Fluorination vs. Chlorination: Effect on the Sensor Response of Tetrasubstituted Zinc Phthalocyanine Films to Ammonia

Abstract

In this work, the effect of fluorine and chlorine substituents in tetrasubstituted zinc phthalocyanines, introduced into the non-peripheral (ZnPcR₄-np, R = F, Cl) and peripheral (ZnPcR₄-p, R = F, Cl) positions of macrocycle, on their structure and chemiresistive sensor response to low concentration of ammonia is studied. The structure and morphology of the zinc phthalocyanines films (ZnPcR₄) were investigated by X-ray diffraction and atomic force microscopy methods. To understand different effects of chlorine and fluorine substituents, the strength and nature of the bonding of ammonia and ZnPcHal₄ molecules were studied by quantum chemical simulation. It was shown on the basis of comparative analysis that the sensor response to ammonia was found to increase in the order ZnPcCl₄-np < ZnPcF₄-np < ZnPcF₄-p < ZnPcCl₄-p, which is in good agreement with the values of bonding energy between hydrogen atoms of NH₃ and halogen substituents in the phthalocyanine rings. ZnPcCl₄-p films demonstrate the maximal sensor response to ammonia with the calculated detection limit of 0.01 ppm; however, they are more sensitive to humidity than ZnPcF₄-p films. It was shown that both ZnPcF₄-p and ZnPcCl₄-p and can be used for the selective detection of NH₃ in the presence of carbon dioxide, dichloromethane, acetone, toluene, and ethanol.

1. Introduction

Ammonia is a highly irritating gas with a pungent smell. The problem of detecting ammonia is important in various sectors of agriculture and industry related to the production and use of fertilizers, as well as refrigeration systems [1]. In the last ten years, the diagnosis of various diseases by determining the concentration of biomarker gases in the exhaled air has attracted the attention of researchers and medical doctors [2,3,4]. It is known that ammonia is one of the biomarkers and its concentration > 1 ppm indicates renal failure in nephritis, atherosclerosis of the renal arteries, toxic affections of kidneys and other diseases [5]. There are a variety of sensing techniques existing for NH₃ detection, among them optical [6,7,8], electrochemical [9], surface acoustic [10,11], field effect transistor [12,13,14], and chemiresistive [15] sensors. Among them, chemiresistive gas sensors have attracted considerable attention in the field of sensor technology due to their advantages such as simplicity, low cost, and miniaturization capability [4,16]. Metal oxides, carbon nanomaterials, conductive polymer, and their hybrid materials are used as active layers of chemiresistive sensors for ammonia detection [17]. Metal phthalocyanines (MPc) also attract noticeable interest for sensing applications [18,19,20] due to their high sensitivity to different analytes, exceptional stability, versatile chemical system, and excellent processability resulting in the manufacture of thin films. Their properties can be tuned by introducing desirable substituents into their aromatic rings. It was shown in previous works that introduction of electron withdrawing substituents (e.g., fluorine) led to an increase of MPc sensor response to reducing gases like ammonia and hydrogen [21,22]. Films of hexadecafluorosubstituted (MPcF₁₆) and tetrafluorosubstituted (MPcF₄-p) metal phthalocyanines with F-substituents in peripheral positions, deposited by vacuum evaporation, were studied as active layers of chemiresistive sensors for ammonia detection [22,23]. It was shown in our previous work [23] that the sensitivity of phthalocyanine films to ammonia increased in the order MPc < MPcF₁₆ < MPcF₄-p (M = Cu, Zn, Co). MPcF₄-p films were shown to be used as active layers for sensors to ammonia with the detection limit reaching 0.1 ppm. Apart from this, they allow ammonia to be detected in the presence of carbon dioxide, some volatile organic compounds and at high humidity. These properties make fluorosubstituted metal phthalocyanines very promising for the detection of low concentration of ammonia in the gas mixtures.

In the case of tetrafluorosubstituted phthalocyanines, the F-substituents can be introduced not only to peripheral positions (Figure 1), but also to non-peripheral ones. The literature analysis has shown that studies of MPcF₄-np thin films were only sporadic [24,25], and their sensor properties have not yet been studied. Chlorination is also a common method used to modify the behavior of the π-conjugated molecular semiconductors [26]. Although Cl (electronegativity is 3.16) is less electronegative than F (electronegativity is 3.98), its empty 3d orbit can help to accept π electrons in the system, resulting in the stronger ability of chlorination to decrease than fluorination to downshift the molecular energy levels in comparison with fluorination. Chlorosubstituted metal phthalocyanines ones are also widely investigated. Similarly to MPcFx, films of chlorosubstituted MPcCl_x can also be deposited by thermal evaporation [27,28,29,30,31]. Parameters of crystal cell of CuPcCl_x (x = 4, 8, 16) have been determined by Fryer and co-authors [32] using the method of electron diffraction, while later the single crystal structure of CuPcF₄-np have been refined [33]. CuPcCl_x was shown to demonstrate semiconductor properties with the lower HOMO and LUMO energy levels compared to unsubstituted CuPc [34,35]. The charge carrier mobility of CuPcCl₁₆ films was shown to be about 10⁻² cm² V⁻¹ s⁻¹ [36]. Achar et al. [37] found that the conductivity of CuPcCl₄ pellets was four orders of magnitude higher than that in the case of unsubstituted CuPc. At the same time, the chemiresistive properties of chlorosubstituted metal phthalocyanines have not yet been investigated.

