(E,E,E)-1,6-Bis(2,4-di­chloro­phenyl)­hexa-1,3,5-triene

The title mol­ecule, C₁₈H₁₂Cl₄, lies about an inversion centre and the hexatriene chain is planar. The torsion angle of the single bond between the planes of the chain and the benzene ring is -8.6 (3)°. The dihedral angle between the planes defined by the chains of adjacent mol­ecules is 50.0 (2)°. The shortest intermolecular distance between the Cl atoms is 3.514 (1) Å. The mol­ecules are joined through [pi]-[pi]-stacking and strong attractive Cl...Cl interactions.

S1. Comment

The structure and properties of (E,E,E)-1,6-diphenylhexa-1,3,5-triene (DPH) and its ring-substituted derivatives have been extensively studied. In the field of photophysics and photochemistry, their highly fluorescent properties and Z–E geometrical isomerization in solution have attracted much attention. Due to the extended π-conjugation systems, solid-state DPHs are also attractive in the field of material science because of the potential use as materials for third-order non-linear optics and optical power limiting, and two-photon absorbing chromophores (Rodenberger et al., 1992; Spangler, 1999; Rumi et al., 2000). We report here the synthesis and structure of the title compound, (I), a symmetrically substituted DPH having Cl atoms on the benzene rings. Our data can be compared with those for the (E,E,E)-isomers of DPH and 1,6-bis(2-methoxyphenyl)hexa-1,3,5-triene (Hall et al., 1989), 1,6-bis(4- and 1,6-bis(2-chlorophenyl)-3,4-dimethylhexa-1,3,5-triene (Stam & Riva di Sanseverino, 1966), and all-(E)-isomers of longer diphenylpolyenes (Drenth & Wiebenga, 1954, 1955). It is well known that 2,4-dichloro-substitution on a benzene ring is very effective to induce solid-state [2 + 2]-photocycloadditions of aromatic olefins (Ramamurthy & Venkatesan, 1987). However, as we reported previously (Sonoda et al., 2001), compound (I) is photochemically stable. In this study, the crystal structure analysis of (I) has been performed in order to find an explanation for its photostability in the solid state.

The molecular structure of (I) is shown in Fig. 1. In the hexatriene chain, the torsion angles [C1—C2—C3—C4 = 177.4 (2)°, C2—C1—C1ⁱ—C2ⁱ = 180°, and C1—C1ⁱ—C2ⁱ—C3ⁱ = 179.1 (2)°; symmetry code: (i) 1 − x, 1 − y, 1 − z; Table 1] are almost 180°, indicating that the plane of the triene chain is planar. Distinct bond alternation is observed; although C—C single bonds [C1—C2 = 1.439 (2) Å and C3—C4 = 1.461 (2) Å] are shorter than the standard value of 1.54 Å, they are significantly longer than C═C double bonds [C1—C1ⁱ = 1.340 (3) Å and C2—C3 = 1.335 (2) Å]. For conjugated olefins, it is expected that C═C double bonds are longer than the standard value of 1.34 Å for isolated olefins. In the present case, however, the C═C bond lengths are all similar to the standard value for mono-olefins. The C═C bond lengths of 1.328 (4) and 1.328 (5) Å for DPH are even shorter than the values for (I) (Hall et al., 1989). It is known that X-ray structures of (E)-stilbene derivatives show unusually short lengths of C═C bonds. These are considered to be artifacts caused by dynamic averaging originating from the torsional vibrations of the C—Ph single bonds (Ogawa et al., 1992; Harada & Ogawa, 2001). It is possible that similar thermal vibrations also exist in the crystals of longer polyenes. The relatively short C═C bonds observed for (I) (and DPH) may therefore be due to the vibrational motions around the C—Ph and/or C—C bonds. C—C—C bond angles in the chain are all somewhat larger than 120°, resulting to minimize the steric interaction between H atoms of the chain and the benzene ring. The C2—C3—C4 angle, for example, is 127.3 (2)°.

The benzene rings are also planar. Distances from the best-plane fit for the ring are −0.042 (2), 0.016 (3) and 0.053 (3) Å for atoms Cl1, Cl2 and C3, respectively. The C5—C4—C9 internal angle of 115.8 (1)° is smaller than 120° to accommodate the large exterior angle described above. The structures of the hexatriene moiety and the benzene ring of (I) are very similar to those reported for the unsubstituted and 2-methoxy derivatives of DPH (Hall et al., 1989).

