Introduction to Molecular Photophysics CHM 5175: Part 2.2 Introduction to Molecular Photophysics Ken Hanson MWF 9:00 – 9:50 am Office Hours MWF 10:00-11:00

Interaction of Light with Matter Rainbows Glasses Mirage Refractometer Moon Light Butterfly Wings Sea Shells Soap Bubbles Two-slit exp Holograms Shadow Blur Sand in Water Sunsets Reflection: Pigmented Color vs structural color structural color changes with angle Refraction: light travels at different speeds Diffraction: bending and spreading out of waves at small openings UV-Vis Fluorometry TA Solar Cells

Interaction of Light with Matter Narrowing Our Focus Absorption/Transmission Visible spectrum Electronic Transitions- electrons excited from one energy level to another. Atomic Molecular Materials Visible Light (hn) Electronic excitation Sample

Hydrogen Absorption hn Energy hn Ground State Excited State does an orbital exist if there is no electron in it Follows Aufbau Principle hn Ground State Excited State

Hydrogen Absorption H H H H H H H “white” light source Hydrogen Sample Prism Line Spectrum Rydberg Formula rydberg constant prism dependent on the wavelength and the angle of refraction

Increasing Complexity Atomic Transitions (movement of electrons) + Molecular Transitions (movement of electron density) 250 e-

Molecular Transitions hn hn Atomic Transitions hn hn Molecular Transitions

Types of Molecular Transitions σ - σ* max < 150 nm p - p* max 200 - 800 nm n - p* max 150 - 300 nm

Types of Molecular Transitions High energy photons methane = 125 nm ethane = 135 nm σ - σ* max < 150 nm Antibonding hn Bonding Ground State Excited State

Types of Molecular Transitions Visible photons benzene = 260 nm tetracene = 500 nm p - p* max 200 - 800 nm Antibonding hn Bonding Ground State Excited State

Types of Molecular Transitions Visible photons acetone = 280 nm pyridine = 270 nm n - p* max 150 - 300 nm Antibonding hn Non-Bonding Ground State Excited State

Types of Molecular Transitions σ - σ* max < 150 nm p - p* max 200 - 800 nm 400 300 200 500 100 p - p* n - p* s - s* Wavelength (nm) Absorption n - p* max 150 - 300 nm

Types of Molecular Transitions [Co(H2O)6]2+ Metal Centered (MC) max 200 –800 nm MnO4- MLCT max 300 –1000 nm LMCT max 300 –1000 nm MMCT max 300 –800 nm

Types of Molecular Transitions Metal Centered (MC) d-d transitions max 200 – 800 nm M M + L t2g eg [CoCl4]2- [Co(H2O)6]2+ 3d and 4d transition metals (+ ligands) Relatively weak (0-1000 M−1cm−1) Early structural determination

Types of Molecular Transitions Metal-to-Ligand Charge Transfer (MLCT) max 300 – 1000 nm eg e- p* hn t2g MLCT p - p* M M + L L Low-lying empty ligand orbital Low oxidation state metal (electron rich) High d orbital energy

Types of Molecular Transitions Ligand-to-Metal Charge Transfer (LMCT) max 300 – 1000 nm eg e- Mn-O4- O2- (p)  Mn7+ Purple t2g p e- M M + L L Cd-S S2- (p)  Cd2+ Yellow Ligand with high E lone pairs (S or Se) Metal with low-lying empty orbitals

Types of Molecular Transitions Metal-to-Metal Charge Transfer (MMCT) max 300 – 800 nm MMCT III e- II eg eg t2g M1 M2 t2g M1 + L M2 + L

Types of Molecular Transitions eg e- eg M1 t2g M2 600 500 400 700 300 MC LMCT MLCT Wavelength (nm) Absorption MMCT t2g p M1 + M2 + L

Complete Diagram σ - σ* σ - p* p - p* n - p* n - σ* Transitions Electronic n - σ* E1 Vibrational Energy Rotational MC MLCT LMCT E0 MMCT