These latest results raise the question of how to choose between fluorinated and chlorinated phthalocyanines when designing materials for chemiresistive sensors with good sensor characteristics.

In this work, the effect of the position of fluorine and chlorine substituents in tetrasubstituted zinc phthalocyanines ZnPcR₄-np and ZnPcF₄-p (R = F, Cl) (Figure 1) on their structure and chemiresistive sensor response to low concentration of ammonia is studied. The structure and morphology of the films of tetrafluoro- and tetrachlorosubstituted zinc phthalocyanines, deposited by thermal evaporation vacuum, were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) methods. On the basis of comparative analysis, the films with the best sensor characteristics (sensitivity, selectivity, detection limits, response and recovery times) were chosen.

2. Materials and Methods

Zinc phthalocyanine derivatives were synthesized according to the techniques described elsewhere [22] from zinc acetate and corresponding phthalonitrile derivatives. 4-fluorophthalonitrile (Aldrich), 3-fluorophthalonitrile (FluoroChem), 4-chlorophthalonitrile, and 3-chlorophthalonitrile (Abcr) were used for the synthesis of ZnPcF₄-p, ZnPcF₄-np, ZnPcCl₄-p, and ZnPcCl₄-np, respectively. 4-chlorophthalonitrile was synthesized from 4-aminophthalonitrile (Acros Organics, Geel, Belgium) according to the procedure described by Antunes and Nyokong [38]. Both ZnPcF₄ and ZnPcCl₄ derivatives contained a statistical mixture of regioisomers due to the various possible positions of fluorine and chlorine substituents, which were not separated because of close parameters of sublimation. The synthesized phthalocyanines were purified by double sublimation in vacuum (10⁻⁵ Torr) at 450–460 °C.

Thin films were deposited by an organic molecular beam deposition technique on glass substrates or glass slides with predeposited Pt interdigitated electrodes. The substrate temperature was about 60 °C. The nominal thickness of the films was about 100 nm.

Crystal structures were determined at 150 K using a Bruker D8 Venture single-crystal diffractometer (Incoatec IμS 3.0 MoKα microfocus tube, PHOTON III C14 charge-integrating pixel array detector, fixed-chi 3-circle goniometer, Cryostream 800 Plus open-flow nitrogen cooler). APEX3 V2018.7-2 software package (SAINT 8.38A, SADABS-2016/2) [39] was used for the raw data collection, data reduction, absorption correction and global unit cell refinement. Obtained hkl datasets were processed in Olex2 v.1.2.10 [40] using SHELXT 2018/2 [41] and SHELXL 2018/3 [42] for the structure solution and refinement, respectively. Thin film XRD patterns were recorded on a Shimadzu XRD-7000 powder diffractometer (Bragg-Brentano scheme with vertical θ-θ goniometer, CuKα sealed tube with Ni foil β-filter, OneSight silicon strip detector).

Atomic force microscopy (AFM) in semi-contact mode with a Ntegra Prima II (NT-MDT, Russia) microscope was used for the characterization of films morphology. The HA_NC (A) tip had the following parameters: length—123 μm, width—34 μm, thickness—3 μm, force constant—17 N/m, resonance frequency—230 kHz. The roughness parameters were calculated using the Nova SPM software according to the standards ISO 4287-1, ISO 4287 and ASME B46.

For testing the chemiresistive sensor response the films were deposited onto the platinum interdigitated electrodes (Dropsens, Spain). The electrodes had the following dimensions: gap between digits—10 μm; number of digits—125 × 2 with a digit length of 6760 μm; cell constant—0.0118 cm⁻¹. The interdigitated electrodes with a deposited phthalocyanine film were placed in a homemade gas flow cell and kept for 15 min under the air flow until their resistance reached a steady value. The scheme of the system for investigation of the chemiresistive sensor response with a photo of the gas flow cell is shown in Figure 2. The required gas flow was regulated using mass flow regulators. The electrical resistance of ZnPc films, which changed when interacting with the analyte gas, was measured using a Keithley 236 electrometer by applying a constant DC voltage (10 V).

Then, the analyte gas of the required concentration and the air used for purging were alternately introduced into the cell. The sensor response of the films was determined after alternation of exposure and recovery periods in a dynamic regime in most experiments. The injection of NH₃ was carried out at the constant air flow rate of 300 mL/min and the exposure time was fixed at 30 s. The determination of the response and recovery times of the sensing layer was performed in a static regime. In this case, the air stream flowed through the test chamber until the resistance reached a state value, and then the valves of the chamber were closed and diluted NH₃ gas was injected into the cell. The resistance was recorded during a period of time necessary to obtain saturation value and calculate the response time of the sensor. Afterwards, the chamber was opened and was purged with air flow. For humidity measurements, the wet carrier gas was prepared by bubbling the carrier gas through distilled water. The relative humidity (RH) inside the cell was controlled with a commercially available humidity meter (MPE-202.013).

