A Theoretical Study on the Photochromic Mechanism of 1-Phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one①

LIU An-Jie JIA Dian-Zeng WU Dong-Ling

LIU Lang GUO Ji-Xi

(Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China)

1 INTRODUCTION

Photochromic compounds are of great interest for optical computer, information storage, light-driven information display device, as well as environmental probes in biomolecule, etc[1-3]. Therefore, the synthesis of novel photochromic systems and the study of the photochemical mechanism are of very important significance.

It is well-known that many Schiff base compounds[4-6]show photochromism. For the past several years, our laboratory has synthesized a large number of pyrazolone photochromic compounds by molecular design and structure modification[7-13].Based on the crystal structures of photochromic compounds, it was concluded that photochromic phenomenon was due to the intramolecular (Intra-PT)or intermolecular proton transfer (Inter-PT)associated with a change in π electron configuration[14,15]through hydrogen bonds (H-bonds), and the non-coplanarity of structure is beneficial to photochromic property[16-18]. In addition, the photochromic processes of some of them are reversible and others are not. These phenomena are very interesting and we think that analyzing their photochromic mechanism is helpful for designing new photochromic materials. Because experimental methods are sometimes insufficient for further achievements on the mechanism of photochromism,theoretical studies are required[16-22].

In this article, 1-phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one (PMCPTSC)[23]has been studied. Previous experimental results indicate that the title compound exhibits photochromic properties when irradiated by 365 nm light at room temperature in solution, and this process is not reversible. In addition, the polarity increase of the solvent favors the photochromism.According to the analysis of the title compound’s crystal structure and hydrogen bond connection diagram (Fig. 1), it is proposed that the photochromic mechanism of PMCP-TSC is an Intra-PT from the enol form to the keto form (Fig. 2).Although previous theoretical studies[24]have done some work on analyzing the molecular structure, the nature of H-bond, the stability and the reactivity of the title compounds in different solvents, it is not sufficient to get insight into the photochromic mechanism. So, this paper will further discuss the photochromic property of the title compound through analyzing the molecular structure, absorption spectra, molecular orbital and stability in the gas and in different solvents.

Fig. 1. Different HB patterns of the title compound

Fig. 2. Proposed photochromic mechanism of the title compound

2 CALCULATION METHODS

All calculations were performed with the Gaussian 03W program[25].

The ground state of enol form, transition state and keto form were calculated with B3LYP/6-311+G (2d, p)method[24]. Frequency calculation at the same level characterized the stationary points as local minima or a first-order saddle point on the potential energy surface. TDDFT method was chosen to obtain the absorption spectrum of keto and enol forms. The polarizable continuum model(PCM)[26]of the self-consistent reaction field theory was used to study the solvent effects on the conformers. Atom-in-molecule theory (AIM)[27-29]is applied to investigate the nature of hydrogen bonds and ring structures of the conformers in different solvents. The NBO analysis was performed by means of the NBO 3.1 program[30]within the Gaussian 03W package. Molecular orbitals were also analyzed to testify the rationality of the photochromic mechanism of the title compound.

3 RESULTS AND DISCUSSION

3. 1 Structure analysis

The title compound’s crystal structure and hydrogen bond connection diagram are presented in Fig. 1.The bond length between O(2)and C(13)atoms is 1.262 ?, which is consistent with the C=O bond length[31-33]and indicates PMCP-TSC existing in the keto form. Geometrical difference of the monomers in the dimer has been found. The S atom is located at the upper and lower positions of the plane of the pyrazolone ring, respectively. However, the experimental and calculated results show that the geometry parameters, the energy and the atomic charges of the monomers are almost the same. The geometrical difference could arise from packing constraints. In Fig. 1, two molecules interact with each other via intermolecular hydrogen bonds(O(2)··H–N(3), 2.954 ?, N(4)··H–N(5), 3.006 ?)and two intramolecular hydrogen bonds (O(2)··H–N(5), 2.792 ?)are also observed. The hydrogen bond geometrical parameters are collected in Table 1. The data indicate that the probable order of the hydrogen bond strength is as follows: O(2)··H–N(5)＞ O(2)··H–N(3)＞ N(4)··H–N(5). The intramolecular O(2)··H–N(5)is most likely to participate in the proton transfer reaction. Thus, a hypothesis that photochromic mechanism of the title compound is due to the intramolecular proton transfer was proposed by us.

