# Positional Tuning of Photophysics and Catalysis in Methoxy-Substituted Heteroleptic Copper(I) Complexes

## TL;DR

This study explores how the position of methoxy groups on copper(I) complexes affects their light absorption and catalytic performance in chemical reactions.

## Contribution

The paper establishes position-dependent structure-property-performance relationships in methoxy-substituted copper(I) photosensitizers.

## Key findings

- Ortho/para-substitution improves absorptivity, emission, and excited-state lifetimes compared to meta/unsubstituted complexes.
- The ortho-substituted complex shows the strongest electron-donating effect and least oxidizing excited state potential.
- Para isomer achieved the highest hydrogen evolution rate and turnover number in photocatalytic reactions.

## Abstract

A series of heteroleptic copper­(I) photosensitizers based
on methoxy-substituted
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline ligands was synthesized
to investigate the influence of substitution patterns on structure
and function. Methoxy groups were introduced in ortho-, meta-, and para-positions of
the phenyl rings. Single-crystal X-ray diffraction and DFT calculations
confirmed the expected tetrahedral geometry with position-dependent
aryl torsion. Photophysical studies reveal that ortho/para-substitution enhances absorptivity, emission
quantum yields, and excited-state lifetimes compared to the meta/unsubstituted complexes. The ortho-substituted complex shows the strongest electron-donating effect,
reflected in the most cathodic ligand reduction
(E1/2red=−2.11V)
and the least oxidizing excited state potential
(E* = 0.46 V). Temperature-dependent luminescence
and emission lifetimes are consistent with thermally activated delayed
fluorescence (TADF) across the series and reveal substitution-controlled
singlet–triplet energy gaps ΔE
ST. Complemented by step-scan FTIR studies, the predominant excited
state was identified and analyzed, highlighting the impact of spin
density location on both energy- and electron-transfer reactivity.
The photocatalytic relevance was demonstrated in three benchmark reactions:
singlet oxygen generation (energy transfer, demonstrated by the photooxidation
of diphenylfuran to cis-dibenzoylethylene), hydrogen
evolution from water, and reductive dehalogenation of aryl halides
(electron transfer). In hydrogen evolution, the para isomer gave the highest initial rate and a TON of 590 at 20 h, while
the ortho isomer remained active up to 36 h with
a TON of 530. Stern-Volmer quenching in THF with TEA confirms a reductive
pathway under these conditions. In the photocatalytic dehalogenation,
activity trends were substrate-dependent, reflecting a balance between
excited-state driving force (E*) and ground-state
reducing power
(E1/2red)
. Together, these results establish clear
position-property-performance relationships to guide Cu­(I) photosensitizer
design.