Probing charge transfer characteristics in a donor–acceptor metal–organic framework by Raman spectroelectrochemistry and pressure-dependence studies

Abstract

The stimuli responsive behaviour of charge transfer donor-acceptor metal-organic frameworks (MOFs) remains an understudied phenomenon which may have applications in tuneable electronic materials. We now report the modification of donor-acceptor charge transfer characteristics in a semiconducting tetrathiafulvalene-naphthalene diimide-based MOF under applied electrochemical bias and pressure. We employ a facile solid state in situ Raman spectroelectrochemical technique, applied for the first time in the characterisation of electroactive MOFs, to monitor the formation of a new complex TTFTC •+ -DPNI from a largely neutral system, upon electrochemical oxidation of the framework. In situ pressuredependent Raman spectroscopy and powder X-ray diffraction experiments performed in a diamond anvil cell revealed blue shifts in the donor and acceptor vibrational modes in addition to contractions in the unit cell which are indicative of bond shortening. This study demonstrates the utility of in situ Raman spectroscopic techniques in the characterisation of redox-active MOFs and the elucidation of their electronic behaviours.

Spectroelectrochemistry: A Powerful Tool for Studying Fundamental Properties and Emerging Applications of Solid-State Materials Including Metal–Organic Frameworks

“…Nevertheless, this cell has proved particularly important for observing the spectroscopic features of MOFs that are otherwise challenging to obtain via ex situ chemical oxidation or reduction. A wide range of MOFs have been interrogated, [5,41,73,[76][77][78][79][80][81][82][83] including the recent observation and mechanistic interpretation of through-space electron transfer in MOFs incorporating cofacially aligned electroactive ligands such as thiazolothiazoles. [73,77,79] A closely related cofacial MOF incorporating tetrathiafulvalene (TTF) ligands, [Cd 2 (Py 2 TTF) 2 (bpdc) 2 ] (Py 2 TTF ¼ 2,6-bis(4 0 -pyridyl)-tetrathiafulvalene; bpdc 2-¼ 4,4 0 -biphenyldicarboxylate) demonstrated the same through-space charge transfer mechanism (Fig.…”

“…A confocal Raman SEC technique was recently reported for MOFs containing donor and acceptor functionalities, namely [(Zn(DMF)) 2 (TTFTC)(DPNI)] (DMF ¼ N,N 0 -dimethylformamide, TTFTC ¼ tetrathiafulvalene tetracarboxylate, DPNI ¼ N,N 0di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide). [38,78] Adapted with permission from the Royal Society of Chemistry. [78] donor-acceptor MOFs have been inspired by the historically rich field of 'organic metals' such as the archetypal tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), which demonstrates near-metallic conductivity.…”

“…[38,78] Adapted with permission from the Royal Society of Chemistry. [78] donor-acceptor MOFs have been inspired by the historically rich field of 'organic metals' such as the archetypal tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), which demonstrates near-metallic conductivity. [110] As shown in Fig.…”

“…9, the cell consisted of a widely available and economical screen-printed electrode to which the MOF was immobilised onto the working electrode. [78] A droplet of electrolyte completed the electrochemical cell. Although reductive SEC was not possible using this cell design, application of an oxidising potential led to an increase in the intensity of vibrations associated with TTFTC þ and DPNI 0 on their increased formation, and a corresponding decrease in vibrations associated with TTFTC and DPNI À .…”

Spectroelectrochemical Sensing: Current Trends and Challenges

“…SEC joins the best of electrochemistry and spectroscopy together during an electron‐transfer process. As can be inferred, this multiresponse and “teamwork” technique has been used in a large number of fields, including, for example, electron transfer processes , chemical reaction mechanisms , side reactions , electrocatalysis , corrosion , conducting polymers , solar cells , memory devices , supercapacitors , synthesis and characterization of nanoparticles , carbon nanomaterials , characterization of metal complexes , liquid/liquid interfaces , electrochromic materials or study of compounds of biological interest , among others.…”