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Abstract:
Time-resolved diffuse X-ray scattering is one powerful
method for monitoring the progression from the creation of local structural
changes inside a crystalline material up to the transformation of the whole
crystalline bulk. In this work, we study the mechanism of phototransformation
of a molecular crystal by time-resolved diffuse X-ray
scattering. Here, an optical excitation source, like a pulsed laser, initiates
structural transformations which are monitored by X-ray scattering
techniques. We have studied the dimerization process of the molecular
switch α-styrylpyrylium (trifluoromethanesulfonate) TFMS, in particular
for understanding whether cooperative effects influence the changes of the
structure in the bulk and its periodicity. Upon illumination with optical
light, α-styrylpyrylium TFMS instantaneously photoswitches. Depending
on the optical fluence, X-ray diffuse planes are observed prior to
phototransformation of the bulk. In the early stages of transformation,
the analysis reveals systems of randomly distributed islands of product clusters with gradual growth in size and amount. The
degree of transformation follows the optical excitation profile, i.e., the spatial absorption of the laser beam. In the present studies,
no influence of cooperativity on the photodimerization process has been found.
