P∩N Bridged Cu(I) Dimers Featuring Both TADF and Phosphorescence. From Overview towards Detailed Case Study of the Excited Singlet and Triplet States.

Thomas Hofbeck,Michel Fleck,Uwe Monkowius,Thomas A Niehaus,Hartmut Yersin

doi:10.3390/molecules26113415

Abstract

We present an overview over eight brightly luminescent Cu(I) dimers of the type Cu2X2(P∩N)3 with X = Cl, Br, I and P∩N = 2-diphenylphosphino-pyridine (Ph2Ppy), 2-diphenylphosphino-pyrimidine (Ph2Ppym), 1-diphenylphosphino-isoquinoline (Ph2Piqn) including three new crystal structures (Cu2Br2(Ph2Ppy)3 1-Br, Cu2I2(Ph2Ppym)3 2-I and Cu2I2(Ph2Piqn)3 3-I). However, we mainly focus on their photo-luminescence properties. All compounds exhibit combined thermally activated delayed fluorescence (TADF) and phosphorescence at ambient temperature. Emission color, decay time and quantum yield vary over large ranges. For deeper characterization, we select Cu2I2(Ph2Ppy)3, 1-I, showing a quantum yield of 81%. DFT and SOC-TDDFT calculations provide insight into the electronic structures of the singlet S1 and triplet T1 states. Both stem from metal+iodide-to-ligand charge transfer transitions. Evaluation of the emission decay dynamics, measured from 1.2 ≤ T ≤ 300 K, gives ∆E(S1-T1) = 380 cm−1 (47 meV), a transition rate of k(S1→S0) = 2.25 × 106 s−1 (445 ns), T1 zero-field splittings, transition rates from the triplet substates and spin-lattice relaxation times. We also discuss the interplay of S1-TADF and T1-phosphorescence. The combined emission paths shorten the overall decay time. For OLED applications, utilization of both singlet and triplet harvesting can be highly favorable for improvement of the device performance.

Highlights

Potential applications of luminescent materials in organic light emitting diodes (OLEDs) have strongly stimulated the development of emitters that are suited for exploiting all singlet (25%) and triplet (75%) excitons [1] generated in the emission layer
We want to focus on complexes that exhibit at ambient temperature both thermally activated delayed fluorescence (TADF) and phosphorescence
We present an overview over a series of P∩N linked Cu(I) dimers of the type of Cu2X2(P∩N)3 with respect to their X-ray structures and with particular focus on photo-luminescence data

Summary

Introduction

Potential applications of luminescent materials in organic light emitting diodes (OLEDs) have strongly stimulated the development of emitters that are suited for exploiting all singlet (25%) and triplet (75%) excitons [1] generated in the emission layer. We want to focus on complexes that exhibit at ambient temperature both thermally activated delayed fluorescence (TADF) and phosphorescence They may be regarded as singlet and triplet harvesting materials. To evaluate the class of materials of Cu2X2(P∩N) complexes (with X = Cl, Br, I), we study eight compounds with respect to their crystal structures and, especially, their emission properties at T = 300 K and 77 K, respectively These data allow us to select one prominent material, Cu2I2(P∩N)3, 1-I with P∩N = 2-diphenylphosphino-pyridine (Ph2Ppy), that shows distinct TADF and phosphorescence at ambient temperature even at high emission quantum yield of ΦPL = 81% at relatively short decay time. Such materials showing both singlet and triplet harvesting character are attractive for applications that require short photoluminescence decay

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