Fluorescence effects, ionic liquids

A case of solvent-driven electronic relaxation has been observed [76] for [Re(Etpy)(CO)3(bpy)]+ in ionic liquids TRIR spectra have shown at early times a weak signal due to the II. state, in addition to much stronger bands of the 3MLCT state. Although no accurate kinetic data are available, the II. state converts to MI.CT with a rate that is commensurate with the solvent relaxation time. Fluorescence up-conversion provided an evidence [10] for population of an upper II. state in MeCN, which converts to CT with a much faster lifetime of 870 fs (Table 1). The solvent dynamic effect on the 3IL—>3CT internal conversion can be rationalized by different polarities of the II. and JCT states, Fig. 11. The solvent relaxation stabilizes the 3CT state relative to II., driving the conversion. 

Generally the REE occurs when a ground state heterogeneity exists and the excited state relaxation is slow [15]. In the case of ionic liquids their generally high viscosities and short fluorescence lifetimes favour this effect. 

Hu, Z. and Margulis, C. J. 2006. A study of the time-resolved fluorescence spectrum and red edge effect of ANF in a room-temperature ionic liquid. J. Phys. Chem. B 110,11025-11028. 

Inner Filter Effect in the Fluorescence Emission Spectra of Room Temperature Ionic Liquids... 

The ionic liquid viscosity change effect was obtained by the bipolar solvent (DMF). Influence of the ionic liquid viscosity has been detected by the fluorescence emission and absorption spectra. Important information is that the new electronic states of P-carotene were found in the mixture of RTIL and DMF (Bialek-Bylka, 2008). 

The [BF4] and [(CF3S02)2N] based imidazolium ionic liquids have non-negligible absorption in the UV-VIS region with the absorption red tail extending far into the visible region. Such effect is also observed in very carefully purified RTILs. Inner filter effects are responsible for distorted emission spectra and nonlinear calibration curves between fluorescence intensity and fluorophore concentration. In a case of samples like RTILs in order to minimize IFE the x-cell ought to be used. Other methods may cause changes in the nanostructure of ions and influence the intensity and shape of the fluorescence spectra. 

A recent study using a pulse-radiolysis technique on liquid benzene solutions has shown that there is an abundant yield of excited singlet ( B2 ) and triplet ( B) states of benzene. The effect of ionic scavengers in this study shows that ions are precursors of both singlet and triplet states. It is concluded that the excited states arise from ion-electron recombination. This recombination is particularly facile in benzene solution owing to the numerous excited states of low energy in benzene, which rapidly thermalize the electrons. The formation of excited species which are relatively unreactive, as implied by the observation of fluorescence from these states, may account for the low (7-value of benzene decomposition in the liquid phase. 

Periodic perturbations of the potential across the Hquid/liquid boundary induce a modulation of the concentration of species located in the interfacial region. By collecting the spectroscopic signals at the same frequency as that of the potential perturbation, employing phase-sensitive detection, the interfacial sensitivity of the measurements is tremendously enhanced, as the contribution from species in the bulk of the electrolyte solutions can be effectively neglected. Based on this principle, Fermfn and co-workers introduced potential-modulated reflectance (PMR) and potential-modulated fluorescence (PMF) to study a variety of processes including ion transfer [22], electron transfer [20], and the specific adsorption of ionic species [15]. 

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