DFT-D3 and TD-DFT Studies of the Adsorption and Sensing Behavior of Mn-Phthalocyanine toward NH3, PH3, and AsH3 Molecules

This study employs density functional theory (DFT) calculations at the B3LYP/6-311+g(d,p) level to investigate the interaction of XH3 gases (X = N, P, As) with the Mn-phthalocyanine molecule (MnPc). Grimme’s D3 dispersion correction is applied to consider long-range interactions. The adsorption behavior is explored under the influence of an external static electric field (EF) ranging from −0.514 to 0.514 V/Å. Chemical adsorption of XH3 molecules onto the MnPc molecule is confirmed. The adsorption results in a significant decrease in the energy gap (Eg) of MnPc, indicating the potential alteration of its optical properties. Quantum theory of atoms in molecules (QTAIM) analysis reveals partially covalent bonds between XH3 and MnPc, and the charge density differenc (Δρ) calculations suggest a charge donation-back donation mechanism. The UV-vis spectrum of MnPc experiences a blue shift upon XH3 adsorption, highlighting MnPc’s potential as a naked-eye sensor for XH3 molecules. Thermodynamic calculations indicate exothermic interactions, with NH3/MnPc being the most stable complex. The stability of NH3/MnPc decreases with increasing temperature. The direction and magnitude of the applied electric field (EF) play a crucial role in determining the adsorption energy (Eads) for XH3/MnPc complexes. The Eg values decrease with an increasing negative EF, which suggests that the electrical conductivity (σ) and the electrical sensitivity (ΔEg) of the XH3/MnPc complexes are influenced by the magnitude and direction of the applied EF. Overall, this study provides valuable insights into the suggested promising prospects for the utilization of MnPc in sensing applications of XH3 gases.


Introduction
Preserving the environment stands as one of the utmost priorities for scientists today.With technological advancements permeating every facet of life, the prevalence of pollutants has escalated to a level that poses a threat to living organisms.Consequently, there arose a necessity to develop and produce nanosensors specifically designed for detecting environmentally harmful gases.Among the hydrides of the fifth group, ammonia (NH 3 ), phosphine (PH 3 ), and arsine (AsH 3 ) merit special attention due to the hazards they pose.NH 3 is a colorless gas with a pungent odor.NH 3 is used in several industries, such as the production of fertilizers, pesticides, hair dyes, plastics, and the textile industry.The inhalation of ammonia can result in severe irritation to the respiratory system [1][2][3].PH 3 is a scentless toxic gas characterized by its spontaneous flammability in the air or when interacting with oxygen.PH 3 has several uses; for instance, it is used to exterminate insects and rodents.It is also used to preserve agricultural crops in warehouses during storage.
Its risks are due to inhalation or absorption through the skin.Inhaling PH 3 gas causes pneumonia and respiratory poisoning and can harm the central nervous system [4,5].AsH 3 is a colorless gas that has a disagreeable fish-like scent.It is used in the steel industry and metal treatment.It is an extremely toxic gas.The risks caused by arsine are abdominal pain, diarrhea, dyspnea, hemolysis, and renal failure [6][7][8].
Earlier research indicates that attempts were made toward the adsorption of these hazardous gases onto the surfaces of various materials.Luo et al. [9] demonstrated that the adsorption of NH 3 , PH 3 , and AsH 3 on graphene is weak, while the doping of graphene with La, Ce, Nd, Pm, Sm, Eu, and Gd improved the adsorption of NH 3 and weakened the adsorption of AsH 3 .Habibi-Yangjeh et al. [4] suggested that Ta/P heptazine graphitic carbon nitride serves as a suitable sensor for PH 3 .Conversely, Ranea et al. [10] found that the V atom of V 2 O 5 is an attractive center for NH 3 , PH 3 , and AsH 3 .Additionally, CdSe functions efficiently as a gas sensor for NH 3 , PH 3 , and AsH 3 gases [11].
Phthalocyanines (Pcs) represent a class of aromatic macrocyclic organic compounds characterized by the molecular formula C 32 H 18 N 8 .Their distinct attributes encompass remarkable thermal and chemical stability, coupled with favorable electrical and optical properties, primarily attributed to their extensive electronic π-conjugated system [12,13].
The enumerated advantages have significantly propelled the utilization of phthalocyanines (Pcs) across diverse domains, including solar cells, semiconductors, catalysis, sensors, and tumor treatment [12,[14][15][16].Notably, research indicates that the incorporation of transition elements into phthalocyanine structures enhances their suitability for a wide range of applications.For instance, the integration of MnPc and FePc into junctions between two single-walled carbon nanotubes results in superior magnetic spin moments compared to CoPc and NiPc, thereby enhancing their performance in spintronic devices [15].Furthermore, investigations into the magnetic properties of transition metal phthalocyanine sheets (TM = Cr-Zn) reveal that only MnPc exhibits ferromagnetic behavior [17].Doping Pcs with Co and Mn has been shown to heighten their redox activity, a crucial aspect for electrochemical applications [16].Given these findings, it can be reasonably anticipated that grafting phthalocyanine with a transition element, particularly Mn, will induce notable changes in its properties as a gas sensor.
To the best of our knowledge, the application of MnPc as an adsorbent or sensor for NH3, PH3, and AsH3 gases has not been explored previously.This study is designed to investigate the adsorption properties of NH 3 , PH 3 , and AsH 3 gases on the MnPc molecule.Additionally, we will explore the impact of an external static electric field on these adsorption characteristics.