Theoretical Calculations

The nature of bonding between zinc phthalocyanines and ammonia was studied by performing a topological analysis of the electron density distribution function in these molecules in the framework of the Quantum Theory “Atoms in Molecules” (QTAIM) [43,44]. These functions were obtained as a result of quantum-chemical calculations carried out in the ORCA software package [45,46], using the DFT B3LYP/def2-SVP method [47,48,49,50,51], semiempirical dispersion potential Grimme [52,53], RI approximation [54,55,56,57,58,59], and auxiliary basis set Def2/J [60]. In the search of the equilibrium geometric structure of the molecules, no symmetry restrictions were used (the point group of symmetry C₁). The spin multiplet was equal to one, which corresponded to energetically more favorable electronic states.

The absence of negative frequencies in the vibrational spectra of the molecules in addition to their minimum total energies was the criterion for achieving an equilibrium geometric structure. The binding energy (E_b) of ZnPcHal₄ with the NH₃ molecule was calculated from the difference in the total energies of the corresponding structure and its components:

$$E_b=E_{{\mathrm{NH}}_3}+E_{{\mathrm{ZnPcHal}}_4}-E_{{\mathrm{ZnPcHal}}_4···{\mathrm{NH}}_3}-\Delta E_{\mathrm{BSSE}},$$

(1)

where ΔE_BSSE is the correction to the binding energy, taking into account the basis set superposition error (BSSE), which was estimated as follows:

$$\Delta E_{\mathrm{BSSE}}=\left(E_{{\mathrm{ZnPcHal}}_4}^{{\mathrm{ZnPcHal}}_4···{\mathrm{NH}}_3}+E_{{\mathrm{NH}}_3}^{{\mathrm{ZnPcHal}}_4···{\mathrm{NH}}_3}\right)-\left(E_{{\mathrm{ZnPcHal}}_4*}^{{\mathrm{ZnPcHal}}_4···{\mathrm{NH}}_3}+E_{{\mathrm{NH}}_3*}^{{\mathrm{ZnPcHal}}_4···{\mathrm{NH}}_3}\right).$$

(2)

The upper index in Equation (2) indicates that the geometries of ZnPcHal₄ and NH₃ were taken from the optimized ZnPcHal₄···NH₃ structure, while the geometric structures of ZnPcHal and NH₃ were not optimized, but only their electronic structures were calculated. The asterisks in the lower indices mean which fragment of the ZnPcHal₄···NH₃ aggregate is considered, whereas the atoms of the other fragment are considered as the points described by the corresponding atomic basis sets.

Additionally, the binding energies were calculated for comparison by another more accurate method, where the structure geometries and zero-point energies were taken from the DFT results, while electronic energies were estimated in the framework of the DLPNO-CCSD(T)/def2-TZVPP approach [50,60,61,62,63] without taking into account the BSSE correction. This approach represents the domain-based local pair natural orbital (DLPNO) approximation implemented to speed up the coupled-cluster method with single and double excitations and perturbative triple excitations (CCSD(T)), employs localized occupied orbitals, and allows us to get the correlation energy as the sum of the correlation energies of the electron pairs [62,63,64].

In the QTAIM approach, an indicator of the binding interaction of two atoms is the presence of the bond critical point (3, −1) (BCP) between them. Herein ω = 3 is a Hessian matrix rank at this point, and σ = −1 is a sum of signs of Hessian matrix eigenvalues λ_i (λ₁ < 0, λ₂ < 0 и λ₃ > 0). The set of values of these parameters, including the Laplacian of the electron density ∇²ρ(r) = λ₁ + λ₂ + λ₃, can be used to estimate the nature of the atoms’ interaction under consideration. In particular, if |λ₁|/λ₃ > 1 (∇²ρ(r) < 1), the so-called shared interaction occurs. The electron density is accumulated and distributed in the interatomic space, which is typical for covalent bonds. In the case where |λ₁|/λ₃ < 1 (∇²ρ(r) > 1), the closed-shell interaction is observed, which is characteristic of ionic, strongly polar covalent, hydrogen and van der Waals bonds [43,44,65,66]. In this case, there is an outflow of electron density from the critical point to the atomic nuclei. There is also an intermediate interaction, in which the value of ∇²ρ(r) is also positive, but the chemical bond is considered covalent if the value of ρ(r) is large. In this case, one resorts to the analysis of additional quantities, such as, for example, the electron energy density h(r). A negative value of this value at the corresponding critical point of the bond is considered a sufficient condition for the bond to be considered covalent [44].

