Quantum-Chemical Study of Electronic Properties of Model Spirooxazines

Abstract

Model spirooxazines were studied via ab initio quantum-chemical calculations. An anomeric interaction between nitrogen lone pair and antibonding orbital of the C_spiro-O bond has been considered by Hartree-Fock SCF level of theory with 3-21G basis set. Extent of the interaction has been shown to depend on planarity of the nitrogen atom, being maximized with its planarity growth. Electronic properties and geometry parameters of model spirooxazines were considered by limited CI under 3-21G basis set. Excitation lengthens the C_spiro-O bond and causes a significant polarization of total wave function, which results in increasing of the system dipole moment and transfering the electronic density from the O2 atom and the phenyl fragment onto the N13=C14 bond in the case of spirooxazine 2 and onto the NO₂ group in the case of nitro-spirooxazine 3. Elongation of the C-O bond under excitation is due to the interaction in spirooxazines. CI calculation of the excited singlet state of a model chromene 4 doesn't show elongation of this bond.

1. Introduction

Spiropyrans (SP) and spirooxazines (SO) are typical organic compounds exhibiting photochromism[1]. Upon ultraviolet irradiation the C_spiro-O bond is broken to form the X-isomer which is then being isomerized into a merocyanine form (Scheme 1).

Scheme 1.

An electronic structure and geometry of SP and structural mechanism of the photochromism have been extensively studied by various authors (see review [2]). Recently photochromism of SO has been of the increased interest. Compared to SP, they show better fatique resistance [3] which provides a possibility of their practical application. Thus, detailed study of a mechanism of the bond cleavage and isomerization of the open form of SO is an important task.

Spectroscopic studies of spiropyrans show that the both first excited singlet and triplet can be involved in the conversion mechanism [4,5]. In SP with the NO₂-containing benzopyran moiety the bond rupture reaction occurs mainly via the triplet state, while in the NO₂-free SP the photoreaction occurs via the singlet state.

This conversion has been extensively studied by various spectroscopic techniques [6,7] and different quantum-chemical calculations [8,9]. A femtosecond transient absorption spectroscopy study [6] of the spirooxazine revealed that the merocyanine is formed only from the first excited singlet.

Another interesting feature of SP and SO is an anomeric interaction between the nitrogen lone pair and an antibonding orbital of the C_spiro-O bond. There are many evidences [2,10] that this interaction facilitates the ring opening reaction. The extent of this interaction depends on the planarity of the nitrogen atom and is maximum when nitrogen lone pair is collinear to the C_spiro-O bond. This interaction results in shortening of the N-C bond and the corresponding elongation of the C_spiro-O bond. For example, a length of the C-O bond in six-membered oxygen-containing heterocycles is equal to 1.41-1.43A, while in 3,3-diphenylpyrano[5,6-f]quinoline it is 1.458A [11], and it is essentially elongated to 1.460-1.496A in SP [2,10]. Thus, our objective was, using quantum-chemical calculations, to check whether this experimentally found elongation is due to the interaction, and to study the role of this interaction in the geometry of the excited state.

2. Computational technique

All calculations were carried out non-empirically by GAMESS [12] program with 3-21G basis set on DEC Alpha 3000/600 computer. To reveal anomeric interaction, constraint geometry optimization of 1 has been used, with a value of the dihedral angle W being changed step by step, while all the other coordinates were allowed to vary subject to energy minimization. The dihedral angles W were used for controlling the N3 atom planarity. Optimized geometries of 1 are summarized in Tables 1 and 2 with constraint parameter W1(C1-N3-C4-C5) and W2(H7-N3-C4-C5), respectively. To check out planarity of N3 in each table we used a sum of the N3 atom valence angles, (N2)={C1-N3-C4 + C4-N3-H7 + C1-N3-H7}, and a dihedral angle W_pyr(C4-N3-C6-C1), which determines a degree of the pyran cycle bending.

The ground state (S₀), the first (S₁) and the second (S₂) excited singlet states of 2, 3 and 4 were computed under Configuration Interaction (CI) with 3-21G basis set and frozen core approximation applied. Active space was constructed from 3 double occupied MO's, two independent MO's with alpha and beta spin, and 3 virtual molecular orbitals. Only the first order excitations were considered. Thus we had 1764 configuration state functions (CSF) included in each CI calculation. Because of the program limitations only partial geometry optimizations were carried out, all the rest parameters were kept from full geometry optimizations under single determinant Hartree-Fock SCF calculation with the same basis set.

