. 2008. 112(40): 97519757.
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35. A. Blake, N.R. Champness, T.L. Easun, D.R. Allan, H. Nowell, M.W. George, J. Jia, X.Z. Sun. ‘‘Photoreactivity Examined Through Incorporation in Metal-Organic Frameworks’’. Nat. Chem. 2010. 2: 688-694. 36. A.E. Nahhas, A. Cannizzo, F. v. Mourik, A.M. Blanco-Rodriguez, S. Za´lis, A. Vlcˇek Jr., M. Chergui. ‘‘Ultrafast Excited-State Dynamics of [Re(L)(CO)3(bpy)]n Complexes: Involvement of the Solvent’’. J. Phys. Chem. A. 2010. 114(22): 6361-6369. 37. S. Archer, J.A. Weinstein. ‘‘Charge-Separated Excited States in Platinum(II) Chromophores: Photophysics, Formation, Stabilization and Utilization in Solar Energy Conversion’’. Coord. Chem. Rev. 2012. 256(21-22): 2530-2561. 38. M.K. Kuimova, W.Z. Alsindi, A.J. Blake, E.S. Davies, D.J. Lampus, P. Matousek, J. McMaster, A.W. Parker, M. Towrie, X.-Z. Sun, C. Wilson, M.W. George. ‘‘Probing the Solvent Dependent Photophysics of fac-[Re(CO)3(dppz-X2)Cl] (dppz-X2 = 11,12-X2-dipyrido[3,2-a:2 0 ,3 0 -c]phenazine); X = CH3, H, F, Cl, CF3)’’. Inorg. Chem. 2008. 47(21): 9857-9869. 39. Y. Sun, Y. Liu, C. Turro. ‘‘Ultrafast Dynamics of the Low-Lying 3 MLCT States of [Ru(bpy)2(dppp2)]2þ’’. J. Am. Chem. Soc. 2010. 132(16): 5594-5595. 40. J.J. Turner. ‘‘Infrared Vibrational Band Shapes in Excited States’’. Coord. Chem. Rev. 2002. 230(1-2): 212-223.
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41. M.W. George, J.J. Turner. ‘‘Excited States of Transition Metal Complexes Studied by Time-Resolved Infrared Spectroscopy’’. Coord. Chem. Rev. 1998. 177(1): 201-217. 42. M.K. Kuimova, W.Z. Alsindi, J. Dyer, D.C. Grills, O.S. Jina, P. Matousek, A.W. Parker, P. Portius, X.Z. Sun, M. Towrie, C. Wilson, J.X. Yang, M.W. George. ‘‘Using Picosecond and Nanosecond TimeResolved Infrared Spectroscopy for the Investigation of Excited States and Reaction Intermediates of Inorganic Systems’’. Dalton Trans. 2003. 21: 3996-4006. 43. J. Dyer, W.J. Blau, C.G. Coates, C.M. Creely, J.D. Gavey, M.W. George, D.C. Grills, S. Hudson, J.M. Kelly, P. Matousek, J.J. McGarvey, J. McMaster, A.W. Parker, M. Towrie, J.A. Weinstein. ‘‘The Photophysics of fac-[Re(CO)3(dppz)(py)]þ in CH3CN: A Comparative Picosecond Flash Photolysis, Transient Infrared, Transient Resonance Raman and Density Functional Theoretical Study’’. Photochem. Photobiol. Sci. 2003. 2(5): 542-554. 44. D.R. Gamelin, M.W. George, P. Glyn, F.-W. Grevels, F.P.A. Johnson, W. Klotzbucher, S.L. Morrison, G. Russell, K. Schaffner, J.J. Turner. ‘‘Structural Investigation of the Ground and Excited States of ClRe(CO)3(4,4 0 -bipyridyl)2 Using Vibrational Spectroscopy’’. Inorg. Chem. 1994. 33(15): 3246-3250.
