In situ Spectroelectrochemical Investigations of the Redox-Active Tris(4-(pyridin-4-

yl)phenyl)amine Ligand and a Zn2+ Coordination Framework

Carol Hua,a Amgalanbaatar Baldansuren,b* Floriana Tuna,b David Collisonb and Deanna M.

D’Alessandroa*

a School of Chemistry, The University of Sydney, New South Wales 2006, Australia. Fax: +61 (2) 9351 3329;

Tel: +61 (2) 9351 3777; E-mail: deanna.dalessandro@sydney.edu.au

bSchool of Chemistry and Photon Science Institute, The University of Manchester, Manchester M13 9PL,

United Kingdom. Tel: +44 (0)161 275 1012; E-mail: amgalanbaatar.baldansuren@manchester.ac.uk

Abstract

An investigation of the redox-active tris(4-(pyridine-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 was conducted using in situ UV/Vis/NIR, 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 delocalised throughout the entire ligand

backbone. This combination of techniques provides comprehensive insights into electronic

delocalisation in a framework system and demonstrates the utility of in situ spectroelectrochemical

methods in assessing electroactive MOFs.

Introduction

Multifunctional materials whose properties can be altered as a function of their redox state are

promising candidates for use in numerous applications.1-5 To obtain a thorough understanding of the

material’s properties in its electrochemically accessible redox states, the application of in situ

characterisation methods are important alongside ex situ bulk techniques. In situ

spectroelectrochemical techniques involve the direct application of an electrical stimulus to the

material, where the spectral response is monitored as a function of the potential applied. 6 While

solution state in situ spectroelectrochemical experiments have been well developed and have been

paired with a number of different spectroscopic techniques such as UV/Vis/NIR, EPR, NMR, IR,

XAS and optical fluorescence,7-9 the analogous solid state in situ spectroelectrochemical experiments

have received far less attention.5 Solid state spectroelectrochemical experiments meet relatively

greater challenges in terms of the experimental set up and subsequent data analysis. 10 Many of the

difficulties are associated with the inherent complex processes that occur during a solid state

electrochemical experiment, which include charge transport and diffusion of ions into the pores of the

porous materials.11 Solid state in situ spectroelectrochemical experiments can be applied to the

analysis of materials that have potential use in electronic swing adsorption, 1 as cathode materials for

battery applications and in electrocatalytic schemes,12 amongst others.

Metal-Organic Frameworks (MOFs) are particularly versatile redox-active materials due to the high

level of systematic control and variation possible. It has previously been shown that incorporation of a

redox-active ligand and/or metal centre will result in a framework that retains the redox activity of the

ligand.13-21 As the properties of the ligand are directly translated into the framework structure, the

thorough investigation and characterisation of the ligand properties in solution provides valuable

insights into the properties of the solid state material.

Triarylamines are tri-substituted aromatic amines in which the central nitrogen is planar and the

pendant aromatic rings are orientated in a “propeller-like” orientation with each of the phenyl rings

canted in the same direction.22 Triarylamines are capable of a one electron oxidation, which can be

achieved through electrochemical or chemical processes to form the radical cation.23 Triarylamine

units bridged by organic linkers have been widely studied in the field of organic mixed-valence

chemistry,24 where powerful insights into the factors that govern electron transfer have been

determined.22 The highly reversible one electron oxidation of triarylamines with their well

characterised electrochemical and spectral properties make them attractive structural motifs for the

design of multifunctional materials.25 The oxidised states of triarylamines have previously been

exploited as hole-transport components in photoconductors and light-emitting diodes.26-34 Thus, redox-

active materials incorporating these functional motifs should exhibit interesting electrical, optical and

fluorescence properties that are sensitive to an applied electrical stimulus.

A detailed understanding of the generation of the triarylamine radical cation has been achieved using

electron paramagnetic resonance (EPR) techniques.35 Continuous wave (c.w.) EPR experiments

provide information about the spin-system-dependent interactions such as the g-matrix and hyperfine

coupling. Pulsed EPR can be applied to most samples of interest and it allows a better discrimination

between different small interactions in the spin-Hamiltonian. For instance, precise measurements of

the smaller hyperfine interactions, which are readily masked by EPR broadening in the c.w.

experiments, of the electron spin with the remote nuclear spins in the environment can be achieved

using techniques such as pulsed ENDOR. The extent of electron delocalisation throughout the radical

species is able to be elucidated by these selective pulsed EPR methods. The quantification of electron

delocalisation is particularly important in the development of lightweight conductive and charge-

transfer materials, which have been demonstrated to exhibit novel and intriguing electronic

properties.24, 35

Herein, we describe the use of multiple in situ spectroelectrochemical experiments in the solution

state paired with UV/Vis/NIR, EPR, and fluorescence spectroscopies for the investigation of the

tris(4-(pyridine-4-yl)phenyl)amine (NPy3) ligand. The extent of electron delocalisation and relevant

information about the electronic and local structures of the electrogenerated radical cation were

determined through pulsed EPR experiments. Solid state in situ spectroelectrochemical experiments

were performed on the novel [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework, allowing an

investigation of the ligand-based processes in the solid state. This multifaceted approach has enabled

deeper insights into the charge delocalisation properties of electroactive MOFs which are of interest

as the basis for optical and electronic devices.

