The solvatochromic behavior and degree of ionicity of a synthesized pentacyano (N-substituted-4,4′-bipyridinium) ferrate(II) complex in different media. Tuning the solvatochromic intensity in aqueous glucose solutions

Article abstract of DOI:10.1016/j.chemphys.2013.12.008

A new solvatochromic pentacyanoferrate(II) complex with a 4,4′-bipyridine based dicationic ligand was synthesized and characterized. Its chromotropic behavior markedly affected by solvent polarity was examined in different media i.e. neat solvents, binary solvent mixtures, and aqueous glucose solutions, and quantified using suitable solvent polarity parameters. The metal to ligand charge transfer (MLCT) energy shows a linear correlation with the Reichardt solvent polarity parameter as well as the Kirkwood dipolarity function and suitable functions expressing polarizability. Using suitable Linear Solvation Energy Relationships (LSERs) it was concluded that mainly specific solute-solvent interactions contribute to the solvatochromic phenomenon. The investigation of the solvatochromic behavior of the title compound in aqueous glucose solutions revealed a different response to medium polarity. Tuning of the solvatochromic intensity of the title compound by changing the type of medium was further studied focusing on aqueous glucose solutions.

Full text of DOI:10.1016/j.chemphys.2013.12.008

Chemical Physics 430 (2014) 29–39
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Chemical Physics
journal homepage: www.elsevier.com/locate/chemphys
The solvatochromic behavior and degree of ionicity of a synthesized
pentacyano (N-substituted-4,40-bipyridinium) ferrate(II) complex in
different media. Tuning the solvatochromic intensity in aqueous
glucose solutions
⇑
Raffaello Papadakis
Aix-Marseille Université, ISM2, UMR CNRS 7313, 13397 Marseille, France
The School of Chemical Engineering, National Technical University of Athens (NTUA), Laboratory of Organic Chemistry, 15780 Athens, Greece
a r t i c l e i n f o
a b s t r a c t
A new solvatochromic pentacyanoferrate(II) complex with a 4,40-bipyridine based dicationic ligand was
synthesized and characterized. Its chromotropic behavior markedly affected by solvent polarity was
examined in different media i.e. neat solvents, binary solvent mixtures, and aqueous glucose solutions,
and quantified using suitable solvent polarity parameters. The metal to ligand charge transfer (MLCT)
energy shows a linear correlation with the Reichardt solvent polarity parameter as well as the Kirkwood
dipolarity function and suitable functions expressing polarizability. Using suitable Linear Solvation
Energy Relationships (LSERs) it was concluded that mainly specific solute–solvent interactions contribute
to the solvatochromic phenomenon. The investigation of the solvatochromic behavior of the title com-
pound in aqueous glucose solutions revealed a different response to medium polarity. Tuning of the sol-
vatochromic intensity of the title compound by changing the type of medium was further studied
focusing on aqueous glucose solutions.
Article history:
Received 6 September 2013
In final form 17 December 2013
Available online 31 December 2013
Keywords:
Solvatochromism
Pentacyanoferrate(II) complex
Solute–solvent interactions
Degree of ionicity
Glucose
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
have been also reported [6–8]. The aforementioned marked med-
ium responsive character reinforced their use as sensors of special
Pentacyanoferrate(II) complexes with suitable electron accept-
ing ligands, are a widely known class of solvatochromic coordina-
tion compounds [1–5]. Their intense solvatochromism resulting
from the solvent polarity dependent metal-to-ligand-charge-trans-
fer (MLCT) is pronounced in systems possessing the electron-
donating FeII center coordinated to an electron-deficient ligand,
such as aryl-substituted 4,40-bipyridines [6], 4-aryl or 4-hetero-
aryl-substituted 40-vinylpyridines [7], or N-methyl-pyrazine [8].
Their medium responsive character has been shown to be affected
by Lewis acidity [6] and hydrogen-bond-donor (HBD) acidity [7], as
well as the dipolarity and polarizability of solvents [6,7,9]. They
present negative solvatochromism attributed to the lowering of
the dipole moment in their excited states with respect to their
ground states [10]. The latter effect results in bathochromic shifts
of the visible MLCT bands in their electronic absorption spectra,
by decreasing solvent polarity. The changes in most of the cases
are obvious by naked-eye [1]. Their intense responses to small
solvent polarity changes (e.g. between water and methanol)
modifiers in mutual aqueous solutions. Pinheiro et al. used a tetra-
cyano RuII solvatochromic complex as a sensor of traces of water in
aprotic organic solvents [11]. In this work the solvatochromic
behavior of a new synthesized and fully-characterized pentacyano-
iron(II) complex in different media, including aqueous glucose
solutions, neat solvents and binary solvent mixtures is examined,
taking into account both specific and non-specific solute–solvent
interactions. To the best of the author’s knowledge, just a few
works related to the use of medium responsive probes in glucose
solutions have been published so far. The work of Spange et al.
for the determination of the values of ENT (normalized Reichardt
polarity scale [10]) of aqueous and DMSO sugar solutions, and
the solvatochromic parameters
a
and p⁄ (HBD acidity and
dipolarity/polarizability parameters respectively of the Kamlet,
Abboud and Taft equation [12]) of sugar DMSO solutions, being
the most significant [13]. Nevertheless the use of pentacyano-
iron(II) complexes for such a purpose has never been reported, al-
beit the use of the latter complexes can give valuable informations
for Lewis and HBD-acidity as well as dipolarity and polarizability of
aqueous glucose solutions. Besides, using different probes leads to
a better understanding of the solvating ability, dielectric effects as
well as specific solute–solvent effects in glucose solutions. The
⇑
Address: Aix-Marseille Université, ISM2, UMR CNRS 7313, 13397 Marseille,
France. Tel.: +33 49128197.
E-mail address: rafpapadakis@gmail.com
0301-0104/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.chemphys.2013.12.008

30
R. Papadakis / Chemical Physics 430 (2014) 29–39
rationalization of the aforementioned effects are of high interest
mainly because of the biological importance of glucose. For that
purpose suitable solvatochromic probes such as the title complex
can be used. Since glucose modifies significantly both dipolarity
[14,15] and polarizability [16] as well as Lewis acidity of water
[13], a pentacyanoferrate(II) complex could serve as suitable solva-
tochromic probe. In this work the degree of ionicity of the title
compound which is a measure of solvatochromic intensity (degree
of charge transfer) in all types of media examined, was determined
and its relation with the medium responsive character of the title
compound is discussed. Rationalizing the different responses of
the MLCT energy of the title compound, strongly dependent on
the type of medium, leads to the ability of controlling solvatochro-
mic intensity.
analyses (TGA) were performed under N2 on a Mettler Toledo
TGA/SDTA 851. Finally cyclic voltammetry measurements were
performed in aqueous NaCl solutions 1 M, using
a glassy
carbon working electrode at scan rates varying between 10 and
500 mV/s. The reference electrode was Ag/AgCl and the counter
electrode was Pt. All measurements were performed at 25 °C.