Probing Organometallic Reactions by Time-Resolved Infrared Spectroscopy in Solution and in the Solid State Using Quantum Cascade Lasers James A. Calladine,a Raphael Horvath,a Andrew J. Davies,a Alisdair Wriglesworth,a Xue-Zhong Sun,a Michael W. George a,b* a
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD United Kingdom Department of Chemical and Environmental Engineering, University of Nottingham Ningbo China, 199 Talking East Road, Ningbo 315100, China b
The photochemistry and photophysics of metal carbonyl compounds (W(CO) 6 , Cp*Rh(CO) 2 (Cp* ¼ g5 -C 5 Me 5 ), and fac-[Re(CO)3(4,4 0 -bpy)2Br] [bpy ¼ bipyridine]) have been examined on the nanosecond timescale using a time-resolved infrared spectrometer with an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning it can monitor small transients with absorbance changes less than 106 DOD. The C–H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we demonstrate that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements. Index Headings: Quantum cascade laser; QCL; Photochemistry; Time-resolved; Infrared; TRIR.
INTRODUCTION Since the early 1980s, time-resolved infrared spectroscopy, a combination of ultraviolet (UV) or visible flash photolysis and infrared (IR) detection, has proved to be an increasingly important tool for studying the photophysics and photochemistry of a wide range of photochemical processes, particularly of organometallic and coordination compounds.1–8 The use of IR detection in flash photolysis experiments has a long history. In 1958, Tanner and King developed a sophisticated IR rapid-scan technique using a dispersive IR spectrometer equipped with a continuously rotating Littrow mirror to obtain spectra on the millisecond timescale.9 This approach was further developed by Received 4 September 2014; accepted 22 October 2014. * Author to whom correspondence should be sent. E-mail: mike. [email protected]. DOI: 10.1366/14-07708
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Pimentel and co-workers over the next 20 years. Siebert and co-workers combined UV flash photolysis and a dispersive IR grating spectrometer with a fast-rise-time mercury cadmium telluride (MCT) IR detector.10 This enabled them to measure transient IR spectra in a ‘‘point-by-point’’ fashion, whereby a single IR frequency is monitored following photoexcitation and the measurement then repeated across the desired spectral region. Time-resolved infrared (TRIR) spectra are built up by plotting the change in IR intensity against frequency. Nowadays, a variety of different methods are available to perform TRIR measurements. Processes on a nanosecond-to-millisecond timescale can be monitored using step-scan FT-IR,3,11 while ultrafast laser-based techniques can be used for those occurring on a femtosecond to microsecond timescale. 5–7,12–16 The recent development of commercially available quantum cascade lasers (QCLs), which operate at room temperature, has enabled these versatile, high-powered sources to find uses in a wide variety of applications. External cavity quantum cascade lasers are useful for TRIR measurements, particularly for monitoring reactions in the condensed phase where broadband tunability is of greater importance than narrow line widths, which are more applicable to gas phase measurements. As such, QCLs have recently found extensive use in nanosecond time-resolved and pulse-radiolysis experiments.17–23
MATERIALS AND INSTRUMENTATION In this paper we use two external cavity quantum cascade lasers (Daylight Solutions) to construct a sensitive point-by-point nanosecond TRIR spectrometer. These lasers are able to provide coverage in the metal carbonyl region, between 1850 and 2100 cm1. The spectrometer is constructed around our previous leadsalt diode laser spectrometer,1 with only minor changes to the optical layout; this allows us to easily change between the two configurations.
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FIG. 1. (a) Traces showing the spectral region and output power of the QCLs at Nottingham (courtesy of Daylight Solutions). (b) FT-IR spectra of the lasers’ output recorded at 10 cm1 intervals.
The output spectrum of the QCLs is shown in Fig. 1a. An FT-IR spectrometer was used to measure the output of the two QCLs, to ensure that single-mode operation was achieved over the range 1850–2100 cm1 (see Fig. 1b). This is important, since the single mode output of the QCL means we do not require a monochromator for separating multi-mode outputs, which is a necessity for IR diode laser spectrometers. The QCLs are particularly attractive for use in TRIR applications where sensitivity or high photon flux are required because they should allow us to detect much smaller changes in absorbance. Cp*Rh(CO)2 (Strem), W(CO)6, (Sigma Aldrich), and CO (BOC, commercially pure grade) were used as received. Fac-[Re(CO)3(4,4 0 -bpy)2Br] was synthesized according to the literature procedure.24 Cyclohexane and cyclopentane (Sigma-Aldrich) were dried by refluxing over CaH2 under an argon atmosphere and degassed prior to use. All experiments were performed in either CO- or Arsaturated solutions at room temperature (22 6 2 8C). Solutions were made using standard Schlenk line techniques in dried, degassed solvents. For the experiments on the point by point QCL and ultrafast ns-TRIR spectrometers, a recirculating flow system was used to flow the solution while data were acquired. This consisted of a sample reservoir, IR cell (Harrick Scientific Corp.) with CaF2 windows (25 3 2 mm, Crystran) spaced at path lengths between 0.3 and 0.5 mm and a peristaltic pump (Cole-Palmer).
