Functionalized cyano-OPVs as melt-processable two-photon absorbers

Article abstract of DOI:10.1039/c2jm15846a

Several cyano-functionalized oligo(phenylenevinylene) (cyano-OPV) dyes were modified with different alkyl substituents with the objective to develop melt-processable, two-photon absorbing materials. The new dyes exhibit two-photon absorption cross-section

Full text of DOI:10.1039/c2jm15846a

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PAPER
Functionalized cyano-OPVs as melt-processable two-photon absorbers†
a
Brian T. Makowski, Joseph Lott, Brent Valle, Kenneth D. Singer and Christoph Weder
a
b
b
ac
*
*
Received 14th November 2011, Accepted 5th January 2012
DOI: 10.1039/c2jm15846a
Several cyano-functionalized oligo(phenylenevinylene) (cyano-OPV) dyes were modified with different
alkyl substituents with the objective to develop melt-processable, two-photon absorbing materials. The
new dyes exhibit two-photon absorption cross-sections in the range of 400–850 GM, can be melt-
processed at temperatures as low as ꢀ100 ꢁC, and offer high thermal stability. The propensity of these
molecules to form excimers also makes them interesting for sensor and data storage applications.
dissolution entropies, leading to limited solubility and high
melting temperatures. Nevertheless, for many devices, such as
optical switches, multiplexers based on macroporous silicon
structures that are locally infiltrated with a neat NLO dye,14 or
multilayered adaptive dielectric mirrors,15 it would be desirable if
the NLO material could be melt-processed at low temperatures
and/or processed from high-concentration solutions. We here
report the investigation of several cyano-substituted oligo(phe-
nylenevinylene)s (cyano-OPVs, Fig. 1), which can offer rather
large two-photon absorption cross-sectionsꢁand are readily melt-
processed at temperatures as low as ꢀ100 C. These properties,
together with their propensity to form excimers, render them
attractive for use in polymer-based optical storage systems that
operate on the basis of TPA-induced switching of the aggrega-
tion state and fluorescence.6
1. Introduction
Two-photon absorption (TPA) is a nonlinear optical (NLO)
process that involves the simultaneous absorption of two
photons of identical or different frequencies, leading to the
excitation of a molecule from the ground state to a higher energy
electronic state.1 TPA-related processes such as nonlinear
transmission and intensity-dependent refraction can be exploited
in a plethora of optoelectronic devices such as waveguides,2
optical switches,3 lasers,4 optical limiters,5 and 3-dimensional
optical data storage systems.6 TPA can also be used to trigger
subsequent events, such as fluorescence or chemical reactions,
which enable applications such as biological imaging,7 photo-
dynamic therapy,8 and 3-dimensional lithography.9 Significant
efforts have been directed towards the development of organic
TPA chromophores that possess large TPA cross-sections (s),
which permit one to exploit NLO processes with lower-energy
light sources. Strategies to maximize a chromophore’s s include
extending the conjugation length and dimensionality,10,11 as well
as the introduction of electron donating/accepting (D/A) groups
to create ‘‘push–pull’’ systems.12 An effective design involves
symmetrically substituted D–p–D chromophores in which two
donors are conjugated by a bridge (p) so that excitation leads to
large changes of the quadrupole moment.13 The integration of
additional electron-accepting groups in the conjugated system,
so that D–p–A–p–D structures are created, further increases s.12
Maximization of s, however, often comes at the expense of
processability, since extended p-systems usually exhibit high
melting and dissolution enthalpies and low melting and
Many low-molecular weight oligo(phenylene vinylene) dyes
(OPVs) have already been reported to display appreciable TPA
cross-sections of between 400 and 2000 GM, depending upon the
detailed chemical structure and also the solvent.16,17 The largest
nonlinearities are seen in OPVs in which donor and acceptor
groups are connected to create D–p–A–p–D structures, with the
objective to maximize the extent of intramolecular charge
aDepartment of Macromolecular Science and Engineering, Case Western
Reserve University, Cleveland, OH, 44106, USA
bDepartment of Physics, Case Western Reserve University, Cleveland, OH,
44106, USA
cAdolphe Merkle Institute and Fribourg Center for Nanomaterials,
University of Fribourg, CH-1723 Marly 1, Switzerland. E-mail: kenneth.
singer@case.edu; christoph.weder@unifr.ch
† Electronic
supplementary
information
(ESI)
available:
Crystallographic data and procedures for the structural analysis of A3;
differential scanning calorimetry traces; absorbance and fluorescence
spectra. See DOI: 10.1039/c2jm15846a
Fig. 1 Chemical structures of the nonlinear optical dyes investigated in
this study. EHO ¼ 2-ethylhexyloxy; DMO ¼ 3,7-dimethyloctyloxy.
