10. 6 Photochemistry. Out-class reading: Levine, pp photochemistry

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1 Out-class reading: Levine, pp photochemistry

2 6.1 Brief introduction of light 1) Photochemistry The branch of chemistry which deals with the study of chemical reaction initiated by light. 2) Energy of photon The photon is quantized energy: light quantum h h C hc Where h is the Plank constant, C the velocity of light in vacuum, the wave-length of the light, and the wave number.

3 6.1 Brief introduction of light

4 6.1 Brief introduction of light 4) Interaction between light and media di I adx I I 0 exp( ax) Lambert s law I a I [1 0 I I0 exp( ax)] I: intensity of light, x: the thickness of the medium, a: the absorption coefficient. Beer s law: I exp( ) a I0 cx

5 6.2 Physical processes of Excitation and decay (1) Photoexcitation: Upon photo irradiation, the molecules or atoms can be excited to a higher electronic, vibrational, or rotational states. A + h A * The lifetime of the excited atom is of the order of 10-8 s. Once excited, it decays at once. IR spectrum

6 A. Jablonski, Efficient of Anti-Stokes Fluorescence in Dyes, Nature, 1933, Jun. 10, Physical processes of Excitation and decay (1) Photoexcitation: The Foundation of a typical Jablonski Diagram

7 6.2 Physical processes of Excitation and decay (1) Photoexcitation: Absorbance Vibrational Relaxation and Internal Conversion Intersystem Crossing Fluorescence Phosphorescence Jablonski diagram

8 6.2 Physical processes of Excitation and decay (1) Photoexcitation: Radiation-less decay Which is which?

9 6.2 Physical processes of Excitation and decay (1) Photoexcitation: Transition Time Scale Radiative Process? Internal Conversion s no Vibrational Relaxation s no Absorption s yes Phosphorescence s yes Intersystem Crossing s no Fluorescence s yes

10 (2) Decay of photoexcited molecules 6.2 Physical processes of Excitation and decay Radiation transition Fluorescence and phosphorescence decay non-reactive decay reactive decay Radiationless transition Vibrational cascade and thermal energy Reaction of excited molecule A * P Energy transfer: A * + Q Q * P

11 6.3 Photochemistry (1) The first law of photochemistry: Light must be absorbed by a chemical substance in order to initiate a photochemical reaction. Grotthuss and Draper, 1818:

12 6.3 Photochemistry (2) The second law of photochemistry / The law of photochemical equivalence One quantum of radiation absorbed by a molecule activates one molecule in the primary step of photochemical process. Einstein and Stark, 1912 E = h F-F = Lh = J mol -1 one einstein

13 6.3 Photochemistry primary step of photochemical process: A chemical reaction wherein the photon is one of the reactant. S + h S * h What is the nature of activation energy of a photochemical reaction?

14 6.3 Photochemistry The primary photochemical process: S + h S * Some primary photochemical process for molecules

15 6.3 Photochemistry Secondary photochemical process Energy transfer: A * + Q Q * donor acceptor Q * P (sensitization), A * :sensitizer Q * +A (quenching), Q:quencher Photosensitization, photosensitizers, photoinitiator

16 6.3 Photochemistry About multi-proton absorption Under high intensive radiation, absorption of multi-proton may occur. A + h A * A * + h A ** Under ultra-high intensive radiation, SiF 6 can absorb 20~ 40 protons. These multi-proton absorption occur only at I = photon s -1 cm -3, life-time of the photoexcited species > 10-8 s. Commonly, I = ~ photon s -1 cm -3, life-time of A * < 10-8 s. The probability of multi-photon absorption is rare.