Supporting Information for: Effects of Temperature and Concentration on the Rate of Photo-bleaching of Erythrosine in Water Joshua K. G. Karlsson, Owen J. Woodford, Roza Al-Aqar and Anthony Harriman* Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom. Corresponding author email address: anthony.harriman@ncl.ac.uk Contents 1. Experimental Details 2. Steady-State Spectroscopic Characterization of Erythrosine 3. Spectroscopic Characterization of the Erythrosine Triplet 4. NMR Characterization Data 5. Overlaid spectra for Erythrosine Photo-bleaching with Monochromatic Light 6. Effect of Light Intensity on the Rate of Photo-bleaching 7. Photo-bleaching of Erythrosine in the Presence of Furfuryl Alcohol 8. Comparison of Photo-bleaching in H 2 O and D 2 O 9. Effect of Dissolved Oxygen 10. Tables of Statistical Data for Kinetic Fits 11. References 12. Bleaching in Deoxygenated Solution S1
1. Experimental Details Erythrosine (also known as Erythrosine B but not to be confused with Erythrosine Y) was donated by Procter and Gamble Co., the purity of which was verified with 700 MHz 1 H and 13 C NMR and then used without further purification. Absorption and emission spectra were recorded on Perkin-Elmer Lambda 35 and Hitachi F-4500 spectrophotometers, respectively. Fluorescence quantum yields were measured against Rhodamine 6G ( F = 94% in ethanol) 1 using optically dilute solutions to avoid re-absorption effects and with a suitable glass cut-off filter being used to exclude stray excitation light. Excited-singlet state lifetimes were recorded using the time-correlated, single photon counting method with a PTI EasyLife-2 instrument, where the excitation source is a 535 nm diode laser. Transient absorption measurements were performed using laser flash photolysis methods with an Applied Photophysics Ltd. LKS-70 setup. The pump laser wavelength (FWHM = 4 ns) was set to 500 nm using an optical parametric oscillator. Samples were deaerated by purging with dry N2 for at least 10 minutes before starting the measurements. Triplet lifetimes were determined at fixed wavelength by averaging at least 10 individual decay records. Photo-bleaching studies were recorded using a bespoke optical rail setup (Figure S1). Illumination was provided by a 523 nm LED (Mouser Electronics) operating at 8 W. A lens was used to focus light into the centre of a 1 cm cuvette at a distance of 20.5 cm from the light source. The temperature was controlled using a heater block connected to a circulating water bath. Stock solution samples of Erythrosine in deionized water were placed in a 1 cm quartz cuvette with a magnetic stirrer to ensure proper mixing during illumination. The progress of photo-bleaching was monitored by recording absorption spectra at regular time intervals (Figures S6-S12). By running multiple repeats of the photo-bleaching experiments, it was found that the light source produced consistent results within a ±5% margin of error. For the temperature-dependent photo-bleaching experiments, samples were equilibrated with air. Samples were typically bleached until less than 25% of the lowest energy absorption band remained. Figure S1. Illustration of the steady-state photo-bleaching apparatus. LED (A), lens (B), heater block/sample holder (C), solar cell detector (D). The solar cell detector was not used for this work but offers a convenient means of recording changes in absorbance with a datalogger. S2
The spot size of the focused LED beam incident on the sample was characterized by using filter paper pre-soaked in Erythrosine solution and dried. Positioning the dyed filter paper in place of a sample cuvette bleached a spot on the paper to the dimensions of the beam. The optical power incident on the sample was measured with a silicon photodiode detector where the output current was corrected for the response curve of the detector. Figure S2. Characteristics of the focused green LED beam incident on the sample. S3
2. Steady-State Spectroscopic Measurements made with Erythrosine Figure S3. Absorption and fluorescence spectra recorded for Erythrosine in deionized water. The molecular formula is given as an inset. 2a. Erythrosine Beer-Lambert Plot Figure S4. Beer-Lambert plot determining the molar absorption coefficient for Erythrosine dissolved in deionized water. S4
3. Spectroscopic Characterization of the Erythrosine Triplet state Figure S5. (a) Transient absorption spectra of Erythrosine in water at 35, 43, 50, 59, 72, 84 120 and 130 s. Inset triplet decay traces recorded at 590 nm. (b) Normalized emission spectra of Erythrosine recorded at 77K in an 80:20 mixture of ethanol-ethylene glycol. Fluorescence trace is in red and phosphorescence in black (isolated by way of using a signal chopper). S5
