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A cryostat for low temperature resonance Raman measurements on operando oxygenated bioinorganic model complexes
Inorganica Chimica Acta xxx (2017) xxx–xxx
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
A cryostat for low temperature resonance Raman measurements on operando oxygenated bioinorganic model complexes B. Grimm-Lebsanft a,b,⇑, C. Brett a,b, F. Strassl c, D. Rukser a,b, M. Biednov a,b, F. Biebl a,b, M. Naumova b,d, A. Hoffmann c, L. Akinsinde a,b, D. Brückner a,b, S. Herres-Pawlis c, M. Rübhausen a,b a
Institut für Nanostruktur- und Festkörperphysik, Universität Hamburg, Notkestrasse 85, 22607 Hamburg, Germany Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22607 Hamburg, Germany Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany d Department Chemie, Universität Paderborn, Warburgerstrasse 100, 33098 Paderborn, Germany b c
a r t i c l e
i n f o
Article history: Received 11 May 2017 Received in revised form 29 June 2017 Accepted 5 July 2017 Available online xxxx
a b s t r a c t Raman spectroscopy is a useful tool for the elucidation of bioinorganic mechanisms. To investigate charge-transfer processes in operando oxygenated bioinorganic samples, we present a new cryostat for liquid samples at low temperature. We reach temperatures below 80 °C inside a Suprasil glass cuvette which is prerequisite for the formation of the oxo species and long-term stability of the sample. Under operando conditions we demonstrate resonance Raman as well as isotope substitution measurements on a bis(l-oxo) dicopper(III) complex. Future applications of our setup are envisioned to be transient Raman, steady-state and time-resolved fluorescence as well as transient absorption spectroscopy of bioinorganic or other charge-transfer complexes. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The copper containing enzyme tyrosinase is part of the melanin production and catalyzes the biological oxidation of phenols such as tyrosine. Most model complexes for tyrosinase are not stable at room temperature when oxygenation occurs but usually stable for several hours when the temperature is below 60 °C [1]. Tyrosinase serves as an archetype for nature’s solution to activate molecular oxygen and use it for mild and selective hydroxylation of phenols. In the last 30 years, a plethora of ligand systems have been used for the stabilization of model complexes [2]. To investigate direct hydroxylation of phenolic substrates bis (l-oxo) dicopper(III) and peroxo dicopper(II) [3] complexes have been used. For the synthesis of bis(l-oxo) dicopper(III) complexes, guanidine ligands have been utilized due to their good N-donor properties and UV/Vis and X-ray absorption spectroscopy confirmed the Cu2O2 species [4–10]. Guanidine ligands offer a facile variation of the substitution pattern [11] and allow the steering of formation and decay rates [12,13]. The complex used here is [Cu(btmgp)]I together with the corresponding bis(l-oxo)
⇑ Corresponding author at: Institut für Nanostruktur- und Festkörperphysik, Universität Hamburg, Notkestrasse 85, 22607 Hamburg, Germany. E-mail address: [email protected] (B. Grimm-Lebsanft).
dicopper(III) complex [14,15]. At 20 °C, the bis(l-oxo) species decays with a rate constant of 1 s1 whereas it is stable for days at 80 °C [16,12]. Since the early 1950s low temperature Raman measurements have been sought after [17–20] as they open up pathways to emerging properties of materials [21–25] at e.g. phase transitions. A longer stability of the sample can also be observed as damage by the incident laser beam is reduced [26]. This is especially valuable for understanding charge-transfer processes in bioinorganic chemistry where temperature-sensitive metal complexes are investigated and resonance Raman spectroscopy unveils bonding modes, crucial vibrations of the active sites and insights into the electronic structure [27]. However, when tuning the incident photon energies through the absorption bands, the sample decays much faster. Other groups use cooled nitrogen gas [28], liquid nitrogen [4], liquid-helium flow cryostats [29], or cooled spinning cells [30] to keep the samples stable for longer time periods. While our previous Raman measurements on similar model complexes have been successful at room temperature [24] and low temperatures as well, leaving the cuvette with the studied complex taken from the cold bath uncooled resulted in a stability of the complex of approximately 90 s until the Cu2O2 species disappeared due to the warm up process, thus making resonance Raman or other time-consuming measurements impossible [31].
