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(1)

Quenching of Excited States

홍익대학교

신동명

(2)

Quenching of Excited States: Introduction

• Quenchers

• Q : Oxygen, Metals (Heavy, transition metals), solvents

• Oxygen 제거: 질소 또는 헬륨으로 degassing (용매가 날라가서 농도 가 바뀔 수 있기 때문에 천천히 해야 한다. )

10-2 M 에서 10-5 M 으로 떨어진다. Copper tubing 또는 Tygon tube 사용

:

여기상태의 life time이 긴 경우

에는 사용할 수 없다.

• Freeze-pump-thaw : 10

-5

Torr (3번 반복)

M M

*

⎯ ⎯→

[Q]

[ ] O

2

= 10

10

X 10

4

= 10

6

k

q

(3)

Quenching of Excited States: Excimer & Exciplex

• Excimer

excimer Monomer

(4)

Quenching of Excited States: Excimer & Exciplex

• Excimer

M M M M*

ΔE

−ΔE

−ΔE’

ΔE’

Temperature dependence of emission으로 ΔET 또는 ΔH를 계산 ΔE ~ 40 KJ/mole (10 kcal/mol) ΔS ~ 80 JK-1 (20 cal K-1 ) for

strongly bound rigid structure molecules a) Unbound state : ΔET = 0

b) Bound state : ΔET = -(ΔE+ΔE’)

(5)

Quenching of Excited States: Excimer & Exciplex

• Excimer formation – fl. Decay curve

Pyrene(5 X10-3 M) in cyclohexane.

Monomer Dimer

Relative intensity P(t)

50 100 150 200 (ns)

0

Excimer formation rate ~ k diff

(6)

Quenching of Excited States: Excimer & Exciplex

• Excimer formation

(n=4)3.73A

(CH

2

)

n

(CH

2

)

n

hv (254nm) hv (> 284nm)

(CH2)3

low temp

Alkyl benzene의 absorption spec.

Excimer fl 보임.

이 물질이

naphthanlene 의 . Excimer fl 보임.

(7)

Quenching of Excited States: Excimer & Exciplex

• Exciplex formation

Exciplex는 dipolar 한 성격때문에 용매의 극성이 증가하면 red shift

농도에 따라서 isosbestic point를 갖으면서

spectrum이 변한다.

(8)

Quenching of Excited States: Excimer & Exciplex

• Exciplex formation f/b electron transfer

Eg. Indole + TCNE

(D*A) (D

+

A

-

)*

exciplex CT exciplex

Exciplex formation – Donor excited

ID EA

D* A D A

(DA)* hv e-transfer

emission

How about acceptor excited?

N

H

(9)

Quenching of Excited States: Excimer & Exciplex

• Exciplex formation f/b electron transfer

Exciplex formation – acceptor excited

ID EA

D A* D A

(DA)*

e-transfer hv emission

(10)

Quenching of Excited States: Excimer & Exciplex

• 예1

Dielectric constant νF (cm-

1)

H2O EtOH

N H

용매와 1:1 complex 형성한다. Exciplex 의 극성으로 인해 낮은 energy 영역에서 형광이 나타난다.

hydrocarbon

낮은 energy 영역에서 형광이 나타나는 이유 는?

(11)

Quenching of Excited States: Excimer & Exciplex

• 예2

• 예3

• 예4 :

Triplet – Triplet annihilation

hv

O *

+

R

CN O

R CN

*

O

R CN

(12)

Quenching of Excited States: Kinetics

• Kinetics

Q M

M

] [ k

* I

Q

⎯ +

⎯ →

⎯→

+

⎯→

⎯→

⎯ +

Q k k

D

F

M h

M h

M ν ν

[ ]

] [ 1

] [ 1

] [ ]

