Synthesis and electronic properties of 1,2-hemisquarimines and their encapsulation in a cucurbit[7]uril host

Article abstract of DOI:10.1002/chem.201400039

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2-hemisquarimines (1,2-HSQiMs), through the microwave-assisted condensation of aniline derivatives with the 1,2-squaraine core is reported. In CH ₃CN, 1,2-HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530nm, as well as an emission band centered at about λ=574nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2-HSQiMs indicate that charge injection into TiO ₂ is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion. Be square! 1,2-Hemiquarimines (see figure), with one unsubstituted or derivatized phenylimine group, can be synthesized from the corresponding 1,2-squaraine. The interesting photophysical and redox characteristics of these dyes make them useful candidates for charge transfer and sensitization studies.

Full text of DOI:10.1002/chem.201400039

DOI: 10.1002/chem.201400039
Full Paper
&
Host–Guest Systems
Synthesis and Electronic Properties of 1,2-Hemisquarimines and
Their Encapsulation in a Cucurbit[7]uril Host
Christian C. De Filippo,[a] Hao Tang,[b] Luca Ravotto,[c] Giacomo Bergamini,[c] Patrizio Salice,[a]
Miriam Mba,[a] Paola Ceroni,*[c] Elena Galoppini,*[b] and Michele Maggini*[a]
Abstract: The synthesis of a new class of robust squaraine
dyes, colloquially named 1,2-hemisquarimines (1,2-HSQiMs),
through the microwave-assisted condensation of aniline de-
rivatives with the 1,2-squaraine core is reported. In CH3CN,
1,2-HSQiMs show a broad absorption band with a high ex-
tinction coefficient and a maximum at around l=530 nm,
as well as an emission band centered at about l=574 nm,
that are pH dependent. Protonation of the imine nitrogen
causes a redshift of both absorption and emission maxima,
with a concomitant increase in the lifetime of the emitting
excited state. Encapsulation of the chromophore into a cu-
curbit[7]uril host revealed fluorescence enhancement and in-
creased photostability in water. The redox characteristics of
1,2-HSQiMs indicate that charge injection into TiO2 is possi-
ble; this opens up promising perspectives for their use as
photosensitizers for solar energy conversion.
Introduction
Squaraines (SQs) I and II are or-
ganic chromophores that are de-
rivatives of squaric acid. Among
the most common SQ structures
are the 1,3-regioisomers[1] (I) ob-
tained by condensation of anhy-
drobases with squaric acid.
These 1,3-SQ derivatives, which
belong to the family of cyanine
dyes, have been used as light harvesters or photosensitizers in
organic[2] and hybrid organic–inorganic solar cells (DSCs)[3] or
photodetectors[4] for their wide and highly tunable absorption
range, as well as other favorable photophysical and electro-
chemical properties.
enough to allow electron transfer to acceptors such as ful-
lerenes[6] or the TiO2 conduction band.[7] In addition, the spec-
troscopic and redox properties of SQ dyes, in particular, the
HOMO–LUMO levels, are tunable by varying the substituents
on the squaric acid moiety.[8] 1,3-SQs also have a broad range
of applications in different areas, such as imaging, nonlinear
optics, and photodynamic therapy.[1] The 1,2-SQ regioisomers
(II) have two cross-conjugated p-electron systems spanning
from the heteroaromatic donor to the carbonyl acceptors and,
for this reason, are regarded as merocyanine-type chromo-
phores. 1,2-SQs can be prepared in excellent yields by con-
densing suitable precursors (i.e., the anhydrobase of the im-
monium salt 2; Scheme 1) with squarate ester 3.[9] The synthet-
ic methods to access 1,2-SQs selectively, however, were devel-
oped only very recently and, to date, remain limited to a few
reports.[9,10] For this reason, 1,2-SQs have not yet been studied
as extensively as their 1,3-SQ counterparts, despite their attrac-
tive properties, including high electrochemical reversibility; in-
creased solubility; absorption in the l=550 nm region, which
is complementary to that of the 1,3-SQs; and low-lying HOMO
levels, which are potentially useful to increase the open-circuit
voltage (VOC) of solar cells. Another class of compounds related
structurally to 1,3-SQs are the core-substituted 1,3-SQs (III) that
Specifically, 1,3-SQ dyes exhibit intense light absorption
bands in the visible/near-infrared (NIR) region of the spectrum,
NIR fluorescence with high quantum yields and nanosecond
lifetimes,[4,5] and excited-state reduction potentials negative
[a] C. C. De Filippo, Dr. P. Salice, Dr. M. Mba, Prof. M. Maggini
Department of Chemical Sciences, University of Padova
Via Marzolo 1, 35131 Padova (Italy)
Fax: (+) 300498275050
E-mail: michele.maggini@unipd.it
[b] Dr. H. Tang, Prof. E. Galoppini
Department of Chemistry, Rutgers University
73 Warren Street, Newark, NJ - 07102 (USA)
E-mail: galoppin@rutgers.edu
[c] L. Ravotto, Dr. G. Bergamini, Prof. P. Ceroni
Department of Chemistry “G. Ciamician”
Via Selmi 2, 40126 Bologna (Italy)
E-mail: paola.ceroni@unibo.it
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400039.