Results and Discussion

Synthesis and Structure

The tris(4-(pyridine-4-yl)phenyl)amine (NPy3) ligand was synthesised via a Suzuki cross-coupling

reaction between tris(p-bromophenyl)amine and 4-pyridyl boronic acid to yield the ligand as a bright

yellow crystalline solid.13, 15 Incorporation of NPy3 into a coordination framework was achieved by

heating the ligand with Zn(NO3)2·6H2O in a mixture of DMF and methanol for 48 hours at 80 °C to

form [Zn(NPy3)(NO2)2·xMeOH·xDMF]n as yellow needles. Zinc oxide was formed during the

synthesis of the framework, but was easily separated by hot filtration of the reaction mixture prior to

cooling and crystallisation of the framework, as was verified by the powder X-ray diffraction pattern

(ESI).

The asymmetric unit of the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework consisted of one NPy3

ligand coordinated by a Zn2+ ion (Figure 1a). The Zn2+ contains a distorted tetrahedral coordination

sphere, where the Zn2+ centre is coordinated to three oxygen atoms and one nitrogen atom. Two

oxygen atoms are bound in a bidentate manner from one NO2- ion, while the third oxygen atom is

bound in a monodentate manner to a second NO2- ion with the nitrogen atom from an NPy3 ligand.

The presence of the NO2 groups bound to the Zn2+ centre and the detection of zinc oxide indicates that

the Zn(NO3)2·6H2O precursor underwent decomposition during the course of the reaction. This may

have been caused by reduction of the nitrate counter-ion by the triarylamine core, which can act an

oxidising agent, resulting in the formation of the nitrite anion. The subsequently formed triarylamine

radical cation was presumably reduced back to the neutral state by the methanol in the reaction

mixture. The presence of the nitrite anion was confirmed by inspection of the IR spectrum, where

peaks at 1327, 1188 and 848 cm-1 corresponded well to the values previously reported (1351, 1171

and 850 cm-1).36

The central triarylamine core from the NPy3 ligand is orientated in a propeller-like configuration,

which correlates well with previous reports of solid state structures incorporating triarylamines

(Figure 1a).37 The framework consists of one 3D network containing a series of (4,4’)-nets. Ordered

layers were observed down the c axis (Figure 1c), whilst pores are found down the a axis (Figure 1b).

A solvent void accessible volume of 17% was calculated using the SQUEEZE function in PLATON.38

This corresponds roughly to the mass loss below 123 °C in the TGA of the framework, which is due

to the liberation of methanol from the pores (ESI).

Figure 1. Solid state structure of the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework where a) asymmetric unit, b) view down the a axis and c) view down the c axis.

Redox Properties

The redox properties of the NPy3 ligand have previously been determined in [(n-C4H9)4N]PF6/CH3CN

electrolyte and are characterised by two broad irreversible oxidation processes (ESI). 13 The process at

0.70 V vs. Fc+/Fc was assigned to oxidation of the pyridyl rings present in the ligand, whilst the

process at 0.96 V vs. Fc+/Fc involves the 1e- oxidation of the triarylamine core to its radical cation.22,

c)b)

a)

23, 39 In light of the favoured use of aprotic solvents for EPR measurements (vide infra), the redox

properties of NPy3 were additionally investigated in [(n-C4H9)4N]PF6/CH2Cl2 electrolyte. NPy3

exhibited markedly different redox properties in this electrolyte, with an irreversible peak observed

upon oxidation, which became increasingly reversible with increased scan rates indicating the

domination of an Electrochemical-Chemical (EC) process at low scan rates (ESI). The anodic shift of

the oxidation peak from 0.75 V to 1.4 V vs. Fc+/Fc upon redox cycling appeared to indicate the

formation of a film on the surface of the working electrode (ESI).

The redox properties of the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework were investigated using

solid state electrochemical experiments in [(n-C4H9)4N]PF6/CH3CN electrolyte, where one irreversible

redox process at 0.89 V vs. Fc+/Fc was observed. This process was due to oxidation of the

triarylamine core to its corresponding radical cation state (ESI) as oxidation of the pyridyl rings

should be inhibited due to its coordination to Zn2+ through the nitrogen donors on the ligand.

UV/Vis/NIR Chemical and Spectroelectrochemical Oxidation

The UV/Vis/NIR spectrum of the neutral [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework displayed

bands >20000 cm-1 due to the charge transfer processes for the d10 Zn2+ metal centre, which are

overlayed on π to π* transitions of the aromatic triarylamine core (ESI). The ex situ oxidation of

[Zn(NPy3)(NO2)2·xMeOH·xDMF]n was attempted using several different chemical oxidants, (ESI)

with vapour diffusion of iodine proving to be the most successful. The appearance of a peak at 16000

cm-1 was indicative of formation of the triarylamine radical cation. The vapour diffusion of bromine

into the framework resulted in its dissolution, while attempted oxidation of the framework with Ce 4+

did not result in any observable redox state change.

The chemical oxidation of NPy3 with NOBF4 in acetonitrile as monitored by solid state UV/Vis/NIR

has previously been reported, where the appearance of a Gaussian-shaped peak at 11800 cm-1 (due to

the localised D0 to D1 transition of the radical cation) and red shift of the band at 23000 cm -1 to 17800

cm-1 upon oxidation was indicative of formation of the triarylamine radical cation.13

The in situ UV/Vis/NIR spectroelectrochemical experiment provides complementary information to

the ex situ chemical oxidation experiments, and provide insights into transient states that may be

unable to be detected using the latter techniques.5 The spectrum of the neutral state of NPy3 is

dominated by a band at 28140 cm-1 due to a HOMO to LUMO (S0 to S1) transition (Figure 2a). This

band was at a lower energy than that for analogous methoxy- and chloro- para-substituted

triarylamine derivatives and may be due to the presence of an extended aromatic system.23

