Solvation dynamics play an important role in many ultrafast photochemical reactions in solutions. [1] In particular, dielectric stabilization can significantly influence the driving force of chemical reactions involving charged intermediates or products, such as electron and proton transfer reactions. [2] Solvent mixtures of varying dielectric constant enable systematic studies on the influence of solvent stabilization on such reactions. Ideally, other solvent parameters such as viscosity and refractive index should remain constant in the mixture. To investigate the effect of solvent stabilization on these ultrafast photochemical reactions, we use broadband femtosecond (fs) fluorescence up-conversion technique, [3] which provides the entire fluorescence spectrum as a function of time with fs time resolution.

Figure. Absorption of Reichardt’s dye in solvent mixtures. (A) Absorption spectrum shift of RD with increasing butyronitrile concentration (B) Reaction field factor (f(x) = (x-1)/(2x+1)) versus E_T(30)

A binary mixture between propyl acetate and butyronitrile is ideally suited for investigating the effect of dielectric stabilization. The dielectric constant (ε_r) increases from 6 to 24.7 upon increasing fraction of butyronitrile from 0 to 1 whereas the viscosity and refractive index remain constant. However, a thorough characterization of the dielectric stabilization in the mixture is necessary before detailed studies on ultrafast reactions. Here in, we present simple and reliable ways for the purification of the solvents and characterize both the static and dynamic dielectric stabilization in this mixture. The static dielectric stabilization is investigated by measuring the steady-state absorption energies of Reichardt's dye whereas the dynamic solvent effects are monitored from the time-resolved fluorescence of standard push-pull dyes. Last, we will demonstrate the influence of dielectric stabilization on both ground- and excited-state acid-base reactions between a photoacid and organic bases in these mixtures.

[1] Tatu Kumpulainen, Bernhard Lang,  Arnulf Rosspeintner, and Eric Vauthey, Chemical Reviews, 2017, 117 (16), 10826-10939.
[2] Tatu Kumpulainen, Arnulf Rosspeintner, Bogdan Dereka, and Eric Vauthey, Journal of Physical  Chemistry Letters, 2017, 8 (18), 4516-4521.
[3] Xin-Xing Zhang, Christian Würth, Lijuan Zhao, Ute Resch-Genger, Nikolaus P. Ernsting, Mohsen Sajadi, Review of Scientific  Instruments , 2011, 82, 063108.