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Viscosity dependence of intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane in alkane solvents as a function of temperature.

Intramolecular excimer formation with 1,5-bis(1-pyrenylcarboxy)pentane, (1PC(5)1PC) is studied as a function of temperature in a series of alkane solvents and in toluene, covering a wide range of solvent viscosities η, from 0.2 to 125 cP. The rate constant k(a) of the monomer → excimer reaction is determined from the effectively single exponential monomer fluorescence decays. For the viscosity dependence of k(a) in n-alkanes, the Stokes-Einstein relation k(a) ∼ η(-1.0) does not hold. Instead, k(a) is proportional to η(-α), with α increasing upon cooling, from 0.56 at 85 °C to 0.86 at -30 °C. The activation energy E(a) of excimer formation with 1PC(5)1PC, always larger than the activation energy E(T/η) of solvent viscous flow, decreases when the solvent viscosity becomes smaller, from 20.7 kJ/mol in n-hexadecane to 11.8 kJ/mol in n-butane, approaching a value of 11-12 kJ/mol for the low viscosity solvents. As the excimer formation process depends on the restricted diffusion of the 1PC end groups as well as on the C-O and C-C rotations in the -O(CH(2))(5)O- chain, the limiting barrier of 11-12 kJ/mol is attributed to the activation energy E(c) of the multiple bond rotations. This fractional viscosity dependence (α < 1.0) is caused by the multidimensional character of the barrier crossing in the excimer formation process. This multidimensional character should also be taken into account in investigations of polymers and biological media employing excimer formation.