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which photostability and decreased aggregation are important
for applications involving charge transfer. The use of molecular
hosts to encapsulate dyes, in solution and on semiconductor
surfaces, is a strategy that is increasingly employed to over-
come such problems, as well as to enhance emission proper-
ties, influence charge transfer, and, generally, to shield the
chromophore and achieve binding control.[12]
CB[7], which is a water-soluble macrocyclic host composed
of seven glycoluryl units and methylene units, is part of the
broader cucurbit[n]uril family (CB[n], typically n=5–8 and
10).[13] CB[n] hosts, especially CB[7], offer significant advantages
over other host molecules, such as crown ethers or cyclodex-
trins, because CB[n] can bind to molecular guests with high
binding affinities.[14] For example, the binding affinities of ada-
mantane derivatives (>1012 mꢀ1)[14b] to CB[7] are at least 107
times higher than that to b-cyclodextrin (104–105 mꢀ1).[15] The
dominant driving forces for CB[n]–guest binding are hydropho-
bic effects and ion–dipole interactions between the carbonyl-
lined portals of CB[n] and the guest cation that lead to a slow
dissociation process of the guest in CB[n] cavities.[16] In particu-
lar, binding to CB[7] stabilizes dyes against photobleaching,[17]
reduces dye aggregation,[18] enhances the dye fluorescence
emission,[19] and may lead to electrochromic effects.[20]
Scheme 1. Synthesis of 1,2-hemisquarimines 1a–c.
have emerged as powerful panchromatic photosensitizers for
DSCs.[11]
Herein, we report a further development of 1,2-SQ chemis-
try, namely, the synthesis of compounds 1a–c through the
condensation of 4 with the corresponding aniline derivative in
the presence of titanium(IV) isopropoxide under MW irradia-
tion (Scheme 1). We have named these new condensation
products hemisquarimines (HSQiMs). The phenylimine func- Results and Discussion
tional group can bear different substituents, including a bro-
Synthesis
mine atom for further functionalization through cross-coupling
chemistry or an alkyl ester group that, upon hydrolysis, can
anchor to TiO2. More importantly, the conjugated 1,2-HSQiM
structure allows, at least in principle, the directionality of elec-
tron flow, in the carboxylic push–pull photosensitizer, from the
electron-rich benzothiazole moiety toward the semiconductor
surface to be maintained. This is a fundamental molecular
design prerequisite for electronic coupling between the LUMO
of the sensitizer and the oxide conduction band in DSCs. Fur-
thermore, herein, the photophysical and electrochemical prop-
erties of 1,2-HSQiMs 1a–c, are presented, along with a study of
the host–guest complex of 1a with cucurbit[7]uril (CB[7]). The
molecular structures and relative dimensions of CB[7] and 1a
are shown in Figure 1. SQs, similar to numerous other organic
dyes, have the tendency to aggregate and, in some cases, they
may also photodegrade. This behavior is particularly observed
on nanostructured semiconducting surfaces such as TiO2, upon
The synthesis of 1,2-HSQiMs 1a–c is shown in Scheme 1. It was
achieved in one step by the condensation of 4[9] with the spe-
cific aniline, in the presence of Ti(OiPr)4 under microwave (MW)
irradiation to overcome the high stability of the 1,2-diketone
system against nucleophilic attack. The investigation of the
effect of MW irradiation indicates that the best conversions
can be obtained within the temperature range of 80–908C for
2.5–8 min of reaction time. Under these conditions, the prod-
uct imines 1a–c were isolated in 21–35% yield after chromato-
graphic purification, which turned out to be a rather difficult
task because highly polar 1a–c tended to irreversibly bind
onto the stationary phase (either silica gel or alumina) of the
chromatographic column. The rest was mostly unreacted 4
and a complex mixture of compounds. In the case in which
aniline was used, TLC over neutral alumina, with toluene/
AcOEt (8:2) with 3% MeOH as the eluent, showed the pres-
ence of nonfluorescent side products with a retention factor
(Rf) of >0.8. Fluorescent 1,2-HSQiM 1a appeared at Rf =0.64,
1
as confirmed by H and 13C NMR spectroscopy and MS analyses
of the isolated product. The nonsymmetrical 1,2-SQ core of
1
1a–c is evidenced in the H NMR spectrum by two resonances
for the alkenyl protons between d=5.5 and 6 ppm (see the
Supporting Information). TLC analysis of the crude mixture
showed traces of fluorescent 1,2-SQiM (Rf =0.73) in which both
carbonyl groups of 4 were condensed with aniline, as observed
by MS analysis of an isolated fraction. Unreacted 4 was detect-
ed as a fluorescent spot at Rf =0.16. Decomposition of the
starting 1,2-SQ substrate and products was observed at tem-
peratures of >908C and prolonged MW irradiation. Although
Figure 1. Molecular structure and dimensions of CB[7] and 1a.
