Time-resolved solvatochromism

Molecular rotors are useful as reporters of their microenvironment, because their fluorescence emission allows to probe TICT formation and solvent interaction. Measurements are possible through steady-state spectroscopy and time-resolved spectroscopy. Three primary effects were identified in Sect. 2, namely, the solvent-dependent reorientation rate, the solvent-dependent quantum yield (which directly links to the reorientation rate), and the solvatochromic shift. Most commonly, molecular rotors exhibit a change in quantum yield as a consequence of nonradia-tive relaxation. Therefore, the fluorophore s quantum yield needs to be determined as accurately as possible. In steady-state spectroscopy, emission intensity can be calibrated with quantum yield standards. Alternatively, relative changes in emission intensity can be used, because the ratio of two intensities is identical to the ratio of the corresponding quantum yields if the fluid optical properties remain constant. For molecular rotors with nonradiative relaxation, the calibrated measurement of the quantum yield allows to approximately compute the rotational relaxation rate kor from the measured quantum yield [Pg.284]

Time-Resolved Spectroscopy. Steady-state solvatochromic techniques provide a reasonable means to study solvation processes in supercritical media (5,17-32,43-45,59-68). But, unless the interaction rates between the solute species and the supercritical fluid are slow, these "static" methods cannot be used to study solvation kinetics. Investigation of the kinetics requires an approach that offers inherent temporal resolution. Fortunately, time-resolved fluorescence spectroscopy is ideally suited for this task. 

W. R. Ware, P. Chow and S. K. Lee, Time-resolved nanosecond emission spectroscopy spectral shifts due to solvent-solute relaxation, Chem. Phys. Lett., 2 (1968) 356-8 W. Rapp, H. H. Klingenberg and H. E. Lessing, Kinetic model for fluorescence solvatochromism, Her. Bunsen-Ges. Phys. Chem., 75 (1971) 883-6. 

Time-resolved emission spectroscopy (TRES), also referred to as time-resolved Stokes shift spectroscopy, enables one to derive information about the dynamics of biopolymer-solvent interactions on the femtosecond to nanosecond time scales, provided that suitable solvatochromic fluorescent probes have been identified. Such probes should exhibit significant Stokes shifts that change with solvent polarity and should have fluorescent lifetimes on the order of the dynamic solvent exchange process or longer. TRES detects solvent dynamics that influences the energy difference between the excited and the ground states of the fluorophore and is insensitive to dynamic processes that are significantly slower than the fluorescence lifetime. 

The steady-state UV-Vis spectra of functionalized l,3-dithiol-2-ylidene anthracene donor units 20-22 contained solvatochromic bands arising from ICT between the 1,3-dithiole donor and the keto, dicyanomethylene, and cyanoimine acceptor groups. Time-resolved spectroscopic studies indicated that at least two excited states may be formed on photolysis and that the lifetime of 170-400ps strongly depended on the solvent polarity <1998CEJ2580>. 

The concept of reduced solvation of the solvatochromic chromo-phore bound to polymer chain with solvent molecules and preferential intramolecular solvation of the solvatochromic molecule with side subtituent of the polymer backbone was supported by means of time-resolved absorption spectrometry of the merocyanines bonded to poly/ methyl methacrylate/. ... 

At about the same time as this work was published, Znamenskiy and Kobrak simulated the absorption spectrum of betaine-30, a commonly used solvatochromic probe molecule, in [C4inim][PF6]. They investigated the interactions responsible for the solvatochromic shift. Because this shift is used experimentally to assess solvent polarity, the calculations can thus provide a direct window into the nature of polarity in ionic Hquid systems. To conduct the study, a single molecule of betaine-30 was immersed in a Hquid containing 200 ion pairs. Twelve independent 1-ns runs were then carried out, and from that the absorption spectrum was computed. They observe two distinct time scales one on subpicosecond time scales and one that is on the order of 100 ps. This result is consistent with previous simulation studies as well as time-resolved fluorescence spectroscopy experiments. Although the actual absorption spectra computed do not agree quantitatively with experimental results, the qualitative features do. 

Redecker, M., Bradley, D., Baldwin, K., Smith, D., Inbasekaran, M., Wu, W., Woo, E. An investigation of the emission solvatochromism of a fluorene-triarylamine copolymer studied by time resolved spectroscopy. J. Mater. Chem. 9, 2151-2153 (1999)... 

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