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transfer (ICT). For example, Pond et al. showed that the
attachment of cyano groups to the central benzene ring in
OPVs with peripheral donor groups increases s by as much as
900 GM.18 Interestingly, the TPA response of OPVs featuring
cyano groups attached to the vinylene bridge, and which may be
viewed as compact D–p–A–p–D–p–A–p–D systems, has been
little explored. The compounds found in the literature all carry
the cyano groups in the vinylene positions close to the central
benzene ring (CN0-OPVs).19 With respect to maximizing s, this
configuration is not ideal, since cyano groups in this position are
known to twist the molecule out of plane,19 thereby limiting
conjugation and ICT.20 By contrast, the placement of cyano
groups on the vinylene position away from the central ring (CN00-
OPVs) results in planar molecules.19 Such CN00-OPVs have
attracted our attention as fluorescent ‘‘chameleon dyes’’ due to
their propensity to form highly fluorescent excimer complexes
and the ability to change their emission color upon (dis)
assembly.21 In this context, the high stability of these molecules
has been exploited to melt-process them either in their neat form
or as blends with a range of different polymers.21 Nonetheless,
the TPA of such CN00-OPVs has received little attention, perhaps
on account of the comparably low TPA properties of CN0-OPVs.
Departing from one CN00-OPV derivative whose TPA cross-
section was recently reported (Fig. 1, A2), we here explore
systematic structural variations with the objective to maximize s
and minimize the melting temperature (Tm) for optimum proc-
essability. All of the new dyes form highly fluorescent excimer
complexes and display significant emission color changes upon
transition from solution to the solid state, which makes them also
interesting for sensor applications21 and optical data storage
schemes.6
(d, 4H), 4.14 (t, 4H), 3.93 (d, 4H), 1.86 (m, 4H), 1.80 (m, 2H),
1.57–1.20 (m, 36H), 0.99 (3t, 18H). Calcd: C, 79.57; H, 9.54; N,
3.31. Found: C, 79.39; H, 9.35; N, 3.43%.
Synthesis of (2Z,20Z)-3,30-(1,4-phenylene)bis(2-(4-((2-
ethylhexyl)oxy)phenyl)acrylonitrile) (B2)
4-(2-Ethylhexyloxy)phenylacetonitrile (340 mg, 1.4 mmol), ter-
ephthaldehyde (91 mg, 0.68 mmol), THF (5.7 mL) and t-buOH
(17 mL) were combined and heated to 50 ꢁC while stirred. t-
buOK (0.136 mL of a 1 M solution in THF) and Bu4NOH
(1.35 mL of a 1 M solution in methanol) were added quickly and
a yellow precipitate formed. The reaction mixture was stirred for
another 15 min, cooled to room temperature, and poured into
acidified methanol (34 mL containing 1 drop of concentrated
acetic acid). The precipitate was filtered off, excessively washed
with methanol, and dried in vacuo at 50 ꢁC to yield B2 in the form
1
of a red powder (240 mg, 60%). H NMR (CDCl3): d ¼ 7.95 (s,
4H), 7.63 (d, 4H), 7.43 (s, 2H), 6.98 (d, 4H), 3.91 (d, 4H), 1.76 (m,
4H), 1.55–1.30 (m, 16H), 0.88 (2t, 12H). Calcd: C, 81.59; H, 8.22;
N, 4.76. Found: C, 81.49; H, 7.85; N, 4.86%.
