Solvent polarity and viscosity

Fluorophores with polar groups are often sensitive to solvent polarity. Interaction between the excited fluorophore and surrounding polar groups lowers the energy of the excited state, which shifts the emission to longer wavelengths. The relative amounts of emission from the relaxed and the unrelaxed states depend upon the 

The effects of solvent polarity are best imderstood by specific examples. To model the fluorescence emission of proteins we examine spectra for iV-acetyl-L-tryptophanamide (NATA). This molecule is analogous to tryptophan in proteins. It is a neutral molecule, and its emission is more homogeneous than that of tryptophan itself. In solvents of increasing polarity the emission spectra shift towards longer wavelengths (Fig. 5). The emission maxima of NATA in dioxane, ethanol and water are 333, 344 and 357, respectively. These solvents are non-viscous, so the emission is dominantly from the relaxed state (Fig. 4). The spectral shifts can be used to calculate the change in dipole moment which occurs upon excitation [6]. More typically, the emission spectrum for a sample is compared with that foimd for the same fluorophore in various solvents, and the environment judged as polar or non-polar. While this approach is qualitative, it is simple and reliable, and does not involve the use of theoretical models or complex calculations. 

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Experimental parameters such as evaporation method, solvent polarity and viscosity, and warming rate during cluster formation were varied. Cluster/crystallite size and particle surface area were monitored. Additional information was gleaned from Mossbauer, Differential Scanning Calorimetry (DSC), and X-Ray Photoelectron Spectroscopy (XPS). 

Traylor TG, Magde D, Marsters J, Jongeward K, Wu GZ, Walda K. Geminate processes in the reaction of nitric oxide with l-methylimidazole-iron(II) porphyrin complexes. Steric, solvent polarity, and viscosity effects. J Am Chem Soc 1993 115 4808. 

The influence of solvent polarity and viscosity on the deactivation of the S., state of donor- acceptor in substituted trans-stilbenes has been determined by measuring the f. 1. u or e s cen c e... 

The reaction mechanism proposed above seems to be the most probable from the kinetic point of view, but must be treated only as a semiquantitative approach until the role of the ion pair dynamics has been exactly recognized. Systematic studies (similar to those performed in acetonitrile solutions) of solvent polarity and viscosity may provide the decisive answer, probably also making possible a more quantitative description with estimates of all the kinetic parameters. 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Very recently, we reported liquid imidazole-borane complexes (scheme 5)57 that are air stable. Judging from their polarity and viscosity (Table 3), they are expected to be a new class of solvent or electrolyte. Preparation of polymer homo-logues of imidazole-alkylborane complexes will also be reported elsewhere in the near future. 

Haidekker MA, Brady TP, Lichlyter D, Theodorakis EA (2005) Effects of solvent polarity and solvent viscosity on the fluorescent properties of molecular rotors and related probes. Bioorg Chem 33(6) 415-425... 

In Section 3.4, structural effects were often discussed in conjunction with the nature of the solvent. As emphasized in the introduction to this book, the fluorescence emitted by most molecules is indeed extremely sensitive to their microenvironment (see Figure 1.3), which explains the extensive use of fluorescent probes. The effects of solvent polarity, viscosity and acidity deserves much attention because these effects are the basis of fluorescence probing of these microenvironmental characteristics and so, later chapters of this book are devoted to these aspects. The effects of polarity and viscosity on fluorescence characteristics in fluid media and the relevant applications are presented in Chapters 7 and 8, respectively. The effect of acidity is discussed in Sections 4.5 and 10.2. This section is thus mainly devoted to rigid matrices or very viscous media, and gases. 

The rate-limiting nucleophilic addition leading to homodimers formally includes the reversible homolysis of the nucleophilic adduct (110) (e.g., Scheme 11). Indeed the successful isolation of the labile intermediates 5c and 5d from both cyclic cations 3 and 4 indicates that the nucleophilic adduct (feLmo) is the critical intermediate leading to the homodimers 6 [compare Eq. (51) with Eq. (49)]. Bond homolysis (111) as the mechanism for the decomposition of the nucleophilic adduct is experimentally supported by kinetics, ESR studies, as well as spin and chemical trapping. Furthermore, the low sensitivity of the first-order rate constant kH (Fig. 7) on solvent polarity and the marked effect of solvent viscosity are both earmarks of the homolytic bond scission (107,108). 

In solution at ambient temperature the 3t 3p equilibrium is established within a few nanoseconds or even faster [31,180]. Rotation into the twisted configuration is a prerequisite for a triplet mechanism the occurrence of the back step 3p - 3t is based on the effects of temperature, viscosity, polarity, and added quenchers on rT (and 4>T). At room temperature the rate constant for the step 3p -> p is kp = (1-3) x 107 s-, that is, similar to that of stilbene. The equilibrium constant K = [3p ]/[3t ] depends on the substituents, the solvent polarity, and the temperature it shows the general trend to be shifted to the trans side with increasing polarity and decreasing temperature [32],... 

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