Results and Discussions
In this study, we examined the adsorption of XH 3 gases (where X = N, P, As) on a MnPc molecule.Initially, we focused on determining the most energetically stable structures of both the individual XH 3 and MnPc molecules.This investigation aimed to understand the influence of the Mn atom on the properties of Pc.Subsequently, we investigated the most energetically stable structures of the XH 3 /Pc complexes.), which are consistent with previous findings [2,3,18,19].The MESP analysis reveals a negative electrostatic potential around the X atom, indicating the presence of a 2p electron lone pair.This suggests that the X atom functions as a nucleophilic center.Moreover, the calculated electrical dipole moments (D) were found to be 1.673, 0.823, and 0.345 Debye for NH 3 , PH 3 , and AsH 3 , respectively, which aligns with the findings of Zhang et al. [18].Consequently, the chemical reactivity of a molecule increases with its dipole moment [14].Therefore, one can anticipate the following trend in chemical reactivity: NH 3 > PH 3 > AsH 3 .Figure 2 illustrates the optimized structures and MESP for Pc and MnPc, while their electronic properties are summarized in Table 1.It was observed that in MnPc, the Mn-N bond length was determined to be 1.958 Å and the N-Mn-N angle was found to be 90 • , which is consistent with previous theoretical and experimental studies [15,20,21].The calculated band gap (E g ) values indicate that both Pc and MnPc are semiconductors, with the presence of the Mn atom leading to a 34.5% reduction in the E g value of the Pc molecule.Furthermore, the binding energy (E b ) for MnPc was more negative compared to that of Pc, implying that the Mn atom enhances the stability of the MnPc molecule.This can be attributed to intramolecular partial charge transfer (PCT) occurring from the Mn atom to the rest of the molecule, resulting in a positive charge of 0.954 e accumulating on the Mn atom.Moreover, the MnPc molecule exhibits lower values of E g , IP, and η, while displaying higher values of E f and ω compared to the Pc molecule.These characteristics indicate that the MnPc molecule is more reactive than the Pc molecule.Additionally, the positive charge on the Mn atom results in a positive electrostatic potential surrounding it, as depicted in Figure 2d.Consequently, the Mn atom can be considered an electrophilic site.It is worth noting that the Pc and MnPc molecules do not exhibit noticeable values of the electrical dipole moment (D) due to the symmetrical distribution of electrical charges on the atoms within these molecules.Figure 3b,e depict the DOS and PDOS for Pc and MnPc molecules, respectively.It is evident that the presence of the Mn atom causes the HOMO to shift upwards by 0.507 eV, while the LUMO is shifted downwards by 0.225 eV.Consequently, the E g value of the Pc molecule decreases from 2.120 eV to 1.388 eV in the MnPc molecule, which is consistent with previous research [21,22].Table 1.Electronic properties of Pc and MnPc.HOMO and LUMO energy levels (eV) for α and β spins, HOMO-LUMO gap (E g , eV), average binding energy per atom (E b , eV), NBO charges (Q, e), ionization potential (IP, eV), chemical potential (µ, eV), hardness (η, eV), electrophilicity (ω, eV), and dipole moment (D, Debye).Furthermore, the UV-vis spectra of Pc and MnPc molecules are displayed in Figure 3g.The Pc molecule exhibits two absorption peaks, known as the Soret band and Q band, at wavelengths of 340 nm and 608 nm, respectively.These findings align with previous studies [22][23][24].On the other hand, the MnPc molecule demonstrates a broad absorption peak at 604 nm.Furthermore, the UV-vis spectra of Pc and MnPc molecules are displayed in Figure 3g.The Pc molecule exhibits two absorption peaks, known as the Soret band and Q band, at wavelengths of 340 nm and 608 nm, respectively.These findings align with previous studies [22][23][24].On the other hand, the MnPc molecule demonstrates a broad absorption peak at 604 nm.

Adsorption of XH 3 on MnPc
The adsorption of XH 3 molecules on the Mn site of the MnPc molecule was investigated in three different adsorption modes, as shown in Figure 4.All modes undergo full optimization, and it was found that modes 1 and 2 resulted in energetically stable complexes.Interestingly, mode 3 reoriented to yield the same complexes as mode 2. Top and side views of the optimized complexes for adsorption modes 1 and 2 are presented in Figure 5.The adsorption properties for modes 1 and 2 are summarized in Table 2.
Molecules 2024, 29, x FOR PEER REVIEW 7 of