Synthesis of (2Z,20Z)-3,30-(1,4-phenylene)bis(2-(4-nitrophenyl)
acrylonitrile) (C1)
Terephthaldehyde (0.529 g, 4.0 mmol) and 4-nitro-
phenylacetonitrile (1.376 g, 8.5 mmol) were combined in
a 250 mL 3-neck round-bottom flask. Methanol (100 mL) was
added and the mixture was stirred at 50 ꢁC until the reactants had
dissolved. Piperidine (0.86 mL) was quickly added via a syringe.
The color of the reaction mixture instantly turned purple and
a yellow precipitate formed after ꢀ2 min. The reaction mixture
was stirred for another 30 min at 50 ꢁC, cooled to room
temperature, and poured into acidified methanol (380 mL con-
taining 8 drops of concentrated acetic acid). The precipitate was
2. Experimental section
Materials and methods
ꢁ
filtered off and dried in vacuo at 50 C overnight. Recrystalliza-
All commercial reagents were used as received unless otherwise
stated in the text. 4-(2-Ethylhexyloxy)phenylacetonitrile,22 and
molecules A1,23 A2,24 A3,22 and B123 were prepared as previously
tion from DMF yielded C1 in the form of an orange powder
(1.18 g, 71%). Calcd: C, 68.24; H, 3.34; N, 13.26. Found: C,
67.57; H, 3.35; N, 13.30%.
1
described. H NMR spectra were recorded on Varian 300 MHz
Synthesis of (2Z,20Z)-3,30-(2,5-bis((3,7-dimethyloctyl)oxy)-1,4-
phenylene)bis(2-(4-nitrophenyl)acrylonitrile) (C2)
and 600 MHz NMR spectrometers. Chemical shifts were
expressed in ppm relative to the internal chloroform peak at
7.26 ppm. Elemental analyses were conducted by Galbraith
Laboratories, Inc.
2,5-Bis(3,7-dimethyloctyloxy)terephthaldehyde (0.200 g, 0.45
mmol) and 4-nitrophenylacetonitrile (0.1597 g, 0.99 mmol) were
combined in a 25 mL 3-neck round-bottom flask. THF (2.6 mL)
and t-buOH (7.8 mL) were added and the mixture was stirred at
60 ꢁC until the reactants had dissolved. Piperidine (0.11 mL) was
quickly added via a syringe. The color of the reaction mixture
instantly turned orange. The reaction mixture was stirred for
another 60 min at 60 ꢁC and subsequently cooled to room
temperature. The solvents were removed using a rotary evapo-
rator. The remaining solid was dissolved in CHCl3, the solution
was filtered through a plug of silica gel, and the solvent was
removed using a rotary evaporator. The crude orange product
was recrystallized from 2-propanol to yield C2 in the form of
a deep red powder (0.200 g, 60%). 1H NMR (CDCl3): d ¼ 8.34 (d,
4H), 8.19 (s, 2H), 7.95 (s, 2H), 7.86 (s, 4H), 4.21 (t, 4H), 1.93 (m,
2H), 1.69–1.12 (m, 14H), 0.98 (d, 6H), 0.84 (d, 12H). Calcd: C,
71.91; H, 7.41; N, 7.62. Found: C, 71.49; H, 7.07; N, 7.58%.
Synthesis of (2Z,20Z)-3,30-(2,5-bis(octyloxy)-1,4-phenylene)
bis(2-(4-((2-ethylhexyl)oxy)phenyl)acrylonitrile) (A4)
4-(2-Ethylhexyloxy)phenylacetonitrile (156 mg, 0.64 mmol), 2,5-
bis(octyloxy)terephthaldehyde (112 mg, 0.29 mmol), THF
(4 mL) and t-buOH (11 mL) were combined and heated to 50 ꢁC
while stirred. t-buOK (0.058 mL of a 1 M solution in THF) and
Bu4NOH (0.58 mL of a 1 M solution in methanol) were added
quickly and the mixture turned green. The reaction mixture was
stirred for another 15 min, cooled to room temperature, and
poured into acidified methanol (30 mL containing 1 drop of
concentrated acetic acid). The precipitate was filtered off,
excessively washed with methanol, and dried in vacuo at 50 ꢁC to
1
yield A4 in the form of an orange powder (151 mg, 60%). H
NMR (CDCl3): d ¼ 7.90 (s, 2H), 7.85 (s, 2H), 7.62 (d, 4H), 6.97
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Synthesis of (2Z,20Z)-3,30-(2,5-bis(octyloxy)-1,4-phenylene)
bis(2-(pyridin-4-yl)acrylonitrile) (D1)
Steady-state photoluminescence spectroscopy
All photoluminescence (PL) measurements were acquired using
a Photon Technology International QuantaMaster 40 spectro-
photometer under excitation at the wavelength of maximum
absorption; the spectra were corrected for instrument
throughput and detector response. Quantum yields in degassed
toluene (n ¼ 1.497) were determined according to the general
protocol given in ref. 34 using Rhodamine 6G in EtOH (n ¼
1.361) as reference. Experiments were conducted with samples
that had an absorbance of 0.05–0.1 in a 1 cm cuvette on a Horiba
Fluorolog 3 with Xenon arc lamp source.
