Squaraines as reporter units: Insights into their photophysics, protonation, and metal-ion coordination behaviour

Article abstract of DOI:10.1002/chem.200800300

The synthesis, photophysical properties, protonation, and metal-ion coordination features of a family of nine aniline-based symmetrical squaraine derivatives are reported. The squaraine scaffold displays very attractive photophysical properties for a signalling unit. These dyes show absorption and weakly Stokes-shifted, mirror-image-shaped emission bands in the visible spectral range and there are no hints of multiple emission bands. The mono-exponential fluorescence decay kinetics observed for all the derivatives indicate that only one excited state is involved in the emission. These data stress the interpretation that squaraines can be regarded as polymethine-type dyes. From a coordination chemistry point of view, the squaraines possess four potential binding sites; that is, two nitrogen atoms from the anilino groups and two oxygen atoms from the central C₄O₂ four-membered ring. These coordination sites are part of a cross-conjugated π-system and coordination events with protons or certain metal ions affect the electronic properties of the delocalised π-system dramatically, resulting in a rich modulation of the colour of the squaraines. The absorption band at around 640 nm is blue-shifted when coordination at the anilino nitrogen atoms occurs, whereas coordination to the C₂O₄ oxygen atoms results in the development of red-shift-ed bands. Addition of more than one equivalent of protons or metal cations could additionally entail mixed N,O- or N,N-coordinated complexes, manifested in the development of a broad band at 480 nm or complete bleaching in the visible range, respectively. Analysis of the spectrophotometric titration data with HYPERQUAD yielded the macroscopic and microscopic stability constants of the complexes. Theoretical modelling of the various protonated species by molecular mechanics methods and consideration of some of the title dyes within the framework of molecular chemosensing and molecular-scale logic gates complement this contribution.

Full text of DOI:10.1002/chem.200800300

FULL PAPER
DOI: 10.1002/chem.200800300
Squaraines as Reporter Units: Insights into their Photophysics,Protonation,
and Metal-Ion Coordination ACHTREUBNG ehaviour
Jose V. Ros-Lis,[a] Ramón Martínez-MµÇez,*[a] FØlix Sancenón,[a] Juan Soto,[a]
Monika Spieles,[b] and Knut Rurack*[b]
Abstract: The synthesis, photophysical
properties, protonation, and metal-ion
coordination features of a family of
nine aniline-based symmetrical squar-
aine derivatives are reported. The
squaraine scaffold displays very attrac-
tive photophysical properties for a sig-
nalling unit. These dyes show absorp-
tion and weakly Stokes-shifted, mirror-
image-shaped emission bands in the
visible spectral range and there are no
hints of multiple emission bands. The
mono-exponential fluorescence decay
kinetics observed for all the derivatives
indicate that only one excited state is
involved in the emission. These data
stress the interpretation that squaraines
can be regarded as polymethine-type
dyes. From a coordination chemistry
point of view, the squaraines possess
four potential binding sites; that is, two
nitrogen atoms from the anilino groups
and two oxygen atoms from the central
C4O2 four-membered ring. These coor-
dination sites are part of a cross-conju-
gated p-system and coordination
events with protons or certain metal
ions affect the electronic properties of
the delocalised p-system dramatically,
resulting in a rich modulation of the
colour of the squaraines. The absorp-
tion band at around 640 nm is blue-
shifted when coordination at the anili-
no nitrogen atoms occurs, whereas co-
ordination to the C2O4 oxygen atoms
results in the development of red-shift-
ed bands. Addition of more than one
equivalent of protons or metal cations
could additionally entail mixed N,O- or
N,N-coordinated complexes, manifest-
ed in the development of a broad band
at 480 nm or complete bleaching in the
visible range, respectively. Analysis of
the spectrophotometric titration data
with HYPERQUAD yielded the mac-
roscopic and microscopic stability con-
stants of the complexes. Theoretical
modelling of the various protonated
species by molecular mechanics meth-
ods and consideration of some of the
title dyes within the framework of mo-
lecular chemosensing and molecular-
scale “logic gates” complement this
contribution.
Keywords: dyes/pigments · fluores-
cence · metal ions · protonation ·
squaraines
Introduction
nonlinear optics,[6] photodynamic therapy,[7] biochemical la-
belling[8] and chromo- and/or fluorogenic probes[9] for vari-
ous targets such as pH,[10] cations[11] and neutral molecules[12]
as well as for the study of self-assembled dye aggregates.[13]
The popularity of squaraines is based on their favourable
spectroscopic properties. These dyes possess typically
narrow and very intense absorption bands (with molar ab-
sorption coefficients e>105) at the red end of the visible
spectral window, and rather resonant yet bright fluorescence
bands of mirror-image shape with fluorescence quantum
yields F>0.1 in polar and aqueous solvents.[14] Attracted by
these features, we have also utilised squaraine derivatives in
a number of chemical signalling systems for cations,[15,16]
anions[17] and neutral molecules.[18,19] Besides functionalisa-
tion of the squaraine backbone with suitable receptor units
in the classical sense of molecular probes following the
“binding site-signalling unit” approach,[20] we especially ex-
plored alternative sensing schemes by exploiting the squar-
In the last 30 years, squaraine dyes[1,2] have attracted strong-
ly increasing attention as deeply coloured and fluorescent
units in many optochemical applications, such as optical re-
cording,[3] solar energy conversion,[4] electrophotography,[5]
[a] Dr. J. V. Ros-Lis, Prof. R. Martínez-MµÇez, Dr. F. Sancenón,
Dr. J. Soto
Instituto de Química Molecular Aplicada
Departamento de Química, Universidad PolitØcnica de Valencia
Camino de Vera s/n, 46022 Valencia (Spain)
Fax : (+34)963-877-349
E-mail: rmaez@qim.upv.es.
[b] M. Spieles, Dr. K. Rurack
Div. I.5, Bundesanstalt für Materialforschung und -prüfung (BAM)
Richard-Willstätter-Strasse 11, 12489 Berlin (Germany)
Fax : (+49)30-8104-5005
E-mail: knut.rurack@bam.de
Chem. Eur. J. 2008, 14, 10101 – 10114
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ainesꢁ unique tendency to undergo reversible bleaching reac-
tions upon nucleophilic attack.[16–18]
SNO3,[24] SNO4,[24] SNO5,[24] SNS2O2,[15,19] SBu,[31] SAb,[18]
SJu,[32] SCar[18] and SJuOH.[32] All the derivatives were char-
acterised by standard techniques, including HRMS and
NMR spectroscopy. In the latter case, the characteristic dou-
Apart from the reactivity mentioned in the previous para-
graph, squaraines show several other peculiarities, arising
from the fact that they can be regarded formally as donor–
acceptor–donor-substituted (intramolecular charge-transfer,
ICT) chromophores, as doubly cross-linked polymethine-
like p-systems or as biradicaloid chromophores (Scheme 1).
Scheme 1. Possible formal electronic structures of squaraines. In B, only
the limiting mesomeric structures are shown, cf. reference [72].