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not fully compatible with 4 and product imines, the use of
highly oxophilic TiIV species and MW heating proved suitable
to give the expected product imines 1a–c in moderate yields.
Further conditions are currently being investigated, in particu-
lar, the use of alternative Lewis acid activation, to increase the
conversions and minimize the formation of side products. IR
and Raman vibrational spectroscopies were used to character-
ize the donor–acceptor nature of both derivatives 4 and 1a.
As mentioned earlier, the structure of 4 contains a cross-conju-
gated p-electron system that links four end groups: two
donor-linked (benzothiazole) units (D) and two carbonyls with
electron-accepting characteristics (A). In analogy with other
merocyanine systems,[21] a few vibrational modes are simulta-
neously active in both the IR and Raman spectra at n˜ =1590,
1500, 1420, and 1160 cmꢀ1, (Figure S1 in the Supporting Infor-
mation); this indicates that the D–A groups in 4 are strongly
coupled. Derivative 1a, in which one of the cyclobutenyl car-
bonyl groups has been replaced by a phenylimine group,
shows IR and Raman spectra similar to those recorded for 4,
except for the signal corresponding to the acceptor units,
which is now split into two components for the carbonyl and
imine groups at n˜ =1490 and 1520 cmꢀ1, respectively (Fig-
ure S1 in the Supporting Information). This also suggests that
in 1a the D–A groups are strongly coupled, which is in agree-
ment with the large dipole moment calculated by DFT.
Photophysical properties in acetonitrile
The absorption spectra of compounds 1a–c in CH3CN exhibit
an almost identical shape with a band maximum at lꢁ530 nm
(Table 1). Interestingly, all spectra present a high molar absorp-
tion coefficient over a wide wavelength range, spanning from
the UV to about l=600 nm. Figure 3 reports the absorption
characteristics of derivative 1a, which can be considered a rep-
resentative example of a 1,2-HSQiM. A modest solvatochrom-
ism is observed: for example, the absorption band maximum
of 1a shifts from l=527 to 534 nm upon going from CH3CN
to DMF.
Table 1. Photophysical properties of the investigated compounds and
their protonated forms in air-equilibrated CH3CN at room temperature.
labs
emax/104
[cmꢀ1 mꢀ1
lem
Fem
t
[nm]
]
[nm]
[ns]
1a
527
466
556
531
466
551
529
467
552
519
5.0
3.3
3.6
6.3
4.5
3.9
6.2
4.2
4.0
11.1
574
635
1.4ꢁ10ꢀ3
2.1ꢁ10ꢀ2
<0.1
1a·H+
0.4
1b
1b·H+
572
640
1.7ꢁ10ꢀ3
1.4ꢁ10ꢀ2
<0.1
0.4
1c
1c·H+
572
639
2.1ꢁ10ꢀ3
1.7ꢁ10ꢀ2
<0.1
0.4
4
542
7.2ꢁ10ꢀ3
<0.1
Theoretical calculations
DFT calculations carried out for 4 and 1a suggest that a syn
conformation is favored for both. In the case of 4, this finding
is in accordance with diffraction analysis data.[9] Also, both mol-
ecules have a large dipole moment (about 10 D). The forma-
tion of the imine leads to decreased delocalization of the fron-
tier orbitals onto the heterocyclic ring and a small increase in
the HOMO–LUMO gap compared with that of 4 (Figure 2).
Figure 3. Absorption (c) and emission (a) spectra of 1a in CH3CN.