3. Results and Discussion

3.1. Crystal Structure of ZnPcCl₄-p and ZnPcCl₄-np

Crystal structure of ZnPcF₄-p has been investigated by our group earlier [67]. ZnPcF₄-p crystallizes in P-1 space group with Z = 1 and is isomorphous to α-polymorphs of unsubstituted phthalocyanines, such as α-CuPc [68] and α-CoPc [69]. ZnPcF₄-p molecules are arranged in stacks (Figure 3a) with a distance of 3.331 Å between the molecules inside the stack (the distance between the planes drawn through all non-hydrogen atoms) and a stacking angle of 25.31° (the angle between the stacking direction and the normal to the plane). The structure of ZnPcF₄-np crystals grown by vacuum sublimation in this work was identical to that described by Hui Jiang et al. [70]. ZnPcF₄-np crystallizes in the P2₁/n space group with Z = 2 and is isomorphous to the β-polymorph of ZnPc [71]. ZnPcF₄-np molecules are arranged in a herringbone pattern (Figure 3b) with a distance of 3.258 Å between the molecules within the stack, a stacking angle of 47.29° and an angle between molecules in adjacent stacks of 85.42°. ZnPcCl₄-p crystallizes in the I4₁/a space group with Z = 8 (Z’ = 0.5). ZnPcCl₄-p molecules are also packed in stacks with a distance of 3.392 Å between the molecules within the stack and a stacking angle 20.81°, while the angle between the molecules in adjacent stacks is 29.12° and the adjacent stacks are rotated 90° relative to each other (Figure 3c).

ZnPcCl₄-np crystallizes in the P2₁/n space group. A distance between the molecules in the stack is 3.309 Å, a stacking angle is 46.51° and an angle between molecules in adjacent stacks is 86.97° (Figure 3d). At first glance, ZnPcCl₄-np is isomorphic to both ZnPcF₄-np and β-ZnPc, but the substituents of the chlorine atoms are too bulky compared to fluorine and hydrogen, which leads to very close contacts between the molecules in adjacent stacks. As a result, one ZnPcCl₄-np molecule is rotated relative to the other in the plane around the central Zn atom by about 13° (Figure 3e). This leads to the fact that all the atoms, except for Zn and Cl, are disordered in two positions with a ratio of 0.56:0.44. Since each Cl atom can already occupy two equivalent positions in the molecule, they are disordered over four positions each, with the ratios of 0.19:0.45:0.29:0.07 and 0.13:0.36:0.44:0.07 ratios. F and Cl atoms are also disordered over two positions each in other tetrahalogenated ZnPc derivatives with the following ratios: 0.56:0.44 and 0.52:0.48 for ZnPcF₄-p, 0.61:0.39 and 0.54:0.46 for ZnPcF₄-np, 0.52:0.48 and 0.54:0.46 for ZnPcCl₄-p.

It should also be noted that the molecules of all four investigated ZnPcHal₄ derivatives remain relatively flat; the maximum deviation of any non-hydrogen atom from the root-mean-square plane is 0.168 Å for ZnPcF₄-p, 0.103 Å for ZnPcF₄-np, 0.232 Å for ZnPcCl₄-p and 0.216 Å for ZnPcCl₄-np (cf. 0.116 Å for ZnPc). The unit cell parameters and refinement statistics for ZnPcF₄-np, ZnPcCl₄-p and ZnPcCl₄-np are given in

Table 1. Unit cell parameters and refinement details.

Compound	ZnPcF4-np	ZnPcCl4-p	ZnPcCl4-np
Empirical formula	C32H12F4N8Zn	C32H12Cl4N8Zn	C32H12Cl4N8Zn
Formula weight	649.87	715.67	715.67
Temperature/K	150.0	150.0	150.0
Crystal system	monoclinic	tetragonal	monoclinic
Space group	P21/n	I41/a	P21/n
a/Å	14.8240(17)	38.3949(13)	14.4126(15)
b/Å	4.8033(7)	38.3949(13)	4.8075(5)
c/Å	17.866(2)	3.6284(2)	20.331(2)
α/°	90	90	90
β/°	108.039(7)	90	110.544(3)
γ/°	90	90	90
Volume/Å3	1209.6(3)	5348.9(5)	1319.1(2)
Z	2	8	2
ρcalcg/cm3	1.784	1.777	1.792
F(000)	652	2864	716
Independent reflections	2293 [Rint = 0.0818, Rsigma = 0.0751]	4075 [Rint = 0.0575, Rsigma = 0.0371]	1895 [Rint = 0.0684, Rsigma = 0.0519]
Data/restraints/parameters	2293/0/226	4075/0/226	1895/310/400
Goodness-of-fit on F2	1.014	1.120	1.042
Final R indexes [I >= 2σ (I)]	R1 = 0.0479, wR2 = 0.0800	R1 = 0.0460, wR2 = 0.0871	R1 = 0.0581, wR2 = 0.1547
Final R indexes [all data]	R1 = 0.0833, wR2 = 0.0910	R1 = 0.0553, wR2 = 0.0900	R1 = 0.1015, wR2 = 0.1806
CCDC deposition №	2077055	2077056	2077057

.

3.2. Structure and Morphology of Thin Films

Thin film XRD patterns of ZnPcHal₄ in the range of 4–30° 2θ are shown in Figure 4 in comparison with the powder patterns calculated using the above-mentioned data on the single-crystal structure and unit cell parameters.

