Deep-red electrophosphorescence from a platinum(II)–porphyrin complex copolymerised with polyfluorene for efficient energy transfer and triplet harvesting

David M.E Freeman,Hugo Bronstein,Silvia Araguas Rodriguez,Franco Cacialli,Kealan J Fallon,Giulia Tregnago

doi:10.1080/21606099.2015.1047473

David M.E Freeman, Hugo Bronstein + Show 4 more

Open Access

https://doi.org/10.1080/21606099.2015.1047473

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Abstract

A series of polyfluorene-based polymers with a range of weight percentages (w/w) of a platinum(II)-containing porphyrin, 5,15-dimesityl-10,20-diphenylporphyrinato platinum(II) (MPP(Pt)), were synthesised and incorporated into organic light-emitting diodes. All polymers showed emission predominantly in the red/NIR region with only those polymers with porphyrin w/w of less than 2% showing residual tails at wavelengths lower than 600 nm, indicating increased emission from the porphyrin as w/w increases. The 2% loading of MPP(Pt) gave the highest efficiency LED (0.48%) and light output (2630 mW/m2).

Highlights

Phosphorescent emitters are of great interest in organic light-emitting diodes (OLEDs) due to their potential to achieve near to 100% internal quantum efficiency.[1]
The good spectral overlap between the MPP(Pt) absorption and the PFO emission (Fig. S1) corroborates this hypothesis: both the Soret band and the Q-band of the Pt(II)–porphyrin significantly overlap with the PFO emission
The radiative lifetime taken near the peak emission wavelength of the PFO (440 nm) confirms the presence of efficient energy transfer from the PFO to the porphyrins

Summary

Introduction

Phosphorescent emitters are of great interest in organic light-emitting diodes (OLEDs) due to their potential to achieve near to 100% internal quantum efficiency.[1] Upon incorporation of heavy metal atoms with strong spin–orbit coupling such as platinum(II) or iridium(III), the system is distorted such that radiative transitions from triplet states, that is, where S = 0, become feasible. Since the first investigation of the electroluminescence (EL) from Pt(II)–porphyrin complexes [2] and Ir(ppy)3,[3] many groups have reported the use of such complexes in blend with a polymer host. Phosphorescent dopant aggregation and phase separation are disadvantages of blend architecture, due to the increased phosphorescent quenching rate.[4,5] While synthetically more challenging, copolymerisation overcomes these disadvantages.[6,7] Having the dopant covalently bonded to the polymer will both increase the rate of energy transfer and reduce loss processes by maintaining the absorbing and emitting moieties at fixed distance, preventing intermolecular interaction between dopant molecules. Still few investigations have been reported for the incorporation of Pt(II)– porphyrin directly in the polymer backbone.[16,17,18] Of the few previously reported polymers, porphyrins are generally incorporated by the β-pyyrollic position.[16,18] Meso incorporation into a polyfluorene polymer has previously been achieved by Xiang but was studied only in the context of oxygen sensing and was synthesised via a less controlled (compared to Suzuki) Yamamoto reaction.[17]

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