RESULTS AND DISCUSSION The increase in photon flux provided by QCLs for the point-by-point TRIR measurement potentially offers a much more simple and sensitive approach compared to other IR sources such as diode lasers or globars. The photochemistry of a typical metal carbonyl complex such as W(CO)6 has been studied previously25 and continues to produce interesting results.26,27 Figure 2 shows the point-by-point TRIR difference spectra 5 ls after photolysis (355 nm) of solutions of W(CO)6 in cyclopentane (5 mM, Fig. 2a and 50 nM, Fig. 2b), saturated with CO (2 atm). Points were recorded every 2 cm1, and
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FIG. 2. Time-resolved infrared difference spectra of W(CO)6 in cyclopentane, recorded using the point-by-point QCL spectrometer: (a) 5 mM and (b) 50 nM solutions. The solid line is a Lorentzian fit to the data.
FIG. 3. Kinetic traces for the reaction of W(CO)5(C5H10) with CO, recorded using the point-by-point QCL spectrometer with a 16-bit oscilloscope. (a) 5 mM (16 laser shots, 1954 cm1) and (b) 1 lM (4000 laser shots, 1958 cm1).
averages of 16 and 256 laser shots were used for traces shown in Fig. 2a and Fig. 2b, respectively. It is clear that upon irradiation the parent m(CO) band at 1982 cm1 is bleached, and two new transient bands, due to the formation of the alkane complex, W(CO)5(C5H10), are produced at 1954 and 1928 cm1. In the presence of excess CO, these bands are found to decay on the microsecond timescale. Original measurements were made with an 8-bit oscilloscope (Tektronix, TDS5032B), and as such the digitization will ultimately limit the signal-to-noise ratio (SNR) of the measurements. To investigate this further we have also monitored this reaction using a 16-bit oscilloscope (Picoscope 5444B), which has the advantage of greatly increased resolution in both the time and intensity domains, allowing for averaging of very small TRIR signals. Figure 3 shows kinetic traces of the decay of W(CO)5(C5H10) for
Root mean square noise was used to calculate the SNR.
FIG. 4. (a) Point-by-point QCL TRIR spectra and (b) ns-TRIR spectra recorded on the Nottingham ultrafast spectrometer, after 266 nm photolysis of a solution of Cp*Rh(CO)2 in cyclohexane in the presence of 2 atm CO shown at 30 and 200 ns after the laser pulse.
SCHEME 1. Photochemical reaction of Cp*Rh(CO)2 in alkane solvents.
two solutions of different concentrations. The trace in Fig. 3a shows a 16-scan average of the band at 1954 cm1 of a 5 mM solution; a SNR greater than 140:1 is obtained. Figure 3b shows the trace acquired on the shoulderà of the same band of a solution with 1 lM concentration. Using an amplification factor of 500 and a 4000-scan average, a change in absorbance of less than 1 3 106 was measured with an SNR of 10:1. We have also investigated the photochemistry of Cp*Rh(CO)2 (Cp*=g5-C5Me5) in alkane solution, assessing the ability of the system to record data on very short, nanosecond timescales, which is limited only by the rise time of the MCT detector (about 10 ns). The reactions of the photochemically generated CO-loss product, Cp*Rh(CO) with alkanes have been studied in detail and are summarized in Scheme 1.28–34 Upon UV photolysis, loss of CO from the parent occurs within a few picoseconds, and the fragment, Cp*Rh(CO), forms a r-complex with the alkane solvent. This species is characterized by a m(CO) band to lower energy of the parent vibrations. The initially formed alkane complex is then observed to undergo rapid C–H activation (within a few tens of nanoseconds) to form an alkyl hydride. The m(CO) band of the alkyl hydride species is found to higher energy of the r-complex, due to the oxidation of the metal from Rh(I) to Rh(III). The TRIR spectra obtained after photolysis (266 nm) of a 0.5 mM solution of Cp*Rh(CO)2 in cyclohexane in the presence of 2 atm of CO is shown in Fig. 4a, and the same experiment carried out on the Nottingham ultrafast instrument is shown in Fig. 4b. Upon photolysis, the parent bands at 2025 and 1962 cm1 are bleached within à
The shoulder was measured to demonstrate that signals with an absorbance less than 106 can easily be obtained.
