Theoretical studies on geometrical properties and photochromic mechanism of two photochromic compounds

Journal of Molecular Structure: THEOCHEM 806 (2007) 197–203 www.elsevier.com/locate/theochem

Theoretical studies on geometrical properties and photochromic mechanism of two photochromic compounds Dong-Ling Wu, Lang Liu, Guang-Fei Liu, Dian-Zeng Jia

*

Institute of Applied Chemistry, University of Xinjiang, Urumqi 830046, PR China Received 8 August 2006; received in revised form 17 October 2006; accepted 20 November 2006 Available online 1 December 2006

Abstract Two organic photochromic compounds containing pyrazolone-ring as photochromic functional unit (1-phenyl-3-methyl-4-benzal-5pyrazolone thiosemicarbazone and 1-phenyl-3-methyl-4-benzal-5-pyrazolone 4-methylthioesmicarbazone) are investigated to deepen our understanding of geometrical properties and the mechanism of photoinduced intermolecular proton transfer by using DFT method. Bader’s atom-in-molecule (AIM) theory is applied to investigate the nature of various hydrogen bonds and their relative strength. Good correlation between hydrogen bond length and electron density at the bond critical point is obtained. The oxygen atom of the pyrazolone ring and the sulfur atom in the thiosemicarbazone part are sites of the most negative concentration of the electrostatic potential and it is assumed that the oxygen atom should be the preferred site to accept the proton. Ó 2006 Elsevier B.V. All rights reserved. Keywords: DFT; Proton transfer; Hydrogen bond; AIM

1. Introduction Hydrogen bond (HB) as a relatively weak interaction has been renewed interest due to its great role in biological phenomena, chemical reactions and crystal engineering, etc. Information on HB is useful to understand physical and chemical properties of various molecules [1,2]. Among those HB systems, proton transfer from a hydroxyl or amino proton to an acceptor as an important reaction plays a crucial role in many biological and photochemical processes [3–5], which not only occurs in solution and in solid phase [6] but also proceeds in the excited states [4,7–9]. Many thiosemicarbazones derived from 4-acyl pyrazolone have been systemically studied in our laboratory [9]. It is found that HB is strong enough to influence the arrangement of molecules in crystals and even the molecular geometry. It is also found that the molecular packing manner in the lattice and the nature of HB mainly influence

*

Corresponding author. Tel.: +86 991 8580032; fax: +86 991 8581006. E-mail address: [email protected] (D.-Z. Jia).

0166-1280/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2006.11.027

proton transfer mechanism [9,10]. These findings make the knowledge of molecular geometry and HB of great importance. Furthermore, we have known that not all the synthesized compounds exhibit photochromism. Thus, modification of the molecules containing pyrazolone gives the possibilities of synthesizing novel compounds with photochromic properties and probing the relationship between geometry and photochromic properties. In order to gain a deeper insight into the relationship between geometry and photochromic properties, two compounds (1-phenyl-3-methyl-4-benzal-5-pyrazolone thiosemicarbazone (PMBP-TSC) and 1-phenyl-3-methyl-4-benzal-5-pyrazolone 4-methylthioesmicarbazone (PMBP-MTSC)) have been studied. Synthesis, structures and photochromic properties have been done previously and it is proposed that the photochromic mechanism is an intermolecular proton transfer (Inter-PT) [9a,9b]. In this paper, apart from investigating the geometry and reactive properties concerned with Inter-PT process, the nature of various HBs has also been investigated by using Bader’s Atoms in Molecules (AIM) theory [11]. The calculated results, merging from natural bond

