Probing the surface polarity of inorganic oxides using merocyanine-type dyes derived from barbituric acid

Article abstract of DOI:10.1039/c2nj20835k

The solid state complexes, solvatochromic and acidochromic behaviour of four merocyanine-type dyes derived from barbituric and thiobarbituric acid and their use as solvatochromic probe molecules for coloured surfaces are described. The dyes were obtained

Full text of DOI:10.1039/c2nj20835k

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PAPER
Probing the surface polarity of inorganic oxides using merocyanine-type
dyes derived from barbituric acidw
Susan Seifert,a Andreas Seifert,a Gunther Brunklaus,b Katja Hofmann,a
Tobias Ru
¨
ffer,c Heinrich Langc and Stefan Spange*a
Received (in Montpellier, France) 28th September 2011, Accepted 23rd November 2011
DOI: 10.1039/c2nj20835k
The solid state complexes, solvatochromic and acidochromic behaviour of four merocyanine-type
dyes derived from barbituric and thiobarbituric acid and their use as solvatochromic probe
molecules for coloured surfaces are described. The dyes were obtained by condensation reaction
of barbituric, N-methylbarbituric, N,N0-dimethylbarbituric and thiobarbituric acid with 4-N,
N-dimethylaminocinnamaldehyde. The dyes were characterised by means of liquid and solid state
1
NMR techniques (1H MAS NMR, H–1H DQ MAS NMR, 13C CPMAS NMR), single-crystal
X-ray analysis, UV/vis and IR measurements. The solvatochromism has been investigated in
43 solvents and interpreted in terms of Kamlet–Taft parameters a, b, and p*. Altogether, the
solvatochromic properties of these four dyes were determined mainly by the hydrogen bond
donating (HBD) ability and the polarisability/dipolarity of a solvent. The interactions of the dyes
with oxidic and metal surfaces were studied. Their perichromic behaviour was compared with that
of established solvatochromic dyes for the determination of a, b, and p*, namely dicyano-bis-
(1,10-phenanthroline)-iron(II)-complex (1), 3-(4-amino-3-methylphenyl)-7-phenyl-benzo-[1,2-b:4,5-b0]-
difuran-2,6-dione (2) and 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzylidene]-D3-
thiazoline (3). The barbituric acid dyes were used to probe the surface polarity of coloured oxides like
tungsten(VI) oxide and iron(III) oxide. The interactions between the dyes and metals like zinc and
aluminium were also investigated.
including rather simplistic single-parameter approaches of
Introduction
e.g. Kosower’s Z-scale,15 Gutmann’s acceptor (AN) and donor
number (DN),16 or Dimroth–Reichardt’s ET30 scale17 as well as
multi-parameter approaches of e.g. Kamlet–Taft’s a, b, and p*
scale,18 or Catalan’s SA, SB, SP and SdP scale.19 Notably, such
measurements are based on monitoring solvent-dependent shifts
of the UV/vis absorption band of a probe dye resulting from
both rather specific (e.g. hydrogen bond donating (HBD) or
hydrogen bond accepting (HBA)) and non-specific interactions
(including dipole–dipole, dipole–induced dipole, or even dispersion
forces), while interactions that chemically alter the probe dye are
disregarded.20 The Kamlet–Taft and Catalan approaches have
been successfully applied to separate these influences,18,19 but
based on practical considerations the simplified Kamlet–Taft
equation was evaluated and used in this work, given as eqn (1),18
The surface polarity of inorganic oxides constitutes an important
factor for many practical applications, and potentially allows for
a deeper understanding of the catalytic activity of inorganic
oxides in heterogeneous catalysis, interactions between oxidic
surfaces and adsorbed polymer films or the integration of
inorganic fillers into organic matrices, respectively.1–13 However,
the term polarity itself has, until now, not been precisely defined
and cannot be merely quantified by means of individual physical
constants such as the relative permittivity or dipole moments.14
In the past, several empirical methods have been introduced to
determine the polarity of solvents via solvatochromic probe dyes,
a Department of Polymer Chemistry, Chemnitz Technical University,
Strasse der Nationen 62, 09107 Chemnitz, Germany.
E-mail: stefan.spange@chemie.tu-chemnitz.de;
Fax: +49 (0) 37 1 53 12 12 39; Tel: +49 (0) 37 1 53 12 12 30
b Department of Physical Chemistry, University of Muenster,
Corrensstrasse 28/30, 48149 Muenster, Germany
n~max = n~max,0 + aa + bb + sp*
(1)
where n~max,0 denotes the value of a solvent reference system such
as the nonpolar medium cyclohexane. The parameter a describes
the HBD ability, b the HBA ability, and p* represents the
dipolarity/polarisability, respectively. In addition, a, b, and s
are solvent-independent coefficients reflecting contributions of
c Department of Inorganic Chemistry, Chemnitz Technical University,
Strasse der Nationen 62, 09107 Chemnitz, Germany
w Electronic supplementary information (ESI) available: CIF file of
compound 4; spectral data and spectra, which are not shown in the
manuscript as well as solvent parameters known from the literature.
CCDC reference number 822677. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2nj20835k
solvent effects to the UV/vis absorption shift n~max
.
c
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474–586 nm, respectively. They show rather broad absorption
bands in HBD media. This effect was first reported by Rezende
et al. and attributed to protonation of carbonyl units comprising
the barbituric acid moiety, rendering these dyes less suitable for
application as perichromic probes of Lewis acid metal oxide
surfaces. Though barbituric and thiobarbituric acid dyes obtained
from retinal, without the presence of strong electron donor
substituents, may exhibit enlarged conjugated p-systems, no
indication of successful proton transfer in HBD media was
found.45 However, the corresponding absorption maxima were
identified in the range of 468–526 nm, thus excluding these dyes
for analysis of strongly coloured surfaces.45
Scheme 1 Solvatochromic (perichromic) dyes 1–3 suitable for
measurement of surface polarities.
The application of solvatochromic probes for the determination
of solvent and surface polarity is rather advantageous since a quite
small concentration of the corresponding dye is required while the
method is rather sensitive and reproducible. Note that, to utilize
solvatochromic probes on surfaces at the microscopic level, the
term perichromism has been suggested.17,21
A rather better perichromic dye may be designed according
to a balance between both the length of the p-system and the
electron donor system thereby yielding UV/vis absorption
maxima larger than 570 nm, and it should not be protonated
in HBD media. In the present work, (thio)barbituric acid
derivatives carrying a N,N-dimethylamino group as electron
donor substituent and two conjugated exocyclic double bonds
(cf. Scheme 2) were explored. In addition, different substituents
were attached to the respective (thio)barbiturate moiety to
systematically vary both the electron-demanding effects and
interactions with the surrounding(s) of the dye.