4. NMR Characterization Data Figure S6. 700 MHz 1 H NMR spectrum recorded for Erythrosine in D2O. Figure S7. 700 MHz 13 C NMR spectrum recorded for Erythrosine in D2O. S6
Figure S8. 700 MHz 1 H NMR spectra of Erythrosine in D2O before and after irradiation with 523 nm light. The sample was irradiated until approximately half of lowest energy transition of Erythrosine was depleted. S7
5. Overlaid Spectra for Erythrosine Bleaching under Monochromatic Light Figure S9. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 195 min) with 523 nm green LED light at 10 o C. Figure S10. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 135 min) with 523 nm green LED light at 20 o C. S8
Figure S11. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 120 min) with 523 nm green LED light at 30 o C. Figure S12. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 105 min) with 523 nm green LED light at 40 o C. S9
Figure S13. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 90 min) with 523 nm green LED light at 50 o C. Figure S14. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 70 min) with 523 nm green LED light at 60 o C. S10
Figure S15. Overlay of absorption spectra for Erythrosine steady-state photo-bleaching (between 0 and 60 min) with 523 nm green LED light at 70 o C. S11
6. Effect of Light Intensity on the Rate of Photo-bleaching 0.018 0.016 0.014 0.012 0.01 k 1 0.008 0.006 0.004 0.002 0 0 20 40 60 80 100 LED power (%) Figure S16. Initial photo-bleaching rate of Erythrosine (k1) in deionised water as a function of light intensity (recorded at 20 o C). Red squares: dye concentration ~ 20 M, Blue squares: dye concentration ~ 3 M. S12
7. Photo-bleaching of Erythrosine in the Presence of Furfuryl Alcohol Figure S17. Overlay absorption spectra for Erythrosine steady-state photobleaching with 523 nm green LED in the presence of 0.05 M furfuryl alcohol. The inset shows a typical kinetic plot. S13
8. Comparison of Photo-bleaching in H2O and D2O Figure S18. Stepwise bleaching of Erythrosine in water (red circles) and D2O (black squares) under identical conditions. The rate of photo-bleaching is accelerated approximately by a factor of two in the presence of D2O. 9. Effect of Dissolved Oxygen Figure S19. Effect on the rate of photo-bleaching where Erythrosine water solution is saturated with oxygen. For comparison the bleaching rate in air equilibrated solution (black squares) versus two repeats of photo-bleaching in oxygen saturated water solution (red circles and blue triangles). Data recorded at 20 o C. S14
10. Tables of Statistical Fits Table S1. Fit of Experimental Data to Equation 1 for Various Initial Concentrations of Erythrosine at 20 0 C. [ER]0 / M IABS / % kf / min -1 [ER]0 / M (a) CC (b) 0.91 14.5 0.0216 0.89 0.99841 3.03 40.7 0.0138 2.92 0.99837 6.77 69.0 0.0136 6.80 0.99973 10.8 84.5 0.0112 11.1 0.99921 19.5 96.6 0.0090 20.5 0.99707 (a) Initial concentration of Erythrosine derived from the intercept to Equation 1. (b) Correlation coefficient from non-linear, least-squares fit. Table S2. Effect of Incident Light Intensity on the Rate of Photo-bleaching of Erythrosine at 20 0 C for an Initial Concentration of ca. 3 M. Intensity / % kf / min -1 [ER]0 / M (a) CC (b) 100 0.0156 3.00 0.99921 72 0.0121 2.98 0.99995 51 0.0090 2.90 0.99862 35 0.0064 2.95 0.99805 21 0.00395 2.96 0.99587 (a) Initial concentration of Erythrosine derived from the intercept to Equation 1. (b) Correlation coefficient from non-linear, least-squares fit. Table S3. Effect of Incident Light Intensity on the Rate of Photo-bleaching of Erythrosine at 20 0 C for an Initial Concentration of ca. 22 M. Intensity / % kf / min -1 [ER]0 / M (a) CC (b) 100 0.00668 21.76 0.99718 72 0.00524 22.00 0.99908 51 0.00365 21.75 0.99933 35 0.00241 21.80 0.99983 21 0.00169 21.70 0.99051 (a) Initial concentration of Erythrosine derived from the intercept to Equation 1. (b) Correlation coefficient from non-linear, least-squares fit. S15
11. Photobleaching with deoxygenated aqueous solutions 0.0 ln (Absorbance) -0.2-0.4-0.6-0.8-1.0-1.2-1.4 0 20 40 60 80 100 120 140 Time / mins Figure S20. Comparison of photobleaching kinetics recorded for Erythrosine in water under illumination from a green ( = 523 nm) LED. The black solid points refer to air-equilibrated solution while the remaining data sets refer to deoxygenated solution. S16
11. References S1. Fischer, M.; Georges, J. Fluorescence Quantum Yield of Rhodamine 6G in Ethanol as a Function of Concentration Using Thermal Lens Spectrometry. Chem. Phys. Lett., 1996, 260,115-118. S17