# Iron N‑Heterocyclic Carbene Photoactive Complexes with Rigid Phenylethynyl Substituents as Ligand π‑System Extensions

## TL;DR

This paper explores how modifying iron complexes with phenylethynyl groups affects their light-absorbing and charge transfer properties, which could be useful for photosensitizing applications.

## Contribution

The study introduces a new approach to extending ligand π-systems in iron complexes to enhance their charge transfer states.

## Key findings

- Phenylethynyl moieties red shift absorption and increase the extinction coefficient of the complexes.
- Modified complexes show 3MLCT state stabilization of about 0.3 eV and longer lifetimes (∼17 ps) compared to the parent complex.
- The electronic effects of substituents on phenylene units had only minor impact on the complexes' properties.

## Abstract

The design of iron complexes with long-lived charge transfer
states
suitable for applications as photosensitizers remains a formidable
challenge. Here, we investigated the effect of an extended ligand
π-system on the ground- and excited-state properties of iron­(II)
complexes with N-heterocyclic carbene (NHC) ligands.
For this purpose, three iron complexes based on the established [Fe­(II)­(pbmi)2]2+ motif (pbmi = (1,1′-(pyridine-2,6-diyl)­bis­(3-methylimidazole-2-ylidene)))
have been modified with phenylethynyl moieties attached to the pyridine
part of the ligand. In general, the introduction of the phenylethynyl
units served to red shift the main absorption band, as well as to
increase the extinction coefficient of the same, compared to the parent
complex. The lowered MLCT energies are in line with the electrochemical
data that revealed substantially easier reduction of the phenylethynyl-modified
ligands, while the potentials of the Fe­(III/II) couple are only moderately
increased. Only minor modifications of the electronic effect intrinsic
to the phenylethynyl moieties could be implemented with bromide and
dimethylamino substituents on the phenylene units. As a result, all
three complexes experience similar stabilization of their 3MLCT states, about 0.3 eV compared to the parent complex, and feature
transient absorption data in line with ES dynamics that are dominated
by a moderately long-lived (∼17 ps) 3MLCT state.
These values exceed the 3MLCT lifetimes reported for the
parent complex (up to 9 ps) and resemble the results for carboxylic
acid and imidazolinium derivatives with comparable 3MLCT
energies and lifetimes.