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orbital (NBO) atomic charges, dipole moment, electrostatic potential and topological analyses offer satisfactory basis for discussing the Inter-PT mechanism. 2. Computational details The DFT calculations with Becke3-Lee-Yang-Parr (B3LYP) exchange-correlation functional [12] and 6-31G* basis sets are carried out by using the Gaussian03 program package [13]. This level of theory has been justified in our previous work [9c]. The structures are all confirmed as true local minima on the potential energy surface by the presence of only real harmonic vibrational frequencies after the corresponding vibrational analysis. Topological analysis has been performed with the AIM2000 program [14]. Molecular electrostatic potentials are obtained after Gaussian calculation and visualized with the GopenMol program [15]. 3. Results and discussion 3.1. Molecular geometry The proposed photochromic mechanism of the compounds is presented in Fig. 1. The calculated parameters are in agreement with that of X-ray diffraction data. However, both compounds predicted longer N2–C9 distance as ˚, compared to experiment by about 0.056 and 0.054 A respectively. Although calculated N2–C9 distance is reliable theoretically [16], taking into account the calculated model only involving in a single molecule, the effect of packing constraints should be considered. On the other hand, the crystal structure may be not pure keto form due to the possibility of solid-state N–H  O/O–H  N tautomeric equilibrium [18]. Therefore, the experimental N2–C9 distance is in the range of related derivatives [9d,9e,17]. In order to study HB interactions, the structure of dimer for PMBP-TSC is also obtained. Geometrical difference of the monomers in the dimer has been found, viz., thiosemicarbazone moiety is located at the upper and lower position of the plane of the pyrazolone ring, respectively (Fig. 2). However, the calculated results show that the geometry parameters, the molecular energy and the NBO

Fig. 1. The structures and proposed photochromic mechanism of the compounds.

Fig. 2. The structure of dimer for PMBP-TSC.

atomic charges of the monomers are almost identical. Geometrical difference and calculated results of PMBP-MTSC are similar to that of PMBP-TSC. The geometrical difference could arise from the effects of steric hindrance within the molecule mainly due to the presence of 4-acyl substitution group of pyrazolone-ring and from packing constraints. The asymmetric distribution of the monomers in the lattice results in various HB modes, and it would be expected to increase the strength of intermolecular HB (Inter-HB). It is notable that the O1 atom is not only involved in an intramolecular HB (Intra-HB) (N4–H  O), but also takes part in dimerization in an intermolecular HB (N2–H  O). This HB pattern (Fig. 3), i.e. bifurcated HB [19] is observed from the two crystal structures. The formation of the bifurcated HBs induces changes in the geometry of molecules in comparison with that of isolated molecules not involved in the Inter-HB. The changes are probably the greatest for the Intra-HB, N4–H  O. For the dimers, two N4–H  O bonds are different (Table 1). For PMBP-TSC, there is only ˚ and one N4–H  O interaction, the bond length is 2.006 A the charge density is 0.0244 au; for PMBP-MTSC, the ˚ /0.0269 au parameters of N4–H  O bonds are 1.962 A ˚ /0.0141 au, respectively. However, the N4– and 2.282 A H  O strength are almost same for the corresponding

Fig. 3. Different HB patterns of PMBP-TSC.

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Table 1 Geometrical and topological parameters (in au) of the existed HBs

PMBP-TSC (N2–H  O) N4–H  O N5–H  N3 (N5–H  O) C5–H  O (C18–H  S) (N4–H  S) (N5–H  S) PMBP-MTSC (N2–H  O) N4–H  O N5–H  N3 C5–H  O (C19–H  S) (N5–H  S) (C18–H  S) (C12–H  S)

D–H

H  A

D  A

D–H  A

qDH

,2q

qHA

,2q

0.80(2) [1.026] 0.93(2) [1.020] 0.88(3) [1.013] 1.04(4) 0.97(3) [1.082] 0.90(6) [1.097] [1.017] [1.022]

1.86(2) [1.890] 2.45(2) [2.006] 2.33(3) [2.223] 2.53(4) 2.47(3) [2.438] 2.84(5) [2.975] [3.172] [2.456]

2.63(2) [2.916] 3.01(2) [2.876] 2.65(3) [2.638] 3.47(3) 2.95(2) [2.992] 3.70(3) [3.849] [3.944] [3.447]