2,5-Bis(3,7-dimethyloctyloxy)-terephthaldehyde (0.150 g, 0.33
mmol) and 4-pyridine acetonitrile hydrochloride (0.115 g, 0.74
mmol) were combined in a 25 mL 3-neck round-bottom flask.
THF (2.0 mL) and t-buOH (5.9 mL) were added and the mixture
was heated to 60 ꢁC, while stirring. H2O (1.0 mL) was added and
the mixture was stirred until the reactants had dissolved. Piper-
idine (0.07 mL) was quickly added via a syringe and the reaction
color instantly turned yellꢁow. The reaction mixture was stirred
for another 90 min at 60 C, cooled to room temperature, and
poured into acidified methanol (50 mL containing 1 drop of
concentrated acetic acid). The precipitate was filtered off and
dried in vacuo at 50 ꢁC overnight. Recrystallization from boiling
methanol to which a few drops of H2O had been added yielded
Determination of solubility in chloroform
The solubility was determined by weighing a known amount of
the respective dye and adding chloroform in small increments
while periodically sonicating the solution until the dye appeared
to be visually dissolved. Since these dyes tend to form excimers
and therefore should emit light at longer wavelengths when not
fully dissolved, the solutions were also observed under 365 nm
UV-light.
1
D1 in the form of an orange powder (0.978 g, 45%). H NMR
(CDCl3): d ¼ 8.73 (d, 4H), 8.25 (s, 2H), 7.95 (s, 2H), 7.58 (d, 4H),
4.20 (t, 4H), 1.93 (m, 2H), 1.72–1.12 (m, 18H), 0.98 (d, 6H), 0.85
(d, 12H). Calcd: C, 77.98, H, 8.41; N, 8.66. Found: C, 77.33; H,
8.22; N, 8.46%.
Synthesis of (2Z,20Z)-3,30-(2,5-bis(octyloxy)-1,4-phenylene)
bis(2-(pyridin-2-yl)acrylonitrile) (D2)
Two-photon absorption measurements
Two photon absorption spectra were measured at wavelengths
between 625 and 725 nm using the open-aperture Z-scan tech-
nique.25 A Ti:Sapphire regenerative amplifier (CPA-2010, from
Clark-MXR) with 200 fs pulse duration and 1 mJ pulse energy
pumped a traveling-wave optical parametric amplifier of super-
fluorescence (TOPAS, Light Conversion Ltd.). The second
harmonic of the signal was passed through a spatial filter to
isolate the lowest-order transverse mode of the beam. The beam
was then split into a signal and reference arm which was used to
divide by shot-to-shot noise. The beam waist and confocal
parameter were determined through measurements of the beam
size using a rotating slit beam profiler (BP104-VIS, Thorlabs
Inc.) at several z-positions and then fitting these points to the
well-known Gaussian beam propagation equation.26
2,5-Bis(3,7-dimethyloctyloxy)-terephthaldehyde (0.150 g, 0.33
mmol) and 2-pyridine acetonitrile (0.087 g, 0.88 mmol) were
combined in a 25 mL 3-neck round-bottom flask. THF (2.0 mL)
and t-buOH (5.9 mL) were added and the mixture was stirred at
ꢁ
55 C until the reactants had dissolved. A solution of Bu4NOH
(3 drops of a 1.0 M solution in methanol) in THF (5.0 mL) was
added drop-wise to the reaction mixture over the course of
10 min. The reaction mixture was stirred for another 60 min at
55 ꢁC, cooled to room temperature, and poured into acidified
methanol (50 mL containing 1 drop of concentrated acetic acid).
