Synthesis, properties and quantum chemical evaluation of solvatochromic pyridinium-phenyl-1,3-thiazol-4-olate betaine dyes

Article abstract of DOI:10.1016/j.tet.2012.12.016

The syntheses and characterizations of four zwitterionic betaines are presented. These dyes possess an uncommon heterocyclic 1,3-thiazol-4-olate donor moiety. The natures of the HOMO/LUMO transitions and of the intramolecular charge-transfer state were assigned with the help of quantum chemical calculations. Multiple intermolecular solute/solvent interactions were discussed using linear solvation energy relationship (LSER) with Kamlet-Taft and Catalán parameters. The dyes show a pronounced negative solvatochromism ranging from λ_max 392 nm in TFE to 820 nm in THF (Δλ=428 nm or 1.65 eV). The X-ray structures of one of the tetrafluoroborate salts and of one of the betaines are discussed.

Full text of DOI:10.1016/j.tet.2012.12.016

Accepted Manuscript
Synthesis, properties and quantum chemical evaluation of solvatochromic pyridinium-
phenyl-1,3-thiazol-4-olate betaine dyes
Roberto Menzel, Stephan Kupfer, Ralf Mede, Helmar Görls, Leticia González, Rainer
Beckert
PII:
S0040-4020(12)01846-7
10.1016/j.tet.2012.12.016
TET 23828
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 22 October 2012
Revised Date: 27 November 2012
Accepted Date: 7 December 2012
Please cite this article as: Menzel R, Kupfer S, Mede R, Görls H, González L, Beckert R, Synthesis,
properties and quantum chemical evaluation of solvatochromic pyridinium-phenyl-1,3-thiazol-4-olate
betaine dyes, Tetrahedron (2013), doi: 10.1016/j.tet.2012.12.016.
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Synthesis, properties and quantum chemical
evaluation of solvatochromic pyridinium-
phenyl-1,3-thiazol-4-olate betaine dyes
Roberto Menzel, Stephan Kupfer, Ralf Mede, Helmar Görls, Leticia González, Rainer Beckert

ACCEPTED MANUSCRIPT
1
Tetrahedron
journal homepage: www.elsevier.com
Synthesis, properties and quantum chemical evaluation of solvatochromic pyridinium-
phenyl-1,3-thiazol-4-olate betaine dyes
Roberto Menzel a, Stephan Kupfer b, c, Ralf Mede a, Helmar Görls d, Leticia González c*, Rainer Beckert a*
a Friedrich Schiller University Jena, Institute of Organic and Macromolecular Chemistry, Humboldtstraße 10, 07743 Jena, Germany.
b Friedrich Schiller University Jena, Institute of Physical Chemistry, Helmholtzweg 4, 07743 Jena, Germany.
c University of Vienna, Institute of Theoretical Chemistry, Waehringer Str. 17, 1090 Vienna, Austria.
d Friedrich Schiller University Jena, Institute of Inorganic and Analytical Chemistry, Humboldtstraße 8, 07743 Jena, Germany.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The syntheses and characterizations of four zwitterionic betaines are presented. These dyes
possess an uncommon heterocyclic 1,3-thiazol-4-olate donor moiety. The natures of the
HOMO/LUMO transitions and of the intramolecular charge-transfer state were assigned with the
help of quantum chemical calculations. Multiple intermolecular solute/solvent interactions were
discussed using linear solvation energy relationship (LSER) with Kamlet-Taft and Catalán
parameters. The dyes show a pronounced negative solvatochromism ranging from λmax 392 nm in
TFE to 820 nm in THF (ꢀλ = 428 nm or 1.65 eV). The X-ray structures of one of the tetra-
fluoroborate salts and of one of the betaines are discussed.
Available online
Keywords:
4-Hydroxy-1,3-thiazoles
Betaine dyes
2012 Elsevier Ltd. All rights reserved.
Solvatochromism
Donor-acceptor system
Linear solvation energy relationship (LSER)
very recently, as probes to determine the degree of substitution in
cellulose derivatives.17
1. Introduction
The majority of the Reichardt’s-type betaine dyes consist of
Compounds of the class of pyridinium N-phenolates are
widely used as solvatochromic, thermochromic, piezochromic
and halochromic functional dyes.1 After the first synthesis of 2,6-
diphenyl-4-(2,4,6-triphenylpyridinio)phenolate (Reichardt’s dye,
30, Figure 1) in 1963,2 the empirical parameter of solvent polar-
ity, the ET(30) values, were established to develop the ET(30) and
N-phenolate donor moieties and only few examples are known
which possess an unconventional purinolate or barbiturate-based
one.18 We choose the 4-
hydroxy-1,3-thiazole
core as an alternative to
the N-phenolate group
in order to extend the
ETN
normalized
(30) solvent polarity scale. The ET(30) values are
simply defined as the molar electronic transition energies of the
Reichardt’s dye, measured in solvents with different polarity.3
The number 30 simply comes from Dimroth´s and Reichardt´s
randomly chosen reference number of the dye in the first publica-
tion. Since then, several different derivatives have been synthe-
sized and several publications and reviews have been published
to explore and explain the characteristics of these compounds.4-6
Numerous betaines were classified in a research article from
Dominguez and Rezende in 2010.7 Additionally, they tried to
give a rationalized view to estimate whether a betaine (also other
dyes) exhibits a negative, inverted or positive solvatochromism.8
Because of their unique spectroscopic properties, they can and
are used mainly to measure solvent polarity of pure solvents and
binary solvent mixtures,9-11 to describe the solvent properties of
ionic liquids,12 as polarity and acid-base responsible dyes for
molecular recognition,13 as optical alcohol sensors in polymer
membranes,14 to examine the electrophilicity of lanthanide shift
reagents,15 to determine alterations in micellar structures,16 and,
scope of available
N
donor-acceptor betaine
dyes. To the best of our
knowledge, this is the
first example where the
anionic compartment
not only acts as the
electron donor, but also
as the chromophore
itself. The class of the 4-hydroxy-1,3-thiazoles was recently
revived by our group due to their outstanding chemical and opti-
cal properties and, additionally, they allow an easy chemical
functionalization toward the application desired. They have al-
ready been used as dyes in polymers, e.g. in a terpolymer as a
FRET energy donor combined with a Ru(II) complex as the
acceptor unit,19 as sensitizers in Grätzel-type dye-sensitized solar
cells (DSSCs),20 and furthermore, as specific sensor molecules
for the detection of fluoride ions,21 or as light-harvesting ligands
O
Figure 1. Structure of the Reichardt’s betaine
30 used as reference compound in this study
Corresponding author. Tel.: +49-3641-9-48230; fax: +49-3641-948212; e-mail: c6bera@uni-jena.de

ACCEPTED MANUSCRIPT
2
Tetrahedron
in Ru(II)-polypyridyl complexes.22,23 They can easily be
angles of the (hetero)aryl rings and of the thiazole, with the sulfur
in an almost perpendicular arrangement due to the missing hy-
bridization, are all in the expected range.28
equipped with various functional groups. Especially the facile
introduction of acceptor or donor moieties makes them suitable
for the construction of betaine dyes which possess a thiazol-4-
olate as donor and a pyridinium as acceptor group. Therefore,
several donor-acceptor dyes consisting of a 1,3-thiazole core, a
triphenyl pyridinium group and a heterocycle in the 2-position at
the thiazole (pyridinyl, pyrazinyl, thiophenyl and furanyl) to vary
the electronic and physical (solubility) properties of the betaines
have been synthesized. Additionally, the solid state structures of
one of the tetrabutylammonium salts and of one of the betaines
derived are provided.
2. Results and Discussions
2.1. Synthesis
The synthetic route and the structures of the betaine dyes are
depicted in Scheme 1. In a first step, the starting materials 1a-1d
were synthesized by a Hantzsch cyclization reaction of the corre-
sponding thioamide with ethyl 2-bromo-2-(4-nitrophenyl)-
acetate.23 Several attempts to reduce the nitro group to the amine,
starting directly from the 4-hydroxy-1,3-thiazoles, resulted in a
hardly separable mixture (due to the very poor solubility of 1a-
1d) and to a very sluggish conversion of the starting material.
