Tuning the polarity and surface activity of hydroxythiazoles - Extending the applicability of highly fluorescent self-assembling chromophores to supra-molecular photonic structures

Article abstract of DOI:10.1039/c5tc03632a

A small library of N,N-diethyl- and -diethanolsulfonamides of differently substituted 2-(2-pyridyl)- and 2-pyrazinyl-4-alkoxythiazoles has been synthesized and investigated in terms of their photophysical properties, electronic structure (quantum chemical

Full text of DOI:10.1039/c5tc03632a

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Tuning the Polarity and Surface Activity of Hydroxythiazoles –
Extending Applicability of Highly Fluorescent Self-Assembling
Chromophores to Supra-Molecular Photonic Structures†
Received 00th January 20xx,
Accepted 00th January 20xx
S. H. Habenicht,a S. Schramm,a S. Fischer,b T. Sachse,b F. Herrmann-Westendorf,b A. Bellmann,ac B.
Dietzek,bd M. Presselt,*bd D. Weiß,a R. Beckert,*a and H. Görlse
DOI: 10.1039/x0xx00000x
www.rsc.org/
A small library of N,N-diethyl- and -diethanolsulfonamides of differently substituted 2-(2-pyridyl)- and 2-pyrazinyl-4-
alkoxythiazoles has been synthesized and investigated in terms of their photophysical properties, electronic structure
(quantum chemical calculations at the CAM-B3LYP/6-31+G(d,p) level of theory) and thin film morphology (Langmuir-
Blodgett- and spin-cast films). By using the Langmuir-Blodgett technique we show how to exert direct control over the
degree of aggregation in thin films made from different thiazole type dyes. In combination with spectroscopic
investigations, we gained an in-depth understanding of the influence of aggregation on the electro optical properties of
thin
films
made
of
the
here
investigated
substances.
be fast and specific systems for fluoride ion detection.8 Recent
research indicates that thiazole-based fluorophores, due to
their matching excitation and emission characteristics, are
potential sensitizers for the highly efficient chemiluminescence
of the 2-coumaranones.9-11 4-Hydroxythiazole derivatives have
furthermore been employed as light-harvesting ligands in RuII
polypyridyl complexes.12 Thanks to their easy functionalization
and tunable optical properties they have been successfully
incorporated as blue-emitting species in polymer backbones,13,
Introduction
The 4-hydroxy-1,3-thiazole chromophore and fluorophore is
structurally very similar to the naturally occurring firefly
luciferin, thus possessing remarkable spectroscopic
characteristics.1, The classical heterocyclic hydroxythiazole
2
core was described by Chabrier et al. in 1949,3 although the
thiazolin-4-one structure, the keto-tautomer of the
hydroxythiazole, had been proposed at that time and was later
proven to be the enol form.4 In the following years, 4-hydroxy-
1,3-thiazoles have been tested for only few applications in
biochemistry, e.g. as inhibitors for several enzymes such as
cyclooxygenase, 5-lipoxygenase and cyclin-dependent kinase
5.4-6 During this period, their ability to fluoresce upon
photoexcitation was never mentioned. Since the revival of
intense scientific investigation on this substance class in 2007,1
several applications of its derivatives have been developed.
Very recently a new derivative has been utilized as a reporter
molecule for fluorescence and mass spectrometric detection
of a model protein.7 Furthermore, they have been reported to
14
as a Förster resonance energy transfer (FRET) energy donor
in a terpolymer alongside a RuII complex as the acceptor unit,15
and as chromophores in donor-π-acceptor (D-π-A) dyes in dye-
sensitized solar cells (DSSCs).16 It was shown that in DSSCs
aggregation of thiazoles reduces the DSSCs performance.16
Recent approaches to avoid dye aggregation by introduction of
atomic spacers and side chains yielded dramatically increased
DSSC efficiencies.17-21 Beyond this already successful approach
of aggregation inhibition, different methods of directed self
assembly and aggregation, such as the formation of hydrogen
bonds and/or π-π-stacking interactions have shown to be
promising approaches for improving the efficiency of
optoelectronic and photovoltaic devices.22-27
We aim for controlling the formation of supramolecular
architectures to be used in photon conversion.28-33 For this
purpose, we utilize self-assembly of amphiphilic molecules,
especially at interfaces.34-41 In the present study we developed
amphiphilic hydroxythiazole derivatives by attaching both
hydrophilic groups and non-polar chains to the fluorophore: a
series of N,N-dialkylsulfonamide substituted 4-alkoxy-1,3-
thiazoles has been synthesized. These compounds can be
tuned in their hydrophilicity, which generally opens up
opportunities for novel applications, e.g. in biomedicine42-44 or
pharmacy.45-47
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The new compounds bear either a 2-pyridyl- or a pyrazinyl-
substituent in 2-position, different alkoxy groups in 4-position
and a p-sulfonamide substituted phenyl ring in 5-position. Both
N,N-diethyl- and N,N-diethanolsulfonamide derivatives have
been prepared from each building block. 2-Pyridyl-substituents
in 2-position facilitate high quantum yields,1 pyrazinyl rings
were introduced to manipulate electronic features of the
compounds.48 In this paper we present both the synthesis as
well as the characterization of the novel thiazoles with a
particular focus on the spectroscopic properties and possible
surface activity. Photophysical properties of the new
sulfonamide containing fluorophores are supplemented by
quantum chemical studies.
N
N
S
N
S
N
(i)
3a-c
(iv)
R1
O
HO
1
2a-c
a: R1 = CH2(CO)OCH3
b: R1 = (CH2)3Cl
c: R1 = CH3
O
O
S
Cl
N
S
N
(ii)
R1
O
O
R2
O
S
N
R2
N
S
4a-c: R2 = CH2CH3
5a-c: R2 = (CH2)2OH
(iii)
Results and Discussion
R1
N
O
Synthesis
Preparation of the starting material
1
followed
a
straightforward synthesis based on Hantzsch cyclization
N
N
S
N
S
between 2-bromo-2-phenylethylacetate and pyridine-2-
R1
carbothioamide. Synthetic procedures can be found in the
12, 15
The O-alkylated derivatives 2a-c were
N
N
N
O
HO
literature.8,
8
9a-c
a: R1 = CH2(CO)OCH3
b: R1 = (CH2)3Cl
c: R1 = CH3
obtained in very good yields by simple Williamson type
etherification. They were then treated with neat chlorosulfonic
acid in order to obtain the sulfochlorides 3a-c (84-91% yield). If
the reaction was carried out in chloroform, no conversion of
the starting materials was observed. The reaction of 3a-c with
diethylamine or diethanolamine yielded new sulfonamides 4a-
O
O
S
Cl
N
S
(v)
R1
N
N
O
10a-c
c
and 5a-c, respectively, in 77-89% yield (Scheme 1, top).