Adsorption of XH3 on MnPc
The adsorption of XH3 molecules on the Mn site of the MnPc molecule was inves gated in three different adsorption modes, as shown in Figure 4.All modes undergo f optimization, and it was found that modes 1 and 2 resulted in energetically stable co plexes.Interestingly, mode 3 reoriented to yield the same complexes as mode 2. Top a side views of the optimized complexes for adsorption modes 1 and 2 are presented Figure 5.The adsorption properties for modes 1 and 2 are summarized in Table 2.   Table 2. Adsorption properties of XH 3 (X = N, P, As) on MnPc.Adsorption energies (E ads , eV), HOMO and LUMO energy levels (eV) for α and β spins, HOMO-LUMO gap (E g , eV), NBO charges (Q, e), and dipole moment (D, Debye).NH3/MnPc < PH3/MnPc < AsH3/MnPc, suggesting that AsH3 exhibited the highest adsorption strength, followed by PH3 and NH3.Furthermore, it can be observed that the adsorption strength in mode 1 was higher compared to mode 2. The distances between the X atom and the Mn adsorbing site (dX-Mn) followed the trend of NH3/MnPc < PH3/MnPc < AsH3/MnPc for both adsorption modes.This indicates that NH3 exhibited the closest proximity to the Mn adsorbing site, followed by PH3 and AsH3, in both modes.In accordance with previous studies [25][26][27], it is well-established that chemisorption is characterized by high adsorption energy (|E ads | ≥ 0.2 eV).Therefore, based on the obtained XH 3 /MnPc complexes in adsorption modes 1 and 2, it can be concluded that the XH 3 molecule undergoes chemical adsorption on the MnPc molecule.It is important to note that a more negative E ads value indicates a higher degree of adsorption.For adsorption mode 1, the trend of adsorption strength was observed as NH 3 /MnPc > PH 3 /MnPc > AsH 3 /MnPc complexes, indicating that NH 3 exhibited the highest adsorption strength, followed by PH 3 and AsH 3 .On the other hand, for adsorption mode 2, the trend was NH 3 /MnPc < PH 3 /MnPc < AsH 3 /MnPc, suggesting that AsH 3 exhibited the highest adsorption strength, followed by PH 3 and NH 3 .Furthermore, it can be observed that the adsorption strength in mode 1 was higher compared to mode 2. The distances between the X atom and the Mn adsorbing site (d X-Mn ) followed the trend of NH 3 /MnPc < PH 3 /MnPc < AsH 3 /MnPc for both adsorption modes.This indicates that NH 3 exhibited the closest proximity to the Mn adsorbing site, followed by PH 3 and AsH 3 , in both modes.
Furthermore, the adsorption of XH 3 molecules resulted in a decrease in the E g value of the MnPc molecule.Specifically, for adsorption mode 1, the E g value decreased to 86.2%, 91.6%, and 92.8% for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.For adsorption mode 2, the E g value decreased to 96.2%, 98.2%, and 97.9% for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.In other words, adsorption mode 1 resulted in a greater reduction in the E g value compared to adsorption mode 2. Notably, the largest decrease in the E g value was observed for the NH 3 /MnPc complex in adsorption mode 1.Additionally, the adsorption of XH 3 molecules led to an increase in the dipole moment.Specifically, for adsorption mode 1, the dipole moment increased to 3.130, 2.083, and 1.576 Debye for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.For adsorption mode 2, the dipole moment increased to 0.949, 0.228, and 0.213 Debye for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.
The results obtained can be further analyzed using several methods, including QTAIM, electrostatic potential (ESP), NBO atomic charge, charge density difference (∆ρ), and PDOS.The QTAIM analysis is particularly useful for understanding the nature of interactions [28][29][30].Previous studies have established certain characteristics for different types of interactions, such as van der Waals interactions, weak hydrogen bonds, and ionic bonds.These characteristics include −G(r)/V(r) > 1, H(r) > 0, and ∇ 2 ρ > 0. Strong interactions are classified by ∇ 2 ρ > 10 -1 au, while weak interactions have ∇ 2 ρ < 10 -1 au.Partially covalent bonds are characterized by ∇ 2 ρ > 0 and 0.5 < -G(r)/V(r) < 1.Therefore, the QTAIM theory can be employed to analyze the topological parameters of the bond critical points (BCP) of type (3, -1) formed between the XH 3 and MnPc molecules.The BCPs, as depicted in Figure 6, and their corresponding parameters are summarized in Table 3.The analysis reveals that in adsorption mode 1, only one BCP exists between the X atom of the XH 3 molecule and the Mn atom of the MnPc molecule.This BCP exhibits a ∇ 2 ρ value greater than zero, specifically 0.171, 0.064, and 0.046 au for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively.Furthermore, the −G(r)/V(r) ratio for these BCPs is greater than 0.5 but less than 1, specifically 0.952, 0.813, and 0.833 for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively.As a result, these BCPs are classified as partially covalent bonds.Moreover, the ∇ 2 ρ values indicate that the interaction in the NH 3 /MnPc complex is characterized as strong, while the interactions in the PH 3 /MnPc and AsH 3 /MnPc complexes are classified as weak.
Table 3.The estimated topological parameters.Electron densities (ρ), Laplacian of charge density (∇ 2 ρ), kinetic electron density (G(r)), potential energy density (V(r)), and energy density (H(r)).All units are in au. the MnPc molecule.These BCPs have positive ∇ 2 ρ values of less than 10 -1 and −G(r)/V(r) ratios greater than 1.Consequently, the first BCPs (X-Mn) are classified as van der Waals interactions, while the second BCPs (H-N) are classified as weak hydrogen bonds.Consequently, the first BCPs (X-Mn) are classified as van der Waals interactions, while the second BCPs (H-N) are classified as weak hydrogen bonds.

