3.1. Molecular structure

In crystals of the pure monomer, (Ia), the nearest contacts between the C=C bonds are observed for pairs of molecules related by centers of symmetry (Fig. 1a, Table 3). The C12=C13 bond axis is parallel to that in the neighboring molecule and the C12C13ⁱ (i: 1 - x, 1 - y, -z) distance is 4.155 (17) Å. This is insignificantly shorter than Schmidt's critical distance (4.2 Å; Ramamurthy & Venkatesan, 1987). However, the - overlapping is excellent, and the C9-C12C13ⁱ, C13=C12C13ⁱ, C12=C13C12ⁱ and C14-C13C12ⁱ angles are in the range 80.0 (8)-99.0 (8)°. The displacement ellipsoids of some C atoms exhibit poor shapes, which may be due to the insufficient number of reflections observed [I > 2(I)]. This lack of observations is the result of the extensive vibration and the loose packing of the molecules in (Ia) (see below).

Table 3 Selected geometric parameters (Å, °)   (Ia) (Ib) (II) Cl-C6 1.720 (16) 1.718 (19) 1.741 (3) C12-C13 1.306 (17) 1.326 (11) 1.538 (3) C12C13i 4.155 (17) 3.891 (11) 1.587 (3) C12*-C13* - 1.534 (14) - C12*-C13*i - 1.534 (14) - Symmetry code: (i) 1 - x, 1 - y, -z.

Figure 1 (a) The structure of a pair of monomer molecules in (Ia), which are related by a center of symmetry. (b) The disordered structure in (Ib), which consists of the monomer (solid bonds) 65.8 (8)% and the dimer (open bonds) 34.2 (8)%. (c) The structure of the dimer molecule in (II). Displacement ellipsoids are plotted at the 50% probability level.

Fig. 1(b) shows the molecular structure in the crystal (Ib), which was illuminated by ambient room light for half a year after crystallization and then irradiated with Cu K radiation for 75 h. The -type photodimer was observed with an occupancy factor of 34.2 (8)%. Another crystal (Ic) picked up from the same batch of (Ib) showed a dimer content of 22.0 (6)%. These results suggest that the crystals had undergone gradual photoreaction under room lighting over about six months. A similar highly sensitive [2+2]-photodimerization was reported for single crystals of 2-benzyl-5-benzylidenecyclopentanone (BBCP), where exposure to daylight provided a similarly slow conversion (Nakanishi et al., 1981).

The shifts of the ethane C atoms in (Ib) during dimerization are comparable to those observed in photodimerization of trans-cinnamamides (Hosomi, Ito & Ohba, 2000). The C12-C12* and C13-C13* atom shifts in (Ib) are 1.22 (1) and 1.16 (1) Å, respectively. The distance between the reaction points of the substrate molecules, C12C13ⁱ, is 3.891 (11) Å, which is shorter by 0.26 (2) Å than the distance in (Ia). This shortening may be due to the contraction of the cell volume as discussed below. Fig. 1(c) shows the structure of the pure photodimer (II) after the single-crystal-to-single-crystal transformation. The C12-C13ⁱ (i: 1 - x, 1 - y, -z) bond distance is 1.587 (3) Å. The atomic coordinates in the pure monomer (Ia) and the pure dimer (II) are compared in Fig. 2. The product molecules in (Ib) adopt approximately the same positions as the molecules in the final pure dimer (II), in contrast to the translational and rotational movement of the product molecules in a BBCP crystal during the transformation (Turowska-Tyrk, 2001). In the present crystal structure, the orientation of the substrate and product molecules has been fixed by the intermolecular N-HO hydrogen bonds (Fig. 3).

Figure 2 A comparison of the atomic positions in (Ia) (solid bonds) and (II) (open bonds). The lattice constants of (Ia) were used with the atomic coordinates for both of (Ia) and (II).

Figure 3 A chain of N-HO hydrogen bonds running along the b axis in (Ia).

