# Steric augmentation of three-coordinate Cu(i) β-diketiminate isocyanide chromophores to achieve microsecond excited-state lifetime

## TL;DR

Researchers improved copper-based photosensitizers by increasing steric bulk, achieving microsecond excited-state lifetimes in copper complexes.

## Contribution

Steric augmentation of ligands in copper(i) complexes extends excited-state lifetimes to over 1 µs through structural design.

## Key findings

- Augmenting β-diketiminate and isocyanide ligands increases excited-state lifetimes up to 1 µs.
- Buried volume and solid angle analysis correlate with longer excited-state lifetimes.
- β-diketiminate steric effects have a stronger influence than isocyanide substituents.

## Abstract

Copper(i) complexes are promising alternatives to noble metal-based photosensitizers. A recently developed class of three-coordinate copper(i) charge-transfer chromophores, pairing β-diketiminate ligands with aryl isocyanides, exhibits excited-state lifetimes that are responsive to the steric profile of the isocyanide, reaching a value of nearly 300 nanoseconds when a substituted m-terphenyl isocyanide was used. In this work, we show that extreme steric augmentation of the β-diketiminate and m-terphenyl isocyanide ligands can lead to further improvements in excited-state lifetime, with one example surpassing 1 µs. This study includes a series of eight compounds of the general formula Cu(RNacNac)(CN-ArX2), where RNacNac is the substituted β-diketiminate and CN-ArX2 is the substituted m-terphenyl isocyanide. All eight compounds are crystallographically characterized, and their excited-state properties are thoroughly evaluated by a combination of steady-state and time-resolved photoluminescence studies. This combined structural/photophysical approach unveils a correlation between common steric parameters, derived from buried volume (%Vbur) and solid angle analysis, and the excited-state lifetime. Our investigation reveals that the steric profile of the β-diketiminate has a much larger impact on the steric profile and lifetime, whereas the m-terphenyl isocyanide substituents have an unsystematic and subtle influence on these two correlated parameters. Nevertheless, judicious pairing of the two ligands is critical for achieving long excited-state lifetimes.

Augmenting the steric profile of both the β-diketiminate ligand and the m-terphenyl isocyanide increases the excited-state lifetime in heteroleptic copper(i) complexes, reaching the microsecond range in one case.