3. Calculations
All correlations were performed using the program QtiPlot.
Structures of different conformers of 4 were generated with Molec-
ular Mechanics conformational searching using Avogadro 1.0.0. The
structure was first drawn in Chemtool and then optimized using
UFF force field. The resulted configuration was further used as in-
put for conformational searching. The donor–acceptor distance
needed for calculation of ground and MLCT-excited state dipole
moments difference, through the methodology of Saito, was calcu-
lated based on the as optimized structure. Van der Waals radius of
compound 4, was calculated using 3 V: Voss volume voxalator,
available on-line. The latter was directly used as the cavity term
in Suppan–Tsiamis equation.
2. Experimental part
2.1. Materials and methods
All products were purchased from ACROS Organics. The sol-
vents were HPLC-grade and they were purified prior to use, accord-
ing to literature [17]. Water was purified with a Barnstead EASY
pure RF compact ultrapure water system and then distilled twice.
Experimental details regarding the synthesis of compounds 1–4,
in Supporting Information: Paragraphs 1.1–1.3.
4. Results and discussion
4.1. Synthesis and characterization of the solvatochromic FeII complex
2.2. Physical measurements
The pentacyanoferrate complex (compound 4) used in this work
possesses a dicationic 4,40-bipyridine based ligand as shown in
Scheme 1. As depicted in Scheme 1 4,40-bipyridine reacts with
the precursor 2 (a 1:1 product of 4-dimethylamino-pyridine: 4-
DMAP and a,a0-dichloro-p-xylene) to give in high yield the desired
ligand (compound 3). After subsequent reaction of the latter with
Na3[FeII(CN)5NH3] in water, solvatochromic compound 4, was pro-
duced as a deep blue solid. Product 4, was isolated as a monoso-
dium salt, very soluble in water and polar organic solvents. All
products were analyzed by means of several spectroscopic tech-
niques (such as FTIR, 1H and 13C NMR, and UV–Vis spectroscopy
as well as elemental analysis, see paragraphs 1.1–1.4, Supporting
Information).
Solvatochromic product 4, is isolated as a hydrated salt, con-
taining 9 water molecules per molecule as confirmed by thermo-
gravimetric and elemental analyses (4.9H2O). The structure of 4,
corresponds to the type of bolaform electrolytes. These kind of
small bolaform amphiphiles containing p-xylene as the hydropho-
bic flexible spacer, have been synthesized in the past for uses in
NMR spectra were obtained using a Varian Gemini 300 spec-
trometer (300 MHz 1H, 75 MHz 13C). 1H spectra were recorded in
D2O at 25 1 °C. The 13C NMR spectra were recorded in D2O con-
taining drops of DMSO-d6. 1H NMR spectra were calibrated by
using residual undeuterated solvent peak. UV–Vis spectra were re-
corded using a Varian CARY 1E UV–Vis spectrophotometer. Regard-
ing the solvatochromism of compound 4, typically solutions with a
concentration of 750 ppm (approx. 1 mM) were prepared right be-
fore any measurement, and measured at 25 1 °C. Each measure-
ment was repeated three times; therefore, each of the values of
MLCT energies listed in Tables 1 and 2 correspond to the average
of three measurements (standard deviation 0.5 nm). IR spectra
were recorded on a Perkin–Elmer Spectrum 1 FTIR spectrophotom-
eter in the solid state (without any preparation of the samples)
using the attenuated total reflectance technique (ATR) in the re-
gion 600–4000 cmꢀ1. Elemental analyses were performed on a Per-
kin–Elmer Elemental Analyzer 2400 CHN. Thermogravimetric
Table 1
MLCT energies of 4 in ten solvents and binary solvent mixtures and corresponding solvent polarity parameters.
a
eb
nc
ET(30)d (kcal/mol)
Ef (kcal/mol)
ETN
e
Solvent
EMLCT (kcal/mol)
H2O
EG
Gly
FA
MeOH
NMF
53.592
47.652
47.258
45.527
42.610
45.684
48.874
47.731
49.810
48.459
78.5
1.3325
1.4318
1.4746
1.4475
1.3264
1.4319
1.3617
1.3583
1.4051
1.3363
63.1
56.3
57.0
55.8
55.4
54.1
56.5
54.9
59.7
58.4
1.000
0.790
0.812
0.775
0.762
0.722
0.797
0.747
0.895
0.856
21.736
14.895
15.371
13.704
14.756
11.907
15.562
14.256
18.355
17.332
41.4
42.5
109.5
33.8
182.4
38.1
30.0
EtOH–H2O, 50% (v/v)g
AcMe–H2O, 50% (v/v)g
EG–H2O, 50% (v/v)g
MeOH–H2O, 50% (v/v)g
44.7
51.9
a
b
c
d
e
f
MLCT energy.
Static dielectric constants of the solvents used, measured at 25 °C [26].
Refractive indices of the solvents used (25 °C) [27].
Reichardt’s solvent polarity scale [23].
Reichardt’s normalized polarity scale [23].
Solvent Lewis acidity parameter of Koppel–Palm equation [23].
g
Values
e [28], n [29], ET(30) [30], and E for these solvent mixtures are determined according to Supporting Information, Paragraphs 2 and 3.

R. Papadakis / Chemical Physics 430 (2014) 29–39
31
Table 2
MLCT energies of 4 in aqueous glucose solutions at eight different concentrations of glucose and corresponding solvent polarity parameters.
a
eb
nc
ET(30)d (kcal/mol)
Ef (kcal/mol)
ETN
e
Glucose concentration (g/L)
EMLCT (kcal/mol)
0
40
80
120
160
200
240
280
320
53.592
53.492
53.492
53.442
53.243
53.144
53.144
53.045
52.947
78.54
77.61
76.64
75.62
74.55
73.44
72.28
71.06
69.80
1.3325
1.3386
1.3446
1.3507
1.3567
1.3628
1.3689
1.3749
1.3810
63.10
62.21
61.57
61.12
60.79
60.55
60.37
60.21
60.08
1.000
0.973
0.953
0.939
0.929
0.921
0.916
0.911
0.907
21.736
20.830
20.170
19.689
19.336
19.071
18.862
18.688
18.533
a
b
c
d
e
f
MLCT energy.
Static dielectric constants of the solutions used, measured at 25 °C [14]. (See also: Supporting Information, paragraph 4).
Refractive indices of the solutions used at 25 °C (Supporting Information, paragraph 4).
Reichardt’s solvent polarity scale [13]. Also: Supporting Information, paragraph 5.