Despite the large number of application-oriented publica-
tions (vide infra), squaraines have also received considera-
ble treatment in fundamental studies that make use of NMR
spectroscopy,[21] X-ray crystallography,[22] quantum chemical
calculations[23] and in various photo-physical studies as a
function of solvent nature,[24,25] chemical structure[26] or tem-
perature.[25] However, several contradictory interpretations
that have appeared throughout the years in the literature
within the context of the use of squaraines as functional
dyes or signalling units in molecular probes and other supra-
molecular ensembles suggest that further insight into the
photophysical performance of squaraine reporters is urgent-
ly required, especially when the use of such chromophores is
extended to areas such as molecular logics,[27,28] multitopic
or allosteric and multichannel chemosensors.[29,30] We report
here the photophysical properties, and a rationalisation of
the colour modulations observed upon interaction with pro-
tons and metal cations, of a family of squaraine-based recep-
tors.
blets in the aromatic region at approximately 6.7 and
8.2 ppm were detected (except for SJu and SJuOH).
As described in the Introduction, the data in Table 1 and
Figure 1 stress the fact that the title dyes show the typical
spectroscopic features of squaraines, that is, narrow and in-
Table 1. Photophysical properties of squaraines in MeCN at 298 K.
abs
max
em
max
l
[nm]
loge [mꢁ1 cmꢁ1
]
l
[nm]
Ff
tf [ns]
SBu
642
636
634
636
640
634
629
662
666
5.20
5.16
5.33
5.18
5.49
5.08
5.01
5.29
5.45
661
658
654
661
661
656
651
683
681
0.087
0.078
0.099
0.074
0.129
0.085
0.138
0.041
0.415
0.46
0.38
0.48
0.43
0.67
0.47
0.71
0.23
2.45
SNO3
SNO4
SNO5
SNS2O2
SAb
SCar
SJu
SJuOH
Results and Discussion
Synthesis and photophysical properties: Nine representative
squaraine dyes were prepared following a general synthetic
route, by condensation of aniline derivates with squaric acid
in butanol/toluene 1:1 v/v with azeotropic removal of
water.[15,18,19,24,31,32] The chemical structures and nomencla-
ture of the prepared compounds are shown here; namely
tense absorption bands with maxima between 630 and
670 nm, full widths at half maximum (fwhm) of 854ꢀ
18 cmꢁ1 (except for SJuOH: fwhm=745 cmꢁ1), and a weak
vibrational shoulder centred at 1434ꢀ30 cmꢁ1 (except for
SJuOH at 1290 cmꢁ1) on the high-energy side. The fluores-
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Chem. Eur. J. 2008, 14, 10101 – 10114

Squaraines as Reporter Units
FULL PAPER
Figure 2. Plot of tf versus Ff; dotted line exemplifies a linear regression
with r=0.998.
a particularly low value. In the case of SJuOH, the deviation
is most likely connected to the stabilisation of the chromo-
phore by internal hydrogen bonds between the hydroxyl and
carbonyl groups and the reduction of torsional motions
around single bonds b1 and b1’, which connect the phenyl
rings and the central four-membered ring (Scheme 2). To
Figure 1. Normalised absorption and fluorescence spectra of the title
dyes in acetonitrile at 298 K. Top (from left to right): SCar (solid line),
SAb (symbols), and SBu (dotted line); middle (from left to right): SNO4
(solid line), SNO5 (solid squares), SNO3 (dotted line), and SNS2O2 (open
circles); bottom: SJu (solid line) and SJuOH (dotted line).
cence spectra of mirror-image shape are rather resonant,
with Stokes shifts between 400 cmꢁ1 (for SJuOH) and
560 cmꢁ1 (for SNO3). The fluorescence excitation spectra
match the lowest energy absorption bands in all cases. Fluo-
rescence lifetime measurements yielded monoexponential
decay kinetics for all the dyes. These last two findings show
that we could not detect any hints for multiple emission
bands[26,33] and indicate that only one excited-state species
emits. These observations are supported by an analysis of
the fluorescence quantum yields and lifetimes by means of
the radiative (kr) and nonradiative (knr) rate constants ac-
cording to Equations (1) and (2).
kr ¼ Ff=tf
ð1Þ
ð2Þ
knr ¼ ð1ꢁFfÞ=tf
The values of kr are very similar for all the compounds in-
vestigated here (0.19ꢀ0.02 nsꢁ1, see Table 2), and a good
linear relationship is obtained between Ff and tf (Figure 2).
From a conformational and transitional point of view, the
emissive states of all the dyes are thus very similar. Differ-
ences, however, are observed for the nonradiative rate con-
stants. Here, SJu shows a particularly high value and SJuOH
Scheme 2. Flexible bonds in the chromophore and their bridging in SJu
and SJuOH.
get better access to the underlying mechanism for the other
squaraines, we focused our attention first on the theory of
the energy-gap law.[34] According to this rule, the degree of
internal conversion is a function of the energy difference be-
tween the excited state and the corresponding ground state,
as reflected by the emission spectrum/maximum. A reduc-
tion in energy gap between these states accelerates internal
conversion. If internal conversion is the only major nonra-
diative process that contributes to quenching, a plot of lnknr
versus the emission maximum should yield a linear correla-
tion. For the present family of compounds, verification of
Table 2. Additional photophysical parameters of the title compounds.
em
max
kr [nsꢁ1
]
knr [nsꢁ1
]
n˜ [cmꢁ1
]
lnknr
SBu
0.19
0.21
0.21
0.17
0.19
0.18
0.19
0.18
0.17
1.98
2.43
1.88
2.15
1.30
1.95
1.21
4.17
0.24
15082
15159
15243
15174
15095
15243
15384
14619
14619
21.4
21.6
21.4
21.5
21.0
21.4
20.9
22.2
19.3
SNO3
SNO4
SNO5
SNS2O2
SAb
SCar
SJu
SJuOH
Chem. Eur. J. 2008, 14, 10101 – 10114
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R. Martínez-MµÇez, K. Rurack et al.
such a relationship has to exclude SJuOH, based on the rea-
sons given above, and SNS2O2, because of the non-negligible
contribution of the d orbitals of the sulfur atoms neighbour-
ing the nitrogen atoms. For SBu, SNO3, SNO4, SNO5, SAb,
SCar, and SJu, a satisfactory linear relationship was indeed
found (Figure 3). If we further consider that the dyes obey-
em
max
Figure 3. Plot of lnknr versus n˜ ; data set excludes points for SNS2O2
and SJuOH (see text), dotted line exemplifies a linear regression with r=
0.935.
ing the relationship shown in Figure 3 carry partly bulky
substituents, which are either very flexible (e.g., SCar) or
more restricted (e.g., SNO3), we can conclude that such sub-
stituents attached to the aniline nitrogen atoms have a negli-
gible influence on torsional motions around bonds b1 and
b1’. Only direct fixation of these bonds by internal hydrogen
bonding as in SJuOH evidently makes
a difference
(Scheme 2). Furthermore, the negligible influence of the
size and topology of the substituents at the aniline groups,
and the fact that SJu, for which the aniline groups are fixed
in a planar conformation, also fits well into the energy-gap
correlation suggest that torsional motions around bonds b2
and b2’ also play only a very minor role in excited state de-
activation (Scheme 2).
Figure 4. Frontier MOs of SBu and model compounds M1–M5 as ob-
tained from the DFT quantum chemical calculations; dashed boxes indi-
cate main region of MO localisation. Similar features of MO localisation
as observed here for SBu have also been found for other squaraines with
more elaborate endgroups, see for example references [73,74].