Adsorption Mode
Figure 7 illustrates the ESP for the MnPc and XH 3 molecules.The ESP was calculated along the Z-axis, with the X and Mn atoms positioned at the origin point.The ESP for the MnPc molecule appears positive and symmetric around the Mn atom along the Z-axis.In contrast, the ESP for the XH 3 molecule exhibits an asymmetric pattern along the Z-axis.In the −Z direction, the ESP curves for NH3, PH3, and AsH3 display minimum negative values of −0.110, −0.037, and −0.021 au, respectively, occurring at distances of -1.303, −1.949, and −2.152 Å from the origin.On the other hand, in the +Z direction, the ESP curves have minimum values of 0.003, 0.002, and −0.003 au at distances of 1.887 Å, 3.373 Å, and 2.828 Å, respectively.Notably, the minima of the curves in the -Z direction are lower than those in the +Z direction.This discrepancy arises from the presence of the X atom's electron lone pair in the -Z direction.These observations may help explain the stronger interaction observed for adsorption in mode 1 compared to mode 2, as well as the trend observed in the adsorption behavior.To analyze partial charge transfer (PCT), NBO atomic charges were computed.In all the examined adsorption structures, the XH3 molecule undergoes a positive charge, suggesting a transfer of charge from the XH3 molecule to the MnPc molecule.This PCT is more significant in adsorption mode 1 than in adsorption mode 2. Consequently, it is reasonable to anticipate that the PCT contributes to the reinforcement of adsorption mode 1.In addition, for adsorption mode 1, the XH3 molecule experiences a loss of charges (Q ) of 0.177, 0.324, and 0.226 e, while the Mn adsorbing site gains charges of 0.127, 0.125, and 0.187 e for NH3/MnPc, PH3/MnPc, and AsH3/MnPc, respectively.This implies that the XH3 molecule not only transfers charge to the adsorbing site but also to the other atoms of the MnPc molecule.Specifically, the XH3 molecule loses more charge than what is gained by the Mn adsorbing site.
It is important to note that the trend of Q does not necessarily align with the trend of Eads, indicating the presence of another mechanism influencing the adsorption process.To further elucidate this, the charge density differences (Δρ) for the XH3/MnPc complexes were analyzed and depicted in Figure 8.The Δρ values exhibit both positive (blue) and negative (red) regions for both the XH3 adsorbate and MnPc substrate.This suggests that charge transfer occurs in two directions, from the XH3 molecule to the MnPc molecule and vice versa.Consequently, a charge donation-back donation mechanism is proposed for In the −Z direction, the ESP curves for NH 3 , PH 3 , and AsH 3 display minimum negative values of −0.110, −0.037, and −0.021 au, respectively, occurring at distances of -1.303, −1.949, and −2.152 Å from the origin.On the other hand, in the +Z direction, the ESP curves have minimum values of 0.003, 0.002, and −0.003 au at distances of 1.887 Å, 3.373 Å, and 2.828 Å, respectively.Notably, the minima of the curves in the -Z direction are lower than those in the +Z direction.This discrepancy arises from the presence of the X atom's electron lone pair in the -Z direction.These observations may help explain the stronger interaction observed for adsorption in mode 1 compared to mode 2, as well as the trend observed in the adsorption behavior.To analyze partial charge transfer (PCT), NBO atomic charges were computed.In all the examined adsorption structures, the XH 3 molecule undergoes a positive charge, suggesting a transfer of charge from the XH 3 molecule to the MnPc molecule.This PCT is more significant in adsorption mode 1 than in adsorption mode 2. Consequently, it is reasonable to anticipate that the PCT contributes to the reinforcement of adsorption mode 1.In addition, for adsorption mode 1, the XH 3 molecule experiences a loss of charges (Q XH 3 ) of 0.177, 0.324, and 0.226 e, while the Mn adsorbing site gains charges of 0.127, 0.125, and 0.187 e for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.This implies that the XH 3 molecule not only transfers charge to the adsorbing site but also to the other atoms of the MnPc molecule.Specifically, the XH 3 molecule loses more charge than what is gained by the Mn adsorbing site.
It is important to note that the trend of Q XH 3 does not necessarily align with the trend of E ads , indicating the presence of another mechanism influencing the adsorption process.To further elucidate this, the charge density differences (∆ρ) for the XH 3 /MnPc complexes were analyzed and depicted in Figure 8.The ∆ρ values exhibit both positive (blue) and negative (red) regions for both the XH 3 adsorbate and MnPc substrate.This suggests that charge transfer occurs in two directions, from the XH 3 molecule to the MnPc molecule and vice versa.Consequently, a charge donation-back donation mechanism is proposed for the adsorption process.Furthermore, Figure 8 illustrates that the ∆ρ values for adsorption mode 1 are higher than those for adsorption mode 2, which corresponds well with the adsorption energies described in Table 2. Lastly, as a result of the adsorption, a redistribution of charges between the XH 3 and MnPc molecules occurs, leading to an increase in the electric dipole moment values of the XH 3 /MnPc complexes.
Molecules 2024, 29, x FOR PEER REVIEW 12 of 24 mode 1 are higher than those for adsorption mode 2, which corresponds well with the adsorption energies described in Table 2. Lastly, as a result of the adsorption, a redistribution of charges between the XH3 and MnPc molecules occurs, leading to an increase in the electric dipole moment values of the XH3/MnPc complexes.Overall, the observed shifts in the HOMO and LUMO energies of the MnPc molecule indicate the modification of its electronic structure due to the interaction with the XH3 molecule in the XH3/MnPc complexes.These changes in the DOS further highlight the influence of the XH3 adsorbate on the electronic properties of the MnPc substrate.
Consequently, for adsorption mode 1, the shifts in the HOMO and LUMO energies lead to a narrowing of the bandgap (Eg) for the NH3/MnPc, PH3/MnPc, and AsH3/MnPc complexes by 13.80%, 8.43%, and 7.21%, respectively.In contrast, for adsorption mode 2, the shifts in the HOMO and LUMO energies are not as significant, resulting in a smaller reduction in the Eg values.Specifically, the Eg values for the NH3/MnPc, PH3/MnPc, and AsH3/MnPc complexes are slightly narrowed by 3.78%, 1.82%, and 2.08%, respectively.Consequently, for adsorption mode 1, the shifts in the HOMO and LUMO energies lead to a narrowing of the bandgap (E g ) for the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes by 13.80%, 8.43%, and 7.21%, respectively.In contrast, for adsorption mode 2, the shifts in the HOMO and LUMO energies are not as significant, resulting in a smaller reduction in the E g values.Specifically, the E g values for the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes are slightly narrowed by 3.78%, 1.82%, and 2.08%, respectively.
where A is a constant, K is Boltzmann's constant, and T is the temperature.As a result, it is reasonable to assume that the XH 3 molecule's adsorption will raise the MnPc molecule's electrical conductivity.Furthermore, the increase in σ in the case of the adsorption mode 1 is greater than in the case of the adsorption mode 2, and in the case of NH 3 , it is greater than in the case of PH 3 and AsH 3 .Consequently, the MnPc molecule exhibits a higher sensitivity for NH 3 molecules than PH 3 and AsH 3 molecules.Our results for mode 1 clarify that MnPc may be useful for XH 3 detection, especially NH 3 detection.
where A is a constant, K is Bol mann's constant, and T is the temperature.As a result, it is reasonable to assume that the XH3 molecule's adsorption will raise the MnPc molecule's electrical conductivity.Furthermore, the increase in σ in the case of the adsorption mode 1 is greater than in the case of the adsorption mode 2, and in the case of NH3, it is greater than in the case of PH3 and AsH3.Consequently, the MnPc molecule exhibits a higher sensitivity for NH3 molecules than PH3 and AsH3 molecules.Our results for mode 1 clarify that MnPc may be useful for XH3 detection, especially NH3 detection.