Indeed, several perichromic probes were used to characterise
the surface polarity parameters a, b, and p*, respectively, of
‘‘white’’ powders, including e.g. cellulose, SiO2, Al2O3, or
alumosilicates via dyes 1–3 shown in Scheme 1.7,22,23
In particular, the dicyano-bis-(1,10-phenanthroline)-iron(II)
complex 1 is a well-established probe molecule for the determina-
tion of both the acceptor number AN and HBD ability a of metal
ions, solvents, binary mixtures of solvents, polymers, particle
surfaces, polymer-modified particles and ionic liquids.7d–f,i,m,24–33
In contrast, the HBA ability b of solvents, polymers, ionic liquids
and surfaces can be quantified with 3-(4-amino-3-methylphenyl)-
7-phenyl-benzo-[1,2-b:4,5-b0]-difuran-2,6-dione (2) while p* can
In particular, such dyes are readily accessible via Knoevenagel
condensation of corresponding (thio)barbituric acid derivatives
with 4-N,N-dimethylaminocinnamaldehyde, and the preparation
of compounds 4, 6, and 7 has been previously reported.39,46–49
Note that dyes 4, 6, and 7 have found interest as nonlinear optic
materials,50 as methionine aminopeptidase-1 inhibitors49 and
laser dyes.51 Further, interactions of heavy metal ions with
compound 739 including its application as photoconductor for
electrophotography52–54 or dye in hair tinting lotions were
described, while comprehensive structural data (e.g. single-crystal
X-ray analysis) as well as investigations concerning their peri-
chromic and acido-chromic behaviour are rather limited to date.
In the current contribution, the perichromic and acidochromic
behaviour of dyes 4–7 was investigated to potentially establish the
compounds for reproducible determination of the Kamlet–Taft
(polarity) parameters of coloured oxidic and metallic materials.
Since metal oxidic surfaces may also bear strong acidic and basic
groups, non-solvatochromic interactions like acid–base reactions
were also considered. Notably, dyes 4–7 were independently
adsorbed onto different ‘‘white’’ oxidic surfaces to probe the
perichromic behaviour in comparison to the rather well-
established dyes 1–3. Since occurrence of self-aggregation or
polymorphism of (thio)barbituric acid derivatives is well known,55
a detailed structural characterisation of solid dyes 4–7 is rather
important. Insights into molecular arrangements of the corres-
ponding dyes were obtained from both single-crystal X-ray
analysis and multi-nuclear solid-state NMR spectroscopy includ-
ing 2D homonuclear dipolar double quantum (DQ) NMR, which
be
identified
using
4-tert-butyl-2-(dicyano-methylene)-5-
[4-(diethylamino)benzylidene]-D3-thiazoline (3).7d,e,i,34–37 Based on
eqn (2)–(4), a, b, or p* can be individually derived from the UV/vis
absorption maxima of the perichromic probe dyes 1–3.7b,34a
a = ꢀ7.49 + 0.46 n~max(1) [10ꢀ3 cmꢀ1 7b
]
(2)
(3)
(4)
b = 3.84 ꢀ 0.20 n~max(2) [10ꢀ3 cmꢀ1 7b
]
p* = 9.475 ꢀ 0.54 n~max(3) [10ꢀ3 cmꢀ1 34a
]
However, experiences in the field of perichromism of strongly
coloured metal oxides such as iron(III) oxide or tungsten(VI)
oxide and industrially important metal powders, including
aluminium, zinc, iron, steel or copper, are up to now rather
limited. Notably, such perichromic probe dyes have to fulfill
several threshold criteria: while the wavelength of the UV/vis
absorption maxima of the considered dye(s) should be larger
than 570 nm, successful and reversible adsorption at either the
metal or metal oxide surface must be associated with a
significant measurable UV/vis shift, which can be difficult to
achieve. Though dyes 1 and 3 have UV/vis absorption maxima
in the range of 570–640 nm, respectively,25a,34d their adsorption
onto metal surfaces did not yield detectable UV/vis spectra,38
therefore rendering a search for alternative dyes necessary.
It is known that both barbituric and thiobarbituric acid
derivatives interact with ions of heavy metals,39–42 while barbituric
acid and N,N0-dimethylbarbituric acid derivatives obtained from
N,N-dimethylaminobenzaldehyde or 4,40-bis(dimethylamino)-
benzophenone exhibit solvatochromic behaviour.43–45 Their
UV/vis absorption maxima are in the range of 450–470 nm and
Scheme 2 (Thio)barbituric acid derivatives 4–7 studied in this work.
c
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has been proven to be highly suitable for the analysis of hydro-
gen-bonded aggregates.56
Results and discussion
Structure of the dyes
Single-crystal X-ray analysis. Crystals of 4 suitable for
single-crystal X-ray structure analysis were obtained by rather
slow diffusion of water into a dimethyl sulfoxide solution of 4.
It crystallizes in the non-standard monoclinic space group P21/n
with one molecule of 4 comprising the asymmetric unit (Fig. 1),
Z = 4, a = 4.566 A, b = 9.625 A, c = 30.386 A, b = 91.6681,
where an E-configuration with respect to the central CQC
double bond (C9–C10) is present.
As anticipated,57 a 3D network structure based on two types
of intermolecular hydrogen bonds among barbituric acid
moieties is formed (N3–H3Nꢁ ꢁ ꢁO2, 2.983(2) A, 167.0(2)1 and
N2–H2Nꢁ ꢁ ꢁO1, 2.870(2) A, 176.6(19)1) leading to a band-like
structure along the crystallographic b-axis. Because of non-
classical hydrogen bonds between the oxygen O3 and the
hydrogen of the methyl group C2 of the aniline ring
(C2–H2Bꢁ ꢁ ꢁO3, 3.159(3) A, 136.01) the bands are further
connected to a 2D-network. Additionally, along the crystallo-
graphic a-axis p–p stacking interactions between the barbituric
units are observed. Due to this reason a weak contact between
the olefinic proton at C11 and the proton of N2 (4.09 A) of
every second molecule is observed. Fig. 2 shows a selected part
of the crystal packing of 4.
Fig. 3 1H and 13C solution NMR spectra of 4 dissolved in DMSO-d6.
The given peak assignment is based on 1H–1H COSY, 1H–1H NOESY,
1
1H–13C HMBC and H–13C HMQC NMR experiments, respectively.
