Emissive solvatochromism

Sackett and Wolff83 measured the emission peaks of Nile Red in various polar solvents, ranging from 16260 cm-1 for acetone to 15040 cm-1 in water. A theoretical study of the absorption and emission solvatochromic properties of Nile Red (67) was presented recently by Han and coworkers84. The HB strength of the solvents around Nile Red was further studied by means of its fluorescence life time by Cser and coworkers85. A fourfold... 

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)... 

Han WG, Liu T, Himo F, Toutchkine A, Bashford D, Hahn KM, Noodleman L (2003) A theoretical study of the UV/visible absorption and emission solvatochromic properties of solvent-sensitive dyes. ChemPhysChem 4(10) 1084-1094. doi 10.1002/cphc.200300801... 

Tables 11-6, 11-7, and 11-8 show calculated solvatochromic shifts for the nucle-obases. Solvation effects on uracil have been studied theoretically in the past using both explicit and implicit models [92, 94, 130, 149, 211-214] (see Table 11-6). Initial studies used clusters of uracil with a few water molecules. Marian et al. [130] calculated excited states of uracil and uracil-water clusters with two, four and six water molecules. Shukla and Lesczynski [122] studied uracil with three water molecules using CIS to calculate excitation energies. Improta et al. [213] used a cluster of four water molecules embedded into a PCM and TDDFT calculations to study the solvatochromic shifts on the absorption and emission of uracil and thymine. Zazza et al. [211] used the perturbed matrix method (PMM) in combination with TDDFT and CCSD to calculate the solvatochromic shifts. The shift for the Si state ranges between (+0.21) - (+0.54) eV and the shift for the S2 is calculated to be between (-0.07) - (-0.19) eV. Thymine shows very similar solvatochromic shifts as seen in Table 11-6 [92],...

The solvatochromic behavior of these dyes in solution can be explained by the comparison of their permanent dipole moments. If the excited state exhibits a larger dipole moment (pii) than the ground state (/i0), it is preferentially stabilized by the more polar solvent, and the energy between these two states decreases, that is, the absorption and emission spectra both shift to the red region. 

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]

Molecular rotors with a dual emission band, such as DMABN or A/,A/-dimethyl-[4-(2-pyrimidin-4-yl-vinyl)-phenyl]-amine (DMA-2,4 38, Fig. 13) [64], allow to use the ratio between LE and TICT emission to eliminate instrument- and experiment-dependent factors analogous to (10). One example is the measurement of pH with the TICT probe p-A,A-dimethylaminobenzoic acid 39 [69]. The use of such an intensity ratio requires calibration with solvent gradients, and influences of solvent polarity may cause solvatochromic shifts and adversely influence the calibration. Probes with dual emission bands often have points in their emission spectra that are independent from the solvent properties, analogous to isosbestic points in absorption spectra. Emission at these wavelengths can be used as an internal calibration reference. 

Compounds are called solvatochromic when the location of their absorption (and emission) spectra depend on solvent polarity. A bathochromic (red) shift and a hypsochromic (blue) shift with increasing solvent polarity pertain to positive and negative solvatochromism, respectively. Such shifts of appropriate solvatochromic compounds in solvents of various polarity can be used to construct an empirical polarity scale (Reichardt, 1988 Buncel and Rajagopal, 1990). 

If solvent (or environment) relaxation is complete, equations for the dipole-dipole interaction solvatochromic shifts can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres and within the assumption of equal dipole moments in Franck-Condon and relaxed states. The solvatochromic shifts (expressed in wavenumbers) are then given by Eqs (7.3) and (7.4) for absorption and emission, respectively ... 

Solvatochromism can be defined as the phenomenon whereby a compound changes colonr, either by a change in the absorption or emission spectra of the molecnle, when dissolved in different solvents. It is one of the oldest of the chromisms, having been described as long ago as 1878, but nowadays it is usual to extend the concept of the solvent to inclnde solids, micelles and films. A textbook published recently covers in detail the theoretical aspects of solvatochromism. Consequently in this section, the theoretical aspects will be dealt with only briefly before moving onto the practical applications of the topic, which have increased noticeably in the last decade. ... 

In solvatochromism, the observed shifts in the maxima of the absorption or emission spectra of molecules are due to differences in the solvation energies of the ground El) and excited states ( ]) as the nature of the solvent is varied, as shown in Figure... 

The solvatochromic shift is the displacement of an absorption or emission band in different solvents. Figure 3.49 shows examples of such shifts, the transition energy being linear with the Onsager polarity function f D). The... 

The equations of solvatochromic shifts represent the difference between the solvation energies of the initial and final states as a function of solvent polarity. For an absorption spectrum the initial state is of course the ground state and the final state is an excited state this could be Sj or S2 or S3, etc. For an emission spectrum the initial state is an excited state (almost always Sj or Tj) and the final state is the ground state. The two main terms are... 

A thermochromic shift is the displacement of an absorption or emission band with the temperature of the solvent. These displacements result from the change in solvent polarity with temperature, the general rule being that the polarity decreases as the temperature increases. These shifts are small compared with solvatochromic effects and are unlikely to lead to state inversion (Figure 3.52). 

As previously noted (see Section II.B.l), the A emission exhibits an important solvatochromic effect due to the appearance of a large dipole moment in the TICT state. The polar interactions between the solute molecule and the polar environment lead to the reorientation of the solvent molecules and to a relaxation of the electronic energy of the TICT state whose manifestation is a spectral shift during the lifetime of the excited state. The competition between the energy relaxation, whose dynamics is strongly viscosity dependent, and the deactivation of the TICT state has been made evident for DMABN78,89 ... 

The electronic absorption, fluorescence and excitation spectra of these compounds indicate the presence of an internal charge transfer (ICT) excited state giving rise to a fluorescence band that displays strong solvatochromism. Both the emission wavelengths and the Stokes shifts increase with solvent polarity, in agreement with a large increase in dipole moment in the excited state. As the chain length increases the... 

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