Table 4 Excess function *l of the mesoionic 2,3-diphenyl-2H-tetrazolium-5-thiolate (2-H) in waterÈorganic solvent mixtures at di†erent mol
fractions (X ) of water calculated from the equation *l \ l [ l
A
Exp
linear
*l/103 cm~1
X
MeCN
Diox
È
0.65
0.30
0.15
[0.01
[0.37
[0.42
[0.38
[0.24
1.10
DMF
Me CO
EtOH
2-PrOH
MeOH
Py
A
2
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
&*l
0.57
0.54
0.55
0.41
0.13
[0.09
[0.15
[0.27
[0.21
2.20
0.27
0.58
0.75
0.83
0.77
0.66
0.52
0.32
0.15
4.87
0.10
0.15
0.06
0.13
0.01
[0.09
[0.20
[0.29
[0.21
[0.23
[0.28
[0.17
[0.13
[0.06
0.006
[0.07
[0.08
[0.10
[0.17
[0.24
[0.32
[0.18
[0.03
[0.02
[0.11
[0.07
[0.11
[0.09
[0.11
[0.14
[0.09
[0.01
[0.36
[0.52
[0.71
[0.72
[0.38
[0.69
[0.75
[0.96
[0.94
[0.02
0.00
0.07
0.06
1.42
[0.72
[1.43
[1.66
[1.18
[0.74
[5.32
inÑuence of the b term on the l
values may be considered
current study is similar. On the basis of these results, it could
be concluded that both the solvent electron-accepting charac-
ter (AN), or capability to donate a proton in a soluteÈsolvent
hydrogen bond (a), as well as the solvent polarity-polarizablity
(n*) properties are the most important factors necessary to
max
negligible. From the data in Table 2, it can be deduced that
the capability of mesoionic derivatives to form hydrogen
bonds with proton donor solvents (as measured by the a term)
plays an important role in determining the shift of the l
max
values. The positive sign of the a coefficients (Table 2) indi-
explain the dependence of the shift of l
values on the
max
cates that the hydrogen bond formed by the mesoionic deriv-
atives in protic solvents may stabilize the ground state rather
than the excited state, resulting in a hypsochromic shift.
nature of the solvent. The relative contributions of these
correlations (Tables 2 and 3) suggest that the shifts of l
max
depend mainly on the AN and a parameters rather than the
Another good correlation was also found when n* and AN
parameters were used as independent variables; the results
obtained are collected in Table 3. These results demonstrate
that both solvent Lewis acidity (measured by AN) and n*
parameters are important to explain the observed variation in
n* parameter. Moreover, the relative contributions of the AN,
a and n* parameters on the shifts of l
were a†ected by the
max
nature of the substituents on the phenyl moieties of the meso-
ionic derivatives (Scheme 1). Thus, the mesoionic derivatives
that have electron-withdrawing substituents (2-F, 3-F, 2-Cl
the shift of the l
values of the mesoionic derivatives with
and 2-OCH ), except 2-OMe and 2-Cl, which have a higher
mesomeric e†ect that might lead to the opposite trend, show,
max
3
the solvent nature, with relative contributions of 76È91 and
9È24% for AN (using the normalized ANN, the coefficient of
AN was multiplied by 54.8)1 and n*, respectively. When the
donor number (DN) of Gutmann, which measures the
electron-donor ability of the solvent27,28 was included in the
correlation, the multiple correlation coefficients did not sig-
niÐcantly di†er from those obtained when only n* and AN
were considered. When the three independent variables were
taken into account the relative contributions for AN, n* and
DN were in the range 70È88, 8È19 and 4È11%, respectively.
These results suggest negligible inÑuence by the DN term on
in general, lower relative contributions for the AN and a
parameters than those containing electron-donating substit-
uents (2-Me, 3-Me and 4-Me); relative contributions for the
n* parameter show the opposite trend. Furthermore, linear
correlations of l0 values from Tables 2 and 3 are obtained
max
with the Hammett p constant (l0 /103 cm~1 \ 19.53 [ 0.40p,
max
r \ 0.998 and l0 /103 cm~1 \ 19.6 [ 0.80p, r \ 0.97, for
max
Tables 2 and 3, respectively). Supporting this, the electronic
transitions of the mesoionic derivatives are highly inÑuenced
by the substituents on the phenyl moieties. The negative
slopes reÑect the observation that the electron-withdrawing
substituents lead to hypsochromic shifts.
the shift of the l
values. The positive sign of the n* coeffi-
max
cient as shown in Table 3 suggests that the ground state is
more polar than the excited state. This correlates well with the
spectral data of the mesoionic compounds, if one assumes that
the CÈS group of the mesoionic compound plays an impor-
tant role in the electronic transition responsible for the long-
wavelength absorption band and therefore its solvatochromic
behavior. This seems an entirely reasonable assumption since
the transition involved is likely to be an intermolecular
charge-transfer transition involving the Cd`ÈSd~ group and
the delocalized p-electron system of the tetrazolium ring of the
mesoionic derivative (see Scheme 1).20 The longest wavelength
band was assigned as an nÈp* transition,3,12 therefore, as the
polarity-polarizability (n*) of the solvent increases, the ground
state is more stabilized than the excited state. This produces a
hypsochromic shift of the absorption band (positive p*
coefficient). Furthermore, the positive AN coefficients indicate
that the mesoionic compounds are able to donate an electron
pair (or a negative charge) to a solvent that has a high Lewis
acidity.29 Therefore, Lewis acidÈbase interactions of the meso-
ionic compound with the solvent molecules would also stabil-
ize the ground state more than the excited state, resulting
again in a hypsochromic shift of the absorption band.
Binary solvent mixtures. Fig. 1 shows a representative
example for the shift of the l
values of the parent mesoionic
max
derivative (2-H) in eight waterÈorganic solvent mixtures vs.
the mol fractions of water (X ). The non-linear dependence of
A
the l
values on X in the bulk solution mixtures indicates
max
A
preferential solvation.4 The deviation from ideality was
ascribed to the speciÐc interactions between, and of the meso-
ionic compound with, the components of the solvent
mixture.30 The extent and type of the preferential solvation
can be obtained from the magnitudes and signs of the depar-
ture of the experimental shift of l
values from the ideal
linear variation. Fig. 1 shows three distinct patterns of prefer-
max
ential solvation for the mesoionic compound 2-H in waterÈ
organic solvent mixtures: (a)
a negative deviation in
waterÈalcohol (EtOH, 2-PrOH and MeOH) and waterÈPy, (b)
a positive deviation in waterÈMe CO and waterÈDMF, and
2
(c) dual behavior, with both positive and negative deviations,
in waterÈMeCN and waterÈDiox solvent mixtures.
Di†erent criteria were used to evaluate the preferential sol-
vation of the mesoionic compounds in di†erent waterÈorganic
solvent mixtures, viz. the excess function (*l), iso-solvation
point (Xiso) and preferential solvation constant (K).
The quality of the Ðts obtained with the presented multi-
parametric correlations for all mesoionic derivatives in the
New J. Chem., 2001, 25, 502È508
505