Note that the 13C NMR signal of the N(CH3)2 group is covered by the
DMSO-d6 signal.
since all atoms that occupy a general position of symmetry
within the unit cell should exhibit a characteristic peak
provided that sufficient spectral resolution can be achieved.58
In addition, in the case of molecular compounds with low
space group symmetry, the integrant unit (which is the first
multiple of the asymmetric unit leading to the integer number
of atoms reflecting the stoichiometry of considered crystal
formula) is often equivalent to the molecule(s) comprising
the crystal, thus rendering ‘‘NMR crystallography’’ feasible.59
NMR analysis. Fig. 3 shows representative 1H and 13C
solution NMR spectra (including the peak assignment) of
these dyes as exemplified for 4 dissolved in DMSO-d6.
The scalar coupling constants of dyes 4–7 are found in the
range of 3JH8–H9 = 14.9–14.5 Hz and 3JH8–H7 = 12.3–12.5 Hz,
thus indicating an E-configuration of the conjugated exocyclic
double bonds, in agreement with the single-crystal X-ray
structure of 4. Indeed, this arrangement allows for formation
of intramolecular rather non-classical hydrogen bonds
between the protons H8 and H7 and the carbonyl oxygens
in positions 4 and 6, as reflected by the significant shift of these
protons to higher ppm values in the 1H solution NMR
spectrum. In the case of 5, a double signal set with quite
similar intensities was observed indicating the likely presence
of two isomers in a 1 : 1 ratio.
Though dyes 5–7 did not yield single crystals suitable for
X-ray analysis, insight into the corresponding spatial arrange-
ments including the molecular configuration could be derived
from both solution and solid state NMR analyses. In the
absence of X-ray data an NMR-based structure determination
would at first require identification of the asymmetric unit,
which in principle can be deduced from 13C-CPMAS spectra
Since compound 7 except for a thiocarbonyl rather than a
carbonyl unit at C2 of the barbiturate moiety reflects the
molecular structure of dye 4, a hydrogen-bonding pattern
and spatial packing similar to that observed for 4 appears
likely. The corresponding 1H MAS and 13C-{1H}-CP MAS
NMR spectra of 4 and 7, however, reveal noticeable differences.
In both cases, two distinct types of NH protons resonating at
either 9.9 and 10.9 ppm (dye 4) or 11.1 and 12.0 ppm (dye 7) can
be identified as reflecting aggregate formation (Fig. 4).
Fig. 1 ORTEP diagram of the molecular structure of 4 with displace-
ment ellipsoids shown at the 50% probability level reflecting the
asymmetric unit.
1
Based on empirical correlations of the H isotropic chemical
shift with OꢁꢁꢁH, NꢁꢁꢁH or OꢁꢁꢁO distances, respectively, in the
case of linear hydrogen bonds as derived from either X-ray
analysis,60 neutron61 or even electron diffraction,62 higher ppm
values indicate stronger hydrogen bonds due to e.g. shorter
heavy atom distances, thus suggesting that the HNꢁꢁꢁO distances
in the solid compound 7 are about 2.87 A or smaller.
Fig. 2 Selected parts of the 3D network formed in solid dye 4. The
packing results from intermolecular hydrogen bonds between barbituric
acid moieties. For the sake of clarity the p–p-stacking is omitted.
c
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Fig. 4 1H MAS NMR spectra of 4 and 7, acquired at 700.2 MHz and
50 kHz MAS.
Fig. 6 1H–1H 2D DQ MAS NMR spectrum of 4 at 700.2 MHz and
50 kHz MAS, acquired under the following experimental conditions:
162t1 increments at steps of 20 ms, relaxation delay 32 s, 16 transients
per increment, and 40 ms DQ excitation time. The F2 projection is
shown on the top.
Fig. 5 13C-{1H}-CPMAS NMR spectra of 4 and 7 acquired at 176.1 MHz
and 40 kHz MAS while applying a contact time of 3 ms.
the methyl groups. Based on the fact that the NH proton at
10.9 ppm has a reasonably strong cross-peak at 18.2 ppm
(DQ: 10.9 + 7.4 ppm), it is assigned to the crystallographic
H2N site, which at least within a threshold distance of about
4.5 A has more contacts to olefinic protons (see ESIw) than the
H3N site (attributed to the peak at 9.9 ppm).
Similar to the case of 4, the 1H–1H DQ MAS NMR
spectrum of 7 exhibits two strong auto-peaks at 24.2 ppm
(DQ: 12.1 + 12.1 ppm) and 22.2 ppm (DQ: 11.1 + 11.1 ppm),
respectively, which can be attributed to the two NH protons.
However, only one of the two NH protons has a strong cross-
peak with the methyl group (DQ: 12.1 + 2.4 ppm), while an
anticipated peak at 13.5 ppm (DQ: 11.1 + 2.4 ppm) is absent.
In addition, the comparatively stronger cross-peaks with
aromatic or olefinic protons at either 20.6 ppm (DQ: 12.1 +
8.5 ppm), 18.8 ppm (DQ: 12.1 + 6.7 ppm) or 17.8 ppm
(DQ: 11.1 + 6.7 ppm) suggest a different spatial packing of
the (thio)barbiturate moieties in compound 7 than found in
dye 4 (Fig. 7).
Nevertheless, the possible occurrence of bond angles even closer
to 1801 may also contribute to increased 1H chemical shifts of the
NH protons and cannot be excluded. Notably, unlike in all other
dyes, observable 13C-{1H}-CPMAS NMR signals (Fig. 5) of the
methyl groups of the aniline ring are isochronic in the case of
compound 4, even though hydrogen bonding of methyl groups
to the oxygen atoms of the barbituric acid moiety can be found in
the single-crystal structure solution of 4.
