Direct Observation of Structural Change in Photochromism

The ditificulty was solved by using two-photon excitation. Two-photon excitation by 730 nm radiation gives molecules with the same energy as one-photon excitation at 365 nm. Because the probability of two-photon excitation is much smaller than that of one-photon excitation, the light intensity hardly decreases in the bulk of the crystal. Thus the two-photon excitation makes it possible to excite the bulk of the crystal and to form enough photoproduct for X-ray analysis. The two-photon excitation mentioned here is a concerted absorption of two photons by individual molecules and does not mean two sequential one-photon absorption events [32]. The IR spectra revealed that the powdered sample of photoproduct obtained by two-photon excitation was identical with that obtained by one-photon excitation. 

The crystal strncture before photo-irradiation was analyzed by X-rays at 90 K. The molecular structure is shown in Fig. 7.6a. The bond distances of C2-01, C1-C7, and C7-N1 are 1.361(1), 1.454(1), and 1.295(1)A, respectively, and the hydrogen atom is bonded to 01. It is clear that the molecule 1 exists in an enol form before photo-irradiation. The photoreaction was performed by irradiation with outpnt pnlse of an optical parametric oscillator with wavelength of 730 nm at room temperatnre. Becanse componnd 1 has no absorption band at 730 nm, the reaction was induced by two-photon excitation. Among the laser-irradiated dark-red crystals, a crystal whose quality did not deteriorate was selected and the intensity data were collected. The nnit-cell volume was increased by 7.89(9) A, which is less than 1 % of the total volume. The difference electron density map showed additional peaks around the original salicylideneaniline molecule. The peaks were assigned and successfully rehned as another molecule (with black bonds) which coexists with the original enol form (with white bonds) in the crystal as shown in Fig. 7.6b. The occupancy factor of the newly produced molecule is 0.104(2). 

Although the transformation demonstrated here involves large structural change and seems difficult to take place in a crystal, it can easily occur through the motion of a pair of benzene rings analogous to the pedal motion of a bicycle, which is extensively studied in the thermochromism of azobenzenes containing similar molecular skeletons to those of salicylideneanilines [33]. 

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