O
O
R2
Derivatives 4a and 5a, both of which are bearing a methyl
S
N
ester function, can be further transformed to the
R2
corresponding carboxylic acids
6, 7 (Scheme 1, bottom).
N
S
(vi)
11a-c: R2 = CH2CH3
12a-c: R2 = (CH2)2OH
Saponification was performed with KOH in aqueous ethanol
and the corresponding carboxylic acid was precipitated with
R1
N
N
O
dilute HCl. 6 and 7 are soluble in alkaline aqueous solutions.
O
S
Compounds that are structurally similar to 4-7 but bearing a
pyrazine ring in 2-position of the thiazole were prepared
following the same synthetic procedures as applied to obtain
4-7. A synthetic protocol for the preparation of the starting
O
R2
4a: X = CH, R2 = CH2CH3
5a: X = CH, R2 = (CH2)2OH
11a: X = N, R2 = CH2CH3
12a: X = N, R2 = (CH2)2OH
N
R2
N
S
N
material
literature.48 Hydroxythiazole
with methyl bromoacetate, 1-bromo-3-chloropropane or
methyl iodide to obtain the ethers 9a-c. Chlorosulfonation of
8
(see Scheme
1
, middle) can be found in the
O
X
O
8
was subsequently alkylated
O
(vii)
9
O
and treatment with diethylamine or diethanolamine afforded
the sulfonamides 11a-c and 12a-c in good to very good yields
(Scheme 1, middle). The methyl ester in both 11a and 12a
O
R2
S
N
R2
6: X = CH, R2 = CH2CH3
7: X = CH, R2 = (CH2)2OH
13: X = N, R2 = CH2CH3
14: X = N, R2 = (CH2)2OH
N
S
N
were cleaved to 13
,
14 using KOH in aqueous ethanol (Scheme
and
1, bottom), analogous to the preparation of
6
7.
O
X
O
Chemical structures of the new compounds were confirmed by
means of NMR, MS, high resolution MS and UV/Vis spectra.
Sulfochlorides 3a-c and 10a-c have only been characterized by
NMR and MS and were rapidly converted to the corresponding
OH
Scheme 1 Top: Synthesis of sulfonamides 4a-c, 5a-c. (i) 2a,b: R1-Br, K2CO3, acetone,
reflux, 88-94%; 2c: CH3I, K2CO3, acetone, reflux, 91%; (ii) HSO3Cl, 0 °C - r.t., 84-91%; (iii)
sulfonamides, as they were highly sensitive to humidity. Even if HN(R2)2, NEt3, CH2Cl2, 77-89%. Middle: Synthesis of sulfonamides 11a-c, 12a-c. (vi) 9a,b:
R1-Br, K2CO3, acetone, reflux, 85-94%; 9c: CH3I, K2CO3, acetone, reflux, 90%; (ii) HSO3Cl,
0 °C - r.t., 51-85%; (iii) HN(R2)2, NEt3, CH2Cl2, 30-92%, Bottom: Synthesis of carboxylic
stored in a closed flask in the freezer overnight, isolated yields
decreased significantly. It is also noteworthy that none of the
acids 6,7 and 13,14. (vii) 1. KOH, EtOH, H2O; 2. HCl. (dil.), 84-97 %.
sulfochlorides exhibited fluorescence emission, neither in
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solution nor in the solid state, most likely due to intersystem atoms in parallelly arranged molecules is 8.2(6) Å (see also:
crossing to the triplet manifold via spin-orbit-coupling.49-51 The Fig.S1-S3, ESI), indicating no π-π-stacking interactions in the
contrary is the case for alkoxy thiazoles 2a-c and 9a-c, the crystalline state.52, 53
corresponding sulfonamides 4a-c
, 5a-c, 11a-c, 12a-c and Single Crystals of 11a that were suitable for X-ray structure
carboxylic acids 6, 7, 13 and 14. All of these show fluorescence analysis were obtained by slow evaporation of a concentrated
both in the solid state and in solution (vide infra), although solution of 11a in EtOAc. The crystal structure is depicted in
containing one or more sulfur atoms each, which can promote Figure 2. Expectedly, the three aromatic rings form a co-planar
intersystem crossing to the triplet manifold as mentioned structure. The pyrazine ring in 2-position of the thiazole core is
above.
twisted 2.17(2)° out of plane (N1-C1-C17-N4: 177.83(12)°) and
the sulfonamide containing phenyl ring is twisted 7.51(2)° (C2-
C3-C4-C5: 172.49(14)°) with regard to the center ring. The
X-Ray Crystal Structures
X-ray crystallographic analysis was performed to confirm glycolic ester moiety is also in plane with the aromatic
successful synthesis of the target compounds, to characterize fluorophore system (C1-N1-C2-O3: 178.81(11)°, C2-O3-C14-
the molecular geometry, as well as to investigate C15: 177.90(11)°, C16-O5-C15-C14: -178.33(12)°, C16-O5-C15-
intermolecular interactions. Single crystals suitable for X-ray O4: 2.7(2)°). Only the sulfonamide moiety, due to the sp3
structure analysis were obtained from slow diffusion of hybridised S2, is significantly tilted towards the phenyl ring in
pentane into a saturated solution of 4c in EtOAc. The crystal 5-position of the thiazole (N2-S2-C7: 107.78(6)°). In the crystal,
structure of 4c is depicted in Figure 1. It reveals the almost the molecules are arranged in stacks where the thiazole cores
planar arrangement of the compound. The torsion angles are located on top of one another and the pyrazine ring of one
between the three aromatic rings are 3.6(2)° (C4-C5-C6-N2) molecule slightly overlaps with the phenyl ring of another (Fig.