3.2. Crystal structure

The amide moiety is involved in intermolecular N-HO hydrogen bonding that forms a chain along b (Fig. 3 and Table 4). These hydrogen bonds were retained after the reaction. The crystal structures of the monomer (Ia) and the dimer (II) are compared in Fig. 4. The atomic displacement parameters U_eq of (II) average ca. two-thirds of those of (Ia), which indicates that the initial structure had loose packing and that dimerization led to a more compact structure with lower atomic vibration. The main changes in lattice constants in the course of the reaction consisted of a shrinkage of the a axis and a decrease in the unit-cell volume. A linear correlation was observed between the cell volume and the degree of conversion (Fig. 5). There have been very few reports of photoreaction that achieves 100% conversion in a single crystal (Nakanishi et al., 1981; Enkelmann et al., 1993; Novak et al., 1993; Suzuki et al., 1994; Hosomi et al., 1998; Leibovitch et al., 1998; Tanaka et al., 2000). In general the cell volume increases during the photoreaction and the crystallinity is degraded by the effects of strain, as reported for the [2+2]dimerization of trans-cinnamamides (Hosomi, Ito & Ohba, 2000), and this degradation prevents the in situ observation of the complete conversion by diffraction techniques.

Table 4 Hydrogen-bonding geometry (Å,°)   D-HA D-H HA DA D-HA (Ia) N5-H5O2i 0.95 2.05 2.942 (13) 155 (Ib) N5-H5O2i 0.95 2.14 3.013 (3) 152 (II) N5-H5O2i 0.95 2.36 3.171 (2) 143 Symmetry code: (i) x, y - 1, z.

Figure 4 Projection of crystal structures of (a) (Ia) and (b) (II) along the b axis.

Figure 5 Cell volume (Å3) versus degree of conversion (%) during the photodimerization.

The unsubstituted cinnamoyl-O,O'-dimethyldopamine, (III), is photostable (Ito et al., 2001). The crystal structure was determined in order to reveal the geometrical factors that prevent [2+2] photodimerization (Ohba & Ito, 2002). The crystal structures of the photoactive 4-chloro derivative (Ia) and the photostable non-substituted compound (III) have some similarities. The space group of (III) is P2₁/a, which is one of the maximal non-isomorphic subgroups of A2/a [a non-standard setting of the space group C2/c observed for (Ia)]. In (III), the chain of N-HO hydrogen bonds lies in the direction of the b axis [5.2431 (5) Å] as observed in (Ia), and the close contacts are observed in a pair of molecules related by a center of symmetry (Fig. 6). However, the C=C double bonds in the cinnamoyl moieties lie far apart and the amido groups are close to each other. The introduction of the 4-chloro group to the cinnamoyl moiety increases the overall length of the molecule, with the result that the C=C bonds in crystals of (Ia) are brought closer together.

Figure 6 The structure of a pair of molecules in (III) that are related by a center of symmetry (Ohba & Ito, 2002).

3.3. Reaction by X-ray radiation

The occupancy factors of the dimer in (Ib) and (Ic) are 34.2 (8)% and 22.0 (6)%, respectively. The larger conversion in (Ib) may be a result of the irradiation with Cu K radiation for 75 h before the data re-collection with Mo K. The decrease in the cell volume of ca. 25 Å³ during the data collection with Cu K radiation corresponds closely to the difference in the cell volume between (Ib) and (Ic). Further evidence that the reaction is induced by X-radiation can be found in the variation in the three standard reflections (from -8.4% to +6.8% in |F_o|) during the Cu K data collection: this variation matches the change in structure factors from (Ib) to (II) that is caused by photodimerization.

The effect on the crystals of Mo K radiation was found to be less than that of Cu K radiation. X-ray intensity measurements on (Ic) were repeated four times in the absence of room light to quantify the influence of Mo K radiation (Table 2). The occupation factor of the dimer increased on average by 2.5 (8)% during the 60 h required for the collection of each data set. The stronger induction of the reaction by Cu K than by Mo K may be due to the greater absorption efficiency of Cu K, although the intensity of the incident X-ray beam for Cu K was stronger than that for Mo K by a factor of ca. 1.5 because of the lower threshold excitation voltage for Cu K.

Although reactions induced by X-ray irradiation of a single crystal were reported for the molecular rearrangement of hirsutic acid (Comer & Trotter, 1966), retro[2+2]cycloaddition of a syn-tricyclooctane derivative (Mori et al., 1994), polymerization of diethyl cis,cis-muconate (Matsumoto et al., 1998; Tashiro et al., 1999), polymerization of 2,4-hexadiynylene bis(p-toluenesulfonate) (Enkelmann et al., 1979) and polymerization of other diacetylene derivatives (Kai & Yamamoto, 1993), the dependence of the reaction rate on the X-ray wavelength has not previously been noted.