Complete Diagram Jablonski Diagram Transitions Electronic Vibrational Energy Transitions Electronic E1 Vibrational S0 Energy Rotational E0

Complete Diagram Jablonski Diagram Second Excited State (S2) Energy First Excited State (S1) S0 Excitation Internal Conversion Fluorescence Ground State (S0) Non-radiative decay

Complete Diagram Jablonski Diagram Ground State S0 hn S1 Energy Ground State S0 Singlet Excited State S1 S0 Excitation Internal Conversion Fluorescence Non-radiative decay

Triplet/Singlet Excited States Triplet Requires e- Spin Flip Unallowed – Conservation of Momentum Allowed by- Spin-Orbit Coupling Lower Energy Nicholas J. Turro, Principles of Molecular Photochemistry

Spin-Orbit Coupling ms = spin of the electron ml = direction of the orbital l = shape of the orbital n = size/energy of the orbital

Spin-Orbit Coupling Quantum Numbers Heavy Atoms Pt, Ir, I... n = Principal l = Angular ml = Magnetic ms = Electron spin Heavy Atoms Pt, Ir, I... ms = spin of the electron ml = direction of the orbital l = shape of the orbital n = size/energy of the orbital Rotating Chair and Bicycle Wheel Nicholas J. Turro, Principles of Molecular Photochemistry

Jablonski Diagram Excitation Internal Conversion Fluorescence Non-radiative decay Intersystem Crossing Phosphorescence S1 T2 Energy T1 S0

Jablonski Diagram of Anthracene Nicholas J. Turro, Principles of Molecular Photochemistry

Other Processes Electron transfer TICT ESIPT Photochemical Reactions Energy T1 Electron transfer TICT ESIPT Photochemical Reactions S0 Excitation Internal Conversion Fluorescence Non-radiative decay Intersystem Crossing Phosphorescence

Excited State Electron Transfer hn e- + A RuIII(bpy)3 + A- e- hn + + RuII(bpy)3 [RuII(bpy)3]* A RuIII(bpy)3 A-

Excited State Electron Transfer Photosynthesis

Excited State Electron Transfer Photocatalytic α-alkylation of aldehydes Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77-80.

Excited State Structural Change Twisted Intramolecular Charge Transfer e- e- Pratt et al. J. Chem. Phys. 2005, 122, 084309

Excited State Structural Change Excited State Proton Transfer ESIPT absorption emission reverse proton transfer Hanson et al. Org. Lett. 2011, 13, 1598

Photochemical Reactions Photopolymerization vision Peachy Printer ($100)

Photochemical Reactions Photolithography

Photochemical Reactions Photoisomerization hn Ground State Excited State

Photochemical Reactions Photoswitches J. Am. Chem. Soc., 2013, 135 (16), pp 5974–5977

“Complete” Jablonski Diagram Product T2 E T1 Product S0 Processes Excitation Fluorescence Phosphorescence Non-radiative decay Internal conversion Intersystem crossing Photochemistry Measurement Technique Absorption Spectroscopy Fluorescence Spectroscopy Transient Absorption Spectroscopy Solar Cell Testing

Side Note: Other Excitations Thermal Excitation

Side Note: Other Excitations Chemical Excitation

Side Note: Other Excitations Sonoluminescence

Side Note: Other Excitations Tribo/Fractoluminescence Nature 2008, 455, 1089–1092.

Side Note: Other Excitations Electroluminescence

Side Note: Dye Structure

Side Note: Dye Structure Bright Blue Common Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing. Royal Blue Baked goods, cereals, snack foods, ice-cream, confections, cherries. Orange-red Gelatins, puddings, dairy products, confections, beverages, condiments. Lemon-yellow Custards, beverages, ice-cream, confections, preserves, cereals. Orange Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections

Molecular Photophysics End Any Questions?