The trans-cis isomerization rates for stiff-diphenylbutadiene (S-DPB) in n-alkane solvents were measured using single photon counting methods and the rotational reorientation times 'T R for S-DPB and trans stilbene were obtained by picosecond polarization spectroscopy. In neither case did 'TR vs viscosity show Stokes-Einstein-Debye (SED) behavior. The values of 'T R were used to calculate the angular velocity correlation frequencies /3 using the Hubbard relation. The variation of isomerization rate with /3 was found to be predicted well by the Kramers equation when barrier frequencies of 154 cm-I for stilbene and 16 cm-I for S-DPB were used. This Kramers-Hubbard fit finesses questions regarding the validity of the one dimensional Kramers model and focuses attention on the SED equation. The dynamical relationship between the torsional friction, important in isomerization, and rotational friction, which determines the overall angular motion of the molecules, is discussed. a) This research was supported by grants from NSF (CHE83-03916) and NIH (GM-12592): and in part by the NSF/MRL program under Grant No. DMR8S19059.

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