π-Expanded Axially Chiral Biaryls and Their Emissions: Molecular Design, Syntheses, Optical Resolution, Absolute Configuration, and Circularly Polarized Luminescence of 1,1′-Bipyrene-2,2′-diols

Abstract

Axially chiral biaryls based on pyrene, 1,1¤-bipyrene-2,2¤-diol derivatives 2, were designed and synthesized from 2-hydroxypyrene. Simple optical resolution was carried out and the absolute configuration was determined by diffraction study. Title compounds 2 and corresponding menthylcarbonate 4 showed clear circularly polarized luminescence (CPL) spectra. The degree of dimensionless Kuhn's anisotropy in the photoexcited state («g em «) of 2 and 4 were ca. 3.6 © 10 ¹4 and ca. 1.2 © 10 ¹3, respectively. Noteworthy properties were as follows: 1) high fluorescent quantum yields of 2 and 4, 0.57 and 0.80, respectively, in marked contrast to that of BINOL, 0.04 and 2) bathochromic shifts of CPL .Circularly polarized luminescence (CPL) is luminescence emitted by a chiral molecule. Because of the high sensitivity of steady-state luminescence, CPL spectroscopy has attracted much attention, particularly in the analysis and evaluation of naturally occurring chiral chromophores, such as chlorophyll (vide infra).1 Although CPL spectroscopy is a relatively new technique, a pioneering review was published as early as in 1977.2 Currently, CPL is regarded as an important chiroptical technique. The applications of CPL have shifted from the analytical chemistry of biological chiral molecules 2 to advanced science using polarized light, such as three-dimensional displays and quantum computing. 3CPL spectroscopy has flourished under the watch of physical chemists. However, the lack of a standard molecule having a large quantum yield (Φ fl ) and a large dimensionless Kuhn's anisotropy in the photoexcited state («g em «) (vide infra), 2 has hindered the development of new materials for CPL. Although 1,1¤-binaphthalen-2,2¤-diol (BINOL) 1 (Chart 1) and its corresponding derivatives are well used in this research field, Φ fl and g em are small (vide infra). In this study, our aim was to design a standard molecule for CPL spectroscopy. The expansion of the π-electronic system seemed to be the most promising strategy, i.e., replacement of the naphthalene moiety of BINOL with other π-expanded polycyclic aromatic hydrocarbons (PAHs). Generally, the expansion of the π-electronic system induces 1) a small HOMOLUMO gap and a resultant low excitation energy, 2) an increase in the fluorescence lifetime, 3) a low redox potential, and so on. 4 These properties are preferred for advanced materials. We focused on pyrene as PAH because it showed strong emission, long lifetime, and high quantum yield, making it one of the standard molecules for photochemistry. 5,6 We designed 1,1¤-bipyrene-2,2¤-diol 2, a pyrene-based axially chiral molecule, with a eye to improving the chiroptical properties of BINOLs.Optically pure title compounds 2 and reference 5 were prepared according to Scheme 1. 7 The issues to be addressed were 1) the solubility improvement of 2 and 2) optical resolution. PAHs are notorious for their poor solubilities and our targets 2, dimers of pyrene, ought to be sparingly soluble materials. To solve this problem, a tertbutyl group was intro...

Synthetic Applications of Oxidative Aromatic Coupling—From Biphenols to Nanographenes

“…The dihedral angle between the two central naphthyl moieties can be altered by chemical modification of the central region; for example, acid‐catalyzed dehydration involving the central OH groups delivers an additional furan system, which in turn decreases the dihedral angle from 86.4° to 14.6°, as predicted by DFT methods. The protected 1,1′‐bi‐2‐pyrenol ( 12 , Figure ) can be prepared by the Fe III ‐ or Cu II ‐mediated oxidative dimerization of the protected precursor of 2‐pyrenol. After dimerization, diastereomers of type 12 can be effectively separated using conventional chromatography and then be deprotected to deliver optically pure 1,1′‐bi‐2‐pyrenols.…”

“…[21] Theo xidative dimerization of larger phenols represents one of the most convenient pathways toward complex 2,2'dihydroxybiaryl compounds.According to the Tsubaki group, butterfly-shaped molecule 11 can be obtained by the Cu IImediated dimerization of dinaphthofuran-6-ol ( Figure 2). [22] Thedihedral angle between the two central naphthyl moieties can be altered by chemical modification of the central region; for example,acid-catalyzed dehydration involving the central OH groups delivers an additional furan system, which in turn decreases the dihedral angle from 86.48 8 to 14.68 8,aspredicted by DFT methods.T he protected 1,1'-bi-2-pyrenol (12, Figure 2) can be prepared by the Fe III - [23] or Cu II -mediated [24] oxidative dimerization of the protected precursor of 2pyrenol. After dimerization, diastereomers of type 12 can Structures of newly designed Mo-based mediators for oxidative coupling.…”

Pyren‐2‐yl‐Substituted Biphenoquinone and p‐Benzoquinone: Pyren‐2‐yl as a Weak Electron‐Withdrawing Substituent

“…Pyrene is a typical π‐expanded PAH that is characterized by its photophysical property and relatively high‐lying HOMO energy level . Utilizing these properties, many pyrene‐based advanced materials have been synthesized, and our research group has contributed to this research field through the synthesis of structurally well‐defined oligopyrenylene, a pyrene‐based PAH, and pyrene dimers emitting circularly polarized luminescence . In this study, we designed pyrene‐substituted quinones 1 a and 2 a .…”

Solvent‐ and Substituent–controlled Circularly Polarised Luminescence of C2‐symmetric Binaphthyl Fluorophores

“…We previously reported that 1,1′‐bi‐2‐naphthol [( R )‐ 3 ] and 2,2′‐diethoxy‐1,1′‐binaphthyl [( R )‐ 6 ], commonly exhibit a negative CPL signal in the weakly‐polar chloroform (CHCl 3 ) . In this work, ( R )‐ 3 and ( R )‐ 6 both exhibited a negative CPL signal in the highly polar dimethylformamide (DMF) (Figures (b) and (b); black dotted line).…”