Molecular Structure, Vibrational Spectrum and Conformational Properties of 4-(4-Tritylphenoxy)phthalonitrile-Precursor for Synthesis of Phthalocyanines with Bulky Substituent

Abstract

By DFT method with B3LYP, PBE, CAM-B3LYP, and B97D functionals, it was found that the molecule 4-(4-tritylphenoxy)phthalonitrile (TPPN) has four conformers. The geometric structure, vibrational frequencies, electronic characteristics, and thermodynamic functions of conformers, as well as the structure and energy of transition states, were determined. IR spectrum of TPPN film contains vibrational bands belonging to different conformers. The assignment of bands was performed basing the distribution of normal vibration energy on internal coordinates. A synchronous electron diffraction/mass spectrometric experiment was performed to determine the structure of conformers in a saturated TPPN vapor. The elemental composition of the ions recorded in the mass spectrum indicates the thermal stability of TPPN at least up to T = 200 °C. The difference in the structure of tetrasubstituted metal phthalocyanines, which can be synthesized from different TPPN conformers, has been shown.

1. Introduction

Currently, considerable attention is paid to phthalocyanines containing bulk trityl, due to their manifestation of liquid crystal properties [1,2,3,4], as well as the prospects for their use in thin-film electronics [5]. In addition, phthalocyanine compounds with high solubility are currently needed [6,7]. An increase in solubility can be achieved by introducing substituents of a certain nature [7,8]. In [5], novel tetra (4-tritylphenoxy) substituted copper, zinc, and cobalt phthalocyanines were obtained by reaction of 4-(4-tritylphenoxy)phthalonitrile (TPPN) (Scheme 1) with CuCl₂, ZnCl₂, CoCl₂ metal salts (scheme in two dimensions of metal substituted phthalocyanine molecules is given in Supplementary Materials, Section S1). It has been shown in Ref. [5] that the presence of bulky 4-tritylphenoxy substituents on the peripheral positions prevents the aggregation in organic solvents, such as CHCl₃, CH₂Cl₂, THF, DMF, DMSO, and adds interesting properties to phthalocyanines, such as high solubility.

It is well known that tetrasubstituted phthalocyanines are usually obtained as a mixture of four constitutional isomers [9]. This can be considered a positive or negative factor, depending on the purpose of applied phthalocyanines. For using these compounds as materials in thin films, it is even better to have an isomeric mixture that will not crystallize very easily [2]. The presence of bulky 4-tritylphenoxy substituents in the β-position can lead to regioselectivity of substituted phthalocyanines.

However, 4-tritylphenoxy substituents have a structural feature that is usually overlooked and which can affect the thermodynamic, spectral, and electro-optical properties of metal phthalocyanines. This feature of 4-tritylphenoxy substituents is related to the conformational properties of TPPN, which is used as the precursor for the synthesis of phthalocyanines with trityl groups as peripheral substituents.

Obtaining reliable experimental data for the 4-(4-tritylphenoxy)phthalonitrile and their subsequent study is an actual problem due to the lack of any experimental work on the structure of this compound.

This paper describes the first experimental study (gas electron diffraction in combination with mass spectrometry, as well as IR spectroscopy) of the molecular structure and conformational properties of 4-(4-tritylphenoxy)phthalonitrile supplemented by quantum chemical calculations.

2. Results and Discussion

2.1. DFT Calculations

2.1.1. Conformational Analysis

The TPPN molecule has six torsion coordinates, which can give rise to several conformers that differ in the mutual orientation of trityl, phenoxy (PhO), and phthalonitrile (PhN) fragments. The atom numbering is shown in Figure 1.

To determine the structure of the found conformers (Figure 2) and the barriers between them, the potential functions of internal rotation were calculated (B3LYP/cc-pVTZ): Around the C_O1-O (Figure 3), C_t-C_O4 (Figure 4), C_N1-O (Figure 5) bonds, and around C_t-C_t1 bond in the trityl group. The ϕ[C_O2C_O1OC_N1], ϕ[C_O3C_O4C_tC_t1], and ϕ[C_O1OC_N1C_N2] dihedral angles were scanned with a step of 2° or 10°.