Reichardt’s normalized polarity scale [13].
Solvent Lewis acidity parameter of Koppel–Palm equation.
Scheme 1. Synthetic route followed for the preparation of solvatochromic compound 4.
coloration technologies [18], rotaxanes and pseudorotaxanes syn-
thesis [19,20] as well as conformational probes of hydrophobic,
complexes with N-substituted-4,40-bipyridines [7]). Nevertheless
the value reported herein is slightly lower than the values reported
by Coe et al. for N-methyl- and N-phenyl-bipyridinium corre-
sponding pentacyanoferrate(II) complexes [7]. This is attributed
to the much different N-substituent of 4,40-bipyridine of
compound 4. It is though closer to the E1/2 for FeII/FeIII reported
by Almaraz et al. for [4-CN-py-FeII(CN)5]3ꢀ were py symbolizes pyr-
idine [24]. The latter observation is an indication that the monocat-
ionic substituent [a,a0-DMAP-xylene]1+ behaves as anticipated as a
worse electron accepting substituent than aryls do. This is consis-
tent with the results of UV–Vis experiments as will be analyzed.
The complexation of the dicationic ligand 3 with pentacyano-
iron(II) induced important shifts in the proton NMR spectra,
especially to protons of 4,40-bipyridyl (Supporting Information,
Fig. S3). All signals became broader after complexation and the
expected splitting patterns were not observed. This has been
reported for other pentacyanoferrate(II) complexes [25].
p-stacking interactions in water solutions [21], or even for the
adsorption of anionic molecules on monolayers for the construc-
tions of multilayers [22]. In case of compound 4, the incorporation
of a pentacyanoferrate(II) group on the bolaform backbone of 3,
renders the title compound capable of developing, in solution,
hydrogen bonds (HBD-solvents with cyano groups of [FeII(CN)5]ꢀ3
)
as well as dipole–dipole interactions. The aforementioned interac-
tions are of great interest for the study of solvatochromism of com-
pound 4 [23]. In Fig. 1, different conformers of the title bolaform
pentacyanoferrate complex are depicted. Both conformers corre-
spond to geometrically optimized structures. In both cases (a)
and (b) 4,40-bipyridine and DMAP branches lie on different sites
of the plane of the xylene bridge, resulting in quasi helical
conformations.
Cyclic voltammetry measurements revealed
a reversible
FeII/FeIII oxidation as shown in Supporting Information, Figs. S1
and S2. E1/2 for FeII/FeIII was determined to be 0.56 V vs Normal
Hydrogen Electrode (NHE). This value is very close to the E1/2
values of published pentacyanoferrate(II) with N-substituted-
4,40-bipyridinium ligands in water [24] (according to Coe et al.
solvent effects become very important for pentacyanoferrate
4.2. UV–Vis spectra and solvatochromism of compound 4
Complex 4, as mentioned, is an intensely solvatochromic com-
pound. Its electronic absorption spectra include two important fea-
tures in the wavelength range 200–900 nm. The lower energy band
Fig. 1. Two different conformers of the anion (4)1-
.

32
R. Papadakis / Chemical Physics 430 (2014) 29–39
appears at about 270 nm, it is sharp and is attributed to the intral-
igand (aromatic)
p⁄ excitation [6,7]. This band is not signifi-
p–
cantly altered in terms of shape, absorbance or position, when
compound 4 is dissolved in solvents of different polarity [6]. The
second band is a significantly broader band of lower absorbance
as compared to the
p–
p⁄ band and appears in the visible region
(533 to even 750 nm) strongly depending on solvent polarity
[6,7,9]. The latter band is attributed to a MLCT band assigned as
dp
(FeII) ? p⁄(L). The value 533 nm (position of MLCT absorption
maximum in water) is very close to the one reported by Coe
et al. for pentacyano (N-methyl-4,40-bipyridinium) ferrate (II)
(534 nm) [7], but shifted hypsocromically as compared to (N-aryl
substituted 4,40-bipyridinium)ferrates (II) [e.g. for pentacyano
(N-p-tolyl-4,40-bipyridinium) ferrate (II), kMLCT(max) = 566 nm, as
reported earlier by the author et al. [6]]. The latter observation
reflects the decrease of the MLCT energy when the nitrogen
substituent of the ligand is in conjugation with the aromatic
4,40-bipyridinium spacer. As shown in Fig. 2, red-shifts as high as
approx. 140 nm (3,840 cmꢀ1) were observed when going from
water to MeOH, which corresponds to a relatively small change
Fig. 3. Vis MLCT band of 4 in glucose aqueous solutions varying glucose concen-
tration between 0 and 320 g/L. The bands absorbances are set in increasing mode
with increasing glucose concentration to better visualize the solvatochromic effect.
in solvent polarity (i.e.
D
k = 137.5 nm for
D
ENT ¼ 0:238, where
ENT is Reichardt’s normalized polarity scale). The vis MLCT band of
4 in different solvents and solvent mixtures, is shown in Fig. 2.
The results are listed in Table 1 along with corresponding solvent
polarity parameters.
separation of the metal
p
and ligand p⁄ orbitals, therefore the
MLCT energy is higher. That is also observed when solvent dipolar-
ity increases. Since the ground state is more polar than the MLCT
excited state, dipolar solvents tend to stabilize more the ground
state increasing the MLCT energy. In both cases (i.e. when either
acidity or dipolarity increases) hypsochromic shifts of the vis MLCT
band are observed. Since increasing glucose content in aqueous
solutions results in lower dipolarity and Lewis acidity of the med-
ium, bathochromism is observed in the vis spectrum of 4, as shown
in Fig. 3. The MLCT energy of 4 in a 320 g/L glucose solution is low-
er by roughly 0.645 kcal molꢀ1 (226 cmꢀ1) than in water. As will be
analyzed, this small change reflects the significantly lower sensi-
tivity on overall solvent polarity, as compared to that observed in
the group of solvents and binary solvent mixtures examined.
Another very interesting feature of the solvatochromic effect of
4 in different media, is the increasing tendency of the bandwidth at
half-height of the MLCT band, which keeps up with the bathochro-
mic effect induced by lowering solvent dipolarity and Lewis acid-
ity. The bandwidths seem to be proportional to the square root
of the MLCT absorption maxima wavenumbers as shown in
Supporting Information, Fig. S6. The impact of glucose concentra-
tion on the MLCT bandwidth is depicted in Fig. 4 (MLCT band-
widths were determined according to Supporting Information,
paragraph 6). A steady linear increase of the MLCT bandwidth is
observed up to 120 g/L. After this concentration the bandwidths
increasing tendency becomes smaller until 320 g/L (which was
the highest glucose concentration examined). These broadening
of the MLCT band upon increasing glucose content (lowering
polarity of the medium) may be associated with the alteration of
solvent (water) relaxation time, an effect which can be related to
water-glucose hydrogen bonding, which gets more intense when
glucose content increases. Interestingly a linear dependence of
the MLCT bandwidths on Reichardt’s solvent polarity scale ET(30)
was observed (Supporting Information, Fig. S7b). Increased MLCT
bandwidths are also observed as going from polar solvents to less
polar ones. This is obvious for instance between water and MeOH
in Fig. 2.