To get a better insight into the chromophoric nature of
the title dyes, SBu and a series of model compounds M1–
M5 (Figure 4) were treated theoretically by using density
functional theory (DFT) with the B3LYP functional and the
6-31G basis set as implemented in Gaussian 03. The main
question is whether squaraines such as SBu more closely re-
metry and delocalisation of the MOs.[40] Here, the situation
is distinctly different for M1 and M2. Whereas both
HOMO and LUMO are symmetric and delocalised over the
entire p system in cyanine M1, they are largely localised on
the respective donor (dimethylaniline) and acceptor (meth-
ylpyridinium) fragments for a classic D–A dye such as M2.
Consequently, M1 displays narrow and structured absorp-
tion and emission bands and a negligible solvatochrom-
ism.[41] In contrast, M2 shows considerably broader absorp-
tion and emission bands (especially in polar solvents) and a
pronounced solvatochromism due to an ICT process involv-
ing donor and acceptor moieties.[42] Comparison of SBu with
a true squarylium cyanine (M3)[43] and other symmetric D–
A–D dyes (M4, M5)[44,45] indicates that SBu more closely
resembles cyanine dyes such as M1 and M3 than ICT or D–
A(–D) dyes such as M2, M4, and M5, the last two also
showing similar spectroscopic features to M2, that is, broad,
solvatochromic bands.
semble cyanine dyes or donor–acceptorACHTRE(UNG –donor)-substituted
stilbene, coumarin or styryl dyes, which are commonly re-
ferred to as “D–A(–D) dyes” in the literature,[35,36] because
they show all the features of ICT dyes.[37] When invoking an
analysis of the frontier molecular orbitals (MOs) of the ge-
ometry-optimised ground-state structures, two features are
important. According to the triad theory of organic dyes,
the first key feature of a polymethinic character is an alter-
nating p electron density, that is, HOMO and LUMO
should show an increasing tendency for alternation upon ap-
proximating the ideal polymethine state.[38] Figure 4 reveals
that both of the simplest models studied here, M1 and M2,
largely fulfil this requirement (see also discussion in refer-
ence [39]). The second important feature relates to the sym-
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Squaraines as Reporter Units
FULL PAPER
Our present experimental and theoretical results, in com-
bination with the lack of a pronounced solvatochromism as
found by Cornelissen-Gude et al.[25] and the negligible influ-
ence of intersystem crossing,[46] suggest that the states of the
squaraines involved in the optical transitions have a more
polymethinic than intramolecular charge-transfer (ICT)
character, often associated with donor–acceptorACTHRE(UNG –donor)-
substituted dyes. In contrast, the moderate fluorescence
quantum yields in acetonitrile observed for all the dyes
except SJuOH and the strong “positive solvatokinetic be-
haviour” (a decrease in emission yield with increasing sol-
vent polarity) reported for such compounds previously[25] in-
dicate that an intramolecular charge separation process in
the excited state, involving large amplitude motions around
b1 and b1’, is nonetheless important. However, this process
does not produce another emissive species, but leads to the
population of an entirely non-emissive species, most likely
connected to the concept of twisted intramolecular charge-
transfer (TICT) states.[47] Support is also furnished by inves-
tigations of solute–solvent complexes[21] or squaraines that
are pretwisted, because of the introduction of bulky sub-
stituents in ortho position to the b1,b1’ bonds,[26] both of
which revealed decreased fluorescence quantum yields.
Figure 5. Spectroscopic behaviour of SNO3 in acetonitrile as a function of
&
^
increasing proton concentration. =initial spectrum, =spectrum at
equimolar ratios of H+ and SNO3 and =endpoint spectrum; dashed
~
lines=intermediate titration spectra.
Spectroscopic studies involving protons: From the point of
view of chemical addressability, the popularity of squaraines
as reporter molecules is mainly based on the fact that they
possess four potential sites of interaction with chemical spe-
cies: two nitrogen atoms from the aniline groups and two
oxygen atoms from the central C4O2 ring. These sites are
part of the same conjugated p system, and interaction with,
for instance, protons or metal ions will dramatically affect
the electronic properties of the chromophore. As a conse-
quence, colour modulations are to be expected (vide infra).
As a first step to assess the influence of the different sub-
stituents at the aniline on these spectroscopic responses, the
protonation behaviours of SNS2O2, SBu, SJu, SJuOH,
SNO3, and SCar were studied in acetonitrile.
The first effect that we will discuss is a hypsochromic shift
of the squaraine absorption band upon addition of one
equivalent of acid, resulting in a colour change from blue to
violet. The titration spectra in Figure 5 show one example:
the appearance of a new absorption band at around 560 nm
with a well-defined isosbestic point for SNO3 in the presence
of up to one equivalent of protons. In accordance with the
shift observed upon exchange of one of the dimethylamino
methinic branch. This interpretation is supported by the fact
that deeply coloured, positively charged, hemicyanine dyes
that carry an amino crown ether group as one terminal unit
undergo strong decolouration upon cation binding.[51] Simi-
lar colour changes upon protonation of the aniline moieties
abs
max
are found for SJuOH for equimolar amounts of acid (l
=
526nm). In this case, the existence of intramolecular hydro-
gen bonds between the OH groups and the oxygen atoms
from the C4O2 central ring decreases the basicity of the
oxygen atoms, favouring protonation at the aniline, but only
one protonation step is observed in the acid concentration
range studied (0.1 mm<cH+ <1 mm).
Features opposite to those found for SNO3 are recognised
when SJu is titrated with up to equimolar amounts of acid.
Again, the typical squaraine band centred at 660 nm disap-
pears. However, for SJu a bathochromically shifted absorp-
tion band develops at about 690 nm, with well-defined iso-
sbestic points again indicating a straightforward equilibrium
SJu+H+QSJuH+ (Figure 6). This bathochromic shift is at-
tributed to the protonation of an oxygen atom of the central
ring. The change in protonation preference is probably con-
nected to the steric fixation of the anilineꢁs nitrogen atom in
groups by a methoxy group, that is, exchange of a strong
abs
max
donor in SDMA with l =627 nm in chloroform for a
weaker donor in USNO, absorbing at 579 nm;[48,49] we attri-
bute this shift to exclusive protonation of one nitrogen
atom. It is known that the small macrocycle in SNO3 stabil-
ises protonation of the nitrogen atom by hydrogen bonding
interactions in nonprotic solvents.[24,50] At higher acid con-
centrations, gradual bleaching of the solution occurs because
of the disappearance of the band at approximately 560 nm
(Figure 5). These changes are attributed to the step from
SNO3H+ to SNO3H22+, switching off also the second poly-
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R. Martínez-MµÇez, K. Rurack et al.
Figure 7. Spectroscopic behaviour of SBu in acetonitrile as a function of
Figure 6. Spectroscopic behaviour of SJu in acetonitrile as a function of
&
^
increasing proton concentration. =initial spectrum, =spectrum at
&
^
increasing proton concentration. =initial spectrum, =spectrum at
equimolar ratios of H+ and SJu and =endpoint spectrum; dashed
~
~
equimolar ratios of H+ and SBu and =endpoint spectrum; dashed
lines=intermediate titration spectra.
lines=intermediate titration spectra.
the julolidine group, which acts as a counterforce for the
transition of a planar (quasi-sp2) aniline nitrogen into a tet-
rahedral (quasi-sp3) anilinium nitrogen. Additionally, the ni-
trogen atomsꢁ lone electron pairs are better delocalised in
the entire julolidine moiety than in an alkylaniline group.