The torsion angle of C2—C3—C4—C5 is −8.6 (3)°, which deviates the most significantly from 0 or 180° in this molecule. The least-squares plane defined by the ring makes an angle of 9.9 (1)° with the least-square plane defined by the hexatriene chain. The twisting around C3—C4 single bond is due to the steric interactions between Cl1 and C3—H atoms, and between C2—H and C5—H atoms. These steric interactions are more important in this case than the complete planar conjugation. It is found that the torsion angle C2—C3—C4—C5, or the dihedral angle for the planes of the ring and the chain, is most sensitive to the substituents introduced on the benzene ring for DPH derivatives. It is reported that the dihedral angle is 1.9° for unsubstituted DPH, and 15.6° for its 2-methoxy derivative (Hall et al., 1989). The molecule of (I) is thus more planar than that of the 2-methoxy derivative.

Fig. 2 shows the crystal structure of (I). The molecules are piled up along the c axis to form parallel plane-to-plane stacks. The angle formed between the planes defined by the chains of adjacent molecules related by the a glide is 50.0 (2)°. The shortest intermolecular distance between the Cl atoms is Cl1···Cl2(x − 1/2, −y + 1/2, z − 1) of 3.514 (1) Å, which is shorter than the van der Waals contact distance of 3.6 Å. Most of the C—Cl···Cl—C distances in aromatic chlorinated compounds lie in the range 3.5–4.2 Å (Ramamurthy & Venkatesan, 1987). The molecules are thus joined through strong attractive Cl···Cl interactions between the adjacent molecules. In addition, it should be noted that the distances between the Cl atoms of the stacking molecules are Cl1···Cl1(x, y, z + 1) of 3.950 (1) Å and Cl1···Cl2(−1/2 + x, 1/2 − y, z) of 4.218 (1) Å.

For the solid-state [2 + 2]-photocycloadditions, it is well known that the distance between the two potentially reactive double bonds should be less than 4.2 Å (Ramamurthy & Venkatesan, 1987). In the crystals of DPH, the distance between parallel double bonds is 7.730 (2) Å (= a) (Hall et al., 1989), which is consistent with its photostability in the microcrystalline state (Sonoda et al., 2001). In the crystals of (I), on the other hand, the distance between double bonds of the neighboring molecules in the stack is 3.950 (1) Å (= c). Thus, it is clear that the attractive Cl···Cl interactions between the stacking molecules strongly reduce the double bond distance in (I) relative to the value for DPH. In spite of the short distance of 3.95 Å, however, the crystalline powder of (I) is photostable (Sonoda et al., 2001). There are several examples of similar exceptions for the 4.2 Å rule in the literature (Murthy et al., 1987). Methyl 4-hydroxy-3-nitrocinnamate, for example, is photostable in the solid state, although the double-bond distance is 3.78 Å. It is likely that the extensive intermolecular hydrogen bonds and C—H···O-type interactions do not permit the easy spatial movement of the double bonds in the lattice for the reaction to proceed. Also in the case of (I), the photostability may possibly be due to the (too) strong attractive Cl···Cl interactions between the adjacent molecules, which prevent any motion of the reactive molecules.

S2. Experimental

The title compound, (I), was prepared by the Wittig reaction of the bis(phosphonium) salt of (E)-1,4-dichloro-2-butene and 2,4-dichlorobenzaldehyde. To a solution of 2,4-dichlorobenzaldehyde (0.63 g, 3.6 mmol) and the salt (1.08 g, 1.7 mmol) in ethanol (10 ml) was added a solution of sodium ethoxide in ethanol (0.30 M, 12 ml). The mixture was stirred under a nitrogen atmosphere at 313 K for 20 h. To the reaction mixture, water (12 ml) was added and the mixture was stirred vigorously. The resulting yellow precipitate was filtered off, washed with water (15 ml), and dried. The crude product (predominantly Z,E,Z) was dissolved in toluene and the solution was refluxed for 24 h with a trace amount of iodine. After cooling, the resulting yellow crystals (E,E,E geometry) were collected and dried at room temperature (yield 52%; m.p. 508 K). Crystals of (I) were grown from a toluene solution. Elemental analysis calculated for C₁₈H₁₂Cl₄: C 58.4, H 3.3, Cl 38.3%; found: C 58.3, H 3.2, Cl 37.7%; HR–MS calculated for C₁₈H₁₂Cl₄: 367.9692, observed 367.9670; IR (KBr): 991 [all-(E) conjugated polyene], 862, 810, 768 cm⁻¹ (1,2,4-trisubstituted benzene ring); ¹H NMR (CDCl₃): δ 7.54 (2H, d, J = 8.6 Hz), 7.38 (2H, d, J = 2.0 Hz), 7.22 (2H, dd, J = 8.6, 2.0 Hz), 6.96 (2H, app. d, J = 15.2 Hz), 6.85 (2H, app. ddd, J = 15.5, 6.3, 3.0 Hz), 6.61 (2H, app. dd, J = 6.3, 3.0 Hz); UV–vis (MeCN): λ_max 367 nm (ε = 55400 ml mol cm⁻¹).