Table 1. Geometrical Parameters of the Existing Hydrogen Bonds

Experimental results indicate that the title compound undergoes photochromism in solution. In order to further study the photochromic mechanism and the solvent effects on the geometries, the keto and constructed enol forms are both optimized and their structural difference in different solvents including water, methanol, tetrahydrofuran (THF)and carbon tetrachloride (CCl4)are investigated in detail.In this part, atom-in-molecule (AIM)theory and Natural Bond Orbital (NBO)theory have continued to be applied to study the geometries and photochromic mechanism of the title compound in different solvents. In the AIM theory, the nature of H-bond can be characterized by the value of electron density ρ(r), the Laplacian of electron density?2ρ(r)and the ellipticity ε at the H-bond critical point (HBCP); the nature of ring can be described by the value of electron density ρ(r)at the ring critical point (RCP). In the NBO theory, the secondorder perturbation energy E(2)can be used to characterize the strength of H-bond. Geometrical and topological parameters as well as stabilization energy of O–H··N/O··H–N H-bonds are collected in Table 2. It shows that O–H··N is stronger than O··H–N, which indicates that the proton transfer is more likely to occur in enol than in the keto form. It also means that the keto form of the title compound is more stable than the enol form in all solvents.Furthermore, because ellipticity ε of H··Y is in agreement with the trend of E(2), ellipticity ε of H··Y can be used to characterize the strength of H-bond.

Table 2. Geometrical and Topological Parameters (a.u)as well as Stabilization Energy (kcal/mol)of O–H··N/O··H–N H-Bonds

In order to discuss the coplanarity of the title compound, we analyze all ring structures which are of its components quantitatively. The perimeter of the rings and the electron density ρ(r)at the ring critical points for the keto and enol forms in different solvents are calculated and listed in Table 3. For the keto and enol forms of the title compound, six rings (ring I (benzene-ring), ring II (H(22)–C(21)–C(20)–N(6)–C(13)–O(2)), ring III (pyrazo-lonering), ring IV (O(2)–C(13)–C(14)–C(9)– N(5)–H(33)), ring V (C(16)–C(15)–C(14)–C(9)– C(10)–H(12))and ring VI (C(8)–N(4)–N(5)–C(9)– C(10)–S(1)))have been studied. The data show that stable ring structures generally have large ρ(r)values(greater than 0.02). In addition, the correlation (Fig.3)between the perimeter and the electron density ρ(r)for the same ring structure is reverse and good linear correlation (0.9432, 0.9869, 0.9994, 0.9997,0.9975 and 0.9789)indicates that perimeter can characterize the electron density and stability for the same ring structure. In other words, the coplanarity for the same ring structure grows with the decrease of perimeter.

Fig. 3. Correlation between the perimeter and the electron density ρ(r)for different ring structures

Table 3. Perimeter of the Rings and the Electron Density ρ(r)at the Ring Critical Points for the Keto and Enol Forms in Different Solvents

3. 2 Keto-enol isomerization

By TDDFT energy calculation and Swizard program[34], we got the absorption spectra (Fig. 4)of keto and enol forms in methanol. Fig. 4 shows the results are accurate enough compared with the experiment ones (Fig. 5). The calculation results of the maximum absorption wavelength for the enol and keto forms are 333 and 369 nm, respectively,and the experimental value for the keto form is 375 nm. The data indicate that the proposed mechanism from enol to the keto form is reasonable because the latter has longer maximum absorption wavelength than the former.