164(2) [178.9] 118.7(2) [141.6] 101(2) [102.7] 150(3) 110(2) [110.4] 160(3) [136.8] [133.8] [163.0]

0.3177

3.7460

0.0271

0.0958

0.3245

3.8107

0.0244

0.0813

0.3271 0.3272

1.6537 1.6564

0.0192 0.0191

0.0838 0.0826

0.2823 0.2833 0.2688

1.0283 1.0424 0.9056

0.0123 0.0155 0.0067

0.0460 0.0562 0.0212

0.3273 0.3162

1.664 1.591

0.0044 0.0177

0.0143 0.04686

0.86 [1.031] 0.86 [1.023] 0.86 [1.014] 0.93 [1.082] 0.96 [1.091] [1.017] [1.098] [1.084]

1.89 [1.846] 2.41 [2.282] 2.21 [2.247] 2.43 [2.417] 2.70 [3.224] [3.088] [2.893] [3.165]

2.70(2) [2.870] 2.86(2) [2.846] 2.61(2) [2.671] 2.92(2) [2.987] 3.09(3) [3.985] [3.864] [3.985] [4.107]

157 [171.6] 113 [132.9] 108 [107.0] 113 [111.5] 104 [127.6] [134.0] [172.3] [145.8]

0.3122

1.5679

0.0319

0.1033

0.3278 0.3190 0.3276 0.3251 0.2537 0.2821 0.2779

1.6595 1.6096 1.6479 1.6425 1.0456 1.0243 0.9704

0.0141 0.0269 0.0228 0.0182 0.0172 0.0128 0.0049

0.0523 0.0869 0.0877 0.0762 0.0600 0.0474 0.0146

0.3251 0.2682 0.2817

1.6425 0.9062 1.0211

0.0051 0.0080 0.0050

0.0168 0.0232 0.0143

The intermolecular HBs are in parentheses. Geometrical parameters of the calculated results are in square brackets.

isolated molecules and stronger than that of the dimers. ˚ /0.0282 au for PMBP-TSC and 1.902 A ˚ /0.0303 au (1.928 A for PMBP-MTSC, respectively.) From the viewpoint of valence bond theory, the interaction between the lone pair of the acceptor N atom and the O–H r* anti-bonding orbital is mainly responsible for the proton transfer from the O to N atom and vice versa. The N  H  O angle and N  H/O  H distance or HB strength play important roles in the proton transfer reaction [20]. Therefore, information on HB is useful to understand the Inter-PT mechanism. Besides IR and NMR measurements, crystal structure determination often provides the strongest experimental evidence for testing the existence and the strength of HB [21]. Here, analysis of HBs is performed with the program PLATON [22]. Possible HB parameters of both experimental and calculated results are summarized in Table 1 and the HBs of TSC are presented in Fig. 3. For strong and medium HBs, H  A distance is usually much less than the corresponding sum of van der Waals radii. The D (donor)– H  A (acceptor) with an angle much further from linearity seems to be the weaker bonding. If we assume that the H  A distance and the D–H  A angle are approximation of HB strength, the probable order of the HB strength is as follows: N2–H  O > N4–H  O > N5–H  N3 > N5–H  O  C5–H  O > C18–H  S for PMBP-TSC and N2– H  O > N4–H  O > N5–H  N3  C5–H  O > C19– H  S for PMBP-MTSC, respectively. The intermolecular

˚ , respectively) N2–H  O HBs (2.63(2) and 2.70(2) A between neighboring molecules should be the strongest HBs, which may facilitate the proton transfer reaction. In addition to the expected HB interactions, two types of unconventional HBs (C–H  O and C–H  S) are observed, which are able to give the extra stabilization for the two dimers. The existence of such complicated HB interactions turns our attention to the possibilities of further investigations with the use of the AIM theory. 3.2. AIM theory applied for the analysis of HB There are different geometrical criteria to detect HBs as well as to describe their strengths. However, it seems that the results of crystal structure which provide information on HB are not sufficient to get insight into the nature of the interaction [23]. Other experimental and theoretical techniques should be applied to study HBs. One of them is AIM theory [11], which has been successfully used nowadays for the description of different chemical bonds and interatomic interactions including HB, chemical bonding in hypervalent and electron deficient polyhedral compounds, etc. [24–26]. In the AIM theory, the nature of bonding between atoms can be characterized by the value of the electron density q(r) and the sign of the Laplacian of the electron density ,2q(r) at the bond critical points (BCP). That is, a large q(r) value together with a large negative ,2q(r) value represents a shared interaction,