Three main processes were observed upon increasing the potential from 0 to 1.0 V in the UV/Vis/NIR

spectroelectrochemical experiment in [(n-C4H9)4N]PF6/CH3CN electrolyte. The first process, where

the potential was increased from 0 to 1.0 V, was characterised by the formation of bands at 23770 and

35000 cm-1, while the band at 28140 cm-1 decreased owing to the oxidation of the nitrogen atoms on

the pyridyl rings to form an N-oxide species (Figure 2a). The loss of the isosbestic points in the

spectroelectrochemical experiment during this first step indicates that multiple processes are occurring

concurrently, where several equilibria need to be established, corresponding to oxidation of the three

pyridyl nitrogen centres. The UV/Vis/NIR solution state spectroelectrochemical experiments on 1,3,5-

tri(4-pyridyl)benzene, which contains three pyridyl rings in a similar geometry with a redox inactive

core, confirmed the formation of a N-oxide species with an increase in the band at 35000 cm-1 (ESI).

The second process, involving the formation of a peak at 13690 cm-1 and a shoulder at 16540 cm-1,

was ascribed to formation of the triarylamine radical cation (Figure 2b). The band at 13690 cm -1 is

due to a HOMO to SOMO (D0 to D1) transition of the triarylamine radical cation, while the shoulder

at 16540 cm-1 arises from interaction of the molecule with the solvent leading to the breaking of

symmetry and a splitting of the degenerate D1 state.23 The final process consisted of oxidation of the

triarylamine radical cation to the dication at 1.2 V (Figure 2c). The peak at 13690 cm -1 due to the

radical cation core decreased in intensity while the band at 32000 cm -1 increased. These bands were

indicative of formation of the dication state, and was assigned to a HOMO to LUMO transition.23

Figure 2. Solution state spectroelectrochemistry on NPy3 in [(n-C4H9)4N]PF6/CH3CN electrolyte over the potential range 0 - 2.05 V where the potential was held at a) 0 - 1.0 V (acquired over 90 mins), b) 1.1 V (acquired over 25 mins) and c) 1.2 V (acquired over 65 mins).

a)

b)

c)

As the Zn2+ centre is redox-inactive, the spectral changes observed in the [Zn(NPy3)

(NO2)2·xMeOH·xDMF]n framework during the in situ solid state spectroelectrochemical experiment

are ascribed to ligand based processes (Figure 3). Upon application of a positive potential of 1.5 V, a

colour change from light yellow to deep green/blue was accompanied by the appearance of an intense

band at 13290 cm-1 and a broad band at 22220 cm-1 (Figure 3a). The band at 13290 cm-1 was assigned

to the π to π* transition of the triarylamine radical cation, which was formed upon oxidation of the

neutral triarylamine core. Upon holding the potential at 1.5 V, the radical cation band decreased,

whilst a band at 21940 cm-1 appeared (Figure 3b), which was due to further oxidation of the radical

cation triarylamine core to the dication state. Application of a reducing potential (of 0 V) led to

reduction of the oxidised [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework (Figure 3c), and a

corresponding colour change from deep blue/green of the oxidised state to the yellow of the neutral

state.

Figure 3. Solid state spectroelectrochemistry on the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework in [(n-C4H9)4N]PF6/CH3CN electrolyte a) upon increasing the potential from 0 to 1.5 V (over 12 mins), b) holding the potential at 1.5 V (over 9 mins), c) reducing the potential from 1.5 to 0 V (over 9 mins).

EPR Spectroelectrochemistry

The solution state EPR spectroelectrochemical experiments for the NPy3 ligand were conducted in

[(n-C4H9)4N]PF6/CH2Cl2 electrolyte at room temperature, where dichloromethane was used as the

solvent instead of acetonitrile because it is less polar and consequently absorbs less microwave

radiation (Figure 4). Continuous wave (c.w.) EPR spectra display strong couplings of the electron spin

to nuclei. The c.w. EPR spectrum in the solution state of NPy3 displayed a signal centred at g ≈

a)

b)

c)

2.0067 containing a 1:1:1 hyperfine splitting with an isotropic value of aiso = 23.0 MHz was observed

when a potential of 1.5 V was applied, along with a colour change from bright yellow to green. These

values agree well with previously reported EPR studies on triarylamine systems.35 As the potential

was increased to 1.8 V, the intensity of the signal increased, indicating the generation of a larger

quantity of the radical. This hyperfine splitting pattern indicated that the radical was primarily

localised on the central nitrogen of the triarylamine and was assigned to the one electron oxidation of

the triarylamine to its radical cation (Figure 4a).35 The EPR spectrum of the electrochemically

generated NPy3 radical was additionally obtained from a frozen solution of the ligand in [(n-

C4H9)4N]PF6/CH2Cl2 electrolyte at 20 K (Figure 4b). From the frozen c.w. spectrum, anisotropic

information is able to be gained. The solid state spectrum was found to exhibit g-factor values of

2.0045 and 2.0037 and anisotropic hyperfine values of 3.3, 58.9 and 3.3 MHz.

A solid state EPR spectroelectrochemical experiment was carried out on [Zn(NPy3)

(NO2)2·xMeOH·xDMF]n where the Zn2+ centre is diamagnetic and EPR silent. Upon application of a

positive potential of 1.0 V to [Zn(NPy3)(NO2)2·xMeOH·xDMF]n , a typical EPR signal with hyperfine

anisotropy was observed (g = 2.002), confirming that the radical was also generated in the solid state

(Figure 4). This signal is comparable to the EPR spectrum of NPy3 in frozen solution and is assigned

to the triarylamine radical cation. As the potential was increased to 1.5 V, the signal intensity

increased correspondingly. The solid state EPR spectroelectrochemical experiment on the [Zn(NPy 3)

(NO2)2·xMeOH·xDMF]n framework demonstrated that the radical cation species were unambiguously

generated, in agreement with the results of the UV/Vis/NIR spectroelectrochemical measurements.