the time resolution of the apparatus (about 10 ns as determined by the rise time of the Kolmar MCT detector and the pulse width of the Nd:YAG laser), and a new band at 1938 cm1 is observed to form. This new band can be readily assigned to the formation of the Rh-cyclohexane r-complex, Cp*Rh(CO)(C6H12), by comparison with previous work.28–34 The r-alkane complex reacts to form the C–H-activated product, Cp*Rh(CO)(C6H11)H, with a lifetime of 26 (63) ns, which is in good agreement to the previously published literature value of 26 (62) ns.34 The lifetime for this process was determined by deconvoluting the detector response function from the measured kinetic traces. In summary, we have shown that the QCL-based system is able to probe very small changes in absorbance, while providing comparable kinetics to the ultrafast system, even on a timescale on tens of nanoseconds. Time-Resolved Diffuse Reflection Spectroscopy. The high photon flux afforded by QCLs has allowed us to combine the time-resolved capabilities of our instrument with a commercially available diffuse reflection (DR) accessory, which is often used for studying solids, but has the potential to be useful in the investigation of photochemical processes in the solid state. In the past, TRIR measurements have been performed by making thin disks from a dilute solution of the molecule of interest in KBr, using a pellet press.35 The DR technique can be used to quickly probe powders (without the need for a hydraulic pellet press) and at a range of temperatures (from 150 to 900 8C), which is not readily achievable using standard transmission cells. The photophysics and photochemistry of mixed-ligand transition metal complexes are an area of intense interest.2,3,36,37 Transition metal a-diimine complexes usually offer a rich manifold of excited states, the relative energies of which can be tuned by donor– acceptor substituents and environmental properties.38,39 Time-resolved infrared spectroscopy is particularly useful for the elucidation of the nature of the lowest excited states of metal carbonyls and cyanides due to the
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FIG. 5.
Schematic diagram showing the setup for time-resolved diffuse reflection, using a Praying Mantis DR accessory.
high oscillator strengths of m(CO) and m(CN) vibrations and the sensitivity of their position and bandwidth to the electron density distribution in the molecule.2,37,40,41 Excitation of a metal complex to produce a triplet metal-to-ligand charge transfer (3MLCT) excited state results in a reduction of electron density at the metal center and, in the case of metal carbonyl complexes, a shift of the m(CO) bands to higher energy by about 50– 60 cm1 relative to the ground state absorptions. For complexes with triplet intraligand (3IL) 3p–p* lowest excited states, both the HOMO and LUMO are localized on the diimine ligand, and the m(CO) bands shift slightly to lower energy (about 5–10 cm1) upon formation of the excited state.42,43 The point-by-point QCL spectrometer was adapted to incorporate a DR module (Praying Mantis, Harrick Scientific Corp.; see Fig. 5). Additional mirrors and lenses were required, and the UV laser was focused onto the sample through the third window of the
sample chamber. To demonstrate the operation of the time-resolved diffuse reflection setup, the photophysics of powdered fac-[Re(CO)3(4,4 0 -bpy)2Br] (diluted with NaCl) was investigated. Figure 6 shows the TRIR difference spectrum 1.4 ls after 355 nm photolysis. A prominent transient band appears at 2012 cm1, which is red-shifted from the parent bleach at 2022 cm1 and is characteristic of an intraligand ( 3p-p*) excited state.42 This is markedly different from the photophysics usually observed in solution, where a 3MLCT state is formed, with m(CO) bands to higher energy.43,44 There is an accompanying transient at lower energy, but this strongly overlaps with the parent bleach, giving rise to the skewed band shape around 1910 cm1. A similar switch in the photophysics of metal-diimine complexes between solution and the solid state has been observed before.35 It was found that the environment of the fac-[Re(CO)3(4,4 0 -bpy)2Br] lowers the energy of the 3 IL state at the expense of the 3MLCT state; this is an interesting way of accessing excited states that are usually unobtainable in solution. This preliminary experiment, utilizing a diffuse reflection accessory for time-resolved infrared measurements, is promising and could be readily extended to investigate a whole range of organometallic complexes and their excited states.