Solvatochromism was also investigated in aqueous glucose
solutions of different glucose concentrations. The results are listed
in Table 2 and the MLCT band shifting depending on glucose con-
centration is depicted in Fig. 3. Increasing dipolarity and/or HBD-
acidity of the medium, induces hypsochromic shifts of the MLCT
vis bands of 4. This behavior corresponds to negative solvatochro-
mism since the ground state is more polar than the excited MLCT
state [6–10]. The effect of glucose concentration on the solvato-
chromic behavior of pentacyanoferrate(II) complexes has never
been reported before. Qualitatively the solvatochromic effect re-
mains the same in case of different media like pure solvents, binary
solvent mixtures and aqueous sugar solutions. Increasing medium
Lewis acidity and HBD-acidity, results in an increased tendency of
development of hydrogen bonds between hydrogen atoms of the
solvent molecules and the cyano groups of compound 4. Thus elec-
tron density is removed from the CN ligands and the p-back-bond-
ing with the metal is increased. The latter increase leads to a higher
4.3. Quantifying solvent effects
Fig. 2. Normalized vis absorption spectra of 4 (MLCT band) in different solvents:
H2O:black (1); EtOH-H2O, 50% (v/v): blue (2); acetone–H2O, 50% (v/v): orange (3);
EG (ethylene glycol): magenta (4); glycerol (Gly): olive (5); FA (Formamide): red
(6); MeOH: green (7). Arrow indicates redshift of MLCT band when decreasing
solvent polarity. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
As mentioned compound 4, exhibits negative solvatochromism
in neat solvents, binary solvent mixtures as well as in aqueous glu-
cose solutions. In order to quantify solvent polarity effects of 4,
arising from the interactions of its molecules with molecules of

R. Papadakis / Chemical Physics 430 (2014) 29–39
33
Fig. 4. Dependence of MLCT bandwidths of 4 (determined at half band height) on
glucose concentration. Error bars correspond to the regression errors (Supporting
Information: paragraph 6). Inset: broadening of vis MLCT band of compound 4 in
aqueous glucose solutions.
the solvent, either specific or not, different solvent polarity param-
eters can be used. It has been pointed out in earlier publications
that the position of MLCT bands of pentacyanoferrate(II) com-
plexes with 4,40-bipyridine based ligands, mainly depends on
Lewis acidity and dipolarity/polarizability of solvents [1,6,9].
Linear correlations of the MLCT energies measured in different sol-
vents with Reichardt’s solvent polarity parameter ET(30) [6,31] as
well as parameters describing solvent Lewis acidity e.g. Gutmann’s
acceptor number (AN) [6,31–33] and hydrogen-bond-donor (HBD)
acidity parameter a(KAT) of the Kamlet–Abboud–Taft (KAT) equa-
tion [9] have been reported. The group of neat solvents/solvent
mixtures used herein is well chosen including both protic and
non-protic solvents, spanning a polarity range, as described by
ET(30) polarity scale of Reichardt, of nearly 9 kcal molꢀ1. MLCT
energies of compound 4 correlate well to Reichardt’s solvent polar-
ity scale, as shown in Fig. 5a. As parameter ET(30) increases, the
MLCT energy increases linearly. The MLCT energies in binary sol-
vent mixtures do not deviate from this linear behavior. The slope
of the obtained line is in the order of magnitude of already de-
scribed pentacyanoferrate(II) complexes, thus exhibiting a typical
sensitivity to solvent dipolarity/polarizability and Lewis acidity.
Additionally, MLCT energies of compound 4 measured in different
solvents and solvent mixtures are correlated slightly better with
the empirical parameter E of Koppel–Palm equation describing
Lewis acidity (Fig. 5b). For a given solvent this parameter is derived
from Reichardt’s parameter ET(30) by excluding the influence of
non-specific effects [23]. The meaning of the latter two correlations
is that solvent Lewis acidity is very important for the observed
solvatochromic effect, thus specific solute–solvent effects seem to
have a significant contribution.
Fig. 5. Dependence of MLCT absorption maxima energies of 4 in neat solvents and
binary solvent mixtures (listed in Table 1) on the solvent polarity parameters ET(30)
(a) and E (b).
On the other hand MLCT energies show poorer correlations with
parameters expressing dipolarity and polarizability of solvents.
Kirkwood polarity function f(e)
(Eq. (1)) and function f(n2)
(Eq. (2)) were used herein as measures of dipolarity and polariz-
ability, respectively. Kirkwood function has been widely used in
the past in order to quantify solvent effects on the spectra of solva-
tochromic compounds [23,34] as well as solvent effects on chemi-
cal reactions [35]. MLCT energies of 4, tend to increase as Kirkwood
function f(e) increases (Fig. 6a; e is the static dielectric constant of
the solvent). This is consistent with the positive slope of the line
shown in Fig. 5a (EMLCT / ET(30)), since ET(30) is also a measure
of dipolarity of solvents. Some deviations from linearity were
observed for solvents FA and NMF. These solvents present very
Fig. 6. Dependence of MLCT absorption maxima energies of 4 in neat solvents and
binary solvent mixtures (listed in Table 1) on the functions f(e
) (a) and f(n2) (b).

34
R. Papadakis / Chemical Physics 430 (2014) 29–39
high static dielectric constants than all the other solvents used
(even water) nevertheless, they are characterized by ET(30) values
lower than water. This is an indication that specific solute–solvent
interactions play here a more important role than specific ones.
Furthermore a linear dependence of EMLCT on function: f(n2)
expressing polarizability was observed. The latter function ex-
presses solvent polarizability (n is the refractive index of the sol-
vent). Interestingly some hydroxylic solvents/solvent mixtures
tend to deviate as shown in Fig. 6b. For most of the selected group
of solvents and solvent mixtures though, the general observation is
that increasing polarizability (as expressed by function f(n2)) re-
sults in decreasing the MLCT energy of 4. The results of all linear
correlations of MLCT energies of 4 in different solvents with differ-
ent solvent polarity parameters are listed in Table S1 of Supporting
Information.
e
ꢀ 1
fð
e
Þ ¼
ð1Þ
ð2Þ
2
e
þ 1
n2 ꢀ 1
fðn2Þ ¼
2n2 þ 1
Correlations of MLCT energies of 4 in glucose aqueous solutions
with the solvent polarity parameters ET(30), E, and functions f(e)
and f(n2) revealed a different sensitivity to solvent polarity (see
Table S1 of Supporting Information). Concerning polarity parame-
ters describing non-specific solute–solvent interactions, the results
obtained were comparable to those in case of neat solvents and
solvent mixtures. First of all regarding the dependence of the MLCT
energies on dipolarity as expressed by Kirkwood’s function f(e) in
glucose aqueous solutions, a linear correlation was observed. The
slope in this case was positive meaning that as glucose content
of a glucose aqueous solution increases causing a drop of the per-
mittivity of the medium, the MLCT energy of 4, decreases linearly.
