Introduction to and 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, 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 with Light. Making Electricity from Light.

8 What is Going On? General

Singlet Manifold

S3

S2 IC

S1 Energy

Photon 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--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 =

*in some molecules, 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 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 (-forbidden transition, hence

T1 state is long-lived) S0

15 Triplet Excited States in Action Light Emitting Diodes in Medicine

Photoredox Catalysts

PHOLED1 Singlet 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 .

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 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: 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