1H–1H double-quantum (DQ) MAS NMR is in general a
highly useful and selective approach to identify close contacts
or spatial proximities of structural moieties and thus can be used
to reveal changes of local hydrogen-bonding environments. In
such a two-dimensional experiment, double-quantum coherences
(DQC) due to pairs of dipolar coupled protons are correlated with
single-quantum coherences resulting in characteristic correlation
peaks. Double-quantum coherences between so-called like spins
appear as a single correlation peak on the diagonal (‘‘auto-peak’’)
while a pair of cross-peaks that are symmetrically arranged on
either side of the diagonal reflect couplings among unlike
spins. DQ peaks appear at the sum frequency of the two
coupled spins and therefore often allow for an increased
spectral resolution. Strong signal intensities at short DQ
excitation times of 1–2 rotor periods (20–40 ms) identify
protons in close spatial proximity while weak DQ signals
typically reflect long-distance contacts.63 Indeed, two strong
auto-correlation peaks at 21.6 ppm (DQ: 10.8 + 10.8 ppm)
and 19.8 ppm (DQ: 9.9 + 9.9 ppm), respectively, in the 1H–1H
DQ MAS NMR spectrum of 4 (Fig. 6) are in excellent agreement
with the self-complementary aggregation of the asymmetric unit
moiety with two neighbouring molecules, stabilized by HNꢁꢁꢁO
hydrogen-bonding. In addition, the rather strong cross-peaks at
13.3 ppm (DQ: 10.9 + 2.4 ppm) and 13.2 ppm (DQ: 9.9 +
3.3 ppm) reflect the close spatial proximity of the NH protons to
In the case of 5, the auto-peak of the NH proton at 22.6 ppm
(DQ: 11.3 + 11.3 ppm) in the 1H–1H DQ MAS NMR spectrum
clearly indicates the presence of hydrogen bonded ‘‘dimers’’ in the
solid compound (consistent with dyes 4 and 7), while a similarly
strong cross-peak at 13.9 ppm (DQ: 11.3 + 2.6 ppm) reflects the
contact of the NH proton with the methyl group. Combined with
the 13C-{1H}-CPMAS NMR spectrum, this allows us to propose
the tentative structure of solid dye 5 displayed in Fig. 8.
Due to the absence of NH protons in compound 6, the
formation of ‘‘dimers’’ based on strong directional hydrogen-
bonding is not possible. However, though weakly stabilized
1
aggregates are in principle feasible, the measured H–1H DQ
MAS NMR spectrum of 6 is not sufficiently conclusive to
propose a resulting structure (ESIw). Nevertheless it should be
noted that the 13C-{1H}-CPMAS NMR spectrum of 6 is
somewhat similar to that of dye 5.
c
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Table 1 UV/vis absorption maxima of compounds 4–7 in selected
solvents, the extent of the solvatochromic shift, as well as the solvent
polarity parameter sets of Kamlet–Taft64
n~max(dye)/103 cmꢀ1
a64
b64
p*64
4
5
6
7
Solvents
Acetone
Dioxane
19.59 19.51 19.47 18.03 0.08 0.43 0.71
20.20 20.12 20.12 18.59 0.00 0.37 0.55
1,2-Ethanediol 17.86 17.92 17.98 16.79 0.90 0.52 0.92
HFIPa,b
Toluene
17.08 17.05 17.09 16.31 1.96 0.00 0.65
19.42 19.46 19.66 17.97 0.00 0.11 0.54
Triethylaminec 21.14 20.79 20.12 19.34 0.00 0.71 0.14
Dl/nm
113
4060
106
3740
88
3030
96
3030
Dn~/cmꢀ1
a
b
1,1,1,3,3,3-Hexafluoroisopropanol. Solvent with the highest batho-
c
chromic shift. Solvent with the highest hypsochromic shift.
Table 2 Extinction coefficient of compound 4–7
e/L molꢀ1 cmꢀ1
Fig. 7 1H–1H 2D DQ MAS NMR spectrum of 7 at 700.2 MHz and
50 kHz MAS, acquired under the following experimental conditions:
162t1 increments at steps of 20 ms, relaxation delay 32 s, 16 transients
per increment, and 40 ms DQ excitation time. The F2 projection is
shown on the top.
Dichloromethane
Acetonitrile
Compound
4
5
6
7
61 000
62 000
55 000
57 000
55 000
55 000
55 000
43 000
The individual influences of either specific and/or non-
specific solvent–dye interactions of both HBA and HBD
solvents on the position of the UV/vis absorption band have
been evaluated by means of multiple regression analysis using
the solvent polarity parameter sets of Kamlet–Taft (see ESIw).
Furthermore, non-HBD and HBD solvents were also considered
separately for discussion.
Reasonable correlations can only be obtained, excluding
several solvents (such as aniline, piperidine, trichloroacetic
and trifluoroacetic acid) from correlation analysis. Notably,
non-solvatochromic reactions including protonation (trifluoroacetic
or trichloroacetic acid) or degradation reactions (piperidine) were
observed. When all tested solvents or solely the non-HBD solvents
are considered, the (thio)barbituric acid derivatives 4, 5, and 7 with
their NH-acidic pyrimidone nitrogen group reveal solvatochromic
behaviour as a function of the three solvent parameters, a, b, and
p*, respectively. In contrast, for explanation of the solvatochromic
behaviour of the dimethyl-functionalised barbituric derivative 6, the
influence of the parameter b can be neglected. Furthermore, the
thiobarbituric acid derivative 7 exhibited consistently lower
transition energies compared to the barbituric acid derivatives
4–6 when measured in the same solvent. The collective results are
in good agreement with the solvatochromic behaviour of similar
barbituric acid merocyanine-type dyes.43,45
Fig. 8 1H–1H 2D DQ MAS NMR spectrum of 5 at 700 MHz and
29 762 Hz MAS, acquired under the following experimental condi-
tions: 96t1 increments at steps of 33.6 ms, relaxation delay 7 s, 32
transients per increment. The F2 projection is shown on the top.
Non-specific interactions. Non-specific interactions are
expressed with the Kamlet–Taft’s p* parameter. As expected
for this class of barbituric acid merocyanine-type dyes,43,45
positive solvatochromism (bathochromic shift of the UV/vis
absorption band with increasing dipolarity/polarisability p* of
the solvent as indicated by a large negative s term) with regard
to this parameter is observed. Therefore, the electronically excited
state exhibits a higher dipole moment and is stabilised by solvents
with rather high polarisability/dipolarity. A charge-transfer from
Solvatochromism
The solvatochromic properties of dyes 4–7 were studied in 43
HBD and HBA solvents (see Table 1 and ESIw).
The observed UV/vis absorption bands were found in the range
of 473–587 nm (dyes 4–6) and 517–613 nm (dye 7), respectively,
irrespective of the dye concentration, exhibiting a moderate to
large solvatochromic UV/vis shift range and high extinction
coefficients in different organic solvents (Tables 1 and 2).
c
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Table 3 Values of solvent-independent correlation coefficients a, b,
and s for the Kamlet–Taft equation: n~max = n~max,0 + aa + bb + sp*
(1), including the wavenumber of the reference system (n~max,0), the
correlation coefficient (r), the standard derivation (sd), and the number
of solvents (n) of 4–7. If not mentioned the significance f was below
0.0001
Kamlet–Taft solvent polarity parameter set
HBD and HBA solvents
Scheme 3 Schematic representation of two possible resonance struc-
tures of dye 4 and explanation of the influences of specific and non-
specific solvent–dye interactions on the solvatochromic behaviour.