No change in the absorption spectral shape was observed
within the concentration range 10ꢀ6–10ꢀ4 m explored; thus
suggesting no aggregation phenomena. Figure 3 also shows
the emission spectrum of compound 1a (dashed line) in
CH3CN at room temperature, with a band maximum at l=
574 nm: no significant difference in the band maximum was
observed for 1,2-HSQiMs 1a–c (Table 1). The emission quantum
yields (Fem; Table 1) are quite low, with a slight increase upon
going from compound 1a to functionalized derivatives 1b and
1c. The fluorescence lifetimes are very short and below the in-
strumental resolution (t<100 ps). Compounds 1a–c exhibit
Figure 2. Contour plots for the HOMO and LUMO orbitals of 4 (a) and 1a
(b).
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photophysical properties very similar to compound 4, apart
from a broader and less intense lowest energy absorption
band. All of the investigated compounds show a very high
photostability upon irradiation at l=436 nm in CH3CN
(Fphotoreact <1ꢁ10ꢀ4). This result is encouraging in view of po-
tential applications of 1,2-HSQiMs in solar energy conversion
devices.
gave a pKa value of 6.3. Because no spectral changes were ob-
served for compound 4, under the same experimental condi-
tions as those used for the characterization of 1a, it is reasona-
ble to assume that protonation occurs at the imine nitrogen.
Fluorescence spectra, recorded upon excitation at an isosbestic
point (lexc =367 nm), show the progressive decrease of the l=
574 nm band with the growth of a 5-fold more intense emis-
sion, with a maximum centered at l=635 nm (Figure 4b). The
same behavior was observed for all investigated compounds,
with a different degree of increase of the quantum yield de-
pending on the specific substituents in the phenyl ring of the
imine function (Table 1). The lifetimes of the emitting excited
state show a concomitant increase to 0.4 ns. The observed
spectral changes are completely reversible upon addition of
tributylamine.
Acid–base behavior in acetonitrile
The absorption spectra of compound 1a upon titration with
CF3SO3H in CH3CN (Figure 4a) show a progressive decrease of
the most intense peak at l=527 nm, with a simultaneous
growth of two bands with maxima at l=466 and 556 nm,
respectively.
The presence of several isosbestic points suggests the direct
transformation of one species into another, which is compati-
ble with a single protonation step. Analysis of the spectral
changes with the SPECFIT software, upon acid addition to 1a,
Electrochemical characterization in acetonitrile
Cyclic voltammograms of compound 1a (Figure 5), recorded in
CH3CN (0.1m tetraethylammonium hexafluorophosphate
Figure 4. Absorption (a) and emission (b) spectra of compound 1a
(c=9.2ꢁ10ꢀ6 m) upon titration with CF3SO3H in CH3CN: from 0 (c) to
1.4 equiv (a) of acid; lex =367 nm. Inset: the normalized changes of ab-
sorbance at l=500 nm (circles) and emission intensity at l=670 nm (tri-
angles) with respect to the added equivalents of acid.
Figure 5. Cyclic voltammetry (CV) results for compound 1a in CH3CN/TEAPF6
at room temperature: scan rate V=0.5 (a), 0.1 (b, c), and 1 Vsꢀ1 (b, a).
For comparison purposes, cyclic voltammograms in b) report the value of
I/V1/2
.
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(TEAPF6)) at room temperature, show two one-electron-transfer
processes both in the cathodic (peaks 1 and 2 in Figure 5a)
and anodic region (peaks 3 and 4 in Figure 5a).
Processes 1, 2, and 4 are chemically and electrochemically
reversible, whereas process 3 is associated with a chemical re-
action, the product of which is re-reduced (corresponding to
the peak marked with * in Figure 5a). Complete chemical re-
versibility is attained at scan rates higher than 1 Vsꢀ1, as re-
ported in Figure 5b. Compound 4 shows a very similar value
of the reduction potential for the first process in the cathodic
region (Table 2). The oxidation processes are shifted to less
Table 2. Oxidation (ox.) and reduction (red.) potentials (E1/2 in V vs. a stan-
dard calomel electrode (SCE)) of compounds 1a and 4 in CH3CN at room
temperature (0.1m TEAPF6).
Figure 6. Absorption spectra for aqueous solutions of 1a in the absence (a;
lmax =474 nm) and in the presence (b; lmax =556 nm) of 250 mm CB[7].
pH 6.9. c) The absorption spectrum for 1a in CH3CN.
Compound
Ox.
Red.