Figure 4. a) EPR spectra of NPy3 at applied potentials of 1.5 (blue) and 1.8 V (red) in [(n-C4H9)4N]PF6/CH2Cl2 electrolyte at 298 K (liquid solution). b) Comparison between the experimental (black) and numerically simulated spectra (red) at 20 K (frozen solution) and c) EPR signal of the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework. The parameters for the numerical simulation are listed in the ESI.

a)

b)

c)

Pulsed EPR

Due to their different spin properties and fundamental nuclear frequencies, separate contributions

from the 14N and 1H nuclei in the EPR spectrum of NPy3 are expected. The use of c.w. EPR has

allowed for one large 14N hyperfine coupling to be determined. Small hyperfine couplings present

from the remaining 1H and 14N are unresolved, however, being hidden under the inhomogeneous

linewidth. The use of pulsed EPR techniques, such as HYSCORE and pulsed ENDOR allows for the

elucidation of hyperfine interactions, which are on the order of the nuclear Zeeman frequencies, that

are too small to be resolved by c.w. EPR.

14N HYSCORE

The field-sweep electron spin echo (ESE) measurement, where a magnetic field sweep is performed,

allows a field value to be selected for time-domain measurements, such as the HYSCORE

experiment. A field value of 346.9 mT was chosen, where only molecules with the appropriate g-

value satisfying the resonance condition was selected.

An important topic in ESEEM/HYSCORE is the characterization of the nitrogens involved in either a

ligation of metal ion centres or a formation of hydrogen bonds with paramagnetic species, and either

case implies spin density delocalization or transfer (vide infra). It is well known that the parameters of

the 14N spin Hamiltonian have the typical values of quadrupole coupling constant 1 – 5 MHz, nuclear

Zeeman frequency νN ~ 1.05 MHz, and isotropic hyperfine constant aiso ~ 0.1 – 5.0 MHz at X-band.40

The anisotropic hyperfine coupling is at least several times smaller than the isotropic constant.

Therefore, the approximation of a pure isotropic hyperfine interaction has been used for the

qualitative consideration of 14N powder-type ESEEM spectra.40-42

To confirm the presence of 14N in the local structure of the radical cation species, HYSCORE

experiments were performed probing for its hyperfine interaction at X-band. The contour peaks in the

(+,+) quadrant arise from weakly coupled nitrogens with predominantly isotropic hyperfine

interactions (Figure 5a). This spectrum features the three narrow contour peaks at ν0 ≈ 0.9 MHz, ν− ≈

2.3 MHz, and ν+ ≈ 3.2 MHz (Figure S6), referred to as the cancellation condition42 in one of the two

electron spin (mS = ±1/2) manifolds with νeff±/K < 1.41 Detailed information on the cancellation

condition for the observation of the three pure (or zero-field) nuclear quadrupole resonance

frequencies is provided in the ESI. These frequencies further determine the quadrupole coupling

constant41, 43, 44 K ≈ 0.92 MHz and asymmetry parameter η ≈ 0.5 (given in Table 1).

There is the cross-correlation of the nuclear quadrupole resonance frequencies with the double-

quantum frequency at νdq ≈ 5.5 MHz, marked as (ν+, νdq+)43, 44 (Figure 5a). These correlation peaks

possess a maximum at (5.5, 3.2) or (3.2, 5.5) MHz, when the double-quantum frequencies correlate

from the opposite mS manifold with νeff±/K > 1 (ESI). From the nuclear quadrupole resonance

frequencies, K and η can be determined, which are used to characterize the chemical type of the 14N

atom and its electronic state.43, 44 The double-quantum frequency provides the isotropic coupling

constant, of the order of aNiso ≈ 2.3 MHz (Table 1). The corresponding numerical simulation45

reproduced the locations of the contour-peaks from ν0, ν−, ν+ and (ν+, νdq+) well (Figure S6). These

contour-peaks are assigned to the weakly coupled nitrogen of the NPy3 ligand. The experimental

results obtained here do not unambiguously distinguish between whether the central or pyridyl N

atom has the greater spin density, however upon comparison with previously reported hyperfine

values for the central 14N, it is likely that the weakly coupled nitrogen is due to the pyridyl N.35 This is

supported by the three quadrupole resonance frequencies determined from the HYSCORE experiment

being in close agreement with the measured 14N quadrupole resonance transitions and quadrupole

coupling constant in different pyridine compounds at 77 K.46 In Zn2+ coordinated pyridine compounds,

for instance, the quadrupole coupling constants range from 0.1 to 0.75 MHz. In ring substituted

pyridine derivatives, the quadrupole coupling constants are in the narrow range of 1 to 1.2 MHz. [47b]

The existence of a non-zero isotropic constant for the interacting 14N of the pyridyl ring therefore

suggests that the unpaired electron spin density was transferred onto this atom and hence the

generated radical was delocalised throughout the entire ligand structure.