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characteristic of covalent bonding; a low q(r) value along with a positive ,2q(r) value indicates a closed-shell interaction typically found in ionic bonding, H-bonding and van der Waals interactions. The existence of HBs in the compounds is confirmed by the presence of a corresponding bond path in the electron density, and their properties are characterized in terms of q(r) at the associated BCPs. Electron densities and their Laplacians (in au) at BCPs are collected in Table 1. It shows that all H  A bonds have low q(r) (ranging from 0.004 to 0.030 au) and positive ,2q(r) values (ranging from 0.01 to 0.10 au), which are in agreement with the values reported by Koch and Popelier [27], where the ranges of q(r) and ,2q(r) values are 0.002–0.034 au and 0.024– 0.139 au, respectively, if HBs exist. Here, H  A bond represents a shared interaction. In contrast, D–H bonds have large q(r) and negative ,2q(r) values, which indicate closed-shell interactions. PMBP-TSC has N5–H  O HB experimentally and the ˚ . However, the AIM analysis for this kind length is 2.53 A of bond reveals that no BCP exists. The reason is that the calculated system is only involved in two molecules. It should be noted that in the two compounds, in addition to the experimentally existed HBs, BCPs also exist for other atoms and they are all weak HBs, such as the N–H  S HB.

The HB energy (EHB) is one of the most important characteristics of HB systems, and known as descriptor of HB strength. The intermolecular EHB is usually calculated as a difference between the energy of the complex and the energy of isolated molecules. The intramolecular EHB strongly depends on the choice of the reference state, and it often cannot be obtained properly [28,29]. However, for the compounds in this paper, there are various HBs including intra-HB and inter-HB, which makes the EHB even hard to obtain. On the other hand, the linear or exponential correlations between EHB and HB distance/topological parameters have been found in previous studies. In this part, the relationship between HB length and topological parameter q(r) is established to confirm that both HB length and q(r) at H  A BCPs relate to the strength of the HB for the studied system, that is, both of them can be applied to estimate the HB strength. The correlations between HB length and q(r) at H  A BCPs are shown in Figs. 4a and 5a, respectively. The correlation is inverse, that is, an increase in bond length corresponds to a decrease in the q(r), which is expected, since increase in distance results in reduced orbital overlap and hence lower q(r) along the bond. From Fig. 4a, it can be seen that N5–H  S bond deviate the linearity. Its bond ˚ ) approaches to C5–H  O (2.438 A ˚ ) bond length (2.456 A

Fig. 4. The correlations between electron density at the BCP and the HB distance for the dimer of PMBP-TSC.

0.035

0.035

0.030

0.030

0.025

0.025

0.020

0.020

0.015

0.015

0.010

0.010

0.005

0.005 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

-0.4 -0.2 0.0

0.2

0.4 0.6

0.8

1.0

Fig. 5. The correlations between electron density at the BCP and the HB distance for the dimer of PMBP-MTSC.