The latter also leads to an increase of the electron density at
the central oxygen atoms. At equimolar ratios, one oxygen
atom is thus protonated. With a higher excess of acid,
bleaching of the band at around 690 nm occurs and a new
band is formed at 470 nm. The latter is very broad and con-
siderably weaker in intensity. Such changes correspond to a
loss of the cyanine nature of the chromophore and suggest
that the doubly protonated species possesses a certain
charge-transfer character, hence the second protonation step
occurs at a nitrogen atom. Interestingly, SCar shows a very
similar behaviour, that is, a bathochromic shift up to equi-
molar ratios of acid. However, because the neighbouring
carbamate groups distinctly reduce the tendency for aniline
protonation, a second protonation step could not be ach-
ieved here in the acid concentration range studied (0.1 mm<
cH+ <1.2 mm).
A third type of response is registered for SNS2O2 and
SBu. Addition of up to one equivalent of acid results in the
simultaneous development of blue- and red-shifted bands,
indicating rather similar basicities of N and O atoms and
statistical protonation at both sites. Furthermore, addition of
an excess of acid to SNS2O2 as well as SBu induces the de-
velopment of a new band at 470 nm, suggesting the forma-
tion of species protonated at one nitrogen and one oxygen
atom (Figure 7).
excited-state charge shift/rearrangement reactions leading to
fast nonradiative decay through conical intersections seem
to be responsible for efficient quenching.[53] Table 3 summa-
rises the observed relationship between the changes in ab-
sorption and the site of protonation.
Table 3. Colour modulation and protonation sites.
Protonated atom
lmax [nm] approx.
none protonated
N
640
56
O
6
N and O
N and N
470
no band
The extent of the colorimetric shift in the squaraine
family is evidently related to protonation at different sites
and a delicate balance between the basicity of the nitrogen
and oxygen atoms in the chromophore. A detailed analysis
of the spectrophotometric titration data has thus been car-
ried out with the HYPERQUAD program (see Experimen-
tal Section). Macroscopic protonation constants logKn were
calculated for the representative compounds SNS2O2, SBu,
SJu, SJuOH and SNO3 and are shown in Table 4. As stated
above, SJuOH is only protonated at the oxygen, whereas
SJu, SNO3, SNS2O2 and SBu are protonated twice. More-
over, for SJu and SNO3 the two protonation steps are well
separated and it is possible to determine the stability con-
stants logK1,2 for individual protonation at the nitrogen and
oxygen atoms. In contrast, in the case of SNS2O2 and SBu,
Unfortunately, none of the protonated species showed a
measurable fluorescence, so it was not possible to obtain ad-
ditional information by employing this technique. However,
such a loss of fluorescence is in agreement with the fact that
already neutral but asymmetric dyes such as USNO show
strongly quenched emission (Ff <10ꢁ3 already in a medium
polar solvent such as CHCl3).[48] Moreover, in their protona-
tion studies of SOHOH, Das et al. found that only the neu-
tral derivative is fluorescent.[52] In the protonated species,
Table 4. Logarithm of the macroscopic (K) and overall (b) protonation
constants for selected squaraine derivatives in acetonitrile (298 K);
logb=ÆlogKn.
Reaction
SNS2O2 SBu
SJu
SJuOH SNO3
[L]+H+Q[LH]+
logK1 5.42(9) 5.9(1) 5.3(1) 5.9(6)
6.3(7)
4.7
11.1(1)
[LH]+ +H+Q
A
2.9
2.9
–
–
[L]+2H+Q
[LH2]2+
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Squaraines as Reporter Units
FULL PAPER
Table 6. Proton affinities for SBu, SJu and SNO3 calculated using the
PM3 semiempirical molecular model through subtraction of the energy
of the deprotonated [E(sq)] form to the energy of the protonated mole-
the complex spectral changes shown in Figure 7 indicate
that the two protonation steps at the nitrogen and oxygen
atoms are not well separated, leading to macroscopic/appar-
ent logK1 values of 5.42 and 5.9, respectively, characterizing
the basicity of the molecules as a whole. These macroscopic
constants, however, fail to provide information on the spe-
cific proton-binding sites. Nevertheless, site-specific basici-
ties can be obtained by determination of the microscopic
constants, logkN/O. These data are shown in Table 5. In com-
cule [EACHTER(UNG sqHn)].
Number of
protons (n)
Protonated
atoms
E
(sqHn)ꢁE(sq) [Kcalmolꢁ1
]
SBu
SJu
SNO3
0
1
–
O
N
0
0
0
74.677.1
60.3
93.8
88.8
90.6
2
3
O-O
N-N
N-O
N-O-O
N-N-O
248.0
221.6247.7
194.1
424.1
428.1
248.9
243.1
209.6
219.1
446.4
452.1
214.3
437.4
443.9
Table 5. Logarithm of the macroscopic (K) and microscopic (k) protona-
tion constants of SNS2O2 and SBu in acetonitrile (298 K).
Reaction
[SNS2O2]+H+Q
[SBu]+H+Q[SBuH]+
logK
[SNS2O2H]+ 5.42(9) 4.61
5.9(1) 5.74
logkN logkO logkN,O logkO,N
A
G
5.34
5.37
3.91
3.063.43
3.18
N
A
behaviour. Whereas N-protonation is favoured in SBu and
SNO3, protonation at an oxygen atom is preferred in SJu.
The calculations for the second protonation step suggest
that in the case of SBu and SJu the complementary atom is
favoured, leading to N+H,OH species in accordance with
the experiments. In addition, the results on SNO3 in Table 6
support N,N-protonation. The third protonation then is en-
ergetically highly unfavourable and could also not be ach-
ieved experimentally for the squaraines studied.
bination with the data of Table 4 it is evident that protona-
tion at the oxygen atoms requires similar acidities (logkO
ꢂ5.3 for SNS2O2 and SBu as well as logK=logkO =5.3 for
SJu), whereas protonation at the nitrogen atoms varies for
all of the dyes. This observation is in agreement with the va-
riety of different substitution patterns at the aniline group,
ranging form logK=logkN =6.3 for SNO3 (stabilisation by
internal hydrogen bonds, see above) to logkN =4.61 for
SNS2O2 (aggravated protonation due to neighbouring sulfur
atoms).
If we look to the second protonation step (logK2), the
values are two to three orders of magnitude lower than the
first protonation constants, in agreement with the additional
electrical work necessary to protonate species that are al-
ready positively charged. Additionally, a detailed look at the
squaraine derivatives able to be protonated twice allows the
identification of two different behaviours. Whereas SNS2O2,
SBu and SJu finally yield N,O-protonated species with
logK2 ꢂ3 for the HL+ +H+QH2L2+ process, the HL+ +H+
QH2L2+ process of SNO3 with logK2 =4.7 produces an N,N-
protonated species. This difference in the second protona-
tion constant can be explained by electrostatic repulsion,
which should be larger for the conjugated N,O-protonation
sites than for the cross-conjugated N,N-protonation sites.