XRD patterns of all ZnPcHal₄ films have single diffraction peaks, which indicate a possible strong preferred orientation of the film. The diffraction peak on the XRD pattern of ZnPcF₄-p film has a noticeable asymmetry on the right side, and the 2θ position of its maximum at 6.71° lies between the peak (001) at 6.59° and the peak (010) at 7.12°, which means that the preferred orientation in the film is not ideal, and although most crystallites are oriented by the plane (001) parallel to the substrate surface, some are oriented by the plane (010) parallel to the surface substrates. The 2θ position of the diffraction peak on the XRD pattern of ZnPcF₄-np film coincides well with the peak (−101) on the calculated powder pattern. The ZnPcF₄-np film has a stronger preferred orientation and better crystallinity compared to the ZnPcF₄-p film, since the diffraction peak on its X-ray image has a significantly lower FWHM (0.15°) compared to that of ZnPcF₄-p (0.5°).

The XRD pattern of ZnPcCl₄-p film has a strong diffraction peak at 6.39° 2θ with FWHM of 0.37°, which corresponds to (220) peak at 6.47° 2θ on the calculated powder pattern. A small peak at 19.62° 2θ corresponds to the peak (660), which belongs to the same group of crystallographic planes as the peak (220). The XRD pattern of ZnPcCl₄-np film has a single weak broad peak at 6.38° 2θ with FWHM of 0.83°, which corresponds to the peak (−101) at 6.55° 2θ on the calculated powder pattern. Its low intensity and high FWHM value suggest that the ZnPcCl₄-np film has very poor crystallinity compared to the films of other investigated phthalocyanines. The angles between the molecules and the substrate surface in ZnPcHal₄ films, calculated on the basis of XRD data, are shown in Figure 5.

The morphology of the films was studied by AFM (Figure 6). The morphology of ZnPcF₄-np and ZnPcF₄-p films is very similar. The surface of both films consists of slightly elongated crystallites with the size reaching 0.07 μm. The RMS values of ZnPcF₄-np and ZnPcF₄-np film are 9.54 and 4.44 nm, respectively. The film of ZnPcCl₄-p consists of the clearly distinguishable smaller crystallites and has RMS of 5.58 nm. The morphology of the ZnPcCl₄-p film differs significantly from that of the films of ZnPcCl₄-p; the film is formed by vague elongated agglomerates, the size of which reaches 0.4 μm. Its RMS value is 8.96 nm.

3.3. Sensor Response of ZnPcHal₄ Films to Ammonia

The effect of the type of halogen substituents and their positions in the phthalocyanine ring on the sensor response toward gaseous ammonia (0.1–50 ppm) was studied in order to choose the material with the best sensor performance. The typical response S of the sensor, defined as S = (R − R_o)/R_o, where R is the resistance of a phthalocyanine film at a certain concentration of NH₃ and R_o is the initial resistance of the film in fresh air, is shown in Figure 7a as an example.

All sensing layers showed a noticeable increase in the resistance when exposed to ammonia. When the ammonia supply was stopped, the resistance was restored to its initial value, which indicated fast response and reversibility of the investigated sensors toward ammonia at room temperature. This behavior is typical for p-type organic semiconductors, and has also been observed for other unsubstituted and tetrafluorinated metal phthalocyanines [23]. Based on the fact that the studied phthalocyanines are p-type semiconductors, it was expected that their exposure to NH₃ would cause an increase in the resistance due to depletion of the positively charged holes by electrons donated by NH₃ molecules [72,73].

The dependence of the sensor response of investigated sensing layers on ammonia concentration is presented in Figure 7b. The sensor response of the investigated layers was found to increase in the order ZnPcCl₄-np < ZnPcF₄-np < ZnPcF₄-p < ZnPcCl₄-p. For example, the sensor response of ZnPcCl₄-p film to 1 ppm NH₃ is 2 times higher than that of the ZnPcF₄-np film and 17 times higher than that of the ZnPcCl₄-np film.

The linear dependence of the sensor response on the ammonia concentration is observed in the range from 0.1 to 1 ppm, and at higher concentrations the curve deviates from the linear one. The detection limits (LOD) of the sensing layers were estimated as 3s/m, where m is the slope of the calibration plot in the linear region, while s is the standard deviation of the sensor response to 1 ppm of NH₃ (Figure 7b inset).

LODs were calculated to be 0.01 ppm for the films of ZnPcCl₄-p and ZnPcF₄-p, while its values were 0.08 and 0.1 ppm in the case of ZnPcF₄-np and ZnPcCl₄-np films, respectively. The response and recovery time of phthalocyanine layers toward ammonia (1 ppm) was also investigated as presented in Figure 8 and summarized in

Table 2. Calculated detection limits and response/recovery times (at 1ppm NH₃) of ZnPcF₄-np, ZnPcF₄-p, ZnPcCl₄-np, and ZnPcCl₄-p sensing layers.

Sensing Layer	Calculated LOD, ppm	Response Time, s	Recovery Time, s
ZnPcF4-np	0.08	35	155
ZnPcF4-p	0.01	45	210
ZnPcCl4-np	0.1	45	280
ZnPcCl4-p	0.01	45	260

.

