Questions of Mirror Symmetry at the Photoexcited and Ground States of Non-Rigid Luminophores Raised by Circularly Polarized Luminescence and Circular Dichroism Spectroscopy. Part 2: Perylenes, BODIPYs, Molecular Scintillators, Coumarins, Rhodamine B, and DCM

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.

Handed Mirror Symmetry Breaking at the Photo-Excited State of π-Conjugated Rotamers in Solutions

“…They provided a possible explanation for the result, which was that although these non-rigid luminophores are made of lighter atoms possessing non-zero spin-orbit interaction (ζ, kcal•mol −1 ) [ζ = 0.1 (C), 0.2 (N), 0.4 (O), 0.7 (F), 1.0 (S)], a large number of photoexcited luminophores (>10 10 -10 16 ) confined in a cuvette interacted with focused incident light beams. Subsequently, the faint ∆E pv on the order of 10 −8 -10 −14 kcal•mol −1 was resonantly amplified to a detectable level to produce CPL chiroptical signals during a synchronized spontaneous radiation process [43]. The authenticity of the results were verified by studying luminophores with an enantiomeric chiral center and achiral D 2h -symmetric fused aromatics (naphthalene, anthracene, tetracene, and pyrene).…”

“…For instance, linear amplification of faint E pv by the connotation of molecules to polymers and crystals [6], and a small influence of PV-originating oscillation on parity conserving electromagnetic (PC-EM)-originating quantum oscillation of imaginary chiral molecules in a double-well (DW) potential [11]. Additional theoretical models of real and imaginary molecules have been referred to in several innovative re-ports, along with experimental attempts [6][7][8][12][13][14][16][17][18]20,[25][26][27][28][29][41][42][43][44][45][46]. Recent reports highlight the advance in the realm of MPV [47][48][49][50].…”

“…Recently, one of the authors (M.F.) and coworkers reported astonishing results from a comprehensive circularly polarized luminescene (CPL) and circular dichroism (CD) spectroscopic data set of nearly fifty non-rigid luminophore rotamers without stereogenic centers [42,43]. On controlling the tunneling barrier (E b ), all these luminophores revealed step-like and consistent alteration in Kuhn's dissymmetry factor (g lum ) with only (-)-sign CPL signals with changes in the viscosity of the achiral solvents.…”

“…Based on previous reports [42,43], we rationally synthesized three non-rigid extended π-conjugated luminophores. These luminophores consist of anthracene (single horizontal and verticle conjugation axis), bis-anthracene (single horizontal and double vertical conjugation axes), and biphenyl moieties (single horizontal conjugation axis) as twistable cores in the right-or left-hand geometries, which commonly carry two floppy trans-styryl moieties as labile pendants that provide excellent solubility to a broad range of organic solvents.…”

“…Comparison of the CPL and photo-luminescence (PL) spectra in methanol and 1,4-butanediol for DSA excited at 420 nm (a), DSBA excited at 390 nm (b), and DSBP excited at 320 nm (c). The glum values at 0-0 PL band extremum of DSA (d), DSBA (e), and DSBP (f) as a function of the viscosity of alcohol, respectively: methanol (0.55 cP), ethanol (1.09 cP), n-propanol (1.96 cP), n-butanol (2.59 cP), n-pentanol (3.47 cP), n-hexanol (4.59 cP), n-heptanol (5.97 cP), n-octanol (7.59 cP), n-nonanol (9.51 cP), n-decanol (11.50 cP), ethylene glycol (16.1 cP), n-undecanol (16.95 cP), 1,3-propandiol (33.0 cP), and 1,4-butanediol (71.0 cP)[42,43]. DSBA and BA…”