Therefore, the hydroxy group of the 4-hydroxy-1,3-thiazoles was
protected as silyl ether by reacting with tert-butyldimethylsilyl
chloride (TBDMSCl), which facilitated the work up process
dramatically. Reduction was carried out with Pd/C and H2
(10 bar) in an autoclave with MeOH or THF as the solvent in
good to very good yields (75-88%). We also used hydrazine
hydrate and Raney nickel in test reactions which, unexpectedly,
led to a more or less desilylation back to the 4-hydroxy-1,3-
thiazoles and only to a poor formation of the products desired.
The transformation of the amines to the pyridinium salts was
carried out as a one-pot (ANRORC) reaction. Firstly, a vinylo-
gous amide, catalyzed with Et3N (1 equiv), is formed in a ring
opening sequence between the 2,4,6-triphenylpyrylium tetra-
fluoroborate and the corresponding aryl amine;24 secondly, the
ring closure is achieved by heating the resulting black mixture
with a twofold excess of AcOH (2 equiv) in EtOH (96%) under
reflux and, thirdly, the silyl ether is cleaved under the acidic
conditions used. The tetrafluoroborate salts synthesized were
deprotonated with tetrabutylammonium hydroxide (TBAOH) in
MeOH in order to obtain the “free” betaines almost quantitatively
(the process is fully reversible and other salts can be obtained).
Figure 2. ORTEP plot and numbering scheme of 4a. Ellipsoid probability
is 50%. Hydrogen atoms (except the calculated atoms of the hydrogen bonds)
and counter ions are omitted. Selected bond lengths (Å) and angles (°) are:
S1-C6 1.733(6), C6-N2 1.316(8), N2-C7 1.369(7), C7-C8 1.384(8), C8-S1
1.731(6), C7-O1 1.350(7), C6-C5 1.471(8), C8-C9 1.460(8), C5-N1 1.343(8),
N3-C12 1.478(8), S1-C6-N2 114.7(4), C6-N2-C7 109.9(5), N2-C7-C8
117.5(5), C7-C8-S1 107.6(4), C8-S1-C 90.3(3), C7-C8-C9-C14 25.97 (5),
S1-C8-C9-C10 26.12(3), S1-C6-C5-N1 24.98(2), N2-C6-C5-C4 20.92(4),
C13-C12-N3-C15 69.48(4), N3-C19-C20-C25 59.06(4), C16-C17-C26-C27
143.04(6).
X-ray structures of Reichardt’s betaines are rare in this field
and only few examples are known.25,29,26 Crystals of compound C
were obtained after various attempts by slow diffusion of MeOH
into DMSO. Therefore, a solution of C in DMSO was covered
with a layer of MeOH directly in an NMR tube which yielded
deep violet very thin needles. The compound crystallized as a
solvate together with one molecule of MeOH and one molecule
of water. Both solvent molecules form strong hydrogen bonds
[O1-O1E 2.806(7) Å and O1-O1W 2.690(9) Å] to the thiazole
oxygen O1. The structure and data are depicted in Figure 3 (addi-
tional refinement data is reported in the ESI S1). The pyridinium
moiety features a similar geometry like 4a. The phenyl rings are
twisted out of the pyridinium plane [e.g. N3-C19-C20-C25
66.23(5) °] to a greater extent probably because of the reduced
bond length between N3 and C12 [1.462(8) Å instead of
1.478(8) Å] leading to an enhanced steric hindrance.
2.2. X-ray structure of 4a and C.
Crystals of the tetrafluoroborate salt 4a suitable for X-ray
structure analysis were obtained by adding acetone directly to the
hot reaction mixture till the product dissolved followed by slowly
cooling down to r.t. One molecule of the compound crystallizes
together with one molecule of ethanol in light yellow crystals.
The structure and data are depicted in Figure 2 (additional re-
finement data is reported in the ESI S1). Two of the molecules of
4a are connected with one solvent molecule ethanol which forms
two medium-strength hydrogen bonds, one from the hydrogen of
the hydroxy group of the thiazole [O1-O1E 2.562(6) Å] and one
to the pyridine nitrogen from the hydroxy group of the ethanol
[O1E-N1 2.814(7) Å]. The phenyl rings at the pyridinium core
are twisted out of planarity due to steric hindrance [e.g. N3-C19-
C20-C25 59.06(4) °], as described for various similar
triphenylpyridinium derivatives.25-27 The phenyl and pyridyl ring
connected to the thiazole also are twisted to some extent [C7-C8-
C9-C14 25.97(5) ° and S1-C6-C5-N1 24.98(2) °]. There are no
dispersive interactions among the two molecules as the distance
between their aromatic rings is too far. The bond lengths and
Figure 3. ORTEP plot and numbering scheme of C. Ellipsoid probability
is 50%. Hydrogen atoms are omitted. Selected bond lengths (Å) and angles
(°) are: S1-C6 1.726(7), C6-N2 1.309(9), N2-C7 1.428(10), C7-C8 1.396(10),
C8-S1 1.752(7), C7-O1 1.278(8), C8-C9 1.405(9), N3-C12 1.462(8), S1-C6-
N2 116.0(6), C6-N2-C7 110.5(6), N2-C7-C8 114.2(7), C7-C8-S1 109.2(5),
C8-S1-C 90.1(4), C7-C8-C9-C14 2.01(6), S1-C8-C9-C10 6.24(4), C13-C12-
N3-C15 71.5(5), N3-C19-C20-C25 66.23(5), C16-C17-C26-C27 22.06(7).

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3
NO2
NO2
NH2
a)
R = 2-pyridyl
MeOH, Pd/C (10%)
H2 (10 bar )
CH2Cl2, Imidazole,
TBDMSCl
S
N
b) R = 2-pyrazinyl
S
N
S
N
R
R
R
c)
d)
R = 2-thienyl
R = 2-furanyl
20 h, r.t.
Yield: 62-96%
3 to 24 h, 50 °C
Yield: 75-88%
OH
O
Si
O
Si
1a 1d
-
2a 2d
-
3a 3d
-
Ph
O
BF4
Ph
Ph
N
Ph
Ph
Ph
Ph
R =
N
N
N
N
B
1. EtOH, TEA (1. equiv.)
1 h, r.t.
BF4
A
Ph
Ph
S
N
S
MeOH, TBAOH
10 min, 40 °C
Yield: > 93%
3a 3d
R
-
R
N
2
. CH3COOH (2 equiv.)
2 h, r.f.
OH
4a - 4d
O
S
C
O
A - D
D
Yield: 46-73%
Scheme 1. Synthesis of the starting materials, cyclization reaction to the pyridinium salts and structures of the four betaine dyes
Synthesis of the starting materials, cyclization reaction to the pyridinium salts and structures of the four betaine dyes A-D.
A-D.
2.3. Electronic Properties and Solvatochromism of the be-
taines
As expected, the thiophen-thiazole-phenyl moiety is almost
completely planar [e.g. C7-C8-C9-C14 2.01(6) °] because of the
change in the electron distribution. The zwitterionic structure
leads to shortening of the single bonds, while the double bonds
are elongated in this fragment. The bond of the deprotonated
oxygen O1 to C7 of the thiazole becomes a double bond and,
therefore, is significantly shortened [O1-C7 1.278(8) Å, com-
pared to C-O saturated 1.43 Å, C=O in ketones 1.22 Å and C=O
in zwitterion forms 1.26 Å]30 while the adjacent bonds are elon-
gated [N2-C7 1.428(10) Å compared to N2-C7 1.369(7) Å for
4a]. This is consistent with the partial quinoid character of the
thiazol-4-olate ring. Also the bond between the thiazole-phenyl is
shortened [C8-C9 1.405(9) Å compared to C8-C9 1.460(8) Å for
4a] because of a partly double bond character in this position
leading to the planarization observed. The thiophene moiety is
displaced but both isomeric forms are planar with respect to the
thiazole (torsion angle < 5 °). Therefore, it can be stated that
deprotonation of the hydroxy group to the corresponding betaines
significantly alters the geometry and the electronic properties.