The potential function of the internal rotation of the O-PhN group around the C_O1-O bond has two deep minima (Figure 3) corresponding to cis-conformers I and II with ϕ[C_O1OC_N1 C_N2] ≈ 0° (

Table 1. Selected characteristics of 4-(4-tritylphenoxy)phthalonitrile conformers according to DFT/B3LYP/cc-pVTZ.

Characteristics	Cis-Forms		Trans-Forms
	I	II	III	IV
∆Etotal, kcal/mol	0.10	0	0.34	0.27
EHOMO, eV	−6.8	−6.8	−6.9	−6.8
ELUMO, eV	−2.4	−2.4	−2.3	−2.3
ELUMO-EHOMO, eV	4.4	4.4	4.6	4.5
μ, D	8.2	8.1	10.1	10.0
Dipole moment direction in I and III conformers
CO1OCN1CN2	−4.2	1.4	−176.1	−178.0
CO4CtCt1Ct2	−2.3	−1.4	−6.2	−1.1
CO3CO4CtCt1	67.6	−116.4	69.8	−115.6
CN1OCO1CO2	99.3	−94.8	99.7	−96.5
min r(HPhN···Htrt)	3.74	4.06	3.79	4.15
ΔG°298, kcal/mol	0.12	0.40	0.63	0.0
ΔG°478, kcal/mol	0.28	0.76	0.95	0.0
χi, mol% T = 298 K	30.4	19.2	13.0	37.4
χi, mol% T = 478 K	29.1	17.6	14.3	39.0
S°298 cal/mol K	197.3	196.2	196.4	198.2
Cv298 cal/mol K	114.3	114.2	114.3	114.3

). The potential function of the internal rotation of the O-PhN group for trans-conformers III→IV with torsion angle ϕ[C_O1OC_N1C_N2] ≈ 180° (Table 1) has a similar form. In cis-conformers, the C_O1–O bond eclipses the C_N1-C_N2 bond, whereas in trans-conformers, the dihedral angle C_O1OC_N1C_N2 is approximately 180°. As a result, cis-conformers I and II differ from the corresponding trans-conformers III and IV by the position of the C≡N substituents in the phthalonitrile moiety (Figure 2).

The barrier between cis-conformers I and II and, accordingly, between trans-conformers III and IV is practically the same and amounts to 6.3 kcal/mol. In transition states TS_I→II and TS_III→IV the dihedral angle ϕ[C_O2C_O1OC_N1] between the planes of the PhO and PhN fragments is close to 0° (Figure 2 and Figure 3).

The second way of the transition between conformers I→II and III→IV can be realized by rotating the PhO-PhN fragment around the C_O4-C_t bond. In this case, in the TS_I→II and TS_III→IV transition states, strong steric interaction occurs between PhO and trityl C(Ph)₃ fragments, which leads to a high energy barrier > 16 kcal/mol (Figure 4).

The difference in the two cis-conformers I and II, as well as in the two trans-conformers III and IV, lies in the different orientation of the PhN group relative to trityl fragment, namely, in conformers I and III, the dihedral angle C_O2C_O1OC_N1 should be defined as φ = +99°, and in conformers II and IV φ = −95° (Figure 3, Table 1).

Figure 5 demonstrates that cis→trans transition can be realized by rotation of the PhN fragment around the O-C_N1 bond. The energy barrier between the cis-conformer II and trans-conformer IV is 5.6 kcal/mol and corresponds to TS_II→IV in which the bond angle C_O1-O-C_N1 between the PhO and PhN fragments is 118°, and dihedral angle ϕ[C_O1OC_N1C_N2] is 90°.

Additional calculations were carried out to estimate the barrier of internal rotation of the phenyl ring of the trityl group around the C_t-C_t1 bond. It has been shown that an independent rotation by ~40° of one of the phenyl fragments leads to a strong steric repulsion between the Ph fragments in trityl. As a result, the phenyl rings cannot rotate but perform synchronous torsional vibrations. The amplitude of these vibrations Δϕ depends on the thermal energy RT and is estimated as ±16° at T = 298 K and ±18° at T = 478 K.

