Influence of Imidazole Substituent Bulkiness on [CuI(PPh3)2N] Complexes with TADF Blue Solid-State Emission

TL;DR

This paper studies how the size of imidazole ligands affects the blue light emission of copper complexes used in OLEDs.

Contribution

The study introduces new Cu(I) complexes with varying alkyl chains and reveals how ligand bulkiness influences TADF properties.

Findings

Bulky ligands result in smaller singlet–triplet energy gaps, enhancing TADF behavior.

Alkyl chain size affects phosphorescence lifetimes and coordination geometry.

Imidazole ligands do not directly participate in emission but influence geometry around the Cu(I) center.

Abstract

Copper­(I) heteroleptic complexes with phosphine- and
nitrogen-containing
ligands are well-known thermally activated delayed fluorescence (TADF)
emitters and are of interest for OLED applications due to their cost-effectiveness.
In this work, we report the design and synthesis of a series of novel
Cu­(I) complexes featuring triphenylphosphine- and imidazole-based
ligands functionalized with varying alkyl chains. This allowed for
the evaluation of purely monodentate ligand systems and the impact
of the ligand bulkiness on the photophysical properties. Time-resolved
spectroscopy, combined with temperature-dependent measurements, revealed
blue solid-state emission and confirmed the TADF behavior across the
series. Room-temperature lifetimes reached as low as 0.5 μs,
facilitated by small singlet–triplet energy gaps ΔE
(S1–T1) between 110 and 60 meV. Bulky
ligands led to the smallest S–T gaps. A decrease in phosphorescence
lifetimes with increasing alkyl chain size suggested enhanced spin–orbit
coupling (SOC), attributed to a more tetrahedral coordination environment.
Quantum chemical calculations revealed that the imidazole moiety does
not participate directly in the frontier molecular orbitals, indicating
that the alkyl chains primarily influence the emission by modulating
the geometry around the Cu­(I) center.

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