Recent developments in In(III) coordination complexes: Singularity in structure-photophysics relationships

Abstract

Indium coordination complexes have recently emerged as a structurally versatile class of main-group luminophores in which excited-state behavior is governed predominantly by ligand architecture rather than intrinsic metal-centered transitions. This review covers structure–photophysics relationships across four representative ligand families—quinolinolate, salen/salophen, dipyrrin-based systems, and porphyrin/phthalocyanine—highlighting how donor–acceptor balance, ligand rigidity and π-conjugation collectively control charge-transfer, intra-ligand and π-delocalized states. Particular emphasis is placed on systems for which quantitative photophysical data (Φ, τ, k_r/k_nr, ISC efficiency) can be correlated with well-defined structural parameters. Comparisons with aluminum and gallium congeners show that indium's softer Lewis acidity and enhanced orbital polarizability generally lead to stronger ICT character, broader emission tunability and, in macrocyclic platforms, efficient triplet formation and singlet-oxygen generation. By analyzing common design motifs—such as μ-oxo-bridged quinolinolates, ICT-active In-salen luminophores and axially modulated In–porphyrin/phthalocyanine macrocycles—this review identifies recurring principles that link localized CT emission to fully delocalized π-systems. The resulting framework provides a basis for rational/systematic ligand modification in In(III) complexes, allowing established aluminum-based design rules to be extended and refined towards indium-based emitters and photosensitizers as prominent optoelectronic materials.