2.1.2. The General Structural Motives and Electronic Characteristics of Conformers

Theoretical calculations performed using four DFT functionals lead to consistent results. All variants of the DFT method predict the presence of four C₁ symmetry conformers for the TPPN molecule. For identical molecular forms, the differences in the corresponding internuclear distances do not exceed 0.002 Å, in the values of bond angles 1°, and less than 7° in the values of torsion angles (Table S1).

All conformers have a similar structure of (Ph)₃C(PhO) fragment. The central site C(C)₄ is a distorted tetrahedron with identical C-C bonds, two C-C-C bond angles that are smaller than the tetrahedral one, and four C-C-C bond angles larger than the tetrahedral one (Table S1). The bond lengths and bond angles in this fragment are the same in all conformers. Differences are observed in the values of torsion angles of cis- and trans-conformers as well as bond lengths in the PhN fragment. In addition, in conformers I and III, the shortest distance H···H between the hydrogen atoms of the trityl group and the phthalonitrile PhN fragment is ~3.7 Å and in conformers II and IV ~4.1 Å.

Table 1 shows the selected characteristics of conformers of 4-(4-tritylphenoxy)phthalonitrile molecule according to DFT/B3LYP/cc-pVTZ (Table S2 shows the characteristics of the conformers calculated with the functionals B97D, CAM-B3LYP, PBE).

All conformers have close energies of frontier orbitals and large dipole moments (Table 1), however, the direction of dipole moments in cis-conformers differs significantly from their direction in trans-conformers.

Figure 6 shows the frontier MOs of conformer II. At the electron transition from HOMO to LUMO, the electron density is transferred from the trityl fragment to phthalonitrile PhN. The frontier orbitals of other conformers have a similar form.

Table 1 lists the relative energies of conformers ∆E_total, relative Gibbs free energies ΔG°_T, and mole fractions χ_i of conformers in saturated vapor calculated for T = 298 K and T = 478 K (the latter corresponds to the temperature of the GED experiment). The difference in the ratio of ∆E_total and ΔG°_T for the four conformers is due to the entropy factor.

The conformers have close electronic energies. The difference in total energy ΔE between conformers I-IV does not exceed 0.4 kcal/mol. This increases the probability that the 4-(4-tritylphenoxy)phthalonitrile may contain different conformations in the unit crystal cell and have a complex conformational composition of the gas phase (Table 1 and Table S2). Since the barriers between conformers are quite high and exceed 5 kcal/mol, conformational transitions are unlikely even at the temperature of GED experiment (the thermal energy RT is less than 1 kcal/mol). Thus, conformational diversity may arise during the synthesis of a compound.

In this case, the mixture of conformers should be characteristic of both the condensed and gas phases. We tried to find the validity of this assumption when interpreting the IR spectra and GED data.

2.2. Vibrational Spectrum

The infrared spectrum for the films of TPPN is shown in Figure 7, along with the calculated spectrum for a mixture of cis- and trans-conformers.

The harmonic vibrational frequencies were calculated for the fully optimized geometries of I–IV conformers at B3LYP/cc-pVTZ level. Analysis of the calculated IR spectra showed that the spectra of the two conformers I and II (cis) and two conformers III and IV (trans) almost coincide (Figure S1).

In

Table 2. Experimental (ν_exp) and theoretical (ω_theor) vibrational frequencies for cis- and trans-conformers of 4-(4-tritylphenoxy)phthalonitrile ^1,2,3.