S3. Refinement

Please describe exactly what was done with the H atoms. Located from differene maps but not refined? Were they constrained?

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1993); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN (Molecular Structure Corporation, 1999); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: TEXSAN; software used to prepare material for publication: TEXSAN.

Figures

	Fig. 1. A view of the molecular structure of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry code: (i) 1 − x, 1 − y, 1 − z.]
	Fig. 2. A packing diagram for (I). The broken lines show the shortest Cl···Cl intermolecular interactions of 3.514 (1) Å.

(I)

Crystal data

C18H12Cl4	Dx = 1.506 Mg m^−3
Mr = 370.10	Melting point: 508 K K
Monoclinic, P21/a	Mo Kα radiation, λ = 0.7107 Å
a = 15.163 (4) Å	Cell parameters from 22 reflections
b = 13.639 (3) Å	θ = 16.0–17.6°
c = 3.950 (1) Å	µ = 0.72 mm^−1
β = 92.39 (2)°	T = 278 K
V = 816.2 (4) Å^3	Prism, yellow
Z = 2	0.50 × 0.10 × 0.10 mm
F(000) = 376.0

Data collection

Rigaku AFC-7R diffractometer	θmax = 27.5°
θ/2θ scans	h = −19→0
2083 measured reflections	k = −17→1
1866 independent reflections	l = −5→5
1584 reflections with F^2 > 2σ(F^2)	3 standard reflections every 150 reflections
Rint = 0.010	intensity decay: 0.5%

Refinement

Refinement on F^2	H-atom parameters not refined
R[F^2 > 2σ(F^2)] = 0.041	w = 1/[σ^2(Fo^2) + (0.05P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.109	(Δ/σ)max = 0.0003
S = 1.50	Δρmax = 0.23 e Å^−3
1866 reflections	Δρmin = −0.49 e Å^−3
100 parameters

Special details

Refinement. Refinement using reflections with F² > −10.0 σ(F²). The weighted R-factor (wR) and goodness of fit (S) are based on F². R-factor (gt) are based on F. The threshold expression of F² > 2.0 σ(F²) is used only for calculating R-factor (gt).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Cl1	0.75109 (3)	0.17343 (3)	0.5787 (1)	0.0519 (1)
Cl2	1.03527 (3)	0.33057 (4)	1.2186 (1)	0.0656 (2)
C1	0.5273 (1)	0.4615 (1)	0.4905 (4)	0.0424 (4)
C2	0.6129 (1)	0.4539 (1)	0.6596 (4)	0.0402 (4)
C3	0.6662 (1)	0.3766 (1)	0.6318 (4)	0.0423 (4)
C4	0.75505 (10)	0.3646 (1)	0.7833 (4)	0.0363 (4)
C5	0.8003 (1)	0.4422 (1)	0.9471 (4)	0.0431 (4)
C6	0.8854 (1)	0.4326 (1)	1.0828 (4)	0.0461 (5)
C7	0.9276 (1)	0.3437 (1)	1.0540 (4)	0.0449 (4)
C8	0.8866 (1)	0.2648 (1)	0.8969 (4)	0.0435 (4)
C9	0.8011 (1)	0.2763 (1)	0.7674 (4)	0.0381 (4)
H1	0.5099	0.4059	0.3400	0.0685*
H2	0.6319	0.5057	0.7984	0.0714*
H3	0.6444	0.3244	0.5057	0.0626*
H4	0.7713	0.5044	0.9704	0.0582*
H5	0.9100	0.4907	1.2046	0.0632*
H6	0.9163	0.2003	0.8841	0.0529*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0525 (3)	0.0365 (3)	0.0660 (3)	0.0011 (2)	−0.0053 (2)	−0.0031 (2)
Cl2	0.0358 (3)	0.0758 (4)	0.0836 (4)	0.0005 (2)	−0.0165 (2)	0.0200 (3)
C1	0.0354 (8)	0.0452 (9)	0.0463 (8)	0.0038 (7)	−0.0029 (7)	0.0012 (7)
C2	0.0359 (8)	0.0419 (9)	0.0424 (8)	0.0042 (6)	−0.0015 (6)	0.0027 (6)
C3	0.0369 (8)	0.0405 (8)	0.0491 (9)	0.0017 (7)	−0.0048 (6)	−0.0021 (7)
C4	0.0329 (7)	0.0381 (8)	0.0378 (7)	0.0034 (6)	−0.0002 (6)	0.0038 (6)
C5	0.0410 (9)	0.0378 (8)	0.0501 (9)	0.0044 (7)	−0.0040 (7)	−0.0009 (7)
C6	0.0415 (9)	0.0464 (9)	0.0497 (9)	−0.0049 (7)	−0.0056 (7)	0.0038 (7)
C7	0.0317 (8)	0.056 (1)	0.0465 (9)	0.0008 (7)	−0.0035 (6)	0.0156 (8)
C8	0.0377 (8)	0.0419 (9)	0.0508 (9)	0.0080 (7)	0.0003 (7)	0.0107 (7)
C9	0.0394 (8)	0.0357 (8)	0.0393 (8)	0.0002 (6)	0.0021 (6)	0.0049 (6)

