Multifunctional Supramolecular Materials Based On Bipyridinium Amphiphiles

Bipyridiniums are extensively applied in the area of supramolecular materials owing to their π-electron deficient nature which allows them to bind with variousπ-electron rich units through donor-acceptor charge transfer(CT)interactions.Herein,a series of multifunctional supramolecular materials based on bipyridinium amphiphiles were investigated,including template-directed micelles,solid-state photochromic and hydrochromic materials,dynamic macroscopic supramolecular helices,and a heatactivated redox supramolecular hydrogel.Firstly,a series of melatonin bioisosteres were found to bind with bipyridiniumbased amphiphiles as templates through donor-acceptor CT interactions and direct their micellization processes.It was found that the formation of CT complexes between templates and the bipyridinium-based amphiphiles is capable of decreasing the Coulombic repulsion among the highly charged bipyridinium groups.This process led to the enlargement of micellar size and enhancement of the surface activity of the bipyridinium-based amphiphiles with a concomitant decrease in critical micelle concentration by up to 70%.This study proved that the biologically relevant melatonin bioisosteres and other commonly used synthetic π-electron rich donors played the same role for template-directed micellization of bipyridinium-based amphiphiles for the first time.Meanwhile,changes in micellar aggregation size and interfacial properties after the template-directed micellization process could help to identify the presence of these melatonin bioisosteres.Subsequently,a series of solid-state photochromic and hydrochromic materials based on 1,8-naphthalimide-viologen derivatives were prepared.The bipyridinium unit serves as a water-sensitive receptor for the hydrochromic system,and a photochromic group capable of generating a radical cation with its own distinct color upon exposure to UV light.These materials have properties which manifest as reversible solid-state fluorescence hydrochromism with changes in relative humidity and reversible solidstate photochromism upon generation of the viologen radical cations with the irradiation of UV light.The photophysical behavior of 1,8-naphthalimide-viologen derivatives in the solid state was relatively unknown and unexplored.Herein,the photochromic and hydrochromic properties of the 1,8-naphthalimide-viologen derivatives with the external UV irradiation and relative humidity changes have been well studied.Meanwhile the mechanism of the photochromic process was evaluated by designing and analyzing controlled experiments.In addition,macroscopic supramolecular helical fibers were also obtained from assembled CT complexes in aqueous media by mixing an achiral bipyridinium-based amphiphile acceptor with a chiral π-electron rich donor,tryptophan(Trp).These supramolecular helical architectures allowed for a quantitative exchange of the chiral tryptophan template with an achiral π-electron rich donor with the retention of their helical chiral handedness on account of the chiral memory effect.In general,the chirality controllable helical architectures completely composed of achiral molecules were obtained through the donor-exchange process based on the chiral memory effect while the chiral template after donor-exchange could be cyclically used.This design strategy provided more opportunities in macroscale chiral translation and constructions of single-handed helices.At last,a heat-activated redox supramolecular hydrogel based on a linear bipyridinium-based amphiphile was prepared.The rust(copper oxide)on the surface of the old copper coins can be reduced and polished by heating the hydrogel-coated coin.This reductive action on the copper metal oxide surface is based on the thermalactivated reduction of bipyridinium to its radical cationic state.This investigation verified that the bipyridinium-based hydrogel could be harnessed based on its redox property to promote the reduction of metal surface oxides.The simple preparation of this hydrogel could have widespread potential applications in rust removal and metal oxide surface cycling.The design and synthesis of multifunctional supramolecular materials based on bipyridinium amphiphiles by employing the commonly used bipyridinium in the supramolecular chemistry of this work has helped to develop and broaden the scope of use for supramolecular self-assembled modulating systems and stimuli-responsive materials.