Abstract
We investigated whether semi-rigid and non-rigid π-conjugated fluorophores in the photoexcited (S1) and ground (S0) states exhibited mirror symmetry by circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy using a range of compounds dissolved in achiral liquids. The fluorophores tested were six perylenes, six scintillators, 11 coumarins, two pyrromethene difluoroborates (BODIPYs), rhodamine B (RhB), and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM). All the fluorophores showed negative-sign CPL signals in the ultraviolet (UV)–visible region, suggesting energetically non-equivalent and non-mirror image structures in the S1 state. The dissymmetry ratio of the CPL (glum) increased discontinuously from approximately −0.2 × 10−3 to −2.0 × 10−3, as the viscosity of the liquids increased. Among these liquids, C2-symmetrical stilbene 420 showed glum ≈ −0.5 × 10−3 at 408 nm in H2O and D2O, while, in a viscous alkanediol, the signal was amplified to glum ≈ −2.0 × 10−3. Moreover, BODIPYs, RhB, and DCM in the S0 states revealed weak (−)-sign CD signals with dissymmetry ratios (gabs) ≈ −1.4 × 10−5 at λmax/λext. The origin of the (−)-sign CPL and the (−)-sign CD signals may arise from an electroweak charge at the polyatomic level. Our CPL and CD spectral analysis could be a possible answer to the molecular parity violation hypothesis based on a weak neutral current of Z0 boson origin that could connect to the origin of biomolecular handedness.
Highlights
Since the mid-19th century, one of the greatest puzzles for scientists has been why life on Earth selected L-amino acids and D-sugars, because the corresponding L/D enantiomers were considered to be energetically identical [1,2,3,4,5,6,7,8,9,10,11,12,13]
To test the MPV hypothesis by further experiments, we investigated whether semi-rigid and non-rigid laser dyes, molecular scintillators, and other fluorophores in achiral liquids are mirror symmetrical by circularly polarized luminescence (CPL) and circular dichroism (CD) spectroscopy
To validate the MPV hypothesis by CPL and CD spectroscopy, the crucial factors to choose are as follows: (i) semi-rigid and non-rigid racemic fluorophores carrying side chains allowing for rotatable freedom and/or flip-flop motional ability, (ii) fluorophores constituting only lighter atoms among the first three periods of the periodic table, and (iii) achiral liquids as solvents to continuously control the Eb value
Summary
Since the mid-19th century, one of the greatest puzzles for scientists has been why life on Earth selected L-amino acids and D-sugars, because the corresponding L/D enantiomers were considered to be energetically identical [1,2,3,4,5,6,7,8,9,10,11,12,13]. Symmetry 2019, 11, 363 flows of solar/thermal energy and low-entropy food [17] This mystery is intimately connected to the origins of life [2,3,4,5,6,7,8,9,10,11,12,13,14] and the accelerating expansion of the universe [18,19]. In 1845, Faraday observed a magneto-optical phenomenon, called the “Faraday effect”, in which a light–matter (LM) interaction causes linearly polarized (LP) light to be rotated clockwise (CW) and counterclockwise (CCW) (from the observer) by passing through achiral lead-containing glass and certain liquids under the influence of a static magnetic field. In 1894, Curie considered that magnetic and electric fields need to align co-linearly or anti-colinearly to produce optically active substances [21]
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