Electrochromic Polycationic Organoboronium Macrocycles with Multiple Redox States

Abstract

Polycationic macrocycles are attractive as they display unique molecular switching capabilities arising from their redox properties. Although diverse polycationic macrocycles have been developed, those based on cationic boron systems remain very limited. We present herein the development of novel polycationic macrocycles by introducing organoboronium moieties into a conjugated organoboron macrocyclic framework. These macrocycles consist of four bipyridylboronium units that are connected by fluorene and either electron-deficient arylborane or electron-rich arylamine moieties. Electrochemical studies reveal that the macrocycles undergo reversible multi-step redox processes with transfer of up to 10 electrons. Switchable electrochromic behavior is demonstrated via spectroelectrochemical studies and the observed color changes are rationalized by correlation with computed electronic transitions using DFT methods.Endowed with unique electrooptical characteristics, conjugated polycations serve as important components in the fields of supramolecular materials and redox-responsive systems. [1] Among them, polycationic macrocycles such as Stoddarts "blue box" [2] generate impressive interlocked structures and exhibit unique molecular switching capabilities. [3] Since their first discovery, numerous derivatives have been synthesized and utilized in supramolecular assemblies, host-guest chemistry, [4] molecular electronics, [5] electron transfer, [6] and electrochromic [7] materials. Although diverse polycationic macrocycles have been reported, [8] those based on cationic boronium moieties remain very limited. [9] Very recently, coordination-driven self-assembly to build up polycationic boron macrocycles was reported by Himmel and co-workers. [9c] These macrocycles are composed of cationic diborane units that are linked together by pyrazine, 4,4'-bipyridine, or 1,2di(4-pyridyl)ethene.Over the course of our studies on organoboron pconjugated materials, [10] we developed a unique class of boracyclophanes with strongly Lewis acidic boron sites, including ambipolar borazine A [11] with its alternating electron-deficient arylboranes and electron-rich arylamines and the highly electron-deficient macrocycles B [12] (Figure 1 a). [13]

Polycationic Redox‐Active Cyclophanes with Integrated Electron‐Rich Diboron Units

“…[8] Jäkle et al synthesized several boracyclophanes with interesting optical and redox properties. [9][10][11][12][13][14] One example is sketched in Figure 1. In these compounds, the B III atoms are Lewis-acidic sites and act as electron-acceptors.…”

Recent Advances in the Synthesis and Applications of Nitrogen-Containing Macrocycles

“…The influence of nitrogen atoms in the macrocycle in comparison to an analogue structure with boron atoms instead of amino functions was presented on polycationic macrocycles, and due to their redox properties, could be used as molecular switches [ 34 ].…”

Access to tetracoordinate boron-doped polycyclic aromatic hydrocarbons with delayed fluorescence and aggregation-induced emission under mild conditions

“…38,39 The location of the BN unit was found to be important for the optoelectronic properties because whether the BN unit could fully conjugate with PAHs is highly dependent on its locations. [40][41][42][43][44] Although both the number of BN units [45][46][47][48][49][50] and different orientations of BN units 6,[51][52][53][54] are believed to exhibit non-negligible impact on the optoelectronic properties and intermolecular interactions, only a few of examples have been documented due to the limited synthetic methodology.…”