Investigations of new potential photo-acid generators: crystal structures of 2-[(E)-2-phenylethenyl]phenol (orthorhombic polymorph) and (2E)-3-(2-bromophenyl)-2-phenylprop-2-enoic acid

In the crystal of the orthorhombic polymorph of compound (I), the molecules are linked into chains by O—H⋯π interactions. In compound (II), carboxylic acid inversion dimers are observed; the dimers are linked into chains by C—H⋯O hydrogen bonds.

The title compounds, C 14 H 12 O, (I), and C 15 H 11 BrO 2 , (II), were prepared and characterized as part of our studies of potential new photo-acid generators. In (I), which crystallizes in the orthorhombic space group Pca2 1 , compared to P2 1 /n for the previously known monoclinic polymorph [Cornella & Martin (2013). Org. Lett. 15,[6298][6299][6300][6301], the dihedral angle between the aromatic rings is 4.35 (6) and the OH group is disordered over two sites in a 0.795 (3):0.205 (3) ratio. In the crystal of (I), molecules are linked by O-HÁ Á Á interactions involving both the major and minor -OH disorder components, generating [001] chains as part of the herringbone packing motif. The asymmetric unit of (II) contains two molecules with similar conformations (weighted r.m.s. overlay fit = 0.183 Å ). In the crystal of (II), both molecules form carboxylate inversion dimers linked by pairs of O-HÁ Á ÁO hydrogen bonds, generating R 2 2 (8) loops in each case. The dimers are linked by pairs of C-HÁ Á ÁO hydrogen bonds to form [010] chains.

Chemical context
Photo-acid generators can be used as additives for creating patterns in a polymer film by irradiation through a mask followed by thermal development and base treatment (Ayothi et al., 2007;Kudo et al., 2008;Steidl et al., 2009). The UV irradiation degrades a small amount of the photo-acid generator in exposed areas, which releases a catalytic amount of a strong acid (commonly triflic acid). This acid subsequently catalyses the degradation of the tert-butylcarboxylate groups of a polymer film in a thermal development step, releasing carboxylic acid groups and isobutene. Treatment with base then solubilizes and removes the degraded polymer film in exposed areas, thereby creating a positive resist image (Ito et al., 1994).
We are exploring new types of organic structures as potential photo-acid generators, which might offer improvements over existing substances. Scheme 1 shows how substituted trans-stilbenes might act as photo-acid generators via sequential photochemical trans-cis isomerization and ringclosing reactions. It should be noted that the photochemical cyclization of stilbenes to phenanthenes in the presence of a hydrogen acceptor such as iodine or propylene oxide is well known (Mallory & Mallory, 2005). However, in the absence of an oxidant, if a leaving group is present at the ring-closure site, as in structure 3, a rapid elimination of HX (structure 5) might occur via a stabilized carbocation intermediate 4. In the absence of an oxidant, the cyclized dihydro-phenanthrene compound 6 will equilibrate back to cis-stilbene 2. Stilbenes can also undergo 2 + 2 photochemical cycloadditions (Fulton & Dunitz, 1947;Shechter et al., 1963), a possible competing reaction, but the molecular structures and morphology may still favour the desired reaction to proceed in a thin film.
As part of these studies, the syntheses and crystal structures of the title substituted stilbenes, (I) and (II), are now described [compound (II) could also be described as a cinnamic acid derivative: the photochemical reactions of this family of compounds were reported by Schmidt (1971)]. Compound (I) is an intermediate in the synthesis, whereas a close analogue of compound (II) has already been shown to undergo photochemical cyclization to a phenanthrene with concomitant release of HCl (Geirsson & Kvaran, 2001). A monoclinic polymorph (space group P2 1 /n) of (I) was reported recently (Cornella & Martin, 2013) although its crystal structure was not described in detail.