3.4. Theoretical Study of the Nature of Bonding between Ammonia and ZnPcHal₄ Molecules

Although observations of changes in the measured current of MPc sensing layers exposed to NH₃ and other analytes are explained by electron transfer, the interaction sites in the phthalocyanine macrocycle or metal center are still a matter of discussion. According to the data of most works, the most energetically favorable cite of NH₃ molecule binding is the central metal of MPc molecules [23,74,75]. In the case of the non-planar lead phthalocyanines, two bond critical points characterized the considered interaction between NH₃ and PbPcF_x were observed: between the Pb and nitrogen atoms as well as between one of the hydrogen atoms of ammonia and the N_β atom of the PbPcF_x macrocycle [76]. These conclusions were based only on quantum chemical calculations. At the same time, the experimental investigations of the interaction sites are sporadic [77]. Chia et al. [77] used in situ X-ray absorption spectroscopy (XAS) for the investigation of interaction of CuPc with NH₃ molecule. The first derivative of XANES suggested low or lack of axial position coordination on the Cu metal center. On the other hand, the interaction on the macrocycle was supported by the EXAFS. From the EXAFS of CuPc with NH₃, the interaction was suggested to be at the benzene ring or bridging nitrogen atom. The experimental investigations of the interaction sites in the case of halogen-substituted MPcs have not yet been carried out.

In this work, the strength and nature of the bonding of ammonia and ZnPcHal₄ molecules were studied using quantum chemical simulation to understand the different effects of chlorine and fluorine substituents. It is known than there are two possible positions for the binding of NH₃ molecule in ZnPcHal₄, namely central metal and substituents. For this reason, we considered the interaction between NH₃ and ZnPcHal₄ both via the central metal and through peripheral and non-peripheral halogen atoms.

The binding energy between the central metal (Zn) and nitrogen of ammonia in ZnPcHal₄····NH₃ aggregates seemed to be very similar and only slightly increased in the order ZnPcF₄-p (−18.4 kcal/mol) < ZnPcF₄-np (−18.4 kcal/mol) < ZnPcCl₄-np (−18.6 kcal/mol) < ZnPcCl₄-p (−18.6 kcal/mol). The energy values were close, but a different trend was obtained with the use of the more accurate DLPNO-CCSD(T)/def2-TZVPP method: ZnPcF₄-np (−18.5 kcal/mol) < ZnPcCl₄-np (−18.8 kcal/mol) < ZnPcF₄-p (−21.2 kcal/mol) < ZnPcCl₄-p (−21.3 kcal/mol). The binding energy was approximately 3 kcal/mol higher in the case of phthalocyanines bearing halogen substituents in peripheral positions.

It is important to note that the interaction of an isolated phthalocyanine molecule with NH₃ is considered in quantum-chemical calculations. In fact, in polycrystalline films, the molecules are packed in stacks of parallel molecules with a distance between the aromatic rings of about 3.3–3.4 Å (Figure 3), which is not enough for the penetration of NH₃ molecules between them to the central metal. Therefore, such interaction seems unlikely or possible only with the terminal molecules in the stacks. This, apparently, explains the results obtained in the work of Chia et al. [77] cited above. In addition, the estimated energies (20–24 kcal/mol) look high enough to be responsible for the sensor response to ammonia, which is easily reversible at room temperature.

Thus, it was also reasonable to consider the interactions of ammonia with ZnPcHal₄ molecules via peripheral and non-peripheral halogen atoms. For this purpose, the topological analysis of the electron density distribution function was carried out. As a result, the corresponding bond critical points were established (Figure 9), and the values of ρ(r) and ∇²ρ(r) at these points were estimated. It was shown that the ZnPcCl₄-np···NH₃ aggregate is characterized by the presence of three bond critical points: two between the hydrogen atoms of the ammonia molecule and the chlorine atom of ZnPcCl₄-np (BCP1 and BCP2, Figure 9) and one between the nitrogen atom of NH₃ and the non-peripheral hydrogen atom of ZnPcCl₄-np (BCP3). Similarly, three BCPs characterizing the interaction of NH₃ molecule with phthalocyanines are observed for ZnPcCl₄-p···NH₃, but, in contrast to the previous case, BCP2 is located between the nitrogen atom of NH₃ and the non-peripheral hydrogen atom of the benzene ring, which contains the chlorine atom involved in the formation of BCP1.

Phthalocyanines with fluorine substituents are characterized by two bond critical points (BCP1 and BCP2). BCP1 is observed between one of the hydrogen atoms of ammonia and the F atom, and BCP2 is between the nitrogen atom of NH₃ and the non-peripheral hydrogen atom. The difference between ZnPcF₄-np···NH₃ and ZnPcF₄-p···NH₃ is that in the case of BCP2, the hydrogen atoms are located on different benzene rings. The parameters of almost all designated BCPs indicate the formation of hydrogen bonds between these pairs of atoms. This follows from the fact that the ρ(r) и ∇²ρ(r) values in almost all cases lie in the ranges of 0.013–0.236 e/ų and 0.578–3.350 e/Å⁵, respectively, which are characteristic of a hydrogen bond (

Table 3. Topological parameters of the ρ(r) function in the bond critical points between atoms of ammonia and ZnPcHal₄ molecules.