The bond lengths measured are in accordance with literature
examples.29
The color of a solution of the tetrafluoroborates 4a-4d in
MeOH changes from yellow to purple or almost black after de-
protonation with TBAOH in MeOH and the betaines precipitated
as purple to deep green solids (ESI S2). In order to gain insight
into the electronic properties of the betaines A-D, firstly UV/Vis
measurements were carried out in CH3CN, a solvent with inter-
mediate polarity and hydrogen bond donor (HBD) and hydrogen
bond acceptor (HBA) character.31 All of the dyes show several
absorption bands of different intensities. The spectra together
with the excited states calculated are shown in Figure 4. The
spectra feature two main absorption bands. The absorption band
of the excitations at 300 nm are independent of the solvent and
are attributed to excitations of the 2,4,6-triarylpyridinium moiety
(the band is also independent of the aryl-1,3-thiazole used).32 As
can be seen from the spectra, the longest wavelength absorption
band at 470-670 nm, along with a red-sided shoulder, results
from a superposition of at least two excited states. Both absorp-
tion bands are prone to differences of solvent properties and
Figure 4. Experimental and simulated UV/Vis spectra of A-D at B3LYP(35)/6-31G(d,p) level of theory in CH3CN at r.t. The calculated excited states
were broadened using a full width of half maximum of 3000 cm-1.

ACCEPTED MANUSCRIPT
4
Tetrahedron
show a negative solvatochromism. The more intense absorption
the compounds are insoluble in more unpolar solvents like hep-
tane, toluene or cyclohexane excluding a better understanding of
the solvatochromic behavior in these solvents. Although that in
some solvents the two possible π-π* and ICT bands are superim-
posed, a clear trend of a pronounced negative solvatochromism
was found. It was possible to simulate and resolve almost all the
superimposed bands in an appropriate manner to obtain the λmax
of absorption by using the free available peak fitting program
Fityk.36 The solvatochromism determined is even more pro-
nounced compared to the standard dye 30. Compounds C and D
exhibit the largest negative solvatochromism measured. This
clearly follows the assumption made in the first part. For C and
D, the additional electron donor in the 2-position (furanyl and
thienyl) supports the electron rearrangement in the molecule from
the negatively charged oxygen to the pyridinium part leading to a
less dipolar S1 state, which is extraordinarily well stabilized
through unpolar solvent. This is contrary to A and B, where the
additional electron acceptor in the 2-position leads to a S1 state
with a higher dipolar character and, therefore, a less pronounced
negative solvatochromism. The regression parameters between
the ET(X) values of A-D and the ET(30) were determined in order
to get a more quantitative description of the solvatochromism. As
expected, no good linear correlations were observed for any
solvents. The HBD solvents (alcohols) especially do not follow
the more or less linear trend of the other solvents, as depicted in
the ESI S3. Two explanations can be found: (i) Contrary to 30,
no steric hindrance due to the two adjacent phenyl group exists in
A-D, allowing hydrogen bond formation of R-OH to the thiazol-
4-olate oxygen and (ii) additional hydrogen bond formation can
be formed to the heterocycle in the 2-position (see also X-ray
structure). Both cases stabilize the ground state to some extent
and, consequently, make the ICT less accessible to changes in
solvent polarity (e.g. changes of R in R-OH). This was also ob-
served and described for similar dyes with pyridines (HBA) at
the phenolic donor moiety.3 Interactions of solutes with the sol-
vent can be quite diverse. Therefore, Kamlet and Taft developed
a more reasonable method to explain solvent properties. Intermo-
lecular interactions of the solute/solvent system can be described
by applying the concept of linear solvation energy relationships
(LSERs, equation 1) using a multiple linear regression.9,37,38
maximum is positioned at λmax = 470 (D) to 540 nm (B) and is
attributed to an excitation located at the heterocycle-1,3-thiazole-
phenyl fragment. The band exhibits a less pronounced negative
solvatochromic behavior, as can be seen from Table 1, compared
to the excitation at the red-sided shoulder. The λmax is shifted
bathochromically in CH3CN for the dyes with the pyridine or
pyrimidine group in the 2-position, representing an electron-
acceptor group, (partially CT-character of the transition) com-
pared to the dyes bearing the furanyl or thienyl moiety (more π-
π*-character). This is consistent with the electronic properties of
the latter two, which contrarily possess more electron-donating
properties. This absorption band was also found for similar do-
nor-π-acceptor (D-π-A) dyes based on 4-methoxy-1,3-thiazoles.23
A methoxy group was connected to the thiazole in the 4-position
in the literature examples which represents a weaker electron-
donor than the deprotonated hydroxy group and, consequently,
the negative solvatochromism for this band was similar, but not
as pronounced compared to the dyes presented here. The lower
energy absorption band (shoulder at the red-side) is the most
solvatochromic one and can be attributed to the intramolecular
charge transfer (ICT) state typical for Reichardt’s-type betaines
(see section 2.4 for a more detailed description and Figure 5 for
the correlated molecular orbitals).5 The position of the band is
strongly affected by the heterocycle in the 2-position. The ab-
sorption maximum in CH3CN varies from 559 nm (A) to 613 nm
(C) and is superimposed for B with the absorption of the higher
energetic π-π*-CT state. It can be assumed that the bathochromic
shift of the longest-wavelength absorption band for C and D is
due to a partial participation of the electron donating thiophene
and furan ring increasing the energy of the HOMO to efficiently
enhance the ICT to the pyridinium group.
To get further insight into the solvatochromism of A-D, meas-
urements in solvents with different polarities and different HBD
(alcohols),33 HBA (acetone, CH3CN, DMSO, DMF) or non-HB
properties (DCM, 1,2-dichloroethane) were carried out.34 The
polarity scale ranges from 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP) [ET(30) 65.3] to THF [ET(30) 37.4]. The most polar sol-
vent used, HFIP, did not yield reproducible results because of the
high acidity (pKa = 9.3),35 which protonates the betaines at least
to a certain extent. The spectra measured were almost identical to
=
ν
α + b×β
+s×π*
%
νmax
%
max,0 +a×
(1)
̶
the spectra obtained from the BF4 salts in HFIP. Unfortunately,
Table 1. Absorption maxima of the betaines A-D (both transitions are stated) and of the reference dye 30 together with their ET(X) values in various solvents
ordered according decreasing ET(30) values at r.t. under normal pressure.
Solvent a
λmax [nm] [ET(A)] b
λmax [nm] [ET(B)] b
λmax [nm] [ET(C)] b
λmax [nm] [ET(D)] b
λmax [nm] [ET(30)] b
HFIP
TFE
MeOH
369, 441 [64.8]
373, 398 [71.8]
393, 446 [64.1]
388, 448 [63.8]
389, 451 [63.4]
493, 559 [51.1]
504, 593 [48.2]
510, 580 [49.3]
500, 619 [46.2]
525, 615 [46.5]
489, 597 [47.9]
462, 590 [48.5]
492, 655 [43.7]
526, 700 [40.8]
497, 734 [39.0]
530, 715 [40.0]
540, 730 [39.2]
ꢀλ = 332 d
390, 460 [62.2]
398, 474 [60.3]
400, 475 [60.2]
435, 485 [60.0]
439, 488 [58.6]
555 (sup.) c
393, 444 [64.4]
376, 392 [72.9]
397, 434 [65.9]
453, 517 [55.3]
462, 522 [54.8]
483, 585 [48.9]
493, 600 [47.7]
503, 624 [45.8]
495, 637 [44.9]
510, 647 [44.2]
488, 635 [45.0]
460, 605 [47.3]
440, 692 [41.3]
522, 722 [39.6]
481, 759 [37.7]
489, 724 [39.5]
526, 820 [34.9]
ꢀλ = 428 d
385, 455 [62.8]
369, 403 [70.9]
413, 527 [54.3]
428, 553 [51.7]
426, 554 [51.6]
463, 613 [46.6]
472, 600 [47.7]
497, 621 [46.0]
477, 635 [45.0]
512, 645 [44.3]
465, 627 [45.6]
446, 593 [48.2]
481, 675 [42.4]
501, 720 [39.7]
482, 756 [37.8]
495, 751 [38.1]
520, 823 [34.7]
ꢀλ = 420 d
438 [65.3]
478 [59.8]
516 [55.4]
551 [51.9]
564 [50.7]
627 [45.6]
634 [45.1]
662 [43.2]
678 [42.2}
678 [42.2]
689 [41.5]
692 [41.3]
702 [40.7]
706 [40.5]
731 [39.1]
750 [38.1]
764 [37.4]
Δλ = 286 d
EtOH
n-Propanol
CH3CN
DMSO
DMF
Acetone
NMP
560 (sup.) c
560 (sup.) c
533, 592 [48.3]
563, 630 [45.4]
522, 575 [49.7]
485, 560 [51.1]
504, 637 [44.9]
540, 653 [43.8]
514, 701 [40.8]
541, 693 [41.3]
554, 730 [39.2]
ꢀλ = 256 d
Benzonitrile
1,2-Dichloroethane
DCM
Pyridine
CHCl3
EtOAc
THF
a HFIP = 1,1,1,3,3,3-Hexafluoro-2-propanol, TFE = 2,2,2-Trifluoroethanol, DCM = Dichloromethane, b ET(X) [kcal mol-1] = hcNA/λmax = 28591/λmax [nm],
c superimposed, assignment would be tentative, d from THF to TFE in nm.

