Quenching of Excited States
홍익대학교
신동명
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 경우에는 사용할 수 없다.
• Freeze-pump-thaw : 10
-5Torr (3번 반복)
M M
*⎯ ⎯→
[Q][ ] O
2= 10
10X 10
−4= 10
6k
qQuenching of Excited States: Excimer & Exciplex
• Excimer
excimer Monomer
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’)
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
Quenching of Excited States: Excimer & Exciplex
• Excimer formation
(n=4)3.73A
(CH
2)
n(CH
2)
nhv (254nm) hv (> 284nm)
(CH2)3
low temp
Alkyl benzene의 absorption spec.
Excimer fl 보임.
이 물질이
naphthanlene 의 . Excimer fl 보임.
Quenching of Excited States: Excimer & Exciplex
• Exciplex formation
Exciplex는 dipolar 한 성격때문에 용매의 극성이 증가하면 red shift
농도에 따라서 isosbestic point를 갖으면서
spectrum이 변한다.
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
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
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?
Quenching of Excited States: Excimer & Exciplex
• 예2
•
• 예3
• 예4 :
Triplet – Triplet annihilation
hv
O *
+
R
CN O
R CN
*
O
R CN
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
0time life
:여기상태 τ
) (
sec 10
~ 10
. rate
controlled diffusion
:
1 1
10
9
용매에 따라 다름
이다 경우
많은
−
− mol
⋅
LkQ
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.
Quenching of Excited States: Kinetics
• Perrin Model
VN
Q기울기 :
] [Q
F
F
φ φ
0Q
Life time에 관계 없고 농도에 비례
기울기 :
] [Q
F
F
φ φ0
time life :여기상태 τ
Qτ k
참고
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
+
⋅⋅
⋅⋅
+
=
⋅⋅
⋅⋅
+
⋅⋅
⋅⋅
=
+
⎯→
⎯
⋅⋅
⋅⋅
⋅ +
−
−
−
⎯
⎯ →
⎯
⎯
⎯⎯
← −
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
의
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
≈
+
→
⋅⋅
⋅⋅
⋅
⋅ =
=
>>
⋅⋅
⋅⋅
+
=
<<
−
−
−
←→
−
Quenching of Excited States: diffusion and viscosity
• Diffusion
• 1, 2, 3 모두 k
diff와 관계가 있다.
(modified)
2000 8
.
: solute
solvent
poise) (in
viscosity :
) sec mol
(l 3000
8
-1 1실제론 작다
값보다 원래
식으론 이
때 작을 차이가
크기 의
와
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
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)
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
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?
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 로
1 A D *
Exciplex A
sD
s1 AD 1 A + D *
Solvated Ion Pair
A + D
A + D +hv k
Dk
Fk
Ck
Rk
QEncounter complex
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
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*
Ms Qs
1
M + Q*
Solvated Ion Pair k12
k21
k23
2
k322 2
Ms Q2
s 7AM + Q + hv
(~15A 까지 e-transfer가능)
Products encounter
complex
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.
Quenching of Excited States: Electron transfer 와 energy transfer
• Inner and Outer change
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
Quenching of Excited States: Free Energy of Electron Transfer
λ
λ ) (
) exp(
2 0
+
= Δ Δ
Δ
= −
≠
≠
G G
RT A G
k
etRxn 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 ≠
Quenching of Excited States: Electron transfer 와 energy transfer
•
Potential energy description of an electron transfer reaction with ΔG = 0Quenching 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
Quenching of Excited States: Electron transfer
•
Inverted regionQuenching of Excited States: Electron transfer
•
Thermodynamics and KineticsQuenching of Excited States: Heavy atom Quenching
• Heavy atom effects
• Exciplex 형성 때 heavy atom이 isc 증가 시킨다.
1 M Q * 1 M + Q * k
12k
21M + Q + hv
heavy atom effect
3 M Q *
3 M Q *
+ISC
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
21 3 M + O *
2
1 M + O *
2
3 3 1
Spin forbidden process
Spin allowed process
Spin allowed process
k diff
Quenching of Excited States: Electron transfer 와 energy transfer
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.
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.
Quenching of Excited States: Electron transfer 와 energy transfer
• 왜 두 process를 동시에 고려하는가?
두 process 같이 일어난다.