and 7.8(2)° (C7-C8-C9-C14). The methoxy group is twisted only S4, ESI). The sulfonamide moieties are tilted towards the next
6.68(19)° out of plane (C15-O1-C7-N2). While S2 is in plane molecule in the stack. The distance between the thiazole cores
with the aromatic ring it is attached to (S2-C12-C13-C14: in the different layers is 3.3(3) Å (see Figure 2), thus indicating
178.42(11)°), the sulfonamide-nitrogen N3 is distinctly tilted π-π-stacking interactions in the crystalline state,52, 53 probably
towards the phenyl ring (N3-S2-C12: 109.17(6)°). A look at the occurring due to the minimization of the overall dipole
molecular packaging of 4c reveals that the molecules are not moment through anti-aligned dipoles.54, 55
arranged parallely due to the steric demand of the ethyl
groups on the sulfonamide moiety. Therefore no π-stacking is
present in the single crystals. Instead, the molecules are
arranged in zigzag like stacks where the orientation of the
molecule is alternating. The distance between two similar
Figure 2 ORTEP plot (50% probability ellipsoids) of 11a. The numbering scheme (top)
and molecular packing (bottom) are shown. Hydrogen atoms were omitted for clarity.
Figure 1 ORTEP plot (50% probability ellipsoids) of 4c The numbering scheme (top)
.
and molecular packing (bottom) are shown. Hydrogen atoms were omitted for clarity.
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Optical Properties
Spectroscopic data (UV/Vis and fluorescence) for 2a-c
and are summarized in Table 1. Since all UV/Vis spectra look
very similar, only one example ( ) is shown in Figure 3. The
, 4a-c, 6
7
6
same is the case for fluorescence emission spectra. All other
spectra can be found in the ESI (Fig. S5-S14).
The absorption spectra of all compounds exhibit a single
structureless band with maxima between 372 and 378 nm,
which can be assigned to S0
range from 2.05 to 3.02
104 M-1 cm-1. A second, shorter-
wavelength absorption band was observed for 2a-c 4a-c and
. Only in 2a 4a and those bands were distinctly separate
and their maxima are 270, 284 and 285 nm, respectively. They
can be assigned to S0 S2 transitions. The emission maxima
ꢀ S1 transitions. Values for ε
×
,
6
,
6
ꢀ
range from 440 to 447 nm. Stokes shifts are very similar for all
compounds and range from 4084 to 4287 cm-1 (0.50-0.53 eV).
This is in good agreement with previous observations for
similar compounds.12,15 The fluorescence quantum yields of
2a-c are 0.85, 0.81 and 0.75, respectively, i.e. they decrease
from 2a to 2c. No such trend was observed for diethyl
Figure 4 UV/Vis absorption (dashed) and fluorescence emission (solid) spectra of 11a,
2×10-6 M in THF.
slightly more intense than the longer wavelength signal. The
opposite is the case for 9b,c, 11b,c, 12b,c, 13 and 14. The
UV/Vis absorption and fluorescence emission spectra of 11a
are depicted in Figure 4. All other spectra can be found in the
ESI (Fig. S15-S23). Stokes Shifts are between 3899 and 4654
cm-1 (0.48-0.58 eV). Fluorescence quantum yields are very
similar for 9a-12c: they range from 0.72-0.79. Only the
carboxylic acids 13 and 14 feature a slightly lower quantum
yield of 0.67. As for the pyridyl derivatives, good fluorescence
quantum yields were achieved for the pyrazine compounds.
The introduction of sulfonamide moieties did not significantly
decrease quantum yields and only those of carboxylic acids are
slightly lower than all others.
sulfonamides 4a-c
(
Φ
F = 0.80-0.83) and diethanol sulfonamides
ΦF = 0.77-0.79). Quantum yields for carboxylic acids
7 are 0.75 and 0.71, respectively and are therefore slightly
5a-c
(
6
and
lower than all others. Overall it can be said that good to very
good quantum yields were achieved for 2a . It is also worth
-
7
mentioning, that the introduction of sulfonamide moieties did
not significantly decrease fluorescence quantum yields.
Spectroscopic data (UV/Vis and fluorescence) for 9a-c and 11a-
14 are summarized in Table 1. The absorption spectra of all
compounds exhibit a single structureless band with maxima
between 385 and 391 nm, which can be assigned to S0
transitions. Values for
range from 2.15 to 2.85 × 104 M-1 cm-1.
A second, shorter-wavelength absorption band was observed
for all pyrazine derivatives. It can be assigned to S0 S2
ꢀ S1
ε
Electronic Structure Calculations
To gain a more detailed insight in the emission and absorption
properties of the here presented sulfonamide substituted 4-
alkoxy-1,3-thiazoles, we applied density functional theory
(DFT) and time dependent density functional theory (TD-DFT)
ꢀ
transitions. The maxima range from 274 to 277 nm for 9a-c
and from 286 to 289 nm for 11a-14 (see ESI). All compounds
9a-14 exhibit two emission maxima between 453 and 470 nm.
to those derivatives of interest (4a-c
13
, 5a-c, 6, 7, 11a-c, 12a-c,
It is noteworthy that for the esters 9a, 11a and 12a, the
,
14). The effect of solvatation (THF) was addressed by
shorter-wavelength emission band is
means of the polarizable continuum model56 as implemented
in Gaussian 09.57
The frontier orbitals of 4a and 11a are depicted in Figure 5 as
illustrative examples. Both HOMOs and LUMOs are mainly
delocalized over the whole molecule with exception of the
glycolic ether in 4-position of the thiazole core and the ethyl
groups of the sulfonamides. Therefore the MO constructs
suggest a significant overlap of the HOMO and the LUMO for
the molecules. This observation nicely corresponds with the
high experimental extinction coefficients and is also reflected
in high theoretical oscillator strengths (vide infra) and their
highly efficient fluorescence.
Moreover Figure 5 clearly shows that the HOMO- and LUMO-
shapes of both compounds 4a and 11a are virtually identic.
Considering that all calculated absorption and emission
properties are only involving these frontier orbitals, this may
serve as a possible explanation for the quite small gap
between the absorption and emission maxima of pyridyl and
Figure 3 UV/Vis absorption (dashed) and fluorescence emission (solid) spectra of 6,
2×10-6 M in THF.