Fig. 4. Calculation results of the maximum absorption wavelength for enol and keto forms in methanol

Fig. 5. UV-vis spectra in methanol

3. 3 Molecular orbital analysis

A powerful practical model for describing chemical reactivity is the Frontier Molecular Orbital(FMO)theory. The important aspect of the frontier electron theory is the focus on the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). Generally, the frontier orbital theory predicts that a site where the highest occupied orbital is localized is a good nucleophile site. Similarly, where the lowest unoccupied orbital is also localized is a good electrophilic site. With the purpose of explaining the photochromic mechanism,orbital analyses of the keto and constructed enol form in methanol were performed (Fig. 6). For enol form, the calculation results indicate that the HOMO orbital contains N(6)and N(7); LUMO orbital contains the N(6), C(13), C(14)and O(2).They have the same symmetry: A. Because there are three interactions (O(2)··H–N(5), O(2)··H–N(3)and N(4)··H–N(5))after reaction, only O(2)is the possible site for reactivity. In addition, the charge quantity on O(2)decreases after reaction, which indicates O(2)is an electrophilic site and shows once again the photochromic process from enol to the keto form is reasonable. For keto form, the calculation results indicate that the HOMO orbital contains benzene ring; LUMO orbital also contains N(6), C(13), C(14)and O(2). Because O(2)is an electrophilic site, it will be impossible that the H(33)transfers to O(2), along with the breaking of N–H and C=O bonds as well as the formation of O–H and C=N bonds.

Fig. 6. HOMO and LUMO orbitals for the enol and keto forms in methanol

3. 4 Stability analysis

Table 4 and Fig. 7 depict the electronic energies and dipole moments of enol, TS and keto forms in gas and different solvents. The results show that the title compound is more stable in all solvents than in gas, and the stabilities of enol, TS, and keto forms grow with the increase of the solvent polarity. In addition, the stability order of the title compound is keto form ＞ enol form ＞ TS form. The relative energy for enol to TS form is 0.02642 a.u in gas,0.02616 a.u in water, 0.02607 a.u in methanol,

0.02590 a.u in THF and 0.02598 a.u in CCl4,indicating that the solvent will decrease the barrier height of pronton transfer and contribute to the reaction from enol to the keto form. It is worth noting that the relative energy for keto form to the TS form is 0.03676 a.u in gas, 0.04020 a.u in water,0.03994 a.u in methanol, 0.03915 a.u in THF and 0.03786 a.u in CCl4. This means that the solvent will be detrimental to the reversible reaction from keto to the enol form.

Table 4. Electronic Energies (E)(a.u), Dipole Moments (μ), and Relative Energies for Enol and Keto Forms to the TS Form (aΔE and bΔE)(a.u)

Fig. 7. Calculated reaction energy profile

The dipole moments of enol, TS and keto forms increase with growing the solvent polarity. The order of dipole moments of the title compound is as below: keto form ＞ TS form ＞ enol form, which further proves that the title compound exists in the keto form.

4 CONCLUSION

Atom-in-molecule (AIM)theory was used to study HBs and ring structures of the title compound.The computational results show that ellipticity ε of H··Y is in agreement with the trend of E(2), which indicates that ellipticity ε of H··Y can be used to characterize the strength of H-bond. In addition, the data show that good linear correlation between the perimeter and the electron density ρ(r)for the same ring structure has been established.

The calculation results of the maximum absorption wavelength for enol and keto forms indicate that the proposed mechanism from enol to the keto form is reasonable.

The analysis of molecular orbitals further accounts for the probable reactive sites and the photochromic mechanism.

The keto form of the title compound is more stable than the enol form in all appointed solvents,and the stability of enol, TS and keto forms grows with the increase of solvent polarity. In addition, the solvent will contribute to the reaction from enol to the keto form.

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