The precipitate was filtered off and dried in vacuo at 50 ꢁC
overnight. Recrystallization from boiling methanol to which
a few drops of H2O had been added yielded D2 in the form of an
1
orange powder (0.117 g, 54%). H NMR (CDCl3): d ¼ 8.87 (s,
Data were acquired from photodiodes in combination with
a gated boxcar integrator (SR200 series, Stanford Research
Systems) while the sample was translated through the focus by
a computer-controlled translation stage. The accuracy of the
Z-scan measurement was probed by measuring the TPA cross-
section of Rhodamine 6G (2.13 ꢃ 10ꢂ3 M in methanol) at
a wavelength of 685 nm and the measured value (76.9 GM27) is
consistent with values in the literature (ꢀ55 to 97 GM28). The
TPA cross-section of each dye was measured in spectroscopic
grade toluene in a 1 mm path-length fused quartz cuvette.
2H), 8.67 (d, 2H), 7.96 (s, 2H), 7.74 (m, 4H), 7.30 (m, 2H), 4.20 (t,
4H), 1.92 (m, 2H), 1.76–1.11 (m, 18H), 0.97 (d, 6H), 0.84 (d,
12H). Calcd: C, 77.98; H, 8.41; N, 8.66. Found: C, 77.27; H, 8.16;
N, 8.67%.
Differential scanning calorimetry
The differential scanning calorimetry (DSC) traces were recorded
using a Mettler-Toledo DSC-1 equipped with a Huber TC-100
cooling regulation system. All traces were recorded in a nitrogen
atmosphere with heating and cooling rates of 10 K minꢂ1, except
3. Results and discussion
for dye A3, which was recorded at a rate of 5 K minꢂ1
.
In connection with the development of 3-dimensional optical
data storage materials, we recently reported the TPA cross-
section of chromophore A2,6 a 1,4-bis-(a-cyano-4-(2-alkyloxy-
styryl))-2,5-dialkoxybenzene derivatized with methoxy and
octadecyloxy groups in the central and peripheral positions,
respectively (Fig. 1).6,24 The nonlinear optical characteristics of
this dye were explored, because its structure offers several
features typically associated with a large s, including extended
UV-Vis spectroscopy
UV-Vis absorbance spectra were recorded in chloroform
(>99.8% containing 0.75% ethanol as stabilizer) on a Perkin
Elmer Instruments Lambda 800 UV-VIS spectrophotometer.
For quantum yield measurements, absorbances were measured
on a CARY 500 UV-VIS-NIR spectrophotometer.
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conjugation,29 a large extent of symmetric charge transfer upon
excitation,13 and donor/acceptor functionalization.12 The TPA
cross-section of A2, determined using the open-aperture Z-scan
method as a function of wavelength in the range of 625 to
725 nm, where the linear absorption is negligible, was found to
reach a maximum of ꢀ650 GM at 675 nm.6 Using this data point,
the Tm (154 ꢁC), the solubility (1 ꢃ 10ꢂ3 M), the UV-Vis
absorption maximum (labs ¼ 371 nm), and the PL emission
maximum (lem ¼ 507 nm) in CHCl3 solution as starting points
(Table 1), we explored systematic structural variations with the
objective to create an OPV with maximum s and solubility and
minimal Tm. Compounds A1,23 A3,22 and A4 with identical
electronic structure, but different peripheral substituents, were
synthesized via the Knoevenagel reaction of (4-alkyloxyphenyl)
acetonitriles, which are readily accessible through alkylation of
the parent (4-hydroxyphenyl)acetonitriles,22–24 with 2,5-dime-
thoxyterephthalaldehyde (A1, A3) or 2,5-dioctyloxytereph-
thalaldehyde (A4). As expected, the UV-Vis absorption and PL
emission spectra of these chromophores in CHCl3 solution
(Table 1, Fig. S2† and ref. 22–24) are essentially identical and
match that of A2 (Fig. 4). Fig. 2, which shows s of selected dyes
as a function of wavelength measured by an open-aperture
Z-scan measurement, reveals that the TPA cross-sections of the
series A1–A4 are comparable, with a maximum of between ꢀ630
and 858 GM at 675 nm (i.e., within the commonly accepted error
of ꢄ15%).30,31 We note that in these symmetric dyes, TPA will