Dye
n~max,0
a
b
s
r
sd
n
Eqn
4
5
6
7
20.22
20.35
20.57
18.82
ꢀ1.17
ꢀ1.12
ꢀ1.10
ꢀ0.80
0.89
0.46
ꢀ1.94
ꢀ1.87
ꢀ1.82
ꢀ1.81
0.88
0.89
0.92
0.90
0.41
0.34
0.28
0.28
39
40
39
38
5a
5b
5c
5d
the N,N-dimethylamino group (donor) to the oxygen atoms in
positions 4 and 6 of the (thio)-barbituric acid moiety (acceptor)
leads to a zwitterionic resonance structure (Scheme 3). In the
electronically excited state this zwitterionic resonance structure II
dominates the neutral resonance structure I, whereas in the
ground state the reverse case is found.
0.66
Non-HBD solvents
4
5
6
7
20.38
20.48
20.78
18.90
ꢀ0.61
ꢀ0.63
ꢀ0.90
ꢀ0.59
1.64
0.93
ꢀ2.51
ꢀ2.27
ꢀ2.09
ꢀ2.10
0.94
0.91
0.89
0.95
0.27
0.26
0.26
0.19
25
26
26
25
5e
5f
5g
5h
1.07
HBD solvents
4
17.74
17.68
17.69
17.00
1.73
1.65
1.45
1.24
ꢀ0.72
ꢀ0.54
ꢀ0.36
ꢀ0.86
0.93
0.92
0.86
0.93
0.25
0.26
0.29
0.20
14
14
14
13
5i
5k
5l
5
6a
7a
Specific interactions. Specific dye–solvent interactions are
expressed by both, the HBD ability (acidity) a and HBA
ability (basicity) b. Interactions of HBD solvents with a
(thio)barbiturate carbonyl moiety lead to a decrease of their
electron density and an increase of the push–pull character of
the chromophores, thus resulting in a bathochromic UV/vis
shift contribution. Indeed, HBA solvents are able to interact
with the NH-acidic pyrimidone nitrogen groups of dyes 4, 5,
and 7, respectively, which increases the electron density of the
(thio)barbituric acid moiety and therefore, the push–pull
character of the dyes decreases. These effects destabilise the
electronically excited state yielding a hypsochromic UV/vis
shift contribution.
5m
a
f = 0.0004 [eqn (5l)], f = 0.0001 [eqn (5m)].
Brønsted acidic solvents takes place, leading to a significant
change in the push–pull character of the dyes.
Acidochromism
Interactions with acids. In strong acids such as trifluoroacetic
acid (TFA) (pKa = 0.26), trichloroacetic acid (pKa = 0.08), with
HCl (pKa = ꢀ7), or when adsorbed onto acidic surfaces
(e.g., aluminosilicate Siral 30), UV/vis absorption bands in the
range of 360–370 nm (dyes 4–6) and at 392 nm (dye 7) are
observed (Fig. 10).
Thus, with increasing HBD strength and polarisability/
dipolarity of the considered solvent, the observable UV/vis
absorption band shifted bathochromically (Fig. 9).
The appearance of UV/vis absorption bands at 360–370 nm
(dyes 4–6) and at 392 nm (dye 7), respectively, is reversible with
addition of NaOH and isosbestic points at 424 nm (dyes 4–6) and
at 453 nm (dye 7) were observed. Therefore, an acid–base
reaction in conjunction with the loss of the push–pull character
of the compounds takes place. Rezende and co-workers43 already
reported on the interactions of acidic media with a compound
The physicochemical results of the correlation square ana-
lysis are different when only HBD solvents are considered
(Table 3, HBD solvents). Using the Kamlet–Taft parameters
the solvatochromic behaviour seems to be only dependent on b
and p*. According to the work of Rezende and co-workers,43
it is likely that the formation of strong hydrogen bonds
between the (thio)barbituric acid moiety of the dyes in more
Fig. 9 UV/vis absorption spectra and photo of 4 dissolved in five
Fig. 10 Acidochromic behaviour of dye 4 adsorbed onto alumino-
different solvents.
silicate Siral 30 (a) or dissolved in EtOH/H2O (4/3) with HCl addition (b).
c
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Table 4 pKa values of dyes 4–7 determined in EtOH : H2O (ratio
4 : 3/vol/vol)
Dye
R1
R2
X
pKa
4
5
6
7
H
H
CH3
H
H
O
O
O
S
0.54
0.60
0.74
0.39
CH3
CH3
H
obtained by condensation of N,N0-dimethylbarbituric acid and
N,N-dimethylaminobenzaldehyde. Their semiempirical calcula-
tions indicated that a protonation occurs preferentially at the
oxygen atoms of the barbituric acid moiety and not at the
nitrogen atom of the N,N-dimethylamino group.43 The pKa
values of dyes 4–7 concerning this protonation were determined
by a UV/vis titration in an ethanol : water mixture (4 : 3 vol/vol)
(Table 4).
Fig. 11 UV/vis absorption spectra of 4 adsorbed onto five different
surfaces [(a) MgO, (b) ZnO, (c) CaCO3, (d) silica gel 60, (e) alumino-
silicate (see the photo)]. Inset: UV/vis absorption spectrum of 4
adsorbed onto magnesium oxide and multipeak fitting.
A clear trend is observed: the higher the degree of methyl-
substitution of the barbituric acid moiety, the higher is the pKa
value of the dyes caused by the +I effect of the substituted
methyl group(s). The thiobarbituric acid derivative has the
lowest pKa value. This can also be observed when the dyes
were adsorbed onto acidic surfaces. In contrast to dye 7,
protonations of dyes 4 and 6 are observed when adsorbed
onto aluminosilicate Siral 30 (Fig. 10).
recognisable. The UV/vis absorption band (3) is assumed to
correspond to the interaction of the dimethylamino group of
molecules interacting with a positive charge, i.e. Mg2+ or Zn2+
.
To evaluate the obtained values of the longest wavelength
UV/vis absorption band of dyes 4–7 adsorbed onto different
surfaces as shown in Table 5, reference polarity parameters a,
b, p* of these surfaces were required.
Therefore, the Kamlet–Taft surface polarity parameters
(a, b, p*) of these surfaces were determined by means of
well-established probes 1–3 and eqn (2)–(4) (Table 6).
Inserting the reference polarity parameters a, b, and p* of
solvents64 and surfaces (Table 6; Table S4 in the ESIw) in
eqn (5a) (from Table 3: result of the multiple regression
analysis of dye 4) leads to a theoretical value for UV/vis
absorption maxima of dye 4 dissolved in different solvents/
adsorbed onto oxidic surfaces. Compared to the measured
UV/vis wavenumbers of 4, only small differences are identified
(Fig. 12). For compounds 5–7, analogous relationships were
obtained (see ESIw), thus rendering dyes 4–7 a viable addition
to the established perichromic probe dyes 1–3, if one or two
surface polarity parameters cannot be determined by probe
dyes 1–3.
