Triplet-Triplet
Annihilation Based
Photon Upconversion
Literature Seminar #1
B4 Yuri Katayama

1

Contents
1. Introduc+on
2. Applica+ons -photocatalysis-biological applica+ons
3. Perspec+ve
4. Summary
2

Contents
1. Introduc+on
2. Applica+ons -photocatalysis-biological applica+ons
3. Perspec+ve
4. Summary3

What is TTA-UC ?
ØTTA-UC = triplet-triplet annihilation photon upconversion
ØPhotochemical phenomenonØLow energy photons are converted to high energy photons.
ØEnergy transfer processes occur.

ex.

The main casts
SensiAzer & Annihilator

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.

4

Mechanism

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.

D = donor = sensi:zer
A = acceptor = annihilator
ISC = intersystem crossing
TTET = triplet-triplet energy transfer
TTA = triplet-triplet annihila:on
TTET and TTA occur through electron exchange via Dexter energy transfer mechanism.

Sensi&zed an&-Stokes delayed ﬂuorescence5History of TTA-UC
1962 Sensi&zed an&-Stokes delayed ﬂuorescence by Parker and Hatchard
2003 TTA-UC at r.t. by Baluschev
2006 Green to blue UC by non-coherent green sunlight by Baluschev

PdOEP

PF2/6

2003. Baluschev
P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.
S. Baluschev et al., Adv. Mater.,2003, 15, 2095.

2006. Baluschev
6

Recent breakthroughs 2015-2020
New direcAons in NIR to Vis molecular
TTA-UCØ Inorganic-organic hybrid NIR to Vis
molecular TTA-UC
Ø Lanthanide-organic complex sensi&zer
based NIR to Vis molecular TTA-UC
Ø NIR to Vis molecular TTA-UC in liquid
crystalsØ Direct S0-to-T1 absorbing sensi&zer based
NIR to Vis molecular TTA-UC
introduced by Kimizuka and Yanai’s researchgroups at Kyushu Univ. in 2016
↪Discussed later
P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.
N. Yanai and N. Kimizuka, Acc. Chem. Res., 2017, 50, 2487.

7

Parameters of evaluaBng TTA-UC
ΔEUC : an:-Stokes shiU<unit = eV>

Ith : threshold excita:on intensity<unit = W cm-2 >ΦUC : upconversion quantum yield<unit = %>!
f ΦISC ΦET ΦTTA ΦFL"
ηUC = ΦUC’ = 2× ΦUCΦUC =f : Spin sta:s:cal factor
the probability of ge]ng excited singlet state aUer
annihila:on of two triplet via TTA
P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.

※ in some cases, ηUC used as ΦUC
ex. PdTNP(sen)/Py5(ann)λex = 720 nm (1.72 eV)
λem = 570 nm (2.18 eV)
ΔEUC = 0.45 eV (720 to 570 nm)

Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

8

Advantages and applicaBons
Advantages
ü Lower excita&on intensity of non-coherent light ↔ conven&onal UC ;two photon absorp&on UC, inorganic materials, nanopar&cles
ü Lower energyü Deep-&ssue penetra&on of NIR

Applica&ons• Photovoltaics• Bioimaging• Photocatalysis• Photodynamic therapy• Sensing• Optogene&cs

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.
F. Li et al., ACS Appl. Mater. Interfaces, 2018, 10, 9883.

9

Contents
1. Introduc+on
2. Applica+ons -photocatalysis-biological applica+ons
3. Perspec+ve
4. Summary
10

Photochemical reacBons via TTA-UC
Background• UV or Vis photoredox reac:ons
×the penetra:on depth of visible light is low→×large-scale reac:ons.
×Need of high energy.
• TTA-UC○Deep penetra:on○No need of high energy○Modiﬁca:on of organic chromophores→ tunable electronic
structures are tunable.

D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

11

PC excited by TTA-UC
Last year, Campos’s group reported photocatalysis via NIR to Vis TTAーUC
and its advantages over Vis or UV photocatalysis though their experiments.
NIR to orange upconversion

Hydrodehalogena:on

0.040 WA large number of ‘lightbulbs’ inside the ﬂask : 1000 :mes less powerful lightsources achieved similar yields to photoredox standard.

D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

12

1[A]* as photoredox catalysts
NIR to blue upconversion

Intramolecular [2+2] cycliza:on

1[An]* of TTBT itself performs as photoredox catalysis.