probe two-photon allowed Ag symmetry states rather than the
one-photon allowed Bu states. Indeed, Z-scan measurements
conducted at pump energies near half the one photon absorption
maxima yielded negligible TPA signal. The common dispersion
among these various chromophores is interesting and worthy of
quantum chemical studies. The processability of these dyes,
however, was found to vary considerably (Table 1). The Tm of
A1, carrying four methoxy groups, is almost 250 ꢁC. Substitution
of the two peripheral methoxy against octadecyloxy groups (A2)
led to a reduction of Tm by almost 100 ꢁC, but proved to decrease
the solubility in CHCl3. The introduction of two peripheral
ethylhexyloxy groups (A3) caused a similar reduction of Tm, but
increased the solubility by an order of magnitude in comparison
to A1. A4, featuring the sterically hindered ethylhexyloxy groups
in the peripheral position and two octadecyloxy groups on the
Fig. 2 TPA cross-section of OPV dyes investigated in this study as
a function of wavelength measured by open-aperture Z-scan measure-
ments (1 GM ¼ 10ꢂ50 cm4 sꢂ1 photonꢂ1) in toluene.
central benzene ring, displayed by far the best processability,
ꢁ
ꢁ
with a melting temperature of 84 C, i.e., 55–70 C lower than
that of A3 and A2, and a higher solubility than A3. This behavior
is consistent with the large melting and dissolution entropies
imparted by both substituent types, in particular the sterically
demanding ethylhexyloxy groups.32–34 Fig. 3 shows this by
comparing differential scanning calorimetry (DSC) traces of the
first heating of A1, A3, and A4 (note that the small endothermic
ꢁ
transition observed for A3 at 137 C is not seen in the second
heating trace and appears to be related to polymorphs; also note
that dye A2 exhibits a solid to liquid crystalline transition below
Tm).24 Chromophores B1 and B2 are simple structural variations
of the A-series and simply lack the two methoxy groups attached
to the central benzene ring. Consistent with the lower degree of
substitution, the solubility of these dyes is somewhat lower than
that of the corresponding A1 and A3 and the melting tempera-
tures are somewhat higher. The absorption and emission spectra
of the B-dyes are blue-shifted and s is considerably reduced
(Table 1 and Fig. 4). These changes are consistent with a reduc-
tion of symmetric charge transfer brought about by the omission
of the two donor groups.
In order to increase the extent of symmetric charge transfer,
and therewith s, C1 and C2, carrying electron-withdrawing nitro
groups in the peripheral positions, were investigated. Not
Table 1 Optical properties and isotropic melting temperatures of the dyes investigated in this study
Abs lmax
in CHCl3/nm
PL lmax in
CHCl3/nm
PL lmax of
solid/nm
Solubility in
CHCl3 (M)
TPA cross-section in
toluene, s (GM)d
Dyea
Tm/ꢁCb
s/Neff
e
A1
A2
A3
A4
B1
B2
C1
C2
D1
D2
365, 436
371, 437
371, 440
371, 434
389
397
374
360, 458
346, 446
353, 442
513, 536
507, 538
508, 541
511, 544
461, 486
464, 487
No PL
553
644
644
619
594
550
552
543
618
575
584
248
154
141
84
279
157
325
191
149
113
2 ꢃ 10ꢂ2
1 ꢃ 10ꢂ3
1 ꢃ 10ꢂ1
2 ꢃ 10ꢂ1
9 ꢃ 10ꢂ3
5 ꢃ 10ꢂ2
8 ꢃ 10ꢂ3
1 ꢃ 10ꢂ2
5 ꢃ 10ꢂ3
—
—
26
33
24
—
15
—
28
14
16
666
858
632
—
378
—
717
359
413
c
ꢅ4 ꢃ 10ꢂ4
526
531
a
b
Absorbance, emission, and thermal data for dyes A1,23 A2,24 A3,22 and B123 were reproduced from earlier publications. The isotropic melting
c
temperature is listed here. Dyes A2 and B1 exhibit solid to liquid crystalline transitions below this temperature.23,24 C1 was not dissolved at this
concentration. The TPA resonance for all dyes was between 650 and 700 nm. In the case of A1, B1, and C1 the solubility in toluene was too low
d
for an accurate determination of s. e All molecules studied have an Neff value (number of pi-electrons effectively involved in conjugation)36 of 26.