Interactions with bases. Interactions of 4–7 with several bases
were investigated. The NH proton bearing (thio)barbituric acid
dyes 4, 5, and 7 shows a different behaviour depending on the base.
These are solvatochromic effects (pyridine, piperidine), deprotona-
tion of the (thio)barbituric acid moiety (tetra-n-butyl ammonium
fluoride [TBAF], NaOH) as well as degradation reactions
(piperidine, TBAF, NaOH). The dyes 4, 5, and 7 are much more
stable in basic media compared to 6. Thus, for the dimethyl-
functionalised barbituric acid 6 only degradation reactions were
observed using piperidine, TBAF, and NaOH, respectively. The
different behaviour between the NH-bearing and the dimethyl-
functionalised (thio)barbituric acid dyes and bases is exemplified by
the use of TBAF in the ESI.w
Interactions with oxidic and metal surfaces
Table 5 Longest wavelength UV/vis absorption band of 4–7
adsorbed onto oxidic surfaces
Interactions with oxidic surfaces. A variety of interactions
between different surface centres and 4–7 can take place. Thus,
UV/vis absorption spectra with a complex shape were often
obtained. Representative UV/vis absorption spectra of 4
adsorbed onto five oxide surfaces are shown in Fig. 11.
Obviously, simple determination of the global UV/vis absorp-
tion maxima from the UV/vis absorption spectra is not adequate
to characterize perichromic interactions in those cases. Rather, in
order to identify the UV/vis absorption band attributable to
perichromic effects, the UV/vis absorption spectra of dyes 4–7
were fitted with several Gaussian functions. An exemplary result is
presented in Fig. 11 for 4 adsorbed onto magnesium oxide. The
red curve is the original UV/vis absorption spectrum, which could
be reasonably deconvoluted with two Gaussian curves (green).
The resulting blue curve is nearly congruent with the original
UV/vis absorption spectrum. A clear difference between the global
UV/vis absorption maximum (1) at 473 nm and the isolated
longest wavelength UV/vis absorption band (2) at 543 nm is
n~max,4/
n~max,5/
n~max,6/
n~max,7/
Sample
103 cmꢀ1
103 cmꢀ1
103 cmꢀ1
103 cmꢀ1
Silica gel 60
Siral 1.5
Siral 15
Siral 30
Siral 60
Siral 80
MgO
CaCO3
ZnO
TiO2 (anatase)
TiO2 (rutile)
Al2O3
17.61
17.42
17.30
16.98
16.84
16.84
18.40a
18.13a
17.71a
17.09a
17.74a
17.57a
16.92a
16.64
17.70
17.42
17.42
17.04
16.92
16.84
18.25a
17.71a
18.19a
17.40a
18.04a
17.47a
17.15a
16.67
18.08
17.79
17.21
17.36
17.39
17.27
18.44
17.86a
17.71a
18.15a
17.75a
18.02
17.22a
16.89
16.92
16.37
16.31
16.13
15.95
15.95
20.23
16.55
16.73a
16.90a
16.81a
16.44a
16.05a
15.91
WO3
Fe2O3
a
Longest wavelength UV/vis absorption band obtained by multipeak
fitting.
c
680 New J. Chem., 2012, 36, 674–684
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Table 6 Kamlet–Taft’s a, b, and p* values of oxide surfaces used in
this work, determined by means of dyes 1–3 and eqn (2)–(4)
Sample
a (1)
b (2)
0.00
p* (3)
Silica gel 60
Siral 1.5
Siral 15
Siral 30
Siral 60
Siral 80
MgO
CaCO3
ZnO
TiO2 (anatase)
TiO2 (rutile)
Al2O3
1.14
1.34
1.75
1.64
1.71
1.62
0.67
0.78
1.56
1.41
1.48
1.32
0.81
1.01
1.09
1.06
1.12
1.11
0.87
0.97
0.71
0.85
0.86
0.94
1.03
1.21
0.43
0.52
0.44
0.51
0.31
0.26
0.29
0.00
a
a
ꢀ0.10
a
WO3
Fe2O3
1.62
a
a
Fig. 13 UV/vis absorption spectra of 4 adsorbed onto zinc/zinc oxide
and aluminium/aluminium oxide powders. *Traces of ZnO at the Zn
surface.
a
The dye is not recommended for the determination of the parameter,
because of photocatalytical decomposition (titanium dioxides), and
strong UV/vis self-absorption of tungsten(VI) oxide and iron(III) oxide.
eqn (5a)–(5c) (Table 3). Therefore, even the polarity parameters
of sophisticated surfaces could be obtained using dyes 4–7 in
combination with one or two established probe dyes.
Adsorption onto metal surfaces. The interactions of 4 and 7
with surfaces of industrially important metal powders such as
iron, copper, nickel, zinc, aluminium, and steel (X65Cr13, 316 L)
were probed. Verification of adsorption was achieved by
comparison of the intensities of the UV/vis absorption band of
the dye solution before and after the adsorption process. In all
cases, particularly with semi-noble metals like copper, only a
small decrease of the extinction was observed, indicating a low
adsorption tendency of 4 and 7 onto these surfaces. Note that the
dried, dye-modified metal powders were measured in reflection
techniques with a special UV/vis reflectance device and the
pristine metal powder as reference sample. However, useful
UV/vis absorption spectra were obtained only for zinc and
aluminium powders (Fig. 13).
Fig. 12 Correlation of experimental found and calculated wavenumbers
of dye 4 dissolved in 38 different solvents ( ) and adsorbed onto ten
oxidic surfaces (’) (longest wavelength UV/vis maximum after multi-
peak fitting ( )), obtained using eqn (5a) and the polarity parameters of
Table 6 and Table S4 (ESIw), respectively.
The UV/vis adsorption spectra of 4 adsorbed onto both zinc
and zinc oxide as well as onto the aluminium and aluminium
oxide surface exhibit the same shape and identical UV/vis
absorption maxima. Therefore, solely interactions between 4
and the oxidic centres of the metal powders were measurable.
Obviously, valuable UV/vis absorption spectra of dyes 1–3
adsorbed onto the considered surfaces are not available,
rendering an adequate determination of the polarity parameters
of these surfaces rather difficult.
