Unveiling controlled breaking of the mirror symmetry of Eu(fod)(3) with alpha-/beta-pinene and BINAP by circularly polarised luminescence (CPL), CPL excitation, and F-19-/P-31{H-1}-NMR spectra and Mulliken charges

摘要

We investigated whether the 'Pfeiffer effect' of optically inactive Eu(fod)(3) (fod = 6,6,7,7,8,8,8-heptafluoro-2,2- dimethyl-3,5-octanedionate) is detectable when it is dissolved in four chiral hydrocarbons, namely, (1S)-/(1R)-alpha-pinene, (1S)-/(1R)-beta-pinene, (1S)-/(1R)-trans-pinane and (S)-/(R)-limonene. Among these solvents, alpha-pinene and beta-pinene induced clear circularly polarised luminescence (CPL) signals in CPL-silent Eu(fod) 3 at the electric dipole (ED)-forbidden/magnetic dipole (MD)-allowed D-5(0)- F-7(1) transition at 593 nm and the ED-allowed/MD-forbidden D-5(0)- F-7(2) transition at 613 nm due to Eu(III). The CPL excitation (CPLE) spectra monitoring the D-5(0)- F-7(1) and D-5(0)- F-7(2) transitions confirmed that these CPL bands are induced by the chirality of alpha-pinene. Eu(fod)(3) in the presence of non-charged chiral aromatic additives, including (S)-/(R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP), (S)-/(R)-[1,1'-binaphthalene]2,2'-diylbis[1,1-diphenyl-1,1'-phosphine-oxide] (BINAPO) and (S)-/(R)-alpha-phenylethylamine (PEA), also showed mirror-image CPL spectra with different extents of dissymmetry factors in their CPL signals. The F-19-NMR spectra revealed that the CPL signals had two different origins, which can be categorized as the Pfeiffer-perturbed system at an outer-sphere and a strong coordinative interaction at an inner-sphere Eu(III). The former was exemplified by alpha-pinene, BINAP and dilute PEA, whereas the latter was observed with BINAPO and neat PEA. The strong coordinative interaction of Eu(fod) 3 with BINAPO was verified by comparing its P-31{H-1}-NMR spectrum with that in the presence of BINAP. The Mulliken charges of Eu(fod)(3) and the chiral compounds obtained with MP2(6-311G) calculations led us to hypothesize that the surplus charge neutralization can act as an invisible attractive force between several ligands in the Eu(III) complexes and CPL-inducible chiral compounds.