Thermotropic behaviour, self-assembly and photophysical properties of a series of squaraines

Article abstract of DOI:10.1080/10610278.2011.622391

A series of squaraine dyes, based on 2,4-bis[4-(N,N-di-n-alkylamino)-2- hydroxyphenyl] squaraine including ethyl, propyl, butyl, pentyl, hexyl and heptyl derivatives, were synthesised by condensation of the corresponding 4-(N,N-di-n-alkylamino)-2-hydroxyp

Full text of DOI:10.1080/10610278.2011.622391

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Thermotropic behaviour, self-assembly and
photophysical properties of a series of squaraines
Maher A. Qaddoura a , Kevin D. Belfield a b , Paul Tongwa c , Jessica E. DeSanto c , Tatiana V.
Timofeeva c & Paul A. Heiney d
a Department of Chemistry , University of Central Florida , Orlando, FL, 32816, USA
b CREOL, The College of Optics & Photonics, University of Central Florida , Orlando, FL,
32816, USA
c Department of Biology & Chemistry , New Mexico Highlands University , Las Vegas, NM,
87701, USA
d Department of Physics and Astronomy , University of Pennsylvania , Philadelphia, PA,
19104, USA
Published online: 03 Nov 2011.
To cite this article: Maher A. Qaddoura , Kevin D. Belfield , Paul Tongwa , Jessica E. DeSanto , Tatiana V. Timofeeva & Paul A.
Heiney (2011) Thermotropic behaviour, self-assembly and photophysical properties of a series of squaraines, Supramolecular
Chemistry, 23:11, 731-742, DOI: 10.1080/10610278.2011.622391
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Supramolecular Chemistry
Vol. 23, No. 11, November 2011, 731–742
Thermotropic behaviour, self-assembly and photophysical properties of a series of squaraines
Maher A. Qaddouraa, Kevin D. Belfielda,b*, Paul Tongwac, Jessica E. DeSantoc, Tatiana V. Timofeevac and Paul A. Heineyd
b
aDepartment of Chemistry, University of Central Florida, Orlando, FL 32816, USA; CREOL, The College of Optics & Photonics,
University of Central Florida, Orlando, FL 32816, USA; Department of Biology & Chemistry, New Mexico Highlands University,
c
d
Las Vegas, NM 87701, USA; Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
(Received 6 July 2011; final version received 26 August 2011)
A series of squaraine dyes, based on 2,4-bis[4-(N,N-di-n-alkylamino)-2-hydroxyphenyl] squaraine including ethyl, propyl,
butyl, pentyl, hexyl and heptyl derivatives, were synthesised by condensation of the corresponding 4-(N,N-di-n-alkylamino)-
2-hydroxyphenol with squaric acid. The thermal behaviour of the series was recorded using both thermogravimetic analysis
and differential scanning calorimetry while their crystalline structures were elucidated via single-crystal X-ray diffraction.
The length of the alkyl chain proved to have a significant effect on both the thermotropic behaviour and the crystalline
structure of the squaraine series. Two derivatives, butyl and heptyl, revealed the presence of liquid crystalline mesophases,
smectic and nematic, respectively, which were confirmed and characterised via polarised light microscopy and X-ray
diffraction. Several of the derivatives formed H- and/or J-aggregates upon thin film formation via spin coating before and
after the thermal annealing treatment as indicated by UV–vis spectroscopy. The molecular and crystal structure, aggregation
and thermal behaviour provide insight into the supramolecular assembly of this important class of materials. Photophysical
measurements revealed large molar absorptivity, reasonably high fluorescence quantum yields and significant fluorescence
anisotropy by making these derivatives suitable candidates for a number of electro-optic and photonics applications.
Keywords: squaraine dyes; crystalline structure; liquid crystalline phase; H-aggregates; J-aggregates; fluorescence
anisotropy
Introduction
structures that have been reported is based on N,N-
dialkylaniline derivatives that have paved the way to
construct a variety of squaraine dyes (12–14). One of
these derivatives emanates from 3-(dialkylamino)phenol
and has been employed to produce relatively stable
squaraine dyes for different studies including aggregation
Squaraine dyes have been a subject of extensive
investigation due to their applications in a number of
important fields such as bio-imaging probes, bio-
conjugation, second generation photosensitisers for
photodynamic therapy, second harmonic-generating
organic dyes, two-photon absorbing materials with large
cross-section values and light harvesting and photocon-
ductor materials in photovoltaic cells (1). While a large
number of patents and papers has appeared regarding their
applications (2–10), limited work has been reported
concerning their thermotropic behaviour including their
liquid crystalline (LC) properties and the correlation of the
crystalline structure to both solid-state aggregation and to
their photophysical properties (11).
(15, 16) and sublimation (17).
Although several compounds based on 3-(dialkylami-
no)phenol, such as methyl, ethyl, propyl and butyl
derivatives of 2,4-bis [4-(N,N-dialkylamino)-2-hydroxy-
phenyl] squaraine (18, 19), were investigated for their
fluorescence and aggregation properties in solution, very
limited work has been devoted to both their supramole-
cular organisation in the solid state and even less to their
thermotropic LC behaviour. In this work, the series is
expanded to include three new compounds, i.e. pentyl,
hexyl and heptyl derivatives, along with a comprehensive
investigation on their thermal behaviour and self-assembly
including LC properties. The photophysical properties of
the entire series (ethyl to heptyl) were also investigated
(Chart 1).
