Simultaneous single molecule tracking (SMT) and single molecule emission dichroism (SMED) measurements are used to investigate the translational and orientational motions of fluorescent probes diffusing within mesoporous silica films. The results afford a quantitative measure of molecular confinement within the mesopores and allow for the accessible pore diameter to be determined with higher precision than can be achieved by common SMT methods alone. In these studies, dual-polarization wide-field video microscopy is used to obtain SMT data simultaneously in two orthogonal polarizations. A rod-shaped perylene diimide dye (C 11 OPDI) is employed as the probe and is excited by circularly polarized light. SMT reveals a predominance of one-dimensional dye diffusion within the mesopores. The SMED data demonstrate that the molecules diffuse in an oriented state, with their long axes parallel, on average, to the long axis of the mesopores. Theoretical expressions quantitatively relating the SMED and SMT results to the extent of C 11 OPDI orientational wobbling are developed. The results are consistent with confined wobbling of the single molecules within the mesopores. An ensemble-averaged wobbling angle of 19 ± 3°is obtained, yielding an estimated accessible pore diameter of 1.3 ± 0.2 nm.

The confined orientational motions of fluorescent dye molecules diffusing along one dimension (1D) within individual silica mesopores are investigated by simultaneous single molecule tracking (SMT) and single molecule emission dichroism (SMED) methods. Four perylene diimide (PDI) dyes of different lengths are employed as the probe species. Wobbling angles exhibited by the individual molecules are measured within cetyltrimethylammonium bromide-filled mesopores. The results show a clear dependence on probe molecule length, attributable to confinement of the molecular motions to small cavities within the surfactant- and solvent-filled mesopores. These results are used to obtain quantitative estimates of the accessible cavity diameters. Histograms of these data reveal a broad distribution of cavity sizes. The average cavity diameters are shown to be largely independent of molecular length and yield a global mean value of 1.06 ± 0.03 nm, corresponding to ∼1/3 the physical diameter of the silica mesopores, as estimated from X-ray scattering data. The difference in physical and accessible pore diameters is attributed to confinement of PDI orientational motions by nanostructuring of the solvent/surfactant medium filling the pores. It is proposed that the PDI molecules are confined to the most hydrophobic regions of the surfactant micelles and that formation of a water-rich solvent layer at the silica/surfactant boundary may also contribute. These results will facilitate a deeper understanding of solute–solvent interactions in nanoconfined systems and are relevant to applications of mesoporous silica materials in solution-phase catalysis and chemical separations.

The effect of the addition of cosolvents in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO(4)]) was probed by the solvent and rotational relaxation studies of coumarin 153 in neat ionic liquid [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using steady-state and time-resolved fluorescence spectroscopy. With gradual addition of cosolvents in the RTIL, both the average solvation time and rotational relaxation times gradually decrease. Addition of cosolvents in the IL decreases the viscosity of the medium. We have optimized the geometry of [Emim][EtSO(4)] and [Emim][EtSO(4)]-cosolvent mixtures by using quantum chemical calculations using density functional theory methods, which show the formation of hydrogen bond between cosolvents with [Emim][EtSO(4)]. With addition of the same amount of alcohols in neat [Emim][EtSO(4)], the rotational relaxation time decreases more compared to the addition of the same amount of water.

The effect of different microenvironments inside various biomimicking supramolecular assemblies of ionic (SDS/CTAB) and nonionic (TX100) micelles and nonionic surfactants (Tween-80/PEG-6000) forming vesicles (niosome) on the photophysical and rotational dynamical properties of 1'-hydroxy-2'-acetonaphthone (HAN) have been studied using steady-state and time-resolved fluorescence spectroscopy. Enhanced fluorescence intensity with a significant blue shift and longer emission lifetime of the caged tautomers of HAN indicate modulation of photophysics of HAN upon encapsulation in both micellar assemblies and the niosome system. The binding constant and free energy change for the complexation of HAN with micelles and niosome demonstrate a comparative study on the binding efficiency of the different assemblies depending on the nature of microenvironments toward HAN. The enhancement in the steady-state anisotropy in niosome solutions compared with that in pure aqueous solution indicates that HAN is located inside the motionally restricted bilayer region of niosome. The fluorescence quenching experiment further reveals the probable location of HAN in micelles and niosome. In TX100 micelles, the obtained lifetime values are 417 ps and 1.63 ns for the caged tautomers, whereas in the comparatively more rigid and confined environment provided by niosome those values are 444 ps and 2.5 ns. The rotational relaxation time constants for the caged tautomers in niosome are also found to be higher than those in micelles. The observed difference in binding ability of the different assemblies is due to the difference in the extent of water penetration and different extent of rigidity around the fluorophore.

In this work, we reported a detailed study of the solvation dynamics of coumarin-480 in [bmim][BF(4)]/BHDC/benzene reverse micelles (RMs) with varying [bmim][BF(4)]/BHDC molar ratio (R) 1.00, 1.25, 1.50, and also study the solvation dynamics at five different temperatures from 15 to 35 °C RMs at [bmim][BF(4)]/BHDC molar ratio 1.25 for the first time. The average solvation time constant becomes slightly faster with the increase in R values at a temperature 25 °C. The solvation dynamics of the RMs with R value 1.25 becomes faster with the increase in temperature. We have also investigated temperature-dependent solvation dynamics in neat [bmim][BF(4)]. The solvation dynamics in neat [bmim][BF(4)] has a substantial temperature effect but for the [bmim][BF(4)]/BHDC/benzene RMs the temperature effect on the solvation dynamics is not that significant. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature. Wobbling-in-cone analysis of the anisotropy data also supports this finding.