Viscosity Dependence of Intramolecular Excimer Formation with 1,5-Bis(1-pyrenylcarboxy)pentane in Alkane Solvents as a Function of Temperature

Abstract

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.

Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1-pyrenyl)propane within 1-Butyl-3-methylimidazolium Hexafluorophosphate and Its Polyethylene Glycol Mixtures

“…This assumption is supported by the fact that for 1Py(3)1Py in n-heptane similar values for the two activation energies (18.4 and 19.3 kJ mol −1 ) were obtained previously. 53 Pre-exponential factors were also found to be very similar. It may be assumed that the relative contributions of k a1 and k a2 to the overall rate constant k a remain independent of temperature to a good extent.…”

“…Under these conditions, the overall rate constant of excimer formation (k a ) can be obtained from the monomer decay time (τ 1 ) and the fluorescence lifetime (τ 0 ) of a model compound which does not form excimer under these conditions. 53 We selected 1-methylpyrene as the model compound. Understandably, the excited-state intensity decay of 1-methylpyrene is found to fit best to a single-exponential function irrespective of the medium, temperature, or emission wavelength (Table S3 presents recovered parameters, including τ 0 , for 1-methylpyrene).…”

“…47,48,53,54 It is clear from these investigations that the excimer emission depends most importantly on the solvent property/composition. 47,48,[53][54][55]58,59 The chain length, chain structure, actual fluorophore chemistry, exact labeling site on the fluorophore and the chain, tether length and its chemistry, temperature, and pressure also play a part. Among the dipyrenylalkanes, 1,3-bis-(1-pyrenyl)propane, [1Py (3)1Py], is perhaps the most investigated as the intramolecular excimer formation dynamics of 1Py (3)1Py is shown to be highly sensitive to the temperature and the solubilizing milieu.…”

Internal Dynamics of Dendritic Molecules Probed by Pyrene Excimer Formation

“…The first study of intramolecular pyrene excimer formation was reported by Zachariasse in 1976 for a series of n-alkanes terminated at both ends with a 1-pyrenemethyl group [151]. It was followed by numerous others that provided information about the ensemble of conformations adopted by the short alkane chain as it folds to bring its two ends in close proximity to form an excimer [152][153][154][155][156][157][158][159][160]. The scope of these studies was then expanded to include the end-to-end cyclization (EEC) of polymer chains by covalently attaching a pyrenyl group at both ends of a series of poly(ethylene oxide) (Py 2 -PEO) [78].…”

“…As for the other analyses, the monomer and excimer decays are fitted gobally to improve the accuracy on the retrieved parameters [178][179][180]. However, our analysis departs in one important manner from the current art in the way the pre-exponential factors in Equation (9) and and in Equation (11) [126,150,176,177] compared to more traditional analyses [74,108,160]. Furthermore, it also enables the calculation of the ratio given in Equation (16) …”