As additional support, the relative hydrogen-bond donat-
ing and accepting abilities (i.e., basicities) of the N and O
atoms were derived in a comparatively simple way from the
protonation energies in the gas phase at particular molecular
sites of the PM3-optimised ground state geometries of SBu,
SNO3 and SJu. The proton affinities were calculated by sub-
tracting the energy of the neutral form from the energy of
Spectroscopic studies involving metal ions: The protonation
experiments provided a first valuable insight into the differ-
ent basicities of the squarainesꢁ heteroatoms as a function of
substituents. To elucidate the additional effect of coordina-
tive forces on the signalling expression of squaraines, we
next investigated the title compoundsꢁ response in the pres-
ence of various metal ions in acetonitrile. Again, spectro-
scopically similar changes as described above can occur, that
is, a blue shift of the band at around 640 nm for cation coor-
dination to the anilino nitrogen or a red-shifted band upon
coordination to the C2O4 oxygen atoms. If, additionally, a
second metal ion is bound, either mixed N,O- or N,N-coor-
dinative complexes can be formed, resulting in the develop-
ment of a broad band at 480 nm or bleaching. The colori-
metric behaviour of the ligands in the presence of one
equivalent or an excess of certain metal cations is summar-
ised in Table 7. Also, for selected systems the stability con-
stants for the formation of complexes have been determined
and are shown in Table 8.
Among the alkali (Li+, Na+, K+) and alkaline earth
metal (Mg2+, Ca2+, Ba2+) cations tested, only Ba2+ was able
to induce changes, and only with the SNO5 derivative. A
hypsochromic shift of 50 nm with a change of colour from
blue to purple was observed upon addition of one equiva-
lent of Ba2+ (Figure 8a). This behaviour agrees with an in-
teraction of the metal ion through the macrocycle and is in
accordance with the ability of N-substituted A18C6to bind
the various protonated species, DEH =E
(sqHnn+)ꢁE(sq).
SBu, SNO3 and SJu were taken as representative examples
because they display a clearly different behaviour upon pro-
tonation (see above). The calculated results are shown in
Table 6. Here, the lower the value of DEH the higher the
ability of the protonation site to accept hydrogen bonds,
that is, the more basic is the site. Table 6reveals that the
theoretical results are in agreement with the spectroscopic
Ba2+ [54] Addition of an excess of Ba2+ resulted in no further
.
change, indicating that SNO5 is only able to bind one cation.
Apparently, the binding constant for A18C6-Ba2+ (logK=
4.55) is too small to overcome the electrostatic repulsion for
a positively charged complex to bind another twofold posi-
Chem. Eur. J. 2008, 14, 10101 – 10114
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10107

R. Martínez-MµÇez, K. Rurack et al.
Table 7. Summarised UV/vis absorption behaviour of the squaraine derivatives upon addition of Ba2+, Hg2+, Fe3+, Pb2+, Cu2+ and H+.
Ba2+ Hg2+ Fe3+ Pb2+ Cu2+
1 equiv excess excess 1 equiv
H+
excess
excess
1 equiv
1 equiv
excess
1 equiv
excess
1 equiv
SNS2O2
SBu
SAb
–
–
–
–
–
–
–
560
–
–
–
–
–
–
–
–
560
–
560
670
560/670
560
560
560/670
560/670
560
bleaching
670
560/670
560
670
670
–
–
–
–
–
–
–
–
560
670
670
560
560
670
670
670
670
No
560/670
560/670
–
670
560
560
–
480
480
–
480
560
bleaching
–
–
670/560
670/560
560
670/560
670/560
500
480
480
500
500
480
480
480
480
SJu
SJuOH
SNO3
SNO4
SNO5
SCar
560
560
500
–
–
560
560
bleaching
–
560/670
560/670
560
670/560
670/560
670/560
670
670/560
670/560
670/560
670
560
560
560
–
–
670
670
670
670
Table 8. Logarithm of the stability constants for the interaction of the squaraine derivatives with Ba2+, Pb2+, Hg2+ and Cu2+ in acetonitrile (298 K).
Reaction
SNS2O2
SBu
SAb
SJu
SJuOH
SNO3
SNO4
SNO5
[L]+Ba2+
Q
A
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
4.55(1)
7.9(7)
4.0
11.9(3)
4.80(2)
–
[L]+Pb2+
Q
A
logK1
logK2
Logb
logK1
logK2
Logb
logK1
logK2
Logb
3.69(3)
–
–
4.91(8)
–
–
4.54(9)
–
–
4.49(7)
–
–
[LPb]2+ +Pb2+
Q
N
Q
Q
G
N
9.2(3)
6
15.9(6)
7.0(3)
3.1
4.45(5)
.7
–
6.6(0)
5.66 5.46.9
12.1(8)
4.29(5)
–
–
5.3(6)
–
–
4.93(8)
–
–
N
Q
G
–
Q
Q
U
–
U
6.3(8)
6.1(5)
6.8(1)
6.5(2)
6.6(2)
6.44(7)
C
Q
E
.0
5.8
5.1
5.5
Q
G
10.1(3)
12.4(3)
11.7(4)
11.7(0)
12.3(0)
11.7(5)
11.98(6)
lent character, which results in a more efficient capture of
the electron pair of the nitrogen.[55] These favourable bind-
ing features are reflected in the stability constant obtained
for the formation of the [SNO5Pb]2+ complex (logK=7.9),
which is almost two orders of magnitude larger than that for
the formation of [SNO5Ba]2+ (see Table 8). Moreover, this
strong interaction of Pb2+ with SNO5 allows a second cation
to be bound in the second crown with logK=4.0 for the
process [LPb]2+ +Pb2+ [LPb2]4+, see Table 8.
QACHTREUNG
Addition of Cu2+, Fe3+ and Hg2+ induced colour changes
for all the squaraine dyes, with Hg2+ exhibiting the most in-
teresting behaviour. In the case of SBu, the butyl chains do
not favour, in general, the interaction of the anilino nitrogen
atoms with cations and the presence of Hg2+ leads to the
evolution of a red-shifted band at 656 nm that is assigned to
coordination of Hg2+ with one oxygen atom of C4O2. Similar
coordinative behaviour has been found for SCar. Moving to
receptors with better aniline ligating units we also start to
observe a band at 560 nm, indicative of partial coordination
at the aniline. This band is not very important for SAb (Fig-
ure 9a), but significant for SNO4 (Figure 9b), indicating
mixed nitrogen and oxygen coordination. Accordingly,
SNS2O2 only forms species in which Hg2+ is bound at the
macrocycle, as indicated by the development of the hyspo-
chromically shifted band at 582 nm (upon addition of up to
one equivalent of Hg2+, Figure 9c). In this case, addition of
more than one equivalent of Hg2+ resulted in a bleaching of
the solution, suggesting the formation of species where two
Hg2+ ions occupy both macrocycles. The stability constants
calculated for the formation of the Hg2+ complexes with
SNS2O2, SBu, SAb, SJu, SJuOH, SNO3, SNO4 and SNO5 are
Figure 8. Spectroscopic behaviour of SNO5 in acetonitrile upon increasing
amounts of a) Ba2+, b) Pb2+. a) =initial spectrum, =endpoint spec-
&
^
&
trum, dashed lines=intermediate titration spectra. b) =initial spectrum,
2+
~
^
=spectrum at equimolar ratios of Pb and SNO5, =endpoint spec-
trum, dashed lines=intermediate titration spectra.
tively charged species. Pb2+ also induces changes for SNO5
(Figure 8b), and to a minor extent with SNO4 and SNO3.