Figure 5. (a) X-band 14N HYSCORE spectrum of NPy3 recorded with τ = 136 ns at 346.9 mT, ~ 9.7 GHz, and 20 K. The nuclear quadrupole frequency peaks assigned to ν0, ν−, ν+ are shown in the (+,+) quadrant with a full stacked (top) and contour (bottom) plots. In the bottom plot, a number of contour lines were deliberately reduced in order to better show the locations of the correlation peaks (ν+, νdq) marked in the stacked presentation. The corresponding numerical simulation is shown in Figure S6 of ESI. (b) X-band 1H HYSCORE spectrum of NPy3 recorded with τ = 136 ns at 346.9 mT, ~ 9.7 GHz is shown with the corresponding numerical simulation (red). (c) The cross-peak of 1H was shown in (ν1

2) vs (ν2

2) coordinates where the red curve was defined by |ν1 ± ν2| = 2|1νH| with the proton Zeeman frequency 1νH ~ 14.7 MHz. The regression analysis of the cross-peak is further shown in Figure S7 of ESI.

b) c)

a)

Table 1: Nuclear quadrupole frequencies and isotropic hyperfine coupling of 14N in NPy3 fulfilling the cancellation condition.40, 41

ν0 (MHz) ν- (MHz) ν+ (MHz) νdq (MHz) aiso (MHz) κ (MHz) η

0.9 2.3 3.2 5.5 2.2 0.92 0.5

1H Hyperfine

To elucidate complete information regarding the coupling of the electron spin to 1H nuclei, three

complimentary techniques were used in this study; 1D pulsed ENDOR (Mims and Davies) and 2D

HYSCORE. As the polarisation of the electron spin transition is significantly larger than that of the

nuclear spin transition, in ENDOR experiments, this polarisation can be partly transferred to the

nuclear transition to enhance the detection sensitivity, especially for anisotropic hyperfine couplings

in a frozen solution. The two 1D pulse-ENDOR experiments used are complimentary to each other

with Davies-ENDOR optimised for hyperfine couplings 5.0 - 50 MHz and Mims-ENDOR for

hyperfine couplings smaller than 5.0 MHz. From the 2D HYSCORE experiment, information

regarding the hyperfine couplings in addition to the isotropic and anisotropic parts of the hyperfine

tensor is able to be elucidated.

1H HYSCORE

The HYSCORE spectrum exhibits well-resolved cross-peaks from 1H nuclei, where the cross-peak

was split symmetrically along the anti-diagonal, with a peak maximum at (17.3, 12.1) or (12.1, 17.3)

MHz [(να, νβ) or (νβ, να)] in the (+,+) quadrant, corresponding to a first-order estimated hyperfine

coupling of ~ 5.2 MHz (Figure 5a).

Analysis of the cross-ridges from nuclei of I = 1/2 spin (e.g. 1H, 15N and 13C) in the (ν12) vs. (ν2

2)

coordinate allows a direct, simultaneous determination of the isotropic aiso and anisotropic T

components of the hyperfine coupling.45, 47, 48 Linear regression of the 1H cross-ridges in the NPy3

spectrum plotted in (ν12) vs. (ν2

2) coordinates (Figure 5b) was analysed in Figure S7 of ESI. Linear

regression gives intersection points with the |να ± νβ| = 2νI curve for each cross-peak.44 These points

uniquely determine the two principal values aiso and T of the hyperfine tensor. There are two possible

assignments to (να⊥, νβ⊥) and (να||, νβ||) for each crossing point and, consequently, two solutions, one for

each assignment.49 Uncertainty in the assignment of ν1 to να or νβ and, respectively ν2 to νβ or να, allows

alternate signs of aiso and T in both solutions (see footnote of Table 2). Tensors obtained from linear

regression analysis are summarized in Table 2 and Table S3, respectively. The isotropic and

anisotropic parts of hyperfine tensor of the 1H nuclei contributing to the HYSCORE spectrum

obtained from the linear regression analysis44, 47, 48 were further used for the numerical simulation

using EasySpin.45 For both combinations of aiso and T, the numerical simulations reproduced the exact

locations of the cross-peak ridges, assigned to the coupled protons (Figure 5b and Figure S8 of ESI).

The cross-peak in the (+,+) quadrant arises from weak hyperfine interactions, satisfying a condition |T

+ 2aiso| < 4νI. If proton hyperfine couplings are smaller than the proton Zeeman frequency, one

normally calls them “weakly coupled” protons. Importantly, this result provides evidence that the

cross-peak was attributable to protons coupled to the unpaired electron spin and its density

delocalization. Non-structural or simply non-coupled 1H, such as matrix and/or solvent protons, would

give rise to the intense contour peak centred at the diagonal point (1νH, 1νH) in the (+,+) quadrant.

Table 2. 1H hyperfine constants calculated from the linear regression analysis of the cross-peak ridges plotted in (ν1

2) vs. (ν22) coordinates, shown in Figure 5c and Figure S7 of ESI.

aHiso (MHz) T (MHz)

5.0 (±0.4) 2.0 (±0.2)

-7.0 (±0.4)a 2.0 (±0.2)aSigns of a and T are relative to the general form of two solutions: (±a1,

±T) and (±a2, ∓T) with equal |2a1iso + T| and |2a2

iso + T|

1H ENDOR

The Davies 1H-ENDOR spectrum for NPy3 displayed several first-order hyperfine coupling with 6.5,

3.0 and 1.6 MHz split from the proton Zeeman (Larmor) frequency 1νH ~14.7 MHz due to the

presence of the coupled protons in the NPy3 ligand. The system was in the high field limit (ν > A/2) as

the signal was centred at the nuclear frequency and split by the hyperfine value. An additional peak at

6.0 MHz was due to matrix effects from the phosphorus (31P) present in the [(n-C4H9)4N]PF6/CH3CN

electrolyte.