[

]) [ (

M

0 0

*

*

Q k

k Q k

k k k

k

Q k

k k

k I

M k

Q k

k k

I

Q D

F Q F

D F

F

Q D

F

F F

F

Q D

F

F F

φ τ φ φ φ

+ + =

+

=

== +

+

= +

=

+ +

=

Q

τ

기울기 :

k

] [Q

F

F

φ φ

0

time life

:여기상태 τ

) (

sec 10

~ 10

. rate

controlled diffusion

:

1 1

10

9

용매에 따라 다름

이다 경우

많은

mol

L

kQ

(13)

Quenching of Excited States: Kinetics

• Perrin Model – Static quenching

The Perrin model assumes:

(a) The donor and acceptor cannot undergo displacements in space during the lifetime of D*.

(b) There exists a volume in space - or more precisely

a "quenching sphere" - about D* whose radius is R and if a quencher molecule is within this quenching sphere, then D* is deactivated with unit efficiency.

(c) If a quencher molecule is outside of the quenching sphere, it

does not quench D* at all.

(14)

Quenching of Excited States: Kinetics

• Perrin Model

VN

Q

기울기 :

] [Q

F

F

φ φ

0

Q

Life time에 관계 없고 농도에 비례

기울기 :

] [Q

F

F

φ φ0

time life :여기상태 τ

Qτ k

참고

(15)

Quenching of Excited States: Kinetics

• Kinetics

complex e

within th const.

rate :

] )[

(

] [

] [

] ][

[ k

exciplex or

complex encounter

Q M

) (

1

* 1

*

* 1

* diff

*

*

1

quenching k

k

Q M

k k

Q M

k Q

M k

Q M

Q M

Q M

C

C

C kC

kdiff k

+

⋅⋅

⋅⋅

+

=

⋅⋅

⋅⋅

+

⋅⋅

⋅⋅

=

+

⎯→

⋅⋅

⋅⋅

⋅ +

⎯ →

⎯⎯

(16)

Quenching of Excited States: Kinetics

• Kinetics

) (

] ][

[ ] [

quenching of

1

* 1

*

C diff C

Q

C diff C

C

k k

k observed k

k

Q k M

k k k

Q M

k rate

= +

= +

⋅⋅

⋅⋅

=

.

k :

. 3

s) d r : M

( k

viscosity sol.

of t Independen

quencher weak

:

. 2

s) d r : (M

solvent

k :

. 1

Q 1

*

1 Q

1

*

*

Q 1

작다

보다는 는

크다 영향이

diff C

C C

diff C

diff C

k k

k

Q Q

M

K k k

k k

k k

Q M

Q

k k

k

+

⋅⋅

⋅⋅

⋅ =

=

>>

⋅⋅

⋅⋅

+

=

<<

(17)

Quenching of Excited States: diffusion and viscosity

• Diffusion

• 1, 2, 3 모두 k

diff

와 관계가 있다.

(modified)

2000 8

.

: solute

solvent

poise) (in

viscosity :

) sec mol

(l 3000

8

-1 1

실제론 작다

값보다 원래

식으론 이

때 작을 차이가

크기 의

η η

η

k RT k RT

diff diff

=

=

이에 적용 안 되는 것은

모양이 구형이 아닌 경 우

Solvent 와 solute의

size 차이가 많이 나는

(18)

Quenching of Excited States: Quenching Process and Mechanism

• Quenching Processes and Quenching Mechanisms

Photochemical Quenching

Products 예: cis-trans isomer.

M

• M* self-quenching

Physical Quenching

Q

Impurity quenching

e- transfer energy transfer heavy atom quenching

(구분이 어렵다. 산화환원 전위 등이 뒷받침 되면 가능)

(19)

Quenching of Excited States: Quenching Process and Mechanism

• Electron Transfer

*

Et3N Et3N

H

H H H

H H

H

H H H H

dimeri- zation

H-abstrac tion

H2O

S1 Ox-Red 쉽다.