http://dx.doi.org/10.1016/j.ica.2017.07.010 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
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2. Experimental To overcome these experimental shortcomings of short-time stability while keeping the flexibility of the UT-3 Raman spectrometer [32] a new low-temperature cryostat was developed (see Fig. 1.). To avoid the usage of liquid gases and minimize vibrations, the cryostat uses a two-way cooling system to reach low temperatures. It consists of a liquid ethanol based closed-cycle Proline RP890 chiller (Lauda, Lauda-Königshofen, Germany) and a multistage Peltier module (TEC4-97-49-17-7-05, Thermonamic, China). The chiller flushes the copper block behind the Peltier module with cooled liquid ethanol. The Peltier module was glued to this copper block with a thin layer of Ceramique 2 heat paste (Arctic Silver, USA). On top of the Peltier module another small copper block was used for better heat transport. The cuvette is pressed against this small copper block with a holder for best thermal conductivity. The cap for the cuvette inlet has two pipe inlets which are used for operando oxygenation and the corresponding overpressure reduction inside the cuvette. With a vacuum pump a pressure of 2102 mbar inside the cryo was reached. A temperature of around 70 °C on the warm side of the Peltier module was reached when using the ethanol chiller only. When the Peltier module was turned on, the temperature behind the cuvette further decreases to around 150 °C which yields a temperature inside the cuvette below 60 °C after 15 min and below 80 °C after 55 min (see Fig. 2.) measured with a Pt100 temperature sensor inside the solution. This makes the cryostat suitable for many complexes. For all wavelengths two Tsunami Ti:Sapphire laser systems, model 3960C-15HP and model 3950-X1BB (both Spectra Physics Lasers Inc., California) were used. They were synchronized using a Lok-to-clock (LTC) system. The fundamental laser line was frequency doubled and tripled with a flexible harmonic generation (FHG) unit, model GWU2 23-PS (GWU-Lasertechnik Vertriebsges.
Fig. 2. Temperature measured inside the solution while cooling. With the setup a temperature below 60 °C was reached after 15 mins and below 80 °C after 55 mins.
Fig. 3. Two Tsunami Ti:Sa lasers were used in the resonance Raman study. The beam was widened by a spatial filter and then focused on to the cuvette inside the cryostat. Raman scattered light was captured with the entrance optics of the UT-3 triple monochromator spectrometer and recorded with a liquid nitrogen cooled CCD camera.
Fig. 1. Schematic setup of the cryostat. The cuvette is cooled by a small copper block (brown) on top of a multistage Peltier module (grey). The copper block (brown) on the backside of the Peltier module is cooled by liquid ethanol. During operation a pressure inside the cryostat of around 2102 mbar is reached. Lower pressures could be achieved but those are not suitable for cuvette operation. The oxygen is supplied via a pipe which leads directly into the cuvette. Overpressure is regulated by a one-way exhaust valve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mbH, Erfstadt, Germany). The laser beam was widened with a spatial filter and then focused on the cuvette inside the cryostat (see Fig. 3.). The focus spot size was around 20 lm in diameter. The used laser power in front of the entrance optics was, depending on the wavelength, between 8.6 mW and 23.7 mW. Pulse width was between 1.5 ps and 2.5 ps. All spectra were normalized to 1 s integration time and 1 mW laser power. Background was subtracted and the resulting spectra were corrected for the spectrometer sensitivity in the respective wavelength regions. The experiments were conducted in a clean room with constant temperature (20.0 °C ± 0.5 °C) and humidity (45% ± 3%). To determine the pulse width of the laser, a small part of the Tsunami fundamental was mirrored out using a glass plate and the reflex then coupled into an autocorrelator (AC) (APE GmbH, Berlin, Germany). For the measurements a custom made half-height Suprasil glass cuvette (Hellma Analytics, Müllheim, Germany) with 1.7 ml sample volume was used. 3. Sample preparation The copper complexes were prepared in an oxygen and water free atmosphere (<0.5 ppm) inside a LABstar glovebox (MBraun,