Structure	BCP	ρ(r), e/Å3	∇2ρ(r), e/Å5	|λ1|/λ3	he(r), a.u.	Eb, kcal/mol
ZnPcCl4-np···NH3	1	0.045	0.674	0.137	1.21·10−3	−0.9 (−0.1 *)
	2	0.045	0.672	0.138	1.18·10−3
	3	0.069	0.628	0.180	3.50·10−4
ZnPcCl4-p···NH3	1	0.059	0.579	0.199	2.16·10−4	−3.5 (−3.7 *)
	2	0.113	0.972	0.238	−6.60·10−5
	3	0.006	0.075	0.140	2.74·10−4
ZnPcF4-np···NH3	1	0.077	1.003	0.194	2.38·10−4	−1.5 (−1.0 *)
	2	0.075	0.665	0.187	2.34·10−4
ZnPcF4-p···NH3	1	0.065	0.845	0.196	3.99·10−4	−3.2 (−3.1 *)
	2	0.110	0.967	0.235	−4.00⋅10−6

* E_b values calculated using the DLPNO-CCSD(T)/def2-TZVPP method.

) [44,78]. Only for BCP3 in ZnPcCl₄-p···NH₃, the values of the electron density and its Laplacian are significantly lower than the lower limit of these ranges, thus the interaction of the corresponding pair of atoms is rather of the Van der Waals type [79].

As a rule, hydrogen bonds are characterized by the closed-shell interaction between two atoms [44,78,79]. This interaction is characterized by small values of ρ(r) and ∇²ρ(r) at the corresponding critical point (3, −1), while the values of ∇²ρ(r) are positive [43,44,65,66]. Besides, the ratio of eigenvalues (|λ₁|/λ₃) of the electron density Hessian matrix at this point in the case of closed-shell interaction is less than one, which is also observed for all considered BCPs (Table 3). However, negative values of the local electron energy h_e(r) in BCP2 in the case of ZnPcCl₄-p···NH₃ and ZnPcF₄-p···NH₃ indicate the intermediate interaction between the corresponding pairs of atoms, which could be an indicator of covalent bonds formation [79]. At the same time, the values of ρ(r) at these points are low enough to suggest the formation of covalent bonds; however, they are significantly higher than the electron density values at all other critical points of the bond considered here. It is this fact that determines a stronger binding of the NH₃ molecule with ZnPc derivatives bearing Cl and F substituents in peripheral positions. Finally, the absolute value of E_b increases in the order ZnPcCl₄-np···NH₃ < ZnPcF₄-np···NH₃ < ZnPcF₄-p···NH₃ < ZnPcCl₄-p···NH₃ (Table 3), which is in good agreement with the experimental data. Moreover, calculated Gibbs free energy change values at 298 K are close to zero in all four cases, indicating the reversibility of the NH₃ molecule binding with phthalocyanines. The same tendency of E_b change was obtained by additional calculations with the use of the DLPNO-CCSD(T)/def2-TZVPP method (Table 3, the values of E_b in parentheses).

In this type of interaction, the binding of NH3 molecules can occur simultaneously with several phthalocyanine molecules, which leads to an increase in the total energy of the interaction. At the same time, the energy remains sufficiently low, which ensures the reversibility of sorption-desorption of analyte molecules.

3.5. Sensor Characteristics of ZnPcCl₄-p and ZnPcF₄-p Films

Sensor characteristics of ZnPcCl₄-p and ZnPcF₄-p films showing the best sensitivity to ammonia were studied in more detail to demonstrate the possibility of their use for the determination of low concentrations of ammonia in air. The reproducibility of the response of ZnPcR4 layers was studied by repeated exposure to 0.4 ppm NH₃ over a certain period of time, as shown in Figure 10a. Both sensors showed reproducible response after gas-on and gas-off cycle measurements for about one hour. The long-term stability of the sensing layers was also tested by an experiment with 0.4 ppm NH₃ at room temperature. The inset of Figure 10a shows that no more than 6% decrease of the sensor response of a ZnPcCl₄-p film to ammonia was observed after 120 days, which testify its good stability. To investigate the effects of the relative humidity (RH), the response of both ZnPcF₄-p and ZnPcCl₄-p films was tested at different RH values. Figure 10b shows the change of the films’ resistance upon exposure to 30–50 ppm NH₃ at 10, 40, and 70% RH. It was shown that an increase in humidity up to 40% did not lead to a change in the initial resistance of the ZnPcF₄-p film and its sensor response to ammonia, but with an increase in humidity up to 70%, a decrease in the sensor response by about 1.5 times was observed (Figure 10c). This decrease in the sensor response can be explained by competitive sorption of the NH₃ and H₂O molecules on the surface of a ZnPcF₄-p film. This behavior is consistent with previously reported results [23,80]. At the same time, the films of ZnPcCl₄-p are more sensitive to humidity. An increase of the humidity led to the more pronounced decrease in the initial resistance of ZnPcCl₄-p film than in the case of ZnPcF₄-p (Figure 10b). In contrast to ZnPcF₄-p films, the sensor response of a ZnPcCl₄-p film to ammonia increased with a growth of humidity up to 70% (Figure 10d).