UV-Vis Spectra Analysis
The influence of XH3 adsorption on the UV-vis spectrum of the MnPc molecule was investigated for adsorption modes 1 and 2. TD-DFT calculations were performed on the optimized structures of XH3/MnPc complexes to predict the UV-vis absorption spectra.In Figure 11a,b, the UV-vis spectra for XH3/MnPc complexes in adsorption modes 1 and 2

UV-Vis Spectra Analysis
The influence of XH 3 adsorption on the UV-vis spectrum of the MnPc molecule was investigated for adsorption modes 1 and 2. TD-DFT calculations were performed on the optimized structures of XH 3 /MnPc complexes to predict the UV-vis absorption spectra.In Figure 11a,b, the UV-vis spectra for XH 3 /MnPc complexes in adsorption modes 1 and 2 are presented, respectively.
For adsorption mode 1, the NH3/MnPc, PH3/MnPc, and AsH3/MnPc complexes exhibit maximum absorption wavelength peaks (λ max ) in the visible region at 524, 552, and 572 nm, respectively.In contrast, Figure 3g illustrates that the λ max of the MnPc molecule is situated at 604 nm.This indicates that the adsorption of XH 3 molecules induces a blue shift in the UV-vis spectrum of MnPc.Consequently, the color of the MnPc molecule may be altered by the adsorption of XH 3 molecules, suggesting the potential utility of MnPc as a naked-eye sensor for the investigated XH 3 molecules.
Conversely, for adsorption mode 2, the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes display λ max in the visible region at 604, 600, and 600 nm, respectively.In this case, XH 3 molecules exhibit no significant impact on the UV-vis spectrum of MnPc.For adsorption mode 1, the NH3/MnPc, PH3/MnPc, and AsH3/MnPc complexes exhibit maximum absorption wavelength peaks (λmax) in the visible region at 524, 552, and 572 nm, respectively.In contrast, Figure 3g illustrates that the λmax of the MnPc molecule is situated at 604 nm.This indicates that the adsorption of XH3 molecules induces a blue shift in the UV-vis spectrum of MnPc.Consequently, the color of the MnPc molecule may be altered by the adsorption of XH3 molecules, suggesting the potential utility of MnPc as a naked-eye sensor for the investigated XH3 molecules.

Thermodynamic Analysis
Given that the majority of gas sensors operate at temperatures below 800 K [37,38], thermodynamic calculations were conducted for the XH3 molecules, MnPc substrate, and XH3/MnPc complexes within the temperature range of 300 to 800 K.As adsorption mode 1 exhibited the most pronounced impact on the electrical and optical properties of the MnPc molecule, subsequent discussions will focus on this particular adsorption mode.
The thermodynamic parameters, including enthalpy difference (ΔH) and free energy difference (ΔG), play a crucial role in characterizing the strength and spontaneity of gas adsorption.In the case of XH3/MnPc complexes, ΔH was computed using Equation (10) and is illustrated as a function of temperature (T) in Figure 12a.Negative ΔH values are indicative of an exothermic reaction, and a more negative ΔH signifies greater stability of

Thermodynamic Analysis
Given that the majority of gas sensors operate at temperatures below 800 K [37,38], thermodynamic calculations were conducted for the XH 3 molecules, MnPc substrate, and XH 3 /MnPc complexes within the temperature range of 300 to 800 K.As adsorption mode 1 exhibited the most pronounced impact on the electrical and optical properties of the MnPc molecule, subsequent discussions will focus on this particular adsorption mode.
The thermodynamic parameters, including enthalpy difference (∆H) and free energy difference (∆G), play a crucial role in characterizing the strength and spontaneity of gas adsorption.In the case of XH 3 /MnPc complexes, ∆H was computed using Equation (10) and is illustrated as a function of temperature (T) in Figure 12a.Negative ∆H values are indicative of an exothermic reaction, and a more negative ∆H signifies greater stability of the products.Figure 12a demonstrates that for XH 3 /MnPc complexes, the ∆H values exhibit negativity across the entire temperature range under consideration.Furthermore, as temperature increases, the ∆H values become less negative.Notably, at a specific temperature, the ∆H value for the NH 3 /MnPc complex is more negative compared to the PH 3 /MnPc and AsH 3 /MnPc complexes.This observation suggests that the interaction between the XH 3 molecule and the MnPc molecule is exothermic.Specifically, the NH 3 /MnPc complex is more stable than the PH 3 /MnPc and AsH 3 /MnPc complexes.Additionally, the stability of NH 3 /MnPc complexes decreases with an increase in temperature.
temperature increases, the ΔH values become less negative.Notably, at a specific temperature, the ΔH value for the NH3/MnPc complex is more negative compared to the PH3/MnPc and AsH3/MnPc complexes.This observation suggests that the interaction between the XH3 molecule and the MnPc molecule is exothermic.Specifically, the NH3/MnPc complex is more stable than the PH3/MnPc and AsH3/MnPc complexes.Additionally, the stability of NH3/MnPc complexes decreases with an increase in temperature.The ∆G values for the XH 3 /MnPc complexes were determined using Equation (11) and are depicted as a function of temperature in Figure 12b.Negative and positive ∆G values signify spontaneous and non-spontaneous reactions, respectively, with low negative ∆G values suggesting a potential for reversing the reaction [39][40][41].Figure 12b shows a linear increase in ∆G values with temperature.At room temperature (300 K), all investigated complexes display negative ∆G values.However, beyond 300, 400, and 600 K for AsH 3 /MnPc, PH 3 /MnPc, and NH 3 /MnPc complexes, respectively, ∆G values turn positive, indicating a non-spontaneous adsorption process.Additionally, at T = 300 K, the ∆G value for the NH 3 /MnPc complex is more negative than that for the PH 3 /MnPc and AsH 3 /MnPc complexes.Consequently, the adsorption process can be more easily reversed for the PH 3 /MnPc and AsH 3 /MnPc complexes than for the NH 3 /MnPc complex.
The thermodynamic adsorption equilibrium constant (K) for the adsorption process was calculated using Equation (12), and log K was plotted against temperature, as shown in Figure 12c.K serves as a crucial parameter in assessing the strength and spontaneity of the adsorption process, with higher K values indicating stronger adsorption [42].Furthermore, K values greater than 1 suggest spontaneous adsorption, while values less than 1 indicate non-spontaneous adsorption.Observing Figure 12c, it is evident that log K for the NH 3 /MnPc complex surpasses that of the PH 3 /MnPc and AsH 3 /MnPc complexes.Consequently, the adsorption of the NH 3 molecule is stronger compared to the adsorption of PH 3 and AsH 3 molecules.Additionally, for the examined XH 3 /MnPc complexes, log K decreases as temperature increases, signifying that elevated temperatures diminish the ability of the MnPc molecule to adsorb XH 3 molecules.Moreover, beyond 300, 400, and 600 K for AsH 3 /MnPc, PH 3 /MnPc, and NH 3 /MnPc complexes, respectively, the K value falls below 1 (log K < 0), highlighting non-spontaneous adsorption.

