1. Chapter 12 Laser-Induced Chemical Reactions 1

2. Contents  Chapter Overview  Organic Chemical Syntheses  Organic Photochemistry  Lasers as a Photochemical Tool  Case Study: Photoisomerization Of Carvone 2

3. Chapter Overview  Laser excitation sources offer the synthetic chemist exceptional control over the outcome of photochemical reactions. The use of lasers in synthetic applications can reveal microscopic details of reaction pathways. 3 laser parameters: Wavelength Tunability Monochromaticity, Intensity Mode of operation (CW vs. pulsed) Precise selections of the reactant(s) to be photoexcited The excited state(s) to be generated The product(s) to be observed

4. Organic Chemical Syntheses The ultimate objectives of an organic synthesis: The preparation of a naturally occurring or physiologically active compound The manufacture of an industrially important material, The development of a new process The generation of a novel structure of theoretical interest or use in reactivity and structural studies. 4 Ordinary chemical reactions rely on the thermal energy of colliding reactant particles to provide the necessary energy of activation.  thermodynamically favorable transition from  Kinetic of the distribution of possible thermodynamically favored products  activation enthalpies, entropies, and kinetic rate constants

5. Organic Photochemistry 5 The principal basis for this preparative scheme is the initiation of chemical reactions involving organic molecules via the absorption of visible or ultraviolet light. Photochemical reactions generate energetic, albeit short-lived, excited states which on thermodynamic grounds provide for a greater range of potential final products. The differing electron configurations of ground and excited states may alter the observed product distribution, widening synthetic possibilities. An excited-state molecule might undergo isomerization, internal rearrangement, or fragmentation (unimolecular processes), or reaction with solvent or solute to produce new chemical species (multimolecular processes). Competing photophysical reactions which dissipate the energy of the excited state: Radiative processes (e.g., fluorescence and phosphorescence) Nonradiative processes (e.g., internal conversion, intersystem crossing, thermal deactivation)

6. The Laser as a Photochemical Tool 6 Significant advantages for the use of lasers in photochemical studies: Monochromaticity Wavelength control (tunability ) High intensity Spatial coherence (low divergence )

7. The Laser as a Photochemical Tool 7 Significant advantages for the use of lasers in photochemical studies: Wavelength control (tunability ) to avoid the simultaneous irradiation of reactants and products or to irradiate a single component in a mixture of reactants laser isotope separation techniques.

8. The Laser as a Photochemical Tool 8 Significant advantages for the use of lasers in photochemical studies: High intensity initiating multiphoton reactions molecule may successively absorb additional photons before the initial excited state is deactivated.

9. The Laser as a Photochemical Tool 9 Significant advantages for the use of lasers in photochemical studies: Spatial coherence (low divergence) Photochemistry in hostile environments, such as flames and plasmas, as well as in systems at significant distances from the experimenter, is readily feasible

10. Case Study: Photoisomerization of Carvone 10 Objective. To illustrate the effectiveness of laser monochromaticity, tunability, intensity, and mode of operation (CW vs. pulsed) in governing the efficiency, yield, and product composition of a photochemical reaction. Laser Systems Employed. A pulsed XeF excimer laser with 350 nm output, a pulsed third-harmonic YAG laser with 354 nm output, a pulsed XeCl excimer laser with 308 nm output, and a continuous wave krypton ion laser with both 350.7 and 356.5 nm output.

11. Case Study: Photoisomerization of Carvone 11 Role of the Laser Systems. To enable selective excitation of a single reactant in a photochemical reaction and sufficient generation of a particular excited state to control the product composition of the photochemical reaction. Useful Characteristics of the Laser Light for this Application. Monochromaticity, wavelength tunability, intensity, mode of operation (CW vs. pulsed). Principles Reviewed. Photolysis, multiphoton processes, selective excitation, singlet and triplet states, photoisomerization, reaction yield. Conclusions. This study demonstrates that by using lasers of appropriate wavelength, intensity, and mode of operation (i.e., pulsed or continuous), careful control is possible of the yields of the two products resulting from the photoisomerization of carvone.

12. Case Study: Photoisomerization of Carvone 12 Early Studies  carvone (I) to carvone-camphor (II) with sunlight for periods of up to one year (yield 9%) was first observed by Ciamician and Silber in 1908. 44% yield of II after irradiation of carvone for 4 days. A higher intermediate concentration of II is observed with black-light irradiation (l max ≈ 355 nm) than with irradiation via the high-pressure mercuryvapor lamp l max ≈ 310 nm). from photoaddition of the solventintramolecular [2+2] photoaddition of two double bonds

13. Case Study: Photoisomerization of Carvone 13 Laser-Induced Investigations Impact of Laser Monochromaticity and Tunability Selective excitation of a single reactant within a mixture of reagents is very important. So laser could be very useful. Zandomeneghi et al. in 1980 used a powerful continuous wave krypton ion laser to irradiate carvone. The aim was to irradiate the long wavelength tail of the absorption band of I and avoid the absorption band of the cycloadduct II. Within 1.5 hours, 88% of carvone was converted to II, with only an 8% yield of III.

14. Case Study: Photoisomerization of Carvone 14 Laser-Induced Investigations Impact of Tunability and Intensity To establish the mechanism of the photoisomerization reaction of carvone, two independent studies investigated the photolysis using four different laser sources (table) Laser/Wavelength (nm) ModeIIIIII XeF/350Pulsed94.42.13.5 YAG(3v)/354Pulsed96.12.31.6 Kr ion/350.7, 356.5CW81.79.39.0 XeCl/308Pulsed982.0

15. Case Study: Photoisomerization of Carvone 15 Laser-Induced Investigations Impact of Tunability and Intensity To account for the differences observed with pulsed vs. CW laser excitation at 350 nm, similar mechanisms for the overall system involving multiphoton processes were proposed by both research groups:  pulsed irradiation at either 308 or 354 nm can form a second excited triplet

16. Case Study: Photoisomerization of Carvone 16 Laser-Induced Investigations Impact of Tunability and Intensity