Volume 83, Issue 4 p. 217-229
Full Paper

CF3 Substitution of [Cu(P^P)(bpy)][PF6] Complexes: Effects on Photophysical Properties and Light-Emitting Electrochemical Cell Performance

Sarah Keller

Sarah Keller

Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, 4058 Basel, Switzerland

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Fabian Brunner

Fabian Brunner

Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, 4058 Basel, Switzerland

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Dr. José M. Junquera-Hernández

Dr. José M. Junquera-Hernández

Instituto de Ciencia Molecular, Universidad de Valencia, 45980 Paterna, Valencia, Spain

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Dr. Antonio Pertegás

Dr. Antonio Pertegás

Instituto de Ciencia Molecular, Universidad de Valencia, 45980 Paterna, Valencia, Spain

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Maria-Grazia La-Placa

Maria-Grazia La-Placa

Instituto de Ciencia Molecular, Universidad de Valencia, 45980 Paterna, Valencia, Spain

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Dr. Alessandro Prescimone

Dr. Alessandro Prescimone

Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, 4058 Basel, Switzerland

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Prof. Dr. Edwin C. Constable

Prof. Dr. Edwin C. Constable

Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, 4058 Basel, Switzerland

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Dr. Henk J. Bolink

Dr. Henk J. Bolink

Instituto de Ciencia Molecular, Universidad de Valencia, 45980 Paterna, Valencia, Spain

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Prof. Dr. Enrique Ortí

Corresponding Author

Prof. Dr. Enrique Ortí

Instituto de Ciencia Molecular, Universidad de Valencia, 45980 Paterna, Valencia, Spain

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Prof. Dr. Catherine E. Housecroft

Corresponding Author

Prof. Dr. Catherine E. Housecroft

Department of Chemistry, University of Basel, BPR 1096, Mattenstrasse 24a, 4058 Basel, Switzerland

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First published: 09 January 2018
Citations: 44

bpy=2,2′-bipyridine

Graphical Abstract

A light touch: Emissive [Cu(P^P)(N^N)][PF6] complexes with P^P=bis[2-(diphenylphosphino)phenyl]ether or xantphos and N^N=CF3-substituted 2,2′-bipyridines are reported (see figure). On going from solution to frozen 2-methyltetrahydrofuran, the excited-state lifetimes increased, thus indicating thermally activated delayed fluorescence. Three compounds were incorporated into light-emitting electrochemical cells. The influence of the ligand substitution pattern on their performance is discussed.

Abstract

Herein, [Cu(P^P)(N^N)][PF6] complexes (P^P=bis[2-(diphenylphosphino)phenyl]ether (POP) or 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos); N^N=CF3-substituted 2,2′-bipyridines (6,6′-(CF3)2bpy, 6-CF3bpy, 5,5′-(CF3)2bpy, 4,4′-(CF3)2bpy, 6,6′-Me2-4,4′-(CF3)2bpy)) are reported. The effects of CF3 substitution on their structure as well as their electrochemical and photophysical properties are also presented. The HOMO–LUMO gap was tuned by the N^N ligand; the largest redshift in the metal-to-ligand charge transfer (MLCT) band was for [Cu(P^P){5,5′-(CF3)2bpy}][PF6]. In solution, the compounds are weak yellow to red emitters. The emission properties depend on the substitution pattern, but this cannot be explained by simple electronic arguments. Among powders, [Cu(xantphos){4,4′-(CF3)2bpy}][PF6] has the highest photoluminescence quantum yield (PLQY; 50.3 %) with an emission lifetime of 12 μs. Compared to 298 K solution behavior, excited-state lifetimes became longer in frozen Me-THF (77 K; THF=tetrahydrofuran), thus indicating thermally activated delayed fluorescence (TADF). Time-dependent (TD)-DFT calculations show that the energy gap between the lowest-energy singlet and triplet excited states (0.12–0.20 eV) permits TADF. Light-emitting electrochemical cells (LECs) with [Cu(POP)+(6-CF3bpy)][PF6], [Cu(xantphos)(6-CF3bpy)][PF6], or [Cu(xantphos){6,6′-Me2-4,4′-(CF3)2bpy}][PF6] emit yellow electroluminescence. The LEC with [Cu(xantphos){6,6′-Me2-4,4′-(CF3)2bpy}][PF6] had the fastest turn-on time (8 min), and the LEC with the longest lifetime (t1/2=31 h) contained [Cu(xantphos)(6-CF3bpy)][PF6]; these LECs reached maximum luminances of 131 and 109 cd m−2, respectively.

Conflict of interest

The authors declare no conflict of interest.