A comparison of the
solution and solid state thermal decay kinetics
of five photochromic spiropyrans with different <i>N</i>-alkyl groups (<b>SP1</b>–<b>SP5</b>) was carried
out in acetonitrile and nanocrystalline suspensions at 298 K. The
change in absorbance at ca. 550 nm was measured as a function of time
for the merocyanine (MC) using transmission UV–vis spectroscopy.
We found that the thermal decay kinetics are slower and follow a biexponential
decay in the solid state compared to a faster, monoexponential decay
that was measured in solution. We observed that, while the kinetic
range measured in solution varies by a factor of 13, the decay kinetics
in the solid state cover a range of ca. 150, indicating that crystal
packing has an influence much greater than that of the effects of <i>N</i>-alkyl substitution. A fluorescence analysis of irradiated
samples of <b>SP1</b> in solution could be used to determine
the formation of the MC species and its subsequent decay. By contrast,
a similar analysis of nanocrystalline suspensions displayed changes
as a function of time that are consistent with self-quenching