Effect of the EF on the Adsorption of XH 3 on MnPc
This study explored the adsorption properties under the influence of an external static electric field (EF), applied within the range of −0.514 to 0.514 V/Å with increments of 0.125 V/Å along the axis perpendicular to the MnPc molecule's plane, as depicted in Figure 13.At each EF step, both XH 3 and MnPc molecules, as well as XH 3 /MnPc complexes, undergo full optimization.The dipole moment plotted against the electric field is illustrated in Figure 14a for free XH 3 gases and Figure 14b for MnPc and XH 3 /MnPc complexes.gated complexes display negative ΔG values.However, beyond 300, 400, and 600 K for AsH3/MnPc, PH3/MnPc, and NH3/MnPc complexes, respectively, ΔG values turn positive, indicating a non-spontaneous adsorption process.Additionally, at T = 300 K, the ΔG value for the NH3/MnPc complex is more negative than that for the PH3/MnPc and AsH3/MnPc complexes.Consequently, the adsorption process can be more easily reversed for the PH3/MnPc and AsH3/MnPc complexes than for the NH3/MnPc complex.
The thermodynamic adsorption equilibrium constant (K) for the adsorption process was calculated using Equation (12), and log K was plo ed against temperature, as shown in Figure 12c.K serves as a crucial parameter in assessing the strength and spontaneity of the adsorption process, with higher K values indicating stronger adsorption [42].Furthermore, K values greater than 1 suggest spontaneous adsorption, while values less than 1 indicate non-spontaneous adsorption.Observing Figure 12c, it is evident that log K for the NH3/MnPc complex surpasses that of the PH3/MnPc and AsH3/MnPc complexes.Consequently, the adsorption of the NH3 molecule is stronger compared to the adsorption of PH3 and AsH3 molecules.Additionally, for the examined XH3/MnPc complexes, log K decreases as temperature increases, signifying that elevated temperatures diminish the ability of the MnPc molecule to adsorb XH3 molecules.Moreover, beyond 300, 400, and 600 K for AsH3/MnPc, PH3/MnPc, and NH3/MnPc complexes, respectively, the K value falls below 1 (log K < 0), highlighting non-spontaneous adsorption.

Effect of the EF on the Adsorption of XH3 on MnPc
This study explored the adsorption properties under the influence of an external static electric field (EF), applied within the range of −0.514 to 0.514 V/Å with increments of 0.125 V/Å along the axis perpendicular to the MnPc molecule's plane, as depicted in Figure 13.At each EF step, both XH3 and MnPc molecules, as well as XH3/MnPc complexes, undergo full optimization.The dipole moment plotted against the electric field is illustrated in Figure 14a for free XH3 gases and Figure 14b for MnPc and XH3/MnPc complexes.Figure 14 presents the z-component of the dipole moment for free XH3 molecules, the MnPc molecule, and XH3/MnPc complexes in response to the applied electric field (EF) along the Z-axis.Notably, the z-component of the dipole moment is considered for this analysis.In Figure 14a, it is evident that the dipole moment of XH3 molecules increases with the magnitude of the EF in the negative direction and decreases with increasing the Figure 14 presents the z-component of the dipole moment for free XH 3 molecules, the MnPc molecule, and XH 3 /MnPc complexes in response to the applied electric field (EF) along the Z-axis.Notably, the z-component of the dipole moment is considered for this analysis.In Figure 14a, it is evident that the dipole moment of XH 3 molecules increases with the magnitude of the EF in the negative direction and decreases with increasing the EF in the positive direction.Furthermore, the dipole moment follows the trend NH 3 > PH 3 > AsH 3 .Moving to Figure 14b, the dipole moment of the MnPc molecule increases with the magnitude of the EF in the negative direction, while it decreases with increasing the EF in the positive direction.The chemical reactivity of a material with its surrounding environment tends to increase with a higher dipole moment [14].The dipole moments of both XH 3 and MnPc molecules are enhanced by negative EF values, leading to an enhancement of the adsorption energy (refer to Figure 15).Conversely, positive EF values result in a decrease in the dipole moment for both the MnPc and XH 3 molecules, leading to inhibition of the adsorption energy (refer to Figure 15).Therefore, the direction and magnitude of the applied EF play a crucial role in determining the adsorption energy (E ads ) for XH 3 /MnPc complexes.ment of the adsorption energy (refer to Figure 15).Conversely, positive EF values result in a decrease in the dipole moment for both the MnPc and XH3 molecules, leading to inhibition of the adsorption energy (refer to Figure 15).Therefore, the direction and magnitude of the applied EF play a crucial role in determining the adsorption energy (Eads) for XH3/MnPc complexes.ment of the adsorption energy (refer to Figure 15).Conversely, positive EF values result in a decrease in the dipole moment for both the MnPc and XH3 molecules, leading to inhibition of the adsorption energy (refer to Figure 15).Therefore, the direction and magnitude of the applied EF play a crucial role in determining the adsorption energy (Eads) for XH3/MnPc complexes.The impact of the EF on the E g values for the MnPc molecule and XH 3 /MnPc complexes is illustrated in Figure 16a.It is observed that the E g value for the MnPc molecule remains relatively constant with varying EF values.In contrast, the E g values for XH 3 /MnPc complexes exhibit changes: they increase as the positive EF rises, reaching 1.22, 1.38, and 1.38 eV for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively, at EF = 0.514 V/Å.Conversely, the E g values decrease with an increasing negative EF, reaching 1.18, 1.22, and 1.23 eV for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively, at EF = −0.514V/Å.This suggests that the electrical conductivity (σ) of the XH 3 /MnPc complexes is influenced by the magnitude and direction of the applied EF.Additionally, the electrical sensitivity (∆E g ) dependency on the EF was investigated using the following equation: EF intensifies the magnitude of ΔEg, making it more negative.Therefore, a positive electric field diminishes the sensitivity, whereas a negative electric field enhances the sensitivity of the MnPc molecule to XH3 molecules.It is noteworthy that the effect of the electric field on the UV-vis spectrum of the XH3/MnPc complexes was investigated, but no considerable effect was observed.In Figure 16b, at EF = 0.0 V/Å, the ∆Eg values were observed to be −13.68%,−8.43%, and −7.21% for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively.This indicates that the adsorption of XH 3 molecules leads to a decrease in the E g value for the MnPc molecule, accompanied by an increase in σ.Furthermore, the MnPc molecule exhibited greater sensitivity to NH 3 compared to PH 3 and AsH 3 molecules.Additionally, the influence of the electric field is evident in the ∆E g values: increasing the positive EF reduces the magnitude of ∆E g , making it less negative.Conversely, increasing the negative EF intensifies the magnitude of ∆E g , making it more negative.Therefore, a positive electric field diminishes the sensitivity, whereas a negative electric field enhances the sensitivity of the MnPc molecule to XH 3 molecules.
It is noteworthy that the effect of the electric field on the UV-vis spectrum of the XH 3 /MnPc complexes was investigated, but no considerable effect was observed.

