Assessing hydrophobic deep eutectic solvents for intramolecular excimer formation

Intramolecular excimer formation by a dipyrenyl probe, 6-(1-pyrenyl)hexyl-11(1-pyrenyl)-undecanoate [1-Py(CH₂)₁₀COO(CH₂)₆1-Py], is used to assess hydrophobic deep eutectic solvents (HDESs) for the purpose. n-Decanoic acid (DA), L(-)-menthol (Men) and thymol (Thy) have been utilized to form HDESs with different pairs of constituents in different molar ratios, namely Men : DA (2 : 1, 1 : 1, and 1 : 2), Thy : DA (2 : 1, 1 : 1, and 1 : 2), and Thy : Men (5 : 1, 2 : 1, 1 : 1, 1 : 2, and 1 : 5). The maximum of the excimer-to-monomer emission intensity ratio, (I_E/I_M)_max, is observed at 343.15-353.15 K for all DESs irrespective of the constitution, and it varies in a narrow range exhibiting no correlation with the dynamic viscosity (η) of the DES which varies between 2.05 and 3.56 mPa s. Excited-state intensity decay data reveal excimer dissociation back to the excited monomer to be negligible in all DESs at lower temperatures (T ≤ 323.15 K); the simplistic Birks scheme is followed at higher temperatures (T > 323.15 K). The rate constant for excimer formation/association, k_a, ranges from (3.00 ± 0.50) × 10⁶ s^-1 to (103 ± 10) × 10⁶ s^-1, which is similar to that reported in other media. The temperature-dependence of the equilibrium constant for excimer formation follows the van't Hoff equation with recovered standard enthalpy (Δ_aH*^⊖) and standard entropy (Δ_aS*^⊖) changes, indicating the reversible intramolecular excimer formation to be exothermic and energetically-favorable but entropically unfavorable. A plot of k_avs. T/η for all the DES systems investigated exhibits a fairly good linear correlation, indicating the adherence to the Stokes-Einstein formulation within the HDESs further emphasizing the homogeneous nature of the solubilizing media. The work helps to highlight the potential of HDESs for intramolecular excimer formation involving non-polar reactants.