Structural commentary
Compound (I) comprises one molecule in the asymmetric unit ( Fig. 1), with the -OH group disordered over two sites in a 0.795 (3):0.205 (3) ratio. For the major disorder component, the C ar -C ar -O-H (ar = aromatic) torsion angle is 172 . The molecule is close to planar and the dihedral angle between the aromatic rings is 4.35 (6) . The bond lengths of the central unit [C6-C7 = 1.4703 (19); C7-C8 = 1.3407 (16); C8-C9 = 1.4720 (18) Å ] are consistent with data from previous studies of similar compounds (Tirado-Rives et al., 1984;Jungk et al., 1984). In the monoclinic polymorph of (I) (Cornella & Martin, 2013), the asymmetric unit consists of a half-molecule, which is completed by crystallographic inversion symmetry and therefore, of course, the aromatic rings are exactly coplanar: the OH group is statistically disordered by symmetry and the corresponding C-C-O-H torsion angle for the monoclinic phase is À175 .
There are two molecules in the asymmetric unit of (II) (Fig. 2). In the first (C1) molecule, the dihedral angles between the carboxylic acid group and the phenyl and bromobenzene rings are 61.52 (6) and 55.43 (5) , respectively; the dihedral angle between the aromatic rings is 54.45 (5) . The equivalent data for the second (C16) molecule are 50.72 (6), 60.28 (5) and 61.48 (6) , respectively. The C1 and C16 molecules have a similar overall conformation with an r.m.s. deviation of 0.183 Å for the overlay fit for all non-hydrogen atoms. Otherwise, their bond lengths and bond angles are unexceptional and fall within the expected range of values.

Supramolecular features
The crystal of (I) features O-HÁ Á Á interactions as the main supramolecular interaction ( The asymmetric unit of (II), showing 50% displacement ellipsoids.

Figure 1
The asymmetric unit of (I), showing 50% displacement ellipsoids. Only the major disordered component for the OH group is shown (the minor component is attached to C14).
In the crystal of (II), both molecules (A and B) form carboxylic acid inversion dimers linked by pairs of O-HÁ Á ÁO hydrogen bonds (Table 2), which generate R 2 2 (8) loops in each case. The (A + A) and (B + B) dimers are in turn linked by pairs of C-HÁ Á ÁO hydrogen bonds to generate [010] chains (Figs. 6 and 7). This hydrogen-bond scheme is 'balanced,' with both O1 and O3 accepting one O-HÁ Á ÁO and one C-HÁ Á ÁO hydrogen bond. The shortest BrÁ Á ÁBr contact distance of 3.6504 (4) Å in the crystal of (II) is slightly shorter than the van der Waals radius sum of 3.70 Å for two Br atoms (Bondi, 1964).  Table 1 Hydrogen-bond geometry (Å , ) for (I).

Figure 3
Part of a [001] chain of molecules in the crystal of (I), connected by O-HÁ Á Á interactions (cyan lines).

Figure 4
The unit-cell packing in (I), viewed approximately down [100]. The O-HÁ Á Á interactions from both disordered components are shown as cyan lines.

Figure 5
The unit-cell packing in the monoclinic polymorph of C
The organic layer was washed twice with water to remove DMF, dried over Na 2 SO 4 , concentrated in vacuo and purified by flash chromatography on silica gel. Part of a [010] chain in the crystal of (II), with O-HÁ Á ÁO hydrogen bonds shown as yellow lines and C-HÁ Á ÁO hydrogen bonds shown as cyan lines.

Figure 7
The unit-cell packing in (II), viewed approximately down [010]. 2-Bromobenzaldehyde (0.5 g, 2.70 mmol) and methyl phenylacetate (0.6 g, 4.0 mmol) in dry DMF (30 ml) were treated with sodium methoxide powder (0.3 g, 5.6 mmol) and refluxed for 4 h. The reaction mixture was then cooled, acidified with dilute aqueous HCl and extracted into CH 2 Cl 2 . The organic layer was washed twice with water to remove DMF, dried over Na 2 SO 4 , concentrated in vacuo and purified by flash chromatography on silica gel. Hexane-diethyl ether (75:25) eluted (II) (65 mg, 8%) as a colourless solid, which was recrystallized from hexane/diethyl ether solution as colourless rods. The starting ester was evidently hydrolysed either during the reaction or at the work-up stage; m/z 300.9866 (M + H) C 15 H 10 O 2 Br requires 300.9870.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Atom H1O in (I) was located in a difference Fourier map and refined as riding in its as-found relative position with U iso (H) = 1.2U eq (O). The other H atoms were placed geometrically (C-H = 0.95 Å , O-H = 0.91 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C,O). The O-bound H atoms in (II) were located in a difference Fourier map and refined with U iso (H) = 1.2U eq (O). The C-bound H atoms were placed geometrically (C-H = 0.95 Å ) and refined as riding atoms with U iso (H) = 1.2U eq (C). ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.