3. Results and Discussion

3.1. Anomeric interaction

We have studied the efficiency of the anomeric interaction between the nitrogen lone pair and the antibonding orbital of the C_spiro-O bond by varying the planarity of the N3 atom. We suppose that an increase of the nitrogen planarity causes a rise of its lone pair energy and a decrease of an energy gap between the n(N3) and anti-bonding(C1-O2) orbitals, making their interaction stronger. In case of the anomeric interaction this will be followed by shortening of C1-N3 and lengthening of C1-O2 bonds. Another behaviour of the length of these bonds depending the N3 planarity would disprove our supposition. To study the problem we used a model spirooxazine 1.

1

According to W_pyr and (N2) values from Tables 1 and 2, planarity of N3 increases from left to right. The C1-N3 and C1-O2 bonds are shortened and elongated, respectively. These results confirm that an increase of the N3 planarity leads to the increase of the anomeric interaction.

Table 1. Constraint optimization of 1 with fixed values of the W₁ parameter.

Parameter	W1=159.0°	W1=164.0°	W1=169.2°*	W1=174.0°
Wpyr(C4-N3-C6-C1),°	155.1	158.5	162.5	166.8
(N3),°	353.4	355.2	357.0	358.5
C1-O2	1.4542	1.4554	1.4563	1.4566
C1-N3	1.4457	1.4442	1.4430	1.4420

* - equilibrium geometry

Table 2. Constraint optimization of 1 with fixed values of the W₂ parameter.

Parameter	W2=14.0°	W2=8.7°*	W2=4.0°
Wpyr(C4-N3-C6-C1),°	161.3	162.5	163.6
(N3),°	354.0	357.0	358.3
C1-O2	1.4560	1.4563	1.4566
C1-N3	1.4453	1.4430	1.4414

* - equilibrium geometry

It can be supposed that changes of the anomeric interaction we have found are due rather to changes of the mutual orientation of the lone pair and anti-bondind orbital of the C-O bond than to the planarity changes of the N3 atom. Actually, it is very difficult to determine theoretically orientation of a normal vector of the nitrogen lone pair in the asymmetric environment to check out this hypothesis. Nevertheless, we suppose this effect to be insignificant. A simple example demonstrates how planarization raises an energy of the lone pair. Table 3 shows results of semiempirical calculation of the NH₃ molecule with C_3V symmetry. In the planar structure of the NH₃ molecule HOMO (nLP-N-H = 90°) is a pure nitrogen lone pair. The HOMO energy has a linear dependence on the nitrogen atom planarity. This proves that the N3 lone pair energy in 1 changes with changing of planarity of this atom.

1- nLP - normal vector of nitrogen lone pair;
2- atomic orbital coefficients in HOMO;
3- equillibrium geometry.

3.2. Properties of excited singlet states

Stucture of photochromic compounds has been extensively studied by X-ray diffraction experiments [2]. Less is known about the nature of their excited states.

In this section an influence of interaction on the length of C_spiro-O bond in the excited state of model spirooxazines 2 and 3 has been considered.

2	3

To establish the nature of the lowest excited singlet states and to reproduce their spins correctly CI was imposed. Results of these calculations are shown in Tables 4 and 5. To fit in our limited computer resources we had to use limited CI with 3-21G basis set. Thus the results presented are assumed to be qualitative.

Two first excited singlet states of 2 and 3 were computed. We assume that limited CI may not correctly reproduce a sequence of the excited states, which doesn't allow us to discuss a difference between the first and second excited states. So we are focusing on determining a photochemically active excited singlet state (PAES) that leads to the opened merocyanine form. To determine it we relied on assumption that excitation should weaken the C_spiro-O bond which will be broken later in the way of transforming the molecule into a merocyanine form. The states in the Tables 4 and 5, marked by asterisk, confirm this criterion. Nevertheless we list both excited singlet states of 2 and 3 in the Tables 4 and 5, respectively, to let a reader make his own choice.

*expected photochemically active excited singlet state

*expected photochemically active excited singlet state

From the total 1764 considered CSF's only limited number of the configuration states have large weighting factors. Tables 4 and 5 list only first three CSF's with the greatest coefficients. It is well known from spectroscopic experiments [14] that in spiropyrans electronic density transfers from an oxygen atom onto vacant molecular orbitals under photoexcitation and formation of PAES. Assuming close similarity in behaviour of spiropyrans and spirooxazines we should examine those MO's of ground states of 2 and 3 where the oxygen atom has significant atomic orbital coefficients. From our calculations HOMO doesn't have considerable coefficients on the oxygen atom, all such orbitals lie lower, they are HOMO-1, HOMO-3, for example, and so on. This explaines why HOMO doesn't play a significant role in the formation of the PAES and testifies for a multiconfigurational nature of the PAES, because excitation proceeds from deep MO's of 2 and 3. Population of those MO's included in CI are given in the Tables 4 and 5; they show that HOMO-1 (CSF 5) plays the most important role in formation of the both S₂ of 2 and S₁ of 3.