[4]	νexp, cm−1	ωtheor 2, cm−1 cis	ωtheor, cm−1 trans	Distribution Energy of Normal Vibrations over Internal Coordinates
	3084 w 4	3101	3101	ν(Ct-H)
3051 (C-H)Ar	3057 m	3073	3073	ν(C-H)t, ν(C-H)PhO
	3028 w	2986	2986	ν(C-H)t
2231 (C≡N)	2233 m	2256 m	2256 m	ν(C≡N)
1593 (C=C)	1593 s	1595 s	1597 s	ν(C-C)PhN, ν(C-C)PhO
1566	1566 w	1561 vw	1558 m	ν(C-C)PhN, δ(CCH)PhN
	1505 sh	1505 m	1505 m	ν(C-C)PhO, δ(CCH)PhO
1487	1491 s	1492 s	1488 s	ν(C-C)PhN, δ(CCH)PhN, ν(C-O)
1442	1442 m	1448 w		ν(C-C)PhN, δ(CCH)PhN
	1419 w		1420 m	ν(C-C)PhN, δ(CCH)PhN
	1311 w	1310 s		ν(C-C)PhN, ν(C-O), δ(CCC)PhN
1288	1282 m	1279 m	1283 s	ν(C-O), ν(C-C)PhN, δ(CCH)PhN
1253 (Ar-O-Ar)	1250 s	1243 s	1248 s	ν(C-O), ν(C-C)PhN, ν(CN-C),δ(CCH)PhN
1213	1213 m	1208 m	1211 m	ν(C-O), ν(C-C)PhN, δ(CCC)PhO,δ(CCH)t
1174	1176 m	1172 m	1174 m	ν(C-O), ν(C-C), δ(CCH)PhO, δ(CCH)PhN
1082	1086 w	1087 w	1092 w	ν(CN-C), δ(CCH) ν(C-O), ν(C-C)PhN, δ(CCC)
	1033 w		1036 vw	ν(C-C)t
1014	1014 w	1017 w	1017 w	ν(C-C)PhO, δ(CCC) PhO, δ(CCH) PhO
956	954 w	953 w	954 w	ρ(C-H)PhO, δ(CCC)
885	890 w	892 w	892 w	ρ(C-H)PhO, τ(CCCC)PhO
825	835 m	854 w	853 w	ρ(CN-C), ρ(C-H)PhO, ρ(O-C), ρ(C-H)PhN
750	752 s	752 w	752 w	ρ(C-H)t, ρ(C-C) t, τ(CCCC)t
700	704 s	709 m	708 m	ρ(C-H)t, δ(CCC)PhN, ρ(C-C)t, τ(CCCC)PhO
632	632 w	632 w	635 w	δ(CCC)C4, δ(CCC)t, ρ(CCN)
	522 w	537 w	536 w	ρ(C-C)PhN, lb(CPhNCNN)

¹ see Figure 1 for atom numbering. PhO—phenoxy fragment, PhN—phthalonitrile fragment, t—trityl fragment. ² scaled values of ω_theor are given. Scale is found as the average value of the ratio ν_exp/ω_calc. The scale value is 0.965 for frequencies in the range of 3500–1650 cm⁻¹ and 0.978 for frequencies in the range of 1650–500 cm⁻¹. ³ relative intensities: w—weak, vw—very weak, s—strong, m—medium. ⁴ internal coordinates: ν(X-Y)—bond stretching, δ(XYZ)—deformation of XYZ valence angle, ρ(XY)—out of plane motion of X-Y bond, lb(XYZ)—bending of XYZ linear fragment, τ(XYVZ)—torsion about Y-V bond_.

, the experimental vibrational frequencies ν_exp of TPPN recorded in this work and in [5] are compared with the calculated frequencies ω_theor, which have the highest intensity in the corresponding spectral region for cis- and trans-conformers (Figure 7).

The values of the experimental frequencies ν_exp in the two works are in good agreement with each other. However, in Ref. [5], low-intensity bands at 1419, 1311, and 1033 cm⁻¹ were not noted, and an erroneous assignment of the frequency at 1253 cm⁻¹ to the C-O-C bending vibration was given.

The correctness of the assignment of frequencies was checked on the basis of the dependencies ω_theor = f(ν_exp), which had a linear character with a correlation coefficient almost equal to 1 (Figure 8).

For all frequencies of normal vibrations, the distribution of potential energy (DPE) over internal vibrational coordinates is calculated. For this, the VibModule [10] program was used. The last column of Table 2 shows the natural coordinates that make the maximum contributions to an individual normal vibration.

The relative intensities are indicated for the vibrational bands. The assignment for all 168 vibrational frequencies of the four conformers of the TPPN molecule is given in Table S3.

