Photochemical Reactivity of Dyes. Transient Absorption Spectroscopy As a Tool

1. Photochemical Reactivity of Dyes. Transient Absorption Spectroscopy As a Tool to Probe Fast Chemical Events Prashant V. Kamat Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 http://www.nd.edu/~pkamat

2. Outline • I. Reactive Intermediates and Fast Kinetic Spectroscopy Techniques • Time-resolved photochemistry • Detection of singlet and triplet excited states using picosecond and nanosecond laser flash photolysis • Radiolysis • Gamma radiolysis and product identification • Pulse radiolysis for spectral characterization and kinetic evaluation • II. Photochemistry of Dyes in Surfactant Solution • Dye aggregation • Triplet-triplet energy transfer processes • Excited state interactions • Sensitization of Semiconductor Surfaces

3. Reactive Intermediates • What are they? • Reactive intermediates are short-lived chemical species that interact with other molecules. • Singlet and triplet excited states • Excited state charge transfer complex • Radical anions and radical cations • Trapped charge carriers • What are the reaction pathways? • Energy transfer in the excited state • Electron transfer to initiate chemical transformation • Dimerization, polymerization, fragmentation,hydrolysis, etc. • Why are they important? • Understanding the problems associated with photostability and degradation mechanism • Improving the stability of the molecules in heterogeneous media

4. Study of Reactive Intermediates Electrochemistry/ESR Mechanistic and Kinetic Aspects of Excited State and Radical Reactions Radiolysis Photochemistry Product Analysis

5. Experimental Techniques • Fast Kinetic Spectroscopy (Pump-Probe Method) • Picosecond and Nanosecond Laser Flash Photolysis • Pulse Radiolysis (Radiation Induced Processes) • Diffuse Reflectance Spectroscopy • In-situ photolytic studies of opaque samples • UV-VIS, FTIR and Emission Spectroscopy • Electrochemistry, Spectroelectrochemistry, Sonochemistry, g-radiolysis and Analytical Techniques Pulsed Laser Probe Probe Pulsed Laser Sample To Detector (or e-pulse) Detector Sample a. Laser flash photolysis (or pulse radiolysis) b. Diffuse reflectance laser flash photolysis

6. Dt pump probe It Probe Laser I0 Pump H2O/D2O cell Spectrograph/ Detector Optical delay rail Picosecond Laser Flash Photolysis • The chemical events in these experiments are initiated by an ultrafast laser pulse (pump) and the photophysical and photochemical events are probed by another ultrafast laser probe pulse. (Mode-locked, Q-switched Continuum YG-501 DP Nd:YAG laser, pulse width ~18ps). • Provides vital information on the mechanistic and kinetic details of chemical events that occur in the timescale of 20 picoseconds to 10 nanoseconds. • The white continuum picosecond probe pulse is generated by passing the fundamental output through a D2O/H2O solution. An optical delay rail employed to control the delay time of the probe pulse enables detection of transients at desired time intervals after the sample excitation..

7. S2 probe S1 pump T1 hn hn’ S0 What is a difference absorption spectrum? DA=Aex-A0 A0 Aex FS+FT+FNR = 1 FS= kf/(kf+knr) Singlet Excited State Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye. • The transient absorption recorded immediately after the laser pulse excitation corresponds to singlet excited state • Triplet excited states accumulate at longer times. • Singlet excited state has a shorter lifetime of 420 ps while triplet excited state has a lifetime of ~10 ms. • The singlet excited lifetimes can also be determined from the emission measurements.

8. S2 T2 S1 T1 hn hn’ DA time S0 Nanosecond Laser Flash Photolysis Since T1 S0 is a forbidden transition the triplet excited states are long-lived. Triplet excited molecules undergo diffusion controlled electron transfer reactions with other solutes. • Nitrogen laser (337 nm / 6 ns) • kinetic absorption spectroscopy • fluorescence lifetimes • 2-pulse experiments • Excimer laser (308 nm / 20 ns) • kinetic absorption spectroscopy • 2-pulse experiments • YAG laser (266, 355, & 532 nm/ 6 ns) • kinetic absorption spectroscopy • fluorescence lifetimes • microwave conductivity • diffuse reflectance • 2-pulse experiments Time-resolved Raman Spectrometer

9. T2 S1 T1 hn S0 Triplet Excited State TH+ + hn1TH+* 3TH+* 3TH+*  TH+ 3TH+* + ZnO  TH•2+ + ZnO(e) Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye. The reactivity of triplet excited thionine can be established using laser flash photolysis. The dye molecules participate in the electron transfer with ZnO colloids. Photoinduced electron transfer processes play an important role in determining the stability of dyes in different environments.