The squaraine dyes that are the subject of this research
have a donor–acceptor–donor chromophore characterised
by sharp and intense absorption bands in the long
wavelength visible accompanied by red to near-IR
fluorescence in liquid solution. They can be synthesised
by condensation of electron-rich aromatics, such as N,N-
dialkylaniline, phenols, benzothiazoles and pyrroles with
squaric acid (4). One of the fundamental electron-rich
*Corresponding author. Email: belfield@ucf.edu
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2011.622391
http://www.tandfonline.com

732
M.A. Qaddoura et al.
Chart 1. n ¼ 2: SQC2OH; n ¼ 3: SQC3OH; n ¼ 4: SQC4OH; n ¼ 5: SQC5OH; n ¼ 6: SQC6OH; n ¼ 7: SQC7OH.
Experimental
3.86 mmol) in 20 ml of 1:1 butanol and toluene. After 6 h
of reflux, the reaction mixture was cooled to room
temperature and the precipitate was obtained. The solvent
was removed on a rotary evaporator in vacuo and the
product was recrystallised in CHCl3 to yield 1.2 g (81%) of
2,4-bis [4-(N,N-dihexylamino)-2-hydroxyphenyl] squar-
aine as shiny green crystals. MP ¼ 1358C. 1H NMR
300 MHz (d ppm TMS, CDCl3): 0.93 (m, 12H, CH3), 1.32
(m, 24H, CH2), 1.63 (m, 8H, CH2), 3.36 (q, 8H, CH2), 6.10
(d, J ¼ 3 Hz, 2H), 6.32 (dd, J ¼ 3 Hz), 7.88 (d, J ¼ 3 Hz,
2H) and 8.00 (d, J ¼ 9 Hz, 2H). 13C NMR (d ppm TMS,
CDCl3): 13.6, 22.3, 26.4, 31.5, 51.04, 98.5, 107.4, 109.8,
132.3, 156.0, 163.8, 164.5, 171.2, 173.1 and 182.6. LC-
MS: theoretical Mþ ¼ 632.4548, found M/z ¼ 632.4537.
Materials
1-Bromopropane (99%), 1-bromohexane (99% þ), 1-
bromoheptane (99%), 3-(di-n-ethyamino)phenol (99%), 3-
(di-n-butylamino)phenol (99%), aminophenol (99%) and
3,4-dihydroxy-3-cyclobutane-1,2-dione (99%) were pur-
chased from Acros Organics and Aldrich, USA; 1-bromo-
pentane was obtained from Eastman Organic Chemicals,
USA and sodium carbonate (99.5%) was purchased from
Spectrum. 3-(Di-n-propylamino)phenol, 3-(di-n-pentylami-
no)phenol, 3-(di-n-hexylamino)phenol and 3-(di-n-heptyla-
mino) phenol were prepared according to procedures given
in the literature (20). SQC2OH, SQC3OH and SQC4OH
were prepared by the previously reported methods (21, 18).
Synthesis
2,4-Bis [4-(N,N-di-n-heptylamino)-2-hydroxyphenyl]
squaraine (SQC7OH)
2,4-Bis [4-(N,N-(di-n-pentylamino)-2-hydroxyphenyl]
squaraine (SQC5OH)
3-(Di-n-heptylamino)phenol (2.08 g, 7.56 mmol) was
added to 3,4-dihydroxy-3-cyclobutane-1,2-dione (0.39 g,
3.4 mmol) dissolved in 20 ml of 1:1 butanol and toluene.
After 6 h of reflux, the reaction mixture was cooled to
room temperature and the precipitate was obtained. The
solvent was removed on a rotary evaporator in vacuo and
the product was recrystallised from CHCl3 to yield 1.4 g
(60%) of 2,4-bis [4-(N,N-diheptylamino)-2-hydroxyphe-
nyl] squaraine as bright yellowish to greenish crystals.
3-(Di-n-pentylamino)phenol (2.66g, 11.2mmol) was added
to 3,4-dihydroxy-3-cyclobutane-1,2-dione (0.7g, 6.1 mmol)
dissolved in 20ml of 1:1 butanol and toluene. After 7 h of
reflux, the reaction mixture was cooled to room temperature
and the precipitate was obtained. The solvent was removed
on a rotary evaporator in vacuo and the product was
recrystallised in CHCl3 to yield 1.02g (60%) of 2,4-bis [4-
(N, N-dipentylamino)-2-hydroxyphenyl] squaraine as bright
yellowish to greenish crystals. MP ¼ 1858C. 1H NMR
300 MHz (d ppm tetramethylsilane (TMS), CDCl3):0.94(m,
12H, CH3), 1.18 (m, 8H, CH2), 1.34 (m, 8H, CH2), 1.62 (m,
8H, CH2), 3.36 (m, 8H, CH2), 6.1 (d, J ¼ 3 Hz, 2H), 6.3 (dd,
J ¼ 3 Hz), 7.87(d, J ¼ 3 Hz, 2H)and8.01(d, J ¼ 9 Hz, 2H).