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Table 1 Absorption and emission properties of 2a-c, 4a-7, 9a-d and 11a-14 in THF at room temperature.
νSt
ε
entry
λ
max,abs. [nm]
λ
max,em. [nm]
ΦF
[104 M-1 cm-1]
2.07
2.31
2.70
2.89
2.63
3.02
2.10
2.89
2.30
2.05
2.42
2.27
2.15
2.21
2.78
2.74
2.85
2.31
2.55
2.81
2.46
2.15
[eV]
0.52
0.53
0.52
0.52
0.52
0.51
0.53
0.52
0.52
0.50
0.51
0.50
0.58
0.57
0.49
0.55
0.54
0.48
0.55
0.55
0.52
0.53
[cm-1]
4155
4287
4215
4165
4154
4084
4237
4154
4204
4033
4103
3996
4654
4609
3948
4430
4364
3899
4430
4430
4207
4294
2a
2b
2c
372
373
374
374
377
378
373
377
377
378
377
385
387
387
385
389
390
385
389
389
391
389
440
444
444
443
447
447
443
447
448
446
446
455
472
471
454
470
470
453
470
470
468
467
0.85
0.81
0.75
0.83
0.81
0.80
0.78
0.77
0.79
0.75
0.71
0.76
0.74
0.72
0.79
0.73
0.75
0.77
0.76
0.75
0.67
0.67
4a
4b
4c
5a
5b
5c
6
7
9a
9b
9c
11a
11b
11c
12a
12b
12c
13
14
All measurements were performed at r.t. Quinine sulfate was used as standard for the determination of quantum yields
(Φ = 0.52).
pyrazinyl derivatives (see Table 3).
The calculated absorption and emission maxima are in a good
agreement with the experimental values. Nonetheless, it must
be said that the calculated absorption wavelengths entail a
larger error (0.19–0.29 eV) than the calculated emission
wavelengths (0.03–0.11 eV). Yet the error is always within the
0.3 eV margin that is typical for TD-DFT calculations58, 59 and
especially for thiazole-based chromophores like firefly
oxyluciferin.60
4a
11a
LUMO
The UV/Vis-absorption as well as the emission properties of
the studied molecules only consists of transitions from the
HOMO to the LUMO (or vice versa). It is noteworthy that all of
the here studied derivatives feature very high oscillator
strengths for their S0
ꢀ S1 transition (0.9180 - 0.9906).
As can be seen in Table 3, the Stokes shifts for all sulfonamide
compound can also be calculated within a reasonable margin
of error (0.14 – 0.20 eV). All theoretical Stokes shifts are
overestimating the experimental value by a mean of 0.16 eV.
Since this offset has a very low standard deviation (0.02 eV),
the calculated Stokes shift can be corrected by the empirical
value of 0.16 eV to give a much smaller discrepancy in
comparison with the experimentally determined values (-0.03
HOMO
Figure 5 Fontier Orbitals (HOMO, LUMO) of 4a and 11a, optimized at the CAM-
B3LYP/6-31+G(d,p) level of theory.
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Table 3 Calculated Stokes shifts from the Absoption to the Emission of S1 in THF.
– 0.04). This procedure may be important considering the in
silico prognosis of new thiazoles for further research.
ETheo Stokes
[eV]
ꢀEExp Stokes
ꢀEExp Stokes corr.
entry
[eV]
[eV]
Table 2 Calculated absorption and emission properties to/from the S1 in THF.
4a
4b
-0.66
-0.69
-0.67
-0.66
-0.68
-0.66
-0.67
-0.66
-0.68
-0.71
-0.69
-0.68
-0.70
-0.70
-0.69
-0.68
0.14
0.17
0.16
0.13
0.16
0.14
0.17
0.15
0.19
0.16
0.15
0.20
0.15
0.15
0.17
0.15
-0.02
0.01
0.00
-0.03
0.00
-0.01
0.01
-0.01
0.03
0.00
-0.01
0.04
-0.01
-0.01
0.01
-0.01
Etheo
entry
f
ꢀEExp [eV]
4c
[eV]
[nm]
5a
Absorption
4a
5b
3.51
3.51
3.48
3.51
3.50
3.48
3.52
3.52
3.44
3.43
3.40
3.44
3.43
3.41
3.45
3.45
353
354
357
353
354
357
352
352
361
361
365
360
362
363
360
359
0.98
0.96
0.98
0.98
0.97
0.98
0.98
0.99
0.94
0.91
0.93
0.94
0.92
0.93
0.94
0.95
0.20
0.22
0.20
0.19
0.21
0.19
0.24
0.23
0.22
0.24
0.22
0.22
0.24
0.23
0.28
0.26
5c
4b
6
4c
7
5a
11a
11b
11c
12a
12b
12c
13
5b
5c
6
7
11a
11b
11c
12a
12b
12c
13
14
Stokes shift energies (ETheo Stokes); error of the calc. to the exp. value (ΔEExp Stokes),
error of the corrected calc. to the exp. value (ΔEExp Stokes corr).
Solubility and Surface Activity
14
To utilize the amphiphilicity of the thiazoles and the resulting
self-assembly at the air-water interface we applied the
Emission
4a
Langmuir-Blodgett (LB) technique for assembling and
deposition of the dyes.36,
2.85
2.82
2.81
2.85
2.82
2.81
2.85
2.86
2.76
2.72
2.71
2.77
2.73
2.72
2.76
2.77
435
440
442
435
440
441
435
434
449
456
458
448
455
457
449
448
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.05
0.04
0.03
0.05
0.05
0.04
0.07
0.08
0.03
0.08
0.07
0.03
0.09
0.08
0.11
0.11
61-64
For this purpose the thiazoles
4b
were dissolved in a volatile solvent (typically chloroform), the
resulting solution was carefully dispersed on the water surface
of the LB-trough and the LB-experiments were started after
4c
5a
solvent evaporation. The LB-isotherms of 4a-c
12a-c 13 and 14 were recorded at a constant compression
speed of 375 mm2/min. Under these conditions the
compounds 12c and 13 did not show any resistance to
, 5a-c, 6, 7, 11a-
5b
c
,
,
5c
6
6,
compression on the LB-trough which suggests that they are
getting dissolved in the water subphase. Substance 14 is not
soluble in organic solvents and was therefore not tested, thus
7
11a
11b
11c
12a
12b
12c
13
we are discussing only the derivatives of types
in the following.