3[An]* (~35 kcal/mol, E•+
1/2 (PC /PC*) = -0.78 V)
3Sub*~45 kcal/mol, E (Sub/Sub-•) = -1.55 V vs Ag/AgCl in MeCN
1/2
TTA-UC mechanism and the reduc:on by TTBT’s 1[An]* .

D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

13

Deep-penetraBng NIR lightbarriers

Experimental set-up with water as the barrier

730 nm

D. N. Congreve, T. Rovis, L. M. Campos
et al., Nature, 2019, 565, 343.

450 nm14

Scale-up experiments via TTA-UC
The problems of visible-light photoredox reac&ons
1. Shallow absorp&on of visible light
2. Low performance in large-scale reac&ons
Overcoming of the problem 1 : deeper penetra:on of NIR light
Applica'on of the Beer–Lambert law toblue and NIR light.
A = εcl(A, absorbance;ε, molar ex:nc:on coeﬃcient;c, concentra:on; l, path length)[Ru(bpy)3]2+ : ε = 15376, c = 5 mol%
PtTPTNP: ε = 2100, c = 0.12 mol%
→ NIR light penetrates deeper than blue
light though the reac:on shown in theright.
15

D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

Scale-up experiments via TTA-UC

mul:-gram scale
The deﬁned shapes were
not observed with bluelamp.

Gel forma:on excited with NIR, not with blue light
D. N. Congreve, T. Rovis, L. M. Campos
et al., Nature, 2019, 565, 343.

16

Contents
1. Introduction
2. Applications -photocatalysis-biological applica+ons
3. Perspective
4. Summary
17

Biological applicaBons• Advantages of TTA-UC for biological applica&ons
ü Long-wavelength light u&lized in the therapeu&c window (600-900nm) -Deep-&ssue penetra&on
ü Non-invasive• Applica&ons
Bioimaging, optogene&cs,
Oxygen quenching

photodynamic therapy

F. N. Castellano et al., ACS Energy LeG., 2020, 5, 2322.

• The challenges to overcome about biological applica&ons
u Oxygen quenching of triplets, photostability
u Cytotoxicity of heavy metal in sensi&zers
u Solubility in aqueous environment

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.
Han, G. et al., Angew. Chem. Int. Ed., 2017, 56, 14400.

18

Prodrug acBvaBon via TTA-UC
In vivo anticancer prodrug activation though TTA-UC was reported by
Han’s research group in 2017.

annihilator

sensi:zerü High ﬂuorescence quantum
yield in the deep-blue region
from 410-500 nm, peaking
at 432 nm (Φf = 87 %)ü Broader and more intense absorp:on (the farred region from 600-700 nm, λex = 615 nm,
ε = 1.77 ×105 M-1cm-1) than 2, 6-diiodo-BODIPY(λex = 525 nm, ε = 85000 M-1cm-1)
ü Long triplet-state life:me : τT = 243.6 μs

Han, G. et al., Angew. Chem. Int. Ed., 2017, 56, 14400.

19

ProperBes of TTA-UC

650 nm excita:on power density
TTA-UC can be observed with the naked eye.
TTA-UC emission spectra of the op:mized concentra:on
BDP-F (20 μM)and PEA (0.2 mM) in degassed toluene,
λex = 650 nm (100 mWcm-2)
ηUC = Φ’UC = 2 × ΦUC = 3.1 %,Δλ = 0.96 eV (650 nm to 432 nm)
Ith = 19.6 mWcm-2
Han, G. et al., Angew. Chem. Int. Ed., 2017, 56, 14400.

20

TTA-CS
430 nmin methyl oleate

Upconversion emission
spectra of TTA-MSNs
at diﬀerent power
intensi?es in PBS andair, 0.5 mg mL-1 ,λex = 650 nm.

ü TTA-UC in methyl oleate is stable in air.
ü ΦUC = 1.0 % (100 mW cm-2) in waterü Photostability in air

O
OH
O

N

Cl

N

OH

Cl
O

O

TTA - UC

N

O

O

O

Cou - CCl

N

Cl

Coumarin

Chlorambucil

Chlorambucil : tumor inhibitor

TTA-CS = TTA upconversion core-shell structured nanocapsule
MSNs = mesoporous silica nanopar:cles

Han, G. et al., Angew. Chem. Int. Ed., 2017, 56, 14400.