CONCLUSIONS
FIG. 6. (a) Time-resolved spectrum of fac-[Re(CO)3(4,4 0 -bpy)Br] in NaCl 1.4 ls after 355 nm photolysis recorded using the DR accessory, showing the presence of the p–p* IL excited state. (b) DR spectrum of the same sample.
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In this article we have described some TRIR spectroscopic investigations using a QCL TRIR spectrometer to probe excited states and reaction intermediates in organometallic and coordination chemistry. Additionally, the sensitivity of the spectrometer has been demonstrated to be very good (DAbs , 106), and data with a high SNR are achievable with only a few seconds of averaging. The time resolution of the instrument is limited by the rise time of the MCT detector, but this is still sufficiently fast ( 10 ns) to allow some important processes, such as C–H activation reactions, to be measured. Finally, we have shown that time-resolved diffuse reflection TRIR measurements are relatively
straightforward using a QCL as the IR source, and that this approach is applicable to the study of solid-state photophysics and photochemical reactions. The application of fast TRIR spectroscopy is likely to continue to be a powerful tool to probe photochemical and photophysical processes, and QCL technology will be increasingly used for this purpose. Increased activity in the QCL field is likely to drive down the cost of such instrumentation. ACKNOWLEDGMENTS We would like to thank Peter Fields, Richard Wilson, David Litchfield, and Mark Guyler for their technical assistance. We thank Jeff Christenson of Harrick Scientific Corp. for very helpful discussions and the generous loan of a Praying Mantis DR accessory. This work was supported by the EPSRC. MWG gratefully acknowledges receipt of a Royal Society Wolfson Merit Award. 1. M.W. George, M. Poliakoff, J.J. Turner. ‘‘Nanosecond TimeResolved Infrared Spectroscopy: A Comparative View of Spectrometers and Their Applications in Organometallic Chemistry’’. Analyst. 1994. 119(4): 551-560. 2. J.R. Schoonover, G.F. Strouse. ‘‘Time-Resolved Vibrational Spectroscopy of Electronically Excited Inorganic Complexes in Solution’’. Chem. Rev. 1998. 98(4): 1335-1356. 3. J.M. Butler, M.W. George, J.R. Schoonover, D.M. Dattelbaum, T.J. Meyer. ‘‘Application of Transient Infrared and Near Infrared Spectroscopy to Transition Metal Complex Excited States and Intermediates’’. Coord. Chem. Rev. 2007. 251(3-4): 492-514. 4. J.A. Smith, M.W. George, J.M. Kelly. ‘‘Transient Spectroscopy of Dipyridophenazine Metal Complexes Which Undergo Photo-Induced Electron Transfer with DNA’’. Coord. Chem. Rev. 2011. 255(21-22): 2666-2675. 5. S. Garrett-Roe, P. Hamm. ‘‘What Can We Learn from ThreeDimensional Infrared Spectroscopy?’’ Acc. Chem. Res. 2009. 42(9): 1412-1422. 6. Z. Ganim, H.S. Chung, A.W. Smith, L.P. DeFlores, K.C. Jones, A. Tokmakoff. ‘‘Amide I Two-Dimensional Infrared Spectroscopy of Proteins’’. Acc. Chem. Res. 2008. 41(3): 432-441. 7. J. Zheng, K. Kwak, M.D. Fayer. ‘‘Ultrafast 2D IR Vibrational Echo Spectroscopy’’. Acc. Chem. Res. 2007. 40: 75-83. 