ACCEPTED MANUSCRIPT
5
max,0 [103 cm-1] a
a
b
c
d
r
δ
Here, the Kamlet-Taft parameters are as follows: α is the HBD
%
ν
acidity, β is the HBA basicity and π* describes the dipolar-
ity/polarizability of the solvent. Derived from the constants a, b
and s, one can selectively determine the influence of either the
HBD or HBA strength or the polarity of the solvent to the energy
needed for excitation. Though there may be some controversies
connected with the simplified Kamlet-Taft equation, it is still one
of the easiest and best approaches to describe solvent effects and
details can be found elsewhere.9,39-41 The results of the multiple
linear regressions can be found in Table 2. HFIP was again ex-
cluded and the parameters used can be found in the literature.9
A
12.70
27.76
13.51
18.23
10.44
8.645
10.94
9.378
10.96 1.249
-3.375 5.992 0.944 1.365
-
-0.628 -24.89 9.597 0.751 2.623
10.58
11.38
11.58
-
-
-
4.219
3.533
-
6.515 0.940 1.350
0.911 1.577
6.396 0.937 1.332
-
B
C
D
7.717 -1.132
2.917
5.903 0.884 1.563
10.90 -0.589 -3.407 8.214 0.980 0.833
8.706 -1.635 -0.846 8.361 0.955 1.023
max,0 corresponds to the absorption maximum [cm-1] in gas phase.
a
%
ν
The neglectable contribution of the HBA basicity expressed
through b is also similar to the LSER using Kamlet-Taft parame-
ters. Regressions without SB yielded only very slightly lower
correlation coefficients r than with this parameter. A very poor r
was obtained for all betaines (illustrated for A) if the HBD acid-
ity of the solvent was excluded from the regression. The main
difference was observed by the differentiation between the po-
larizability and the dipolarity of the solvent. While an increasing
polarizability SP causes a bathochromic shift (negative value for
c), an increasing dipolarity SdP causes a hypsochromic shift
(positive value for d). Nonetheless, the overall shift of the ab-
sorption maximum is hypsochromic in more polar solvents due to
a higher contribution of SdP in all relevant regressions. This is
also consistent with the dipolar nature of the betaines. It is worth
mentioning that the intensity of the longest wavelength absorp-
tion diminished when solvent polarity decreased, similar to be-
taines presented in the literature,13,46 This would be the main
obstacle for an application of A-D as solvent polarity indicators.
Table 2. Regression coefficients a, b and s obtained for different multiple
linear regressions of the Kamlet–Taft equation 1 for 16 solvents (13 for B)
with correlation coefficient (r) and standard deviation (δ).
max,0 [103 cm-1] a
a
b
s
r
δ
%
ν
Comp.
A
8.027
19.67
10.20
14.00
8.862
6.128
5.743
8.065
-
3.450
2.679
-
3.095
3.345
1.780
0.579
7.347
-4.588
6.652
-
7.135
9.712
10.94
0.966
0.305
0.927
0.924
0.925
0.967
0.966
1.021
3.635
1.427
1.461
1.199
1.007
0.850
7.907
6.817
5.286
8.403
7.066
B
C
D
a
max,0 corresponds to the absorption maximum [cm-1] in reference system
%
ν
cyclohexane (α, β and π* = 0 by definition).
First of all, the relevant regressions obtained all show a very
good correlation coefficient of ≥ 0.925. The regression coeffi-
cients are positive, indicating a negative, hypsochromic solvato-
chromism with increasing solvent dipolarity/polarizability and
HBD or HBA strength. This is in accordance with the assumption
that the S0 state is more stabilized as a result of solvation with
polar HBD molecules than the S1 state. In all calculations using
all three coefficients, b has the smallest influence, which is rea-
sonable due to a missing HBD group in the betaines. Conse-
quently, the regression excluding b, as exemplified for A, also
yielded a very good r of 0.927, contrary to a multiple regression
only using b and s as solvent parameters which yielded an unfea-
sible r of 0.305. In summary, the solvatochromic behavior of the
betaines A-D is basically only influenced by the HBD acidity
leading to interactions with the thiazol-4-olate (and minor with
the heteroatom of the heterocycle in the 2-position) and from the
polarity/polarizability of the solvent.
2.4. Quantum Chemical Calculations on A-D
Density functional theory (DFT) and its time-dependent vari-
ant (TDDFT)47 were applied in order to gain a deeper insight into
the electronic and structural properties of the dyes. All calcula-
tions were performed with the GAUSSIAN 09 program.48 The
effects of solvation (CH3CN, ε = 35.688, n = 1.344) were ad-
dressed with a polarized continuum model.31 The ground state
equilibrium structures of A-D were optimized using a functional
based on B3LYP49,50 and denoted as B3LYP(35),51 combining
35% of exact-exchange, 58.5% of non-local B8852 exchange, and
the LYP correlation along with the 6-31G(d,p) double-ζ basis
set.53 Subsequent analyses of the harmonic vibrational frequency
confirmed that the four stationary points of A-D correspond to
minima of the respective potential energy surfaces. Excited state
properties, such as excitation energies, oscillator strengths and
excited state wavefunctions, were computed at the TDDFT level
of theory with the same exchange correlation functional and basis
set as the ground states. This functional allows a reasonable
description of 4-hydroxy-thiazole based dyes as confirmed previ-
ously.23 The absorption spectra of the four dyes were simulated
from the first 30 singlet excited states.
Additionally, the alternative solvent scales SP, SdP, SB and
SA42-45 from Catalán et al. with the mathematical expression
shown in equation 2 were studied. They are considered to be
slightly more precise than the Kamlet-Taft parameters when
describing multiple solute-solvent interactions (especially in the
differentiation between solvent dipolarity and polarizability), but
generally yield very similar results.
%
%
=ν max,0 + a × SA + b × SB + c× SP + d × SdP
(2)
ν max
The ground state equilibrium geometries of A-D show similar
structural features. The phenyl moieties at the pyridinium ring are
twisted out of planarity, while the torsion of the phenyl rings in
the ortho-position is significantly enhanced (57 to 62°) contrary
to the phenyl ring in the para-position (33°). Additionally, the N-
phenyl ring is twisted by approximately 70° due to steric reasons.