4, 5, 11 and 12
Figure 6 depicts the isotherms of 4a-c and 11a-c. The
isotherms of 5a-c and 12a,b can be found in the ESI (Fig.
S24). The corresponding values for collapse area (Ac) and
pressures ( c) are shown in Table 4. The collapsing point here
is defined as the inflexion point of the isotherm39 that is
generated by a pressure-induced phase- transition.65 The third
parameter that is considered to compare the isotherms to
each other is Aext, which is the hypothetical area per molecule
of the liquid phase at a surface pressure of 0 mN/m (see Figure
6). An exemplary fit of the linear region of the graph that was
used for the determination of Aext is depicted in Figure 6 for
,
7
π
14
All transitions H→L, absorpꢀon and emission energies (Etheo); oscillator
strengths (f), error of the calc. to the exp. value (ΔEExp), H = HOMO, L =
LUMO.
4b
.
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Table 4 Concentrations of the spread solutions and calculated collapse parameters of
the LB-isotherms.
substance
4a
c [mmol/l]
2.17
πc [mN/m]
Ac [Ų]
Aext [Å ²]
44
12
14
15
-
33
70
27
-
4b
2.15
74
4c
2.48
42
5a
2.03
20
5b
2.01
-
-
24
5c
2.30
-
-
18
7
2.09
-
-
18
11a
11b
11c
12a
12b
2.16
23
30
27
-
28
30
32
-
62
2.14
56
2.47
40
Figure 6 LB-isotherms of 4a-c and 11a-c, recorded with a constant barrier speed of 5
2.02
25
mm/min. For the measurement
a volume of 50µL was spread on the trough
2.00
-
-
27
(concentrations are presented in Table 4). An exemplary linear fit is shown for 4b.
Aext, value of the area per molecule at the theoretical pressure of
π
= 0; Ac and πc,
values of the area per molecule and the pressure at the collapse-point of the
isotherm.
The isotherms of 5a-c, 7, 12a and b exhibit an almost linear
curve progression at high surface-pressure and no signs of
phase transition (Figure 6). Moreover, their Aext value is
already very low before the compression starts (around 25 Å2).
The same LB-behavior is found for Lauric acid and is explained
by its slight solubility in water.61 We therefore assume that 5a-
As discussed qualitatively above, the most obvious polar
anchors of the chromophores are the sulfonamide groups
highlighted by green ellipsoids in Figure 7. To estimate the
possible weight of further polar moieties we computed the
electrostatic potential at the van-der-Waals surface as a sum
over contributions due to partial charges at every atomic site
(see Figure 7). Partial charges have been partitioned using the
MMFF9466 force field after quantum chemical optimizations
considering polarizable continuums as artificial environment.
In each case, the methyl ester group is pointing away from the
rest of the molecule, an unlikely conformation for a single
molecule at an air water interface. Instead, the methyl ester
group will most likely bend into the water phase. However,
possible anchor groups at the interface can clearly be
identified by their high polarity in decreasing order of
anchoring strength from Figure 7: methyl esters, sulfuryl
moieties, hydroxyethyl groups, pyrazinyl and pyridyl rings.
Figure 7 reveals that, additionally to the sulfuryl moiety, the
methyl ester and the pyrazinyl/pyridyl rings show considerable
polarity. Therefore, 4a and 11a appear likely to be anchored
with one long side of the chromophore on the water surface,
thus explaining the medium sized experimentally determined
molecular areas of 44 and 62 Ų, respectively. As can be seen
in Figure 7, the pyrazinyl ring in 11 exhibits a lower dipole
moment making it a slightly worse anchor as compared to the
c, 7, and 12a,b are also slightly water soluble. This seems
reasonable due to the two hydroxyl-groups on the
sulfonamide moiety which increase the hydrophilic properties.
4a-c show a sharp collapse-point at relatively low pressures
(12 - 15 mN/m, see Figure 6 and Table 4). This indicates a
typical phase-transition as observed for many long-chain
amphiphiles.61 As opposed to 4a-c, isotherms of 11a-c show
several weak points of inflection, particularly 11b, and their
collapse pressures are significantly and systematically higher,
while the only molecular structural difference to
replacement of the pyridyl moiety by the pyrazinyl ring.
A very dense packing in the Langmuir monolayers in the trough
is observed for derivatives 12 that have alcohols attached
4 is the
5, 7,
to the sulfonamide group to make it even more polar. Thus the
polar heads provide a strong hydrophilic anchor, which causes
small molecular areas in the dense Langmuir films of about
20 Ų as shown in Table 4. In contrast, derivatives of types
4
and 11, which are bearing short alkyl chains instead of alcohols
attached to the sulfonamide, are showing significantly larger
molecular areas ranging from 40 to 70 Ų because of the
weaker hydrophilic anchor. Additionally, the molecular areas
of 4a-c and 11a-c depend on the substituents introduced at
the 4-hydroxy-group of the thiazole. Expectedly, a- and b-type
substitutions induce stronger interactions of the
chromophores with the subphase as compared to the methyl-
(c)-substitution, thus causing generally somehow larger
molecular areas. Remarkably, the molecular area of 4b is
exceptionally large (Ac = 70 Å2 and Aext = 74 Å2) and indicates a
pyridinyl-ring in
at lower surface pressures than substances
fact that substances of type 11 stack well in the solid state (see
Figure 2), in contrast to substances , we conclude that
4
, thus allowing 11 to leave the flat orientation
4. Considering the
4
substances of type 11 gradually move into an upright
orientation as the surface pressure increases. Substances of
type 4, however, cannot stack in such an upright orientation
flat orientation of
4
at the water surface (largest molecular
flat surface approximately
causing the highly ordered structure to break down completely
at a certain well-defined surface pressure. Figure S25 (see ESI)
spatial demand on
5 Å × 14 Å ≈ 70Ų).
a
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Figure 8 AFM images of LB- (a, b) and SC-films (c, d) of 11a, deposited on quartz glass
(π = 5 mN/m). Individual scale bars are shown to the right of every image.
uneven (Figure 8d). Thus, smooth thin films with rather
homogeneous molecular orientation are obtained via the LB-
technique whereas spin-casting yields apparently stochastic
molecular orientations and cluster formations and shows a
significant amount of random substance accumulation.