This result is in agreement with the larger radius of Pb2+
,
which apparently fits well into the larger cavity of the SNO5
receptor. Figure 8b shows that the spectra of SNO5–Ba2+
and SNO5–Pb2+ are similar, with the development of a band
at 576nm. However, in the case of Pb 2+ and SNO5, the pres-
ence of more than one equivalent of metal ion leads to the
disappearance of the absorption band. The latter can be at-
tributed to a stronger Pb2+···N interaction with a more cova-
10108
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10101 – 10114

Squaraines as Reporter Units
FULL PAPER
Figure 9. Spectroscopic behaviour in acetonitrile upon increasing amounts of Hg2+ for a) SAb, b) SNO4 and c) SNS2O2. a) and b) =initial spectrum,
&
2+
~
&
^
^
=endpoint spectrum, dashed lines=intermediate titration spectra. c) =initial spectrum, =spectrum at equimolar ratios of Hg and SNS2O2,
=
endpoint spectrum, dashed lines=intermediate titration spectra.
shown in Table 8. All these squaraine derivatives except
SNS2O2 form 1:1 metal-to-ligand complexes. For SNS2O2,
the [LHg2]4+ species is also found. Moreover, whereas SJu,
SJuOH and SNO5 display nitrogen coordination, SBu and
SCar show oxygen coordination with the Hg2+ cation. Addi-
tionally, for the ligands SAb, SNO3 and SNO4 for which
mixed N,O-coordination was found, the microscopic con-
ordinated (logK=6.7 for [LHg]2+ +Hg2+ [LHg2]4+).
QACHTREUNG
Again, the second binding constant is lower than the first
due to electrostatic repulsion, but is still larger than those
2+
ꢁ
found for O Hg coordination.
Cu2+ shows a much more homogeneous behaviour than
Hg2+. This is exemplified in Figure 10, which shows the
colour modulations in the SBu–Cu2+ system. Similar graphs
ꢁ
ꢁ
stants for Hg O and Hg N coordination have also been cal-
culated (Table 9). As expected, derivation of the macroscop-
Table 9. Logarithm of the macroscopic (K) and microscopic (k) stability
constants for the interaction of the squaraine derivatives SAb, SNO3 and
SNO4 with Hg2+ in acetonitrile (298 K).
Reaction
[SAb]+Hg2+
[SNO3]+Hg2+
[SNO4]+Hg2+
logK
logkN
logkO
G
Q
Q
Q
G
4.29(5)
4.91(8)
4.49(7)
2.82
4.78
3.964.35
4.27
4.32
G
G
C
G
ic and microscopic binding constants yields comparable
oxygen coordination constants (4.27<logK<4.45). These
values are rather large for oxygen–Hg2+ coordination when
one considers that no macrocycle or chelating arm is present
to stabilise the complex, but agree with the existence of a
partial negative charge at the oxygen. On the contrary, the
Figure 10. Spectroscopic behaviour of SBu in acetonitrile upon increasing
amounts of Cu2+
.
=initial spectrum, =spectrum at equimolar ratios
&
^
of Cu2+ and SBu, =endpoint spectrum, dashed lines=intermediate ti-
~
tration spectra.
nitrogen–Hg2+ binding constants show higher scattering
2+
ꢁ
(2.82<logK<9.2). The largest logK=9.2 for N Hg bind-
and logK were obtained for SAb, SNO3, SNO4, SNO5 and
SCar (see Table 8). At low metal-to-dye ratios a band devel-
ops at 660 nm and is assigned to the interaction of Cu2+
with a central oxygen atom from the C4O2 group. Further
addition of Cu2+ results in the elimination of this band and
the appearance of a new, broad absorption band at about
430 nm, indicating the formation of a bimetallic complex
with Cu2+ ions coordinating at one oxygen and one nitrogen
atom. In contrast to this general behaviour, interaction of
SNS2O2 with Cu2+ results in a hypsochromic shift (band at
ing is found for the formation of [(SNS2O2)Hg]2+, indicating
a very strong interaction of the sulfur-containing macrocycle
with the mercuric cation. A recently reported system con-
taining the same aza–thia–oxa macrocycle appended to a
phenoxazinone dye showed logK=6.07 for the formation of
the Hg2+ complex in water.[56] Furthermore, the strong inter-
action of Hg2+ with the aza–thia–oxa macrocycle prevents
metal coordination at the central C4O2 group and produces
the 1:2 complex [(SNS2O2)Hg2]4+ with both macrocycles co-
Chem. Eur. J. 2008, 14, 10101 – 10114
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10109

R. Martínez-MµÇez, K. Rurack et al.
Table 10. 2-Inputs logic truth tables for colour modulations of SNS2O2
624 nm) because of metal coordination through the macro-
cycle. This shift with Cu2+ is remarkably lower than with
Hg2+ (lmax =582 nm), suggesting weaker interaction of Cu2+
with the nitrogen bridge head atom. Additionally, the pres-
ence of an excess of Cu2+ induces an elimination of the
band due to the simultaneous Cu2+ coordination by both
macrocycles of SNS2O2.
with Hg2+ concentrations as input.
Input A
Input B
Output 1
Output 2
Output 3
Hg+ (1 equiv) Hg2+ (1 equiv) 640 nm band 560 nm band Bleaching
0
1
0
1
0
0
1
1
1
0
0
0
0
1
1
0
0
0
0
1
Finally, Fe3+ also shows a diverse and rather complex be-
haviour that is summarised in Table 7.
Signalling paradigms: Squaraines are very attractive and
popular scaffoldings for chemically addressable colour mod-
ulations. As shown above, chemical inputs such as protons
or metal ions are readily transformed into dramatic spectro-
scopic changes, creating various output patterns. Such versa-
tile chemical input–optical output features are of timely sig-
nificance and have been widely applied in two closely relat-
ed fields: molecular chemosensing and molecular-scale
“logic gates”. In the former, optical signals are used to
detect the presence of certain chemical species in solu-
tion.[37,57] In the second approach, chemical species are treat-
ed as inputs that control, through Boolean logic rules, the
output, which is usually a light signal.[27,58] In the chemosens-
ing field, the goal is often selectivity, that is, a unique chemi-
cal species should induce a unique change in colour or fluo-
rescence. In contrast, molecular logic looks for more com-
plex systems, which usually contain multiple binding sites
and show certain unique response patterns, exemplifying the
desired logic operation, in the presence of either or both
inputs (three-input systems are of course even more com-
plex).
AND, and XOR logics.[59] Additionally, Table 11 shows the
optical response for the system SJu (processor), H+ (input
A), and Hg2+ (input B), which results in NOR, ID-A, and
Table 11. 2-Inputs logic truth tables for SJu with H+ and Hg2+ as inputs.