The remaining 1,3-thiazol-4-olate fragment is completely planar,
contrary to the X-ray structure of 4a presented in Figure 2 or 4-
methoxy-1,3-thiazole-based dyes previously studied.20 Naturally,
the reason is the zwitterionic structure of the dyes confirmed by
the bond length alternation (BLA) indices,54,55 defined as the
difference between the average C-C and C=C bond lengths of -
0.0239 Å, -0.0211 Å, -0.0128 Å, and -0.0118 Å for the dyes A to
D. Hence, single bonds are shortened, while double bonds are
stretched. This effect is illustrated for the C-O bonds with bond
In this multiple regression, SA corresponds to the acidity, SB
to the basicity, SP to the polarizability and SdP to the dipolarity
of the solvent and the values used can be found in the original
publication from Catalán et. al.45 The results of the multiple
linear regressions are summarized in Table 3. The coefficients
obtained are in accordance with the ones from the Kamlet–Taft
equitation. The correlation coefficients for all important calcu-
lated regressions are > 0.884. The positive coefficients a and d
(expressing SA and SdP) show the highest contributions, indicat-
ing a negative solvatochromism with higher HBD acidity and
dipolarity of the solvent.
Table 3. Regression coefficients a, b, c and d obtained for different multiple
linear regressions of the Catalán equation 2 for 16 solvents (13 for B) with
correlation coefficient (r) and standard deviation (δ).

ACCEPTED MANUSCRIPT
6
Tetrahedron
lengths of approximately 1.25 Å consistently with the solid
2.84 eV, while the oscillator strength decreases with the mixing
ratio of these transitions from 0.553 to 0.432. In the case of D, a
further medium strength (0.1971) ICT state (S3) of inverted mix-
ing ratio was found at 2.89 eV. In fact, this state is correlated to
the shoulder at the higher energetic side of the band at 460 nm. In
summary, the results of the quantum chemical calculations corre-
late very well with experimental data and allowed an unambigu-
ous assignment of the electronic states to the absorption bands.
The excitation energies obtained of the S1 are underestimated by
-0.21 to -0.37 eV, while the energies of the S2 are overestimated
by 0.13 to 0.21 eV. For reasons of comparison, the excited states
of A have also been studied using the long-range corrected
CAM-B3LYP functional; however, the deviations from the ex-
periment were significantly increased and results are not shown
here.
state structure of C. The dipolar ionic structure was also con-
firmed by natural bond orbital (NBO) analysis and the corre-
sponding electrostatic potentials of the dyes (for representations,
see ESI S4-S7).
A Lorentzian function with a full width of half maximum of
3000 cm-1 was employed to broaden the transitions based on the
calculated oscillator strengths of the first 30 singlet excited states.
The absorption spectra of the dyes feature an intense band in the
visible region at approximately 470-540 nm together with a less
intense band at 550-620 nm, while the UV region shows a very
intense band at approximately 300 nm. Furthermore, B exhibit a
minor band located at 387 nm. The composition of the transitions
in the visible range are revealed in Table 4 and the molecular
orbitals (MOs) are shown in Figure 5, while a summary of the
predominant excited states in the UV region is provided in ESI
S8 and respective MOs are shown in ESI S9. The absolute ener-
gies of the MOs depicted in Figure 5 are additionally given in
ESI 10. The lowest energy band is determined by the S1 states
and was found to be an ICT state from the HOMO located at the
thiazol-4-olate-phenyl fragment to the LUMO located at the
pyridinium moiety (see Figure 5). Oscillator strengths (f) from
0.145 to 0.162 point to medium strength absorbing states. The
excitation energy for C and D was calculated to 1.81 and 1.82 eV
and increases to 1.85 and further to 1.90 eV for A and B, respec-
tively. On the other hand, the second more intense absorption
bands in the Vis region of A and B result from less pronounced
ICT states (S2) from the HOMOs to the LUMO+1 orbitals. The
latter are mainly centered at the thiazole and the pyri-
dine/pyrazine rings. Substitution of the pyridine moiety (A) with
pyrazinyl (B) decreases the excitation energy from 2.68 to
2.49 eV, while f is also decreased from 0.520 to 0.414. The ex-
cited state wavefunctions of the S2 states of C and D (see Table
4) feature a pronounced mixing of a transition from the HOMO
Table 4. Calculated excitations of A-D of the singlet excited states in CH3CN
in the visible region. Main contributions to the wavefunction (weight), verti-
cal excitation energies (ꢀEe) oscillator strengths (f) and deviations from
experimental peak values (ꢀꢀEExp).
∆Ee
∆∆Eexp
[eV]
Weight
[%]
State
Transition
f
[eV]
[nm]
A
99
S1
S2
HOMO→LUMO
1.85
2.68
672
463
0.149
0.520
-0.37
HOMO→LUMO+1
96
+0.17
B
98
S1
S2
HOMO→LUMO
1.90
2.49
651
497
0.162
0.414
-
HOMO→LUMO+1
99
+0.19
C
98
S1
S2
HOMO→LUMO
1.82
2.70
679
459
0.156
0.553
-0.30
HOMO→LUMO+1
HOMO→LUMO+2
81
18
+0.13
D
99
S1
S2
HOMO→LUMO
1.81
2.84
685
437
0.145
0.432
-0.21
HOMO→LUMO+2
HOMO→LUMO+1
60
38
+0.16
to
the
1,3-thiazole-thienyl/furanyl
fragment
(LUMO+1/LUMO+2) and a transition from the HOMO to the
pyridinium ring (LUMO+2/LUMO+1). This mixing is accompa-
nied by an increase of the S2 excitation energy to 2.70 and
S3
HOMO→LUMO+1
HOMO→LUMO+2
60
39
2.89
429
0.197
+0.21
LUMO+2
LUMO+1
LUMO
HOMO
A
B
C
D
Figure 5. Molecular orbitals involved in the main singlet excited states of the dyes A-D at B3LYP(35)/6-31G(d,p) level of theory in CH3CN contributing to
the absorption in the visual region.

ACCEPTED MANUSCRIPT
7
3. Conclusion
4.2. General procedure for the preparation of the 4-
hydroxy-1,3-thiazoles
This paper presents the synthesis and thorough characteriza-
tion concerning the optical properties of unusual Reichardt’s-type
betaines. The commonly used phenolic donor group was substi-
tuted by a thiazol-4-olate which acts as donor as well as chromo-
phoric unit. The possible different transitions were unambigu-
ously assigned and described using spectroscopic, multiparamet-
ric correlation (LSER) and quantum chemical methods. The
longest wavelength absorption band is of intense ICT character
and prone mainly to changes in HBD acidity and polarity of the
solvents. Compound D proved to be best qualified to meassure
changes in solvent properties while compound A and B are less
suited due to a more pronounced superimposition of the S0→S1
and S0→S2 transition. For further research the focus will be
placed on the synthesis of dyes with a (hetero)aryl in the 4-
position of the thiazole bearing stronger electron donor groups to
influence the ICT (to obtain better resolved absorption bands)
and/or different moieties (alkyl chains, fluorinated chains,
sulfonates) to obtain a distinct solubility. Also the connections
with other heterocycles (e.g. pyrrole) increasing possible interac-
tions with solvent molecules and influencing the optical proper-
ties will be considered. Finally, interactions with molecules mim-
icking nucleic bases pairs to obtain sensor molecules for molecu-
lar recognition, as shown from Bolz et al.,13 will be the subject of
subsequent studies.
A suspension of the thioamide (10 mmol) and ethyl 2-bromo-
2-(4-nitrophenyl)acetate (13 mmol, 1.3 equiv.) in toluene
(20 mL) under a nitrogen atmosphere were heated under reflux
conditions for 4 h. An orange solid precipitated. Pyridine was
added to the hot solution and the mixture was further stirred for
additional 4 h while cooling down to r.t. The solid was filtered
off and washed with EtOH and pentane. The pure compounds
were obtained as crystalline solids after recrystallization from
EtOH/DMF.
4.2.1. 2-(Furan-2-yl)-5-(4-nitrophenyl)thiazol-4-ol (1d). Brown
crystals (1.70 g, 59%). MP >330 °C decompose. 1H NMR
(250 MHz, DMSO-d6): δ = 12.59 (s, 1H), 8.23 (d, J 9.0 Hz, 2H),
7.98-7.86 (m, 3H), 7.12 (d, J 3.5 Hz, 1H), 6.75 (dd, J 3.5, 1.8 Hz,
1H). 13C NMR (63 MHz, DMSO-d6): δ = 161.14, 152.60, 147.74,
145.68, 144.24, 138.88, 125.75, 124.28, 113.08, 110.45, 104.69.
