# High-Resolution Tuning of Iridium(III) 4′-Aryl-terpy Chromophores: A Hammett Parameter-Guided General Methodology for Systematic Property Control through Orbital Decoupling

## TL;DR

This paper introduces a method to fine-tune the properties of iridium complexes using Hammett parameters, enabling precise control over their electronic and photophysical characteristics.

## Contribution

A general methodology is proposed for high-resolution tuning of iridium(III) chromophores using Hammett parameters to decouple and control orbital properties.

## Key findings

- Hammett parameters strongly correlate with redox potentials and energy gaps in iridium complexes.
- HOMO and LUMO are effectively decoupled, allowing independent control over their properties.
- The methodology is validated through experimental and computational data for emissive complexes.

## Abstract

A series of six phenyl-substituted
4′-phenyl-2,2′:6′,2″-terpyridine, RPhTerpy, iridium­(III) complexes of the form [Ir­(RPhTerpy)­(ppy)­Cl]­(PF6), where ppy is C^N cyclometalated 2-(phenyl)­pyridine,
and with Hammett parameters spanning 1.69 units, were synthesized
using microwave-assisted reaction procedures and characterized via
physical (cyclic voltammetry, NMR, crystallography) and photophysical
methods (absorption, emission, time-correlated single photon counting,
Franck–Condon line shape analysis). The iridium complexes’
redox potentials, electrochemically determined ground state HOMO–LUMO
gap (eHLG), photophysically determined energy gap between ground-
and excited-state (E
00, 77 K), estimated
excited-state reduction potential (E(Ir*/–)), and time-dependent density functional theory predicted HOMO–LUMO
gap and lowest energy transition (LET) correlate strongly to the ligands’
Hammett parameters, suggesting that the Hammett parameter can be used
as a convenient method to model and fine-tune the physical and photophysical
characteristics for this series of Ir­(III) complexes. These analyses
reveal similar correlations when applied to data for emissive complexes
previously reported. Experimental and computational modeling data
indicate that the HOMO and LUMO are effectively decoupled, suggesting
independent control over both is possible and offering a general methodology
for high-resolution design of desired characteristics.