Introduction to Photochemistry and Light Upconversion CT Japan Photochemistry Workshop Tomoyasu Mani Department of Chemistry University of Connecticut 10/29/2019 1 Department of Chemistry at the University of Connecticut • @ Storrs, CT • 26 tenure‐track or tenured professors • Located in the Chemistry Building • 65 cutting‐edge research and teaching labs 2 From Low to High Upconversion Based on Triplet-Triplet Annihilation https://mani.chem.uconn.edu/photochem-workshop/ 3 Energy Flows High to Low Potential Energy © Science Media Group. ©The McGraw‐Hill 4 Energy Flows Low to High ?? Potential Energy ©The McGraw‐Hill 5 What is Photochemistry? The chemistry concerned with the chemical effects of light. Generally, a chemical reaction is caused by using UV, visible, infrared light. 700 nm 600 nm 500 nm 400 nm 6 Why Important? Photosynthesis is Driven by Light! We can see “inside” by Light! 7 Converting Light to Something Else. Making Molecules with Light. Making Electricity from Light. 8 What is Going On? General Jablonski Diagram Singlet Manifold S3 S2 IC S1 Energy Photon Fluorescence Absorption Ground State IC = internal conversion 9 Photoexcitation = Excess Energy Singlet Manifold Excited S3 S2 IC S1 Energy Photon Fluorescence Absorption Ground Ground State State IC = internal conversion 10 Fluorescence Emission BLUE Light https://cen.acs.org/biological-chemistry/biotechnology/Chemistry-Pictures-Laser-activated/97/web/2019/08 11 What is Going On? General Jablonski Diagram Singlet Manifold Triplet Manifold S3 T3 S2 T2 IC Triplet-triplet absorption S1 2J Energy ISC T1 Photon Fluorescence Phosphorescence* Absorption Ground State IC = internal conversion ISC = intersystem crossing *in some molecules, quantum yield of phosphorescence can be as high as 0.3-0.5 at room temperature.12 Photoexcitation = Excess Energy Singlet Manifold Triplet Excited S3 Excited S2 IC S1 Energy Photon Fluorescence Absorption Ground Ground State State IC = internal conversion 13 Spins: Singlet vs Triplet State Singlet S=0 MS=0 S1 isc T1 h phos S0 14 Triplet S=1 Singlet S=0 MS=0 S1 isc M =1 M =0 M =-1 T1 S S S h phos (spin-forbidden transition, hence T1 state is long-lived) S0 15 Triplet Excited States in Action Light Emitting Diodes Photodynamic Therapy in Medicine Photoredox Catalysts PHOLED1 Singlet Oxygen Generation4 Organic Synthesis2 Oxygen Sensing in vivo New Technologies Singlet fission etc… Images from 1Universal Display Corporation 2Schultz, D. M.; Yoon, T. P., Science, 343, 1239176 (2014) 3Sakadžić, S. et al.; Nature Methods, 7, 755 (2010) 4Castano, A. P. et al., Nat. Rev. Cancer, 6, 535 (2006) Phosphorescence lifetime3 16 A Peculiar Case of Molecular Oxygen (O2) 3 The ground electronic state is Triplet! Labeled as O2. This is GOOD for us. But, this may NOT be good for triplet excited states of other molecules as they can readily react with molecular oxygens. T1 + 3O2 → GS + 1O2 So, if you want to use triplet excited states, you need to work around the problem of molecular oxygen, 17 Energy Flows From High to Low BLUE Light https://cen.acs.org/biological-chemistry/biotechnology/Chemistry-Pictures-Laser-activated/97/web/2019/08 18 Photon Upconversion: Convert Low-Energy Photon to High-Energy Photon The sequential absorption of TWO or more photons leads to the emission of light (ONE photon) at shorter wavelength than the excitation wavelength. Lower Energy Higher Energy 700 nm 600 nm 500 nm 400 nm Normal Upconversion 19 What is Triplet-Triplet Annihilation (TTA)? T1+T1 [T1T1] S1 + S0 • Two triplet excited states form one singlet excited state (S1) and one singlet ground state (S0). • It leads to delayed fluorescence from S1 • TTA is opposite to singlet fission. 20 Sensitized Triplet-Triplet Annihilation Convenient way to produce TTA. Visible absorption by sensitizers with excellent extinction coefficients. One of the upconversion mechanisms. Delayed fluorescence A S1 F S k TTA S1 kISC F B S k TTET k TTA T1 AT h B 1 k TTET Visible excitation Energy Transfer (TTET) S A S0 S0 Donor‐Sensitizer Acceptor‐Annihilator 1Parker, C.A.; Hatchard, C. G.; Proc. Chem. Soc. London 1962, 147. 21 Picture is taken from Sing-Rachford, T. N.; Castellano, F. N.; Coord. Chem. Rev. 2010, 254, 2560. TTA Experiment: Green to Blue Upconversion Delayed fluorescence A S1 F S k TTA S1 kISC F B S k TTET k TTA T1 h AT Visible excitation B 1 k TTET BLUE GREEN S A S0 S0 Donor‐Sensitizer Acceptor‐Annihilator 22 DPA Blue Filter PdOEP 23 Optical Setup How do we see Blue Light? Sample Green Laser Pointer Filter 24 What are you going to do with Molecular Oxygen? 1. Conventional Method Deoxygenate solutions = Remove Oxygen 2. What We Do Use Gels*! “Embed” sensitizers and annihilators in gel matrix = Block Oxygen Diffusion *Duan, P.; Yanai, N.; Nagatomi, H.; Kimizuka, N. J. Am. Chem. Soc. 2015, 137, 1887–1894. 25.