The binding energies of H₂O with ZnPcCl₄-p and ZnPcF₄-p molecules, calculated using the same method as with ammonia, were shown to be −3.0 and −2.7 kcal/mol, respectively. A stronger interaction between H₂O and ZnPcCl₄-p may promote the adsorption of more water molecules on the film surface. In the conditions of high humidity a continuous water layer appears to form on the surface of ZnPcCl₄-p film and physisorbed water layers become to show a liquidlike behavior. Similar behavior was also described by authors of previous work [81,82], and the mechanism describing the increase in the conductivity of sensitive layers at high RH of the environment is known as the Grottuss mechanism [81]. Protons (H⁺) formed as a result of water dissociation can move more easily through the water layer, which leads to an increase in the conductivity of the sensing film. With the introduction of NH₃, the formation of NH⁴⁺ and OH⁻ ions generated from the dissociation of NH₃ in water may facilitate an increase in the sensor response, similar to the case described by other researchers [82,83].

Selectivity is another important factor for the application of chemical sensors. To investigate the selectivity, the response of the layers of ZnPcF₄-p and ZnPcCl₄-p to various analytes, viz. carbon dioxide, dichloromethane, acetone, toluene, ethanol, was tested (Figure 11). It is obvious that the response of both sensing layers to NH₃ was much higher than to interfering analytes. It is necessary to mention that the concentration of ammonia given in the diagram is much less than that of the other analytes. These results show that the sensors are highly selective against the tested interfering analytes, especially at their low concentrations, which makes them attractive for detecting ammonia in gas mixtures.

The sensor response of a ZnPcCl₄-p film to ammonia was also measured in the mixture of air with carbon dioxide (10,000 ppm) (Figure 12). It was found that the presence of 10,000 ppm CO₂ has virtually no effect on the sensitivity of the ZnPcCl₄-p film to ammonia, e.g., the (R − R_o)/R_o value increased from 0.38 to 0.44 when 10,000 ppm CO₂ was added to the gas mixture containing 20 ppm NH₃. It was also shown (Figure 12) that the heating of the films to 100 °C did not cause noticeable changes of the sensitivity of ZnPcCl₄-p film to ammonia.

Some examples of chemiresistive sensors on the basis of phthalocyanine films described in the literature are presented in

Table 4. Sensor characteristics of various phthalocyanines toward ammonia.

Sensing Layer	Method	Concentration Range, ppm	Minimal Investigated Concentration, ppm	Response/ Recovery Time, s	Temp. Range, °C	Refs.
CuPc	chemiresistive	0.5–2	0.5	60/120 (0.5 ppm)	RT	[77]
CoPc	chemiresistive	20–100	20	~60/240 (23 ppm)	RT	[84]
CoPcR8, R is 5-(trifluorome-thyl)-2-mer-captopyridine	chemiresistive	0.3–50	0.3	20/40 (5 ppm)	RT	[80]
PdPc	chemiresistive	10–50	10	25/50 (10 ppm)	RT	[21]
FCrPc	chemiresistive	40–100	40	10/13 (40 ppm)	100–400	[85]
ZnPcF16	SPR *	100–200	100	10/30 (100 ppm)	RT	[86]
ZnPcF4-p	chemiresistive	0.1–50	0.1	45/210	RT	this work
ZnPcCl4-p	chemiresistive	0.1–50	0.1	45/260 (1 ppm)	RT	this work

* Surface plasmon resonance.

and compared with the results obtained in this work. Both ZnPcF₄-p and ZnPcCl₄-p films exhibit the lower detection limit in comparison with the films of other metal phthalocyanines.

4. Conclusions

In this work, the effect of the position of fluorine and chlorine substituents in tetrasubstituted zinc phthalocyanines ZnPcR₄-np and ZnPcF₄-p (R = F, Cl) on their structure and chemiresistive sensor response to low concentration of ammonia was studied. The structure and morphology of their films deposited by thermal evaporation in vacuum were investigated by XRD and atomic force microscopy methods. It was shown that both position and type of substituents affect the structure of single crystals and films of zinc phthalocyanines. In the case of ZnPcF₄-p, the molecules are arranged in parallel stacks similarly to α-polymorphs of unsubstituted phthalocyanines, while ZnPcF₄-np, ZnPcCl₄-p and ZnPcCl₄-np form crystals and films with herring bone arrangements of phthalocyanine macrocycles. According to XRD data, the films of have strong preferential orientation of crystallites relative to the substrate surface; however, the ZnPcCl₄-np film has poorer crystallinity compared to the films of other phthalocyanine derivatives.

The chemiresistive sensor response of the investigated layers to ammonia was found to increase in the order ZnPcCl₄-np < ZnPcF₄-np < ZnPcF₄-p < ZnPcCl₄-p, which is in good agreement with the values of bonding energy between hydrogen atoms of NH₃ and halogen substituents in the phthalocyanine rings. ZnPcCl₄-p films demonstrate the maximal sensor response to ammonia with the calculated detection limit of 0.01 ppm. Additionally, the sensors have good long-term stability and reproducibility. It was shown that both ZnPcF₄-p and ZnPcCl₄-p and can be used for the selective detection of ammonia in the presence of carbon dioxide, dichloromethane, acetone, toluene, and ethanol.