Figure 7 Electrostatic potential on the van-der-Waals surfaces of molecules 4a, 4b, 11a
and 11b (top to bottom) in different orientations (left to right) for geometries
optimized at the CAM-B3LYP 6-31+g(d,p) level of theory including solvent effects using
the polarizable continuum model. Red, blue and black correspond to
a positive,
negative and vanishing electrostatic potential, respectively. Green and orange ellipses
emphasize the two highly polar groups, namely the sulfonamide and the glycolic ester
moieties, respectively.
The absorption spectra of 11a and 11b (chloroform solution,
LB- and SC-film) are shown in Figure 9 (for the solid state
samples the absorbance at the minimum was set to zero). The
structureless band which appears at 390 nm in CHCl3 for 11a
and 11b is red-shifted to 400 nm in the SC films. The LB films of
5 monolayers (ML) have been deposited at very high surface
pressure (23 mN/m for 11a and 28 mN/m for 11b) and are
expected to show a high molecular ordering. As seen from
Figure 9 a higher molecular order in the LB-films as compared
to the SC-films is revealed by the vibrational progression that is
significantly better resolved in the spectra of the LB-films as
compared to the SC-films (a simple estimation of the coupling
energy in the vibrational progression gives an energy
difference of about 1350±100 cm-1). Stochastic molecular
orientations, like in the SC-films, are also present in Langmuir-
films in the quasi liquid phase at low surface pressures. Thus,
the transition from the disordered to the ordered phase can be
shows that due to the hydroxy-ethyl groups attached to the
sufonamide in
5 and 12, the sulfonamide gets even more polar
and takes precedence, thus supposedly causing upright
chromophore orientations.
Morphology and Optical Properties of Langmuir, LB and Spin Cast
Films
To find out how the optical properties change with varying LB
conditions, Langmuir-Blodgett-films (LB) were deposited on
quartz-glass and characterized by means of UV/Vis-
spectroscopy, fluorescence-measurements and atomic force
microscopy (AFM). Spin-coated (SC) films and solutions were
prepared as references. The two compounds 11a and 11b
were investigated more closely because their LB isotherms
differ significantly. Another advantageous feature of these two
dyes is their comparatively high collapse-point, which allows
varying π over a larger scale.
mimicked by systematically varying the surface pressure at
deposition of the Langmuir-films.61,
67
The corresponding
changes in the spectral shapes and vibrational progressions are
shown in Figure 10. Interestingly the intensity distribution in
the vibrational progression is altered from SC- to LB-films.
Hence the potentials of the associated states are altered due
to intermolecular interactions.
Employing AFM, we probed the surface morphology of LB- and
SC-films of 11a deposited onto a quartz glass surface to
investigate the evenness of the films and the influence of the
respective deposition method on film morphology. Height
images as well as phase images, which probe the local stiffness
of the film surface, are shown for 11a in Figure 8. The LB and
SC films have RMS surface roughnesses of 0.45 and 3.20 nm,
respectively. Thus, the surface roughness of the SC film,
expectedly, is significantly higher than that of the LB film. This
can also be seen in the height images (see Figure 8a,c). A very
narrow distribution of the AFM phase angle was observed for
the LB film (Figure 8b) while the SC film shows significantly
larger phase angles in areas where the surface is especially
The dependence of the fluorescence spectra of 11a and 11b on
the surface pressure was investigated directly for the
Langmuir-films on the LB-trough surface by means of a
custom-made
fluorescence
online-monitoring
setup.
Fluorescence spectra of 11a that were normalized to the
molecular density (same surface concentration) as obtained
from the LB-isotherms are depicted in Figure 11. The
fluorescence intensities increase with increasing surface
8 | J. Name., 2012, 00, 1-3
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Figure 9 UV/Vis-spectra of 11a (left) in solution (CHCl3; c= 20 µg/ml), as SC film (c = 2.16×10-3 M; 30 rps for 30 s) and as LB film (5 ML; π = 20 mN/m). The intensity of the LB film was
increased by factor 4 for clarity. UV/Vis-spectra of 11b (right) in solution (CHCl3; c= 100 µg/ml), as SC film (c = 2.14×10-3 M; 30 rps for 30 s) and as LB film (5 ML; π = 32 mN/m).
Figure 10 UV/Vis spectra of LB films at different π of 11a (5mL deposited at 4, 8, 12, 16, 20 and 23 mN/m, top) and 11b (4, 8, 12, 16, 20 and 28 mN/m, bottom). The spectra have
been normalized with regard to the molecule density per surface area.
pressure, which may be explained with a reduced misfit further increasing the surface pressure, the shape of the
between the irradiation angle of excitation (intermediate fluorescence spectra changes: the band at 470 nm decreases
incidence) and chromophore orientations. As can be seen from while a second band at 542 nm almost retains its intensity,
Figure 11, no disctinct change in the spectral shape or finally dominating the fluorescence spectrum at the rather
wavelength of the fluorescence maximum occurs.
large surface pressure of 27.55 mN/m.
In contrast to 11a, whose fluorescence intensity is increasing The spectral changes of 11b with increasing surface pressure
with increasing surface pressure, the fluorescence intensity of can be explained in the following manner: At low surface
11b is almost constant when normalized with regard to the pressures one can expect stochastically distributed dye
molecule density for low pressures (<1 mN/m) as shown in molecules without appreciable intermolecular interaction,
Figure 12. For these low surface pressures the fluorescence similar to solutions or amorphous phases. Indeed the optical
spectra become narrower for increasing surface pressures, but properties of these LB-films are very similar to those of the
the maximum is retained at 460 nm, which is about the same dissolved samples. Further increase of the surface pressure is
value as found for THF-solutions of 11b (see Table 2). When expected to cause alignment of the dyes and possibly
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pressure, its intensity relative to the band at 470 nm (i.e.
I(542 nm)/I(470 nm)) is plotted versus the surface pressure in
the inset in Figure 13.