It should be noted that most bands in the IR spectrum possess a mixed nature (Table 2). For this reason, it is possible to discuss only the predominant contribution of various internal coordinates to the normal vibration. The exceptions are the stretching vibrations of C-H bonds, which are completely separated from other vibrations, and the stretching vibrations of C≡N bonds.

The bands in the range 1593–1491 cm⁻¹ correspond to the vibrations of ν(C-C)_PhN, δ(CCC)_PhN, δ(CCH)_PhN in the phthalonitrile fragment. The bands at 1282, 1250, 1213, and 1176 cm⁻¹ can be assigned to ν(C-O)_PhN, ν(C-O)_Ph stretching vibrations in combination with ν(C-C), δ(CCH) modes. Note that in [1,5], the band at 1250 cm⁻¹ is assigned to the Ph-O-Ph bending vibration instead of the C-O stretching vibration (although the force constant stretching of C-O bond (5.38 mdn/Å) is significantly higher than the force constant deformation of the C-O-C angle (1.20 mdn/Å)).

The frequencies associated with bending vibrations of the central site C(C)₄, phthalonitrile PhN, phenoxy PhO fragments, and the trityl group C(Ph)₃, according to calculations, are in the range below 1000 cm⁻¹. The low frequency region of the IR spectrum shows the presence of bands at 835, 752, 704, 632, 522 cm⁻¹ associated with out-of-plane vibrations of the C-H, C_PhN-C_N bonds.

The main differences between the calculated IR spectra of the cis- and trans-conformers are in the intensity of the band at 1566 cm⁻¹ as well as the presence of characteristic bands at 1311 cm⁻¹, 1442 cm⁻¹ belonging to cis-conformers, and 1033 cm⁻¹, 1419 cm⁻¹ belonging to trans-conformers (Table 2).

The bands at 1442 cm⁻¹ (cis) and 1419 cm⁻¹ (trans) correspond mainly to the stretching vibrations of the C-C bonds in the phthalonitrile fragment. It should be noted that the r(C_N3-C_N4)_PhN bond length in cis-conformers is shorter than in trans-conformers (

Table 3. Experimental and theoretical parameters of cis- and trans-conformers.

Parameter 1,2	cis-Conformer I		trans-Conformer IV
	DFT	GED	DFT	GED
Ct-CO4	1.554	1.542(6) 4	1.553	1.542(6)
CO4-CO3	1.402	1.409(3)	1.402	1.409(3)
O-CO1	1.396	1.392(14)	1.395	1.391(14)
C≡N	1.152	1.148(7)	1.152	1.148(7)
CN3-C	1.429	1.424(3)	1.429	1.425(3)
C t2-H	1.078	1.101(6)	1.078	1.102(6)
CO4CtCt1	105.1	103.7(2.3) 4	104.9	103.6(2.3)
CO3CO4Ct	118.7	118.3(0.9)	118.7	118.2(0.9)
CO2CO3CO4	121.9	121.8(0.8)	121.9	121.8(0.8)
CN1OCO1	120.3	120.3(4.6)	120.3	120.4(4.6)
Ct3Ct2H	118.6	fixed	118.6	fixed
Ct1CtCO4Ct1′	121.2	120.5(2.2) 4	121.1	120.5(2.2)
CO3CO4CtCt1	−53.5	−57.2(3.0)	−54.1	−56.6(3.0)
CN1OCO1CO2	99.3	82.3(27.3)	−96.5	−83.9(27.3)
Distorted tetrahedron CC4
C-C 3	1.554	1.543(6)	1.554	1.543(6)
C-C-C 3 (4)	111.6	111.3(2.5)	111.7	111.4(2.5)
C-C-C 3 (2)	105.3	105.8(3.3)	105.1	105.6(3.3)
Parameters of PhN fragment
CN2-CN3	1.395	1.402(3)	1.389	1.396(3)
CN3-CN4	1.407	1.414(3)	1.411	1.418(3)
CN4-CN5	1.400	1.407(3)	1.395	1.402(3)
CN5-CN6	1.379	1.386(3)	1.385	1.392(3)
CN1CN2CN3	119.8	119.8(0.8)	120.3	120.2(0.8)
CN5CN6CN1	120.1	120.1(0.8)	119.6	119.6(0.8)
Parameters of Ph fragment of trityl group
C-C 3	1.391	1.397(3)	1.391	1.397(3)
C-C-C 3	120.0	120.0(1.2)	120.0	120.0(1.2)

