Tunable Capacitive Behavior in Metallopolymer-based Electrochromic Thin Film Supercapacitors.

Volumetric capacitance is a more critical performance parameter for rechargeable power supply in lightweight and microelectronic devices as compared to gravimetric capacitance in larger devices. To this end, we report three electrochromic metallopolymer-based electrode materials containing Fe2+ as the coordinating metal ion with high volumetric capacitance and energy densities in a symmetric two-electrode supercapacitor setup. These metallopolymers exhibited volumetric capacitance up to 866.2 F cm-3 at a constant current density of 0.25 A g-1. The volumetric capacitance (poly-Fe-L2: 544.6 F cm-3 > poly-Fe-L1: 313.8 F cm-3 > poly-Fe-L3: 230.8 F cm-3 at 1 A g-1) and energy densities (poly-Fe-L2: 75.5 mWh cm-3 > poly-Fe-L1: 43.6 mWh cm-3 > poly-Fe-L3: 31.2 mWh cm-3) followed the order of the electrical conductivity of the metallopolymers and are among the best values reported for metal-organic systems. The variation in the ligand structure was key toward achieving different electrical conductivities in these metallopolymers with excellent operational stability under continuous cycling. High volumetric capacitances and energy densities combined with tunable electro-optical properties and electrochromic behavior of these metallopolymers are expected to contribute to high performance and compact microenergy storage systems. We envision that the integration of smart functionalities with thin film supercapacitors would warrant the surge of miniaturized on-chip microsupercapacitors integrated in-plane with other microelectronic devices for wearable applications.

Ligand-Controlled Electrochromic Diversification with Multilayer Coated Metallosupramolecular Polymer Assemblies.

Designing surface-confined molecular systems capable of expressing changes in functional properties as a result of slight variations in chemical structure under the influence of an external stimulus is of contemporary interest. In this context, we have designed three tetraterpyridine ligands with variations in their core architecture (phenyl vs tetraphenylethynyl vs bithiophene) to create spray-coated electrochromic assemblies of iron(II)-based metallosupramolecular polymer network films on transparent conducting oxide substrates. These assemblies exhibited molecular permeability and spectroelectrochemical properties that are in turn dictated by the ligand structure. Electrochromic films with high coloration efficiencies (up to 1050 cm2/C) and superior optical contrast (up to 76%) with a concomitant color-to-color redox transition were readily achieved. These functional switching elements were integrated into sandwich-type electrochromic cells (CE up to 641 cm2/C) that exhibited high contrast ratios of up to 56%, with attractive ON-OFF ratios, fast switching kinetics, and high operational stability. Every measurable spectroelectrochemical property of the films and devices is an associated function of the ligand structure that coordinates the same metal ion to different extents. While exhibiting a ligand-structure induced differential metal coordination leading to porosity and spectroelectrochemical diversification, these assemblies allow the creation of electrochromic patterns and images by a simple spray-coating technique.

Thermochromic Color Switching to Temperature Controlled Volatile Memory and Counter Operations with Metal-Organic Complexes and Hybrid Gels.

Temperature is often not considered as a precision stimulus for artificial chemical systems in contrast to the host-guest interactions related to many natural processes. Similarly, mimicking multi-state volatile memory operations using a single molecular system with temperature as a precision stimulus is highly laborious. Here we demonstrate how a mixture of iron(II) chloride and bipyridine can be used as a reversible color-to-colorless thermochromic switch and logic operators. The generality of the approach was illustrated using CoII and NiII salts that resulted in color-to-color transitions. DMSO gels of these systems, exhibited reversible opaque-transparency switching. More importantly, optically readable multi-state volatile memory with temperature as a precision input has been demonstrated. The stored data is volatile and is lost instantaneously upon withdrawal or change of temperature. Simultaneous read-out at multiple wavelengths results in single-input/multi-output sequential logic operations such as data accumulators (counters) leading to volatile memory states. The present system provides access to thermoresponsive materials wherein temperature can be used as a precision stimulus.