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but its q(r) is larger than that of the latter. The deviation is reasonable because N5–H  S bond is a conventional HB. The linear correlation coefficients for PMBP-TSC and PMBP-MTSC amount to 0.964 and 0.947, respectively. In order to allow us to compare contacts with different accept (A) centers, the difference between the sum of H and A van der waals radii (The van der waals radii are those introduced by Bondi.) [30] and H  A distance Eq. (1) is taken into account. The linear correlation coefficients (Figs. 4b and 5b) amount to 0.989 vdw DrHA ¼ Rvdw H þ RA  r HA

ð1Þ

and 0.970, respectively. It is obvious that the application of Eq. (1) results in much better linear correlation between the HB length and q(r). From Figs. 4b and 5b, it is seen that the H  A distance of conventional HBs is usually less than the corresponding sum of van der Waals radii and the stronger HBs correspond to the greater DrH  A; while the H  A distance of unconventional week HBs is either less than or more than the corresponding sum of van der Waals radii. Furthermore, in both cases, the quadratic correlation (The fitted curve corresponds to the equation q(r) = A + B1 (rH  A/DrH  A) + B2 (rH  A/DrH  A)2.) is statistically better than the linear ones. Their correlation coefficients R2 are 0.952/0.992 for PMBP-TSC and 0.984/ 0.994 for PMBP-MTSC, respectively. The order of HB strength based on the AIM results is as follows: N2–H  O > N4–H  O > N5–H  N3 > N5– H  S  C5–H  O > C18–H  S > N4–H  S for PMBPTSC and N2–H  O > N4–H  O > N5–H  N3 > C5–H  O > C–H  S  N–H  S for PMBP-MTSC, respectively. Good correlation between q(r) and HB length indicates that the topological parameter can describe the HB strength of the title compounds and their derivatives. 3.3. Keto–enol isomerization Based on the proposed photochromic mechanism, the enol form of the title compounds is optimized by using

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the same theoretical level. We have also attached, for comparison purpose, the experimental parameters of HPMBP [31] and DP4FBP-PSC [9f], which exist in enol forms in the crystal states. The results indicate that geometrical parameters of the two enol forms are similar and also show sufficient coincidence with the experimental results of the compared derivatives. The C7–O1 bond is coincident with ˚ (PMBPthe single bond with bond distance of 1.339 A ˚ TSC) and 1.338 A (PMBP-MTSC), respectively, slightly ˚ ). The difference longer than that of DP4FBP-PSC (1.330 A is mainly attributed to the ignorance of the intermolecular interactions. The presence of 4-acyl substitution group of the three compounds results in the notable deviations from the values of the HPMBP. The difference of the geometrical parameters between the studied enol forms and the DP4fBP-PSC are rather small. We emphasize charges of correlative and adjacent atoms in keto–enol isomerization. NBO atomic charge analysis shows that the atomic charge of the H atom (on N2 atom) increases along the process from 0.425 (PMBP-TSC)/0.424 (PMBP-MTSC) for keto form to 0.524/0.525 for enol form. The negative charge of the N2 atom is 0.410 (PMBPTSC)/0.412 (PMBP-MTSC) for keto and 0.304/ 0.305 for enol, which decreases along the process. That of O7 is 0.643 (PMBP-TSC)/0.642 (PMBP-MTSC) for keto, 0.692/0.696 for enol, which shows that the negative charges of these O atoms increase along the process. The C7 atomic charge (from 0.648 (PMBP-TSC)/0.647 (PMBP-MTSC) for keto to 0.557/0.559 for enol) shows the decreased net charges along the process. The results show that electrons are redistributed after the formation of enol form. In the gas phase the enol form is more stable than the keto form by about 0.75 and 1.06 kcal/mol, respectively. On the basis of the calculated bond ellipticity (e) (Table 2) which is considered as a quantitative measure of the bond p-character [32,33], the p-delocalization in the pyrazolone-ring is estimated. The lowering of the e (N2–C9) values (0.0743 and 0.0767 for keto forms, respectively) is in

Table 2 Topological properties (in au) of the electron density and bond ellipticity (e) of the pyrazolone-ring for enol form and keto form, respectively ,2q

q N1–N2 N2–C9 C8–C9 C7–C8 N1–C7

0.3457/0.3331 0.3560/0.3087 0.2883/0.3229 0.3077/0.2801 0.3266/0.2955

e

0.6158/0.6174 0.8699/0.8221 0.8071/0.8764 0.8116/0.6962 0.9364/0.8520

[0.3455/0.3329] [0.3552/0.3085] [0.2917/0.3237] [0.3087/0.2792] [0.3260/0.2954]

[0.6532/0.6184] [1.1069/0.8165] [0.6666/0.8822] [0.9842/0.6936] [1.0992/0.8437]

0.1098/0.1730 0.2357/0.0743 0.2079/0.3200 0.3319/0.1920 0.3282/0.1444

[0.1434/0.1745] [0.2556/0.0767] [0.5274/0.3270] [0.3879/0.1908] [0.3005/0.1385]

(The results of PMBP-MTSC are in square brackets.)