¹ See Figure 1 for atom numbering. ² distances in Å, bond angles in deg, ³ averaged values ⁴ Error limit given in parentheses was estimated as σ = ((0.002 σ)² + (2.5 σ_LS)²)^1/2 for distances and σ = 3 σ_LS for angles, where σ_LS is the standard deviation from LS refinement.

), which is reflected in the difference in the ν(C-C)_PhN frequency.

An analysis of the recorded vibrational frequencies allows us to conclude that the IR spectrum indicates the presence of the cis- and trans-conformers of TPPN in the condensed state.

2.3. GED Structural Analysis

Geometry of each conformer of the TPPN molecule possessed C₁ symmetry and was described by 79 independent parameters. To reduce the correlation, the number of independent parameters was restricted to 14: C_t-C_O4, C_O4-C_O3, O-C_O1, C_N3-C, C-N, C_t2-H bond distances, C_O4C_tC_t1, C_O3C_O4C_t, C_O2C_O3C_O4, C_N1OC_O1, C_t3C_t2H valence angles, and C_O3C_O4C_tC_t1, C_N1OC_O1C_O2, and C_t1C_tC_O4C_t1′ dihedral angles (Figure 1). The remaining 65 independent parameters were linked to the above-mentioned 14 parameters by the difference between non-equivalent structural parameters of the same type obtained from B3LYP/cc-pVTZ calculations.

VibModule program [10] was applied to calculate the vibrational corrections Δr = r_h1 − r_a and the starting values of root-mean-square vibrational amplitudes of each conformer at the temperature of the GED experiment using the harmonic approximation and also taking into account the non-linear interrelation between internal and Cartesian vibrational coordinates. Starting values of the geometrical parameters and vibrational amplitudes were taken from B3LYP/cc-pVTZ calculations. The dependent parameters were determined in terms of the geometrically consistent r_h1-structure. The amplitudes were refined in groups corresponding to the different peaks on the radial distribution curve. The analysis of the electron diffraction intensities was carried out using the modified KCED-35 program [11].

Theory predicts that despite the different mutual orientations of the phthalonitrile and trithyl groups, the corresponding bond lengths and most nonbonded distances of conformers I–IV are very close. This indicates that the electron diffraction intensities may be insensitive to the conformational composition of this compound.

The calculated Gibbs free energies at the temperature of the GED experiment 478 K indicate that the ratio between the mole fractions of the cis- and trans-conformers in the gas phase is close to 1 (Table 1). In addition, DFT calculations predict that the cis I and trans IV forms predominate in vapor over the cis II and tarns III forms.

To assess the ability of the GED method to identify TPPN conformers I-IV, the theoretical functions sM(s) were calculated separately for each conformer based on structural parameters (bond lengths, angles, vibrational amplitudes, and shrinkage corrections) obtained using B3LYP/cc-pVTZ level (Figure S2), as well as theoretical radial distribution functions f(r) (Figure S3). Only a small difference in the f(r) functions is observed in the range of 3–8 Å for the cis- and trans-conformers.

The least-squares analysis of GED experimental intensities was performed under the assumption that the vapor contains separate conformers, as well as under the assumption that the vapor contains cis- and trans-conformers in mole percent of 30/70, 50/50, and 70/30. In these versions of the least squares analysis, the value of the disagreement factor R_f varied in the range of 4.13–4.28%. In accordance with the Hamilton criterion at 0.05 significance level [12], GED data do not allow giving preference to any variant of the vapor composition.

Despite the lack of success in the experimental solution of the conformational problem of TPPN, the GED method still allows us to obtain reliable structural parameters of the molecule. Table 3 shows that the calculated geometric parameters of equilibrium configurations of cis- and trans-conformers agree satisfactorily with the experimental ones.

