Closo- or Nido-Carborane Diphosphane as Responsible for Strong Thermochromism or Time Activated Delayed Fluorescence (TADF) in [Cu(N^N)(P^P)]0/.

Adrián Alconchel,Pilar García-Orduña,Olga Crespo,M Concepción Gimeno

doi:10.1021/acs.inorgchem.1c03092

Adrián Alconchel, Pilar García-Orduña + Show 2 more

Open Access

https://doi.org/10.1021/acs.inorgchem.1c03092

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Abstract

Ortho-closo or ortho-nido-carborane-diphosphanes have been selected to prepare the heteroleptic cationic or neutral [Cu(N^N){(PPh2)2C2B10H10}]PF6 (1) and [Cu(N^N){(PPh2)2C2B9H10}] (2) [N^N = 2-(4-thiazolyl)benzimidazole], respectively. Complexes 1 and 2 display very different emissive behavior. Neutral complex 2 exhibits TADF (time activated delayed fluorescence) which has been studied both as powder and PMMA composite with similar ΔE(S1 – T1), τ(T1), and τ(S1) in both phases. Cationic complex 1 displays a much lower quantum yield than 2 and does not show TADF, but it exhibits a significant thermochromic luminescence, and its emission is very dependent on the medium. Theoretical studies show that metal–ligand (M–diphosphane) to ligand (L′, diimine) transitions, MLL′CT, are responsible of the transitions which originate the emissive properties, but with very different contribution of the copper center, carborane cluster, and diphosphane phenyl rings for 1 and 2.

Highlights

The synthesis and study of heteroleptic copper complexes [Cu(N^N)(P^P)]n (n = 0, 1) represents a growing field in the study of emissive coordination complexes.[1−12] One evident reason is the use of a cheaper metal (Cu) compared to those widely selected for the design of phosphors for emissive devices, such as Ir, Pt, or Au
In these systems, phosphorescence and TADF13 should be favored. Both emissive mechanisms lead to an increment in the number of useful transitions for an emissive device. These facts have prompted an important effort in the synthesis and study of heteroleptic copper complexes [Cu(N^N)(P^P)]0/+ for which the emission energies mostly lie in the yellow-green region
The neutral compound [Cu(N^N)(dppnc)] exhibits TADF and singlet and triplet lifetimes; in addition, the ΔE(S1 − T1) is almost unmodified from the solid to PMMA composites at 5% wt

Summary

Introduction

Substitution of a mostly planar diimine by a nonplanar ligand, a diphosphane, in the heteroleptic species diminishes the flattening distortion of the tetrahedral geometry in the excited state, compared with the homoleptic bis(diimine) copper complexes. This flattening distortion enables quenching mechanisms of the luminescence. In these systems, phosphorescence and TADF (time activated delayed fluorescence)[13] should be favored. Both emissive mechanisms lead to an increment in the number of useful transitions for an emissive device. Studies of composites are growing,[20−25] including those in which these derivatives are tested in OLED devices,[26,27] revealing the potential of these systems

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