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Garching, Germany) with a concentration of 15 mM in 99% propionitrile (Sigma–Aldrich Chemie GmbH, Germany). The solvent was degassed by repetitive ‘‘freeze–pump–thaw” cycles and dried over a 3 Å molecular sieve, as we found a huge influence of the water content of the solvent on the distinct oxygen based vibration. After cooling the precursor solution to below 80 °C, oxygen was run through the sample until the complex was formed. This can be observed by a distinct change in color of the solution. The cryostat was then placed inside the entrance optics of the UT-3 Raman spectrometer. With a micrometer screw a focal depth of around 50 lm inside the cuvette was adjusted. 4. Results and discussion Upon oxygenation of the precursor [Cu(btmgp)]I at below 80 °C, the solution turns from yellowish to a dark red. This change can be distinctly seen in the Raman spectra (see Fig. 5a.) as compared to the spectrum of the precursor (green) a sharp peak at around 596 cm1 rises for the oxygenated complex (blue) which can be identified as the breathing mode of the Cu2O2 rhomb of the complex (see Fig. 4). With its 596 cm1 it fits well into similar bisl-oxo dimer copper cores [33–36]. To verify the oxygen-based vibration mode isotope substitution was performed (see Fig. 5b.). Instead of 16 O2 ; 18 O2 (Campro Scientific, Berlin) was used for oxygenation. A shift of the 596 cm1 peak to 571 cm1 or a delta of 25 cm1 was observed. Also some 16O2 residue can still be seen in the 18O2 spectrum which resulted from not completely flushing the pipes with 18O2 before oxygenation. Other publications show similar 16O2/18O2 shifts for bis(l-oxo) dicopper complexes [38]. The Gaussian [39] DFT calculations (see Fig. 5c.) with TPSSh [40]/def2-TZVP[41] and empirical dispersion correction with Becke-Johnson damping [42–44] predict this socalled breathing mode at 619 cm1 (18O2: 591 cm1) with an isotope shift of 28 cm1 and are thus in excellent agreement with the obtained experimental value. A resonance Raman study was conducted with eleven separate incident laser wavelengths (250.2 nm, 282.4 nm, 333.2 nm, 359.7 nm, 364.5 nm, 369.9 nm, 375.1 nm, 380.7 nm, 399.9 nm, 423.7 nm and 500.2 nm). As can be seen in Fig. 6., the first part of the spectrum up to 550 cm1 is dominated by solvent and glass
Fig. 5. a) Raman spectra of the [Cu(btmgp)]I (green) and the oxygenated sample (blue). One can clearly see a peak appearing at 596 cm1 which can be assigned to the breathing mode of the Cu2O2 rhomb of the complex. Stars denote solvent peaks. Different glass signal in the range of 320 cm1 to 520 cm1 due to slightly different cuvette positions can be observed. b) Raman spectra of [Cu(btmgp)]I oxygenated with 16O2 (blue) and 18O2 (red). A blue shift of 25 cm1 is observed. Stars denote solvent peaks. Spectra were normalized to the 541 cm1 solvent peak respectively for better comparison. c) Unscaled DFT spectra for [Cu(btmgp)]I with 16O2 (blue) and 18O2 (red) show a shift of 28 cm1 which is in excellent agreement with the experiment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
signal of the cuvette. The ratio between the solvent peak at 541 cm1 and the 596 cm1 mode changes only slightly for different incident wavelengths. In Fig. 7. we show the normalized peak intensities I0 of the 596 cm1 vibration. They were normalized using the incident photon energy dependent absorbance to I0 ¼ I ð1 expð2:3 c dÞÞ , with the concentration c = 15 mM, the optical path length d = 100 lm and the extinction coefficient. This yields a distinct resonance at around 370 nm (3.35 eV) incident photon energy (error bars included in the figure). Also a second resonance around 358 nm (3.46 eV) can be seen. In the 400 nm region we see a resonance shoulder which is supported by the components of the absorption curve. The possible double resonance at 370 nm and 358 nm has an energy difference of at least 110 meV. This can be explained by ingoing and outgoing resonance with 2 hxv ib 148 meV of the 596 cm1 vibration [45]. Noise in the spectra was four orders of magnitude lower than the errors of the fits and thus neglected. In the resonance region (350 nm to 380 nm) several measurements have been performed to validate the peak intensities. Additional errors can result from slightly different concentrations of the sample, different focal depths inside the cuvette as well as different cuvette position inside the cryostat. The errors for all remaining incident photon energies were approximated to take these variations into account. 1
Fig. 4. Molecule structure of [Cu2O2(btmgp)2]2+ and the calculated eigenmodes of the Cu2O2 breathing mode [37].