This is consistent with the results in case of the group of solvents
and solvent mixtures. Nevertheless, a smaller slope in case of glu-
cose solutions was observed, as shown in Fig. 7b (see also Support-
ing Information Table S1). Such a linear correlation is anticipated
since the permittivity of a glucose aqueous solution is decreasing
linearly with increasing glucose concentration (Supporting
Information, Fig. S4), whereas the MLCT energy tends to decrease
linearly as glucose concentration increases (Fig. 7a). Small discrep-
ancies of the sensitivities (slopes) of MLCT energies on parameter
f(e) were observed within glucose concentration range 0–320 g/L
(values listed in Table S1 of Supporting Information). On the other
hand a decreasing tendency of EMLCT of 4 is observed when the
function f(n2) increases (Fig. 7c). This is also consistent with
the behavior observed in most of the solvents (shown in Fig. 6b).
The slope in that case was slightly smaller than in case of solvents
(excluding MeOH and MeOH/H2O 50%). Some small but notable
discrepancies of the sensitivities (slopes) of MLCT energies on
parameter f(n2) were also observed within glucose concentration
range 0–320 g/L (values listed in Table S1 of Supporting
Information).
Nevertheless MLCT of 4, responds in a much different way to
changes of Lewis acidity in glucose-aqueous solutions, than does
in neat solvents and solvent mixtures. As shown in Fig. 8a, starting
form neat water and up to roughly 120 g/L the MLCT energies seem
to be practically insensitive to the changes of ET(30) (slope: 0.070,
Table S1, Supporting Information). In glucose concentration range
120 6 Cglu 6 320 g/L, the sensitivity of MLCT energies of 4 on
ET(30) becomes higher (slope: 0.380, that is approx. 5.5 times
greater than observed for range 0 < Cglu 6 120 g/L). This is also
obvious in Fig. 8a. This bigger slope is still significantly lower than
the slope (sensitivity) obtained for the examined group of solvents
and binary solvent mixtures (Supporting Information: Table S1 and
Fig. S11a). Correlations of the MLCT energies of 4 with the Lewis
Fig. 7. Dependence of MLCT absorption maxima energies of 4 in glucose aqueous
solutions, on glucose concentration (a), and polarity functions f(
) (b) and f(n2) (c).
Error bars in plots are based on the resolution uncertainty, which corresponds to an
uncertainty of 0.05 kcal/mol.
e
acidity of solvents parameter E (Fig. 8b), result in the same pattern
as in case of EMLCT / ET(30) correlations.
The latter effect has been reported before in case of solvatochro-
mic organic betaines exhibiting intramolecular charge transfer
[36,37]. In those cases different sensitivity on ET(30) was also

R. Papadakis / Chemical Physics 430 (2014) 29–39
35
transformed to D(1 ꢀ d)+
-p
-A(1 ꢀ d)ꢀ in the excited state (MLCT
excited state for 4). For 4, this transition can be written as: FeII- -N+
p
? FeIII- -N+ꢀ. When d is very close to the value 0.5, it is obvious that
p
the ionicity between the ground and excited state is roughly the
same, thus the difference between ground and excited dipole mo-
ments approaches zero. Taking into account that the driving force
of solvatochromism is the aforementioned difference, no solvato-
chromism is expected when d ꢁ 0.5, whereas maximization of the
intensity of solvatochromism is anticipated when d = 1. Saito points
out that the difference (lg
when d ꢁ 0.5, as implied by Eq. (3).
ꢀ
le) decreases and approaches zero
ðle
ꢀ
lgÞ ¼ ð1 ꢀ 2dÞ ꢂ er
ð3Þ
In this equation g is the ground state dipole moment,
l
le is the
excited state dipole moment, e is the electron charge, and r is the
distance between Dd+ and Adꢀ. In order to estimate d, Saito et al.
introduced a method which takes into account that Reichardt’s sol-
vatochromic zwitterionic dye depicted in Supporting Information,
Scheme S2, when d = 1. It is then concluded that ionicity of a com-
pound of the type Dd+
-p
-Adꢀ can easily be determined by Eq. (4):
a
a1
¼ ð2d ꢀ 1Þ
ð4Þ
where a1 and a are the slopes of the lines determined by Eq. (5) (a1
for Reichardt’s dye, a for the solvatochromic dye of interest).
ECT ¼ aET ð30Þ þ b
ð5Þ
In Eq. (5), ECT is the charge transfer energy for a given solvato-
chromic dye of the type Dd+ -Adꢀ, ET(30) is the Reichardt’s solvent
-p
polarity scale and b is the intercept of this linear equation corre-
sponding to the charge transfer energy when ET(30) is minimized
to zero (the case of TMS: tetramethylsilane or gas phase [23]). a1
is the slope for Reichardt’s zwitterionic dye and a the slope deter-
mined for a solvatochromic dye of the type: Dd+ -Adꢀ. It is obvious
-p
that for Reichardt’s dye it is: a1 = 1, because d = 1. Assuming now
that for the solvatochromic dye: Dd+ -Adꢀ it is
= 1 when ionicity
is equal to 1 (d = 1), the ionicity Dd+ -Adꢀ can easily be determined
-
p
p
a
-
Fig. 8. Dependence of MLCT absorption maxima energies of 4 in glucose aqueous
solutions, on solvent polarity parameters ET(30) (a) and E (b).
through Eq. (4). As mentioned before for a given solvatochromic
compound, it is possible to observe different sensitivity on solvent
polarity (as expressed by parameter ET(30)) within a given range of
solvent polarity. In those cases this effect can be attributed in
terms of the theory analyzed above, to the alteration of the ionicity
of the solvatochromic compound within this range of solvent
polarity. According to those regression values and Eq. (4) three dif-
ferent cases are recognized within solvent polarity range:
54.1 6 ET(30) 6 63.1 kcal molꢀ1, including all types of media exam-
ined herein. The corresponding three ionicity values of 4, regarding
these three cases, are mentioned in Fig. 9.
observed depending on the type of solvents. These differentiations
within a given range of solvent polarity are connected to the dipole
moment differences between the ground and excited state of sol-
vatochromic molecules [36–38]. Analyzing the different polarity
in ground and excited states can provide insights of the nature of
solvatochromism and rationalize those different behaviors
depending on the nature of the medium.
