7-Cyanoindole fluorescence as a local hydration reporter: application to probe the microheterogeneity of nine water-organic binary mixtures

Abstract

Considerable efforts have been devoted to the development of spectroscopic probes that are sensitive to water and can be used to monitor, for example, biological and chemical processes involving dehydration or hydration. Continuing this line of research, herein we show that 7-cyanoindole can serve as a sensitive fluorescence probe of hydration as its fluorescence properties, including intensity, peak wavelength and lifetime, depend on the amount of water in nine water-organic solvent mixtures. Our results indicate that 7-cyanoindole is not only able to reveal the underlying microheterogeneity of these binary solvent systems, but also offers distinct advantages. These include: (1) its fluorescence intensity increases more than ten times upon going from a hydrated to a dehydrated environment; (2) its peak wavelength shifts as much as 35 nm upon dehydration; (3) its single-exponential fluorescence decay lifetime increases from 2.0 ns in water to 8-16 ns in water-organic binary mixtures, making it viable to distinguish between differently hydrated environments via fluorescence lifetime measurements; and (4) its absorption spectrum is significantly red-shifted from that of indole, making selective excitation of its fluorescence possible in the presence of naturally occurring amino-acid fluorophores. Moreover, we find that for seven binary mixtures the fluorescence lifetimes of 7-cyanoindole measured at solvent compositions where maximum microheterogeneity occurs correlate linearly with the peak wavenumbers of its fluorescence spectra obtained in the respective pure organic solvents. This suggests that the microheterogeneities of these binary mixtures bear certain similarity, a phenomenon that warrants further investigation.

Chemical signatures of surface microheterogeneity on liquid mixtures

“…The solvent effects on CIs reactions in our ternary mixtures are closely related to the x w -dependences of the intensity, peak emission wavelength, and decay lifetime of 7-cyanoindole fluorescence in W:AN, W:THF, and W:DIO mixtures (see below). 28 The correlations between the degree of dissociation of 2-naphthol and the rates of tert-butyl chloride hydrolysis with the composition of W i AN j clusters as functions of x w in W:AN mixtures ( Fig. S14) 23,25,26,91,92 support our interpretation that the R vs x w trends of Fig.…”

“…Note that not only AN, THF, and DIO but also dimethylsulfoxide, methanol, ethanol, isopropanol, and tert-butanol all show evidence of microheterogeneity. 28 The implication is that microheterogeneity (both at shorter and longer time scales) may be a general phenomenon of water-hydrotrope mixtures and, therefore, that "molecularly homogeneous" aqueous mixtures may not exist. There is also thermodynamic evidence on microheterogeneity in aqueous mixtures.…”

“…The characteristic times of X and W exchange between X m W n clusters with the rest of the solution, however, are much shorter, as revealed by the fact that 7-cyanoindole fluorescence in W:AN, W:THF, and W:DIO mixtures decays as single exponentials in the 2-15 ns range. 28 Single exponential decays suggest that 7-cyanoindole fluoresces while embedded in X m W n clusters of average composition, rather than from an ensemble of X m W n clusters. Therefore, averaging takes place via X and W exchange in the sub-ns range scale.…”

“…These solvents are fully miscible, but are known to display bulk microheterogeneity. 28 A previous sum-frequency spectroscopy study showed that the surface of W:AN mixtures undergoes a sharp transition into a structurally uniform interfacial layer below water molar fraction x w ≈ 0.93. 51,53,63 Related molecular dynamics calculations refined such scenario and revealed that the outermost interfacial layers are actually microheterogeneous, being populated by disjointed W:AN clusters in proportions that depend on x w in a non-trivial way.…”

“…[23][24][25][26] The universality of this phenomenon opens up the possibility that molecular environments could be engineered in solvent mixtures for steering the course of chemical reactions in unanticipated ways. 12,[27][28][29] It also leads to the realization that spontaneous compartmentalization at the mesoscale in complex liquid mixtures, by potentially supporting disjoint but coupled chemical processes in apparently homogeneous systems, is an alternative mechanism to the reaction-diffusion Turing model of the onset of complexity and pattern formation in Nature. [30][31][32] Water is a major component of natural systems; however, it is rarely present as a neat water, but as a solvent or distributed in diverse materials.…”