Temperature dependence of solvation dynamics of probe molecules in methanol-doped ice and in liquid ethanol

We have studied the solvation statics and dynamics of coumarin 343 and a strong photoacid (pK* ∼ 0.7) 2-naphthol-6, 8-disulfonate (2N68DS) in methanol-doped ice (1% molar concentration of methanol) and in cold liquid ethanol in the temperature range of 160-270 K. Both probe molecules show a relatively fast solvation dynamics in ice, ranging from a few tens of picoseconds at about 240 K to nanoseconds at about 160 K. At about 160 K in doped ice, we observe a sharp decrease of the dynamic Stokes shift of both coumarin 343 and 2N68DS. Its value is approximately only 200 cm^-1 at ∼160 K compared to about 1100 cm^-1 at T ≥ 200 K (at times longer than t > 10 ps). We find a good correlation between the inefficient and slow excited-state proton-transfer rate at low-temperature ice, T < 180 K, and the dramatic decrease of the solvation energy, as measured by the dynamic band shift, at these low temperatures. We find that the average solvation rate in ice is similar to its value in liquid ethanol at all given temperatures in the range of 200-250 K. The surprisingly fast solvation rate in ice is explained by the relatively large freedom of the water hydrogen rotation in ice I _h.