13C NMR (d ppm TMS, CDCl3): 13.85, 22.03, 26.91, 28.84,
51.57, 98.24, 107.37, 109.31, 132.5, 155.49, 163.8, 172 and
183. liquid chromatography-mass spectrometry (LC-MS):
theoretical Mþ ¼ 576.3922, found M/z ¼ 576.3895.
1
MP ¼ 1198C. H NMR 300 MHz (d ppm TMS, CDCl3):
0.90 (m, 12H, CH3), 1.32 (m, 32H, CH2), 1.60 (m, 8H,
CH2), 3.37 (q, 8H, CH2), 6.11 (d, J ¼ 3 Hz, 2H), 6.31 (dd,
J ¼ 3 Hz), 7.88 (d, J ¼ 3 Hz, 2H) and 8.03 (d, J ¼ 9 Hz,
2H). 13C NMR (d ppm TMS, CDCl3): 14.29, 22.3, 26.4,
27.8, 29.3, 31.5, 51.6, 98.7, 107.4, 109.6, 131.8, 156.0,
163.5, 164.0, 171.7, 173.1 and 182.6. LC-MS: theoretical
Mþ ¼ 688.5174, found M/z ¼ 688.5189.
Instrumentation and methods
2,4-Bis [4-(N,N-di-n-hexylamino)-2-hydroxyphenyl]
squaraine (SQC6OH)
Thermogravimetric analysis (TGA) was carried out using
a TGA Q5000 thermogravimetric analyser (TA Instru-
ments) at a heating rate of 108C/min. The thermotropic
behaviours of all compounds were determined by a
3-(Di-n-hexylamino)phenol (1.3 g, 4.7 mmol) was added
to 3,4-dihydroxy-3-cyclobutane-1,2-dione (0.44 g,

Supramolecular Chemistry
733
combination of differential scanning calorimetry (DSC)
and polarised optical microscopy. A TA Instruments
Q1000 DSC was used to determine the thermal transitions
during heating and cooling cycles; all heating and
cooling rates were 108C/min; thermal transitions were
read from reproducible second scans. An Olympus BX51
polarised optical microscope (magnification £ 40)
Cambridge CB2 1EZ, UK (Fax: þ44-(0)1223-336033 or
e-mail: deposit@ccdc.cam.ac.uk).
Powder and LC phase X-ray diffraction studies
Temperature-dependent X-ray diffraction measurements
of powder samples employed a Bruker-Nonius FR591
equipped with both
a DP70 microscope digital
˚
fine-focus generator with a Cu target (l ¼ 1.542 A),
camera and an Instec HCS302 heating/cooling stage was
used to detect and image the phase transitions by
observing thin samples between a clean glass slide and a
cover slip.
Osmic confocal optics and a Bruker HiStar wire detector.
Measurements were obtained at a fixed sample–detector
distance of 11 cm. The scattered intensity was recorded for
scattering vectors in the range 0.2 A21 # q ( ¼ 4p sin
˚
(u)/l) # 1.7 A21. Measurements were obtained between
˚
room temperature and the clearing point upon both heating
and cooling. The heating and cooling rates were both
108C/min, and the data were collected for 5 min at each
temperature. The primary data analysis was carried out
using Datasqueeze (http://www.datasqueezesoftware.
com). Thin films were prepared by spin coating a 2%
solution of the dye in CHCl3 at 2000 rpm.
Single-crystal X-ray diffraction study
Single crystals of the series of squaraine dyes were grown
by slow evaporation from CHCl3 for single-crystal X-ray
diffraction, which was carried out with a Bruker SMART
APEX II diffractometer with CCD area detector (graphite
˚
monochromated Mo-Ka radiation, l ¼ 0.71073 A) and v-
scans with a 0.5o step in v at 100 K. The semi-empirical
method SADABS was applied for absorption correction
for all compounds; data reduction and further calculations
were performed using Bruker SAINT þ and SHELXTL
NT program packages. The structures were solved by
direct methods and refined by the full-matrix least-squares
technique against F 2 with the anisotropic temperature
parameters for all non-hydrogen atoms except SQC5OH,
where the disordered C-atoms were refined in isotropic
approximation because of the disorder of the side chains.
All hydrogen atoms were calculated from the geometrical
point of view within the riding model except for the
hydroxyl groups in SQC3OH and SQC4OH, where they
were located from the Fourier electron density synthesis.
In the three compounds, the hydroxyl groups are
disordered over two positions with the occupancies
0.75:0.25 in SQC4OH, 0.85:0.15 in SQC5OH and
0.57:0.43 in SQC6OH and alternate with hydrogens in
these positions. In the molecule SQC5OH that resides on
an inversion centre, two alkyl side chains are disordered
over two positions with the occupancies 0.60:0.40 and
0.56:0.44, respectively, while in the molecule SQC6OH
which occupies general position three of four alkyl chains
are disordered over two positions with the occupancies
0.54:0.46; 0.63:0.37 and 0.63:0.37, respectively. The
details of crystal structures are given in the Supporting
Information, available online. The X-ray data and the
details of the structure refinement are summarised in Table
S1; the selected geometry parameters are given in Table S2
and the hydrogen-bonding geometry is given in Table S3.