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Raman spectroscopy as well as isotope substitution with a peak shift supported by DFT calculations. Also, first Raman spectra on copper complexes with an aromatic bis(guanidine) ligand measured with this setup have been published [46]. Future applications of the setup can be transient Raman, fluorescence or transient absorption spectroscopy of liquid samples. Acknowledgements We acknowledge the financial support by DFG FOR1405 ‘‘BioCTDyn”. References
Fig. 6. Resonance Raman study of 16O2 oxygenated [Cu(btmgp)]I for ten separate wavelengths. An offset was added to each spectrum for better visibility. Stars denote solvent peaks. Only very weak peaks were seen in the 250 nm spectrum (not shown).
Fig. 7. Normalized intensities with error bars (red) of the 596 cm1 peak shown against the incident photon energy. Peak intensities have been normalized to the wavelength dependent absorption of the sample (grey). A distinct resonance behaviour can be seen at 370 nm and 358 nm. Also a resonance shoulder can be observed at 400 nm. Black lorentzians show the fit of the absorption spectrum.
5. Conclusion and outlook In conclusion, we have presented a new cryostat for operando oxygenated resonance Raman measurements at the UT-3 Raman spectrometer which does not need cold gases like nitrogen or helium. The temperature sensitive samples have shown a longterm stability of the bis(l-oxo) species for several days when oxygenated below 80 °C. This has enabled a detailed characterisation of the used bis(l-oxo) dicopper(III) complex using resonance
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A cryostat for low temperature resonance Raman measurements on operando oxygenated bioinorganic model complexes B. Grimm-Lebsanft a,b,⇑, C. Brett a,b, F. Strassl c, D. Rukser a,b, M. Biednov a,b, F. Biebl a,b, M. Naumova b,d, A. Hoffmann c, L. Akinsinde a,b, D. Brückner a,b, S. Herres-Pawlis c, M. Rübhausen a,b a
Institut für Nanostruktur- und Festkörperphysik, Universität Hamburg, Notkestrasse 85, 22607 Hamburg, Germany Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22607 Hamburg, Germany Institut für Anorganische Chemie, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany d Department Chemie, Universität Paderborn, Warburgerstrasse 100, 33098 Paderborn, Germany b c
a r t i c l e
i n f o
Article history: Received 11 May 2017 Received in revised form 29 June 2017 Accepted 5 July 2017 Available online xxxx
a b s t r a c t Raman spectroscopy is a useful tool for the elucidation of bioinorganic mechanisms. To investigate charge-transfer processes in operando oxygenated bioinorganic samples, we present a new cryostat for liquid samples at low temperature. We reach temperatures below 80 °C inside a Suprasil glass cuvette which is prerequisite for the formation of the oxo species and long-term stability of the sample. Under operando conditions we demonstrate resonance Raman as well as isotope substitution measurements on a bis(l-oxo) dicopper(III) complex. Future applications of our setup are envisioned to be transient Raman, steady-state and time-resolved fluorescence as well as transient absorption spectroscopy of bioinorganic or other charge-transfer complexes. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The copper containing enzyme tyrosinase is part of the melanin production and catalyzes the biological oxidation of phenols such as tyrosine. Most model complexes for tyrosinase are not stable at room temperature when oxygenation occurs but usually stable for several hours when the temperature is below 60 °C [1]. Tyrosinase serves as an archetype for nature’s solution to activate molecular oxygen and use it for mild and selective hydroxylation of phenols. In the last 30 years, a plethora of ligand systems have been used for the stabilization of model complexes [2]. To investigate direct hydroxylation of phenolic substrates bis (l-oxo) dicopper(III) and peroxo dicopper(II) [3] complexes have been used. For the synthesis of bis(l-oxo) dicopper(III) complexes, guanidine ligands have been utilized due to their good N-donor properties and UV/Vis and X-ray absorption spectroscopy confirmed the Cu2O2 species [4–10]. Guanidine ligands offer a facile variation of the substitution pattern [11] and allow the steering of formation and decay rates [12,13]. The complex used here is [Cu(btmgp)]I together with the corresponding bis(l-oxo)
⇑ Corresponding author at: Institut für Nanostruktur- und Festkörperphysik, Universität Hamburg, Notkestrasse 85, 22607 Hamburg, Germany. E-mail address: [email protected] (B. Grimm-Lebsanft).
