Phthalocyanines (Pcs) have been widely used as organic dyestuffs since their first synthesis early last century because of their intense absorption of light in the visible and ultra-violet regions and their high chemical and thermal stabilities [1,2]. They have also attracted considerable interest due to their applications in modern science and technology on the basis of their special electric and photoelectric properties: semiconductivity, photoconductivity and luminescence .
The benzo rings of Pc compounds can be substituted by heterocycles such as thiophene, thionaphthalene, pyridine, or pyrazine, thus forming heterocyclic Pc analogues. The aza-Analogues of the Phthalocyanines (aza-Pcs) have been extensively studied over the past three decades. Potential applications include their use as textile bleaching agents, photoinactivators for controlling growth of microorganisms, catalysts for oxygen reduction, materials for eletrochromic displays, and media for optical data storage with large memory capacity, inhibitor of thermal degradation of polymers and photosensitizers for photodynamic therapy of cancer [4-9].
The synthesis of heterocyclic Pc analogues was initially reported in 1937 by Linstead and co-workers [10,11]. A significant contribution to the development of the aza-Pcs chemistry was made by Galpern and Lukyanets  who described the synthesis and properties of a series of aza-Pcs. Previously, Density Functional Theory (DFT) methods have been extensively employed investigating Pcs and their derivatives by Zhang et al [13-18]. We have also approached the molecular, electronic structures and vibrational spectra for metal-free (TPdPzH2), N,N-Dideuterio (TPdPzD2) and Magnesium (TPdPzMg) tetra-2,3-pyridino-porphyrazines and metal-free (TPyPzH2), N,N-Dideuterio (TPyPzD2) and Magnesium (TPyPzMg) tetra-2,3-pyrazino-porphyrazines [19,20].
Moreover, theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of transition metal phthalocyanines and naphthalocyanine have been performed using the density functional theory, and ideal results have been achieved [21,23]. All the previous research works demonstrated that the density functional B3LYP method performs well in the calculation for phthalocyanines and their analogues, thus it should also be appropriate for the tetra-3,4-pyridino-porphyrazine complexes.
Computer simulation as an attractive alternative to experiment can provide valuable information. So, to get more insights into the molecular and electronic structures, it is worthwhile to conduct quantum chemistry calculations. In the present paper, we report theoretical calculations of molecular, electronic structures of metal-free Tetra-3,4-Pyridino-Porphyrazine (TPdPzH2*), its N,N-Dideuterio Derivative (TPdPzD2*) and Magnesium complex (TPdPzMg*). The substitution effect of the N atoms and the isotopic effect of D atoms on these properties of these compounds were discussed. Some interesting and meaningful results were obtained.
The structures of the tetra-3,4-pyridino-porphyrazine are shown in Figure 1. The 6-31G(d) basis set was used at the density functional B3LYP level for geometry optimization and calculations. The default Mulliken method was used for atomic charge calculation. The Berny algorithm using redundant internal coordinates was employed in energy minimization and tight convergence criteria were used throughout . C4h symmetry for TPdPzMg* and C2h for TPdPzH2* and TPdPzD2* in the input structures were detected and then enforced by the program. Using the energy-minimized structures generated in the previous step, normal coordinate analyses were carried out. All calculations were carried out using Gaussian09 program.
Results and Discussion
At density functional B3LYP level, the energy-minimized structure of TPdPzMg* calculated has C4h symmetry, and both TPdPzH2* and TPdPzD2* have C2h symmetry. No imaginary vibration is predicted in the frequency calculations, indicating that the energy-minimized structures are true energy minima. Corresponding structural parameters are illustrated in Figure 1. Whats interesting is that N,N-Dideuterio-Derivative (TPdPzD2*) cannot change the structural parameters in metal-free-derivative (TPdPzH2*), thus only the bond lengths and bond angles of TPdPzH2* herein are discussed. Considering the substitution effect of the N atoms on the molecular structure, except that N-H bond lengths maintain six significant digits to compare, the other structure parameters retain four.