The comparison of the experimental functions sM(s) and f(r) with the theoretical curves for the mole percent of 50/50 cis/trans vapor model are shown in Figure 9 and Figure 10, respectively.

The performed analysis of IR spectra, data, and the results of quantum chemical calculations indicates the existence of cis- and trans-conformers of TPPN, both in the condensed and in the gaseous state, and GED/MS data do not contradict this conclusion.

Figure 11 shows examples of positions of 4-tritylphenoxy substituents that can occur in tetrasubstituted metal phthalocyanines (MPc) during their synthesis by reaction of cis- (a) or trans-conformers (b) 4-(4-tritylphenoxy)phthalonitrile with metal salts. There are significant differences in the structure of the MPc conformers (a and b), which can lead to a difference in their physicochemical properties. Thus, for tetra-cis-substituted MPcs, there is a high probability of manifestation of regioselectivity, which reduces the steric repulsion between adjacent bulky substituents (Figure 11a). In the case of tetra-trans-substituted MPcs, regioisomers are possible. In addition, the possibility of the formation of tetrasubstituted MPcs with different cis- and trans-orientations of 4-tritylphenoxy substituents within the same metal complex cannot be ruled out.

A series of copper (4-tritylphenoxy)phthalocyanines was synthesized in [4]. The authors note that the synthesized compounds exhibit mesomorphic properties and suggest that their mesomorphism originates from thermal fluctuations due to the free rotation of the bulky substituents. However, our data on barriers of internal rotations (Figure 3, Figure 4 and Figure 5) show that the assertion of the authors of [4] about the free rotation of phenoxy and trityl groups is untenable. Nevertheless, it seems that the copper(II) phenoxy(phthalocyaninato) complex with trans-oriented 4-tritylphenoxy substituents has no serious steric obstacles to the formation of a columnar mesophase (Figure 11b).

3. Methods and Materials

3.1. Synthesis

Synthesis of TPPN was carried out according to the scheme used in [7,13], which is closely repeated in [4] and is described in detail in Supplementary Materials (Section S1).

The product was characterized by mass spectrometry (LDI–TOF, negative mode), ¹H NMR (CDCl₃), ¹³C NMR (CDCl₃), FTIR spectrum, and elemental analysis methods. The results are given in Supplementary Materials Section S1.

3.2. The Infrared Spectrum

IR spectrum was recorded for the films deposited from a solution on the KRS-5 slides in the range of 400–4000 cm⁻¹ using an Avatar 360 FTIR spectrometer. (Details in Section 2.2).

3.3. Synchronous Gas Electron Diffraction and Mass Spectrometric Experiment

The gas phase electron diffraction patterns and mass spectra were recorded simultaneously using the technique described in Refs. [14,15] at two nozzle-to-plate distances (338 and 598 mm). The conditions of the synchronous gas-phase electron diffraction/mass spectrometric (GED/MS) experiments are shown in Table S4.

The electron wavelength was obtained by polycrystalline ZnO. The optical densities were measured by a computer-controlled modified MD-100 microdensitometer [16] with a step of 0.1 mm along the diagonal of the plate. A 10 × 130 mm region was scanned. The number of equidistant scan lines was 33. The total intensity curves were obtained in the ranges s = 1.3–16.7 Å⁻¹ and s = 3.0–28.6 Å⁻¹.

Simultaneously with the registration of electron diffraction patterns, the mass spectra of TPPN vapors were recorded. According to the mass spectrometric data, only TPPN is present in the vapor, no volatile impurities are observed.

Table 4. Most intensive ions in mass spectrum of TPPN recorded during the GED experiment at 478 K (U_ioniz = 50 V).

