Chirality induction using circularly polarized light into a branched oligofluorene derivative in the presence of an achiral aid molecule

Abstract

Chirality induction to a uniform, star-shaped fluorene oligomer with a central truxene moiety (T3) was achieved using circularly polarised light in the presence of achiral fluorene or phenanthrene. Induction to T3 alone was difficult, suggesting that close chain packing realised through interactions of T3 with small molecules plays a role in chirality induciton.Circularly polarised light (CPL) was proposed to be a source of chirality for the synthesis of asymmetric organic molecules for the first time in 1874, 1,2 and various experimental examples of CPL-based chiral synthesis have been reported thereafter. 3,4Target materials include small molecules, polymers and supramolecular ensembles. Since chiral polymers find a wide range of applications, facile methods of synthesis using CPL are important. 5 As for CPL-based chiral polymer preparation, a polymethacrylate bearing a side chain containing an azobenzene moiety was for the first time made optically active using CPL; 6 polymers having azobenzene moieties buried in the main-chain have also been studied. 7 We have recently reported a preferred-handed helix formation of linear poly(9,9-di-n-octylfluoren-2,7-diyl) (PDOF) using CPL as a chirality source, where the twist bias around the single bonds connecting the aromatic groups was controlled by chirality of light.8,9a PDOF became optically active in a thin film form on CPL irradiation and exhibited intense circular dichroism (CD) bands. The optical activity of PDOF arises from a 5/1-helical conformation of the polymer generated from a random conformation as disclosed through metadynamics simulation of stereomutation of the polymer chain. 9Through the simulations, inter-chain interactions as well as chain-tosubstrate interactions were found to be important in chirality induction. The conformational transformation is triggered by a "twist-coplanar" transition of the Ar-Ar unit on photoexcitation. 10 Selective photoexcitation of a right-or lefthanded twist modifies a chiral twist into an achiral coplanar conformation which can be deactivated into both right-and left-handed twists, leading to a decrease in population of the selectively excited twist where a preferred-handed twist (helix) is formed. 8,9a,11Herein, we report chirality induction to a monodisperse, welldefined, star-shaped oligofluorene with a central truxene core and terfluorene arms (T3, Scheme 1). 12 T3 has a higher luminescence quantum yield than linear polymer systems and also has a high nonlinear absorption coefficient. 12b On the basis of these characters, T3 is a promising candidate for photo electronic material, and addition of chirality would widen its range of application. Chirality induction to T3, however, is a challenge since the solid-state form of T3 does not have close packing of chains due to its branched structure, while chain-tochain interactions have been found to be important in chirality induction to linear PDOF 8,9a .Scheme 1. Structures of compounds used in this work.

Learning how planarization can affect dichroic patterns in polyfluorenes

“…Conjugate polymers are continuously raising widespread attention owing to their versatile features spanning applications from organic electronics, photovoltaic devices towards nanomedicine 1 . Their flexible uses stem from the typical molecular organization, which confers them a specific chemical scaffold 2–11 . For instance, amorphous arrangements have been so far related to spurious cis ‐defects arising during chemical synthesis 12 or owing to their backbone flexibility 13 .…”

Surface-induced enantiomorphic crystallization of achiral fullerene derivatives in thin films

“…[30][31][32] However, the mixing of the dopants affects the lm structure and interferes with the properties of the organic semiconductors. The application of chiral stimuli to the materials, such as irradiation with circularly polarized light, can also induce enantiomorphic structures, [33][34][35] but the enantiomorphic effects are generally weak. To our knowledge, chiral induction during crystallization has not been reported for thin-lm organic semiconductors.…”

Helical screw sense bias in chiral polyfluorene stimulated by solvent

“…Chemists have successfully realized the helical chirality switching by means of external stimuli, such as solvent, temperature, light irradiation, pH and so on. [21][22][23][24][25][26][27][28][29][30][31] In particular, intrinsic chirality exists in many organic solvents (e.g., limonene), and it tends to influence the solute conformation, then results in complete helical chirality inversion. Gin and Moore demonstrated that [This article is part of the Special Issue: Proceedings of 28th International Symposium on Molecular Chirality, Heidelberg 2016.…”

“…Interestingly, supramolecular helical handedness of chiral materials is not invariable, actually, they display conformational transformations under certain treatments. Chemists have successfully realized the helical chirality switching by means of external stimuli, such as solvent, temperature, light irradiation, pH and so on . In particular, intrinsic chirality exists in many organic solvents (e.g., limonene), and it tends to influence the solute conformation, then results in complete helical chirality inversion.…”