=> e-transfer에 의하여

전하가 생성되어 극성 물

질이 생성되어, 용매

(20)

Quenching of Excited States: Quenching Process and Mechanism

• Electron Transfer

1 *

AD

; Exciplex

A D Polar solvents에서 형성이 잘 된다.

Polar solvents에서는 안정화되기 때문에 쉽게 떨어져 나간다.

Non-polar solvent에서는 exciplex형태로 남아 있으려 한다.

Solvent Polarity를 증가시키면:

Exciplex fl. 감소 (charged species 로 전환된다.) Fl. Lifetime 감소

Fl. Lifetime의 감소 폭이 fl. Intensity 감소 폭 보다 적다. Why?

(21)

Quenching of Excited States: Quenching Process and Mechanism

• Electron Transfer

• Fl. Lifetime의 감소 폭이 fl. Intensity 감소 폭 보다 적다. Why?

Fl. Lifetime: exciplex에서 kD, kR process로 감소

Fl. Intensity: kD, kR process로 감소 + encounter complex에서 kQ

1 A D *

Exciplex A

s

D

s

1 AD 1 A + D *

Solvated Ion Pair

A + D

A + D +hv k

D

k

F

k

C

k

R

k

Q

Encounter complex

(22)

Quenching of Excited States: Quenching Process and Mechanism

• Fl. Lifetime의 감소 폭이 fl. Intensity 감소 폭 보다 적다. Why?

1 A D*

Exciplex As Ds

1AD 1A + D*

Solvated Ion Pair

A + D

A + D +hv

kD kF kC

kR kQ

Encounter complex

F Q

C C

D R

F

D R

F F

k k

k

k k

k y

probabilit

k k

exciplex k

τ

τ

F

F F

k

) k formation exciplex

( of

Yields Quantum

lifetime 1

= +

+

× +

=

Φ

=

+

= +

=

Solvent polarity

=> kQ

=> ΦF

(23)

Quenching of Excited States: Free Energy of Electron Transfer

• Rehm and Weller

by released energy

E.

Coulombic :

affinity electron

: ) / (

potential ionization

: ) /

(

) / ( )

/ (

2

00 2

23

a e

A A

E

D D E

a E A e

A E D

D E

G

= − − − Δ

Δ

+

+

ε

ε

1

M Q

*

M

s Qs

1

M + Q

*

Solvated Ion Pair k12

k21

k23

2

k32

2 2

Ms Q

2

s 7A

M + Q + hv

(~15A 까지 e-transfer가능)

Products encounter

complex

(24)

Quenching of Excited States: Free Energy of Electron Transfer

• Rehm and Weller

Eq.

[ ]

: :

:

1 ) )( 1

1 2

1 2

( 1 :

:

4

) (

:

) /

exp(

) /

exp(

25 . 0 1

10 0 . k 2

) / ( )

/ ( 2

5 2

2 1

2

2 23 23

* 23

* 23 23

10 Q

00 2

23

D n d

D n

d r

e r G G G

RT G

RT G

a E A e

A E D

D E G

o o i

o i

− +

= +

=

Δ

= + Δ

Δ

Δ +

Δ +

= ×

Δ

= Δ

+

λ λ λ

λ λ

λ λ

λ

ε

Inner sphere recognition term (donor acceptor 사이의 through bond electron transfer에서 orientation 과 같은 것에 의하여 결정

Outer sphere recognition term

D-A 사이의 거리

용매의 Optical dielectric const.

Static dielectric const.

Encounter complx. 에서 sip 형성에 필요한 activation Energy.

(25)

Quenching of Excited States: Electron transfer 와 energy transfer

• Inner and Outer change

(26)

Quenching of Excited States: Free Energy of Electron Transfer

• Rehm and Weller Eq.