Consequently, coloured oxides including tungsten(VI) oxide
and iron(III) oxide as well as photocatalytic active titanium
dioxides (anatase and rutile) were investigated. For these
oxidic surfaces not all necessary polarity parameters are
available based on the well-established probe dyes 1–3, e.g.
due to photocatalytic decomposition of dye 2 (titanium dioxides),
and strong UV/vis self-absorption of tungsten(VI) and iron(III)
oxide, respectively (see ESIw). Compounds 4–7 were adsorbed
onto these solids. For tungsten(VI) oxide and titanium dioxides
(anatase and rutile) multi-peak fitting was essential because of the
complex shape of the UV/vis absorption bands of 4–7 adsorbed
onto these surfaces. The b values of tungsten(VI) oxide and rutile
were calculated using eqn (5e)–(5m) (Table 3), the longest
wavelength UV/vis absorption bands, and the polarity para-
meters a and p* from Table 5. For tungsten(VI) oxide a b value of
0 and for rutile a b value of 0.4 were obtained. In the case of
anatase, photocatalytic decomposition of the considered dyes
was anticipated. The a value for iron(III) oxide of 1.4 was
calculated using eqn (5c) and a b value of 0.4 was obtained using
Experimental
General
Solvents were dried and freshly distilled before use. N-Methyl
urea (Acros, 97%), malonic acid (ABCR, 99%), trifluoroacetic
acid (Riedel-de Haen, +99%), trichloroacetic acid (Riedel-de
Haen, 99%), piperidine (Alfa Aesar, 99%), TBAF (1 mol Lꢀ1
in tetrahydrofuran, ABCR), 1,1,1,3,3,3-hexafluoroisopropanol
(fluorochem, 99%), 2,2,2-trifluoroethanol (Acros, 99.8%)
were used without further purification. Metals and metal
oxides were used as received (BET, purity and source are
provided as ESIw).
c
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The synthesis and purification of N-methylbarbituric acid
and dyes 1–7 have been described previously.34d,47,65–67
fibre optics was used. Solutions were measured in transmission
with precision quartz cells and an immersion cuvette TSM 5A
(Zeiss) while powder spectra were received in reflection with a
special reflectance device with unmodified powders as a reference.
Spectral analysis was performed with Win-Aspect (version 1.3.1).
For multiple regression analysis the Origin Pro (version 8.1)
statistic program from OriginLab Corporation was used. Multi-
peak fitting was done with IGOR Pro (version 6.2.0.0).
The determination of the pKa values of dyes 4–7 was done in
mixtures of ethanol and water (volume ratio 4 : 3) by titration
with 4.6 mol Lꢀ1 HCl-solution. The UV/vis spectra were recorded
with an immersion cuvette TSM 5A (Zeiss). Simultaneously the
pH-value was determined with the pH-measuring electrode
(Vario pH from WTW, GmbH & Co KG Weilheim, Germany)
(accuracy of measurement: ꢂ0.01 pH). The determination of
pKa-values was done by means of the Henderson–Hasselbalch
equation.68
4, dark violet solid, yield: 92% (80%);47 elementary analysis:
C15H15N3O3 (285.298 g molꢀ1): found (calc.) [%]: C: 62.99
(63.15), N: 14.85 (14.73), H: 5.14 (5.30); 1H NMR (DMSO-d6),
d [ppm]: 3.06 (s, 6H, N-CH3 (aniline)), 6.80 (d, 2H, 12, J12,11
8.63), 7.55 (d, 2H, 11, J11,12 8.66), 7.62 (d, 1H, 9, J9,8 14.92),
7.97 (d, 1H, 7, J7,8 12.92), 8.22 (dd, 1H, 8), 10.98 (s, br, 1H,
NH), 11.04 (s, br, 1H, NH); 13C NMR (DMSO-d6), d [ppm]:
39.8 (N-CH3 (aniline))*, 110.4 (5), 112.2 (12), 119.1 (8), 122.8
(10), 131.6 (11), 150.5 (2), 152.9 (13), 155.6 (7), 156.1 (9),
163.5/163.7 (4/6). Crystals of compound 4 for single-crystal
X-ray structure analysis were obtained by slow diffusion of
water into a dimethylsulfoxide solution of 4.
Sample preparations for perichromic measurements were
done according to the following procedure. Dyes 1, 4–7 were
dissolved in dichloromethane; dyes 2 and 3 were dissolved in
cyclohexane. 5 mL of the dye solution per 0.1 g of the metal/
metal oxide powder was used. The suspensions were shaken
for ten minutes under exclusion of light, decanted and dried
under vacuum.
5, dark violet solid, yield: 83%; elementary analysis:
C16H17N3O3 (299.325 g molꢀ1): found (calc.) [%]: C: 63.98
(64.20), N: 14.01 (14.04), H: 5.57 (5.73); diastereomeric mixture,
1H NMR (DMSO-d6), d [ppm]: 3.06 (s, 6H, N-CH3 (aniline)),
3.16 (s, 3H, N-CH3 (barbituric acid)), 6.80 (d, 2H, 12, J12,11 8.91),
7.56 (d, 2H, 11, J11,12 8.94), 7.57 (d, 2H, 11, J11,12 8.94), 7.65
(d, 1H, 9, J9,8 14.89), 8.00 (d, 1H, 7, J7,8 12.35), 8.02 (d, 1H, 7, J7,8
12.35), 8.25 (dd, 1H, 8), 8.26 (dd, 1H, 8), 11.19 (s, br, 1H, NH),
11.24 (s, br, 1H, NH); 13C NMR (DMSO-d6), d [ppm]: 26.7/27.3
(N-CH3, (barbituric acid)), 39.3 (N-CH3 (aniline))*, 110.2/110.3
(5), 112.2 (12), 119.0/119.2 (8), 122.7/122.8 (10), 131.6/131.7 (11),
150.8/150.9 (2), 152.9/152.9 (13), 155.7/156.2 (7), 156.3/156.5 (9),
162.3/162.4/162.8/163.3 (4/6).
ATR-FT-IR spectra were obtained with a Golden Gate
ATR accessory (LOT-Oriel GmbH & Co. KG, Darmstadt,
Germany) using a BioRad FT-IR 165 spectrometer (Bio-Rad
Laboratories, Philadelphia, PA, USA).
Liquid-NMR-measurements were performed with a Varian
UNITY INOVA 400 and a Bruker Avance 250 NMR spectro-
meter with a 1H-resonance of 400 MHz and 250.13 MHz,
respectively, and a 13C-resonance of 100 MHz and 62.90 MHz,
respectively. The solvent residue signals were used as internal
standards. Coupling constants are given in Hz.