1a
+0.52
+0.16
+0.75
+0.28
ꢀ1.82
ꢀ2.07
ꢀ1.80
provided by the cavity of CB[7] for 1a was similar to that pro-
vided by organic solvents.[21] The isotherm for 1a binding to
CB[7] was determined by adding stock solutions of CB[7] to
a solution of 1a (Figure 7). The data fit well with a 1:1 CB[7]/
1a binding model and the equilibrium binding constant for
CB[7]–1a complex was determined as (4.18ꢂ0.09)ꢁ104 mꢀ1,
according to three independent experiments.
4
positive potentials and the first process is again associated
with a chemical reaction, and attains reversibility at high scan
rates. The electrochemical HOMO–LUMO gap (ꢁ2.0 eV) is in
good agreement with the spectroscopic energy gap (E0-0
ꢁ2.2 eV, as estimated from the emission maximum reported in
Table 1). From the spectroscopic energy (E0-0) and the ground-
state redox potential for the oxidation process (Eox), we can es-
timate the redox potential for the excited state (E*ox) by the
approximated formula: E*ox =EoxꢀE0-0. In the case of 1a, E*ox =
ꢀ1.82 V versus SCE is substantially higher than the conduction
band of TiO2, which allows activationless charge injection into
TiO2.
Properties of 1a in aqueous solutions and encapsulation in
CB[7]
The absorption and emission properties of 1a were studied in
aqueous solutions and at different pH values, in a solvent and
under conditions relevant to encapsulation in CB[7]. The ab-
sorption and emission spectra of 1a were strongly pH depen-
dent (Figure S2 in the Supporting Information). The fluores-
cence spectrum for 1a at pH 5.7 was very weak and increased
about 4-fold when the pH was decreased to 1.1, as shown in
the inset of Figure S2 in the Supporting Information. The com-
plexation of 1a in CB[7] was studied in deionized water
(pH 6.9) and under acidic solutions, in which the non-protonat-
ed (1a@CB[7]) and protonated (1aH+@CB[7]) forms of the
complex were predominant, respectively. The shape of the ab-
sorption spectrum for 1a in deionized water changed dramati-
cally upon the addition of CB[7]: the band maximum shifts
from l=474 to 556 nm (Figure 6), which indicates the forma-
tion of the 1a@CB[7] complex. The absorption spectrum of
1a@CB[7] was comparable to that of 1a in CH3CN (curve c in
Figure 6), suggesting that the hydrophobic microenvironment
Figure 7. Binding isotherm for 1a (2.5 mm) binding to CB[7] (0–63 mm) in de-
ionized water (pH 6.9). Following the arrow: [CB[7]]=0 (g), 3.34, 9.97,
23.0, 32.5, and 62.9 mm (c). Inset: Absorbance change of 1a at l=556 nm
with the addition of CB[7]. The curve represents the fit of data to the model
of 1:1 CB[7]/1a binding with a value of (4.18ꢂ0.09)ꢁ104 mꢀ1 recovered for
the equilibrium binding constant (K).
The formation of 1a@CB[7] in deionized water was probed
by steady-state fluorescence (Figure S3 in the Supporting Infor-
mation). The addition of CB[7] to an aqueous solution of 1a
led to a 21-fold enhancement of fluorescence intensity, and
a redshift of fluorescence emission maximum from l=488 to
621 nm (inset to Figure S3 in the Supporting Information). The
shape of the absorption spectrum of 1a in water at pH 1.6
changed with the addition of CB[7], although large shifts were
not observed (Figure S4 in the Supporting Information). The
broadening of the spectrum of 1a, compared with that ob-
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served in the presence of CB[7], is ascribed to the coexistence
of 1a and 1aH+. The formation of the 1aH+@CB[7] complex
at pH 1.6 was confirmed by the 4.5-fold enhancement and
sharpening of the spectral bands upon CB[7] addition in the
steady-state fluorescence spectra (Figure S5 in the Supporting
Information).
Stabilization of 1a by complexation with CB[7]
Compound 1a was photostable in CH3CN, and no UV/Vis spec-
tral changes were observed for solutions of 1a in CH3CN ex-
posed to ambient light for days. On the contrary, rapid photo-
bleaching of 1a in water was observed under room fluorescent
light. Specifically, exposure of 2.5 mm solutions of 1a to room
fluorescent light for 3 min led to a loss of 15% of 1a (Figure S6
in the Supporting Information). Moreover, the extent of photo-
bleaching was reduced by the addition of CB[7], as suggested
by the decrease of the amplitude of the decay (Figure 9).
