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Cite This: J. Org. Chem. XXXX, XXX, XXX−XXX
Circularly Polarized Luminescence of Boronic Acid-Derived Salicylidenehydrazone Complexes Containing Chiral Boron as Stereogenic Unit Vicente G. Jiménez,† Fábio M. F. Santos,‡ Silvia Castro-Fernández,† Juan M. Cuerva,† Pedro M. P. Gois,‡ Uwe Pischel,*,§ and Araceli G. Campaña*,† †
Department of Organic Chemistry, University of Granada, Campus Universitario Fuentenueva, E-18071 Granada, Spain Research Institute for Medicines (iMed.ULisboa), Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-003 Lisbon, Portugal § CIQSO−Center for Research in Sustainable Chemistry and Department of Chemistry, University of Huelva, Campus de El Carmen s/n, E-21071 Huelva, Spain
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‡
S Supporting Information *
ABSTRACT: Racemic mixtures of boronic acid-derived salicylidenehydrazone (BASHY) complexes were enantiomerically resolved. The chiroptical properties of the stereoisomers, containing an asymmetric boron as the only stereogenic unit, are translated into mirror-imaged electronic circular dichroism spectra and circularly polarized luminescence (CPL, dissymmetry factors of 3−5 × 10−4) is observed. The spectral position of the CPL emission is determined by the push− pull character of the dye. These features expand the functional scope of the brightly emitting BASHY dye platform.
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based on boronic acid-derived salicylidenehydrazone (BASHY) complexes.49−51 These dyes are part of the larger family of heterocyclic boronate complexes with a tetracoordinated (stereogenic) boron center.52−57 Following our recently published work,49,50 it can be affirmed that these fluorophores are strong light absorbers (∼50000−60000 M−1 cm−1 for the long-wavelength absorption band at ∼470 nm), emit with maxima between 500 and 600 nm, and have high quantum yields (Φfluo ≈ 0.5−0.6) in nonpolar solvents. These complexes contain a single asymmetric boron center, and given the fact that we had focused so far on the racemic dyes, we became interested in separating the enantiomers and testing them as SOM-CPL systems. For this purpose, we selected dyes 1−5 from a recently prepared and photophysically characterized larger pool of BASHY dyes;50 see Figure 1. This selection was motivated by the previous observation49,50 that the varying push−pull character of 1−4 allows for fine-tuning the emission wavelength of the racemic dyes and thus, would also provide a means to control the spectral position of the CPL signal. Dye 5 was chosen for being a BASHY dimer with two chiral boron centers, embracing the additional challenge of also separating the chiroptically inactive meso diastereoisomer. The racemic dyes were prepared according to a recently published protocol, implying the condensation of the corresponding salicylidenehydrazone ligands and phenyl boronic acid (dyes 1−4) or a fluorene-derived bisboronic
ircularly polarized luminescence (CPL) is a phenomenon that has attracted growing attention in recent years. Although CPL from organometallic lanthanide complexes is a very well studied case,1−3 the observation of this effect for purely organic systems has created considerable interest only much more recently.4−6 Higher-order supramolecular assemblies 7−11 and small organic molecules (SOMs), e.g., helicenes,12−19 have been in the limelight of the related efforts. This has resulted in the design of architectures with interesting photophysical properties20−22 and applications as molecular switches/sensors,23−29 ratiometric probes,30 and in lightemitting devices,31−33 integrating CPL as output signal. A strong focus has been laid on the investigation of tetracoordinate organoboron dyes, among them boron dipyrromethenes (Bodipy), as SOM-CPL architectures.34 This statement refers to structures that show helical chirality,19,35−39 axial chirality,40,41 propeller chirality,42 or that contain chiral auxiliary ligands,43 leading to chiroptically active fluorophores with very attractive photophysical properties. These include, for example, strong absorption of visible light (>500 nm) and very efficient red-shifted emission. Cyclic ketones with one or more asymmetric carbon atoms as the only stereogenic units are pioneering examples for the observation of CPL.44−48 However, these chromophores absorb and emit usually at relatively short wavelengths (
