Chiral exciton coupling of merocyanine dyes within a well defined hydrogen-bonded assembly

Abstract

Multichromophoric hydrogen-bonded assemblies 13⅐(BAR)6 are studied that bear a remarkably close resemblance to commelinin, a naturally occurring assembly responsible for an intense blue color of flowers. The incorporated chromophores exhibit a hypsochromic shift in the UV͞visible (Vis) absorption maximum (⌬ max ‫؍‬ 14 nm) compared with the free chromophores. In addition, the chiroptical properties of incorporated chromophores can be rationally controlled by changing the supramolecular chirality of the assembly. These properties have been used to study the stability of this type of assembly with UV and CD spectroscopy at concentrations far below the NMR sensitivity threshold (10 ؊4 M). The determined C50% values of 2-3 M in benzene show the extremely high stability of these hydrogen-bonded assemblies. Most chromophores are susceptible to changes in their optical properties depending on their chemical environment. This principle has been actively exploited for the development of optical sensors (1-3) and devices (4-6). Along these lines, Nature generates a wide variety of different fruit and floral colors from different noncovalent complexes of the natural anthocyanin chromophores (7,8). These colors cover almost the complete visible spectrum, despite the fact that the structural diversity in the building blocks is surprisingly low. Commelinin is a blue complex in which a total of six anthocyanins and six flavocommelins are pairwise coordinated around two central magnesium cations (Fig. 1a) (9). The blue color of commelinin is a direct result of the self-assembly process driven by coordination of the anthocyanins to magnesium and completely disappears on dissociation of the complex. In addition, commelinin is strongly CD active as a result of exciton coupling between the stacked anthocyanin units (Fig. 1b). The left-handed screw is a result of chiral induction by sugar moieties present in the components. Numerous synthetic assemblies that contain multiple chromophores in a cyclic (10-13) or polymeric array (14-17) have been reported. However, none of them can match natural assemblies in the sense that they are both molecularly well defined and display (chir)optical properties that are different from the separate components. In this paper we report hydrogen-bonded assemblies that bear a remarkably close resemblance to commelinin ( Fig. 1 c and d). The similarity is not only expressed by the supramolecular structure, but also by a similar change in both the optical and chiroptical properties of the chromophores on formation of the assembly. These spectral changes enable us to study the stability of these assemblies at micromolar concentrations, far below the 1 H NMR sensitivity threshold.The barbiturate BAR has a chromogenic donor--acceptor system with a charge-transfer (CT) absorption band in the visible range at max ϭ 478 nm in chloroform ( 478 ϭ 8. ) (Fig. 3a). Evidence for the quantitative formation of assembly 1 3 ⅐(BAR) 6 was obtained from 1 H NMR spectroscopy in CDCl 3 . The spectrum shows the characteristic ...

Gelation‐Assisted Control over Excitonic Interaction in Merocyanine Supramolecular Assemblies

“…[7] Other examples include merocyanine assemblies supported by specific noncovalent interactions such as multiple hydrogen bonds, which show drastic changes in their optical and electronic properties. [8] We report herein that the aggregation of cholesterolappended merocyanines with minor structural differences in various organic solvents results in the formation of organogels with dramatically different chromophore packing, thus giving rise to distinct optical properties through excitonic interactions.…”

“…[5] This structural character also makes these dyes attractive as supramolecular components because different chromophore packing or spatial arrangements result in prominent changes in their optical properties through excitonic interactions. [6][7][8] Covalently tethered merocyanine dimers are interesting examples in that their optical properties can be tuned through the control of the relative orientation of two merocyanine chromophores by selecting the media. [7] Other examples include merocyanine assemblies supported by specific noncovalent interactions such as multiple hydrogen bonds, which show drastic changes in their optical and electronic properties.…”

Photoswitchable Exciton Coupling in Merocyanine–Diarylethene Multi‐Chromophore Hydrogen‐Bonded Complexes

“…[6] We herein report a supramolecular system where exciton interactions of chromogenic components can be controlled through the conformational change of photochromic components. Previous our study on supramolecular complexation between merocyanine dye 1 [7] (Figure 1 a) and bismelamine (BM) receptors [8] through multiple hydrogen bonds demonstrated that the oligomethylene linker length of the receptors can control the J-(BM3) and H-type (BM12) exciton interactions of 1 (Figure 1 b). [9] From the spectroscopic and the NMR studies, it was suggested that the formation of discrete oligomers (1 + BM3) and folded supramolecular polymers (1 + BM12) are responsible for such contrasting exciton interactions.…”

Induction of Liquid Crystallinity by Self‐Assembled Molecular Boxes

“…[19] The assemblies, which are held together by 36 hydrogen bonds, are thermodynamically stable, even at a concentration of 5 mm. [20] The calix [4]arene dimelamine 1 was functionalized with octadecyl chains to promote the self-organization of the double-rosette assemblies into a liquid-crystalline phase. The long alkyl chains are connected to the terminal benzamide moieties of 1 to prevent their interference with the network of hydrogen bonds that holds the double rosette together.…”

“…6 (1 mm in [D 8 ]toluene) at 298 K. shown). [20,22,23] The optically active double-rosette assemblies (with P or M helicity) exhibit a very strong induced CD signal at a wavelength of 350-250 nm, [20] owing to the dissymmetric arrangement of the many chromophoric units, whereas the building block 1 is nearly CD inactive. The intensity of the CD signal of 1 3 ·((R)-propCYA) 6 does not decrease after several thermal cycles, evidencing the high thermal stability of the assembly.…”