dicopper(III) complex [14,15]. At 20 °C, the bis(l-oxo) species decays with a rate constant of 1 s1 whereas it is stable for days at 80 °C [16,12]. Since the early 1950s low temperature Raman measurements have been sought after [17–20] as they open up pathways to emerging properties of materials [21–25] at e.g. phase transitions. A longer stability of the sample can also be observed as damage by the incident laser beam is reduced [26]. This is especially valuable for understanding charge-transfer processes in bioinorganic chemistry where temperature-sensitive metal complexes are investigated and resonance Raman spectroscopy unveils bonding modes, crucial vibrations of the active sites and insights into the electronic structure [27]. However, when tuning the incident photon energies through the absorption bands, the sample decays much faster. Other groups use cooled nitrogen gas [28], liquid nitrogen [4], liquid-helium flow cryostats [29], or cooled spinning cells [30] to keep the samples stable for longer time periods. While our previous Raman measurements on similar model complexes have been successful at room temperature [24] and low temperatures as well, leaving the cuvette with the studied complex taken from the cold bath uncooled resulted in a stability of the complex of approximately 90 s until the Cu2O2 species disappeared due to the warm up process, thus making resonance Raman or other time-consuming measurements impossible [31].
http://dx.doi.org/10.1016/j.ica.2017.07.010 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.
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2. Experimental To overcome these experimental shortcomings of short-time stability while keeping the flexibility of the UT-3 Raman spectrometer [32] a new low-temperature cryostat was developed (see Fig. 1.). To avoid the usage of liquid gases and minimize vibrations, the cryostat uses a two-way cooling system to reach low temperatures. It consists of a liquid ethanol based closed-cycle Proline RP890 chiller (Lauda, Lauda-Königshofen, Germany) and a multistage Peltier module (TEC4-97-49-17-7-05, Thermonamic, China). The chiller flushes the copper block behind the Peltier module with cooled liquid ethanol. The Peltier module was glued to this copper block with a thin layer of Ceramique 2 heat paste (Arctic Silver, USA). On top of the Peltier module another small copper block was used for better heat transport. The cuvette is pressed against this small copper block with a holder for best thermal conductivity. The cap for the cuvette inlet has two pipe inlets which are used for operando oxygenation and the corresponding overpressure reduction inside the cuvette. With a vacuum pump a pressure of 2102 mbar inside the cryo was reached. A temperature of around 70 °C on the warm side of the Peltier module was reached when using the ethanol chiller only. When the Peltier module was turned on, the temperature behind the cuvette further decreases to around 150 °C which yields a temperature inside the cuvette below 60 °C after 15 min and below 80 °C after 55 min (see Fig. 2.) measured with a Pt100 temperature sensor inside the solution. This makes the cryostat suitable for many complexes. For all wavelengths two Tsunami Ti:Sapphire laser systems, model 3960C-15HP and model 3950-X1BB (both Spectra Physics Lasers Inc., California) were used. They were synchronized using a Lok-to-clock (LTC) system. The fundamental laser line was frequency doubled and tripled with a flexible harmonic generation (FHG) unit, model GWU2 23-PS (GWU-Lasertechnik Vertriebsges.
