Solvent-Responsive Luminescence of an 8-Hydroxyquinoline-Modified 1H-Imidazo[4,5-f][1,10]phenanthroline Ligand and Its Cu(I) Complexes: Excited-State Mechanisms and Structural Effects

TL;DR

This paper explores how different solvents affect the light emission of copper complexes, revealing how their structures influence luminescence behavior for sensor applications.

Contribution

The study reveals solvent-dependent excited-state mechanisms in Cu(I) complexes modified with an 8-hydroxyquinoline ligand.

Findings

Free ipqH2 shows dual emission via ESPT and tautomerization in alcohol/DMSO mixtures.

Cu(I) complexes exhibit red-shifted emission in water-rich DMSO due to (IL + ML)CT states.

Flexible diphosphine ligands enhance DMF sensitivity, while rigid ligands show weaker solvent responses.

Abstract

Understanding how solvents influence the luminescence behavior of Cu(I) complexes is crucial for designing advanced optical sensors. This study reports the synthesis, structures and photophysical investigation of an 8-hydroxyquinoline-functionalized 1H-imidazo[4,5-f][1,10]phenanthroline ligand, ipqH2, and its four Cu(I) complexes with diphosphine co-ligands. Photoluminescence studies demonstrated distinct solvent-dependent excited-state mechanisms. In DMSO/alcohol mixtures, free ipqH2 exhibited excited-state proton transfer (ESPT) and enol-keto tautomerization, producing dual emission at about 447 and 560 nm, while the complexes resisted ESPT due to hydrogen bond blocking by PF6− anions and Cu(I) coordination. In DMSO/H2O, aggregation-caused quenching (ACQ) and high-energy O–H vibrational quenching dominated, but complexes 1 and 2 showed a significant red-shifted emission (569–574 nm) with high water content due to solvent-stabilized intra-ligand charge transfer and metal-to-ligand charge transfer ((IL+ML)CT) states. In DMSO/DMF, hydrogen bond competition and solvation-shell reorganization led to distinct responses: complexes 1 and 3, with flexible bis[(2-diphenylphosphino)phenyl]ether (POP) ligands, displayed peak splitting and (IL + ML)CT redshift emission (501 ⟶ 530 nm), whereas complexes 2 and 4, with rigid 9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos), showed weaker responses. The flexibility of the diphosphine ligand dictated DMF sensitivity, while the coordination, the hydrogen bonds between PF6− anions and ipqH2, and water solubility governed the alcohol/water responses. This work elucidates the multifaceted solvent-responsive mechanisms in Cu(I) complexes, facilitating the design of solvent-discriminative luminescent sensors.

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