Crystallographic data for the structures in this paper have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication numbers CCDC
829763–829767. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road,
Absorbance measurements were obtained using Agilent
8453 UV–vis spectrophotometer; the steady-state emission
and excitation anisotropy studies were carried out using a
Photon technologies, Inc. (PTI) QuantaMaster spectro-
fluorimeter in 10mm spectrofluorometric quartz cuvettes
with C , 1026 M and Rhodamine 6G as reference.
Fluorescence spectra were corrected for the spectral
responsivity of the PTI emission monochromator and
photomultiplier tube. Gas chromatography-mass spec-
trometry analyses were conducted in the Mass Spectrometry
Services, Chemistry Department at the University of
1
Florida. H and 13C NMR analyses were carried out in
CDCl3 (referenced to TMS at d 0.0 ppm) using Varian NMR
spectrometers (500 or 300 MHz for 1H and 75MHz for 13C).
Results and discussions
A series of six squariane dyes varying in the length of their
peripheral alkyl groups were investigated for their thermal
properties, phase transitions, crystalline structure, aggre-
gation and photophysical properties. The synthetic
procedure for their preparation involved an etherification
with the corresponding 18 alkyl bromide with 3-
aminophenol, followed by a subsequent condensation
reaction between the 3-(di-n-alkylamino)phenol derivative
with squaric acid in a mixture of butanol and toluene (14,
22) as shown in Scheme 1.
The crystal structures
Crystal structures of SQC2OH and mono-hydroxylated
analogue of SQC4OH were reported previously (23, 24).
We present herein the first report on the crystal structures
of SQC3OH, SQC4OH, SQC5OH, SQC6OH and

734
M.A. Qaddoura et al.
In the molecule SQC5OH, significant deviations of
disordered pentyl chains in ‘up’ and ‘down’ directions
relative to the core were observed. In SQC6OH, two hexyl
side chains are extended in the plane of the main core,
while the other two are oriented nearly perpendicular to
the central planar fragment. In the molecule SQC7OH, the
heptyl chains are located in planes nearly parallel to the
central core ‘above’ and ‘below’ this plane.
It is known that in some cases solid crystalline
precursors can demonstrate a 3D structure similar to 1D or
2D structures of LC phases. The packing in the crystal
might also demonstrate the type of molecular associates
that can be formed by certain materials in solution. The
careful analysis of the molecular packings (Figures 1 and 2)
revealed some similarities within this series of compounds.
The molecules SQC3OH, SQC4OH and SQC5OH form
H-bonded ribbons at the position of the cores in one plane
that might be considered as 1D associates (Figure 1). The
packing of these ribbons essentially differs in SQC3OH
and in SQC4OH and SQC5OH. The short propyl chains
in SQC3OH do not impede the stacking of the parallel
ribbons in the partially overlapping mode giving rise to the
layers strengthened by weak CH· · ·O hydrogen bonding.
The layers are packed in a herringbone mode. In the
crystals SQC4OH and SQC5OH, the butyl and pentyl
side chains prevent the overlapping of the ribbons,
providing their identical herringbone arrangement in
both structures being quite similar to the non-hydroxylated
and mono-hydroxylated analogous of SQC4OH (23). The
SQC4OH herringbone arrangement is also consistent with
the structure previously reported using scanning tunnelling
microscopy investigation (25).
Scheme 1. The general procedure to synthesise the series of
2,4-bis[4-(N,N-di-n-alkylamino)-2-hydroxyphenyl] squaraines;
n ¼ 2: SQC2OH; n ¼ 3: SQC3OH; n ¼ 4: SQC4OH; n ¼ 5:
SQC5OH; n ¼ 6: SQC6OH; n ¼ 7: SQC7OH.
SQC7OH. The crystal structure of SQC2OH was not
repeated as we were unable to obtain suitable single
crystals. Of the compounds studied, only SQC3OH
crystallises in the non-centrosymmetric P21 space group,
while four other compounds crystallise in the centrosym-
metric space groups P21/c (SQC4OH, SQC6OH), P21/n
(SQC5OH) and P-1 (SQC7OH). The molecules
SQC3OH and SQC6OH occupy general positions with
the whole molecule in the asymmetric unit, while
SQC4OH, SQC5OH and SQC7OH reside on inversion
centres. The planarity of the central core that includes a
four-membered ring and two phenyl rings is sustained by
the presence of the conjugation and the hydrogen bonding
between the hydroxyl groups and the carbonyl atoms in
which the geometry parameters are very close in all
molecules except SQC6OH where deviations were caused
by the crystal structure disorder. The CZC as well as CZO
distances in the squaraine central fragment reflect its high
symmetrical electron distribution (Supporting Infor-
mation, available online, Table S2). The squaraine
structures also demonstrate a quinoid character of both
phenyl substituents that have donor and acceptor groups in
para positions. Despite the similarity of the central
molecular fragment and its high symmetry, the molecules
differ significantly by the orientations and conformations
of the flexible alkyl substituents. The short alkyl chains of
SQC3OH and SQC4OH located on both sides of the
central core have different orientations and conformations.