These observations indicate that complexation with CB[7] in
aqueous solution stabilizes 1a kinetically and thermo-
dynamically.
A series of absorption spectra were recorded for the aque-
ous solution of 1a@CB[7] upon addition of HCl to check
whether 1aH+@CB[7] could be formed (Figure 8). The de-
crease in the absorption peak at l=556 nm and the increase
in the peak at l=432 nm upon addition of HCl indicated the
formation of 1aH+@CB[7].
Figure 8. Absorption spectra for 1 mm 1a/250 mm CB[7] at different pH
values. Following the arrow: pH 6.9 (c), 4.0, 3.6, 3.4, 3.1, and 2.6 (g).
Inset: Fluorescence spectra for 1 mm 1a/250 mm CB[7] at different pH values.
Figure 9. Temporal evolution of the absorption intensity of an aqueous solu-
tion of 1a (2.5 mm) in the absence (*) or presence (*) of 250 mm CB[7] fol-
lowing irradiation at l=430 nm. The black curve represents the fit of data
with a monoexponential decay function. The absorption intensities were re-
corded at l=474 nm for free 1a in water or at l=556 nm for 1a in the
presence of CB[7] and were normalized to 100% at time 0.
The protonation of 1a@CB[7] to form 1aH+@CB[7] was con-
firmed in the steady-state fluorescence experiments in which
the increase in the fluorescence peak at l=488 nm and the
decrease of that at l=621 nm were observed with the addi-
tion of HCl (Figure 8, inset). It is likely that 1a, which is too
large to fit completely in CB[7], as indicated in Figure 1, is only
partially encapsulated in the CB[7] cavity. The phenyliminium
moiety of 1aH+ is a suitable binding site for CB[7] because the
phenyl ring can fit into the CB[7] cavity by hydrophobic effects,
and the iminium ion can coordinate to the carbonyl groups at
the portal of CB[7] by ion–dipole interactions. The observation
that pH changes influence the binding behavior is also consis-
tent with this picture because the imine group is the protona-
tion site for 1a. The binding affinity of 1aH+ to CB[7] should
be higher than that of 1a to CB[7] ((4.18ꢂ0.09)ꢁ104 mꢀ1) due
to the additional driving force (i.e., ion–dipole interaction be-
tween 1aH+ and CB[7]). However, the full protonation of 1a
was achieved only in strongly acidic aqueous media (i.e., at pH
ꢁ0.2), in which competition between the hydronium ion with
1aH+ for the binding sites of CB[7] was strong and the ionic
strength was high.[16a] The determination of the binding con-
stant for 1aH+@CB[7] would require the development of a so-
phisticated methodology, which falls outside of the scope of
this paper.
Conclusion
SQ compounds have attracted increasing attention in the past
decade for applications in bioimaging, photovoltaics, and nu-
merous other technologically important areas of materials sci-
ence and photochemistry, but the 1,2-isomers have only
become synthetically accessible in recent years and remain rel-
atively little explored. We have reported herein the synthesis
and photophysical and electrochemical properties of the first
imine derivatives of symmetric 1,2-SQs prepared from ethyl-
benzothiazole. 1,2-Hemiquarimines carrying one phenylimine
group, either unsubstituted (1a) or derivatized in the meta po-
sition for added functionality with a bromine atom (1b) or a
ꢀCO2Me (1c) group, were synthesized in one step from the
corresponding 1,2-diketones by MW irradiation and in the
presence of a Lewis acid. In CH3CN, the reported HSQiMs ex-
hibited high molar absorption coefficients and fluorescence in
the visible region. The imine group could be protonated and
the resulting species showed an absorption band at lower
energy and a redshifted fluorescence with a higher emission
quantum yield. From the electrochemical measurements, the
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(standard used: solution of perylene in ethanol, F=0.92.[26] Fluo-
rescence lifetime measurements were carried out on an Edinburgh
FLS920 spectrofluorimeter, equipped with a TCC900 card for data
acquisition in time-correlated single-photon counting experiments
and Hamamatsu H5773-04 detector in a cooled housing with an
LDH-P-C-405 pulsed diode laser (0.1 ns time resolution). Global fit-
ting of absorption and emission spectra was performed with the
SPECFIT software.[27] The estimated experimental errors are 2 nm
on the band maximum, 5% on the molar absorption coefficient
and fluorescence lifetime, 10% on Ka, and 15% on the fluorescence
quantum yield.
redox potential of the lowest excited state was substantially
higher than that of the conduction band of TiO2; an important
property for photovoltaic applications. Encapsulation in molec-
ular hosts was successfully used to prevent aggregation, in-
crease photostability, and enhance the photophysical proper-
ties of organic fluorophores or chromophores,[16,19] and this
strategy was applied to aqueous solutions of 1a, which were
less photostable than those in CH3CN. Encapsulation of 1a in
CB[7] led to the formation of a 1:1 complex (1a@CB[7]) with
a binding constant of 104 mꢀ1. Complexed 1a exhibited fluores-
cence enhancement and increased photostability in water.