Geometric parameters (Å, º)

Cl1—C9	1.747 (2)	C4—C5	1.404 (2)
Cl2—C7	1.741 (2)	C4—C9	1.395 (2)
C1—C1^i	1.340 (3)	C5—C6	1.383 (2)
C1—C2	1.439 (2)	C5—H4	0.96
C1—H1	0.99	C6—C7	1.378 (3)
C2—C3	1.335 (2)	C6—H5	0.99
C2—H2	0.93	C7—C8	1.378 (2)
C3—C4	1.461 (2)	C8—C9	1.383 (2)
C3—H3	0.92	C8—H6	0.99
Cl1···Cl2^ii	3.514 (1)	C4···C6^iii	3.589 (2)
C1···C2^iii	3.582 (2)	C7···C8^iv	3.578 (2)
C3···C5^iii	3.565 (2)	C7···C9^iv	3.594 (2)
C4···C5^iii	3.562 (2)
C1^i—C1—C2	125.3 (2)	C6—C5—H4	118.0
C1^i—C1—H1	118.9	C5—C6—C7	118.7 (2)
C2—C1—H1	115.7	C5—C6—H5	116.0
C1—C2—C3	123.8 (2)	C7—C6—H5	125.2
C1—C2—H2	118.0	Cl2—C7—C6	119.4 (1)
C3—C2—H2	118.3	Cl2—C7—C8	119.1 (1)
C2—C3—C4	127.3 (2)	C6—C7—C8	121.5 (2)
C2—C3—H3	116.8	C7—C8—C9	118.5 (2)
C4—C3—H3	116.0	C7—C8—H6	121.3
C3—C4—C5	121.9 (1)	C9—C8—H6	120.2
C3—C4—C9	122.2 (1)	Cl1—C9—C4	120.3 (1)
C5—C4—C9	115.8 (1)	Cl1—C9—C8	116.8 (1)
C4—C5—C6	122.5 (2)	C4—C9—C8	123.0 (2)
C4—C5—H4	119.5
Cl1—C9—C4—C3	−2.9 (2)	C2—C3—C4—C9	172.9 (2)
Cl1—C9—C4—C5	178.5 (1)	C3—C4—C5—C6	−178.1 (2)
Cl1—C9—C8—C7	−178.7 (1)	C3—C4—C9—C8	177.2 (2)
Cl2—C7—C6—C5	179.2 (1)	C4—C5—C6—C7	0.5 (3)
Cl2—C7—C8—C9	180.0 (1)	C4—C9—C8—C7	1.2 (2)
C1—C1^i—C2^i—C3^i	179.1 (2)	C5—C4—C9—C8	−1.3 (2)
C1—C2—C3—C4	177.4 (2)	C5—C6—C7—C8	−0.6 (3)
C2—C1—C1^i—C2^i	180.0	C6—C5—C4—C9	0.5 (2)
C2—C3—C4—C5	−8.6 (3)	C6—C7—C8—C9	−0.2 (3)

Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) x−1/2, −y+1/2, z−1; (iii) x, y, z−1; (iv) x, y, z+1.