Handbook of Solvents - George Wypych - ChemTech - Ventech!

610 Javier Catalán
Figure 10.3.7. Plot of the experimental UV/Vis absorption
maxima,v max, of 4-nitropyridine N-oxide vs. the
predicted v max values according to eq. [10.3.24].
multi-parameter fit of the data for 22 of the solvents revealed that the solvatochromism is
seemingly sensitive to solvent acidity only:
v c.t. = 11.23(±0.75)SA + 21.41(±0.184) [10.3.25]
with n = 22, r = 0.959 and sd = 0.66 kK.
An interesting situation is encountered in the analysis of electronic transitions that results
from chromophores which, assisted by solvent polarity, can undergo a change in the
electronic state responsible for the fluorescence emission; this is a frequent occurrence in
systems involving a TICT mechanism. 68,88-90 In this situation, a plot of Stokes shift against
solvent polarity is a bilinear curve depending on whether the solvents are non-polar (where
the fluorescence is emitted from the normal excited state of the chromophore) or polar
enough for the transition to take place from the more polar state. Unless the solvents used
are carefully selected, it is venturesome to assume that the electronic states will be inverted
simply because the variation of solvatochromism with a function of solvent polarity is
bilinear.
Recently, Sapre et al. 91 showed that a plot of the maximum fluorescence of Neutral
Red (NR) against the solvent polarity function Δf is clearly bilinear (see Figure 10.3.8). Accordingly,
they concluded that, in solvents with Δf > 0.37, the emitting state changes to a
much more polar, ICT state. The analysis of this spectroscopic data in the light of our scales
reveals that, in fact, the solvatochromism of NR is normal, albeit dependent not only on the
polarity of the solvent (SPP), but also on its acidity (SA) (see Figure 10.3.9):
v NR
max = -4.31(±0.57)SPP - 1.78(±0.19)SA + 21.72(±0.47) [10.3.26]
with r = 0.974 and sd = 0.21 kK.
Figure 10.3.8. Plot of the fluorescence maxima,
v max(em), for the Neutral Red in different solvents vs. the
solvent polarity function Δf [Δf = (ε-1)/(2ε +1)-(n 2
-1)/(2n 2 +1)].