Abstract
A novel mechanism for chiroptical activity inversion based on the electronic structure of metal complexes without Λ- or Δ-type structure change was demonstrated spectroscopically and theoretically. To demonstrate the mechanism, a europium (Eu(III)) complex with chiral (+)-3-(trifluoroacetyl)camphor (+tfc) and achiral triphenylphosphine oxide (tppo) was prepared. The steric and electronic structures of the Eu(III) complex were adjusted by additional achiral tppo and coordinating acetone molecules, and were characterised by 1H NMR, photoluminescence, and emission lifetime measurements. The optical activity of the Eu(III) complex in solution was evaluated by circularly polarized luminescence (CPL) measurements. CPL sign inversion, which was independent of Λ- or Δ-type structure changes from the spectroscopic viewpoint, and a drastic CPL intensity enhancement were observed depending on the external achiral molecules around Eu(III) ion. These phenomena provide the first clarification of optical activity change associated with electronic structure rather than chiral coordination structure-type (Λ or Δ) under external environments.
Highlights
IntroductionThe CD and circularly polarized luminescence (CPL) signals are dependent on the molecular chirality, which is influenced by the external field
The magnitude of optical activity is estimated by a dissymmetry factor (g), which is composed of electric and magnetic field components related to the electronic transition[15]
Note that chiroptical activity is related to the electronic transition of the Eu(III) complex; the magnitude and sign could be affected by the chiral electronic structure of the Eu(III) ion surrounded by external ligands (Fig. 1a)
Summary
The CD and CPL signals are dependent on the molecular chirality, which is influenced by the external field. The magnitude of optical activity is estimated by a dissymmetry factor (g), which is composed of electric and magnetic field components related to the electronic transition[15]. Their components are governed by the molecular structure in the surrounding environment. We targeted chiral lanthanide complexes to clarify the influence of electric and magnetic field components on optical activities. Their sharp luminescence bands comprise electric and magnetic field components of 4f-4f transitions[26] Their components in the electronic transitions are dominated by external organic molecules around the lanthanide ion. We demonstrate a mechanism for the optical activity inversion based on the chiral electronic structure of the Eu(III) complex without Λ- or Δ-type structure change spectroscopically and theoretically, for the first time
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