Experimental Section
General Procedures and Molecular Characterization:
Reagents were purchased from commercial sources and were
used directly unless otherwise stated. All solvents were of
reagent grade and were dried according to common practice
and distilled prior to use. Reactions were monitored by TLC,
which was carried out on 0.2 mm Merck silica gel plates (60
F254). 1H and 13C NMR spectra were recorded on Bruker Avance
250 and 400 spectrometers, chemical shifts (δ) are given
relative to signals arising from the solvent. Mass spectra were
recorded using a Finnigan MAT SSQ 710. Melting points were
determined with a Cambridge Instruments Galen III apparatus
(Boëtius system) and are uncorrected.
Figure 11 Fluorescence of the Langmuir films of 11a recorded at different π. The
spectra were corrected by subtraction of a spectrum of the empty LB-trough and
normalized with regard to the molecule density. Inset: Dependence of corrected
fluorescence intensity on π.
Langmuir and LB-Films: A KSV NIMA “alternate small” LB-
trough with a Wilhelmy balance was employed for the
π-A-isotherm measurements as well as the LB film
preparations. A chloroform solution (for concentrations see
Table 4) of the compound was placed at ambient temperature
formation of H-aggregates that are weakly fluorescent.68-70
A
further possibility for the steady reduction of the fluorescence
at 470 nm might by an increasing misfit between the
irradiation angle and chromophore orientations. However, the
542 nm-fluorescence that dominates the fluorescence
spectrum at high surface pressures at deposition can be clearly
assigned to an excimer emission as known from e.g. different
perylene derivatives.71-73 Other groups report similar findings
for excimer emission bands in LB-films of pyrene,74
indocarbocyanine75 or merocyanine dyes.76 Furthermore, the
absence of a vibrational progression in the fluorescence
spectrum supports the assignment to an excimer.
onto an aqueous subphase of ultrapure water (18 m
ꢁ cm).
The solution was spread on the surface with a pipette. In all
cases films were compressed after 10 minutes allowing the
chloroform to evaporate. The samples and the isotherms were
fabricated with a constant compression rate of 375 mm2/min
(barrier speed: 5 mm/min). The deposition of the layers on the
quartz glass (30mm × 5mm × 1mm) was done using Y-type
dipping (dipping speed:
5 mm/min) and keeping the
Figure 13 shows that the LB-films produced from 11a exhibit
the same spectral features as their Langmuir-analogues
discussed above, except showing no systematic intensity
trend, probably due to inhomogenities that arose during the
deposition. In case of 11b all the essential features discussed
for the Langmuir-films above are retained after deposition of
corresponding LB-films as shown in Figure 13, particularly the
monolayer under constant pressure. Prior to the transfer, the
substrates were treated with chloroform, acetone and iso-
propanol in the ultrasonic bath (each solvent 3 times for 10
min). The glass slides were stored under iso-propanol over
night in order to make them hydrophilic.
Atomic Force Microscopy: AFM was carried out using a Veeco
Digital Instruments Dimension 3100 AFM in tapping mode with
a silicon tip (radius < 8 nm) at 300 kHz with a force constant of
≈ 40 N m−1 (Budget Sensors Tap 300-G).
intense eximer fluorescence at π = 28 mN/m. To show the
evolution of the excimer band with increasing surface
Figure 12 Fluorescence of the Langmuir films (11b) for several different π. The spectra were corrected by subtraction of a spectrum of the empty LB-trough and normalized to the
molecule density per area.
10 | J. Name., 2012, 00, 1-3
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Figure 13 The fluorescence spectra of the LB films deposited at different pressures normalized to the molecule density (left: 11a; right: 11b). Inset: Ratio of the fluorescence
intensities at 542 and 470 nm at different π.
Spectroscopic Methods: UV/Vis spectra (THF) were recorded the here presented compounds. Fluorescence emission was
with a PerkinElmer LAMBDA 45 UV/Vis spectrometer, emission calculated according to Kasha82 from the S1 equilibrium
spectra (THF) were recorded using
a JASCO FP-6500 structure. The effects of solvatation (THF) have been
spectrofluorometer. Measurements of the fluorescence addressed state specific for all calculations of ground and
intensity were carried out on a PerkinElmer LAMBDA 45 excited states properties by means of the polarizable
UV/Vis spectrometer and JASCO FP-6500 spectrofluorometer continuum model56 as implemented in Gaussian 09.
in the perpendicular excitation–emission geometry, while the Synthesis of the compounds: Detailed synthetic procedures
absorbance at the excitation wavelength used was adjusted to can be found in the Supporting Information.
be between 0.04 and 0.05. The calculation of fluorescence Crystal Structure Determination: The intensity data were
quantum yields was done according to the following collected on
equation:77
a
Nonius KappaCCD diffractometer, using
radiation. Data were
graphite-monochromated Mo-K
α
corrected for Lorentz and polarization effects; absorption was
taken into account on a semi-empirical basis using multiple-
scans.83-85 The structure was solved by direct methods
(SHELXS)86 and refined by full-matrix least squares techniques
against Fo2 (SHELXL-97)86. All hydrogen atoms were located by
difference Fourier synthesis and refined isotropically. Mercury
(Version 3.3 (Build RC5), Cambridge Crystallographic Data
Centre) was used for structure representations.
ꢂ
ꢃr ꢄꢅ
ꢀ ꢁ ꢀ
,
ꢃ
ꢄrꢅ
r
ꢂr
where
Φ
is the quantum yield, I is the corrected integrated
emission intensity, A is the absorbance at the excitation
wavelength and n is the refractive index of the solvent. The
subscript r refers to a reference fluorophore of known
quantum yield. Here we used quinine sulfate (
Φ
= 0.52)78. All
Crystal Data for 4c : C19H21N3O3S2, Mr = 403.51 gmol-1, yellow
prism, size 0.096 x 0.086 x 0.075 mm3, orthorhombic, space
group P n a 21, a = 22.1463(3), b = 10.5116(1), c = 8.1653(1) Å,
V = 1900.83(4) Å3 , T= -140 °C, Z = 4, ρcalcd. = 1.410 gcm-3, µ
(Mo-Kα) = 3.06 cm-1, multi-scan, transmin: 0.6921, transmax:
0.7456, F(000) = 848, 14865 reflections in h(-28/28), k(-13/13),
l(-10/10), measured in the range 2.67° ≤ Θ ≤ 27.48°,
thiazoles were excited as close as possible to their absorption
maximum while staying inside the excitation range given in the
literature.79
Spectroscopic measurements of the compound films: The
fluorescence spectra were recorded on a home build setup.
