Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 589
Scheme III.
ize the scale by assigning a zero value to tetramethylsilane and a unity value to water, using
the following expression:
N
ET =(ET(solvent) -ET(TMS))/(ET(water) -ET(TMS))=(ET(solvent) -30.7) / 32.4 [10.3.6]
N
The corresponding ET values (between 0 and 1) allow one to rank all solvents studied.
This is no doubt the most comprehensive solvent scale (it encompasses more than 300 solvents)
and also the most widely used at present. 1,13 The probe (1) exhibits solubility problems
in non-polar solvents that the authors have overcome by using a tert-butyl derivative
(2). One serious hindrance to the use of this type of probe is its high basicity (pKa = 8.64 49 ),
which raises problems with acid solvents.
10.3.3.4 The Py scale of Dong and Winnick
The Py scale 50 is based on the ratio between the intensity of components
(0,0) I1 and (0,2) I3 of the fluorescence of monomeric pyrene (3) in various
solvents. It was initially established from 95 solvents 50 and spans values
from 0.41 for the gas phase to 1.95 for DMSO. This scale is primarily used
in biochemical studies, which usually involve fluorescent probes. However,
it poses problems arising largely from the difficulty of obtaining precise values
of the above-mentioned intensity ratio; this has resulted in divergences
among Py values determined by different laboratories. 51 One further hindrance
is that the mechanism via which low-polar solvents enhance the intensity of symmetry-forbidden
vibronic transitions through a reduction in local symmetry is poorly
understood. 52
10.3.3.5 The Z scale of Kosower
The Z scale 53,54
is based on the strong solvatochromism of the 1-ethyl-4-
(methoxycarbonyl)pyridinium iodide ion-pair (4) and defined by the position (in kcal mol -1 )
of the maximum of its first absorption band, which has marked intermolecular charge-transfer
connotations according to Scheme IV (the excited state of the chromophore is much less