21

In vitro eﬀecBveness

TTA-MSNs : insigniﬁcant toxicity
TTA-CS : signiﬁcant toxicity upon irradia:on
100 mWcm-2, 60 min

The release dose and dura:on can be:trated by using the far-red light.
Han, G. et al., Angew. Chem. Int. Ed.,2017, 56, 14400.

Bright-green: living cells
Red : dead cells
22In vivo eﬀecBvenessGroup2 : TTA-CS itself cannot inhibit tumor growth.
Group3 : The low power of LED has low
photothermal and or other eﬀects for cancer cellliving.
Group4 : The deep-blue upconversion-induced
chlorambucil release from TTA-CS leads to tumor:ssue abla:on.
ü Insigniﬁcant weight lossü No no:ceable sign of organ damage
ü No observable inﬂamma:on from a serum
analysis

The ﬁrst example of eﬀec:ve TTA-UC-induced an:cancer prodrug photorelease in vivo
and in vitro with a bio-compa:ble far-red LED lightHan, G. et al., Angew. Chem. Int. Ed., 2017, 56, 14400.

23

Contents
1. Introduction
2. Applications -photocatalysis 1 & 2-biological applica+ons 1
3. Perspective
4. Summary
24

Strategy to achieve eﬃcient TTA-UC
ΦUC : unconversion quantum yield③

f : Spin sta:s:cal factor
the probability of ge]ng excited singlet stateaUer annihila:on of two triplet via TTAΦUC =!
f ΦISC ΦET ΦTTA ΦFL"①②

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.

Main strategies
1. Facile TTA
2. Facile TTET
- Extend the phosphorescence life:me of sensi:zers- Tune the energy of the excited states
3. Reduce the energy loss during ISC
25

Tailor annihilators to eﬃcient TTET
SensiQzers

Annihilators : Py0-Py5

650 nm : 0.1 W cm-2
720 nm : 100 mWcm-2
theoreQcal maximum of ΦUC = 1
Energy : esQmated by TD-DFT

3[Sen]*> 3[An]* & 2× 3[An]* > 1 [An]*653 nm, 100 mW/cm2

π conjuga:on → red-shiUed λabs & λem
Py 4 & Py 5 → T1 of them lower than T1 of PdTNP
higher ΦTTET , ΦUC
Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

26

Molecular engineering for TTA

The deﬁni:on in this paper
ΦUC = ΦISC ΦTET ΦTTA ΦPL × 100
The limit is 50 %Hypothesis
By molecular engineering and tuning E(S1), ΔE[S1-2T1]<0,high ΦTTA will be achieved.
M. Y. Sfeir, D. N. Congreve, L. M. Campos et al., J. Am. Chem. Soc., doi:org/10.1021/jacs.0c06386

27

Tetracene derivaBves
stability, solubility

Finding-CN ( a strong EWG) can lower E(S1)

Excitation with a 730 nm laser

M. Y. Sfeir, D. N. Congreve, L. M. Campos et al., J. Am. Chem. Soc., doi:org/10.1021/jacs.0c06386

28

Upconversion quantum yieldΦUC = ΦISC ΦTET ΦTTA ΦPL × 100
ΦUCs = ΦUC/ ΦPL × 100 = ΦISC ΦTET ΦTTA
Concentrated : high ΦUCs, low ΦPL and ΦUC
Dilute : low ΦUCs, high ΦPL and ΦUC

Result
6 : the lowest E(S1), the largest ΔE[S1-2T1],and the highest ΦUCs / ΦUC

M. Y. Sfeir, D. N. Congreve, L. M. Campos et al., J. Am. Chem. Soc., doi:org/10.1021/jacs.0c06386

29TTA-UC based on S – T absorpBon
Ø In 2016, direct S0-to-T1 absorbing sensi:zer based NIR to Vis molecular TTA-UC
introduced by Kimizuka and Yanai’s research groups at Kyushu Univ. in 2016
Although the S-T absorp:on is spin forbidden, large spin-orbital coupling occurs forproperly designed Os(II) complexes, which allows S-T absorp:on in the NIR region withlarge absorp:on coeﬃcients.
Advantage
Minimize the energy loss during ISC→ larger ΔEUC
2016
2017

○ ΔEUC = 0.86 eV (938 nm to 570 nm)
× low ΦUC = 0.0024 % due to τP = 12 ns○ ΔEUC = 0.97 eV (724 nm to 462 nm)○ ΦUC = 1.35 % due to τP = 207 ns

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529.
30
N. Yanai and N. Kimizuka, Acc. Chem. Res., 2017, 50, 2487.