We can speculate that similar aggregates can be
formed for both SQC6OH and SQC7OH in solution but
not in crystals where close packing most likely prevents
such arrangement. Nonetheless, associates resembling
ribbons that can be seen in both these structures (Figure
2(A), (D)). In the crystals SQC6OH and SQC7OH, the
molecular cores are oriented in one direction and they are
surrounded by long alkyl substituents that impede the self-
association of the molecules in the planar ribbons (Figure
2). In SQC7OH, the CH· · ·O bonds link the parallel
molecules in the stair-case-like ribbons. Both these
arrangements might be considered as a LC precursor, but
our observations showed that it was realised only in
SQC7OH that was confirmed by polarised light
microscopy (PLM) texture observations.
Thermotropic behaviour
The length of the peripheral alkyl group appeared to exert
a profound effect on the thermal stability for each
compound. It was observed that the shorter the alkyl
group, the higher the decomposition temperature (Table 1).
This observation can be explained by the intermolecular

Supramolecular Chemistry
735
Figure 1. Fragments of crystal packing in SQC3OH, SQC4OH and SQC5OH. SQC3OH: (A) H-bonded ribbon, (B), (C) the
herringbone packing of the ribbons; SQC4OH: (D) H-bonded ribbon, (E) herringbond arrangement of the ribbons with short CH· · ·O
contacts between them, (F) crystal packing; SQC5OH: (G) H-bonded ribbon, (H) herringbond arrangement of the ribbons with no evident
short CH· · ·O contacts between them, (I) crystal packing.
interactions including both hydrogen bonding and van der
Waals’ forces for electron-rich regions for the lower
peripheral alkyl chain structure. However, by increasing
the length of the hydrophobic alkyl chains, the
intermolecular forces become weaker resulting in low
melting points.
Although the cooling cycles did not show recrystalli-
sation for the series of squaraine compounds except for
SQC3OH (Figure 3(B)), the PLM analysis confirmed the
recrystallisation process (Figures 4–7). This behaviour
may be due to the sample preparation and the container
and supercooling effect. Whilst the DSC pan offered a
smooth and clean surface that hindered the alkyl
squaraines to pack and recrystallise owing to both the
smaller surface area and smaller London attractions of the
branched alkyl chain (26). This has been previously
reported, possibly involving the reorientational dynamics
during the supercooling process (27, 28) and increased
mobility of the alky chains (29). The surface of the glass
slides used in PLM likely caused the induction of both the
nucleation and the crystal growth process due to surface
scoring or defects during the cooling process at room
temperature. However, upon cooling very slowly at
The thermotropic behaviour was investigated for each
of the squaraine dyes using DSC (Figure 3). The n-propyl,
n-pentyl and n-hexyl derivatives (Figure 3(B),(D),(E),
respectively) revealed sharp melting transition tempera-
tures in addition to weak crystalline transitions for n-
pentyl (Figure 3(D)) and n-hexyl (Figure 3(E)) derivatives,
as observed by PLM, whereas the n-butyl (Figure 3(C))
and n-heptyl (Figure 3(F)) analogues exhibited phase
transitions. Only the ethyl derivative did not show any
melting process but rather only decomposition
(Figure 3(A)).

736
M.A. Qaddoura et al.
Figure 2. Fragments of crystal packing for SQC6OH and SQC7OH. SQC6OH: (A) chain of corrugated molecules, (B), (C) the
alignment of the molecules in the crystal, H-atoms are omitted; SQC7OH: (D), (E) H-bonded stair-case ribbon – top and side views, (F)
crystal packing.
Table 1. Summary of the thermotropic properties of the squaraine derivatives.
Decomposition
(8C)
Phase transition-1
(8C)
Enthalpy
(KJ mol21
Phase transition-2
(melting point, 8C)
Enthalpy
(KJ mol21
)
)
SQC2OH
SQC3OH
SQC4OH
SQC5OH
SQC6OH
SQC7OH
240
227
224
226
228
232
n/a
n/a
164
n/a
n/a
99
n/a
n/a
9.08
n/a
n/a
24.5
n/a
221
196
185
135
119
n/a
53.4
37.6
50.4
33.4
24.8
18C/min, the phase transition signals for SQC7OH began
to appear on the cooling curve as seen in Figure 4.
The SQC3OH squaraine also revealed a sharp melting
point followed by recrystallisation upon cooling; any
further heating above 2218C resulted in abrupt decompo-
sition (Figure 3(B)). It was discovered that SQC4OH and
SQC7OH exhibited LC phases; the mesophase occurrence
is dependent on the intrinsic properties of the dye such as
the conformation and arrangement of the molecule and the
intermolecular interactions. Typically, the generation of a
LC phase implies the loss of long-range crystalline order.