Ion	M/E	Relative Abundance, %
[M]+ = [C33H22N2O]+, parent ion	462	52
[M-C3H3-C2H2]+	397	41
[M-C6H5]+	385	100
[C(C6H5)3(C6H4)]+	319	40
[M-(C6H5)2]+	308	43
[C(C6H5)3]+	243	65
[M-(C6H5)3]+	231	44
[M-(C(C6H5)3)]+	219	40
[M-(C6H5)3-CN]+	205	38
[M-(C(C6H5)3)-CN]+	193	40
[OC6H4CC6H5]+	181	35
[C(C6H5)2]+	166	63
[(C4H4)2CC3H3]+	155	42

shows that the mass spectra of electron ionization (U_ioniz = 50 V) contain signals of the parent ion [M]⁺ ([C₃₃H₂₂N₂O]⁺) and lighter ions formed as a result of dissociative ionization of molecules: Ions formed upon breaking C_t-C_t1 bonds (see Figure 1 for atom numbering), with the loss of one [M-(C₆H₅)]⁺, two [M-(C₆H₅)₂]⁺ and three [[M-(C₆H₅)₃]⁺ phenyl groups of the trityl fragment, and additionally the loss of a substituent CN [M-(C₆H₅)₃-CN]⁺, ions with loss of trityl fragment [M-C(C₆H₅)₃]⁺ and group CN [M-C(C₆H₅)₃-CN]⁺, trityl ion [C(C₆H₅)₃]⁺, as well as the high intensive [C(C₆H₅)₂]⁺ ion. In addition to ions associated with the detachment of individual groups, the mass spectrum contains ions related to the dissociative ionization of trityl phenyl fragments with the elimination of C₂H_n and C₃H_n groups ([M-C₃H₃-C₂H₂]⁺ and [(C₄H₄)₂CC₃H₃]⁺. The most intense is the ion with stoichiometry [M-C₆H₅]⁺.

3.4. Computational Details

Quantum-chemical modeling of the TPPN molecule was implemented using the Gaussian 09 program [17]. The calculations were performed using the density functional theory (DFT) method, variants of hybrid functionals: B3LYP [18,19,20], PBE [21], CAM-B3LYP [22], as well as the B97D functional [23], which considers dispersion interactions. The atoms of carbon, oxygen, nitrogen, and hydrogen were described using correlation-consistent three-exponential valence bases cc-pVTZ [24].

Using the DFT/B3LYP/cc-pVTZ method, the potential energy surface (PES) was scanned along torsion coordinates, and the presence of four conformers was established. For each conformer of the TPPN molecule, the geometry optimization was performed (see Section 2.1) without symmetry restrictions. The calculation of the frequencies confirmed the correspondence of the optimized structures to the PES minima, and the calculated frequencies were used in the interpretation of the experimental IR spectrum.

4. Conclusions

The TPPN compound is thermally stable up to at least 200 °C, as evidenced by the data of the GED/MS experiment. This property of TPPN can be used in modeling CVD processes for obtaining thin films for various purposes.

The presence of bulky 4-tritylphenoxy substituents in the β-position can lead to regioselectivity of substituted phthalocyanines.

Using the DFT method with B3LYP, PBE, CAM-B3LYP, and B97D functionals, it was shown that the molecule TPPN has four conformers with the same structure of the -Ph-C(Ph)₃ fragment and different positions of the -O-PhN group. All conformers have a significant dipole moment and similar energies of frontier orbitals. At the HOMO → LUMO transition, the electron density is transferred from the trityl to the phthalonitrile fragment.

The structure of transition states between conformers is determined. The close values of the total energy of the conformers and high transition barriers suggest that the conformational diversity of TPPN is formed during the synthesis of the compound.

All vibrational bands in the experimental IR spectrum of TPPN have been assigned. A slight difference was noted in the theoretical IR spectra of the conformers, which, however, made it possible to determine that TPPN consists of a mixture of conformers in the condensed state.

The elemental composition of the ions registered in the mass spectrum of electron ionization has been established in saturated vapor TPPN.

GED study of saturated vapor TPPN was performed. The experimental geometrical parameters of the conformers are found, which agree with the calculated ones within the experimental error. Joint analysis of GED and QC data indicates that at T = 478 K, the gas phase of TPPN consists of a mixture of conformers.

It can be assumed that the use of TPPN for the synthesis of tetrasubstituted metal phthalocyanines will lead to the formation of conformers that differ significantly in the spatial structure of peripheral fragments and retain the conformational features of TPPN.

Moreover, within the same complex, 4-tritylphenoxy substituents can be in different conformations. This feature of such complexes can be used for practical purposes.