The viscosity dependence of the photoisomerization of trans‐stilbene in compressed liquid ethanol shows deviations from a simple power law description in the viscosity range from 1 to 4 mPa s. Corresponding deviations are observed in the solvents methanol, n‐propanol, and n‐butanol. This behavior is attributed to a competition between solvent relaxation and barrier crossing in the S₁ state of trans‐stilbene. The relative time scales of barrier crossing and solvent relaxation change as the pressure increases, because the dielectric relaxation rate of the solvent decreases more rapidly with increasing viscosity than the barrier crossing rate. Consequently, the reaction takes place in an increasingly retarded solvent environment which no longer relaxes completely around the changing charge distribution of the solute along its reaction path, giving rise to ‘‘dielectric friction.’’ In contrast to trans‐stilbene, the corresponding reaction of diphenylbutadiene in n‐alkanols shows a much weaker sensitivity to solute‐solvent interaction and, consequently, a simple inverse viscosity dependence of the photoisomerization rate is observed in all alkanols such as described by the Kramers–Smoluchowski theory. This significant difference is probably caused by smaller sudden polarization effects along the reaction path in diphenylbutadiene. The observed dependence of the trans‐stilbene barrier crossing rate on pressure is compared either to a model with density dependent effective barrier height, or to a simple continuum model of the frequency dependence of the dielectric friction in the limit of weak coupling. Neither model works well unless a very strong viscosity dependence of the dielectric relaxation time of the solvent (τ_D∝η¹⁰) is employed to obtain agreement with the observed viscosity dependence of the barrier crossing rate.

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