Transcript No Slide Title

The energy gap law for triplet states in Pt-containing
phenylene ethynylene polymers and monomers
Joanne S. Wilson, Nazia Chawdhury, Richard Friend, Anna Köhler
University of Cambridge, Cavendish Laboratory, Cambridge, United Kingdom
Muna R.A. Al-Mandhary, Muhammad Khan
Paul Raithby
Sultan Qaboos University, Sultanate of Oman
University of Cambridge, Dept. of Chemistry, United Kingdom
0. Introduction
To investigate this we:
• Use a model system of polymers and monomers containing Pt where the T1 state emits.
• Measure phosphorescence  get decay rates of the triplet state.
• Relate decay rates to properties of the materials.
Direct phosphorescence from triplet T1 states has now
been observed in a few conjugated polymers such as
polyfluorenes[1] and polyphenylene-ethynylenes[2].
But:
in all these materials the triplet T1 state is at high energy.
 phosphorescence was never observed in the red spectral range.
2. Photoluminescence
1. Materials
P(C4H9)3
Pt
Polymer
R
n
P(C4H9)3
T
P1
R=
T
5.
O
S
S
Ph
Ph
N
N
6.
S
S
3.
S
7.
N
S
P3
T
S
1
S
T
P5
1
1
T
1
1
S
1
1
P6
N
T
8.
4.
1
S
1
P4
S
N
1
S
1
A bsorption (a.u.)
O
2.
T
P L Intensity (a.u.)
1.
P2
S
1
S
1
The relative
intensity of triplet T1
emission reduces
with T1 energy,
while the singlet S1
to triplet T1 energy
gap is constant at
0.7 eV.
1
4. Decay rates - results
1.2
1.6
2.0
2.4
2.8
So the non-radiative and radiative decay
rates are:
20 K
1.0
P2
P1
P4
0.1
P7
P6
P8
P5
20
40
60
Time (ms)
nr
knr = (1- ΦP) / τT
The lifetime t of the
triplet T1 emission
reduces also with
T1 energy from
112 ms to 0.2 ms
P3
80
100
k r = ΦP / τ T
120
6. Summary
5. Decay Mechanism
The Triplet decay is controlled by the
non-radiative mechanisms (knr > kr).
k (s-1)
12
108
10
20 K
106
S1
8
6
1.2
T1
1.4
1.6
1.8
2.0
2.2
2.4
(2)
For these Pt-containing materials ΦISC  1
3.2
Polymers
300 K
ΦP = ΦISC kr τT
knr = (1-(ΦP /ΦISC)) / τT
18
16
(1)
Combining (1) and (2):
1
E nergy (eV )
0
Non-radiative decay rates (knr = (1-ΦP)/τT)
S
1
τT = 1/(kr+ knr)
P8
Intensity of Emission (a.u.)
We use a conjugated platinum containing
polymer since the inclusion of platinum makes
the triplet state emissive and therefore
accessible via spectroscopy. The spacers R
are chosen to tune the optical absorption
across the whole visible spectral range.
ln k
Experimentally,we can measure the lifetime
τT and the PL quantum yield ΦP of the triplet
emission. These are related to the radiative
and non-radiative decay rates kr and knr and
the efficiency of intersystem crossing ΦISC in
the following way:
P7
T
14
3. Decay rates
2.6
104
102
S0
Triplet Energy (eV)
100
kr of S1in organic molecules
knr of T1
kr of T1 in Pt-polymer
kr of T1 in organic molecules
 knr increases exponentially with
decreasing triplet energy knr  exp(-ΔE)
At best (for Pt-polymer with T1 at 2.4 eV) knr  kr
Radiative decay rates (kr = ΦP / τT)
Radiative decay
• Via dipole emission
• By Strickler-Berg law
3
7 10
monomer
polymer
3
6 10
3
-1
kr (s )
5 10
Configuration coordinate (Q)
kr <μ>2(ΔE)3
3
4 10
Non-radiative decay
• Via phonons emission
• By energy gap law[3,4]:
knr  exp (-γΔE / ω)
3
3 10
• Cubic ΔE dependence • Exponential ΔE dependence
 red phosphorescence
is difficult to detect
3
2 10
3
1 10
0
0
2
4
6
8
10
3
12
14
16
3
(Triplet Energy) (eV )
Triplet emission in materials containing
Pt-partially allowed  kr ~ 103 s-1
kr is determined by:
kr <μ>2(ΔE)3
• Large ΔE  large kr
• Large ΔE and small phonon
energy ω  low knr
• knr  exp (-γΔE / ω)
 High energy triplets intrinsically have
the most efficient emission.
• Emission occurs via a multi-phonon emission
process - through vibration of bonds in the
material.
 Control of the phonon energy ω is needed.
 Rigid materials will have less non-radiative
decay.
References
Acknowledgments
[1] D. Hertel et al., Adv. Mater.13, 65 (2001)
[2] A. Köhler et al., submitted
[3] R. Englman et al., J. Mol. Phys. . 18, 145, (1970)
[4] W.Siebrand et al., J. Chem. Phys. 47, 2411, (1967)
The Royal Society, London, UK
Peterhouse, Cambridge, UK
EPSRC, UK
Sultan Qaboos University, Oman
Cambrige Display Technology, Cambridge, UK
This work is published as
J. Wilson et al., J. Am. Chem. Soc. 123, 9412, (2001)