A smectic phase is characterised by short-range, liquid-
like positional order within well-defined sheets or layers,
while in a nematic phase the molecules have long-range
orientational order of the long axis director but only short-
range positional order. Upon further heating all order is
lost, forming the isotropic liquid (30, 31).
Compound SQC4OH displayed two phase transitions
upon heating, as seen in Figure 3(C). Although crystal-
lisation upon cooling was not observed by DSC, the phase
transition was clearly observed by PLM upon cooling
(Figure 6(C)). The transition temperatures and the
enthalpy of transitions, determined by DSC, are shown
in Table 1. At 1648C, 9.08 kJmol21 of energy was released
attributed to the disruption of the translational order of the
crystal lattice, likely ascribed to the crystalline-smectic-
like mesophase. However, at 1968C, a significantly larger
enthalpy value (37.6 kJmol21) was recorded due to major
disruption in the molecular packing to both the positional
and orientational order, forming anisotropic liquid. The
mesophase for SQC4OH was observed and identified by
the texture morphology observed by PLM upon cooling of
the molten crystals. The principle of the phase
identification depends on observing the microscopic
textures by PLM, characterised by the defects, the elastic

Supramolecular Chemistry
737
Figure 3. The thermotropic behaviour for the series of squaraine dyes. (A) The DSC thermogram for SQC2OH, no melting point was
detected, it decomposed at 2408C. (B) The DSC thermogram for SQC3OH, it exhibited a melting point at 2218C followed by
recrystallisation at 2208C. (C) DSC thermogram for SQC4OH revealed a phase transition at 1648C and a melting temperature at 1968C.
(D) DSC thermogram for SQC5OH showed a melting point at 1858C. (E) The DSC thermogram for SQC6OH exhibited a melting point
at 1358C. (F) DSC thermogram for SQC7OH revealed a phase transition at 998C and a melting temperature at 1198C.
deformation and the birefringence between two glass
slides. The interaction of the polarised light with the
topology of the defects and the deformation elasticity, in
combination with the birefringence, results in character-
istic texture morphology for each LC phase (32, 33). When
external forces are applied, e.g. shear stress on the slides
during sample preparation, the difference in the bulk
elasticity and the fluidity (elastic constants) for each phase
results in a distinguished texture morphology (34). Figure
6(A) presents the growth of the mesophase from the melt
upon cooling. The fan-shaped nuclei are characterised by
extinction crosses that appear dark when the optical axis is
parallel to either the polariser or the analyser. In Figure
6(B), the transition from the mesophase to the crystalline
texture was revealed; while Figure 5(C) illustrates the
crystalline texture at thin thickness exhibiting different
birefringence.
The DSC for SQC5OH showed only a sharp melting
point at 1858C as can be seen in Figure 3(D). However,
PLM revealed the presence of a phase transition assumed
to be between crystalline phase; the crystalline-to-
Figure 5. The recrystallisation texture of pentyl and hexyl
derivatives of squaraine dyes as can be seen under PLM; the
coloured texture is a result of the birefringence. (A) The
crystalline texture of SQC5OH. (B) The crystalline texture of
SQC6OH.
Figure 4. The DSC cooling cycle at 18C/min for SQC7OH
reveals the phase transitions at 988C and 948C.

738
M.A. Qaddoura et al.
Figure 6. Phase transitions for SQC4OH. (A) The growth of
the mesophase from the melt upon cooling at 1968C; the fan-
shaped nuclei are characterised by extinction crosses that appear
dark when the optical axis is parallel to either the polariser or the
analyser. These extinction crosses rotate along the direction of
the rotating polariser, indicating that the liquid crystal is optically
positive. (B) The transition from the mesophase to the crystalline
morphology at 1648C. (C) The crystalline texture morphology for
thin film of SQC4OH.
Figure 8. Powder X-ray diffraction patterns for SQC7OH.
Left: false colour images of measured intensity; right: radial plots
of circularly averaged intensity. (A) Crystalline structure at 998C,
showing Bragg peaks. (B) Nematic phase at 1038C, showing
diffuse liquid-like peak at 58(the peak at wider angle is primarily
due to the glass capillary and alkyl groups).
Figure 7. (A) Crystalline-to-crystalline phase transition at
1408C for SQC5OH; the difference in colour in both the
morphologies is attributed to deformations and birefringence (the
difference between extraordinary ray and ordinary ray of the
polarised light when entering the sample). (B) Crystalline-to-
crystalline transition for SQC6OH at 1108C; each phase has a
slightly different type of topological defect and deformation. The
interaction of the polarised light with the deformation and defect
for each phase results in different path differences and
birefringence.
and the enthalpy of transitions determined by DSC are
shown in Table 1; at 998C, 24.5 kJmol21 of energy was
released due to the disruption of the translational order,
which might be ascribed to the solid crystalline-
mesophase transition. Furthermore, at 1198C, the energy
released was 24.8 kJ mol21 attributable to a major
disruption of the molecular packing in both the positional
and orientational order to become an isotropic liquid.
