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

1. The energy gap law for triplet states in Pt-containing phenylene ethynylene polymers and monomers 1. 5. k (s-1) 108 kr of S1in organic molecules 106 6. knr of T1 2. 104 S1 krof T1 in Pt-polymer 102 T1 kr of T1 in organic molecules S0 100 Potential energy Configuration coordinate (Q) 7. 3. 8. 4. 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: Direct phosphorescence from triplet T1 states has now been observed in a few conjugated polymers such as polyfluorenes[1] and polyphenylene-ethynylenes[2]. • 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. But: in all these materials the triplet T1 state is at high energy.  phosphorescence was never observed in the red spectral range. 2. Photoluminescence 3. Decay rates 1. Materials 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 krandknrand the efficiency of intersystem crossing ΦISC in the following way: Polymer R = 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. τT = 1/(kr+ knr) (1) ΦP = ΦISCkrτT (2) Combining (1) and (2): knr = (1-(ΦP/ΦISC)) / τT 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. For these Pt-containing materials ΦISC 1 So the non-radiative and radiative decay rates are: knr = (1- ΦP) / τT The lifetime t of the triplet T1 emission reduces also with T1 energy from 112 ms to 0.2 ms kr = ΦP / τT 4. Decay rates - results Non-radiative decay rates (knr = (1-ΦP)/τT) 6. Summary The Triplet decay is controlled by the non-radiative mechanisms (knr > kr). 5. Decay Mechanism  knr increases exponentially with decreasing triplet energy knr exp(-ΔE) At best (for Pt-polymer with T1 at 2.4 eV) knr kr • knrexp (-γΔE / ω) •  High energy triplets intrinsically have the most efficient emission. Radiative decay rates (kr = ΦP / τT) Radiative decay 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 • Via dipole emission • By Strickler-Berg law • kr<μ>2(ΔE)3 • 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. • Cubic ΔE dependence • Large ΔE large kr 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) Triplet emission in materials containing Pt-partially allowed kr ~ 103 s-1 Acknowledgments The Royal Society, London, UK Peterhouse, Cambridge, UK EPSRC, UK Sultan Qaboos University, Oman Cambrige Display Technology, Cambridge, UK kr is determined by: kr<μ>2(ΔE)3