MS (EI): m/z (%) 288 (100) [M+]. Anal. Calcd for C13H8N2O4S:
C, 54.16; H, 2.80; N, 9.72; S, 11.12. Found: C, 53.19; H, 2.80; N,
9.73; S, 11.24.
4.3. General procedure for the preparation of the silylethers
To a suspension of the corresponding 4-hydroxy-1,3-thiazole
(10 mmol) and imidazole (20 mmol) in CH2Cl2 (100 mL, dry)
was added tert-butyldimethylsilyl chloride (12 mmol). The solu-
tion was stirred under a nitrogen atmosphere for 24 h, washed
with H2O (3×50 mL), dried over MgSO4 and concentrated till
dryness in vacuo. The silyl ethers were purified using column
chromatography (silica, CHCl3) or by recrystallization from
EtOH/CH2Cl2 by slowly evaporating of the latter, yielding the
compounds as yellow to orange-brown crystalline solids.
4. Experimental section
4.1. General
5-(4-Nitrophenyl)-2-(pyridin-2-yl)thiazol-4-ol (1a)
5-(4-
nitrophenyl)-2-(pyrazin-2-yl)thiazol-4-ol (1b), 5-(4-nitrophenyl)-
2-(thiophen-2-yl)thiazol-4-ol (1c) and ethyl 2-bromo-2-(4-
nitrophenyl)acetate were synthesized according to literature
procedures.23,20,56 Furan-2-carbothioamide is commercially avail-
able or can be prepared from the corresponding nitrile and H2S in
EtOH in the presence of Et3N.57 All other chemicals used were
reagent grade and purchased from Sigma-Aldrich or Alfa Aesar.
Solvents were purified according to standard procedures. Sol-
vents for UV/Vis were of analytical grade and purchased from
4.3.1. 4-((tert-Butyldimethylsilyl)oxy)-5-(4-nitrophenyl)-2-
(pyridin-2-yl)thiazole (2a). Yellow fluffy crystalline solid
(3.56 g, 85%). MP 224.0-224.4 °C. 1H NMR (300 MHz, CDCl3):
δ = 8.60 (d, J 4.7 Hz, 1H), 8.21 (d, J 9.0 Hz, 2H), 8.05 (d, J
7.9 Hz, 1H), 7.93 (d, J 9.0 Hz, 2H), 7.80 (td, J 7.8, 1.6 Hz, 1H),
7.33 (ddd, J 7.4, 4.6, 0.9 Hz, 1H), 1.06 (s, 9H), 0.44 (s, 6H). 13C
NMR (75 MHz, CDCl3): δ 162.90, 158.28, 150.62, 149.54,
145.43, 139.00, 137.03, 126.56, 124.79, 124.02, 119.23, 113.12,
25.90, 18.15, -3.98. MS (EI): m/z (%) 413 (15) [M+], 356 (100).
Anal. Calcd for C20H23N3O3SSi: C, 58.08; H, 5.61; N, 10.16; S,
7.75. Found: C, 58.17; H, 5.78; N, 10.21; S 7.33. UV/Vis
(CH2Cl2): λmax (log ε): 223 (4.22), 275 (4.07), 394 (4.39).
1
Sigma-Aldrich. H-, 13C-NMR and the corresponding correlation
spectra were recorded on a Bruker AC-250 (250 MHz) and AC-
400 (400 MHz) spectrometer. Chemical shifts (δ) are given rela-
tive to solvents. UV/Vis data of the compounds were collected on
a Lambda 19 from PERKIN-ELMER. Elemental analysis was
carried out on a Leco CHNS-932. Mass spectra were measured
either on a Finnigan MAT SSQ 710 (EI) or MAZ 95 XL (FAB)
system. MALDI TOF MS was performed on a Bruker Ultraflex
TOF/TOF mass spectrometer equipped with a 337 nm nitrogen
laser operated in the reflectron mode using an acceleration volt-
age of 25 kV, and dithranol (Bruker, #209783) was used as ma-
trix. TLC was from Merck (Polygram SIL G/UV254, aluminum
oxide 60 F254). The material for Column chromatography was
also obtained from Merck (Silica gel 60).
4.3.2. 4-((tert-Butyldimethylsilyl)oxy)-5-(4-nitrophenyl)-2-
(pyrazin-2-yl)thiazole (2b). Yellow orange solid (4.00 g, 96%).
MP 190.0 °C. 1H NMR (250 MHz, CDCl3): δ = 9.31 (d, J
1.5 Hz), 8.61 (d, J 2.5 Hz), 8.56 (dd, J 2.5, 1.5 Hz), 8.31-8.18
(m), 8.03-7.90 (m), 1.07 (s), 0.45 (s).13C NMR (63 MHz, CDCl3):
δ = 159.87, 158.60, 146.16, 145.81, 145.32, 143.98, 141.12,
138.46, 126.85, 124.10, 114.35, 25.89, 18.19, -3.98. MS (EI): m/z
(%) 414 (15) [M+], 357 (100). Anal. Calcd for C19H22N4O3SSi: C,
55.05; H, 5.35; N, 13.51; S, 7.73. Found: C, 54.96; H, 5.05; N,
13.30; S, 7.46.
X-Ray: The intensity data for the compounds were collected
on
a Nonius KappaCCD diffractometer using graphite-
monochromated Mo-Kα radiation. Data were corrected for Lor-
entz and polarization effects but not for absorption effects.58,59
The structure was solved by direct methods (SHELXS) and re-
fined by full-matrix least squares techniques against Fo2
(SHELXL-97).60 All hydrogen atoms were included at calculated
positions with fixed thermal parameters. All non-hydrogen atoms
were refined anisotropically.60 XP (SIEMENS Analytical X-ray
Instruments, Inc.) was used for structure representations.
4.3.3. 4-((tert-Butyldimethylsilyl)oxy)-5-(4-nitrophenyl)-2-
(thiophen-2-yl)thiazole (2c). Orange crystalline solid (3.01 g,
72%). MP 185.0-185.3 °C. 1H NMR (250 MHz, CDCl3): δ =
8.26-8.16 (m, 2H), 7.90-7.82 (m, 2H), 7.52 (dd, J = 3.7 Hz,
1.0 Hz, 1H), 7.42 (dd, J = 5.0 Hz, 1.0 Hz, 1H), 7.10 (dd, J =
5.0 Hz, 3.8 Hz, 1H), 1.06 (s, 9H), 0.42 (s, 6H). 13C NMR
(63 MHz, CDCl3): δ = 157.96, 156.53, 145.31, 139.09, 137.56,
128.72, 128.31, 126.56, 126.31, 124.25, 109.40, 26.11, 18.34,

ACCEPTED MANUSCRIPT
8
Tetrahedron
-3.86. MS (EI): m/z (%) 418 (20) [M+], 361 (100). Anal. Calcd
4.4.4. 4-(4-((tert-Butyldimethylsilyl)oxy)-2-(furan-2-yl)thiazol-5-
1
for C19H22N2O3S2Si: C, 54.52; H, 5.30; N, 6.69; S, 15.32. Found:
C, 54.32; H, 5.33; N, 7.01; S, 15.58.
yl)aniline (3d). Yellow solid (1.40 g, 75%). MP 93.0-93.3 °C. H
NMR (400 MHz, CDCl3): δ = 7.58-7.52 (m, 2H), 7.45 (dd, J 1.7,
0.6 Hz, 1H), 6.86 (dd, J 3.4, 0.6 Hz, 1H), 6.73-6.66 (m, 2H), 6.50
(dd, J 3.4, 1.8 Hz, 1H), 3.72 (s, 2H), 1.02 (s, J 2.8 Hz, 9H), 0.34
(s, J 3.2 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ = 154.88,
149.81, 148.16, 145.18, 143.01, 128.14, 122.42, 115.26, 112.81,
112.26, 107.62, 26.13, 18.32, -3.91. MS (EI): m/z (%) 372 (70)
[M+], 315 (100). Anal. Calcd for C19H24N2O2SSi: C, 61.25; H,
6.49; N, 7.52; S, 8.61. Found: C, 61.45; H, 6.59; N, 7.36; S, 8.85.
