1.2 1.6 2.0 2.4 2.8 3.2

Energy (eV)

T1 S

1

T1

T1

T1

T1 S

1

S1

S1

S1

P1

P2

P4

P6

P8

PL

In

ten

sit

y (

a.u

.)

P7

P3

P5

T1 S

1

T1

T1

S1

S1

Ab

so

rptio

n (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.

The energy gap law for triplet states in Pt-containing phenylene ethynylene polymers and monomers

Joanne S. Wilson, Nazia Chawdhury, Richard Friend, Anna KöhlerUniversity of Cambridge, Cavendish Laboratory, Cambridge, United Kingdom

Muna R.A. Al-Mandhary, Muhammad Khan Paul RaithbySultan Qaboos University, Sultanate of Oman University of Cambridge, Dept. of Chemistry, United Kingdom

0. Introduction

References

[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)

This work is published as J. Wilson et al., J. Am. Chem. Soc. 123, 9412, (2001)

1. Materials 3. Decay rates

4. Decay rates - results

6. Summary

Pt RnP(C4H9)3

P(C4H9)3

Polymer

R =

O

O

S

N

S S

NN

PhPh

S S S

N NS

1.

2.

3.

4.

5.

6.

7.

8.

2. Photoluminescence

0.1

1.0

0 20 40 60 80 100 120

Inte

nsi

ty o

f E

mis

sio

n (

a.u

.)

Time (ms)

P2

P8P6

P4

20 K

P5

P7

P1

P3

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.

The lifetime of the triplet T1 emission reduces also with T1 energy from 112 ms to 0.2 ms

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:

τT = 1/(kr+ knr) (1)

ΦP = ΦISC kr τT (2)

Combining (1) and (2):

knr = (1-(ΦP /ΦISC)) / τT

For these Pt-containing materials ΦISC 1

So the non-radiative and radiative decay rates are:

kr = ΦP / τT

knr increases exponentially with decreasing triplet energy knr exp(-ΔE)

At best (for Pt-polymer with T1 at 2.4 eV) knr kr

Non-radiative decay rates (knr = (1-ΦP)/τT)

Radiative decay rates (kr = ΦP / τT)

Triplet emission in materials containing Pt-partially allowed kr ~ 103 s-1

kr is determined by: kr <μ>2(ΔE)3

T1

S0

Configuration coordinate (Q)

Pot

entia

l ene

rgy S1

5. Decay Mechanism

Non-radiative decay

• Via phonons emission• By energy gap law[3,4]:

knr exp (-γΔE / ω)

• Exponential ΔE dependence red phosphorescence is difficult to detect

• Large ΔE and small phonon energy ω low knr

Radiative decay

• Via dipole emission• By Strickler-Berg law

kr <μ>2(ΔE)3

The Triplet decay is controlled by thenon-radiative mechanisms (knr > kr).

• 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.

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.

• 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.

To investigate this we:

knr = (1- ΦP) / τT

• Large ΔE large kr

• Cubic ΔE dependence

0

1 103

2 103

3 103

4 103

5 103

6 103

7 103

0 2 4 6 8 10 12 14 16

monomerpolymer

kr (

s-1)

(Triplet Energy)3 (eV3)

6

8

10

12

14

16

18

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

ln k

nr

Triplet Energy (eV)

300 K

20 K

Polymers

108

106

104

102

100

knr of T1

k (s-1)

kr of T1 in Pt-polymer

kr of T1 in organic molecules

kr of S1in organic molecules

AcknowledgmentsThe Royal Society, London, UK Peterhouse, Cambridge, UKEPSRC, UK Sultan Qaboos University, OmanCambrige Display Technology, Cambridge, UK