Methods
The interaction of XH 3 gases onto the MnPc molecule is investigated utilizing DFT [43] calculations at the B3LYP/6-311+g(d,p) level of theory.D3, Grimme's dispersion that considers the long-range interactions, is considered [44,45].Yang et al. [46] state that B3LYP gave good performances for metal phthalocyanines.Additionally, B3LYP is utilized for transition metal-doped porphyrins [31,47], and the 3D transition metal complexes for hexaazabipy H2 and gave reliable results [48].Geometrical optimizations to obtain the most energetically stable structures for free XH 3 gases, MnPc bare molecule, and XH 3 /MnPc complexes are accomplished.The UV-vis absorption spectra for Pc and MnPc molecules, as well as the XH 3 /MnPc complexes, are estimated by Time-Dependent DFT (TD-DFT) calculations.To cover all the expected electronic transitions in the investigated range (0-1000 nm), a satisfactory number of excited states (n = 15) was projected.To estimate the relative stability of the Pc and MnPc molecules, the average binding energy for each atom (E b ) was estimated according to Equation (3) [39].The more negative the E b value, the more stable the molecule.
where n is the total number of atoms of the molecule, E molecule is the total energy of the molecule-optimized structure, and E i is the single atom's total energy.The relative chemical reactivity of molecules is estimated according to the following parameters: the HOMO-LUMO energy gap (E g ), the ionization potential (IP), the chemical potential (µ), hardness (η), and electrophilicity (ω).The high reactive molecule is characterized by low values of E g , IP, µ, and η and high value of ω [49][50][51].
The strength of the XH 3 -MnPc interaction is judged in terms of the adsorption energy (E ads ), which is assessed by Equation ( 8).The strong XH 3 -MnPc interaction is accompanied by more negative E ads values.
Thermodynamic calculations are performed in the range of T = 300-800 K to evaluate the enthalpy difference (∆H) and Gibbs free energy difference (∆G) for the XH 3 /MnPc complexes by Equations ( 10) and (11), respectively [52].∆H = H XH 3 /MnPc − (H MnPc + H XH 3 (10) ∆G = G XH 3 /MnPc − (G MnPc + G XH 3 (11) where H XH 3 /MnPc , H MnPc , and H XH 3 are the enthalpies while G XH 3 /MnPc , G MnPc , and G XH 3 are the Gibbs free energies for the XH 3 /MnPc complex, MnPc bare molecule, and free XH 3 gases, respectively.The thermodynamic adsorption equilibrium constant (K ads ) is a crucial adsorption factor.A K ads value higher than 100 is essential for the sorbent to be a useful application.K is evaluated by Equation ( 12) [18].
The adsorption behavior under the influence of an external static electric field (EF) was investigated.The EF was applied from −0.514 to 0.514 V/Å with a step of 0.125 V/Å (0.0025 au) along the axis, which was perpendicular to the plane of the MnPc molecule.Gaussian 09 software [53] was used for all calculations.GaussSum 3.0 was used to show the density of states (DOS) for the structures under investigation [54].To estimate the atomic charges for the structures under examination, a thorough natural bond orbital (NBO) analysis was carried out using the NBO program version 3.1 [55].A Quantum Theory of Atoms in Molecules (QTAIMs) examination was performed utilizing the Multiwfn 3.7 software package [56].

Conclusions
An investigation into the interaction of XH 3 gases (X = N, P, As) with the MnPc molecule, employing DFT calculations and external electric field modulation, has provided valuable insights into the adsorption behavior and its consequences on the electronic and optical properties of MnPc.The confirmed chemical adsorption of XH 3 onto MnPc, supported by QTAIM analysis indicating partially covalent bonds, underscores the relevance of this study in elucidating the nature of the XH3/MnPc interaction.
The significant reduction in the energy gap (E g ) of MnPc upon XH 3 adsorption, accompanied by the observed blue shift in the UV-vis spectrum, not only reveals the sensitivity of MnPc to XH 3 molecules but also suggests its potential application as a visual sensor.The thermodynamic calculations establish the exothermic nature of the interactions, with NH 3 /MnPc emerging as the most stable complex.The temperature-dependent stability of NH 3 /MnPc complexes adds a nuanced understanding of the dynamics of the adsorption process.
Moreover, the influence of an external electric field on the adsorption energy (E ads ) highlights the tunability of MnPc's sensitivity to XH 3 gases.The magnitude of the applied electric field plays a pivotal role, emphasizing the importance of external factors in tailoring the adsorption characteristics.
In conclusion, this study not only contributes to the fundamental understanding of XH 3 /MnPc interactions at a molecular level but also opens up avenues for potential applications in sensor technologies.The versatility of MnPc, demonstrated through its responsiveness to external stimuli and the dynamic nature of the adsorption process, suggests promising prospects for its utilization in sensing applications.Further exploration in this direction could lead to the development of innovative materials for gas sensing and related technologies.