4.3.4. 4-((tert-Butyldimethylsilyl)oxy)-2-(furan-2-yl)-5-(4-
nitrophenyl)thiazole (2d). Brown thick needles (2.50 g, 62%).
MP 166.0-166.3 °C. H NMR (250 MHz, CDCl3): δ = 8.26-8.15
(m, 2H), 7.95-7.81 (m, 2H), 7.52 (d, J 1.5 Hz, 1H), 6.98 (d, J
3.5 Hz, 1H), 6.56 (dd, J 3.4, 1.7 Hz, 1H), 1.05 (s, 9H), 0.40 (s,
6H). 13C NMR (63 MHz, CDCl3): δ = 158.47, 152.51, 149.11,
145.36, 144.22, 139.14, 126.37, 124.24, 112.67, 109.75, 109.69,
26.09, 18.32, -3.87. MS (EI): m/z (%) 402 (50) [M+], 345 (100).
Anal. Calcd for C19H22N2O4SSi: C, 56.69; H, 5.51; N, 6.96; S,
7.97. Found: C, 56.71; H, 5.49; N, 6.87; S, 7.66.
1
4.5. General procedure for the preparation of the pyridinium
tetrafluoroborate salts
To a suspension of the corresponding amine (2.0 mmol) and
2,4,6-triphenylpyrylium tetrafluoroborate (2.0 mmol) in EtOH
(20 mL) in a round bottle flask was added Et3N (2.0 mmol). The
mixture turned deep brown while the educts dissolved and was
stirred for 30 min at r.t. followed by the addition of AcOH
(4.0 mmol) and heating under reflux conditions for additional
2 h. The product precipitated during the reaction. The product
was dissolved directly in the flask with little acetone at the reflux
temperature after the reaction was finished (no further precipitate
occurred). After cooling down to r.t. the product crystallized as a
yellow solid which was filtered off, washed with cold EtOH and
pentane and dried in vacuo.
4.4. General procedure for the reduction of the nitro group to
the corresponding amines
The corresponding nitro compound (5.0 mmol) was dissolved
in THF (50 mL) in an autoclave (Büchi AG, model: miniclave
steel) and a spate Pd/C was added. Hydrogen was applied (max.
10 bar) under continuous stirring at r.t, or if the reaction proceeds
slow, at slightly elevated temperature (max. 50 °C). After the
reaction was finished (3-10 h) as indicated by TLC (silica,
CHCl3; the educts elute at the front, while the amines do not
elute) the catalyst was filtered off and the remaining solution was
concentrated in vacuo. A short gel filtration (silica, CHCl3 to
CHCl3/EtOAc 2:1) yielded the amines as yellow solids or oils
which solidify after a while.
4.5.1. 1-(4-(4-Hydroxy-2-(pyridin-2-yl)thiazol-5-yl)phenyl)-2,4,6-
triphenylpyridin-1-ium tetrafluoroborate (4a). Orange crystalline
solid (0.93 g, 72%). MP >255 °C decompose. 1H NMR
(400 MHz, DMSO-d6): δ = 8.65 (s, 2H), 8.60 (d, J = 4.7 Hz, 1H),
8.34 (d, J = 6.9 Hz, 2H), 8.00-7.91 (m, 2H), 7.74-7.64 (m, 3H),
7.57 (d, J = 8.7 Hz, 2H), 7.53-7.35 (m, 13H), 4.34 (s, 1H). 13C
NMR (100 MHz, DMSO-d6): 161.89, 159.99, 156.44, 155.51,
149.75, 137.80, 136.49, 133.40, 133.07, 132.54, 130.03, 129.72,
129.08, 128.79, 128.19, 125.27, 125.19, 125.14, 118.64, 108.23.
4.4.1. 4-(4-((tert-Butyldimethylsilyl)oxy)-2-(pyridin-2-yl)thiazol-
5-yl)aniline (3a). Yellow solid (1.59 g, 83%). MP 132.9-
1
133.2 °C. H NMR (250 MHz, CDCl3): δ = 8.56 (d, J 4.8 Hz,
1H), 8.02 (d, J 7.9 Hz, 1H), 7.74 (td, J 7.8, 1.7 Hz, 1H), 7.61 (d,
J 8.6 Hz, 2H), 7.22 (m, 1H), 6.70 (d, J 8.6 Hz, 2H), 3.74 (s, 2H),
1.04 (s, 9H), 0.37 (s, 6H). 13C NMR (63 MHz, CDCl3): δ =
157.97, 155.23, 151.74, 149.46, 145.46, 136.93, 128.35, 123.80,
122.56, 118.91, 116.71, 115.24, 77.16, 26.14, 18.35, -3.84. MS
(EI): m/z (%) 383 (100) [M+], 326 (90). Anal. Calcd for
C19H22N2O4SSi: C, 62.62; H, 6.57; N, 10.95; S, 8.36. Found: C,
62.69; H, 6.89; N, 10.94; S 8.11.
−
MS (EI): m/z (%) 559 (<1) [M-BF4 ]+, 426 (<1), 307 (100). Anal.
Calcd for C37H26BF4N3OS: C, 67.54; H, 4.65; N, 6.06; S, 4.62.
Found: C, 67.50; H, 4.72; N, 6.12; S, 4.56.
4.5.2. 1-(4-(4-Hydroxy-2-(pyrazin-2-yl)thiazol-5-yl)phenyl)-
2,4,6-triphenylpyridin-1-ium tetrafluoroborate (4b). Yellow
crystalline solid (0.95 g, 73%). MP >250 °C decompose. 1H
1
4.4.2. 4-(4-((tert-Butyldimethylsilyl)oxy)-2-(pyrazin-2-yl)thiazol-
NMR (400 MHz, DMSO-d6): H NMR (400 MHz, DMSO-d6): δ
5-yl)aniline (3b). Red orange solid (1.52 g, 79%). MP 155.0-
= 12.27 (s, 1H), 9.17 (d, J = 15.4 Hz, 1H), 8.81-8.60 (m, 4H),
8.37 (d, J = 7.1 Hz, 2H), 7.75-7.64 (m, 3H), 7.61 (d, J = 8.6 Hz,
2H), 7.56-7.45 (m, 6H), 7.45-7.32 (m, 6H). 13C NMR (100 MHz,
DMSO-d6): 160.26, 159.05, 156.42, 155.52, 145.87, 145.26,
144.63, 139.87, 136.90, 133.42, 133.10, 132.91, 132.57, 130.05,
129.75, 129.20, 128.85, 128.23, 125.39, 125.19, 109.55. MS (EI):
1
155.2 °C. H NMR (250 MHz, CDCl3): δ = 9.27 (d, J 1.3 Hz,
1H), 8.50-8.45 (m, 2H), 7.64-7.57 (m, 2H), 6.73-6.66 (m, 2H),
3.77 (s, 2H), 1.04 (s, 9H), 0.38 (s, 6H). 13C NMR (63 MHz,
CDCl3): δ = 155.66, 154.73, 147.11, 145.85, 144.09, 143.81,
140.99, 128.40, 121.88, 118.23, 115.13, 26.05, 18.28, -3.92. MS
(EI): m/z (%) 384 (90) [M+], 327 (100). Anal. Calcd for
C19H24N4OSSi: C, 59.34; H, 6.29; N, 14.57; S, 8.34. Found: C,
59.31; H, 6.29; N, 14.53; S, 8.17.
−
m/z (%) 561.1 (1) [M-BF4 ]+, 427 (1), 307 (100).
C36H25BF4N4OS: C, 66.68; H, 3.89; N, 8.64; S, 4.94. Found: C,
66.96; H, 3.90; N, 8.77; S, 4.55.
