Carol Hua

Exploiting stable radical states for multifunctional properties in triarylamine-based porous organic polymers

Redox-active porous organic polymers (POPs) have enormous potential in applications ranging from electrocatalysis to solar energy conversion. Exploiting the different electronic states offers exciting prospects for controlling host–guest chemistry, however, this aspect of multifunctionality has to date, remained largely unexplored. Here, we present a strategy for the development of multifunctional materials with industrially sought-after properties. A series of hydrophobic POPs containing redox-active triarylamines linked by ethynyl (POP-1), 1,4-diethynylphenyl (POP-2) and 4,4′-diethynylbiphenyl (POP-3) bridges have been synthesised and characterised by NMR and EPR spectroscopy, as well as spectroelectrochemistry and computational modelling. The facile electrochemical or chemical oxidation of the POPs generate mixed-valence radical cation states with markedly enhanced adsorption properties relative to their neutral analogues, including a 3-fold improvement in the H2 uptake at 77 K and 1 bar, and an increase in the isosteric heat of adsorption for CO2.

Redox tunable viologen-based porous organic polymers

The use of an organic donor–acceptor polymer containing a viologen electron acceptor and triarylamine electron donor as a platform in the development of multifunctional materials is presented. The highly robust porous organic polymer (POP) system allows for exploration of the interplay between electronic and host–guest interactions in the synthesized polymers, POP-V1, which contains a redox-active triarylamine core and POP-V2, which contains a redox-inactive benzene core, where each of the redox states present can be reversibly accessed. The degree of charge transfer in addition to the H2 and CO2 gas adsorption properties of the polymer are able to be tuned as a function of the electronic state which has important implications for the potential applications of these polymers in optical, electrochromic and solar cell devices.

A Mn(II) coordination framework incorporating the redox-active tris(4-(pyridin-4-yl)phenyl)amine ligand (NPy3): electrochemical and spectral properties

A new multifunctional coordination framework, [Mn(NPy3)Cl·3MeOH·DMF]n, containing the redox-active tris(4-(pyridine-4-yl)phenyl)amine ligand (NPy3) was synthesised and characterised. The redox properties of the framework were investigated, and the oxidised state of the framework was generated chemically and spectroelectrochemically. This material exhibits multifunctionality by virtue of its ability to switch the fluorescence ‘on’ and ‘off’ with the redox state.

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal–Organic Frameworks

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

Spectroelectrochemical properties of a Ru(II) complex with a thiazolo[5,4-d]thiazole triarylamine ligand

A new bis-chelating ligand containing a triarylamine electron donor core fused with a thiazolo-thiazole electron acceptor, N,N′-(thiazolo[5,4-d]thiazole-2,5-diylbis(4,1-phenylene))bis(N-(pyridin-2-yl)pyridin-2-amine) (1) has been synthesised. This non-innocent ligand exhibits interesting electronic and spectral properties that can be tuned as a function of its redox state. In particular, modulation of the electronic state can be used to turn the fluorescence on and off. A dinuclear Ru(II) terpyridine complex, [Ru2(tpy)2Cl2(1)](PF6)2 (2) was subsequently synthesised and the properties of each of the accessible redox states explored using in situ spectroelectrochemical techniques.

In Situ Spectroelectrochemical Investigations of the Redox-Active Tris[4-(pyridin-4-yl)phenyl]amine Ligand and a Zn2+ Coordination Framework

An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal-organic framework (MOF) [Zn(NPy3)(NO2)2·xMeOH·xDMF]n (MeOH = methanol and DMF = N,N-dimethylformamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (EPR), and fluorescence spectroelectrochemical experiments. Through this multifaceted approach, the properties of the ligand and framework were elucidated and quantified as a function of the redox state of the triarylamine core, which can undergo a one-electron oxidation to its radical cation. The use of pulsed EPR experiments revealed that the radical generated was highly delocalized throughout the entire ligand backbone. This combination of techniques provides comprehensive insight into electronic delocalization in a framework system and demonstrates the utility of in situ spectroelectrochemical methods in assessing electroactive MOFs.

In Situ Spectroelectrochemical Investigations of RuII Complexes with Bispyrazolyl Methane Triarylamine Ligands

The synthesis and characterization of two triarylamine ligands, 4-(di(1H-pyrazol-1-yl)methyl)-N-(4-(di(1H-pyrazol-1-yl)methyl)phenyl)-N-phenylaniline (TPA-2bpm) and tris(4-(di(1H-pyrazol-1-yl)methyl)phenyl)amine (TPA-3bpm), containing the bispyrazolylmethane moiety and its RuII terpyridine complexes are presented. The redox properties of the ligands and RuII complexes are explored in detail through cyclic and square-wave voltammetry in addition to in situ UV-vis-near infrared, electron paramagnetic resonance, and fluorescence spectroelectrochemistry. It was demonstrated that the triarylamine radical cation was able to be generated, and further, TPA-2bpm underwent an electrochemically induced dimerization process.

Redox-State Dependent Spectroscopic Properties of Porous Organic Polymers Containing Furan, Thiophene, and Selenophene*

A series of electroactive triarylamine porous organic polymers (POPs) with furan, thiophene, and selenophene (POP-O, POP-S, and POP-Se) linkers have been synthesised and their electronic and spectroscopic properties investigated as a function of redox state. Solid state NMR provided insight into the structural features of the POPs, while in situ solid state Vis-NIR and electron paramagnetic resonance spectroelectrochemistry showed that the distinct redox states in POP-S could be reversibly accessed. The development of redox-active porous organic polymers with heterocyclic linkers affords their potential application as stimuli responsive materials in gas storage, catalysis, and as electrochromic materials.

Controlling the Uptake and Regulating the Release of Nitric Oxide in Microporous Solids

Representative compounds from three classes of microporous solids, namely, metal-organic frameworks (MOFs), hybrid ultra-microporous materials (HUMs), and porous-organic polymers (POPs), were investigated for their nitric oxide gas uptake and release behavior. Low-pressure sorption studies indicated strong chemisorption of NO on the free amine groups decorating the MOF UiO-66-NH2 when compared to its non-amine-functionalized parent. The HUMs demonstrated reversible physisorption within the low-pressure regime, but interestingly in one case there was evidence for chemisorption following pressurization with NO at 10 bar. Significant release of chemisorbed NO from the UiO-66-NH2 and one of the HUMs was triggered by addition of acid to the medium, a pH change from 7.4 to 5.4 being sufficient to trigger NO release. An imidazole-based POP exhibited chemisorption of NO at high pressure wherein the ring basicity facilitated both NO uptake and spontaneous release upon contact with the aqueous release medium.