The Mims 1H-ENDOR spectra recorded with τ = 100, 178, 200 and 250 ns were determined to contain

four different hyperfine coupling values for 1H at aiso = 1.6, 3.0, 5.0 and 6.5 MHz centred around 1νH

(Figure 6b and ESI). These hyperfine couplings were assigned to the four chemically distinct

environments present in the NPy3 ligand, where the 1H in the ortho position gives rise to the largest

hyperfine coupling value (6.5 MHz), with the 1H furthest from the central N giving rise to the smallest

coupling value (1.6 MHz), indicating that the radical generated at the triarylamine core was

delocalised to the furthest hydrogen on the pyridyl ring from the triarylamine nitrogen. This concurs

with the large and broad linewidth of the nitrogen splitting seen in the continuous wave EPR spectrum

of the radical.

Figure 6. The X-band (~9.7 GHz) pulsed ENDOR spectra of a frozen solution of NPy3 in [(n-C4H9)4N]PF6/CH3CN electrolyte at 20 K. a) Davies 1H-ENDOR spectrum and the corresponding numerical simulation, b) Mims 1H-ENDOR spectra recorded with different τ = 100, 178, 200, 250 and 500 ns. The 1H signals split from the proton Zeeman frequency centred at 1νH = 14.7 MHz where * indicates the peak due to the 31P in the electrolyte.

Fluorescence Spectroelectrochemistry and Chemical Oxidation

NPy3 has previously been reported to fluoresce light blue in solution upon excitation with a UV lamp

(λex = 365 nm).13 Emission spectra of the ligand in both solution and solid state were obtained upon

excitation into the π to π* transition of the triarylamine core at 380 nm (26315 cm -1) (ESI).13, 50, 51 The

in situ fluorescence spectroelectrochemical experiment was performed to investigate the direct effect

of the application of a positive potential to the system (ESI). The experiment was performed in the

spectroelectrochemical cell as used for the UV/Vis/NIR spectroelectrochemical experiments, where

the cell was placed at 90° to the excitation beam in the sample holder of the fluorimeter. NPy3 as a

solution in [(n-C4H9)4N]PF6/CH3CN electrolyte exhibited a broad emission peak at 460 nm (λex = 380

nm), which decreased in intensity as a positive potential of 1.5 V was applied and the triarylamine

was oxidised to its radical cation state. Acetonitrile was used as the solvent instead of

dichloromethane as it is more polar and therefore able to better stabilise the radical cation formed. The

trend observed in the spectroelectrochemical experiment, where fluorescence of NPy3 was quenched

upon formation of the triarylamine radical cation, corresponded well with the ex situ chemical

oxidation of the NPy3 ligand where the fluorescence was quenched upon oxidation (ESI).

The [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework retained the fluorescence of the ligand (ESI),

which was not surprising as diamagnetic d10 Zn2+quench the fluorescence of the ligand. An emission

peak at 570 nm was observed when the sample was irradiated at 400 nm with a relatively large

Stoke’s shift of 170 nm, which was higher than that observed for the ligand (~50 nm). And as found

for the NPy3 ligand, the fluorescence exhibited of [Zn(NPy3)(NO2)2·xMeOH·xDMF]n was quenched

upon chemical oxidation.

a) b)

Conclusions

Through the combination of solution state UV/Vis/NIR, EPR and optical fluorescence in situ

spectroelectrochemical techniques, we have unambiguously confirmed the formation of the radical

cation in the NPy3 ligand and the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework. The triarylamine

radical cation has been characterised in detail through a range of pulsed EPR hyperfine techniques.

The radical was found to be delocalised through the extended π system due to the near planar

conformation of NPy3. Continuous wave and pulsed EPR experiments revealed that the greatest spin

density was located on the central nitrogen of the triarylamine. The incorporation of the ligand into

the [Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework allowed for investigation of the ligand in the solid

state using in situ UV/Vis/NIR and EPR spectroelectrochemical experiments. The c.w. EPR spectrum

of the framework was spectrally similar to that of the NPy3 ligand as a frozen solution containing

many of the same characteristics. The demonstration that the combination of solution and solid state

in situ spectroelectrochemical experiments provides a valuable and comprehensive understanding of

the material as a function of the redox state is particularly important in the development of materials

for electronic swing adsorption, as cathode materials in batteries and in electrocatalysis.5

Footnote

[Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework Formula C66H48N11O6Zn2, M 1221.89,

orthorhombic, Pbcn space group (#60), a 12.3866(10), b 27.373(2), c 18.5603(16) Å, V 6293.0(9) Å3,

Dc 1.290 g cm-3, Z 4, crystal size 0.15 × 0.12 × 0.1 mm, colour yellow, habit block, temperature

150(2) Kelvin, λ(MoKα) 0.71073 Å, μ(MoKα) 0.821 mm-1, T(CRYSALISPRO, AGILENT

TECHNOLOGIES, VERSION 1.171.37.35)min,max 0.76, 0.86, 2θmax 52.58, hkl range -15 15, -34 34, -

23 23, N 161470, Nind 6504(Rmerge 0.1463), Nobs 4883(I > 2σ(I)), Nvar 397, residuals* R1(F) 0.0859,

wR2(F2) 0.2625, GoF(all) 1.081, Δρmin,max -1.696, 6.133 e- Å-3. CCDC 1473613-1473615