Summary of perspecBve
Ø The strategies to improve each process based on the mechanism
can realize eﬃcient TTA-UC.
Ø Besides adding func&onal groups or extending the conjugated πsystem, the novel routes are being developed.
Ø In addi&on to achieving the high upconversion quantum yield, thedevelopment of TTA-UC in aqueous solu&on is required.

N. Yanai and N. Kimizuka,
Acc. Chem. Res., 2017, 50,
2487.
31

Contents
1. Introduction
2. Applications -photocatalysis-biological applica+ons
3. Perspective
4. Summary32

SummaryØ The 21 century have seen tremendous breakthroughs in the ﬁeld of
TTA-UC, and this trend will go on.
Ø The highest upconversion quantum yield has been updated
recently.
Ø By u&lizing the advantages of TTA-UC, many applica&ons will be inuse prac&cally, especially in the ﬁeld of photovoltaics. It may beable to contribute to energy problems in the future.
Ø The biggest drawback of TTA-UC is oxygen quenching, and further
developments solving this are required.

Thank you for kind listening !
33

Appendix
34

Dexter energy transfer
Dexter energy transfer : a non-radia:ve electron exchange

P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529. 35

Upconversion yield in detail

F. N. Castellano et al., ACS Energy LeG., 2020, 5, 2322.

36

f : spin staBsBcal factor
f : spin sta:s:cal factor
2×AT1

AS 1/91
AT 3/92
AQ 5/91

Theore:cal maximum : 11 %But ..
AT & AQ decay to AS
2
1
1

J. Zhao et al., RSC Advances, 2011, 1, 937.
P. Bharmonia et al., Chem. Soc. Rev., 2020, 49, 6529. 37

Requirements of eﬃcient TTA-UC
The requirement for sensi:zers / annihilators
1. The light harves:ng ability of the triplet sensi:zer for eﬃcient TTET.
The metal complexes shows weak absorp:on in the visible region. Withhigh concentra:ons of the triplet sensi:zers, TTET will be more eﬃcient.
2. The triplet excited state quantum yield of the sensi:zer must be high.
Most sensi:zers are transi:on metal complexes. φISC is close to 1.
3. The life:me of the triplet excited state of the sensi:zers should be long.
The triplet sensi:zers need diﬀusion in the system and collide withannihilators. The longer life:me makes the encounter more likely.
4. The energy levels of the triplet state of sensi:zers and the triplet state ofannihilaotrs
5. About annihilators, 2 × ET1 > ES1
6. Φf of annihilators should be high
J. Zhao et al., RSC Advances, 2011, 1, 937.

38

SensiBzers
Pd(II)/Pt(II) porphyrin complexes○Intense absorption of the visible region○ the long-lived triplet excited state×Tuning excitation/absorption wavelength is difficult.
Transi:on metal complexes realize eﬃcient ISC due
to the heavy atom eﬀect and popula:on of triplet
excited state.
Organic triplet sensi:zers

The n-π* transi:on facilitates ISC.
The iodine atom does due to themoderate heavy atom eﬀect.
PPO

×Modiﬁca:on of the molecular structure
J. Zhao et al., RSC Advances, 2011, 1, 937.

39

Sensitizers
Sensi:zers of BODIPY deriva:ves○Absorp:on : 510-629 nm
The varia:on due to extension of the π-conjuga:on○Weak ﬂuorescence○The life:me of triplet excited state : 〜66.3 μs (long)○ Many deriva:ves( )J. Zhao et al., RSC Advances, 2011, 1, 937.