Input A
Input B
Output 1
Output 2
Output 3
H
C
ACHTREUNG
0
1
0
1
0
0
1
1
1
0
0
0
0
0
1
1
0
1
0
1
ID-B Boolean algebraic functions. Some other different
logics can be designed bearing in mind the response of the
different ligands in the presence of protons or metal cations
at different concentrations (Table 7 summarises this behav-
iour) and the corresponding stability constants. The title
dyes thus belong to the class of multifunctional dyes that
contain two or more sites for an interaction with chemical
species and that respond to more than a single input. Such
molecules are commonly referred to as “reconfigurable”
molecular logic gates, that is, as systems in which it is possi-
ble to observe different logic expressions from one unique
molecular entity.[60]
With respect to squaraines, the results for our title com-
pounds presented here suggest that those dye/input combi-
nations that give diverse photometric responses might be
suitable for the design of molecular logic devices, while se-
lective binding of a guest with only a specific group on the
dye can result in selective chromo-fluorogenic chemosen-
sors. In the following, we will review the above given results
in view of these two concepts.
Molecular chemosensing: From the point of view of chemo-
sensing, the behaviour of most of the title compounds is far
from being suitable, because of the considerably low selec-
tivity observed. However, this is relatively common when
using a poorly coordinating solvent such as acetonitrile. In
acetonitrile, there is no serious competition by the solvent
and many metal ions can often interact even with nonsup-
ported coordination sites, such as the oxygen atoms of the
central ring in our case. When moving from acetonitrile to
acetonitrile–water mixtures or to neat water, the competi-
tive nature of the solvent increases and the behaviour of the
dyes changes dramatically. In fact, none of the squaraines
shows a remarkable colour modulation in neat aqueous so-
lution in the presence of metal ions except for SNS2O2. This
lack of signalling is observed even in acetonitrile–water mix-
tures with a relatively low content of around 3 wt% water.
This behaviour originates from the strong solvation energy
of the metal ions in the presence of water, which eventually
disfavours metal–ligand interaction. Additionally, the behav-
iour showed by the SNS2O2 receptor is highly selective and
only the thiophilic Hg2+ cation is able to induce a colour
Molecular-scale logic gates: The cation-induced modulations
of the squarainesꢁ absorption spectrum involve the appear-
ance or disappearance of various bands located at about
640, 560, 670, and 480 nm as well as complete bleaching in
the visible region, and can be divided into four cases: single
N-, single O-, N,O- and exclusive N,N-coordination. Thus,
coordination at different sites could open different channels
related to the absorption properties of the final ensemble.
This is a rich behaviour that could be of interest in the de-
velopment of molecules with intrinsic logic capability. Mo-
lecular logics is an interesting concept and many examples
have been developed since it was noticed that the interac-
tion of a receptor molecule with certain species (input) can
yield response (output) patterns that follow Boolean logic
rules.[27,58] Tables 10 and 11 summarise some general re-
sponse patterns. For instance, Table 10 shows how the ab-
sorption response of the SNS2O2/Hg2+ system obeys NOR,
10110
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Chem. Eur. J. 2008, 14, 10101 – 10114

Squaraines as Reporter Units
FULL PAPER
modulation from purple to colourless in water. The response
of SNS2O2 to several metal ions in buffered acetonitrile–
addressable squaraines will be useful in the future design of
functional supramolecular ensembles based on this class of
dyes.
water 20:80 (v/v) solutions (HEPES 0.08m, pH 6.90, cSNS O
=
2
2
2”10ꢁ6 molLꢁ1) is shown in Figure 11, in which the absorb-
ance of the dye at 640 nm in the presence of 100 equivalents
of certain metal cations is displayed.[61] The system shows a
Experimental Section
remarkable behaviour with respect to the detection of Hg2+
,
with a very low detection limit of 10ꢁ8 m and a complex sta-
bility constant of logK=7.5.
General remarks: All commercially available reagents were used without
further purification. Air/water-sensitive reactions were performed in
flame-dried glassware under argon. Acetonitrile and CH2Cl2 were dried
with CaH2 and distilled prior to use. 16-Phenyl-16-aza-1,4,7,10,13-pen-
taoxacyclooctadecane,
cane, 10-phenyl-10-aza-1,4,7-trioxacyclododecane, N,N-di
thoxy)]aniline, 10-phenyl-10-aza-1,4-dioxa-7,13-dithiacyclopentadecane
13-phenyl-13-aza-1,4,7,10-tetraoxacyclopentade-
AHCTREUNG
and 2-[(2-butylcarbamoiloxiethyl)phenylamine]ethyl ester were prepared
following known procedures.[15,18,19,24,31,32]
Physical measurements and instrumentation: The NMR spectra were re-
corded with a Varian Gemini 300 spectrometer. Chemical shifts are re-
ported in ppm downfield from the TMS signal. Spectra taken in CDCl3
were referred to the residual CHCl3.
General synthesis: SNO3,[24] SNO4,[24] SNO5,[24] SBu,[31] SJu[32] and
SJuOH[32] were prepared according to literature procedures. For SAb,
SCar and SNS2O2, we employed the general synthetic procedure for
squaraines.[62] N,N-Dibutyl aniline (3 mmols) and squaric acid (1.37
mmols) were dissolved in a 1:1 butanol/toluene mixture (25 mL). The
crude reaction mixture was placed in a Dean–Stark apparatus, for the
azeotropic removal of water, and heated under reflux for 5 h. On cooling
the solution to room temperature green needles of the dyes precipitated.
The solids were filtered off and washed with hexane to obtain the final
products as green-blue solids.
Figure 11. Absorbance at 640 nm for SNS2O2 in the presence of 100
equivalents of metal cations in acetonitrile/water 20:80 (v/v) solutions,
HEPES 0.08m pH 6.90, cSNS O =2”10ꢁ6 molLꢁ1
.
2
2
Synthesis of bis(4-CAHTRE{UGN bis[2-(2-methoxy-ethoxy)-ethyl]}aminophenyl)squar-
aine (SAb): Yield: 85.8 mg, 9.3%; 1H NMR: (CDCl3): d=3.35 (s, 12H;
OCH3), 3.48 (t, J=6.6 Hz, 8H; CH2CH2NAr), 3.57 (t, J=7.0 Hz, 8H;
CH2CH2O), 3.66 (t, J=6.5 Hz, 8H; CH2CH2O), 3.73 (t, J=6.8 Hz, 8H;
CH2CH2O), 6.79 (d, J=9.1 Hz, 4H; ArH), 8.33 ppm (d, J=9 Hz,
4H;ArH); 13C NMR (CDCl3): d=51.29, 58.93, 68.23, 70.44, 71.72, 112.49,
119.88, 113.09, 153.75, 183.13, 188.56ppm; mass IE high resolution calcd
for C36H52N2O10: 673.3700; found: 673.3696.