*R1 = Σ||Fo| - |Fc||/Σ|Fo| for Fo > 2σ(Fo); wR2 = (Σw(Fo2 - Fc

2)2/Σ(wFc2)2)1/2 all reflections

w=1/[σ2(Fo2)+(0.1476P)2+21.3790P] where P=(Fo

2+2Fc2)/3

Experimental procedures

Distilled and degassed acetonitrile or dichloromethane (dried over CaH2) were used for all

electrochemical experiments. DMF was dried over activated CaSO4 then distilled under reduced

pressure. Methanol was distilled over Mg/I2. Microanalyses were carried out at the Chemical Analysis

Facility – Elemental Analysis Service in the Department of Chemistry and Biomolecular Science at

Macquarie University, Australia. The tris(4-(pyridin-4-yl)phenyl)amine (NPy3) ligand was

synthesised according to literature procedures.13, 15

[Zn(NPy3)(NO2)2·xMeOH·xDMF]n . NPy3 (22.4 mg, 5.00 × 10-5 mol) and Zn(NO3)2·6H2O (42.0 mg,

1.41 × 10-4 mmol) were dissolved in a mixture of DMF (1.0 mL) and MeOH (1.0 mL) and heated at

80 °C for 48 hours. A white solid (zinc oxide) was observed to form adhered to the edges of the vial.

The solution was filtered whilst hot and allowed to cool slowly to room temperature upon which thin

yellow needles formed (30 mg, 49%). Elemental Analysis: Found C, 61.68; H, 5.37 and N, 14.36%;

Calculated for C132H96N23Zn4O12·7.6DMF: C, 61.87; H, 5.00; N, 14.62%.

Physical characterizations

General Details. Thermogravimetric analysis was performed under a flow of nitrogen (0.1 L min 1)

on a TA Instruments Hi-Res Thermogravimetric Analyser from 25-600 °C at 1 °C min 1. Powder

X-ray diffraction (PXRD) data were obtained on a PANalytical X’Pert PRO Diffractometer producing

Cuα (1.5406 Å) radiation, where the sample was lightly ground prior to analysis. FT-IR was

performed on samples over the range 4000-400 cm-1 on a Perkin-Elmar Spectrum Two ATR

spectrometer with a resolution of 4 cm-1

Crystallography. A yellow block like crystal was attached with Exxon Paratone N to a short length

of fibre supported on a thin piece of copper wire inserted in a copper mounting pin. The crystal was

quenched in a cold nitrogen gas stream from an Oxford Cryosystems Cryostream. A APEXII-FR591

diffractometer employing mirror monochromated MoKα radiation generated from a rotating anode

was used for the data collection. Cell constants were obtained from a least squares refinement against

9933 reflections located between 5.69 and 49.11º 2θ. Data were collected at 150(2) Kelvin with ω

scans to 52.58º 2θ. The data integration and reduction were undertaken with SAINT and XPREP,52

and subsequent computations were carried out with the WinGX graphical user interface. 53 The

structure was solved in the space group Pnma (#62) by direct methods with SIR97,54, 55 and extended

and refined with SHELXL-2014/7.56 An empirical absorption correction determined with SADABS57,

58 was applied to the data. The non-hydrogen atoms in the asymmetric unit were modelled with

anisotropic displacement parameters. A riding atom model with group displacement parameters was

used for the hydrogen atoms. A remaining large residual electron density near Zn2 may be due to

disorder of the Zn which exists as a low occupancy disorder component. This possibility has been

demonstrated in an alternate model named "ZnNPy3_Znpartial".

Solid State UV/Vis/NIR Spectroscopy. UV/Vis/NIR spectra were obtained on the samples at room

temperature using a CARY5000 Spectrophotometer equipped with a Harrick Praying Mantis

accessory over the wavenumber range 5000-40000 cm-1. BaSO4 was used to acquire the baseline

spectrum. Spectra are reported as the Kubelka-Munk transform, where F(R) = (1−R)2/2R (R is the

diffuse reflectance of the sample as compared to BaSO4).

Solid State Electrochemistry. Solid state electrochemical measurements were performed using a

Bioanalytical Systems Electrochemical Analyser. Argon was bubbled through solutions of 0.1 M

[(n-C4H9)4N]PF6 dissolved in distilled CH3CN. The cyclic voltammograms (CVs) were recorded using

a glassy carbon working electrode (1.5 mm diameter), a platinum wire auxiliary electrode and an Ag

wire quasi reference electrode. The framework sample was mounted on the glassy carbon working

electrode by dipping coating the surface of the glassy carbon working electrode into a paste made of

the ground powder sample in CH3CN. The CH3CN was allowed to evaporate in air to yield a thin film

of the framework on the electrode surface. Ferrocene was added as an internal standard upon

completion of each experiment. All potentials are quoted in V versus Fc+/Fc.

Solid State Spectroelectrochemistry (Vis/NIR). In the solid state, the diffuse reflectance spectra of

the electrogenerated species were collected in situ in a 0.1 M [(n-C4H9)4N]PF6/CH3CN electrolyte over

the range 5000-25000 cm-1 using a Harrick Omni Diff Probe attachment and a custom built solid state

spectroelectrochemical cell.10 The cell consisted of a Pt wire counter electrode and a Ag wire quasi-

reference electrode. The solid sample was immobilised onto a 0.1 mm thick Indium-Tin-Oxide (ITO)

coated quartz slide (which acted as the working electrode) using a thin strip of Teflon tape or the

addition of LiClO4-intercalated PVC (to a diameter of 1 cm). The applied potential (from –2.0 to +2.0

V) was controlled using an eDAQ potentiostat. Continuous scans of the sample were taken and the

potential increased gradually until a change in the spectrum was observed.