As shown in Figure 1, the N-H bond length is 1.01421 Å for TPdPzH2*, which is slightly longer than that of TPdPzH2 (1.01418 Å), and longer than that of H2Pc (1.01399 Å) . These indicate that the N-H bond strengths of the three molecules vary with the sequentially decreasing order of H2Pc>TPdPzH2>TPdPzH2*, which also implies that the acidity of the inner N-H bonds of the three molecules sequentially increase with the trend of H2Pc<TPdPzH2<TPdPzH2*. On the other hand, the electron-withdrawing effect of the annulated pyridine ring increases the acidity of the inner N-H bonds, which corresponds to the experimental result .
Analysis of the structure parameters of TPdPzMg* shows that the average size of the central hole (Nc-Nc distance) is 4.021 Å, which is larger than those of MgPc (4.017 Å), but smaller than that of TPdPzMg (4.024 Å) . This indicates that the size of the central hole increases with the order of MgPc<TPdPzMg*< TPdPzMg, which implies that the farther the heterocyclic N atoms in the benzo rings from 16-membered ring are, the smaller they influence the size of the central hole. That is to say, the size of 16-membered ring in TPdPzH2* is nearer to that of H2Pc.
Electronic structures and molecular orbitals
The energies of the molecular orbitals from HOMO-5 to LUMO+3 of TPdPzH2* and TPdPzMg* are comparatively listed in Table 1 and shown in Figure 2. Herein, HOMO and LUMO represent the highest occupied molecular orbital and the lowest unoccupied molecular orbital, respectively. Figure 3 shows the scheme of HOMO and LUMO of TPdPzH2* and TPdPzMg*. It is worth noting that N, N-dideuterio-derivative (TPdPzD2*) cannot change the molecular orbital energy levels in metal-free-derivative (TPdPzH2*). In this paper, the energies of TPdPzH2* are only discussed.
There are two characteristic absorption bands (the Q band and Soret or B band) in UV-visible spectra of porphyrazines and their analogues with fused heterocycles. The Q band in the visible region originates from the HOMO→LUMO electronic transition . In practical application, people pay attention to Q band. Due to the extension of π-conjugation, the HOMO-LUMO gaps of TPdPzH2* and TPdPzMg* (2.146 and 2.165eV) are smaller than those of H2Pz and MgPz (2.627 and 2.651 eV), indicating that the Q bands of TPdPzH2* and TPdPzMg* shift to a longer wavelength side than those of H2Pz and MgPz . According to Linstead, some Pz derivatives show Q band peaks at 535-617 nm, while the TPdPz* compounds at 574-672 nm [27,28]. These data agree very well with the above-mentioned calculation results.
The HOMO-LUMO gaps of TPdPzH2* and TPdPzMg* (2.146 and 2.165 eV) are found to be a bit larger than those of H2Pc and MgPc (2.049 and 2.157 eV), and smaller than those of TPdPzH2 and TPdPzMg (2.266 and 2.285 eV) [13,19]. This straightens out the hypsochromic shift of the Q band in the electronic spectra of these aza phthalocyanine analogues, on the other hand, interprets electronic spectra in the visible region of TPdPz* compounds are identical to the spectra of the corresponding Pcs due to the HOMO-LUMO gaps of them are near each other [27,29]. According to Galpern, the substitution of the CH groups in the Pc benzo rings adjacent to the porphyrazine macrocycle with N atoms causes a remarkable hypsochromic shift in the visible part of the electronic absorption spectrum. The magnitude of this shift increases from tetra-aza-Pcs (20-25 nm, 550-780 cm-1) to the octa-aza derivatives (40-60 nm, 870-1260 cm-1) depending on the nature of the central metal atom [30,31].
The energy levels of the HOMO-5 to LUMO+3 orbitals for TPdPz* compounds all decrease compared with those of the corresponding Pc and TPdPz compounds. Moreover, the magnitude of all the energy levels is in the order of TPdPz*<TPdPz< Pc, which indicates that the farther the heterocyclic N atoms in the benzo rings from the 16-menmbered ring are, the lower the orbital energy level decrease.