Fig. 2. Temperature measured inside the solution while cooling. With the setup a temperature below 60 °C was reached after 15 mins and below 80 °C after 55 mins.
Fig. 3. Two Tsunami Ti:Sa lasers were used in the resonance Raman study. The beam was widened by a spatial filter and then focused on to the cuvette inside the cryostat. Raman scattered light was captured with the entrance optics of the UT-3 triple monochromator spectrometer and recorded with a liquid nitrogen cooled CCD camera.
Fig. 1. Schematic setup of the cryostat. The cuvette is cooled by a small copper block (brown) on top of a multistage Peltier module (grey). The copper block (brown) on the backside of the Peltier module is cooled by liquid ethanol. During operation a pressure inside the cryostat of around 2102 mbar is reached. Lower pressures could be achieved but those are not suitable for cuvette operation. The oxygen is supplied via a pipe which leads directly into the cuvette. Overpressure is regulated by a one-way exhaust valve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mbH, Erfstadt, Germany). The laser beam was widened with a spatial filter and then focused on the cuvette inside the cryostat (see Fig. 3.). The focus spot size was around 20 lm in diameter. The used laser power in front of the entrance optics was, depending on the wavelength, between 8.6 mW and 23.7 mW. Pulse width was between 1.5 ps and 2.5 ps. All spectra were normalized to 1 s integration time and 1 mW laser power. Background was subtracted and the resulting spectra were corrected for the spectrometer sensitivity in the respective wavelength regions. The experiments were conducted in a clean room with constant temperature (20.0 °C ± 0.5 °C) and humidity (45% ± 3%). To determine the pulse width of the laser, a small part of the Tsunami fundamental was mirrored out using a glass plate and the reflex then coupled into an autocorrelator (AC) (APE GmbH, Berlin, Germany). For the measurements a custom made half-height Suprasil glass cuvette (Hellma Analytics, Müllheim, Germany) with 1.7 ml sample volume was used. 3. Sample preparation The copper complexes were prepared in an oxygen and water free atmosphere (<0.5 ppm) inside a LABstar glovebox (MBraun,
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Garching, Germany) with a concentration of 15 mM in 99% propionitrile (Sigma–Aldrich Chemie GmbH, Germany). The solvent was degassed by repetitive ‘‘freeze–pump–thaw” cycles and dried over a 3 Å molecular sieve, as we found a huge influence of the water content of the solvent on the distinct oxygen based vibration. After cooling the precursor solution to below 80 °C, oxygen was run through the sample until the complex was formed. This can be observed by a distinct change in color of the solution. The cryostat was then placed inside the entrance optics of the UT-3 Raman spectrometer. With a micrometer screw a focal depth of around 50 lm inside the cuvette was adjusted. 4. Results and discussion Upon oxygenation of the precursor [Cu(btmgp)]I at below 80 °C, the solution turns from yellowish to a dark red. This change can be distinctly seen in the Raman spectra (see Fig. 5a.) as compared to the spectrum of the precursor (green) a sharp peak at around 596 cm1 rises for the oxygenated complex (blue) which can be identified as the breathing mode of the Cu2O2 rhomb of the complex (see Fig. 4). With its 596 cm1 it fits well into similar bisl-oxo dimer copper cores [33–36]. To verify the oxygen-based vibration mode isotope substitution was performed (see Fig. 5b.). Instead of 16 O2 ; 18 O2 (Campro Scientific, Berlin) was used for oxygenation. A shift of the 596 cm1 peak to 571 cm1 or a delta of 25 cm1 was observed. Also some 16O2 residue can still be seen in the 18O2 spectrum which resulted from not completely flushing the pipes with 18O2 before oxygenation. Other publications show similar 16O2/18O2 shifts for bis(l-oxo) dicopper complexes [38]. The Gaussian [39] DFT calculations (see Fig. 5c.) with TPSSh [40]/def2-TZVP[41] and empirical dispersion correction with Becke-Johnson damping [42–44] predict this socalled breathing mode at 619 cm1 (18O2: 591 cm1) with an isotope shift of 28 cm1 and are thus in excellent agreement with the obtained experimental value. A resonance Raman study was conducted with eleven separate incident laser wavelengths (250.2 nm, 282.4 nm, 333.2 nm, 359.7 nm, 364.5 nm, 369.9 nm, 375.1 nm, 380.7 nm, 399.9 nm, 423.7 nm and 500.2 nm). As can be seen in Fig. 6., the first part of the spectrum up to 550 cm1 is dominated by solvent and glass