Solution State Spectroelectrochemistry (EPR). The procedure and cell set up used were those as

previously described.59 A three-electrode assembly based on simple narrow wires (A–M Systems) as

electrodes where Teflon coated platinum (0.20 and 0.13 mm coated and uncoated diameters,

respectively) and silver wires (0.18 and 0.13 mm coated and uncoated diameters, respectively) were

used for the working and quasi-reference electrodes respectively, and a naked platinum wire

(0.125 mm) as the counter electrode. The bottom 1 cm of the Teflon coated wires were stripped (using

an Eraser International Ltd., RT2S fine wire stripper). The working electrode was positioned lowest

such that the redox product of interest was generated at the bottom of the tube and was well separated

from the counter electrode. The naked platinum wire counter electrode ensures a greater surface area

than the working electrode, while the Teflon coating on the working and reference electrodes prevents

short-circuiting. The electrodes were soldered to a narrow three-core microphone wire. The cell used

was made by flame sealing the tip of a glass pipette. The potential was controlled with a portable

microAutolab II potentiostat and the EPR spectra obtained using an EMX Micro X-band EPR

spectrometer with 1.0 T electromagnet. The operating microwave frequency at 20 K (9.794538 GHz)

was different to the frequency used at room temperature (9.875069 GHz) due to the size of the cavity.

Solid State Spectroelectrochemistry (EPR). The same cell set up as described for the solution state

spectroelectrochemistry was used for the solid state experiments. The sample of interest was wrapped

in a small piece of platinum mesh (~ 5 mm × 3 mm) lengthwise and twisted to ensure the sample

remained immobilised. The exposed end of the working electrode was carefully wrapped in a spiral

shape around the platinum mesh. A small piece of platinum mesh (~ 5 mm × 4 mm) was rolled up

lengthwise and attached in a similar fashion to the counter electrode to ensure that the surface area of

the counter electrode was larger than that of the working electrode.

Pulsed EPR Experiments

The X-band pulse EPR measurements were performed on a Bruker ElexSys E580 spectrometer at 20

K. A standard dielectric ring Bruker EPR cavity (ER4118X-MD5) and (ER4118X-MD4) were used,

which were equipped with an Oxford CF 935 helium flow cryostat.

The 2D ESEEM spectra, so-called hyperfine sublevel correlation spectra (HYSCORE) Hoe86,60 were

recorded employing the sequence /2--/2-t1--t2-/2--echo with microwave pulses of length /2 = 16

ns and /2=32 ns, =136 ns, starting times t1,2=200 ns, and time increments Δt1,2=16 ns. The intensity

of the inverted echo following the fourth pulse is measured with t2 and t1 varied and constant .

Unwanted features from the experimental echo envelopes were removed by using a four-step phase

cycle Gem90.61 In both dimensions 256 data points were collected. The relaxation decay was

subtracted by baseline corrections (fitting by polynomials of 3-6 degree) in both time domains,

subsequently applying apodization (Hamming window) and zero-filling to 1024 data points in both

dimensions. After 2D fast Fourier transformation absolute value spectra were calculated. Analysis of

the cross-ridges in (ν1)2 vs. (ν2)2 coordinates allowed for the simultaneous determination of the

isotropic and anisotropic components of the hyperfine matrix.47, 48

Davies ENDOR experiments were performed using the pulse sequence π-RF-π/2-τ-π-τ-echo. RF pulse

of 47 μs was generated by the Bruker “DICE” system and amplified by a 60 dB gain ENI A-500 RF

amplifier. Mims ENDOR experiments were performed using the pulse sequence π/2-π/2-RF-π/2-τ-

echo.

Solution State Spectroelectrochemistry (Fluorescence). The Optically Semi-Transparent Thin-

Layer Electrosynthetic (OSTLE) cell used for the solution state UV/Vis/NIR spectroelectrochemical

experiment, path length 0.65 mm, was adapted for use in a fluorimeter by placing the OSTLE cell

perpendicular to the excitation and emission windows. Solutions for the spectroelectrochemial

experiment contained 0.1 M [(n-C4H9)4N]PF6/CH3CN supporting electrolyte and ca. 1 μM of the

compound. Appropriate potentials were applied by using an eDAQ e-corder 410 potentiostat and the

current was carefully monitored throughout the electrolysis. The electrogenerated species were

obtained in situ, and their emission spectra were recorded at a scan rate of 100 nm min 1 at regular

intervals throughout the electrolysis.

Supporting Information Available: Synthetic details of the ligand, crystallographic table,

electrochemistry, thermal gravimetric analysis, UV/Vis/NIR spectra, pulsed EPR experiment analysis

details, IR, PXRD and UV/Vis and fluorescence spectroelectrochemistry for the NPy3 ligand and

[Zn(NPy3)(NO2)2·xMeOH·xDMF]n framework. This material is available free of charge via the

Internet at http://pubs.acs.org.

Acknowledgements

We thank Dr Alistair Fielding and the EPSRC UK National EPR Research Facility and Service at the

University of Manchester for support with EPR measurements as well as Dr. Peter Turner at the

University of Sydney for helpful advice regarding the crystallographic structure determination of the

[Zn2(NPy3)2(NO2)2·xMeOH]n framework. We gratefully acknowledge support from the Australian

Research Council.

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For Tables of Contents Only

Synopsis

The one electron oxidation of the tris(4-(pyridine-4-yl)phenyl)amine (NPy3) ligand to its radical cation has been extensively characterised via UV-Vis, EPR and fluorescence spectroelectrochemical experiments on the ligand, and a zinc-based metal-organic framework. Pulsed EPR measurements reveal that the radical generated is highly delocalised throughout the ligand backbone.