Powder X-ray diffraction (Figure 8) was utilised to
identify and confirm the phase transitions. A crystalline
phase was observed at room temperature (see Figure 7),
while above 998C a phase with a nematic-like broad peak
crystalline transition can be seen in Figure 7(A). The
crystal packing of SQC5OH (Figure 1) determined by
single-crystal X-ray crystallography may provide some
rationale to account for the crystalline-to-crystalline
transition. This type of constrained packing impedes the
molecule from undergoing considerably favourable
translational and rotational effect towards forming a LC
phase. However, it is only capable of undergoing a
crystalline-to-crystalline transition as this requires mini-
mum translational and rotational energy. The same
argument pertains to SQC6OH. Similarly, SQC6OH not
only demonstrates a sharp melting point, as can be seen in
the DSC thermogram (Figure 3(E)), but also a weak phase
transition when observed by PLM. The unknown phase
transition is assumed to be crystalline-to-crystalline
transition, see Figure 7(B).
at 0.374 A21 was observed that is consistent with the DSC
˚
data. At 1298C, the signal disappeared completely, most
likely due to transition to the isotropic phase.
The texture observations for SQC7OH were con-
ducted using PLM upon cooling. Figure 9 shows the
appearance of natural texture of the nematic phase without
showing the Schlieren texture.
Aggregation in the solid state
Aggregation of squaraines in the microcrystalline state has
been reported for squaraine structures bearing the N-
pyrrolidine group for xerographic applications (35, 36).
Spin casting of a symmetrical squaraine, 2,4-bis[4-(N,N-
di-isobutylamino)-2,6-dihydroxyphenyl]squaraine, was
investigated for enhancing the power conversion efficiency
The DSC thermogram for SQC7OH (Figure 3(F))
clearly demonstrates the presence of a phase transition
upon heating. Although a weak transition was observed
upon cooling by DSC, this phase transition was clearly
evident by PLM upon cooling. The transition temperatures

Supramolecular Chemistry
739
and formation of a head-to-tail arrangement (J-aggregate,
red shift) (45–47).
One can see from Figure 10(A) that, upon spin coating
of SQC3OH, both large slip angles and smaller ones result
in multiple H-aggregates at 503 and 566 nm and J-
aggregates at 732 and 787 nm compared with the monomer
absorbance in CHCl3 solution at 647 nm. After thermal
annealing, the vibrational and the rotational energies and
collisions result in a new state of equilibrium and new state
of molecular arrangement manifested in a wider slip angle
and formation of primarily of H-aggregates.
In Figure 10(B), a large slip angle appears dominant
upon spin coating SQC4OH, resulting in prevailing H-
aggregation at 546 nm. However, after thermal annealing,
the slip angle became smaller, resulting in an H-aggregate
shifted to a slightly longer wavelength (607 nm) triggering
the formation of J-aggregates (767 nm).
Similarly, SQC5OH underwent formation of H-
aggregates upon spin coating (absorption at 537 nm)
while the annealing process exerted little influence on the
slip angle, resulting in shifting the H-aggregate absorption
from 537 to 570 nm (Figure 10(C)). Finally, both
SQC6OH and SQC7OH tended to form H-aggregates in
spin-coated films before and after thermal annealing
without any likely change in the slip angle (Figure
10(D),(E)). The herringbone packing in SQC3OH,
SQC4OH and SQC5OH structures may account for the
capability of the molecules to change the slip angle and to
shift the aggregation after thermal annealing. Otherwise,
the alignment of the molecules along one direction in
SQC6OH and SQC7OH may account for their inability to
change the slip angle and alter aggregation after thermal
annealing. These types of solid-state aggregates may be
advantageous in certain electro-optic applications, e.g. in
enhancing both photovoltaic cell performance and
efficiency (48, 49).
Figure 9. The natural texture of the nematic for SQC7OH.
for photovoltaic cells (37). Formation of a Langmuir–
Blodgett film was utilised to study the aggregation of
monolayers of selected squaraines on the surface (15, 38–
41). However, limited studies have been reported to
systematically investigate the effect of molecular structure
on the aggregation properties in solid-state films (24, 42–
44). Hence, we investigated the aggregation behaviour of a
series of squaraine compounds in the solid state (thin
films) using a spin coating method on quartz substrates to
probe the effect of the peripheral alkyl chain on their
aggregation behaviour before and after thermal annealing
and to correlate their aggregation properties with their
crystalline structures. Exciton theory was used as a model
to explain the shifts of the absorption band before and after
the thermal annealing process. Based on the molecular
exciton theory, the angle observed between the long axis of
the squaraine molecule and the straight line passing
through the centre of the molecular aggregates is called the
slip angle and is considered essential in determining the
type of aggregates, i.e. a large slip angle results in a
parallel plane-to-plane stacking and formation of a
sandwich-type arrangement (H-aggregate, blue shift),
whereas a smaller slip angle results in end-to-end stacking
Photophysical properties
The absorption and fluorescence emission of several
symmetrical and unsymmetrical squaraines with diverse
molecular entities were reported (18, 50). Here, the
photophysical properties for the series of squaraines based
on 3-(di-n-alkylamino)phenol derivatives were investi-
gated while exploring the possible effect of the peripheral
alkyl chain on their photophysical properties. The series of
the synthesised squaraine dyes was characterised by higher
molar absorptivity, smaller Stokes shifts and relatively
narrow absorption bands in solution, making this series
particularly attractive for applications requiring narrow
and specific excitation wavelength ranges.