8. J.P. Toscano. ‘‘Structure and Reactivity of Organic Intermediates as Revealed by Time-Resolved Infrared Spectroscopy’’. In: D.C. Neckers, G. Von Bu¨neau, editors. Advanced Photochemistry. Hoboken, NJ: John Wiley and Sons, 2001. Vol. 26. Pp. 41-91. 9. K.N. Tanner, R.L. King. ‘‘Infra-red Spectra of Free Radicals’’. Nature. 1958. 181: 963-965. 10. F. Siebert, W. Ma¨ntele, W. Kreutz. ‘‘Flash-Induced Kinetic Infrared Spectroscopy Applied to Biochemical Systems’’. Biophys. Struct. Mech. 1980. 6(2): 139-146. 11. G.D. Smith, R.A. Palmer. ‘‘Fast Time-Resolved Mid-Infrared Spectroscopy Using an Interferometer’’. In: J.M. Chalmers, P.R. Griffiths, editors. Handbook of Vibrational Spectroscopy. Chichester, UK: John Wiley and Sons, 2006. Vol. 1. Pp. 625-640. 12. M. Fayer. Ultrafast Infrared and Raman Spectroscopy. New York, NY: Marcel Dekker, 2001. Vol. 2. 13. A. Remorino, R.M. Hochstrasser. ‘‘Three-Dimensional Structures by Two-Dimensional Vibrational Spectroscopy’’. Acc. Chem. Res. 2012. 45(11): 1896-1905. 14. E.T.J. Nibbering, T. Elsaesser. ‘‘Ultrafast Vibrational Dynamics of Hydrogen Bonds in the Condensed Phase’’. Chem. Rev. 2004. 104(4): 1887-1914. 15. G.M. Greetham, D. Sole, I.P. Clark, A.W. Parker, M.R. Pollard, M. Towrie. ‘‘Time-Resolved Multiple Probe Spectroscopy’’. Rev. Sci. Instrum. 2012. 83: 103107-5. 16. G.M. Greetham, P. Burgos, Q. Cao, I.P. Clark, P.S. Codd, R.C. Farrow, M.W. George, M. Kogimtzis, P. Matousek, A.W. Parker, M.R. Pollard, D.A. Robinson, Z.-J. Xin, M. Towrie. ‘‘ULTRA: A Unique Instrument for Time-Resolved Spectroscopy’’. Appl. Spectrosc. 2010. 64(12): 1311-1319. 17. G. Hancock, S.J. Horrocks, G.A.D. Ritchie, J.H. v. Helden, R.J. Walker. ‘‘Time-Resolved Detection of the CF3 Photofragment Using Chirped QCL Radiation’’. J. Phys. Chem. A. 2008. 112(40): 97519757.
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41. M.W. George, J.J. Turner. ‘‘Excited States of Transition Metal Complexes Studied by Time-Resolved Infrared Spectroscopy’’. Coord. Chem. Rev. 1998. 177(1): 201-217. 42. M.K. Kuimova, W.Z. Alsindi, J. Dyer, D.C. Grills, O.S. Jina, P. Matousek, A.W. Parker, P. Portius, X.Z. Sun, M. Towrie, C. Wilson, J.X. Yang, M.W. George. ‘‘Using Picosecond and Nanosecond TimeResolved Infrared Spectroscopy for the Investigation of Excited States and Reaction Intermediates of Inorganic Systems’’. Dalton Trans. 2003. 21: 3996-4006. 43. J. Dyer, W.J. Blau, C.G. Coates, C.M. Creely, J.D. Gavey, M.W. George, D.C. Grills, S. Hudson, J.M. Kelly, P. Matousek, J.J. McGarvey, J. McMaster, A.W. Parker, M. Towrie, J.A. Weinstein. ‘‘The Photophysics of fac-[Re(CO)3(dppz)(py)]þ in CH3CN: A Comparative Picosecond Flash Photolysis, Transient Infrared, Transient Resonance Raman and Density Functional Theoretical Study’’. Photochem. Photobiol. Sci. 2003. 2(5): 542-554. 44. D.R. Gamelin, M.W. George, P. Glyn, F.-W. Grevels, F.P.A. Johnson, W. Klotzbucher, S.L. Morrison, G. Russell, K. Schaffner, J.J. Turner. ‘‘Structural Investigation of the Ground and Excited States of ClRe(CO)3(4,4 0 -bipyridyl)2 Using Vibrational Spectroscopy’’. Inorg. Chem. 1994. 33(15): 3246-3250.