Table 3 Dipole moment date of the two molecules (unit: Debye) PMBP-TSC

Enol Keto

PMBP-MTSC

XX

YY

ZZ

Total

XX

YY

ZZ

Total

3.1331 0.0932

3.3214 9.1708

1.5646 2.6750

4.8266 9.5535

3.0247 1.2464

3.1621 8.6065

1.7453 2.8531

4.7110 9.1523

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agreement with the N–C single bond of the keto forms. Other values which are greater than zero is indicative of the absence of pure single and pure double bonds in the pyrazolone-ring. Besides the e, the q(r) of these bonds show that the delocalization in the pyrazolone-ring of the enol form is greater than that of the keto form. This feature is another indication of structural instability of the keto form. Why did the crystal structures exist as instable keto forms rather than enol forms? It can be explained from two sides. Table 3 gives the calculated dipole moment. It shows that the dipole moment date of keto form is greater than that of enol form. The different dipole moment indicates that probably in polar media the difference between tautomers would be smaller or even that the keto form would become the most stable [34]. On the other hand, the higher polarity indicates the stronger intermolecular interaction resulting in more compact molecular packing. Thus the keto form is likely to be the favored form as it is involved in the crystallization processes. In addition to this reason, the crystals cannot totally avoid the sunlight when crystallization should also be taken into consideration. Because according to experimental studies [9a,9b], colorless crystalline samples change to yellow after irradiation. With the evaporation of the solvent without avoiding the sunlight, we obtained the yellowish transparent crystal, which implies that the compounds have already existed as keto forms in the crystal state. Furthermore, a quantitative description of the compounds in the isomerization process is provided by the theoretically estimated electrostatic potential. The 3D electrostatic potential contour maps of keto forms of PMBP-TSC (Fig. 6) and PMBP-MTSC (Fig. 7) show

Fig. 7. The 3D electrostatic potential contour map of keto form for PMBP-MTSC.

the most negative regions associated with O (V(r)min = 0.075 au for PMBP-TSC, 0.076 au for PMBP-MTSC) and S (V(r)min = 0.079/0.084 au for PMBP-TSC,0.080/0.085 au for PMBP-MTSC). While other negative regions associated with N3, N5 and N4 with values range from 0.03 to 0.04 au. Much weaker values (Vmin > 0.02 au) are found above and below the two aromatic rings, and the N1, N2 atoms. Thus, we infer that O and S should be the favored sites for the proton acceptor in the proton transfer reactions. Nevertheless, S atom is involved in a weaker HB interaction while O atom forms the strongest HB interaction, which implies that it is O rather than S atom accepts the proton easily in the proton transfer reaction. 4. Conclusions

Fig. 6. The 3D electrostatic potential contour map of keto form for PMBP-TSC.

Different types of HB of the two compounds have been investigated. For experimentally existed HB, the order of strength arranged by HB length is almost identical to that from the AIM results. Furthermore, additional inter- and intramolecular HBs have been found based on AIM calculation. Good correlation between electron density and HB length has been established, which indicates that the electron density at H  A BCPs can well describe the HB strength of the title compounds and their derivatives. The conjectural enol forms in the keto–enol tautomeric process are obtained. Enol forms are favored over the keto forms in the gas phase, but the existed crystal structures are the keto forms. The reasons are the dipole moments of keto forms are greater than that of the enol forms and the crystals cannot totally avoid the sunlight in the process of crystallization.

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