10. Radiolysis of Water H2O —^^^ OH•, H•, eaq, H+, H2O2, H2 G(X) = number of molecules of X/100 eV absorbed G(eaq)=G(•OH)= 2.7 G(H) =0.6; G(H2) =0.45; G(H2O2) = 0.7 At pH4, OH• and eaq are the major reactive species that survive during the ionization of water Reductive Conditions: ……….alcohol as a hydroxyl radical scavenger (CH3)3-COH + •OH — (CH3)2-•CH2-COH + H2O (k=6.0x108 M-1s-1) Oxidative Conditions: ……….N2O as an electron scavenger eaq + N2O + H2O — N2 + OH•+ OH- (k=9.1x109 M-1s-1) Secondary Oxidizing Radicals: OH•+ N3- — N3•+ OH- (k=1.2x1010 M-1s-1) eaq + S2O82-— SO4•-+ SO42-

11. Gamma Irradiators • The short-lived reactive intermediates of water radiolysis for low LET radiation (g- or X-rays with energies above 30 keV) are eaq, •H and •OH. • In the presence of oxygen, hydrated electrons and H atoms are converted into O2- and HO2. • •OH  H+ + O- (pKa 11.9) • HO2 H+ + O2- (pKa 4.9) • By adjusting the pH and O2 concentration one can produce eaq, •H, •OH, O2-, O- and HO2• species NDRL has three cobalt-60 gamma irradiators, with radiation intensities of about 2, 6 and 20 kilocuries, respectively. These sources are programmable to give exposures ranging from minutes to days. After irradiation, samples can be analyzed by a variety of methods, including optical and infrared absorption spectroscopy, high-performance liquid chromatography, ion chromatography and mass spectrometry.

12. t, min a 0 b 5 c 15 d 40 e 60 f 90 O S NH N=N 3 2 SO 3 Reaction with Hydroxyl Radicals Radiolysis of 5mM Acid Yellow 9 solution in N2O saturated aqueous solution Four major products were identified from the Electron spray mass spectral analysis of the reaction mixture. Das, Kamat, Padmaja, Au, Madison, J. Chem. Soc. Perkin Trans. 2, 1999, 1219-1224

13. Electron beam Pulse Radiolysis Linear accelerator characteristics Nominal beam energy: 8 MeV RF source: 20 MW, 2856 Mhz klystron Pulse duration: 2 to 100 nanosec, 1.5ms Pulse frequency: 1 to 60 Hz Maximum beam current: 4 amps Nominal beam diameter: 5 mm Pulse-to-pulse dose stability: ±1% Manufacturer: Titan Beta, Dublin CA • An 8 MeV linear electron accelerator is the experimental centerpiece of the radiation chemistry effort. This instrument is capable of delivering pulses of electrons ranging from 1 nanosecond to 1.5 microseconds in duration. These pulses are delivered to a sample cell where they ionize molecules in the sample, a process called pulse radiolysis. • The ions and electrons rapidly recombine, but in the process produce large quantities of free radicals. If the sample is an aqueous solution, the radicals produced in greatest quantities are the hydroxyl radical (•OH), the hydrogen atom and the hydrated electron (eaq–). • The free radicals react with molecules dissolved in the water to produce the chemical species that are the subject of our studies.

14. Dt, ms a 2 b 5 c 8 d 16 500 nm 370 nm Reaction with Sulfate Radical Anions eaq + S2O82-— SO4•-+ SO42- dye + SO4•-—dye •++ SO42- k= 11010 M-1s-1 370 nm Acid Yellow 9 in water at pH 7 500 nm pKa 5.5 Das, Kamat, Padmaja, Au, Madison, J. Chem. Soc. Perkin Trans. 2, 1999, 1219-1224

15. Scope of Future Research Time-resolved transient studies of hair colorants • Primary photochemical events • Characterization of singlet and triplet excited states • (Spectra, lifetimes, quenching rate constants, pKa) • Photochemistry in heterogeneous media • Effect of surfactants, polymers, colloids and proteins • (dye aggregation effects, excited state properties) • Photostability of dyes during long term exposure • Wavelength and energy dependence • Product analysis • Reactivity of dyes with oxidizing and reducing radicals • Spectral characterization of transients using pulse radiolysis • Kinetics and mechanistic details • Product analysis • Influence of heterogeneous media on the reactivity of dyes