Visible-light sensitized luminescent europium(III) molecular materials are of considerable importance because their outstanding photophysical properties make them well suited as labels in fluorescence-based bioassays and low-voltage driven pure red-emitters in optoelectronic technology. One challenge in this field is development of visible-light sensitizing ligands that can form highly emissive europium(III) complexes with sufficient stability and aqueous solubility for practical applications. Indeed, some of the recent reports have demonstrated that the excitation-window can be shifted to longer-wavelengths in europium(III)-β-diketonate complexes by appropriate molecular engineering and suitably expanded π-conjugation in the complex molecules. In this review, attention is focused on the latest innovations in the syntheses and photophysical properties of visible-light sensitized europium(III)-β-diketonate complexes and their application as bioprobes for cellular imaging. Furthermore, luminescent nanomaterials derived from long-wavelength sensitized europium(III)-β-diketonate complexes and their application in life sciences are also highlighted.
A novel class of efficient visible light sensitized antenna complexes of Eu(3+) based on the use of a series of highly conjugated β-diketonates, namely, 1-(1-phenyl)-3-(2-fluoryl) propanedione, 1-(2-naphthyl)-3-(2-fluoryl)propanedione, 1-(4-biphenyl)-3-(2-fluoryl) propanedione, and 2,2'-bis(di-p-tolylphosphino)-1,1'-binaphthyl oxide as an ancillary ligand has been designed, synthesized, characterized and their photophysical properties investigated. The coordination geometries of the typical Eu(3+) complexes were calculated using the Sparkle/PM3 model. Photophysical properties of europium complexes benefit from adequate protection of the metal by the rigid phosphine oxide ligand against non-radiative deactivation and efficient ligand-to-metal energy transfer exceeding 50% as compared to precursor samples. The replacement of the phenyl group with the naphthyl or biphenyl groups in the 3-position of the fluoryl based β-diketonate ligand remarkably extends the excitation window of the corresponding Eu(3+) complexes towards the visible region (up to 500 nm). The highly conjugated β-diketonate ligands sensitize efficiently the luminescence of Eu(3+) ions with quantum yields ranging from 19 to 43 % in the solid state, which is among the highest reported for a visible sensitized Eu(3+)complex. The theoretical quantum efficiencies from the Sparkle/PM3 structures are in good agreement with the experimental values, clearly attesting to the efficacy of the theoretical models.
The unique four-level photocycle characteristics of excited-state
intramolecular proton transfer (ESIPT) materials enable population
inversion and large spectral separation between absorption and emission
through their respective enol and keto forms. This leads to minimal
or no self-absorption losses, a favorable feature in acting as an
optical gain medium. While conventional ESIPT materials with an enol–keto
tautomerism process are widely known, zwitterionic ESIPT materials,
particularly those with high photoluminescence, are scarce. Facilitated
by the synthesis and characterization of a new family of 2-hydroxyphenyl
benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic
ESIPT lasing material (HBT-Fl2). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified
spontaneous emission (ASE) threshold of 5.3 μJ/cm2 with an ASE peak at 609 nm but also exhibits high ASE photostability.
Coupled with its substantially large Stokes shift (≈236 nm
≈10,390 cm–1) and an extremely small overlap
of excited-state absorption with ASE emission, comprehensive density
functional theory (DFT) and time-dependent DFT studies reveal the
zwitterionic characteristics of HBT-Fl2. In opposition
to conventional ESIPT with π-delocalized tautomerism as observed
in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through
the fluorene conjugation. This structural motif provides a design
tactic in the innovation of new zwitterionic ESIPT materials for efficient
light amplification in red and longer-wavelength emission.
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