Compared with H2Pc and MgPc, the reduction of HOMO of TPdPzH2* and TPdPzMg* are 0.698 and 0.693 eV, while the reduction of LUMO are 0.601 and 0.685 eV, respectively, and the decrease of HOMO are slightly larger than those of LUMO . This is due to the presence of the p-deficient annulated pyridine rings in the TPdPz* unit as compared with the isoelectronic benzene rings in the Pc unit. The lowering of HOMO for TPdPzH2* and TPdPzMg* compared with those of TPdPzH2 and TPdPzMg are 0.238 and 0.238 eV, which are smaller than those of LUMO, 0.358 and 0.358 eV, respectively . The possible reason is that the heterocyclic N atoms on TPdPz* compounds are farther from the central 16-membered ring than those on TPdPz compounds, and they affect the energy levels of the LUMO orbitals are larger than those of the HOMO ones. This interprets the HOMO-LUMO gaps of TPdPzH2* and TPdPzMg* are smaller than those of TPdPzH2 and TPdPzMg.
Both phthalocyanine and aza-Pcs compounds contain basic porphyrazine structural units and additional p-conjugated rings. This sequential increase in the number of p-conjugated aromatic rings gives rise to increased polarization of basic porphyrazine ring, which causes the change of p-electron energy. The atomic charge of the core Pz fragment and the external N atoms for TPdPzH2* and TPdPzMg* compounds are comparatively listed in Table 2. It is worth noting that N,N-dideuterio-derivative (TPdPzD2*) cannot change the atomic charge in metal-free-derivative (TPdPzH2*), thus the atomic charge of TPdPzD2* herein is not discussed. In order to show the difference between TPdPzH2* and TPdPzH2, the significant digits of the atomic charges are maintained three except that four significant digits retained by Hc. The atom symbols for TPdPzH2* and TPdPzMg* are shown in Figure 4. Compared with the TPdPz compounds, the charges of the central Nc for the TPdPz* compounds become less negative, while the meso-N atoms (Nm) become more negative, which means that the charges of these atoms for TPdPz* compounds are nearer to those of Pc compounds .
In addition, all the other charges of the corresponding atoms in the core Pz fragment for TPdPz* compounds are nearer to those of Pc compounds. This implies that the farther the heterocyclic N atoms in the benzo rings from the central 16-membered ring are, the smaller they influence the charges of the atoms on the core Pz fragment. That is to say, the atom charges of the core Pz fragment in TPdPz* compounds are nearer to those of Pc, which basically agree with the experimental electronic spectra in the visible region of TPdPz* compounds being identical to the spectra of the corresponding Pcs .
The atomic charge of the central Hc (0.4355 e) of TPdPzH2* is larger than that of TPdPzH2 (0.4348 e) and H2Pc (0.4345 e), which is consistent with the acidity of the inner N-H bonds of the three molecules (H2Pc<TPdPzH2<TPdPzH2*) mentioned above in the molecular structure. In addition, the atom charge of the magnesium (0.894 e) is larger than that of TPdPzMg (0.888 e), which is larger than that of MgPc (0.876 e).
According to our calculation, we found that the farther the heterocyclic N atoms in the benzo rings from 16-membered ring are, the smaller it influence the size of the central hole, the bond lengths and bond angles of 16–membered ring, the HOMO-LUMO gaps, and the charges of the atoms on the core Pz fragment. That is to say, TPdPz* compounds are nearer to Pc compounds than TPdPz in structure, energies of the molecular orbitals, and atomic charges. The acidity of the inner N-H bonds of the three molecules sequentially increase with the order of H2Pc<TPdPzH2<TPdPzH2*.
The authors thank the National Natural Science Foundation of China (Grant No. 20501011).
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Zhongqiang Liu, Associate Professor, Research Center of Theoretical and Computational Chemistry, School of Chemistry and Chemical Engineering, Qilu Normal University, Jinan 250200, PR China, Tel: +86 531 66778025, E-mail: email@example.com
Liu Z and Zhang X. Density functional calculations on the molecular, electronic structures of metal-free, N,N-dideuterio and magnesium tetra-3,4-pyridino-porphyrazines (2019) Edelweiss Chem Sci J 2: 40-44.
Tetra-3,4-pyridino-porphyrazine, DFT method, Molecular structure, Electronic structure, Phthalocyanines.