title: Supramolecular assembly of achiral alkynylplatinum(II) complexes and carboxylic β-1,3-glucan into different helical handedness stabilized by Pt⋅⋅⋅Pt and/or π-π interactions.
abstract: Two-component ensembles of alkynylplatinum(II) terpyridine (tpy) complexes and carboxylic β-1,3-glucan (CurCOOH) have been investigated by using spectroscopic and microscopic techniques, as well as time-dependent UV/Vis absorption and circular dichroism (CD) experiments. Microscopic images of [Pt(tpy){C≡CC6 H4 (CH2 NMe3 -4)}](OTf)2 (1) have revealed spherical nanostructures, whereas helical fibrous structures of different lengths, depending on the concentration of complex 1 and CurCOOH, were observed. The helical assemblies have been found to show low-energy metal-metal-to-ligand charge transfer (MMLCT) absorption and triplet MMLCT ((3) MMLCT) emission, which are indicative of Pt⋅⋅⋅Pt and/or π-π interactions between the complex 1 molecules. Interestingly, the ensemble has been demonstrated to show different handedness and even a change in handedness over time under different experimental conditions. Low temperatures, low concentrations of CurCOOH, high concentrations of complex 1, or successive addition of CurCOOH into complex 1 solution favor the formation of right-handed helical assemblies, whereas high temperatures, high concentrations of CurCOOH, low concentrations of complex 1, or single-batch addition of CurCOOH into complex 1 solution result in a fast chiroptical inversion of the ensemble solution, giving rise to the left-handed helical assemblies as the dominant species. The results have been rationalized by considering the competing kinetically and thermodynamically controlled assembly-elongation of the ensemble, which leads to the formation of right-handed and left-handed helical assemblies, respectively. The change in the handedness of the ensemble has been demonstrated to stem from the formation of two-component assemblies with supramolecular interactions, in contrast to the template- induced chiral amplification commonly observed in other systems.
authors.affiliation: Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong (P. R. China).
authors.forename: Clive Yik-Sham
authors.initials: CY
authors.lastname: Chung
authors.forename: Shun-ichi
authors.initials: S
authors.lastname: Tamaru
authors.forename: Seiji
authors.initials: S
authors.lastname: Shinkai
authors.forename: Vivian Wing-Wah
authors.initials: VW
authors.lastname: Yam
keywords: chirality; circular dichroism; helical structures; platinum; supramolecular chemistry