Unexpected Molecular Flip in Solid-State Photodimerization

Three cocrystals of the light-stable compound 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol
(I) with light-sensitive molecules 1,2-dimethyl-2­(1H)-pyridinone (a), 6-methyl-2­(1H)-pyridinone (b) and 2-methyl-2­(1H)-pyridinone
(c) were exposed to UV light. It was found that the molecules
undergo molecular flip perpendicular to the molecular plane (rotation
of ∼180°). In the first two cocrystals, the light-sensitive
molecules are disordered, which means that the space provided for
them is larger than needed for ordered molecules. Therefore, rotation
can take place. Moreover, in I-b, the flip is temperature
dependent and takes place without exposure to UV light. Crystal structures
at four different temperatures enable one to estimate the activation
energy of the flip to be 9.72 kJ/mol. The kinetics of the reaction
of I-c was studied at room temperature and revealed a
sigmoidal behavior with Avrami exponent of n = 0.95(6)
that could be explained by the JMAK model for crystal growth. It means
that the nucleation rate is constant over time and that the reaction
is homogeneous with equal probability to occur in any region of the
sample. This could be explained by the fact that the voids where the
reaction and the flip take place are isolated from each other.