Figure 1
Figure1displays the geometrical structure, density of states (DOS), highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and the molecular electrostatic potential (MESP) for XH 3 molecules.The X-H bond lengths and H-X-H angles were determined as follows: NH 3 (1.01Å, 108.3 • ), PH 3 (1.42Å, 93.61 • ), and AsH 3 (1.52Å, 92.36 • ), which are consistent with previous findings[2,3,18,19].The MESP analysis reveals a negative electrostatic potential around the X atom, indicating the presence of a 2p electron lone pair.This suggests that the X atom functions as a nucleophilic center.Moreover, the calculated electrical dipole moments (D) were found to be 1.673, 0.823, and 0.345 Debye for NH 3 , PH 3 , and AsH 3 , respectively, which aligns with the findings of Zhang et al.[18].Consequently, the

Figure 2 .
Figure 2. (a,b) The optimized structures and (c,d) molecular electrostatic potential (MESP) in atomic units for Pc and MnPc, respectively.

Figure 2 .
Figure 2. (a,b) The optimized structures and (c,d) molecular electrostatic potential (MESP) in atomic units for Pc and MnPc, respectively.

Figure 7
Figure 7 illustrates the ESP for the MnPc and XH3 molecules.The ESP was calculated along the Z-axis, with the X and Mn atoms positioned at the origin point.The ESP for the MnPc molecule appears positive and symmetric around the Mn atom along the Z-axis.In contrast, the ESP for the XH3 molecule exhibits an asymmetric pa ern along the Z-axis.

Figure 6 .
Figure 6.Bond critical points of type (3, −1): (a-c) for adsorption mode 1 and (d-f) for adsorption mode 2 for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.In adsorption mode 2, the NH 3 /MnPc complex exhibits one BCP between the N atom of the NH 3 molecule and the Mn atom of the MnPc molecule, with a ∇ 2 ρ value of 0.030 au and a −G(r)/V(r) ratio of 0.897.This BCP is classified as a weak, partially covalent bond.On the other hand, for the PH 3 /MnPc and AsH 3 /MnPc complexes, two BCPs are observed between the XH 3 molecule and the MnPc molecule.The first BCP (X-Mn) is formed between the X atom of the XH 3 molecule and the Mn atom of the MnPc molecule, while the second BCP (H-N) is formed between an H atom of the XH 3 molecule and an N atom of the MnPc molecule.These BCPs have positive ∇ 2 ρ values of less than 10 -1 and −G(r)/V(r) ratios greater than 1.

Figure 8 .
Figure 8. Charge density difference (∆ρ) at 0.001 au isovalue (a-c) for adsorption mode 1 and (d-f) for adsorption mode 2 for NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc, respectively.Red and blue colors refer to negative and positive ∆ρ values.The partial densities of states (PDOS) for the XH 3 /MnPc complexes are presented in Figure 9 for adsorption mode 1 and Figure 10 for adsorption mode 2. By comparing the DOS of the XH 3 molecule prior to adsorption (Figure 1) with those after adsorption (Figures 9 and 10), significant changes are evident, indicating the interaction between the XH 3 molecule and the MnPc molecule.As a result of this interaction, the HOMO of the MnPc molecule undergoes a shift in energy.Specifically, the HOMO is shifted from −4.798 eV to −5.079, −5.136, and −5.163 eV for the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes, respectively.This shift in energy reflects the influence of the XH 3 molecule on the electron density distribution and electronic structure of the MnPc molecule.Similarly, the LUMO of the MnPc molecule experiences an energy shift following the

Figure 9 .
Figure 9. HOMO, PDOS, and LUMO for adsorption mode 1 (a) NH 3 /MnPc, (b) PH 3 /MnPc, and (c) AsH 3 /MnPc.The dashed line refers to the Fermi level.Overall, the observed shifts in the HOMO and LUMO energies of the MnPc molecule indicate the modification of its electronic structure due to the interaction with the XH 3 molecule in the XH 3 /MnPc complexes.These changes in the DOS further highlight the influence of the XH 3 adsorbate on the electronic properties of the MnPc substrate.Consequently, for adsorption mode 1, the shifts in the HOMO and LUMO energies lead to a narrowing of the bandgap (E g ) for the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes by 13.80%, 8.43%, and 7.21%, respectively.In contrast, for adsorption mode 2, the shifts in the HOMO and LUMO energies are not as significant, resulting in a smaller reduction in the E g values.Specifically, the E g values for the NH 3 /MnPc, PH 3 /MnPc, and AsH 3 /MnPc complexes are slightly narrowed by 3.78%, 1.82%, and 2.08%, respectively.Furthermore, Equation (1) represents the relationship between the electrical conductivity (σ) and the HOMO-LUMO gap (E g ).This equation suggests that a smaller E g corresponds to a higher electrical conductivity[31][32][33][34][35][36].

Figure 13 .
Figure 13.Electric field direction relative to the XH3/MnPc complex (X = N, P, and As).

Figure 13 .
Figure 13.Electric field direction relative to the XH 3 /MnPc complex (X = N, P, and As).

Figure 14 .
Figure 14.The dipole moment vs. the electric field for (a) free XH3 gases and (b) MnPc and XH3/MnPc complexes.

Figure 14 .
Figure 14.The dipole moment vs. the electric field for (a) free XH3 gases and (b) MnPc and XH3/MnPc complexes.

Figure 16 .
Figure 16.(a) E g and (b) ∆E g % versus the electric field for MnPc and XH 3 /MnPc complexes.