Prashant V. Kamat Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556

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

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 Reactive Intermediates

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

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,  -radiolysis and Analytical Techniques Probe Sample Detector Pulsed Laser a. Laser flash photolysis (or pulse radiolysis) Probe Pulsed Laser Detector To Sample b. Diffuse reflectance laser flash photolysis Experimental Techniques (or e-pulse)

—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 D 2 O/H 2 O 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.. Picosecond Laser Flash Photolysis tt pump probe Optical delay rail Pump Laser Probe ItIt I0I0 H 2 O/D 2 O cell Spectrograph/ Detector

Singlet Excited State  S +  T +  NR = 1  S = k f /(k f +k nr ) S1S1 T1T1 S0S0 S2S2 h h ’ pump probe 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. Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye. A0A0 A ex  A=A ex -A 0 What is a difference absorption spectrum?

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 S1S1 T1T1 S0S0 T2T2 S2S2 h h ’ Time-resolved Raman Spectrometer Nanosecond Laser Flash Photolysis time AA Since T 1  S 0 is a forbidden transition the triplet excited states are long-lived. Triplet excited molecules undergo diffusion controlled electron transfer reactions with other solutes.

Triplet Excited State S1S1 T1T1 S0S0 T2T2 h Difference absorption spectrum recorded following 532 nm laser pulse excitation of thionine dye. TH + + h  1 TH +*  3 TH +* 3 TH +*  TH + 3 TH +* + ZnO  TH 2+ + ZnO(e) 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.

Radiolysis of Water H 2 O —^^^  OH, H, e aq, H +, H 2 O 2, H 2 G(X) = number of molecules of X/100 eV absorbed G(e aq )=G( OH)= 2.7 G(H) =0.6; G(H 2 ) =0.45; G(H 2 O 2 ) = 0.7 At pH  4, OH and e aq are the major reactive species that survive during the ionization of water Reductive Conditions: ……….alcohol as a hydroxyl radical scavenger (CH 3 ) 3 -COH + OH —  (CH 3 ) 2 - CH 2 -COH + H 2 O (k=6.0x10 8 M -1 s -1 ) Oxidative Conditions: ……….N 2 O as an electron scavenger e aq + N 2 O + H 2 O —  N 2 + OH + OH  (k=9.1x10 9 M -1 s -1 ) Secondary Oxidizing Radicals: OH + N 3  —  N 3 + OH  (k=1.2x10 10 M -1 s -1 ) e aq + S 2 O 8 2   —  SO 4   + SO 4 2 

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. Gamma Irradiators — The short-lived reactive intermediates of water radiolysis for low LET radiation (  - or X-rays with energies above 30 keV) are e aq, H and OH. — In the presence of oxygen, hydrated electrons and H atoms are converted into O 2  and HO 2. OH  H + + O  (pKa 11.9) HO 2  H + + O 2  (pKa 4.9) — By adjusting the pH and O 2 concentration one can produce e aq, H, OH, O 2   O  and HO 2 species

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

—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 (e aq –). —The free radicals react with molecules dissolved in the water to produce the chemical species that are the subject of our studies. Pulse Radiolysis Electron beam Linear accelerator characteristics Nominal beam energy: 8 MeV RF source: 20 MW, 2856 Mhz klystron Pulse duration: 2 to 100 nanosec, 1.5  s 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

 t,  s a 2 b 5 c 8 d 16 Reaction with Sulfate Radical Anions e aq + S 2 O 8 2   —  SO 4   + SO 4 2  dye + SO 4  —  dye  + SO 4 2  500 nm 370 nm Acid Yellow 9 in water at pH 7 pKa 5.5 k= 1  M -1 s -1 Das, Kamat, Padmaja, Au, Madison, J. Chem. Soc. Perkin Trans. 2, 1999, nm 500 nm

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