Solid state NMR measurements. 13C-{1H}-CPMAS NMR
spectra were collected at either 9.4 T (Bruker Avance 400) or
16.4 T (Bruker Avance 700) spectrometers, equipped with com-
mercially available double-resonance probes capable of MAS
(magic angle spinning). The spectra were measured either at
100.6 MHz in a 4 mm standard zirconium oxide rotor (Bruker)
spinning at 12 kHz or 176.1 MHz in a 1.3 mm zirconium oxide
rotor (Bruker) spinning at 40 kHz. Cross polarization with
contact times of 3–5 ms was applied to enhance the sensitivity,
where the recycle delay was set to 32 s. All 13C NMR spectra
were collected with TPPM proton decoupling.
1H MAS NMR data were recorded on a 700 MHz Bruker
Avance spectrometer. The experiments were carried out using
either 2.5 mm rotors at MAS spinning frequencies of 30 kHz or
1.3 mm rotors at MAS spinning frequencies of 50 kHz and recycle
delays of 5–64 s. The back-to-back recoupling sequence was used
to excite and reconvert double-quantum coherences,69 applying
the States-TPPI method70 for phase-sensitive detection. Further
details are given in the figure captions of the respective 2D
spectra. All experiments were performed at room temperature.
The spectra were referenced externally with respect to tetra-
methyl silane using tetrakis(trimethylsilyl)silane as secondary
(solid) standard (13C: 3.55 ppm, 1H: 0.27 ppm).
6, dark violet solid, yield: 75%; elementary analysis:
C17H19N3O3 (313.351 g molꢀ1): found (calc.) [%]: C: 64.94
(65.16), N: 13.46 (13.41), H: 5.92 (6.11); 1H NMR (DMSO-d6),
d [ppm]: 3.07 (s, 6H, N-CH3 (aniline)), 3.20 (s, 6H, N-CH3
(barbituric acid)), 6.81 (d, 2H, 12, J12,11 9.0), 7.58 (d, 2H, 11,
J11,12 8.9), 7.69 (d, 1H, 9, J9,8 14.8), 8.06 (d, 1H, 7, J8,7 2.5),
8.28 (dd, 1H, 8); 13C NMR, (DMSO-d6), d [ppm]: 27.4
(N-CH3, (barbituric acid)), 39.2 (N-CH3 (aniline))*, 111.8
(12), 119.1 (8), 123.0 (10), 131.4 (11), 151.2 (2), 152.8 (13),
156.0 (7), 156.4 (9), 161.5/162.1 (4/6).
7, dark blue-violet solid, yield: 79% (91%);47 elementary
analysis: C15H15N3O2S (301.365 g molꢀ1): found (calc.) [%]:
C: 59.09 (59.78), N: 13.67 (13.94), H: 4.91 (5.02), S: 10.48
1
(10.64); H NMR (DMSO-d6), d [ppm]: 3.09 (s, 6H, N-CH3
(aniline)), 6.82 (d, 2H, 12, J12,11 9.37), 7.58 (d, 2H, 11, J11,12
8.98), 7.73 (d, 1H, 9, J9,8 14.45), 8.00 (d, 1H, 7, J7,8 12.50), 8.25
(dd, 1H, 8), 12.09 (s, br, 1H, NH), 12.13 (s, br, 1H, NH); 13C
NMR (DMSO-d6), d [ppm]: 39.5 (N-CH3 (aniline))*, 110.0 (5),
112.0 (12), 119.2 (8), 122.6 (10), 131.9 (11), 153.2 (13), 156.2
(7), 157.7 (9), 161.1/161.8 (4/6), 177.9 (2).
*The N-CH3 (aniline) signal was localised by HMQC
experiments, because it was covered with the DMSO signal
in the 13C NMR spectra.
Instrumentation
For the UV/vis measurements, a diode array spectrometer MCS
Single-crystal X-ray structure analysis of 4 was performed
with an Oxford Gemini S diffractometer at 105 K equipped
400 from Carl Zeiss Jena GmbH (Jena, Germany) with glass
c
682 New J. Chem., 2012, 36, 674–684
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with a graphite monochromator, utilising Cu Ka radiation
(l = 1.54184 A). The structure was solved by direct methods
and refined by full matrix least-square procedures on F2 using
the SHELX71 software package. All non-hydrogen atoms were
refined in the anisotropic approximation against F of all
observed reflections. All hydrogen atoms were added on
calculated positions, except for NH protons which were found
in difference Fourier synthesis. The figures were created using
the ORTEP-III und POV Ray program package.
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Crystal parameters of 4 are reported as follows: dark violet,
C15H15N3O3, M = 285.30 g molꢀ1; monoclinic, P21/n
,
a = 4.5663(2) A, b = 9.6254(5) A, c = 30.3857(14) A,
b = 91.668(5)1, V = 1334.96(11) A3, Z = 4, DC
=
1.420 g cmꢀ3, 8822 reflections collected in the 4.82–65.53
o-range, 2277 unique. The final R was 0.1530 for all data.
Elemental analyses were obtained on a Vario EL supplied
from Elementaranalysengerate GmbH (Hanau, Germany).
Conclusions
The solid state packing and the formation of self-complementary
aggregates within the structure of dyes 4, 5 and 7 are governed by
intermolecular hydrogen bonds between carbonyl oxygens and
NH protons of the barbituric acid moiety as well as the methyl
groups of the N,N-dimethylaniline ring, as evidenced by solid
state NMR and X-ray analysis. (Thio)barbituric acid derivatives
4–7 have a moderate to high solvatochromic range, and high
extinction coefficients in different organic solvents. The solvato-
chromic behaviour of these compounds is dominated by both the
dipolarity/polarisability properties and hydrogen bond donating
ability of the considered medium. In strong acidic media, proto-
nation of the compounds was observed resulting in a strong
hypsochromic shift of the UV/vis absorption band of 4–7. In
strong basic media, the NH-bearing (thio)barbituric acid deriva-
tives 4, 5, and 7 show solvatochromic interactions, deprotonation
and degradation. In addition, the dimethyl-functionalised barbi-
turic acid derivative 6 is less stable in basic media and solely
degradation reactions were observed. Adsorbed onto metal
surfaces, interactions between the dyes and oxidic layers were
detected, while complex UV/vis adsorption spectra were
obtained upon adsorption onto oxidic surfaces. In comparison
with the well-established solvatochromic probe dyes 1–3, the
(thio)barbituric acid derivatives 4–7 comprise rather promising
candidates for probing surface polarity parameters of coloured
oxidic materials.
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22 L. C. Fidale, C. Ißbrucker, P. L. Silva, C. M. Lucheti, Th. Heinze
and O. A. El Seoud, Cellulose, 2010, 17, 937.
Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie is gratefully acknowledged.
We thank P. Kempe for the determination of the BET values.
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