Multifunctional Coordination Polymer Exhibiting Reversible Mechanical Motion Allowing Selective Uptake of Guests and Leading to Enhanced Electrical Conductivity

Abstract

A remarkably flexible, multifunctional, 2D coordination polymer exhibiting an unprecedented mode of reversible mechanical motion, enabling pores to open and close, is reported. Such multifunctional materials are highly sought after, owing to the potential to exploit coexisting electronic and mechanical functionalities that underpin useful technological applications such as actuators and ultrasensitive detectors. The coordination polymer, of composition Mn­(F4TCNQ)­(py)2 (F4TCNQ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracycanoquinodimethane; py = pyridine), consists of Mn­(II) centers bridged by F4TCNQ dianions and coordinated by py molecules that extend above and below the 2D network. Exposure of Mn­(F4TCNQ)­(py)2, in its collapsed state, to carbon dioxide results in a pore-opening process at a threshold pressure for a given temperature. In addition to carbon dioxide, a variety of volatile guests may be incorporated into the pores, which are lined with electron-rich F4TCNQ dianions. The inclusion of electron-deficient guests such as 1,4-benzoquinone, nitrobenzene, maleic anhydride, and iodine into the pores is accompanied by a striking color change associated with a new host–guest charge-transfer interaction and an improvement in the semiconductor behavior, with the iodine adduct showing an increase in conductivity of almost 5 orders of magnitude. Experimental and density functional theory calculations on this remarkable multifunctional material demonstrate a reduction in the optical band gap with increasing electron affinity of the guest.

A 1D Cu(i)–I-pyrazine coordination polymer with controlled pressure-induced phase transition and opto-electronic response depending on mechanical stimuli, temperature, and CuI content

“…25,26 On the other hand, a highly important objective, in addition to trying to synthesize new compounds with more relevant electrical properties, is to find ways to improve the electrical conductivity in existing systems. In this regard, examples of CPs based on Cu( i )-X double chains have been reported, where their electrical properties can be modified by external factors such as temperature, 27 light irradiance, 18 gas adsorption, 28,29 or doping. 30 Additionally, it should be mentioned that studies related to the variation of their luminescence with pressure are scarce.…”

Electrical conductivity and DFT investigations of a 2D CuI-TCNQII− framework

“…Although many X 4 TCNQ 2À coordination polymers have been reported, 15 there have been relatively few conductivity studies. 16,25 In this current work two-point pressed-pellet conductivity measurements reveal a value of s = 9.8 Â 10 À8 S cm À1 at 300 K. Variable temperature measurements between 250 and 300 K, show an increase in conductivity with temperature, indicating semiconductor behaviour. Current density-electric field strength plots for [Cu 2 (Me 2 pz)(TCNQ)] are presented in Fig.…”

“…Charge-transfer interactions commonly occur in X 4 TCNQ 2À (X = H or F) based compounds and in certain instances result in unusual and interesting properties. [15][16][17][18] The degree of charge transfer can be estimated by various methods 5,[21][22][23][24] that are commonly linked to an estimation of the of the charge (q) of the TCNQ species. The Kistenmacher equation is an empirical relationship represented by the formula:…”

“…Although the dianion is susceptible to air oxidation it can be stabilised by coordination to metal centres and through its participation in charge transfer interactions with p-acceptors. [7][8][9][10][11][12][13][14][15][16][17][18] Not surprisingly, given that each of the nitrile groups has the potential to act as a donor group, there has been long-term interest in metal-TCNQ materials. Cu(TCNQ) has been the focus of particular attention, as its semi-conducting properties depend upon the method of preparation.…”

“…As part of the general interest in metal complexes of TCNQ, we have directed our efforts towards the generation of TCNQ 2À coordination polymers. [7][8][9][10][11][12][13][14][15][16] As part of an investigation into Cu I 2 (TCNQ ÀII ) and Cu I 2 (F 4 TCNQ ÀII ) coordination polymers we reported the synthesis and structure of [Cu 2 (Me 2 pz)(TCNQ)] (Me 2 pz = 2,5-dimethylpyrazine). 14 This 2D framework displayed structural features that warranted further attention.…”