The difference between the the excited MLCT state dipole mo-
ment and the ground state dipole moment, calculated through
Eq. (3), considering a distance between Fe(II) and N+ (para quatern-
ized nitrogen) of approx. 9 Å (Supporting Information, Fig. S8) is
4.4. Ionicity of 4 in different media
As mentioned the overall solvatochromic sensitivity in glucose
solutions on solvent polarity changes as described by Reichardt’s
polarity scale, is significantly lower than in neat solvents. Accord-
ing to Saito et al. it is possible to determine the ionicity of a mole-
equal to (le
ꢀ g)a = ꢀ32D, for case (a) of Fig. 9. The negative sign
l
is consistent with the negative character of observed solvatochro-
mism (the ground state more polar than the MLCT excited state).
The same equation gave a significantly lower dipole moment dif-
ference in case (b) of Fig. 9, regarding the case of glucose aqueous
solutions with concentrations within the range 120 6 Cglu 6 320 g/L
cule of the type Dd+ -Adꢀ [38], where D represents the electron
-p
donating group and A the electron accepting group of a solvato-
chromic compound. represents the -linkage of A and D [38]. Fi-
p
p
nally d is the ionicity of the compound in its ground state [38]. In
case of compound 4, the Fe(II) center plays the role of the electron
donor, being able to donate one electron (by being thus oxidized to
Fe(III)) to the quaternary nitrogen of the neighboring pyridinium
ring of the ligand. The latter acts as the electron withdrawing
group, and charge transfer occurs through the aromatic backbone
of 4,40-bipyridyl, as depicted in Supporting Information,
i.e. (le
ꢀ
l
g)b = –16.4 D. As anticipated Eq. (3) results in a lower
(le
ꢀ
lg)b difference since the slope of Eq. (5) in that case is lower
(roughly half as compared to the slope in case of solvents). Finally
concerning the concentration range 0 < Cglu 6 120 g/L (Fig. 9(c))
Eq. (3) gave (le
ꢀ g)c = ꢀ3D. The as mentioned results are of
l
course consistent with the theory developed by Saito et al. Lower
sensitivity of the experimental charge transfer energies to param-
eter ET(30) reflects a lower dipole moment difference between the
Scheme S1. The molecule Dd+ -Adꢀ, in the ground state, is thus
-p

36
R. Papadakis / Chemical Physics 430 (2014) 29–39
value of the dipole moment difference will be
(
le
ꢀ
lg)w =
ꢀ11.4D ꢁ (le ꢀ g)s = ꢀ11.8D (where the index w denotes the bal-
l
anced value). This is very important since two different methods
for determining differences between excited and ground state di-
pole moment result in very close results. Furthermore taking into
account a general linear correlation in the whole glucose concen-
tration range (0 < Cglu 6 120 g/L) as shown in Fig. S11b gives
according to Eq. (3): (le
to the balanced dipole moment difference (le
as mentioned results prove that the use of the Van der Waals ra-
dius of 4, (5.01 Å) as the cavity radius ( ) in Eq. (6) is not mistaken.
ꢀ
l
)
g glu = ꢀ11.5D. This value is very close
ꢀ
l )
g w = ꢀ11.4D. The
a
This observation is supported by the fact that even small errors of
the cavity radius introduce higher errors to the determined
dipole moments through the term a
3. Unfortunately equation (6),
probably because of poor correlation of the MLCT energies of
4, measured in solvents of Table 1, resulted in a value of
Fig. 9. MLCT of 4 in different media, with respect to ionicity of 4. Calculated
differences between MLCT excited and ground state dipole moments through Eq.
(
le
ꢀ g)a ꢁ ꢀ22D. This value is smaller from the calculated dipole
l
(3): (a) (le
ꢀ lg)a = ꢀ32D, (b) (le ꢀ lg)b = ꢀ16.4D, and (c) (le ꢀ lg)c = ꢀ3D.
moment difference through Eq. (3) (ꢀ32D), though still higher as
compared to the one determined in glucose solutions. (Values
ꢀ22D and ꢀ32D were determined using Suppan’s and Saito’s
method respectively; see Supporting Information: Tables S1 and
S2 and Figs. S9–S11a). This discrepancy is anticipated since corre-
MLCT excited state and the ground state. Interestingly the values
obtained for 4 by means of the described methodology, in case of
the glucose solutions, are close to the ones obtained through an-
other method, the method of Suppan and Tsiamis [39]. This theory
allows the determination of ground and excited dipole moments of
solvatochromic compounds by using only absorption spectral data
[39,40]. That is very important for non-emitting compounds (for
instance compound 4). Eq. (6) is an extension of earlier Suppan’s
equation [41] containing only the first term which is the perma-
nent dipole interaction term. The second term represents the sta-
bilization difference of the ground state and excited state dipoles
lation coefficients (R) for correlations EMLCT vs [
EMLCT vs
u
(e
) ꢀ
u
(n2)] and
u
(n2), regarding EMLCT measured in solvents, were rather
low, whereas as mentioned EMLCT correlate excellently with
ET(30), rendering Saito’s method much more reliable in that case.
The as determined values of (le
methods are listed in Table S2 of Supporting Information.
ꢀ
lg) for 4, by means of different
4.5. Solvatochromic sensitivity of 4
(
lg and le respectively) [39]. Since the MLCT energy of 4 in glucose
solutions correlated very well with both polarity functions
and
(n2), Eq. (6) can give accurate results (Supporting Informa-
The fact that the ionicity of 4 differs depending on the medium,
can explain the observed alteration of the medium responsive
character of 4, as depicted in Fig. 9. In order to answer why such
alteration of the ionicity occurs, the analysis of contribution of dif-
ferent solvent polarity parameters on solvatochromism in neat sol-
vents, binary solvent mixtures, as well as in glucose solutions, is
needed. For that matter Linear Solvation Energy Relationships
(LSER’s) can be employed. [9,23,45–48] Herein the multiparamet-
ric equation of Koppel and Palm was used for different reasons.
First of all, in order to include the solvatochromic shifts measured
in glucose solutions in an LSER, one is obliged to know or to deter-
mine the values of the corresponding LSER parameters for each of
the examined glucose solutions. Unfortunately as also mentioned
in introduction, apart from Reichardt’s polarity scale of aqueous
glucose solutions (determined by Spange et al. [13]) refractive indi-
ces and static dielectric constants, no other solvent polarity param-
eters have been determined up to now. This decreases the number
of applicable LSER’s. Koppel–Palm equation in its extended form,
correlates a physicochemical quantity X with four parameters
[23,49] (see Eq. (8)). Two parameters describing non-specific sol-
u(e)
u
tion, Figs. S9 and S10). Ground and excited state dipoles are consid-
ered as collinear in case of 4. For the determination of both terms
of the biparametric linear equation (6), the value 5.01 Å was used
for the cavity radius (a). The latter is the Van der Waals radius of
4, determined as described in calculations part. This approach
has been applied in the past for the determination of ground and
excited state dipole moments when no other data concerning the
cavity term are available [42–43] (e.g. the density of the solute
[44]). Finally the scaling factor (4peo
ꢀ1
)
where eo is vacuum per-
mittivity, was used for SI units [41].
