Microheterogeneity and Fluorescence Dynamics in Imidazolium Chloride-Acetamide Low Transition Temperature Mixtures

Herein, we investigated the local organization, fluorescence dynamics, and dynamic heterogeneity in low transition temperature mixtures (LTTMs) composed of 1-butyl-3-methylimidazolium chloride (BmimCl; T_m = 343 K), 1-ethyl-3-methylimidazolium chloride (EmimCl; T_m = 353 K), and acetamide (ACM; T_m = 352 K) using steady-state and time-resolved fluorescence spectroscopy. Both mixtures exhibit pronounced excitation-wavelength-dependent emission behavior, with an overall shift of ~ 170 nm in the emission maxima, suggesting the presence of various local molecular environments within the mixtures. Fluorescence lifetime analyses reveal three decay components (~ 300 ps, ~ 1.7-1.9 ns, and ~ 5.5-5.9 ns), indicating tri-exponential fluorescence decay with contributions occurring over different timescales. Time-resolved fluorescence anisotropy studies at different excitation wavelengths at 298 K yield rotational relaxation times ([Formula: see text]) of 18.34-20.65 ns and 12.69-16.55 ns for BmimCl-ACM and EmimCl-ACM, respectively. Furthermore, the temperature-dependent reorientational dynamics exhibit a fractional viscosity dependence, with exponents of 0.47 for BmimCl-ACM and 0.85 for EmimCl-ACM, confirming the presence of dynamic heterogeneity in these LTTMs, with the former system displaying a greater extent of heterogeneity. Complementary 2D ¹1H-¹H NOESY experiments reveal stronger cross-peaks for BmimCl-ACM, suggesting closer spatial association between the components compared with the EmimCl-ACM system. Overall, the combined optical spectroscopy and NMR results demonstrate that BmimCl-ACM forms a more strongly associated molecular structure compared to the EmimCl-ACM system. These findings underline how subtle variations in imidazolium cation structure govern molecular organization and dynamics in LTTMs, providing useful guidelines for the rational design of environmentally benign solvent systems.