ANINDYA DUTTA Lectures 1 and LECTURE 2.pptx

GOOD FOR COLLEGE

1. CH 806: Molecular Photochemistry

2. Introduction 3 parts, 9 classes each 1 st part: concludes with the class on February 1 Topics to be discussed in the 1 st part: Introduction to and techniques of electronic spectroscopy Fluorescence quenching F ӧ rrster Resonance Energy Transfer (FRET) Excited State Proton and Electron Transfer processes Thermally activated delated fluorescence (TADF) Singlet fission Evaluation for the 1 st part: Examination: 20 marks Term paper: 14 marks We will distribute topics for term paper. Read at least 6 papers published in the related area, of which at least 3 should have been published AFTER 2018 . Upload a 5 page (Times New Roman, writeup based on your study this paper at “ AD Project report ” link in Moodle, by March 10 Mark distribution: Coverage: 3, Clarity and coherence: 5, Summary (own thought) : 3, Critique: 3

3. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO

4. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO     state  

5. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO     state  

6. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO         state   state  

7. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO         state   state  

8. Electron Configuration: Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       Electronic states:   HOMO HOMO-1 LUMO             Energy depends on configuration including spin state   Room temperature: Only is populated   state   state   state   state  

9. Molecular orbitals, electronic configurations and electronic states Formaldehyde H H O   *       HOMO HOMO-1 LUMO   Singlet and triplet states

10. Spin selection rule:   Selection rules: allowed and forbidden transitions Formaldehyde H H O   *         HOMO HOMO-1 LUMO             × state   state   state   state  

11. Spin selection rule:   Selection rules: allowed and forbidden transitions Formaldehyde H H O   *         HOMO HOMO-1 LUMO             × Orbital selection rule: symmetry of spatial wavefunctions × state   state   state   state  

12. Spin selection rule:   Breakdown of Selection rules : partially allowed transitions Formaldehyde H H O   *         HOMO HOMO-1 LUMO             Orbital selection rule: symmetry of spatial wavefunctions Spin-orbit coupling Vibronic coupling Less stringent state   state   state   state  

13. Spin selection rule:   Lambert-Beers law: quantification of brightness of colour Orbital selection rule: symmetry of spatial wavefunctions Spin-orbit coupling Vibronic coupling Less stringent               = molar concentration in cm             Absorbance Molar extinction/ absorption coefficient Probability of transition Type of transition Fully allowed 3 to 5 Spin allowed, orbitally forbidden 0 to 3 Orbitally allowed, spin forbidden -5 to 0 Type of transition Fully allowed 3 to 5 Spin allowed, orbitally forbidden 0 to 3 Orbitally allowed, spin forbidden -5 to 0 Unit: M -1 cm -1 https://tinyurl.com/UVspectra

14. Solvent effect   *     Protonation: vanishes   Polar (especially protic) solvents: : hypsochromic (blue) shift : small bathochromic (red) shift   Charge transfer (CT): Donor (D) Acceptor (A) Low energy Broad, structureless Strong solvent effect Intramolecular/ intermolecular/ with solvent Metal ion complexes: MLCT, LMCT Visible and near infrared sensitive photorefractive polymers for holographic display applications - Scientific Figure on ResearchGate. Available from: https:// www.researchgate.net /figure/5-Typical-absorption-band-in-a-charge-transfer-CT-complex-Note-that-there-is-an_fig4_234436092 [accessed 22 Jun, 2021]

15. Fate of electronic excited states

16. Jablonski diagram Fluorescence and phosphorescence Fluorescence spectrum: Mirror image of absorption spectrum Absorption Fluorescence So T 1 S 1 S 2 heat 10 -15 sec 10 -9 sec 10 -6 to 1 sec 10 -12 sec 10 -9 sec 10 -13 sec Phosphorescence Kasha’s rule

17. How is the fluorescence emission spectrum recorded? Fluorescence emission and excitation spectra is fixed, is scanned. is recorded   What kind of spectrum do we get if is fixed, is scanned and is recorded?   Fluorescence excitation spectrum Map of the absorption spectrum unless there is Ground state heterogeneity Impurity

18. Emission quantum yield: Strength of emission 18 Emission lifetime: Slower decay: longer lifetime Stronger emission Time Log of intensity Fluorescence quantum yield and lifetime

19. Jablonski diagram Fluorescence and phosphorescence Fluorescence spectrum: Mirror image of absorption spectrum Absorption Fluorescence So T 1 S 1 S 2 heat 10 -15 sec 10 -9 sec 10 -6 to 1 sec 10 -12 sec 10 -9 sec 10 -13 sec Phosphorescence Kasha’s rule

20. Excited state processes LE l 1 NS l 2 GS Often, the second fluorescence peak is absent, as the NS state is nonfluorescent: Energy Gap Law Sometimes, only the Stokes shifted peak is observed In such cases, the fluorescence quantum yield of the normal peak is decreased F .I. l 1 l 2 wavelength Large Stokes Shift

21. Importance of excitation spectra: an example Barboza et al. J. Phys. Chem. C 2015 , 119 , 6152−6163 Crude Supernatant Recrystallized Always record excitation spectra Choi and coworkers . Phys. Chem. Chem. Phys. 2012 , 14 , 15677−15681 Khan, Vaidya , Mhatre , Datta , J. Phys. Chem. B 2016 120 , 10319-10326 ✗ ✓ 21

22. Acids are more acidic in the excited state, bases are more basic Phenols: Delocalozation of electrons on O - : Stabilization of the phenoxide ion - p * transition: Delocalization involves lower energy p orbitals Further stabilization of the phenoxide ion Electron donating groups Photoacidity and Photobasicity

23. Photoacidity and Photobasicity Acids are more acidic in the excited state, bases are more basic Phenols: Delocalozation of electrons on O - : Stabilization of the phenoxide ion - p * transition: Delocalization involves lower energy p orbitals Further stabilization of the phenoxide ion Electron donating groups Aromatic carboxylates: Withdraw p electrons from the ring High electron density on O - : Efficient proton abstraction - p * transition: p * electrons are easier to withdraw More efficient proton abstraction Electron withdrawing groups

24. An example: 2-naphthol pK a = 9.2, pK a * = 2! Absorption spectra pH titration curves Fluorescence spectra pH 3: protonated ground state, partially deprotonated excited state Acids are more acidic in the excited state J. R. Lakowicz , Principles of fluorescence spectroscopy, 3 rd ed. 2006

25. Photoisomerization p-p * transition Double bond in ground state, Single bond in excited state Cis-trans isomerization Viscosity dependent rates Retinal polyenes Bacteriorhodopsin Wikipedia, Introduction to Laser Spectroscopy, H. Abramczyk Elsevier

26. Strong polarity dependence Intra/intermolecular Protein markers Dye-sensitized solar cells Photoinduced Charge transfer Wikipedia, J. Mater. Chem. C, 2016,4, 2731-2743; Chem. Commun ., 2016,52, 1182-1185 Light-induced change in redox potential

27. Triplet states Heavy atom effect Long lived phosphorescence Photosensitizers Organic electronics Intersystem crossing Wikipedia; Annu . Rev . Phys. Chem . 2021. 72:617–40 Mater. Chem. Front., 2020,4, 589-596

28. No class on January 11 and 15, 29, Feb 1: Make up classes: January 10, 17, 24, 31. 6:00 PM-7:30 PM? Lab visit (optional): Priya Bhandari ( [email protected] ), Ankit Kumar ( [email protected] ) More about experiments: http://tinyurl.com/ADUFSlectures , Lectures 1 – 15 Announcements