4.4.3. 4-(4-((tert-Butyldimethylsilyl)oxy)-2-(thiophen-2-
4.5.3. 1-(4-(4-Hydroxy-2-(thiophen-2-yl)thiazol-5-yl)phenyl)-
2,4,6-triphenylpyridin-1-ium tetrafluoroborate (4c). Yellow-
orange solid (0.60 g, 46%). MP 262.0-263.5 °C. 1H NMR
(400 MHz, DMSO-d6): δ = 12.06 (s, 1H), 8.66 (s, 2H), 8.36 (d, J
= 8.0 Hz, 2H), 7.74 (dd, J = 5.1 Hz, 1.0 Hz, 1H), 7.72-7.63 (m,
3H), 7.60 (dd, J = 3.7 Hz, 1.0 Hz, 1H), 7.55-7.30 (m, 14H), 7.17
(dd, J = 5.0 Hz, 3.8 Hz, 1H). 13C NMR (100 MHz, DMSO-d6):
159.06, 156.44, 155.48, 155.34, 136.27, 136.26, 133.42, 133.12,
133.08, 132.56, 130.03, 129.75, 129.50, 129.11, 128.84, 128.77,
128.20, 127.14, 125.18, 124.83, 104.64. MS (EI): m/z (%) 565
yl)thiazol-5-yl)aniline (3c). Orange solid (1.75 g, 88%). MP
1
175.0-175.5 °C. H NMR (250 MHz, CDCl3): δ = 7.58-7.49 (m,
2H), 7.40 (dd, J = 3.7 Hz, 1.0 Hz, 1H), 7.31 (dd, J = 5.0 Hz,
0.9 Hz, 1H), 7.04 (dd, J = 5.0 Hz, 3.7 Hz, 1H), 6.73-6.65 (m,
2H), 1.03 (s, 9H), 0.36 (s, 6H). 13C NMR (63 MHz, CDCl3): δ =
154.29, 51.87, 145.16, 138.58, 128.06, 127.92, 126.89, 124.83,
122.43, 115.26, 112.47, 26.17, 18.35, -3.90. MS (EI): m/z (%):
388 (100) [M+] 331 (95). Anal. Calcd for C19H24N2OS2Si: C,
58.72; H, 6.22; N, 7.21; S, 16.50. Found: C, 58.72; H, 5.95; N,
6.95; S 16.83.
−
(1) [M-BF4 ]+, 427 (1), 307 (100). Anal. Calcd for
C36H25BF4N2OS2: C, 66.26; H, 3.86; N, 4.29; S, 9.83. Found: C,
66.30; H, 3.70; N, 4.21; S, 9.76.

ACCEPTED MANUSCRIPT
9
4.6.4. 2-(Furan-2-yl)-5-(4-(2,4,6-triphenylpyridin-1-ium-1-
yl)phenyl)thiazol-4-olate (D). Dark green solid (512 mg, 93%).
MP >250 °C decomp. H NMR (100 MHz, DMSO-d6) δ = 8.58
1
4.5.4. 1-(4-(2-(Furan-2-yl)-4-hydroxythiazol-5-yl)phenyl)-2,4,6-
triphenylpyridin-1-ium tetrafluoroborate (4d). Orange solid
(0.88 mg, 63%). MP >230 °C decomp. 1H NMR (400 MHz,
DMSO-d6): δ = 12.08 (s, 1H), 8.66 (s, 2H), 8.36 (d, J 7.4 Hz,
2H), 7.88 (s, 1H), 7.74-7.63 (m, 3H), 7.53-7.36 (m, 14H), 7.02
(d, J 2.9 Hz, 1H), 6.71 (s, 1H). 13C NMR (100 MHz, DMSO-d6):
δ = 159.80, 156.44, 155.45, 151.01, 147.82, 145.22, 136.24,
133.41, 133.12, 132.53, 130.00, 129.72, 129.08, 128.82, 128.19,
125.17, 124.76, 112.94, 109.73, 104.57. MS (Micro-ESI): 549
(s, 2H), 8.35-8.29 (m, 2H), 7.72-7.62 (m, 4H), 7.56-7.30 (m,
12H), 7.01 (d, J = 9.0 Hz, 2H), 6.76 (d, J = 3.1 Hz, 1H), 6.58 (dd,
J = 3.3 Hz, 1.8 Hz, 1H). 13C NMR (100 MHz, DMSO-d6): δ =
156.72, 154.62, 150.00, 148.48, 143.28, 139.42, 133.55, 133.53,
132.26, 130.56, 129.72, 129.64, 128.70, 128.09, 127.66, 125.16,
120.58, 112.28, 107.18, 92.38. MS (MALDI-TOF, dithranol):
m/z 549.32 for C36H25N2O2S ([M + H]+). Requires: 549.16. Anal.
Calcd for C36H24N2O2S: C, 78.81; H, 4.41; N, 5.11; S, 5.84.
Found: C, 78.55; H, 4.11; N, 5.44; S, 5.73.
−
[M-BF4 ]+. Anal. Calcd for C36H25BF4N2O2S: C, 67.94; H, 3.96;
N, 4.40; S, 5.04. Found: C, 67.85; H, 3.91; N, 4.44; S, 4.77.
Acknowledgments
4.6. General procedure for the preparation of the betaines
The authors thank the Thuringian Ministry for Education, Sci-
ence and Culture (grant #B514-09049, Photonische Mizellen
[PhotoMIC]) for financial support.
To a suspension of the corresponding tetrafluoroborate salt
(1.0 mmol) in MeOH (15 mL) was added tetrabutylammonium
hydroxide (1.1 mmol). The mixture was stirred for 1 h at 40 °C.
The deep purple to black precipitate (Et2O was added if the com-
pound did not precipitate sufficiently) was filtered off after cool-
ing down to r.t. and was washed with little EtOH and Et2O and
dried in vacuo.
Supplementary Material
Refinement data and X-ray structure file for 4a and C, Picture
of the betaines as solids and in solution (CH3CN), linear correla-
tion between the ET(X) and ET(30) values, Graphical representa-
tions of the NBO analysis, summary of the excited states in the
UV region, 1H and 13C NMR spectra of the tetrafluoroborate salts
and the betaines. Crystallographic data (excluding structure fac-
tors) has also been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication (CCDC-
902653 for 4a and CCDC-911688 for C). Copies of the data can
be obtained free of charge on application to CCDC, 12 Union
4.6.1. 2-(Pyridin-2-yl)-5-(4-(2,4,6-triphenylpyridin-1-ium-1-
yl)phenyl)thiazol-4-olate (A). Dark purple solid (543 mg, 97%).
1
MP >245 °C decompose. H NMR (400 MHz, DMSO-d6): δ =
8.56 (s, 2H), 8.48 (d, J = 4.5 Hz, 1H), 8.31 (d, J = 6.8 Hz, 2H),
7.88 (d, J = 7.9 Hz, 1H), 7.80 (td, J = 7.8 Hz, 1.4 Hz, 1H),
7.73-7.58 (m, 5H), 7.53-7.44 (m, 4H), 7.42-7.34 (m, 6H), 7.30
(dd, J = 6.6 Hz, 5.5 Hz, 1H), 7.06 (d, J = 8.9 Hz, 2H). 13C NMR
(100 MHz, DMSO-d6): 173.44, 158.63, 156.72, 154.76, 151.61,
149.17, 139.43, 136.96, 133.52, 133.49, 132.29, 131.25, 129.78,
129.65, 128.69, 128.11, 127.73, 125.18, 123.61, 121.30, 118.63,
109.46, 97.44. MS (MALDI-TOF, dithranol): m/z 560.32 for
C37H26N3OS ([M + H]+). Requires: 560.18. Anal. Calcd for
C37H25N3OS: C, 79.40; H, 4.50; N, 7.51; S, 5.73. Found: C,
79.23; H, 4.53; N, 7.39; S, 5.67.
Road,
Cambridge
CB2
1EZ,
UK
or
at
www.ccdc.cam.ac.uk/data_request/cif.
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