"
#
!
lg ꢂðl!g
ꢀ
l!eÞ
ð
le2
ꢀ
lg2
Þ
1
ECT
¼
ð
u
ðeÞ ꢀ
u
ðn2ÞÞ þ
u
ðn2Þ
4
peo
a3
a3
þ const:
where
2ðx ꢀ 1Þ
ð6Þ
ð7Þ
ute–solvent interactions, (one expressing polarization: Y(
e) where
u
ðxÞ ¼
2x þ 1
The dipole moment difference (le
obtained through Eq. (6) regarding the concentration range
0 < Cglu 6 320 g/L was (le g)s = ꢀ11.8 D (the index s denotes
the value obtained through Eq. (6)). It is obvious that (lg e)c =
3D < (lg e)s = 11.8D < (lg e)b = 16.4D. As mentioned MLCT
energies of 4 measured in glucose solutions correlate linearly to
polarity functions
) ꢀ
(n2) and (n2) in the whole concentra-
tion range 0 < Cglu 6 320 g/L. Thus it is not possible to obtain more
than one different values (le g) in the aforementioned concen-
e
is the static dielectric constant of a solvent, the other expressing
ꢀ
l
g) of 4 in glucose solutions
polarizability: P(n2) where n is the refractive index of a solvent)
then two more describing specific solute–solvent effects. The latter
two include both solvent Lewis acidity, expressed by parameter E
(values of E listed in Tables 1 and 2) and Lewis basicity expressed
by parameter B (to the best of the author’s knowledge no data for
this parameter are available for glucose solutions). Coefficients a1
to a4 represent the susceptibilities of physicochemical quantity X
to the four parameters of equation 8 and are determined through
linear regression. The most important solvent parameters to de-
scribe solvatochromism of pentacyanoferrate(II) complexes which
do not bear substituents giving rise to significant overall Lewis
acidity, are parameters describing dipolarity/polarizability of sol-
vents as well as solvents’ Lewis acidity. The latter is a general
ꢀ
l
ꢀ
l
ꢀ
l
ꢀ
l
u
(e
u
u
ꢀ
l
tration range. Nevertheless the value 11.8D is very well balanced
between the values obtained through application of Saito’s theory.
Since (le
ꢀ
l
g)c = ꢀ3D is valid in the range 0 < Cglu 6 120 g/L and
(le
ꢀ
l
g)b = ꢀ16.4D in the range 120 6 Cglu 6 320 g/L, the balanced

R. Papadakis / Chemical Physics 430 (2014) 29–39
37
observation which has been made by different authors in the past
[1,6,9,31–33]. According to Koppel and Palm, parameter E for a gi-
ven solvent can be derived through Eq. (11) [23] (E, ET(30) as well
tribution of medium Lewis acidity. However the contribution of
polarizability is lower than that of dipolarity. Obviously specific
solute–solvent interactions play a very important role. Hydrogen-
bonding between solvent molecules acting as Lewis acids, and
cyano groups of solute molecules, is the expected dominating
specific interaction. A good way to rationalize the different chro-
motropic behavior of 4 depending on the medium is to combine
the information derived by applying Koppel–Palm equation, with
the change in sensitivity of the MLCT energies of 4, on each of
as coefficients of Y(
ET(30), , and n of glucose solutions, E can be determined (listed
values in Table 2).
e
) and P(n2) in kcal molꢀ1). Thus knowing
e
Xð
e
; n2; E; BÞ ¼ Xo þ a1Yð
e
Þ þ a2Pðn2Þ þ a3E þ a4B
ð8Þ
where:
the three parameters of Koppel–Palm equation (Y(e
), P(n2), and
e
e
ꢀ 1
E). The slopes of the corresponding correlations are considered as
measures of sensitivities. In the histograms of Fig. 11, the sensitiv-
ities of MLCT energies of 4 (in kcal/mol) on the aforementioned
three parameters, in all different types of media are depicted
(herein classified in three categories according to the medium
type: solvents and binary solvent mixtures, aqueous glucose
solutions with concentration varying within 0 < Cglu 6 120 g/L,
and finally aqueous glucose solutions with concentration varying
Yð
eÞ ¼
ð9Þ
þ 2
and
n2 ꢀ 1
n2 þ 2
Pðn2Þ ¼
ð10Þ
ð11Þ
E ¼ ET ð30Þ ꢀ 25:10 ꢀ 14:84 ꢂ Yð
e
Þ ꢀ 9:59 ꢂ Pðn2Þ
within 120 6 Cglu 6 320 g/L). As shown, there is
a gradual
A second reason which excuses the choice of using Koppel–
Palm equation is that it gives the possibility of separately deter-
mining the contribution of dipolarity and polarizability, by means
of functions Y and P. Nevertheless, Koppel–Palm will be used here-
in in its reduced form containing three parameters for reasons ana-
lyzed above: Y, P, and E [50]. Correlation of MLCT energies of 4, in
all group of solvents, solvent mixtures and glucose solutions with
the aforementioned parameters, resulted in linear equation (12)
[51].
decreasing of all sensitivities of 4 on Y(
e
), P(n2), and E (SY( ), SP(n)
,
e
SE respectively) when going from the set of solvents and solvent
mixtures (of Table 1) to glucose aqueous solutions. This decrease
becomes very intense in case of parameter E, practically leading
to insensitivity of MLCT energy to Lewis acidity of the medium in
case of glucose-aqueous solutions with concentration varying
within 0 < Cglu 6 120 g/L. This is also observed in case of SY(
,
e
)
S
P(n), but in case of sensitivity on dipolarity SY( is far less intense
e)
than in case of SE and slightly less than SP(n). Taking into account
the results obtained through Koppel–Palm equation (%PE > %PY >
%PP), it is possible to deduce that in case of glucose solutions (espe-
cially at low concentrations of glucose) the solvatochromic dye
seems to practically respond only in changes of dipolarity and
slightly in changes of polarizability of the medium. On the other
hand in case of solvents and solvent mixtures, compound 4 mainly
responds to Lewis acidity changes (and rather less to dipolarity and
polarizability), and it seems that this is the main tendency which
shapes the overall contribution pattern. The answer why such an
alteration of the solvatochromic behavior of 4 is observed in differ-
ent media, has thus to be linked with glucose-water specific inter-
actions which are very intense, as observed in the past [52,53].
