Letter pubs.acs.org/OrgLett
Circularly Polarized Luminescence from Chiral Spiro Molecules: Synthesis and Optical Properties of 10,10′Spirobi(indeno[1,2‑b][1]benzothiophene) Derivatives Ko Takase,† Keiichi Noguchi,‡ and Koji Nakano*,† †
Department of Organic and Polymer Materials Chemistry, and ‡Instrumentation Analysis Center, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan S Supporting Information *
ABSTRACT: Chiral spiro polycyclic aromatic compounds with thiophene and/or thiophene S,S-dioxide units were synthesized as a new family of circularly polarized luminescence (CPL) materials. Oxidation of the thiophene unit to its S,S-dioxide provided significant changes in photophysical properties such as much higher fluorescence quantum yield and positive solvatochromism in photoluminescence spectra. Density functional theory calculations qualitatively demonstrated the observed absorption properties. The prepared chiral spiro compounds exhibit intense CPL signals with a relatively large dissymmetry factor for small organic molecules.
S
(Figure 1). In fact, a number of chiral spiro polycyclic aromatic compounds have been synthesized since the first report on enantiomerically pure 9,9′-spirobifluorene derivatives in 1969.7 However, their chirality has attracted much less attention. Most of them have been evaluated and used as racemic mixtures. Nevertheless, there have been some reports that focused on their chirality. For example, chiral spiro polycyclic aromatic compounds have been applied to molecular recognition8 and asymmetric synthesis.9 Chiroptical properties such as circular dichroism (CD) and optical rotatory power (ORP) have also been investigated for several compounds.7,10 However, studies on chiroptical properties are still limited. In particular, to the best of our knowledge, there have been only two reports on circularly polarized luminescence (CPL).11 CPL has attracted greater attention owing to its potential applications in display devices,12 biological probes,13 sensors,14 and so on. Thus, a variety of CPL materials based on chiral small organic molecules, chiral lanthanide complexes, and helical polymers/ aggregates have been developed,15,16 and new molecular designs are still required for further development of CPL materials. In this context, we designed the chiral spiro polycyclic aromatic compound 1 as a new family of chiral building blocks for CPL materials (Figure 1). Oxidation of a sulfide group to a sulfonyl group in thiophene-containing π-conjugated compounds has been reported to significantly change their electronic properties.17 Thus, we investigated oxidation of 1 to 2 and 3 with one and two thiophene S,S-dioxide moieties, respectively. Herein, we report the synthesis of compounds 1− 3 and their chiroptical properties including CPL activity.
piro polycyclic aromatic compounds consist of two planar biaryls that are connected perpendicularly by the sp3hybridized spiro carbon atom (Figure 1). This structural feature
Figure 1. Spiro polycyclic aromatic compounds with two biarylderived π-conjugated systems.
imposes high steric demand on them and hinders their intermolecular aggregation. It also significantly limits the orbital interaction between the two biaryl-derived π-conjugated systems, while the spiroconjugation allows the orbital interaction to some extent.1 Owing to these characteristics, spiro polycyclic aromatic compounds have been recognized as one of the most promising building blocks for organic functional materials in organic photovoltaics,2 organic lightemitting diodes,3 and so on.4 Among them, 9,9′-spirobifluorene is a prototypical and very widely studied skeleton.5 Replacement of the benzene ring(s) in the 9,9′-spirobifluorene core with heterocycle(s) has also been demonstrated to develop new heterocyclic building blocks.6 Spiro polycyclic aromatic compounds derived from two dissymmetric biaryls are chiral molecules with C2 symmetry © 2017 American Chemical Society
Received: July 28, 2017 Published: September 19, 2017 5082
DOI: 10.1021/acs.orglett.7b02337 Org. Lett. 2017, 19, 5082−5085
Letter
Organic Letters The synthetic route to the designed compounds 1−3 is shown in Scheme 1. 3-Bromo-2-(2-bromophenyl)benzo[b]Scheme 1. Synthetic Route of Spiro Compounds 1−3 and the Structure of Model Compounds
Figure 2. (a) UV/vis absorption and (b) normalized photoluminescence spectra of spiro compounds 1−3 and their model compounds 7 and 8 in CH2Cl2 and (c) CD and CPL spectra of spiro compound 1 in CH2Cl2.
absorption bands in the wavelength region of