40

Annihilators
Basic annihilators

ET1 = 2.07 eV ET1 = 1.53 eV

ET1 < 1.32 eV

J. Zhao et al., RSC Advances, 2011, 1, 937.

The requirements
1. The energy level : 2 × ET1 > ES1
2. High φf
3. Tunable ET1
41
4. Good photochemical stability

Threshold excitaBon intensity• Ith : threshold excita:on intensity
Ith = (αΦET 8πDTa0)-1(τT)-2
α : absorp:on coeﬃcient at the excita:on wavelengthΦET : sensi:zer to annihilator triplet energy transfer eﬃciency
DT : diﬀusion constant of annihilator triplets
a0 : annihila:on distance between annihilator triplets
τT : life:me of annihilator triplets
Larger sensi:zer absorp:on, eﬃcient TTET, faster triplet diﬀusion, longer emi…er
triplet life:me are required to achieve a low Ith.

dependence.

low intensity : the emission from TTA exhibits quadra:chigh intensity : the TTA becomes dominant for emi…er
triplet decay, emission intensity.
N. Yanai and N. Kimizuka, Acc. Chem. Res., 2017, 50, 2487.
42

ConcentraBon!
ηUC = f ΦISC ΦET ΦTTA ΦFL"
ΦFL : quantum yield of annihilator
ﬂuorescence
74 % (100 μM) → 66 % (10 mM)
due to the inner ﬁlter eﬀect[Ir(C6)(acac)] = 100 μM, [TIPS-Nph] = 10 mM
in deaerated THF
ηUC = 20.5 % (445 nm laser)
The highest Vis-to-UV TTA-UC
Case : the broad absorp:on of sensi:zers
High sensi:zer concentra:on
Low sensi:zer concentra:on→ weak absorp:on at the UV region→ reabsorp:on, quenching the upconverted→ high visible absorbance, high ηUCemission, back energy tranfer

43
N. Yanai and N. Kimizuka et al., Angew. Chem. Int. Ed., doi : 10.1002/anie.202012419

Eosin Y as a photocatalyst
Neumann’s research group reported the reductive dehalogenation.
Eosin Y is used as a photoredox catalyst.

EYH2 = Eosin Y

Vishal Srivastavaa and Praveen P. Singh, RSC Adv., 2017, 7, 31377.

44

CombinaBon TTA-UC with PC
The generality of FDPP/PcPc in TTA-UC

45

[Ru]-catalyzed [2+2] cyclizaBon
The reduction potential ofRu*(bpy)32+ = -0.89 V vs SCE

Yoon, T. P. et al., J. Am. Chem. Soc., 2008, 130, 12886.

46 53, 793
E. P. Farney and T. P. Yoon, Angew. Chem. Int. Ed., 2014,

[Ru]-catalyzed azide sensiBzaBon

The reduc:on poten:al of 1 = -1.81 V vs SCE
Ru*(bpy)32+ : -0.89 V vs SCE
Triplet excited state of 1 : ET = 45.4 kcal mol-1
Ru*(bpy)32+ : ET = 46 kcal mol-1→Not SET, but sensi:za:on of 1 by the long lived
Ru*(bpy)32+ triplet stateRu*(dtbbpy)32+ : ET = 47 kcal mol-1
E. P. Farney and T. P. Yoon, Angew. Chem. Int. Ed., 2014, 53, 793.

47

ATRP by perylene as PC(E1/2 (13/13-•) = -1.58 V vs Ag/AgCl in MeCN.

Miyake, G. M. and Theriot, J. C., Macromolecules, 2014, 47, 8255.
D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

48

Experimental set up

D. N. Congreve, T. Rovis, L. M. Campos et al., Nature, 2019, 565, 343.

49

Perylene derivaBves and PdTNP
The definition of the upconversion quantum yieldΦUC = f ΦISC ΦTTET Φcp Φf ≤ 1
Φcp : contact pair formation efficiency for annihilator
ΦTTA = Φcp × 𝑓ηTTA = 2 × ΦTTA

PdTNP
A long triplet excited :meτT = 65 μs

Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

50

Perylene derivaBves and PdTNP
TTET

TTA

Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

51

Perylene derivaBves and PdTNP
ΦUC = f ΦISC ΦTTET Φcp Φf
ηTTA = 2 × ΦTTA = 2 ×(Φcp × 𝑓 )ΦTTA → ηTTA

Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

52

Photocatalysis uBlizing TTA-UC

53
Han, G. et al., J. Am. Chem. Soc., 2020, 142, 18460.

Proposed mechanism

54Extending the lifeBme
Extending the life:me of triplet excited state of sensi:zers

3MLCT/1IL excited state equilibrium

IL = intraligand

108.0 μsτ0 = 0.6 μs

τ0 = 2.6 μs

J. Zhao et al., RSC Advances, 2011, 1, 937.

τ0 = 17.9 μs

C. Kerzig and O. S. Wenger, Chem. Sci., 2018, 9, 6670.

55