Although the length of the peripheral alkyl group may
slightly increase the electron-donating ability of the
nitrogen atom, and, consequently, lead to an increase in the
fluorescence quantum yield, this effect was negligible.

740
M.A. Qaddoura et al.
Figure 10. (A) Comparison of absorption spectra for SQC3OH: (1) after spin coating and thermal annealing, (2) after spin coating and
(3) in CHCl3 solution, lmax ¼ 647 nm. (B) Comparison of absorption spectra for SQC4OH: (1) after spin coating and thermal annealing,
(2) after spin coating and (3) in CHCl3 solution, lmax ¼ 648 nm. (C) Comparison of absorption spectra for SQC5OH: (1) after thermal
annealing, (2) after spin coating and (3) in CHCl3 solution, lmax ¼ 648 nm. (D) Comparison of absorption spectra for SQC6OH: (1) after
thermal annealing, (2) after spin coating and (3) in CHCl3 solution, lmax ¼ 650 nm. (E) Comparison of absorption spectra for SQC7OH:
(1) after thermal annealing, (2) after spin coating and (3) in CHCl3 solution, lmax ¼ 653 nm.
Rather, we observed high fluorescence quantum yields for
SQC2OH and SQC3OH consistent with the literature
data and with an increase in molecular rigidity for the short
alkyl chain derivatives (51). However, the rest of the series
exhibited a moderate and nearly constant fluorescence
quantum yield with no significant effect as a function of
alkyl chain length (Table 2). The reasonably high
luminescence quantum yields suggest the potential of
these dyes for bio-imaging and laser applications.
anisotropy value of 0.4 indicates perfectly collinear
transition dipoles, i.e. the orientation of the absorption
and emission transition dipole moment are parallel, which
is in accordance with a strong single low-energy
absorption band (S0 ! S1 transition) (52). Fluorescence
anisotropy spectra for the series followed a consistent
trend as seen in Figure 11. They are characterised by a
maximum excitation anisotropy value between 0.3 and 0.4
corresponding to S0 ! S1 transitions. Upon moving to
shorter wavelengths, one encounters a change in the slope
of the excitation anisotropy. At ca. 500 nm, a negligible
absorption is observed corresponding to a one-photon
forbidden S0 ! S2 transition. At around 400 nm, a
Steady-state fluorescence excitation anisotropy is
another important property measured for all members of
the series. This technique often reveals important
information regarding electronic transitions and a
molecule’s rotational dynamics. A maximum excitation
Table 2. The linear and nonlinear photophysical properties for the series of squaraine dyes.
Molar absorptivity, 1
(L Mol21 Cm21 £ 104)
lmax CH3CN
(nm)
Emission
(nm)
Stokes shift
(nm)
Fluorescence quantum yield
SQC2OH
SQC3OH
SQC4OH
SQC5OH
SQC6OH
SQC7OH
17
24
20
19
24
19
640
643
645
645
645
653
654
658
660
661
662
665
14
15
15
16
17
12
0.73
0.59
0.42
0.40
0.38
0.37

Supramolecular Chemistry
741
Figure 11. Normalised absorption (green), emission (red) and steady-state fluorescence excitation anisotropy (blue) spectra for the
entire squaraine series. These are characterised by the maximum excitation anisotropy value approximately between 0.3 and 0.4 at long
wavelength corresponding to S0 ! S1 transitions.
transition is observed corresponding to higher energy
S0 ! Sn transitions.
molecules, resulting in a promising new class of liquid
crystals based on the squaraine molecular structure.
Supporting Information Available. Syntheses of
SQC2OH, SQC3OH, SQC4OH, TGA thermograms,
bond length data, molecular structures derived from
single-crystal X-ray crystallographic data, crystallo-
graphic data and hydrogen-bonding interaction data from
X-ray crystal structures are presented.
Conclusions
The alkyl chain in the series of squaraine compounds
exerts a profound effect on the properties of these dyes.
For each carbon atom added to chain, a new behaviour is
revealed that is manifested in different crystalline
structures and thermotropic phase transitions. The
difference in crystalline structures is evident during the
aggregation in the microcrystalline state: only SQC3OH,
SQC4OH and SQC5OH formed H- and/or J-aggregates
upon spin coating before and after thermal annealing
owing to the formation of the herringbone structure that
may account for the capability of the molecules to change
the slip angle and to shift the aggregation after the
annealing process. Otherwise, SQC6OH and SQC7OH
exhibited only H-aggregation before and after annealing,
possibly explained by having only parallel molecular
arrangement in the crystal that accounts for their inability
to change the slip angle and to shift their aggregation after
thermal annealing treatment.
Acknowledgements
We wish to acknowledge the National Science Foundation (CHE-
0832622 – KDB and TVT, DMR-0934212 – TVT and DMR-
0520020 – PAH) and the National Academy of Sciences (PGA-
P210877) for support of this work.
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