This consists of an Isoplane 320 Spectrograph with a cooled
Pixis CCD-camera from Princeton Instruments. As excitation
sources fiber coupled 5mW laser modules with different
wavelength were used. Long pass filters with low self-
fluorescence from ITOS were placed in front of the
spectrograph to block scattered excitation light. The collected
data were afterwards corrected by a self-written Labview
program. This removes cosmic rays, filter self-fluorescence and
accounts for the setup response function.
completeness
= 0.0189, 4276 reflections with Fo > 4
restraints, R1obs = 0.0217, wR2obs = 0.0588, R1all = 0.0222, wR2
Θ
max = 99.7%, 4347 independent reflections, Rint
σ
(Fo), 328 parameters, 1
all
= 0.0593, GOOF = 1.076, Flack-parameter 0.01(4), largest
difference peak and hole: 0.206 / -0.188 e Å-3.
Crystal Data for 11a : C20H22N4O5S2, Mr = 462.54 gmol-1,
light_yellow prism, size 0.140 x 0.128 x 0.106 mm3, triclinic,
space group P ī, a = 7.0339(1), b = 12.5011(3), c = 12.5338(3) Å,
Theoretical Methodology: After an initial systematic
conformational search with MMFF66, the best geometries were
α
= 76.065(1), β = 89.022(1), γ
= 75.506(1)°, V = 1034.54(4) Å3 ,
81
optimized using at the CAM-B3LYP/6-31+G(d,p)80, level of
T= -140 °C, Z = 2, ρcalcd. = 1.485 gcm-3, µ (Mo-Kα) = 2.99 cm-1,
multi-scan, transmin: 0.7186, transmax: 0.7456, F(000) = 484,
6495 reflections in h(-9/9), k(-16/16), l(-16/10), measured in
the range 1.74° ≤ Θ ≤ 27.50°, completeness Θmax = 99%, 4710
theory as implemented in Gaussian 0957. Ground state
optimization and its validation via frequency calculation were
followed by the calculation of the six lowest singlet excited
states. This provided insight into the absorption properties of
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* To whom correspondence should be addressed: c6bera@uni-
jena.de and martin.presselt@uni-jena.de
independent reflections, Rint = 0.0129, 4448 reflections with Fo
(Fo), 368 parameters, 0 restraints, R1obs = 0.0316, wR2
> 4σ
=
obs
0.0794, R1all = 0.0339, wR2 = 0.0817, GOOF = 1.067, largest
all
difference peak and hole: 0.421 / -0.373 e Å-3.
1
2
U.-W. Grummt, D. Weiß, E. Birckner and R. Beckert,
Pyridylthiazoles: Highly Luminescent Heterocyclic
Compounds, J. Phys. Chem. A, 2007, 111, 1104–1110.
E. Täuscher, D. Weiß, R. Beckert, J. Fabian, A. Assumpção and
H. Görls: Classical heterocycles with surprising properties:
Conclusions
The present work describes the synthesis and photophysical
properties of novel fluorescent and amphiphilic sulfonamide-
substituted hydroxythiazole derivatives. Optical properties of
these compounds have been investigated and quantum
chemical calculations (CAM-B3LYP/6-31+G(d,p) level of theory)
were performed in order to explain the experimental results.
the 4-hydroxy-1,3-thiazoles, Tetrahedron Lett., 2011, 52
2292-2294.
,
3
4
P. Chabrier, S. H. Renard and K. Smarzewska: Sur les
composes organiques soufres. II., Bull. Soc. Chim. France,
1949, 237.
F. A. J. Kerdesky, J. H. Holms, J. L. Moore, R. L. Bell, R. D.
Dyer, G. W. Carter and D. W. Brooks: 4-Hydroxythiazole
inhibitors of 5-lipoxygenase, J. Med. Chem., 1991, 34, 2158-
2165.
F. A. J. Kerdesky, C. D. W. Brooks, K. I. Hulkower, J. B. Bouska
and R. L. Bell: Conversion of cyclooxygenase inhibitors into
hydroxythiazole 5-lipoxygenase inhibitors, Bioorg. Med.
Structure-property relationships have emerged from
a
combination of experimental spectroscopic data and
DFT/TDDFT calculations. Absorption and emission properties
as well as quantum yields agree nicely with the quantum
chemical calculations.
5
6
We further we showed how to exert direct control over the
degree of aggregation in thin films made of different thiazole
type dyes. Upon variation of the different substituents in 2-
and 4-position and on the sulfonamide moiety (i.e. altering
their relative polarity / anchoring strength) the chromophores
can be oriented relative to the subphase surface of the LB-
trough. In combination with spectroscopic investigations, we
gained an in-depth understanding of the influence of
aggregation on the electro optical properties of thin films
made of the here investigated substances. In-depth
investigations to gain detailed information on the molecular
packing of our compounds in Langmuir and LB films are
currently ongoing. A next step is to perform investigations on
active layers or possibly entire devices. Work along these lines
is currently in progress and will be reported in due course.
Chem., 1997, 5, 393-396.
R. M. Rzasa, M. R. Kaller, G. Liu, E. Magal, T. T. Nguyen, T. D.
Osslund, D. Powers, V. J. Santora, V. N. Viswanadhan, H.-L.
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Acknowledgements
The authors thank the Bundesministerium für Wirtschaft und
Energie (BMWi KF2258002CS2) and the Bundesministerium für
Bildung und Forschung (BMBF FKZ 03EK3507) for financial
support. S. F is grateful for financial support from the Deutsche
Forschungsgemeinschaft DFG (PR 1415/2). St. S. gratefully
acknowledges the Friedrich-Ebert-Stiftung for financial
support. T. S. Acknowledges the German Federal
Environmental Foundation for his fellowship. Additionally, we
thank Dr. W. Günther and G. Sentis for recording the NMR
spectra and Mrs. Schönau and Mrs. Heineck for recording the
mass spectra.
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Processing of 4-alkoxythiazole sulfonamides via Langmuir-Blodgett technique gave an
insight into the influence of aggregation on the electro optical properties of thin films.