Fig. 5. a) Raman spectra of the [Cu(btmgp)]I (green) and the oxygenated sample (blue). One can clearly see a peak appearing at 596 cm1 which can be assigned to the breathing mode of the Cu2O2 rhomb of the complex. Stars denote solvent peaks. Different glass signal in the range of 320 cm1 to 520 cm1 due to slightly different cuvette positions can be observed. b) Raman spectra of [Cu(btmgp)]I oxygenated with 16O2 (blue) and 18O2 (red). A blue shift of 25 cm1 is observed. Stars denote solvent peaks. Spectra were normalized to the 541 cm1 solvent peak respectively for better comparison. c) Unscaled DFT spectra for [Cu(btmgp)]I with 16O2 (blue) and 18O2 (red) show a shift of 28 cm1 which is in excellent agreement with the experiment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
signal of the cuvette. The ratio between the solvent peak at 541 cm1 and the 596 cm1 mode changes only slightly for different incident wavelengths. In Fig. 7. we show the normalized peak intensities I0 of the 596 cm1 vibration. They were normalized using the incident photon energy dependent absorbance to I0 ¼ I ð1 expð2:3 c dÞÞ , with the concentration c = 15 mM, the optical path length d = 100 lm and the extinction coefficient. This yields a distinct resonance at around 370 nm (3.35 eV) incident photon energy (error bars included in the figure). Also a second resonance around 358 nm (3.46 eV) can be seen. In the 400 nm region we see a resonance shoulder which is supported by the components of the absorption curve. The possible double resonance at 370 nm and 358 nm has an energy difference of at least 110 meV. This can be explained by ingoing and outgoing resonance with 2 hxv ib 148 meV of the 596 cm1 vibration [45]. Noise in the spectra was four orders of magnitude lower than the errors of the fits and thus neglected. In the resonance region (350 nm to 380 nm) several measurements have been performed to validate the peak intensities. Additional errors can result from slightly different concentrations of the sample, different focal depths inside the cuvette as well as different cuvette position inside the cryostat. The errors for all remaining incident photon energies were approximated to take these variations into account. 1
Fig. 4. Molecule structure of [Cu2O2(btmgp)2]2+ and the calculated eigenmodes of the Cu2O2 breathing mode [37].
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Raman spectroscopy as well as isotope substitution with a peak shift supported by DFT calculations. Also, first Raman spectra on copper complexes with an aromatic bis(guanidine) ligand measured with this setup have been published [46]. Future applications of the setup can be transient Raman, fluorescence or transient absorption spectroscopy of liquid samples. Acknowledgements We acknowledge the financial support by DFG FOR1405 ‘‘BioCTDyn”. References
Fig. 6. Resonance Raman study of 16O2 oxygenated [Cu(btmgp)]I for ten separate wavelengths. An offset was added to each spectrum for better visibility. Stars denote solvent peaks. Only very weak peaks were seen in the 250 nm spectrum (not shown).
Fig. 7. Normalized intensities with error bars (red) of the 596 cm1 peak shown against the incident photon energy. Peak intensities have been normalized to the wavelength dependent absorption of the sample (grey). A distinct resonance behaviour can be seen at 370 nm and 358 nm. Also a resonance shoulder can be observed at 400 nm. Black lorentzians show the fit of the absorption spectrum.
5. Conclusion and outlook In conclusion, we have presented a new cryostat for operando oxygenated resonance Raman measurements at the UT-3 Raman spectrometer which does not need cold gases like nitrogen or helium. The temperature sensitive samples have shown a longterm stability of the bis(l-oxo) species for several days when oxygenated below 80 °C. This has enabled a detailed characterisation of the used bis(l-oxo) dicopper(III) complex using resonance
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