Chemistry Reference
In-Depth Information
systems. In a very recent example, we employed electronic effects to stabilize the
product of a spiropyran molecule, which was then analyzed with steady-state ex situ
X-ray photodiffraction [
23
]. The structure refinement showed that the two halves of
the molecule undergo out-of-plane and in-plane shifts so that, as a result of the
opening of the spiropyran heterocyclic system, the molecule bends slightly.
Recently, we have also reported the observation of the electron transfer between
free radical molecules in the solid state [
24
]. Around room temperature, crystalline
1,3,5-trithia-2,4,6-triazapentalenyl radical undergoes paramagnetic-to-diamagnetic
phase transition between violet (high-temperature) and yellow (low-temperature)
phases, accompanied by wide thermal hysteresis. Excitation by pulsed laser light of
the low-temperature diamagnetic phase, in which the molecules are dimerized,
causes an inter-dimer electron transfer and local instability within the stacks. The
structure of the photoinduced phase was refined from the powder diffraction pattern
of the biphasic mixture.
Photocyclization reactions which are not accompanied by drastic color changes
characteristic for the photochromic systems have also been investigated by X-ray
photodiffraction [
25
,
26
] and neutron diffraction [
27
]. In 1998, Scheffer published
two structures of partially reacted single crystals of salts that undergo Norrish type II
(Yang) photocyclization following hydrogen abstraction [
28
]. In one of the crystals,
60% of the cyclobutanol salt was produced and directly observed. The prospects of
the technique for studying similar systems were illustrated by the recent application
by Garcia-Garibay and collaborators to the determination of the ring-extended
intermediate during the photoreaction of
-santonin [
29
], one of the oldest
antiparasitic drugs. The analysis of the partially reacted crystals helped to identify
unequivocally the mechanism of this reaction.
The examples of ex situ steady-state X-ray photodiffraction utilized to follow the
photodimerizations of olefin bonds in a single-crystal-to-single-crystal (or nearly
so) manner are ubiquitous in the chemical literature.
2
The interest of solid-state
chemists in this reaction dates back to the work of Cohen and Schmidt [
30
,
31
], and
it has become much of a “guinea pig” in organic solid-state photochemistry. In
1993, Enkelmann and collaborators published two seminal papers in the
Journal of
the American Chemical Society
[
32
] and in
Angewandte Chemie
[
33
], where they
presented a series of structures of
a
-
trans
-cinnamic acid crystals reacted to various
extents. These reports laid the way for a plethora of later studies on the olefin
photodimerization reaction. The convenience of the high conversion and the simple
mechanism, combined with the relatively small structural perturbation that it
requires, has turned this reaction into a very useful tool to probe intermolecular
a
2
It may be argued whethere these examples, which have usually employed X-ray diffraction
analysis of previously UV irradiated crystals, fall within the domain of the X-ray photodiffraction
methods or solid state photochemistry, and whether the term 'X-ray photodiffraction' should be
reserved only for time-dependent studies. Although that in the original publications many of these
examples have not been labeled as such, because they involve application of XRD methods to
study photochemical reactions, we believe that they should be considered as part of the X-ray
photodiffraction method in its broadest definition.