These observations make complexation with CB[7] a promising
strategy for further expanding the applications of 1a. Current
work is aimed at selecting alternative Lewis acid activation to
increase the 1,2-HSQiM yield and at investigating the binding
of 1a@CB[7] on nanostructured TiO2 films for charge transfer
and sensitization studies.
Photochemical experiments in organic solvents
Photochemical experiments were performed by using a medium-
pressure mercury lamp equipped with an interference filter (Oriel)
to select l=436 nm. The irradiated solution was contained in
a spectrophotometric cell and stirred continuously. The intensity of
the incident light was measured by the ferrioxalate actinometer.[28]
Photophysical measurements in aqueous solutions
Experimental Section
UV/Vis absorption spectra were collected on a Varian Cary 500 UV/
Vis/NIR spectrophotometer at room temperature. Steady-state fluo-
rescence emission spectra were collected on a Horiba Fluorolog-3
instrument equipped with a xenon short-arc lamp source at room
temperature. The excitation wavelength was set to l=430 nm
with a monochromator bandwidth of 3 nm for both excitation and
emission monochromators. The same fluorimeter was used to pho-
tobleach the sample when studying the photostability of 1a. The
excitation wavelength was selected with a monochromator. UV/Vis
and fluorescence spectra were corrected by subtracting a baseline
spectrum for a solution containing all chemicals, except 1a. The
pH values were determined by using an Accumet pH meter (AB15
plus, Fisher Scientific), which was calibrated daily by using three
standard pH buffers (pH 2.00, 4.00, and 10.00). Quartz cuvettes
(10ꢁ10 mm, Starna Cells, Inc.) were employed in the UV/Vis ab-
sorption and fluorescence experiments, unless stated otherwise.
Materials
Unless otherwise stated, all reagents and solvents were used as re-
ceived from Sigma–Aldrich. N-Ethyl benzothiazolium iodide 1,[9] 4,[9]
13a]
and CB[7][23,
were synthesized according to published proce-
dures. Flash chromatography was performed over SiO2 (Macherey–
Nagel 60m, 0.04–0.063 nm, 230–400 mesh, dried at 1308C for
60 min) or neutral alumina (Acros Organics, 50–200 mm, activity 1,
dried at 1308C for 30 min). The concentration of the CB[7] stock
solutions was determined by titration with a known concentration
of cobaltocenium hexafluorophosphate, according to the method
developed by Yi and Kaifer.[24] Deionized water was freshly pre-
pared by using a water purification system (Millipore synergy-UV;
ꢃ18.2 MWcm; pH 6.9). Stock solutions of aqueous HCl (1.0m) in
deionized water, 1a (394 mm) in CH3CN, and CB[7] (733 mm) in de-
ionized water were prepared and used to obtain lower concentra-
tion solutions. Solutions containing derivative 1a were handled
and stored in the dark to prevent photodegradation.
Binding isotherm determination
The binding isotherm for 1a binding to CB[7] in aqueous solutions
was collected by the addition of stock solutions of CB[7] to an
aqueous solution of 1a (2.5 mL) and by measuring the UV/Vis
spectra for 1a in the presence of different concentrations of CB[7].
The dependence of the change in the signal for the absorbance at
l=556 nm on the concentration of CB[7] was fit with a 1:1 CB[7]/
1a binding model [Eq. (1)][29] by using the program Origin (Orgin-
Lab Inc.).
Instrumentation
1H and 13C NMR spectra were collected on 300 and 75 MHz Bruker
spectrometers, respectively, at 300 K. [D6]DMSO and all deuterated
solvents were purchased from Sigma–Aldrich and used as received.
IR spectra were recorded on a Thermo Scientific Nicolet 6700 FTIR
spectrometer. Raman spectra were recorded with an Invia Renish-
aw Raman microspectrometer by using the l=633 nm laser line of
an He–Ne laser at room temperature with a low laser power. The
high-resolution mass spectra (HRMS ESI-TOF) were collected on
a Mariner Perceptive Biosystem. Melting points were recorded on
a Thermo Scientific Q20 DSC apparatus. 1,2-HSQiMs were prepared
on a CEM Discover MW apparatus, equipped with air-cooled and
power-control systems.