[ exp( / ) exp( / ) ]

25 . 0 1

10 0 .

k 2

*

23 23

10

Q

+ Δ

G RT

+ Δ

G RT

= ×

0 10

-10 -20

-30 1010 L/mol sec

KQ

Kdiff

KQ α exp(-ΔG23*/RT)

ΔG23

exothermic Kcal/mol

(27)

Quenching of Excited States: Free Energy of Electron Transfer

λ

λ ) (

) exp(

2 0

+

= Δ Δ

Δ

= −

G G

RT A G

k

et

Rxn coordinate

energy

D*-A

D+-A-

내려가면 Ea 줄고 Rate 증가

Avoid crossing D*-A

D+-A-

옆식에서

이 - 로 많아 지게

되면 이 더 커지게

된다.

G0

Δ

Δ G

(28)

Quenching of Excited States: Electron transfer 와 energy transfer

Potential energy description of an electron transfer reaction with ΔG = 0

(29)

Quenching of Excited States: Free Energy of Electron Transfer

λ λ 4

) (

) exp(

2 0 +

= Δ Δ

Δ

= −

G G

RT A G

ket

energy D*-A

D+-A-

Reaction coordinate 에 변화가 적은 경우

D*-A

D+-A-

이 - 로 많아 지게

되면 이 더 커지게

된다.

이것의 의미는?

G0

Δ

Δ G

Inverted region kQ

(30)

Quenching of Excited States: Electron transfer

Inverted region

(31)

Quenching of Excited States: Electron transfer

Thermodynamics and Kinetics

(32)

Quenching of Excited States: Heavy atom Quenching

• Heavy atom effects

• Exciplex 형성 때 heavy atom이 isc 증가 시킨다.

1 M Q * 1 M + Q * k

12

k

21

M + Q + hv

heavy atom effect

3 M Q *

3 M Q *

+

ISC

(33)

Quenching of Excited States: Oxygen Quenching

• Oxygen quenching effects

• Diffusion에 의해 highly excited triplet exciplex 형성한 다음 singlet oxygen까지 형성한다.

3 MO *

2

1 M + O *

2

highly excited triplet exciplex

n

ic 3 *

MO

2

1 3 M + O *

2

1 M + O *

2

3 3 1

Spin forbidden process

Spin allowed process

Spin allowed process

k diff

(34)

Quenching of Excited States: Electron transfer 와 energy transfer

(35)

Quenching of Excited States: Electron transfer 와 energy transfer

Electron Exchange mechanisms

A theory of energy transfer by electron exchange was worked out by Dexter:

kET (exchange) = KJ exp(–2 rDA/L)

K is related to specific orbital interactions.

J is the normalized spectral overlap integral, where normalized means that both the emission intensity (ID) and extinction coefficient (eA) have been normalized to unit area on the wave number scale.

J, by being normalized does not depend on the actual magnitude of eA.

rDA is the donor-acceptor separation relative to their van der Waals radii, L.

(36)

Quenching of Excited States: Electron transfer 와 energy transfer

Förster (Coulombic) vs. Dexter (exchange)

The rate of dipole-induced energy transfer decreases as R–6 whereas the rate of exchange-induced transfer decreases as exp–(2r/L). This means that kET(exchange) drops to negligibly small values (relative to the donor lifetime) as the intermolecular (edge-to-edge) distance increases more than on the order of one or two molecular diameters (5-10Å).

• The rate of dipole-induced transfer depends on the oscillator

strength of the D* -> D and A -> A* radiative transitions, but the rate of the exchange-induced transfer is independent of the oscillator strength of the D* -> D and A -> A* transitions.

• The efficiency of energy transfer (fraction of transfers per donor lifetime ~ kET /kD) by the dipole mechanism depends mainly on the oscillator strength of the A -> A* transition (since a smaller oscillator strength for D* -> D is compensated by a slower radiative rate

constant), whereas the efficiency of energy transfer by the exchange interaction cannot be directly related an experimental quantity.

(37)

Quenching of Excited States: Electron transfer 와 energy transfer

• 왜 두 process를 동시에 고려하는가?

두 process 같이 일어난다.

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