Cooperative formation of high-spin species in a photoexcited spin-crossover complex

The dynamics of the photoinduced phase transition was investigated in a prototypical spin crossover complex, $[\mathrm{Fe}{(\mathrm{ptz})}_{6}]{({\mathrm{BF}}_{4})}_{2}\phantom{\rule{0.3em}{0ex}}(\mathrm{ptz}=1\text{-propyltetrazole})$, under photoexcitation at 77 K near the transition temperature $({T}_{\mathrm{c}}=130\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. When the excitation power $I$ exceeds $\ensuremath{\approx}1\phantom{\rule{0.3em}{0ex}}\mathrm{mW}∕{\mathrm{mm}}^{2}$, we observed an ``acceleration'' of the creation rate of the density ${n}_{\mathrm{HS}}$ of the high-spin (HS) species after a characteristic incubation period. The ``acceleration'' is interpreted in terms of the negative pressure effect; the photocreated HS species with a larger ionic radius expands the volume to cause the structural phase transition.