Polyhydroxy compounds including sugars have a structure-break-
ing effect in water, because of the strong hydrogen bonding be-
tween glucose and water molecules [52]. Glucose can thus affect
the hydration of the solute molecules (e.g. molecules of the solva-
tochromic dye 4). As observed in the past, partial dehydration of
hydrophilic molecules such as N,N-dimethylformamide (DMF)
[52] or inorganic salts (such as NaCl, KCl etc.) [53] becomes
energetically harder when increasing glucose content. The solvato-
chromic complex salt (4) which possesses five cyano groups coor-
dinated to iron(II) per molecule, as well as other ionic parts along
the molecule (i.e. two quaternized nitrogen atoms) is highly hydro-
philic. Consequently increasing glucose content stabilizes the
hydration shell of solvatochromic dye molecules. Since this effect
is observed even at very low glucose concentrations, hydrogen-
bond formation between solute (4) and water molecules which
(according to the analysis based on Koppel–Palm equation) is the
most dominant solute–solvent interaction, the gradual decrease
of Lewis acidity as glucose content increases, does not affect inten-
sely the cyano-water hydrogen bonding. Consequently it results in
moderate solvatochromic shifts and thus to moderate sensitivity
SE. In comparison with glucose solutions (with concentrations be-
tween 0 and 320 g/L) neat solvents lead to much higher sensitivi-
ties to Lewis acidity, since comparing the solvating abilities of
water and polar organic solvents used herein, water presents by
far the highest Lewis acidity and Hydrogen-Bond-Donor ability.
Thus the effect of different solvent leads to a much more significant
EMLCT ðkcal molꢀ1Þ ¼ 2:74 þ 25:24 ꢂ Yð
e
Þ þ 16:89 ꢂ Pðn2Þ
þ 1:12 ꢂ Eðkcal molꢀ1
Þ
ð12Þ
The success of Eq. (12) in describing solvatochromism of 4, is
proved by the high correlation coefficient (R = 0.92) of the experi-
mental values of EMLCT vs calculated ones, as shown in Fig. 10.
Based on the regression values of Eq. (12), and according to the
treatment described in Supporting Information, Paragraph 10) the
relative contributions of the parameters of Koppel–Palm relation-
ship Y, P, and E (respectively %PY, %PP, %PE) follow the decreasing
sequence: %PE = 78% > %PY = 12.5% > %PP = 9.5%. The meaning of this
sequence is that for the given data set (containing neat solvents,
binary solvent mixtures as well as glucose solutions), Lewis acidity
as expressed by polarity parameter E, contributes the most to the
observed solvatochromism of 4. The contribution of dipolarity
and polarizability of the medium is minor as compared to the con-
Fig. 10. Experimentally obtained EMLCT values vs calculated EMLCT values through
Eq. (12).

38
R. Papadakis / Chemical Physics 430 (2014) 29–39
inversed in glucose solutions where EMLCT shows an excellent lin-
ear correlation with f( ), reflecting the fact that dipolarity becomes
e
more important in glucose solutions. The same thing, in a different
extent is observed for polarizability as expressed by parameter
P(n2).
5. Conclusion
A new pentacyanoferrate solvatochromic complex was synthe-
sized and fully characterized. Its solvatochromic behavior was very
intense as examined in different media (pure solvents, binary sol-
vent mixtures and glucose aqueous solutions). The general conclu-
sion based on suitable Linear Solvation Energy Relationship was
that the medium responsive behavior of the complex, is mainly af-
fected by solute–solvent specific interactions. Hydrogen bond for-
mation mainly affects the chromotropic phenomenon observed,
with the solvent molecules acting as HBD-acids, readily interacting
with cyano groups of the complex. Nevertheless it was observed
that, quantitatively, the solvatochromic behavior of the pentacy-
anoferrate complex differed strongly depending on the type of
medium. The solvatochromism was more intense in the examined
group of pure solvents and binary solvent mixtures (including both
HBD and HBA solvents) than was in aqueous glucose solutions. The
‘‘driving force’’ of solvatochromism which is the difference be-
tween ground and MLCT-excited state dipole moments, was much
smaller in case of glucose solutions, reflecting the moderate or
almost negligible (at lower glucose concentrations) solvatochro-
mism. This discrepancy was attributed to the different environ-
ment, the molecules of the complex ‘‘feel’’ and the intense and
steadily high hydrogen bond donor acidity of glucose solutions in
water. These results reveal an interesting phenomenon narrowly
connected to water–glucose interactions.
Acknowledgments
Professor Dr. Tsolomitis Athanase is gratefully acknowledged
for fruitful discussions as well as Dr. Deligkiozi Ioanna for her help
with some UV–Vis measurements. The author would also like to
thank Dr. Marius Réglier for his encouragement.
Appendix A. Supplementary data
Fig. 11. Histograms showing the sensitivities of measured MLCT energies of 4, to (a)
dipolarity expressed by function Y(e
), (b) polarizability expressed by function P(n2),
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.chemphys.
2013.12.008.
and (c) Lewis acidity of the medium expressed by parameter E of Koppel and Palm.
Values obtained through linear regression analyses between MLCT energies of 4 and
corresponding solvent polarity parameters. (1) solvent set of Table 1, (2) glucose
solutions 120 6 Cglu 6 320 g/L, and (3) glucose solutions 0 < Cglu 6 120 g/L. Error
bars in all cases are based on the regression errors of the slops of the corresponding
lines obtained through linear regression.
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sensitivity SE. This is why Suppan–Tsiamis method used for the
determination of dipole moments of 4, gave much better results
in case of aqueous glucose solutions (results consistent with Saito’s
method) but failed in case of the set of solvent/binary solvent mix-
tures, where sensitivity to Lewis acidity is much higher (since Sup-
pan–Tsiamis method does not consider any specific solute–solvent
effects).
Furthermore by comparing the ratios
kcal molꢀ1 [54] (regarding the set of solvents) and
Y(
)]glu = 707.7 kcal molꢀ1 [54] (regarding glucose solutions: the
[D
E/D
Y(
e)]sol = 131.6
[
D
E/
D
e
whole concentration range examined) it is apparent that the in-
crease of glucose in solution, brings a smaller decrease of dipolarity
than Lewis acidity. This is also in agreement with the results of the
correlations between EMLCT and f(e) and E concerning the set of sol-
vents (of Table 1) and glucose solutions. Although for neat solvents
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[
D
E/
D
Y(
e
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E and DY(e) correspond to the differences of
e
D
), respectively, between water and water/acetone 50%
D
Y(
e)]gluDE and
DY(e) correspond to the differences of
E
e
)
respectively, between water and glucose aqueous
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solution with concentration 320 g/L.