Generally, single configuration excited singlet states is constructed only from HOMO-LUMO excitations, but as follows from our calculations the studied systems (2 and 3) and probably spirooxazines at all are exclusions from this very simple rule. In other words one can state an important role of electronic correlation for correct prediction of electronic properties and geometries of spirooxazines.

Optimized geometries of 2 and 3 are given in Table 6. Excitation results in significant geometry changes. As will be shown later this is due to essential polarisation of the total wave function and charge redistribution. The most important geometry changes are related to the C1, O2, N13, C14 atoms and phenyl fragment.

The C1-O2 bond is shortened in S₁ and elongated in S₂ state of 2 (Table 6). We suppose the S₂ state is the PAES for 2 where cleavage of the C1-O2 bond takes place. We assume that the more precise CI can reorder the states and bring this S₂ to be the first excited singlet state for 2.

It is interesting to note that excitation lenghtens N13=C14 bond both in 2 and 3, populating its vacant antibonding orbital. Later we will show that electronic density is transfering on this bond in the excited state. Elongation of the C1-O2 and N13=C14 bonds and shortening of the C1-N3 one take place in S₁ state of 3 and here this effect is more evident than that of S₂.

Significant elongation of the C7-C12 and C9-C10 bonds in S₁ of 3 and S₂ of 2 shows an important role of the phenyl fragment in polarisation of total wave function under excitation. The maximum elongation of these bonds takes place in S₁ of 3 and S₂ of 2 again, that confirms special nature of these states.

Electronic densities and charge redistribution in S₀, S₁ and S₂ of 2 and 3 are shown in Table 7. They demonstrate that electronic density leaves the O2 atom in S₂(2) and S₁(3). The N13=C14 bond is a single acceptor of electronic density in S₂(2). All the density transfers onto NO₂ group in S₁(3), since this group is a very strong acceptor. The N3 atom doesn't change its electronic population significantly in both molecules. As a non trivial result we have found that phenyl group looses the density under excitation in greater extent than O2 atom in S₂(2) and S₁(3). It is possible because phenyl fragment has six carbon atoms versus one oxygen atom.

From the standpoint of polarization it is interesting to consider the value of dipole moments in excited states (Tables 1 and 2). As one can see excitation increases a value of total dipole moment of the molecules. The maximum dipole moment is for S₂(2) and S₁(3) which we assume to be the PAES. The value of dipole moment of S₁(3) is probably overestimated, although it should be greater than that of S₂(2).

3.3. Chromene

As it was above mentioned the length of C-O bond in spiropyrans and spirooxazines is elongated in their ground state due to anomeric interaction. A model chromene 4 doesn't have such interaction.

4

In this context it would be interesting to compare the length of the C-O bond in the excited state of 2, 3 and 4. Results of CI calculation of 4 are summarized in Tables 8. Two first excited states of 4 were computed.

The length of C1-O2 bond in ground state of 2, 3 and 4 is 1.4530, 1.4629 and 1.4469, respectively (Tables 6,8). Thus the role of anomeric interaction in elongation of the C-O bond in the ground state of 2 and 3 is undoubted. In the excited states S₂(2) and S₁(3) this bond is elongated, compared to the ground state. Shortening of the C-O bond have been found in both excited states S₁ and S₂ of 4 in contrast to spirooxazines, which confirms an assumption about important role of interaction for the value of the C-O bond in an excited state.

4. Conclusions

Model spirooxazines were studied via ab initio quantum-chemical calculations. These results demonstrate that anomeric interaction between nitrogen lone pair and antibonding orbital of C_spiro-O bond plays important role in elongation of the C-O bond both in the ground state and the excited one. It is very important to use CI for correct reproducing of an excited singlet state of spirooxazines because of its multiconfigurational nature. Excitation results in increasing of a total dipole moment of the system. Upon excitation electronic density transfers from O7 atom and phenil fragment onto N13=C14 bond in the case of spirooxazine 2 and onto NO₂ group in the case of nitro-spirooxazine 3.

Acknowledgements

The authors are grateful to Russian Foundation for Basic Research, grant number 96-03-32046, and to International Science Foundation, grant number IFN000 for support of these investigations.

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