DlmaxK½CB½7ꢄꢄ
ð1Þ
Dl ¼
1 þ K½CB½7ꢄꢄ
in which DImax represents the signal changes between two condi-
tions in which 1a is free in water and completely bound to CB[7];
K represents the equilibrium binding constant for 1a/CB[7] com-
plexation.
Photophysical measurements in organic solvents
The experiments were carried out in solutions of air-equilibrated
acetonitrile at 298 K. UV/Vis absorption spectra were recorded on
a PerkinElmer l650 spectrophotometer. Fluorescence spectra were
obtained with a Varian Cary Eclipse spectrofluorimeter, equipped
with a Hamamatsu R928 phototube. Fluorescence quantum yields
were measured by following the method of Demas and Crosby[25]
Electrochemistry
The experiments were carried out in argon-purged CH3CN at room
temperature. In CV, the working electrode was a glassy carbon
electrode (0.08 cm2), the counter electrode was a Pt spiral, and
a silver wire was employed as a quasi-reference electrode (AgQRE).
Chem. Eur. J. 2014, 20, 6412 – 6420
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6418
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
The potentials reported were referenced to SCE by measuring the
AgQRE potential with respect to ferrocene (E1/2 =0.39 V vs. SCE for
Fc+/Fc). The concentration of the compounds examined was of
the order of 1ꢁ10ꢀ3 m; 0.1m TEAPF6 was added as a supporting
electrolyte. Cyclic voltammograms were obtained with scan rates
in the range of 0.05–10 Vsꢀ1. The estimated experimental error on
the E1/2 value was ꢂ10 mV.
126.9, 126.7, 126.5, 126.0, 123.7, 123.0, 122.8, 122.3, 122.0, 121.8,
111.2, 110.5, 82.2, 60.6, 14.2, 14.1, 11.6, 11.5, 11.4 ppm; FTIR (NaCl,
thin film): n˜ =3062–2872, 1715, 1652, 1587 cmꢀ1; HRMS (ESI-TOF):
m/z calcd for C33H30N3O3S2 [M+H]+: 580.172; found: 580.167.
+
Acknowledgements
P.C. thanks the European Commission (ERC StGPhotoSi 278912)
for financial support. C.D.F. thanks FIS SpA for a PhD scholar-
ship. MIUR (FIRB project NANOSOLAR RBAP11C58Y), the Univer-
sity of Padova (Progetto Strategico HELIOS STPD08RCX5, PRAT
CPDA119117/11), and Regione del Veneto (SMUPR n. 4148, Polo
di ricerca nel settore fotovoltaico) are gratefully acknowledged
for financial support. E.G. is grateful to the National Science
Foundation (CHE-1213669) and to the Petroleum Research
Fund (PRF 4663-AC10) for research support, to Rutgers Univer-
sity for a Sabbatical Leave, and to the University of Padova for
a Visiting Scientist Grant.
DFT calculations
Geometry optimization and DFT calculations were performed by
using the Spartan ‘10 software package (Wavefunction, Inc.). DFT
geometry optimization of the ground states of 4 and 1a was per-
formed by using the density functional B3LYP 6-31G* in vacuum.
1,2-HSQiMs 1a–c: General procedure
A solution of 4 in a suitable aniline was introduced into a MW
tube, equipped with a stirring bar under a nitrogen atmosphere.
Ti(OiPr)4 was added to this solution and the resulting suspension
was heated to 80–908C by MW irradiation under vigorous stirring
for 2.5–8 min. The resulting suspension was treated with a saturat-
ed aqueous solution of ammonium chloride (50 mL) and extracted
with CH2Cl2 (3ꢁ75 mL). The combined organic layers were washed
with 2m HCl (30 mL), dried over MgSO4, and evaporated under re-
duced pressure. The residue was purified by flash column.
Keywords: cucurbiturils
·
dyes/pigments
·
host–guest
systems · photochemistry · sensitizers · squaraines
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169.5, 166.1, 159.7, 155.9, 151.6, 148.1, 141.1, 140.6, 137.4, 128.8,
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+
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1
gave a red solid with green metallic flecks (45.9 mg, 21%). H NMR
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7.7 Hz, 1H), 7.50–7.05 (m, 10H), 5.95 (s, 1H), 5.77 (s, 1H), 4.32 (q,
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+
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Published online on April 3, 2014
Chem. Eur. J. 2014, 20, 6412 – 6420
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim