Synthesis and characterization of new azobenzene-containing bis pentacyanoferrate(II) stoppered push-pull [2]rotaxanes, with α- And β-cyclodextrin. Towards highly medium responsive dyes

Article abstract of DOI:10.1016/j.dyepig.2014.10.005

The solvatochromic behavior of novel synthesized azo-containing viologen-based rotaxanes, combining push-pull linear and α or β-cyclodextrin macrocyclic components is examined. These rotaxanes are stoppered by pentacyanoferrate(II) units. The latter stabi

Full text of DOI:10.1016/j.dyepig.2014.10.005

Accepted Manuscript
Synthesis and characterization of new azobenzene-containing bis
pentacyanoferrate(II) stoppered push-pull [2]rotaxanes, with α- and β-cyclodextrin.
Towards highly medium responsive dyes
Ioanna Deligkiozi, Evangelos Voyiatzis, Athanase Tsolomitis, Raffaello Papadakis
PII:
S0143-7208(14)00402-1
10.1016/j.dyepig.2014.10.005
DYPI 4556
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 August 2014
Revised Date: 5 October 2014
Accepted Date: 7 October 2014
Please cite this article as: Deligkiozi I, Voyiatzis E, Tsolomitis A, Papadakis R, Synthesis and
characterization of new azobenzene-containing bis pentacyanoferrate(II) stoppered push-pull
[2]rotaxanes, with α- and β-cyclodextrin. Towards highly medium responsive dyes, Dyes and Pigments
(2014), doi: 10.1016/j.dyepig.2014.10.005.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and characterization of new azobenzene-containing bis
pentacyanoferrate(II) stoppered push-pull [2]rotaxanes, with α- and β-
cyclodextrin. Towards highly medium responsive dyes.
a
b
a
a,c
Ioanna Deligkiozi , Evangelos Voyiatzis , Athanase Tsolomitis Raffaello Papadakis*
,
a
School of Chemical Engineering, Laboratory of Organic Chemistry, National Technical University of Athens,
b
1
5780 Athens, Greece; Eduard-Zintl-Institut fuer Anorganische und Physikalische Chemie, Technische
c
Universität Darmstadt, D-64287 Darmstadt, Germany; Department of Chemistry - BMC Uppsala University,
5123 Uppsala, Sweden.
7
e-mail: rafpapadakis@gmail.com
Abstract: The solvatochromic behavior of novel synthesized azo-containing viologen-based
rotaxanes, combining push-pull linear and α or β-cyclodextrin macrocyclic components is examined.
These rotaxanes are stoppered by pentacyanoferrate(II) units. The latter stabilize these systems and
furthermore act as strong electron donors. These units are linked to strong electron withdrawing
viologen units thus giving rise to an exceptionally intense solvatochromic behavior. Suitable solvent
polarity scales and Linear Solvation Energy Relationships (LSERs) were employed in order to
rationalize the solvent polarity effects observed, both specific and non-specific ones. The results for
the title compounds as well as their Cyclodextrin-Free-Dumbbell (CFD) like analogue, are compared
to recent published solvatochromic data regarding similar smaller pentacyanoferrate(II) complexes as
well as other highly solvatochromic compounds. Structural effects on the solvatochromic intensity as
well as the contribution of different solvent-solute interactions are rationalized and quantified.
Keywords: Solvatochromic azo-dyes, rotaxanes, cyclodextrins, pentacyanoferrate(II) complexes,
metal-to-ligand-charge-transfer (MLCT), push-pull system.
1
. Introduction
Rotaxanes are a well-known family of interlocked compounds which have been intensively
[
1-5]
studied the last few decades.
They consist of one or more macrocyclic compounds threaded
through one or more linear skeletons mimicking this way in their topologically simplest binary form
a wheel-axis system. When these parts of such a wheel-axis system are stoppered by appropriate
bulky side groups the resulting compound is called “rotaxane”; if the previous mentioned conditions
are not fulfilled then the compound is called “pseudorotaxane”. The linear and macrocyclic parts can
[
6]
be held together by means of various non-covalent interactions, such as hydrogen-bonding,
hydrophobic,
[
1,7]
[8]
or donor-acceptor interactions. Heretofore many different rotaxanes have been
synthesized as a result of a big number of combinations of macrocycles, linear skeletons, different
[
1,9-11]
interactions of the latter two, as well as various stoppering groups.
In many cases the resulting
compounds are multifunctional as a result of the combination of one or more properties such as
[
12-15]
[16-17]
[18]
photochromic,
electronic,
photoconductive, useful for various applications in molecular
switches, molecular machines, sensors etc. These properties can lead to the development of novel
promising materials for future hi-tech applications in Key Enabling Technologies (KET). Viologen
based azobenzene-containing linear skeletons along with cyclodextrins (CDs) is such a possible
[
10,19-20]
combination.
Systems with the aforementioned structural basis constitute a very important
[
21-23]
[24]
[24-
family of dyes because of both the high importance of azobenzene based dyes and the redox
plus photo-activity of viologens as well as the strong electron accepting character of the latter.
[
25]
2
7]
Besides, novel push-pull systems containing the azo-group in conjugation with various aromatic
or heteroaromatic units have been recently reported aiming at the development of novel dyes

ACCEPTED MANUSCRIPT
[
28-30]
exhibiting solvatochromism and non-linear-optical (NLO) properties.
CDs play also a very
important role acting as important auxiliaries in textile dyeing preventing the degradation of the azo-
[
31]
chromophore.
Finally if this is allowed by the structure of linear skeleton, CD can undergo
controlled shuttling movements induced, by various stimuli, most importantly by light
[
10,32-33]
irradiation.
Nevertheless to the best of our knowledge, the number of published azo-
containing viologen based CD-rotaxanes, exhibiting extended π-conjugation, constitutes just a small
[
34]
part of the published works in this research field presumably because of the difficulties during
[10]
synthesis as well as isolation of such compounds. In this work we report the synthesis of novel
push-pull 4,4′-bipyridine based ferrocyanide(II) stoppered azobenzene containing rotaxanes with α-
and β-cyclodextrin (CD), and their corresponding cyclodextrin-free-dumbbell (CFD) compound. The
latter compounds belong to the family of push-pull dyes (D-π-Α: D being an electron donor, π: a π-
conjugated system, and A: an electron withdrawing moiety) with the general structure depicted in
Figure 1. In previous works our group has shown the intense solvatochromic character of small
[
35-37]
II
ferrocyanide(II) complexes of 4,4′-bipyridines,
narrowly connected to the Fe electron donating
ability, and the strong electron accepting ability of monoquaternized viologen based ligands, (the
latter provide an aromatic skeleton as well). Conjugation of the bipyridine ligand in those cases is a
very important structural feature which strongly affects the medium responsive metal to ligand
charge transfer (MLCT) of such complexes. This work is targeted towards the study of the synthesis
of the title dyes, and their intense solvatochromism. In order to study their medium responsive
behavior a series of water-ethylene glycol (EG) binary mixtures have been used as different media
triggering measurable responses of the dyes investigated, upon alteration of EG molar fraction
(inducing alteration of the polarity of the medium). As will be analyzed, the intense solvatochromism
of the synthesized dyes can be dramatically affected by small changes of medium polarity even
between water and EG.
Figure 1. (a) General representation of a D-π-Α dye. D being an electron donor, π: a π-conjugated
system, and A: an electron withdrawing moiety and (b) the π-conjugated linker, electron donor, and
electron accepting units, in case of a (N-aryl-substituted 4,4′-bipyridine) ferrate (II) complex studied
herein.
2
. Experimental Section
2
.1 Physical Measurements
1
NMR spectra were obtained using a Varian Gemini 300 spectrometer (300 MHz H, 75 MHz
1
3
1
13
o
1
C). Both H and C-NMR spectra were recorded in D O at 25 ±1 C. The H NMR spectra were
2
1
3
calibrated by using residual undeuterated solvent as an internal reference (4.79 ppm) and C-NMR
spectra were calibrated using the [D ]DMSO signal at 39.52 ppm.
[
38]
UV-Visible spectra were
6
recorded using a Perkin-Elmer Lamda 25 UV/Vis spectrophotometer. All measurements were
o
performed at 25 ±1 C. The slit width and data interval were set to 1 nm, and the scan speed in all
measurements was 960 nm/min. The concentrations of the solutions used (prepared right before each
measurement) were in all cases 40 ppm. Freshly prepared binary solvent mixtures (water/EG) were
used for the study of solvatochromism of all compounds 3 and 4a-b. These binary mixtures were
prepared by mixing the appropriate volumes of each solvent, using high accuracy pipettes (Gilson)
o
samples were prepared at 25 C). The reported absorption maxima energies correspond to the
average of three measurements. IR spectra were recorded on a Perkin-Elmer Spectrum 1 FTIR
spectrophotometer in the solid state (without any preparation of the samples) using the attenuated

ACCEPTED MANUSCRIPT
-1
total reflectance technique (ATR) in the region 600-4000 cm . Melting points were determined in
open capillary tubes using a Gallenkamp HFB-595 melting point apparatus and are uncorrected.
Elemental analyses were carried out using a Perkin-Elmer Elemental Analyzer 2400 CHN.
Thermogravimetric analyses (TGA) were performed under N on a Mettler Toledo TGA/SDTA 851,
2
o
-1
in the temperature range 25–300 °C, with a heating rate of 10 C.min . Scanning Electron
Microscopy (SEM) images as well as Energy-dispersive X-ray (EDX) spectroscopy analyses were
obtained using a FEI Quanta 200 electron microscope. Finally Powder-XRD (PXRD) measurements
were carried out using a Siemens D5000 diffractometer using Cu Ka1, radiation λ= 1.5405 Å, and a
graphite monochromator.
2
.2 Materials
Sodium nitroprusside and chloro-2,4-dinitrobenzene were purchased by ACROS Organics; p-
azodianiline (ADA) and 4,4′-bipyridine was purchased from Fluka. All of them were used without
any further purification. Finally α-CD and β-CD, were obtained from Fluka in 99.0% purity and were
[39]
dried in vacuo over P O prior to use. All solvents were dried according to standard protocols.
2
5
Water (used for both synthetic procedures and UV-Vis spectrophotometry measurements) was
purified using a Barnstead EASYpure RF compact ultrapure water system and then distilled twice.
[
39]
Ethylene glycol (EG) was purchased by ACROS Organics and it was dried according to literature.
2
.3 Synthetic procedures and characterization details
2
.3.1 Synthesis of N-(2,4-dinitrophenyl)bipyridinium chloride (Compound 1).
[
36,40]
Compound 1, was synthesized according to a previously reported method,
which allows
easier handling (since 1 as a chloride salts is extremely hygroscopic). To a solution of chloro-2,4-
dinitrobenzene (4.30g, 21mmol in 35ml EtOH) 4,4′-bipyridine was added (3.35g, 21mmol) and the
solution was refluxed for 24h. The dark brown solution after cooling at room temperature was added
to dry diethylether (350ml), with stirring. A golden-brown precipitate was obtained which was
filtered by suction and washed several times with diethyl ether. The solid was stored under vacuum
over anhydrous P O (highly hygroscopic solid which turns to a sticky brown solid when exposed to
2
5
air) to give 6.25g (17.42mmol) 82%. The resulting dry solid was used without any purification in the
1
13
[36,40]
next steps. ( H and C NMR data have been reported in earlier publications
)
II
2
.3.2 Synthesis of the complex Fe (CN) NH ·3H O.
5
3
2
[41]
This compound was prepared according to the Kenney, Flynn and Gallini method.
Sodium
nitroprusside (15.00g, 0.0526 mol) was finely powdered and then added to concentrated NH OH
4
o
(28%; 60ml) containing anhydrous sodium acetate (5.00g). The reaction mixture was kept at 4 C
overnight, and the formed bright yellow solid was filtered off by suction. The solid product was
reprecipitated twice from an aqueous solution by adding cold ethanol. Finally, the product was dried
under vacuum over anhydrous CaCl for about a week until its weight was stabilized. The yield of
2
the reaction was about 50% (8.60g, 0.0263mol).
2
.3.3 Synthesis of compound 2.
[
10]
(
Slightly altered method of our previously published synthetic procedure ). Compound 2, was
synthesized in the solid state as follows: compound 1 (1.01 g, 2.83mmol) and ADA (0.3 g, 1.41
mmol) were dissolved in EtOH (until complete dissolution of the reactants), in an open round flask

ACCEPTED MANUSCRIPT
placed in an oil bath, and heated gently until all quantity of EtOH was removed, thus leaving back a
homogenous mixture of pasty consistency. The temperature of the oil bath was then regulated at
o
1
60–170 C, a condenser was added, and the reacting mixture was heated overnight (in the absence
of any solvent). After this period, microcrystalline yellow colored 2,4-dinitranilin (by-product) was
formed on the upper part of the flask (through sublimation). After cooling at room temperature, the
latter was removed and the remaining sticky solid mass was easily dissolved in dry MeOH (also
dissolving possibly-remaining quantity of 2,4-dinitranilin). Compound 2 as a chloride salt was
precipitated by adding dry Et O. The precipitate was filtered, washed with Et O and dried in vacuo
2
2
1
for several hours. Red brown powder (0.56 g, 70%); H NMR (300 MHz, D O): δ (ppm) = 9.43 (d, J
2
=
6.3 Hz, 4H; C H N), 8.97 (d, J= 6.3 Hz, 4H; C H N), 8.74 (d, J = 7.2 Hz, 4H; C H N), 8.35 (d, J =
5 5 5 5 5 5
13
8.4 Hz, 4H; Ph), 8.29 (d, J = 4.8 Hz, 4H; C H N), 8.18 (d, J = 8.1 Hz, 4H; Ph); C NMR (75MHz,
5 5
D O/D -DMSO): δ (ppm) = 153.48, 152.78, 150.80, 145.48, 144.22, 140.86, 126.56, 125.37, 124.27,
2
6
122.29.
2
.3.4 Synthesis of solvatochromic compound 3 (CFD compound).
In a round bottom flask equipped with a magnetic stirrer, an aqueous solution of 2 (0.100 g,
0
,177 mmol, in 25mL H O) was added. Then two equivalents of the complex salt
2
II
Fe (CN) NH ·3H O (solid) were added (0.116 g, 0,354 mmol). The color of the solution
5
3
2
immediately turned to deep blue. The reaction mixture was stirred in the dark under argon
atmosphere for 24 h. After that period 150 mL of EtOH (6-fold volume) was added and the resulting
o
blue mixture remained at 4 C in the dark for another 24 h to yield a deep-blue precipitate. The
precipitate was then filtered by suction and washed with EtOH and then Et O. Finally the blue
2
o
powder was dried in vacuo over P O for several hours. 88% ; 188 mg), blue powder; mp >250 C;
2
5
1
H NMR (300 MHz, D O): δ (ppm) = 9.30(d, J= 6.6 Hz, 8H; C H N), 8.65 (d, J = 6.8 Hz, 4H;
2
5
5
C H N), 8.29 (d, J = 8.6 Hz, 4H; Ph), 8.07 (d, J = 8.6 Hz, 4H; Ph), 7.76 (d, J = 6.3 Hz, 4H; Ph).
5
5
-1
-1
-1
FTIR ν = 2044 (CN), 1633 (N=N), 1031 cm . UV/Vis, H O, λ (logε )): 556(3.718 M cm ;
MLCT: dπ(Fe )→π* (bpy). PXRD: 2θ = 21.47, 32.0, 25.6, 17.1, 51.8 degrees. Elem Anal. for:
2
nm
max
·
II
C H Fe N Na ·14(H O), calc C: 41.74; H: 4.34; N: 18.54; found (%): C: 41.57; H: 4.09; N: 18.50.
42
24
2
16
4
2
TGA: loss of ca 13.6 water molecules per molecule of 3.
2
.3.5 General Synthetic Procedure for dyes 4a-b
In a round bottom flask equipped with a magnetic stirrer, 0.100 g (0.177 mmol) of compound 2
and 25 mL of water were added, and the mixture was stirred for a couple of minutes. To this mixture
an excess (0.887 mmol or 5 equivalents) of cyclodextrin (α-CD for compound 4a, β-CD for 4b) was
added. Immediate dissolution of compound 2 was observed after the addition of cyclodextrin in both
cases, indicating the formation of the corresponding pseudorotaxane in each case (see Scheme 1: step
ii). The solution was stirred for 30 minutes at room temperature. Then 0.116 g (0.354 mmol; 2
II
equivalents) of the complex salt Na [Fe (CN) NH ]·3H Ο were added in the solution of the
3
5
3
2
pseudorotaxane. Immediately the color of the solution turned deep-blue. The reaction mixture was
then stirred in the dark and under an Ar atmosphere at room temperature for 24 h. After that period
an excess of EtOH was added to the mixture to induce precipitation of the desired product. (In case
of product 4b, an equal volume of EtOH: 25mL was added because of maximum solubility of β-CD
in 50% binary mixture water/EtOH; in case of product 4a, a five-fold volume of EtOH: 125 mL was
o
added). After that, the mixture was kept at 4 C overnight and then it was filtered by suction. The
collected blue powder in each case was washed with cold EtOH and then with Et O. The solid in
2

ACCEPTED MANUSCRIPT
each case was finally dried over P O in vacuo until stabilization of its mass. Attempts of
2
5
recrystallization failed for both 4a and b.
2
.3.5.1 Synthesis of solvatochromic dye 4a (α-CD containing [2]rotaxane)
o
1
4
5%, (175 mg) blue powder; mp: 200 C (dec.); H NMR (300 MHz, D O): δ (ppm) = 9.27 (s,
2
4
H; C H N), 9.25(s, 4H; C H N), 8.64 (d, J = 6.7 Hz, 4H; C H N), 8.29 (d, J = 8.4 Hz, 4H; Ph), 8.07
5 5 5 5 5 5
(
=
d, J = 8.2 Hz, 4H; Ph), 7.78 (s, 4H; Ph), 5.072 (s, 6H; α-CyD), 4.04-3.34 (m, 36H; α-CyD). FTIR ν
-1
-1
-1
2046 (CN), 1631 (N=N) cm . UV/Vis, H O, λ (logε ): 583(3.866 M cm ; MLCT:
2 nm max
II
dπ(Fe )→π* (bpy); PXRD: broad: 2θ =20-30 °. Elem Anal. for: C H Fe N Na O ·15(H O), calc.
7
8
84
2
16
4
30
2
(%) C: 42.59; H: 5.22; N: 10.19; found (%): C:42.83; H: 5.51; N: 9.91. TGA: loss of ca 14.6 water
molecules
2
.3.5.2 Synthesis of solvatochromic dye 4b (β-CD containing [2]rotaxane)
o
1
5
8
5
7%, (240 mg) blue powder; mp: 200 C (dec.); H NMR (300 MHz, D O): δ (ppm) = 9.28 (m, 8H),
2
.65 (d, J = 5.6 Hz, 4H), 8.31 (d, J = 8.2 Hz, 4H), 8.08 (d, J = 8.6 Hz, 4H), 7.80 (d, J = 3.7 Hz, 4H),
.11 (s, 7H), 3.98 (m, 10H), 3.88 (m, 23H), 3.79 (s, 2H), 3.66 (s, 10H), 3.59 (s, 10H). FTIR ν = 2048
CN), 1633 (N=N), 1006 cm . UV/Vis, H O, λ (logε )): 564(3.921 M cm ; MLCT: dπ(Fe )→π*
-1
-1
-1
II
(
2 nm max
(
bpy). PXRD: broad: 2θ = 20-30°. Elem Anal. for: C H Fe N Na O ·16(H O), calc. (%) C:
84 94 2 16 4 35 2
42.40; H: 5.34; N: 9.42; found (%): C: 42.64; H: 5.12; N: 9.64. TGA: loss of ca 15.8 water molecules
3
. Calculations
Deconvolutions of UV-Vis spectra and all regression analyses were performed using the program
Qti-Plot. Regarding the deconvolution of UV-Vis spectra, the latter were considered as the result of
convolution of four Gaussian type bands (Supporting Information, Paragraphs 1 to 3).
DFT calculations were employed to complement our experimental studies. All computations
[
42]
[43]
were carried out using the GAMESS(US) software. The B3P86 hybrid functional with the 6-
11++G(d,p) basis set were employed for the optimization of the molecular structures. The geometry
3
of compound 3 was optimized in vacuum. Thus, both polarization and diffuse functions were used to
describe accurately the properties of the hydrogens as well as of the heavy atoms. The final structures
[
44]
and molecular orbitals were visualized using the Molekel 4.3 program.
4
. Results and Discussion
4
.1 Synthesis and characterization of dyes 3 and 4a-b.
For the synthesis of the title dyes the synthetic route depicted in Scheme 1 was followed. As
[
10]
shown the precursor compound 2 was initially synthesized. In a previous publication of our group
we described the solvent-free Zincke reaction between compound 1 and ADA, a reaction which leads
to compound 2 in relatively high yield. Herein we report an improved method for its preparation
(described in the experimental section). As mentioned previously this reaction (in our hands) does
not proceed in solution-phase. This is an important step which provides a fully conjugated linear
skeleton which is the basis of all compounds 3 and 4a-b studied herein. Compound 3 was then
II
synthesized by reaction of compound 2 with two equivalents of the complex Fe (CN) NH , during
5
3

ACCEPTED MANUSCRIPT
which ammonia molecules are substituted by the pyridines of precursor 2. The reaction is very rapid
and a direct color change can be observed (form yellow to deep-blue) upon mixing the two reactants.
The same procedure is followed for the preparation of rotaxanes: compounds 4a and 4b. In those two
cases the precursor after supramolecular binding to α- or β-CD respectively in aqueous solutions,
resulted in the corresponding pseudorotaxanes (that is the non-stoppered linear compound after self-
assembling with α- or β-CD). Then the ferrocyanide(II) complexation-stoppering reaction took place
in situ, finally leading to the desired products. The as described reaction sequence corresponds to the
[
1]
so-called “threading approach”. The latter is an important synthetic strategy for rotaxanes of CDs.
The relatively high affinity of CD to the azobenzene moiety of various linear compound has been
exploited for other published rotaxanes; then in situ stoppering results in corresponding rotaxanes in
high yields. Importantly as depicted in Scheme 1, the slippage of the CD macrocycle (either α- or β-)
II
through the Fe (CN) barriers of the CFD compound is not feasible because of the bulkiness of them,
5
rendering the threading-capping approach the only suitable for the preparation of rotaxanes 4a-b.
Scheme 1. Synthetic route followed for the preparation of compounds 3 and 4a-b; (i) solvent-free
Zincke reaction; (ii) self-assembling (threading) of CD macrocycles through compound 2, in water
II
(
formation of pseudorotaxanes); (iii) coordination to –Fe (CN) (in situ stoppering for compounds
5
4
a-b in aqueous solutions); (iv) slipping synthetic approach for compounds 4a-b: not possible.
The reason why it was chosen to study the synthesis of the new two rotaxanes 4a-b, and the CFD
compound: 3 along with their medium responsive behavior, is because on one hand, comparing
between the spectral fingerprints of rotaxanes and their corresponding CFD analogue easily leads to
[10]
confirmation of successful rotaxanation reactions.
This is certainly important since the
crystallization of compounds similar to 4a-b (aiming at solving the structure) is usually problematic,
therefore one mainly relies on a combination of various spectral data. On the other hand as will be
analyzed, both the CFD compound and the rotaxanes show significant structure-depending medium
responsive behavior. Consequently the study of structural effects is important. Various spectroscopic
techniques as well as scanning electron microscopy (SEM) and powder X-ray Diffraction (XRD)
techniques were employed for the characterization of compounds 3 and 4a-b.
NMR spectroscopy is one of the most important techniques used. As shown in Figures 2,
concerning compound 4a, significant shifts are observed upon rotaxanation, in both aromatic and α-
CD protons (Fig. 2).
1
Figure 2. (a) H NMR spectra of compound 2, (b) CFD: compound 3, and (c) compound 4a.
1
1
(
d) Partial H NMR spectra of neat α-CD, (e) compound 4a, (f) overlaid H NMR spectra of α-CD
and 4a; all spectra recorded in D O.
2
1
The most important features of the H-NMR spectra of compound 4a, which clearly denote the
presence of the α-CD ring, are the signals of the latter (annotated with full-pentagons in Figure 2, left
panel). Additionally, as shown in Figures 2b and 2c, significant downfield shifts were observed for
the protons of the two (equal para-substituted) phenyl rings of the dumbbell-like compound upon α-
CD incorporation. This deshielding of the phenyl ring protons is induced by the presence of the α-
CD macrocycle, the internal hydrogen atoms of which interact with the phenyl rings protons. As
shown in Figure 2a-c, the most deshielded protons are as expected the neighboring ones to the
quaternized positively charged and therefore strong electron withdrawing nitrogen atoms (the so-
called ortho-protons). A doublet is observed in each case. Interestingly for both cases 2b and 2c,
corresponding to compounds 3 and 4a, both being pentacyanoferrate(II) complexes, the most
downfield shifted signal is in each case a multiplet as shown in magnification inserts of Figures 2b

ACCEPTED MANUSCRIPT
and 2c (denoted with circles). This is a strong indication of the complex formation after step (ii) of
the synthetic route (Scheme 1). The two convoluted signals correspond to the aforementioned
protons neighboring to quaternized nitrogens and to the ones neighboring to the nitrogen atoms
II
bound to the Fe centers (also much deshielded). Nevertheless it is obvious that in case of compound
4a: [2]rotaxane this signal is better resolved as compared to the compound 3. This comes as a result
of the shifts of both types of ortho-protons in case of compounds 4a-b (the neighboring ones to the
II
aryl-substituted quternized nitrogen, and the ones neighboring to the Fe coordinated nitrogen atom;
the first shifting upfield, the latter downfield as compared to compound 3). This is clearly an effect
deriving from the incorporation of α-CD in case of 4a (similarly of β-CD in case 4b; see
experimental section). The CD macrocycle does not stop close enough to the ending nitrogen atoms;
however, because of the conjugation of the molecules, the incorporation of CD (in both cases)
1
induces significant changes in the H-NMR spectra of the rotaxanes formed, as compared to the CFD
“mother” compound. Another strong indication of this effect is based on the fact that the
aforementioned shifts were not observed in the spectra of an artificial 1:1 mixture of the dumbbell-
like compound and α-CD or β-CD, implying that the CD macrocycles in both cases are threaded
through the linear CFD, and not just mixed with CFD in a 1:1 proportion. Furthermore in Figure 2
(right panel) the shifts of α-CD for 4a, are presented in comparison to neat α-CD.
Regarding the β-CD containing [2]rotaxane (4b) it was observed that mainly H3 and H5
protons of β-CD, located inside the cavity were affected by the formation of the inclusion complex
H3 are located close to the wider rim of cyclodextrin whereas the H5 hydrogens form a ring near
(
[
1]
the narrower rim of the methylene: H6 hydrogens). Significant up-field shifts upon threading of β-
CD were observed in both cases. The exterior hydrogens (the so-called H1, H2 and H4) are located
on the exterior of the cavity and most probably stay unaffected by the rotaxanation reaction. The
aforementioned up-field shifts as well as the peak appearing at δ=3.79 ppm clearly designates H3
and H5 inner protons interact with aromatic azobenzene region of the CFD compound. It is
noteworthy that the H3 protons (3.98 ppm, triplet) in cyclodextrin showed significant up-field shifts.
In the presence of CFD the H3 signals from β-cyclodextrin give rise to the formation of a singlet
peak. In the rotaxane the β-cyclodextrin H3 signals are shifted up-field, such that one signal of the
triplet is obscured by the signal due to H6 and another new signal appears at 3.79 ppm.
Another technique used for the characterization of compounds 3 and 4a-b was Scanning
Electron Microscopy (SEM). In the past electron microscopy has been used for the characterization
of the morphology of rotaxanes. A general observation in many cases of CD containing rotaxanes is
[10,18]
that the latter are amorphous.
When the CFD compounds exhibit higher degree of crystallinity,
comparing the morphologies monitored by SEM at a certain magnification, is very important as it
renders the discrimination of rotaxanes and CFD compounds possible.
Figure 3. SEM images of neat-crystalline α-CD (a), CFD compound: 3 (b), [2] rotaxane: 4a
(c), artificial mixture of CFD compound and α-CD (molar ratio 1:1) (d). Scale bars correspond to
100 ꢀm; magnification: ×400 in all cases.
As shown in Figure 3, although compound: 3, is microcrystalline (Figure 3b), compound 4a presents
a rather amorphous nature (Figure 3c). Interestingly in Figure 3d: SEM photography of an artificial
1:1 mixture of α-CD and compound 3 (molar ratio) it is possible to observe crystals of α-CD (for
comparison see also Figure 3a: neat crystalline α-CD) and smaller crystals of compound 3. The
identity of the latter was confirmed using EDX by spotting on the different types of crystalline
regions. The morphology of the aforementioned artificial mixture is clearly different than that of the
amorphous compound 4a. This is a clear indication that this compound is not a physical mixture of
α-CD and the CFD compound but a new compound. Similar observations have been reported in

ACCEPTED MANUSCRIPT
previous works of our group for compounds 5 and 6 (see Scheme 2). The difference of the latter two
[
10]
compounds with 3 and 4a, is the variation of the ending bulky groups (see Scheme 2).
In accordance with the SEM observations, powder XRD measurements verify the amorphous
character of the rotaxanes. As shown in Figure 4, the CFD compound (3) shows several reflections
between 2θ: 20 and 35 degrees. Nevertheless 4a and 4b, show a large broad distributed in 2θ range
10-40 degrees, which is reviled as a main characteristic of an amorphous phase. However such a
broadening appears also, in roughly the same 2θ range in case of compound 3 along with some well-
resolved sharp peaks at 2θ: 21, 26 and 31 degrees. The same behavior has been reported by our
[
10]
group
regarding the two analogues of compounds 3 and 4a, compounds 5 and 6 respectively,
depicted in Scheme 2, which differ to the stoppering groups (in case of 5 and 6 that is 2,4-
dinitrophenyl groups) and to the overall charge of the dumbbell like ions (negative for 3 and 4a: -4;
positive for 5 and 6: +4). All the above imply that the formation of complexes between CD (either α-
or β-) macrocycles and the viologen based backbone, leads to final materials with lower degree of
crystallinity, the latter being an important discrimination between rotaxanes of that type and their
corresponding CFDs. Similar observations regarding changes in morphology upon inclusion of
[45]
different substrates in cyclodextrins have been reported by different groups.
Figure 4. Powder XRD patterns of CFD compound: 3 (a) and of [2]rotaxane 4a (b).
Scheme 2. Structure of tetracationic rotaxanes (4a-b) and CFD compound (3) possessing
ferrocyanide stoppers (studied herein), and tetraanionic metal-free [2]rotaxane (6) and CFD
[
10,18]
compound (5),
all of them having the same precursor: compound 2.
Additionally the shapes of thermogravimetric (TG) curves of the CFD compound (3) and the
rotaxanes (4a-b), where much different as depicted in Figure 5. All compounds get dehydrated in
o
temperatures lower than 175 C (all three are pentacyanoferrate complexes thus they are always
isolated as hydrated salts; 4a and 4b lost about 13% of their weight, whereas 3 almost 20% of its
o
weight, in water; see experimental section). However when heated above 175 C (in the range 175–
3
00 °C) the rotaxanes tend to decompose whereas the CFD compound remains stable. This behavior
[
46]
has been reported in the past for several complexes of α-,β-, and γ-CD with organic compounds.
The latter observation is an additional evidence of the formation of interlocked compounds 4a-b.
Figure 5. Thermogravimetric (TG) data of compounds 3 and 4a-b for the temperature range 25
o
to 300 C.
Furthermore the analyses of bulk regions of compounds 3 and 4a-b, through the EDX technique,
clearly verifies the existence of the CD in compounds 4a and 4b (α- for 4a, β- for 4b). Elemental
analyses gave even more accurate results (including hydrogen); the exact values in any case are
listed in the experimental section. All the above analysis clearly supports the proposed structure of
dyes 3 and 4a-b. Important structural information was also obtained through the sound UV-Vis
spectrophotometric study of these dyes, analyzed in depth below.

ACCEPTED MANUSCRIPT
4
.2 Theoretical Studies
4
-
The geometry optimized structure of the anionic part of the CFD compound (3 ) was calculated
using Density functional theory (DFT) methods. DFT methods have been successfully applied in the
past for the calculation of structure and properties of pentacyanoferrate(II) complexes similar to 3
[
40]
and 4a-b. The aforementioned structure optimized in vacuum is depicted in Figure S1 and the
most important bond-lengths are shown in Figure 6.
4
-
Figure 6. Calculated bond lengths of the B3P86/6-311++G(d,p) optimized complex anion 3
in vacuum).
(
4
-
As shown in the geometry optimized structure of the anion 3 (Figure S1 of Supporting
Information) the two pyridinium rings of each 4,4′-bipyridine unit, are coplanar. This is in agreement
II
with other published optimized structures of pentacyanoferrate(II) with Fe coordinated to 4,4′-
[40]
bipyridine based ligands.
Interestingly the two planes of the bipyridine units are rotated as
compared to the diazobenzene medium plane, leading to a system of three planar units connected as
depicted in the cartoon image of Figure S1 of Supporting Information (dihedral angles shown in the
same figure).
-4
Furthermore as shown in Figure 7, the HOMOs of the anionic part of the dye 3 are derived
mostly from d excitations of the iron center but also by some transitions associated to the azo group
(
Figure 7a) and the LUMOs are dispersed along the conjugated electron deficient 4,4′-bipyridyl units
II
III
of the ligand. That is anticipated as Fe behaves as a strong electron donor being oxidized to Fe
see discussion in paragraph 4.4.1). Additionally the electron deficient parts of 3, the 4,4′-bipyridyl
(
II
units, can efficiently receive one electron each from the donating Fe center. Because of the aromatic
character of these heterocyclic units, it is anticipated to observe the LUMOs spread along the
aforementioned units. The illustrated HOMOs and LUMOs are in line with the experimentally
observed push-pull character of the studied dyes, as will be analyzed thoroughly next.
Figure 7. Illustrations of the molecular orbitals of the complex anion of dye 3 (a) HOMO and b)
LUMO) calculated on the B3P86/6-311++G(d,p) basis /vacuum (0.02 contour plots).
4
.3 UV-Vis Spectrophotometric analysis
UV-Vis spectrophotometry is a very important technique for both structural elucidation of dyes
and the study of their chromotropic behavior. There are four main features observed in the UV-Vis
spectra of compounds 3 and 4a-b, all obvious in different solvents. First of all, all three compounds
present a sharp, high energy, intense band appearing at around 260-270 nm corresponding to the
π→π* transition of the π molecular orbitals of the aromatic 4,4′-bipyridinium units of the linear
dumbbell like molecules. Generally this band is almost insensitive to solvent polarity changes or
structural differentiations eg incorporation of CD macrocycles (rotaxanation), or the presence of
various substituents (either electron withdrawing or donating ones, as shown in previous

ACCEPTED MANUSCRIPT
[
35-37]
publications).
The second band appearing also in the near-UV-region of the spectra of 3, 4a-b, is
a sharp, intense band attributed to the π→π* transition of the azo-group. This constitutes a
characteristic band for all azo-compounds. The molar absorption coefficient of this band (for the
4
−1
−1
trans azo isomer) is varying within 2–3×10 M ꢁcm and it is positioned in roughly 340-350 nm.
Additionally a much less intense and broader band attributed to the n→π* transition of the azo-group
is observed. This characteristic band is centered around 390-400 nm and it is the reason of the yellow
color of many azobenzene dyes (eg compound 2 has an intense yellow color). The study of this band
is very important especially for the E-Z isomerization of many azo-compounds because of the much
higher molar absorption coefficient of the corresponding Z isomer (herein only the E-isomers of all
dyes are investigated). Importantly this band often appears as a shoulder of the π→π* (azo) band,
and in many cases the deconvolution of the UV-Vis spectrum is necessary for its study (see for
instance Figure 8: UV-Vis spectra of 3 and 4a-b in water). The molar absorption coefficient of this
band is much smaller since the corresponding transition is geometry forbidden. It has been shown in
the past that the latter two bands (characteristic ones of azo dyes) respond quite intensely to medium
polarity changes, especially when the azo group is incorporated in the π-conjugated system of a D-π-
[
28-30]
A dye (see Figure 1a).
o
Figure 8. UV-Vis spectra in water at 25 C, (broken line: recorded data) and Gaussian bands
after deconvolution (details in Supporting Information) for (a) CFD compound 3, (b) [2]rotaxane 4a,
and (c) [2]rotaxane 4b.
The last feature, which appears in the electronic spectra of 3,4a-b, is a Vis band attributed to the
II
MLCT (dπ((Fe ) → π*(bpy), where bpy stands for 4,4′-bipyridine). This is a typical feature for
pentacyanoferrate(II) complexes with 4,4′-bypyridine based ligands. This band is very broad and
[
35-37]
markedly sensitive to structural changes but mainly to solvent polarity changes.
The main
medium responsive characteristic of this band is its position. Shifts of even 300 nm can be observed
upon increases or decreases of the polarity of the medium, therefore making solvent effects
observable even by naked-eye. Furthermore broadening or sharpening of the band as well as
increases or decreases of the molar absorption coefficient can be observed, strongly correlated with
solvent polarity. These intense responses render the complexes of this type very attractive for the
[
35-37]
study of their solvatochromism.
are shown after deconvolution.
In Figure 8 the UV-Vis spectra of compound 3, 4a-b in water
As noticed, UV-Vis spectrophotometry constitutes an additional means of structure verification.
II
It is for example easy to verify the successful fixation of the stoppering groups -Fe (CN) moieties
5
by basically observing the presence of MLCT band in the Vis spectra of compounds 4a-b. This band
is of course absent in the electronic spectra of compound 2 (precursor, see Figure 9b). Additionally
one can also observe small shifts and changes of the molar absorption coefficients induced by the
inclusion of the linear π-conjugated skeletons in the cavities of α- or β-CD macrocycles. This
[
10]
observation is typically happening upon inclusion and it has been also reported for 5 and 6.
Furthermore the MLCT Vis bands are slightly shifted bathochromically in both cases of rotaxanes.
This effect is attributed to bond-polarization in the phenyl rings triggered by the interactions of
hydrogen atoms of the latter and the CD internal hydrogen atoms. This intercomponent polarization
effect possibly leads to inductively increased electron withdrawing ability of the neighboring
quaternized nitrogen atom of 4,4′-bipyridine, therefore influencing the MLCT process. Such an effect
could be thought as analogue of adding a mild electron accepting substituent on the phenyl rings
[
35,36]
(such structural effects have been previously described by our group).
The comparison between
the electronic spectra of the two “homologous” α-CD containing [2]rotaxanes: 4a and 6, bearing

ACCEPTED MANUSCRIPT
II
different types of stoppering units (Fe (CN) and 2,4-dinitrophenyl or DNPh respectively) is also
5
very interesting. As expected the MLCT band is the main differing feature, being present in the
electronic spectra of 4a but not in case of 6 (Figure 9a). The other bands mentioned above,
corresponding to the azo group (typical for azobenzene based dyes) exist in both cases. Nevertheless
two more differences are obvious. First of all the n→π* band of 6 is much more intense. In case of
4a it appears as a non-clear shoulder of the π→π*band, of much smaller absorbance, as seen in
Figure 9a. Additionally for 4a the π→π* band exhibits a relatively higher molar absorption
coefficient. Compound 6, differs from 4a, in the type of stoppering group; a strong electron
II
withdrawing (DNPh) for the first and a strong electron donating (-Fe (CN) ) group for the latter.
5
Intense charge transfer is closely related to the structure of a dye as observed in many recent papers
[
26-30, 36-37]
readily affecting its medium responsiveness.
This can explain the intense medium
responsive behavior of 3,4a-b, and the negligible solvatochromism of compounds 5-6.
Figure 9. Comparison of the UV-Vis spectra of compounds: (a) 4a and 6, differing only in the
stoppering groups (see Scheme 2; 4a measured in water, 6 as a hexafluorphosphate salt in
-2
DMSO, both in concentration: 1.82×10 mM); (b) 4a and precursor 2 (both in water; concentration:
-2
2
.62×10 mM); (c) 3, 4a, and 4b (all in water; normalized UV-Vis spectra).
4
.4 Solvatochromism of compounds 3 and 4a-b.
4
.4.1 General Aspects
Solvatochromism is a very important phenomenon which many different kinds of dyes exhibit
and it corresponds to the change of the electronic spectra of a dye upon alteration of the polarity of
the medium in which such a dye is dissolved. In many cases such changes are observable even by
[
47]
naked-eye. In general terms the solvatochromic behavior of a compound is narrowly connected to
[47,48]
its structure.
Push-pull dyes of the general type D-π-A (Figure 1) either organic or metal
complexes, tend to respond to solvent polarity changes because of the possibility of adopting
different electron distributions along the solvatochromic molecules depending on the polar nature of
[
47-48]
a solvent.
The design and synthesis of novel dyes of the aforementioned type is currently of
[
49,50]
high interest since such compounds can serve as molecular sensors,
probes of solvent
or in some cases can become the basis of novel empirical parameters expressing
[
47,51]
polarity,
solvent polarity,
ET(30). The latter serves as a scale of dipolarity/polarizability and Lewis acidity of solvents.
[
51-52]
the most prominent being Reichardt’s dye and the corresponding polarity scale
[
51]
Compounds 3 and 4a-b, are intensely solvatochromic. Herein we attempt to rationalize and
quantify the solvatochromic effect observed in solutions of these compounds in polar solvents
namely water, ethylene glycol (EG) and their binary mixtures. It is noteworthy that all three
compounds exhibit high solubility in polar hydroxylic solvents, predominantly in water, as they tend
to form strong hydrogen-bonds with molecules of these solvents with the nitrogen atoms of CN
[
35-37]
groups acting as strong Hydrogen Bond Acceptor (HBA) bases.
The main result of the latter
specific solvent-solute interaction is the increase of the energy gap between the ground and MLCT
II
excited state as depicted in Figure 10a. Hydrogen bonding of the type S-H····NC-Fe (CN) -L,
4
(where S-H is a Hydrogen-Bond Donating (HBD) solvent and L is the ligand coordinated to iron-II)
is a typical interaction observed in all cases of pentacyanoferrate complexes when dissolved in HBD
[35-37]
solvents
and leads to a hypsochromic shift of the MLCT band upon increasing solvent HBD

ACCEPTED MANUSCRIPT
character. The latter described specific interaction is not the only demonstrating an important role on
the solvatochromic behavior of pentacyanoferrate complexes such as 3 and 4a-b. Solvents which can
act as Lewis bases, thus being capable of channeling electron density to the iron-II center can also
influence the electron distribution along a solvatochromic molecule. This is also a specific effect
which generally leads to a decrease of the energy gap between the ground and the MLCT excited
state upon increasing Lewis basicity of a solvent. Finally, non-specific interactions between solvent
molecules and the molecules of the pentacyanoferrate complexes (solute) demonstrate a significant
role as well. The main effect in that case, derives from the tendency of dipolar solvents to stabilize
more the ground state than the MLCT excited state, because the latter is less dipolar as it exhibits no
charge-separation (Figure 10a). This effect corresponds to the so-called negative
[51]
solvatochromism, in other words: hypsochromic shifts of the charge transfer band upon increasing
solvent dipolarity (the resulting color change in the two neat solvents H O and EG, is obvious by
2
naked-eye, as shown in Figure 10b.
Solvatochromism of compounds 3 and 4a-b was studied in water/EG binary mixtures. Water and
EG were chosen as two solvents capable of dissolving these dyes but also because of their high
stability in those solvents and their mixtures. Unfortunately other polar solvents such as DMSO or
formamides such as DMF readily lead to decomplexation of pentacyanoferrate complexes. The
maximum absorption wavenumbers of all features of the UV-Vis spectra of the dyes 3,4a-b, recorded
in EG and water are listed in Table 1. MLCT and n→π* energies measured in water/EG mixtures
along with corresponding polarity parameters are shown in Tables S1 and S2, of Supplementary Data
File. (All wavelength values for the 4 different features for all binary mixtures studied are mentioned
in paragraphs S1 to S3 of Supplementary Data file including figures of the deconvoluted spectra in
any case).
Table 1. UV-Vis absorption data for compounds 3 and 4a-b in water and EG
Figure 10. a) Molecular orbital diagram in the ground and MLCT excited state for a complex
II
of the type LFe (CN)
5
(L: ligand) in water and EG. b) The electronic spectra of the dye 4a in water
and EG (insert: color change observed)
4
.4.2 Quantifying The intense medium responsive character of compounds 3 and 4a-b
As shown in Figure 10b and Tables 1 and S2 (Supporting Information) compounds 3 and 4a-b,
exhibit marked color changes in solution. High shifts mainly of the MLCT and n→π* bands (the
latter in smaller extent) are induced when going from pure water to pure EG. It is noteworthy that
water and EG are two solvents which present several similarities with regard to their polarity (both
are structured solvents, presenting high HBD-acidity and HBA-basicity being able to specifically
interact with solute molecules) with EG being though less dipolar than water. As shown in the
histogram of Figure 11, for the given rather small solvent polarity change (see Table 1; polarity
N
Τ
parameters differences: ꢂΕ = –0.21; ꢂπ* = – 0.17; ꢂα = –0.27; ꢂβ = 0.05) compounds 3, and 4a-b,
respond even more than Reichardt’s betaine: RB (Supplementary Data File Scheme S1) which is a
highly solvatochromic compound. In the same histogram the sensitivity to the polarity change (from
water to EG) of another widely known organic betaine (Brooker’s betaine: BB; Supplementary Data
File Scheme S1) is also shown. The changes of charge transfer energies follow the sequence 3 ≈ 4b >
-1
[51]
[53]
4
a > RB > BB (ꢂΕCT in kcal.mol : 3: –9.87; 4b: –9.78; 4a: –8.67; RB: –6.8 ; BB: –4.3 ). The
latter observation is an indication of the remarkable solvatochromic behavior of the dyes 3 and 4a-b.

ACCEPTED MANUSCRIPT
It is noteworthy though that these pentacyanoferrate(II) complexes suffer low solubility in solvents
of low polarity. This issue could be possibly solved by synthesizing pentacyanoferrate(II) complexes
containing tetralkylammonium counter cations. Nevertheless the synthesis and isolation is not
straightforward. Another interesting observation is that the compounds studied here (all containing 2
pentacyanoferrate(II) units per molecule) respond more intensely to solvent polarity changes than the
pentacyanoferrate (II) complexes containing only one unit, as shown in the histogram of Figure 11a
for compounds 7a-e (Supporting Information, Scheme S1). The latter mono-pentacyanoferrate(II)
complexes show negative solvatochromic shifts (– ꢂΕ ) which correlate very well with Hammett’s
CT
constants, reflecting the importance of the nature of substituent in the phenyl ring attached to
quaternary N of 4,4′-bipyridine based ligand (Figure 11b). The –ꢂΕCT observed is still high being
closer to RD as shown in Figure 11a.
In general this increased solvatochromic intensity of multi pentacyanoferrate(II) units containing
Prussian blue analogues, was also recently reported for poly-pentacyanoferrates in water/EtOH
[
54]
II
mixtures.
Here though the connection of bpy-Fe (CN) to a diazobenzene unit is a structural
5
feature which leads to higher medium responsiveness. The aforementioned key-features can be very
important for the design of novel highly solvent-polarity responsive Prussian blue analogues and
solvatochromic dyes and materials.
Figure 11. a) Histogram showing the negative solvatochromic shift (–ꢂECT) observed between
water and EG for different dyes exhibiting charge transfer namely compounds 3,4a-b examined here,
compounds 7a-e (Structures in Supporting Information Scheme S1), Reichardt’s Betaine (RB) and
Brooker’s Betaine (BB) and b) Hammett plot of the observed –ꢂΕCT for compounds
pentacyanoferrate(II) complexes 7a-e.
4.4.3 Using LSERs to describe the solvatochromism of 3,4a-b.
Different solvent polarity parameters can be used in order to describe the solvatochromism of a dye.
In most of the cases one should ideally take into account all types of intermolecular interactions in
solution: both specific and non-specific but also solvent-solute and solvent-solvent interactions. In
many cases suitable solvent polarity parameters obtained by specific solvatochromic probes (eg
[
51]
ET(30) deriving from the study of solvatochromism of RD) are used. Previously we have shown
that the medium responsive character of pentacyanoferrate(II) complexes with ligands based on 4,4′-
bipyridines can sufficiently be described and rationalized using Linear Solvation Energy
Relationships (LSERs) such as the so-called Kamlet-Abboud-Taft (KAT) equation (equation S1,
[
35]
Supplementary Data File).
Herein two different types of equations were employed for that
[
27,55]
purpose: the KAT equation as well as an equation similar to Krygowski-Fawcett equation
EMLCT = EMLCT,0 + α'·ET + b'·β Equation S2; details in Supplementary Data File Paragraph 4:
N
(
Equations). Both of them can describe solvent effects on a physicochemical quantity in terms of both
specific and non-specific solvent-solute interactions. Equation S3 does that by including Reichardt’s
N
polarity scale in its normalized form: ET describing a mixture of Lewis acidity and
dipolarity/polarizability of a solvent, along with the parameter β (of KAT equation) the latter being a
measure of HBA-basicity of a solvent. MLCT energies show much more significant responses to
solvent polarity changes and contribute the most to the overall solvatochromic effect. (As mentioned
in paragraph 4.2, n→π* (-Ν=Ν-) bands are also medium responsive, showing though a minor
contribution, and they are discussed more next). The results of the correlations of the MLCT
Energies with the KAT equation parameters for compounds 3,4a-b, are listed in Table 2.

ACCEPTED MANUSCRIPT
Table 2. Results of the correlation of experimental MLCT energies of 3 and 4a-b with KAT equation
parameters.
KAT equation is proved to be a successful tool for the prediction, quantification, and rationalization
of the solvent effects on the MLCT energies of 3,4a-b. This is confirmed by the high correlation
coefficients of the plots of Figure 12 (plots a1,b1, and c1) reflecting the excellent fitting of the
calculated and experimental MLCT energies (EMLCT) for all compounds examined.
Furthermore, in all cases, increasing dipolarity (expressed in KAT equation by parameter π*) and/or
Lewis/HBD-acidity (expressed in KAT equation by parameter α) results in the increase of MLCT
energies. This is in line with the discussion of paragraph 4.2, and it is concluded by the positive
regression values obtained of the coefficients of parameters π* and α respectively (but also by the
positive slopes obtained in all cases, for the correlations of the type EMLCT vs E (30) as the latter as
T
mentioned is a measure of dipolarity/polarizability and acidity of a solvent; Supplementary Data File,
Figure S2). Nevertheless according to the results listed in table 2, the contribution of
dipolarity/polarizability as described by the parameter π*, is rather high in all cases of the dyes
examined. Some important differences from dye to dye were observed and should be attributed as
structural effects. Interestingly the CFD dye (3) and the corresponding [2]rotaxane with β-CD (4b),
show a very similar response to solvent polarity changes. In both cases the most important solvent-
solute interactions are non-specific ones, as implied by the high contribution of the
dipolarity/polarizability parameter π* as shown in Table 2; contributions of different parameters
examined were calculated according to Paragraph 4g of Supplementary Data File). In both cases
solvent-solute specific interactions are of lower importance as parameters α and β, contributed less
than 35% in both cases (conbined; see Table 2). Nevertheless HBD-acidity was proved to be more
important for the [2]rotaxane dye 4b, than HBA-basicity, and vice versa for dye 3. This effect
reflects the lower susceptibility of the CFD compound to the basic character of the medium as
compared to the β-CD containing [2]rotaxane, and that is because aggregation of HBAccepting
solvent molecules around the region of the azo-group is rather hindered in case of compound 4b.
Most interestingly compound 4a, exhibited a much different behavior as compared to both
dyes 4b and 3. In that case, as shown in Table 2, all three KAT parameters contribute significantly to
the MLCT solvent polarity responsive character. In other words there is a big drop of Pπ* in that case
and the specific solvent-solute interactions as expressed by parameters α and β, contribute to roughly
62%. This inversion in the contribution pattern for 4a, could be attributed to the size of the cavity of
α-CD which is smaller as compared to β-CD and therefore gives rise to significant host-guest
[
56]
interactions between the encapsulated dumbbell and the internal of the CD cavity.
This is the
reason why the contribution pattern is altered also as compared to the CFD dye 3. Additionally the
aforementioned interactions lead to a more pronounced bathochromism of the MLCT bands in case
of α-CD when dissolved in neat EG (λMLCT(3) = 697 nm ; λMLCT(4a) = 709 nm; λMLCT(4b) = 698 nm;
see Table 1). Furthermore 4a, showed somewhat lower solvatochromic response to medium polarity
changes (see Figure 11). Similar results are provided when using equation S2 (Supplementary Data
File). In that biparametric equation, specific solvent-solute interactions (with solvent molecules
N
T
behaving as HBD-acids) and non-specific ones are described by only one parameter: E . The
basicity (specifically HBAccepticity) is described by polarity parameter β (see KAT equation). In all
N
T
cases of dyes examined, the contribution of parameter E , was major (more than 90%), though
parameter β, contributed to a minor extent (for details see Table S4 Supporting Information).
Qualitatively this is in line with the results obtained through the analysis of contribution of the KAT
equation parameters. KAT equation though gives the possibility to split the dipolarity and

ACCEPTED MANUSCRIPT
[
51]
polarizability from Lewis and HBD-acidity giving this way more reliable results. An additional
interesting point is that in case of dye 4a (α-CD containing [2]rotaxane) the regression value of the
coefficient β, in both KAT equation and LSER (see Tables 2 and S3 of Supplementary Data File) was
positive. This was not the case for complexes 3 and 4b, where increasing of solvent basicity leads to
decrease of the MLCT energy. One possible explanation could be that because of the smaller size of
α-CD as mentioned previously the interactions of solvent molecules with the encapsulated azo group,
acting rather as a good HBAccepting moiety, is less pronounced in case of the dye 4a. This effect
(which occurs in a lesser extent in case of 4b and disappears completely in case of the CFD
compound: 3) causes the decrease of contribution of parameter α.
Figure 12. Plots of calculated MLCT energies through KAT equation and experimentally obtained
for the dyes 3:a1; 4a:b1; 4b:c1. Corresponding plots (calculated vs experimental) in case of equation:
N
T
EMLCT = EMLCT,0 + α'·E + b'·β: a2 for 3, b2 for 4a, and c2 for 4b.
4.4.4 The role of the n→π* transition of the azo group
As already mentioned, all dyes examined in this work exhibit a band in the near-UV/Vis region,
giving rise to a yellow color and attributed to the n→π* transition of the azo groups. This band also
contributes to the color change that all three dyes exhibit as a result of its response to solvent polarity
changes. Obviously that happens to a much smaller extent as compared to the MLCT band. There is
a linear dependence between the maximum absorbance of the n→π* in any case, and Reichardt’s
polarity scale E (30) as shown in the plots of Figure S3 of Supplementary Data File. Increases of
T
solvent polarity (dipolarity, polarizability, as well as Lewis acidity as expressed by E (30), lead to
T
decreased absorption maximum wavelengths (or increased absorption energies). This effect has been
[
51]
reported previously for push-pull azo dyes. As also shown in Figure S4 of Supplementary Data
File, the n→π* absorption maxima wavelengths, exhibit a linear dependence on EG molar fraction
(regarding the water/EG binary mixtures) and finally reach a plateau after approx. x(EG) > 0.5 for
all dyes 3,4a-b. This effect is clearly connected to preferential solvation of the dyes by EG as
dielectric non-ideality is almost negligible in water/EG mixtures (see plot of Kirkwood’s function vs
EG molar fraction Figure S5; Supporting Information). Additionally in the plots of Figure S6
(Supplementary Data File) n→π* energies show very poor linear correlations with Kirkwood
function meaning that what observed is mainly a preferential solvation effect rather than a dielectric
non-ideality effect. Furthermore the very good correlation of the n→π* energies with Reichardt’s
polarity scale is attributed to the analogous non-linear dependence of the latter scale on EG molar
fraction in binary H O/EG mixtures. Interestingly the dependence of maximum MLCT Energies on
2
EG molar fraction, also supports preferential solvation by EG. As shown in Figure S4 of Supporting
[
57]
Information, the plots EMLCT vs x(EG) and Enπ* vs x(EG) indicate this preferential solvation result.
According to the aforementioned results, n→π* transition, which also leads to absorptions in the
Vis region for all dyes, contributes to the observed solvatochromic effect by increasing the yellow
color contribution upon reducing solvent polarity. That is why the color of all dyes in EG is green (as
a result of the blue color, because of the MLCT absorptions, and yellow color, because of the n→π*
transition of the azo dyes 3,4a-b).
4
.5 Photochemical stability of the dyes

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It is widely known that azo dyes can undergo photochemical E-Z isomerizations and this
property makes them very attractive for new photoresponsive compounds and materials with a wide
[
21-23, 58-61]
range of applications.
Nevertheless in the case of the dyes 3 and 4a-b, the presence of the
II
ending bulky groups –Fe (CN) , renders the compounds rather photchemically unstable. Irradiation
of pentacyanoferrate(II) complexes results in decomplexation and the loss of the -Fe (CN)
5
II
5
[
62]
[35,63]
groups,
and that is pronounced in solvents of relatively high Lewis Basicity.
Nevertheless
these groups are also the ones which render these compounds markedly solvatochromic. Attempts to
study the photo-responsive character of the dyes 3 and 4a-b, failed because of the decomplexation
even after 30 min of irradiation with UV-light (365 nm). Nevertheless the title compounds are very
stable in solution when not exposed to UV-irradiation. Exposure to ambient light in water or
hydroxylic solvents even for 24 h does not result in significant reduction of the MLCT absorbance of
the compounds, and therefore compounds 3 and 4a-b, can be thought as rather stable
pentacyanoferrate(II) complexes, suitable for applications based on their solvatochromism. This is
II
consistent with the fact that pyridinium-Fe (CN) are rather stable because of the high affinity of
5
II
[64,65]
Fe (CN) to strong π-accepting ligands like pyridines, pyrazines etc.
5
5
. Concluding Remarks
Three new pentacyanoferrate(II) solvatochromic compounds bearing conjugated electron-deficient
ligands have been synthesized and fully characterized. The impact of the incorporation of α- or β-CD
to the mother CFD compound on the solvatochromic behavior of these dyes was quantified and it has
been shown that all compounds show a very intense medium responsive character, much stronger
even than other known markedly solvatochromic dyes such as other previously described
pentacyanoferrate(II) dyes, Reichardt’s betaine and Brooker’s merocyanine. The CFD compound (3)
and the β-CD corresponding [2]rotaxane (4b) exhibited similar susceptibilities to medium polarity
changes (in the region between the polar solvents water and EG), but the α-CD [2]rotaxane (4a)
showed a somewhat weaker response. The study of rotaxanes of chromotropic compounds (like 3)
with CDs shows that the inclusion complexes retain the strong chromotropic character of the mother
CFD dye, providing also stabilization of the azo-dye.
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56] Similar observations have been made for the dye 6 (Scheme 2) see also references 10 and 18.
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TABLES
Table 1. UV-Vis absorption data for compounds 3 and 4a-b in water and EG
o
o
water at 25 C
EG at 25 C
Compound
λmax (nm) Emax (kcal/mol) assignment
λmax (nm) Emax (kcal/mol)
2
3
391
5
2
3
65
37
107.89
84.84
73.12
51.42
107.08
83.85
74.26
49.04
106.29
84.09
74.26
50.69
π→π* (bpy)
π→π* (-N=N-)
n→π* (-N=N-)
d→π* (MLCT)
π→π* (bpy)
π→π* (-N=N-)
n→π* (-N=N-)
d→π* (MLCT)
π→π* (bpy)
273
343
410
697
277
345
406
709
274
344
406
698
104.73
83.35
69.73
41.02
103.22
82.87
70.42
40.33
104.35
83.11
70.42
40.96
3
56
67
41
4
4
a
385
5
2
3
83
69
40
π→π* (-N=N-)
n→π* (-N=N-)
d→π* (MLCT)
b
385
64
5
Table 2. Results of the correlation of experimental MLCT energies of 3 and 4a-b with KAT equation
parameters.
EMLCT,0
kcal/mol)
20.8286
-6.1656
13.0401
b
c
d
e
e
e
f
Compound
a
s(kcal/mol) a(kcal/mol) b(kcal/mol)
%Pπ*
%Pα
%Pβ
rss
(
3
21.6477
22.4658
23.3810
4.3214
23.1008
8.3522
-4.0810
9.8294
-1.5421
68.8
38.2
69.1
13.7
39.3
24.7
17.5
22.5
6.2
17.4888
3.4513
1.2144
4
4
a
b
a
b
c
d
e
f
Intercept of KAT equation; MLCT energy in case where: π* = α = β = 0 (cyclohexane).
Regression value of the coefficient of parameter π* expressing solvent dipolarity/polarizability.
Regression value of the coefficient of parameter α expressing solvent HBD-acidity.
Regression value of the coefficient of parameter β expressing solvent HBΑ-basicity.
Relative contributions for parameters α,β, and π* (indicated with corresponding subscript)
Residual Sum of Squares.

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FIGURE CAPTIONS
Figure 1. (a) General representation of a D-π-Α dye. D being an electron donor, π: a π-conjugated
system, and A: an electron withdrawing moiety and (b) the π-conjugated linker, electron donor, and
electron accepting units, in case of a (N-aryl-substituted 4,4′-bipyridine) ferrate (II) complex studied
herein.
1
Figure 2. (a) H NMR spectra of compound 2, (b) CFD: compound 3, and (c) compound 4a. (d)
1
1
Partial H NMR spectra of neat α-CD, (e) compound 4a, (f) overlaid H NMR spectra of α-CD and
a; all spectra recorded in D O.
4
2
Figure 3. SEM images of neat-crystalline α-CD (a), CFD compound: 3 (b), [2] rotaxane: 4a (c),
artificial mixture of CFD compound and α-CD (molar ratio 1:1) (d). Scale bars correspond to 100
ꢀ
m; magnification: ×400 in all cases.
Figure 4. Powder XRD patterns of CFD compound: 3 (a) and of [2]rotaxane 4a (b).
Figure 5. Thermogravimetric (TG) data of compounds 3 and 4a-b for the temperature range 25
o
to 300 C.
4
-
Figure 6. Calculated bond lengths of the B3P86/6-311++G(d,p) optimized complex anion 3 (in
vacuum).
Figure 7. Illustrations of the molecular orbitals of the complex anion of dye 3 (a) HOMO and b)
LUMO) calculated on the B3P86/6-311++G(d,p) basis/vacuum (0.02 contour plots).
o
Figure 8. UV-Vis spectra in water at 25 C, (broken line: recorded data) and Gaussian bands after
deconvolution (details in Supporting Information) for (a) CFD compound 3, (b) [2]rotaxane 4a, and
(c) [2]rotaxane 4b.
Figure 9. Comparison of the UV-Vis spectra of compounds: (a) 4a and 6, differing only in the
stoppering groups (see Scheme 2; 4a measured in water, 6 as a hexafluorphosphate salt in DMSO,
-
2
-
both in concentration: 1.82×10 mM); (b) 4a and precursor 2 (both in water; concentration: 2.62×10
2
mM); (c) 3, 4a, and 4b (all in water; normalized UV-Vis spectra).
Figure 10. a) Molecular orbital diagram in the ground and MLCT excited state for a complex of
II
the type LFe (CN)
5
(L: ligand) in water and EG. b) The electronic spectra of the dye 4a in water and
EG (insert: color change observed)

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Figure 11. a) Histogram showing the negative solvatochromic shift (–ꢂECT) observed between water
and EG for different dyes exhibiting charge transfer namely compounds 3,4a-b examined here,
compounds 7a-e (Structures in Supporting Information Scheme S1), Reichardt’s Betaine (RB) and
Brooker’s Betaine (BB) and b) Hammett plot of the observed –ꢂΕCT for compounds
pentacyanoferrate(II) complexes 7a-e.
Figure 12. Plots of calculated MLCT energies through KAT equation and experimentally obtained
for the dyes 3:a1; 4a:b1; 4b:c1. Corresponding plots (calculated vs experimental) in case of equation:
N
T
EMLCT = EMLCT,0 + α'·E + b'·β: a2 for 3, b2 for 4a, and c2 for 4b.
Scheme 1. Synthetic route followed for the preparation of compounds 3 and 4a-b; (i) solvent-free
Zincke reaction; (ii) self-assembling (threading) of CD macrocycles through compound 2, in water
II
(
formation of pseudorotaxanes); (iii) coordination to –Fe (CN)
5
(in situ stoppering for compounds
4a-b in aqueous solutions); (iv) slipping synthetic approach for compounds 4a-b: not possible.
Scheme 2. Structure of tetracationic rotaxanes (4a-b) and CFD compound (3) possessing
ferrocyanide stoppers (studied herein), and tetraanionic metal-free [2]rotaxane (6) and CFD
[
10,18]
compound (5),
all of them having the same precursor: compound 2.

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