Conclusion
In conclusion, we have prepared and characterised a family
of squaraine dyes and have studied their interaction and
their photophysical properties. Squaraines such as the title
compounds closely resemble cyanine dyes consisting of two
substituted aniline moieties and a central four-membered
squaric acid ring. In all cases, the squaraine derivatives show
two distinct coordination sites at the anilino nitrogen and at
the oxygen from the C4O2 central ring. Diverse colorimetric
behaviour has been observed in the squaraine derivativesꢁ
interaction with protons and metal cations. In order to ra-
tionalise this colorimetric behaviour, we have performed
semiempirical calculations and determined the stability con-
stants of the complexes. Thus, the visible band of the squar-
aines at around 640 nm is blue-shifted when coordination at
the anilino nitrogen atom occurs, whereas coordination to
the C2O4 oxygen atoms results in the development of red-
shifted bands. Addition of more than one equivalent of pro-
tons or metal ions could additionally lead to the formation
of mixed N,O- or N,N-coordinated complexes, entailing the
development of a broad band at 480 nm or bleaching, re-
spectively. The presence of more complex species involving
more than two coordinated metal ions has not been ob-
served. Finally, the use of these coordinating dyes for the
development of suitable chemical sensors and molecular-
scale “logic gates” has been discussed. We believe that the
present approach of approximating the signal expression of
Synthesis of bis(4-CAHTRE{UGN bis[2-(butyl carbamic acid)-ethyl]}aminephenyl)squar-
aine (SCar): Yield: 174.1 mg, 15.2%; 1H NMR: d=0.85 (t, J=6.6 Hz,
12H; CH2CH3), 1.20–1.25 (m, 8H; CH2CH2CH3), 1.35–1.43 (m, 8H;
CH2CH2CH2), 3.09 (q, J=5.9 Hz, 8H; NHCH2CH2), 3.69 (t, J=6.6 Hz,
8H; CH2CH2NAr), 4.23 (t, J=6.7 Hz, 8H; CH2CH2O), 5.16(m, 4H;
CONHCH2), 6.79 (d, J=8.6Hz, 4H; Ar H), 8.31 ppm (d, J=8.4 Hz, 4H;
ArH); 13C NMR (CDCl3): d=13.97, 20.14, 32.16, 41.09, 50.91, 61.48,
113.17, 120.70, 133.73, 154.51, 156.42, 183.41, 190.55 ppm; mass FAB high
resolution calcd for C44H64N6O10: 836.4721; found: 836.4684.
Synthesis of bis[4-(1,4-dioxa-7,13-dithia-10-aza-cyclopentadecane)phe-
nyl]squaraine (SNS2O2): Yield: 213.9 mg, 21.3%; 1H NMR (CDCl3): d=
2.77 (t, J=7.4 Hz, 8H; SCH2), 2.93 (t, J=7.7 Hz, 8H; SCH2), 3.64 (s,
8H; OCH2), 3.81 (m, 16H), 6.73 (d, J=9.1 Hz, 4H; ArH), 8.38 ppm (d,
J=8.9 Hz, 4H; ArH); 13C NMR (CDCl3): d=189.94, 183.43, 153.17,
133.81, 120.62, 112.89, 74.47, 70.94, 52.66, 32.13, 30.06 ppm; mass FAB
high resolution calcd for C36H49N2O6S4 MH+: 733.247350; found:
733.247373.
Optical spectroscopy: UV/Vis spectra were recorded on a Bruins Instru-
ments Omega 10 and a Perkin Elmer Lambda 35 spectrophotometer,
steady-state fluorescence spectra on
a Spectronics Instruments 8100
fluorometer (908 standard geometry, polarisers at 08 and 54.78 in excita-
AHCTREUNG
tion and emission). All photophysical measurements were carried out at
298ꢀ1 K with dilute solutions with optical densities between 0.03 and
0.05 at the absorption maximum. Fluorescence quantum yields (Ff) were
determined relative to rhodamine 101 in ethanol (Ff =1.00ꢀ0.02)[63] with
uncertainties of ꢀ3% (for Ff >0.2) and ꢀ6% (for 0.2 >Ff >0.02), re-
spectively. All the fluorescence spectra were corrected as described in
reference [64]. The Stokes shifts were calculated from the differences be-
Chem. Eur. J. 2008, 14, 10101 – 10114
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10111

R. Martínez-MµÇez, K. Rurack et al.
eap1
eap2
ðka=kbÞcbea1 þ cbeb1
ðka=kbÞcbea2 þ cbeb2
tween the maxima of the absorption and emission bands after conversion
to the energy scale, taking into account another correction step.[65] Fluo-
rescence lifetimes (tf) were measured with a unique laser impulse fluor-
ometer with picosecond time resolution described by us in an earlier pub-
lication[66] and modified according to a description in reference [67]. The
fourth harmonic output of the signal of the OPA was used for excitation,
the fluorescence was collected at right angles (polariser at 54.78, mono-
chromator with bandwidths of 4, 8, and 16nm), and the decays were re-
corded by the single photon timing method (typical instrumental re-
sponse functions of 25–30 ps full width at half maximum, experimental
accuracy of ꢀ3 ps). The laser beam was attenuated using a double prism
attenuator from LTB and typical excitation energies were in the nano-
watt to microwatt range (average laser power). The fluorescence lifetime
profiles were analysed with a PC using the software package Global Un-
limited V2.2 (Laboratory for Fluorescence Dynamics, University of Illi-
nois). The goodness of fit of the single decays as judged by reduced chi-
squared (c2R) and the autocorrelation function C(j) of the residuals was
always below c2R <1.2.
¼
ð10Þ
This can be easily rearranged to give Equation (11):
eb1ꢁðeap1=eap2Þeb2
ka
kb
¼
ð11Þ
ðeap1=eap2Þea2ꢁea1
The relation between the micro- and the macroscopic constants is given
by Equation (12):
K ¼ ka þ kb
ð12Þ
Working out K and replacing it in Equation (11) we are able to obtain
Equation (13):
K
kb
¼
ð13Þ
½feb1ꢁðeap1=eap2Þeb2g=fðeap1=eap2Þea2ꢁea1gŠ þ 1
Theoretical studies: For the discussion in the photo-physical section,
ground-state geometries were optimised using density functional theory
(DFT) with Beckeꢁs three-parameter hybrid exchange functional[68] in
conjunction with the Lee–Yang–Parr gradient-corrected correlation func-
tional (B3LYP functional)[69] with the 6-31G basis set as implemented in
Gaussian 03.[70] All the optimised structures were confirmed as energy
minima by normal coordinate analysis. For the discussion of the protona-
tion results, quantum chemical calculations at semi-empirical level (PM3,
within restricted Hartree–Fock level) were carried out in vacuo with the
aid of Hyperchem V6.03. The Polar Ribiere algorithm was used for the
optimisation. The convergence limit and RMS gradient were set to
In this equation, K, eap1 and eap2 data can be calculated by HYPER-
QUAD. On the contrary ea1, eb1, ea2 and eb2 were estimated from similar
compounds displaying protonation only at one site.
Acknowledgements
We thank the Ministerio de Ciencia y Tecnología (project MAT2003-
08568-C03 and CTQ2006-15456-C04-01/BQU) for support. J.V.R.-L.
thanks the Generalitat Valenciana for a Postdoctoral Fellowship.
0.01 kcalmꢁ1
.
Calculation of macroscopic binding constants: Spectrophotometric titra-
tions were carried out in acetonitrile using a cuvette thermostated at 25ꢀ
0.18C. The titrant was added by means of a micropipette. The computer
program HYPERQUAD 2000, version 2.1, was used to calculate the pro-
tonation and stability constants.[71] A typical titration experiment had
about 4000 points (50 titration curves measured at 80 wavelengths) for
each system. An excessive b limit of 0.33 and a maximum of 50 iterations
were employed during the refining process.
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ð3Þ
½AŠ
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ð5Þ
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caea1 þ cbeb1
¼
ð9Þ
10112
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10101 – 10114

Squaraines as Reporter Units
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Received: February 18, 2008
Revised: June 24, 2008
Published online: September 11, 2008
10114
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