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expected effect on the processability, dropping Tm to below
200 ꢁC and increasing the solubility by almost two orders of
magnitude. The absorption spectrum shows a slight red-shift
compared to the B-series on account of the strongly electron
withdrawing nitro substituents. However, unlike dye B1, the
fluorescence of C1 was completely quenched, likely because
the nitro groups stabilize the excited state via inductive effects.35
The fluorescence intensity of C2 is much higher than that of C1,
presumably on account of an increased ICT, resulting in
a smaller HOMO/LUMO gap. The s of C2 is among the highest
measured here. Interestingly, however, it does not surpass the
values of the A-series, which possesses weaker electron accepting
functional groups (cyano), likely because it lacks the same
compact D–p–A–p–D–p–A–p–D motif.
Fig. 3 Differential scanning calorimetry (DSC) traces (first heating,
heating rate 10 ꢁC minꢂ1) showing the melting transitions of dyes A1, A3,
and A4.
In the case of D1 and D2, the peripheral 4-alkyloxyphenyl
moieties were substituted with slightly less electron-rich 4-pyr-
idinyl (D1) and 2-pyridinyl (D2) groups. By and large, the optical
properties of these molecules are comparable to those of the A-
series, although the TPA cross-section is somewhat lower
(359 GM for D1 and 413 GM for D2). The melting points of the
dyes are at the low end of the spectrum (149 ꢁC for D1 and 113 ꢁC
for D2), and their solubility is high. This is clearly the result of the
3,7-dimethyloctyloxy groups, which induce large melting and
dissolution entropies. The difference in melting points reflects the
lower symmetry of D2 in comparison to D1.
Several CN00-OPVs, including A1 and A2, have previously
been shown to form excimers upon aggregation so that solutions
and solid materials display rather different fluorescence spectra.
This ‘‘chameleon behavior’’ is the result of intramolecular
interactions and in the case of static (as opposed to dynamic)
complexes strongly depends on the molecular packing.21e The
previously reported dyes of the A-series (A1–A3) display green
emission in CHCl3 solution, with lem ¼ 507–511 nm (Table 1).
Their solid-state emission is orange-red, with identical lem
¼
644 nm for A123 and A2.24 The solid-state emission of A322 is
blue-shifted by 25 nm compared to A1 and A2, presumably due
to a less compact crystal structure by the bulky ethyl-
hexylgroups. In fact, the crystal structure for A1 shows a strong
twisting of the molecular backbone, a likely effect of accom-
modating the strong intermolecular charge transfer inter-
actions.21e The twisting is much less pronounced in A3; its crystal
structure reveals a rather planar molecule, suggesting that the
bulky substituents reduce the intermolecular charge transfer due
to less effective orbital overlap between adjacent molecules
(Fig. S3†).
The absorption and fluorescence spectra of the new dyes A4,
B2, C2, and D1, both in chloroform and solid state, are shown in
Fig. 5, together with pictures of the corresponding samples. The
quantum yield of A4 and B2, which serve as prototype for the
respective series, was for both dyes determined to be 0.47 in
toluene solution. As can be seen, the emission spectra of solution
and solid state (also evident from the digital images shown in
Fig. 5) of all dyes display pronounced differences, and in
particular A4 and B2 show featureless broadened solid-state
spectra that are indicative of excimer formation.23 Aggregation
experiments in mixed solvent systems (MeOH/CHCl3 for B2, C2
and THF/H2O for A3; Fig. S4†) have not revealed any changes in
the absorption spectra that are indicative of ground-state inter-
actions. This observation is in line with the previous findings for
Fig. 4 (a) UV-Vis absorbance and (b) fluorescence spectra (excitation at
365 nm) of solutions of dyes A4, B2, C2, and D1 in chloroform (ca. 2–5 ꢃ
10ꢂ5 M). (c) Digital images of solutions of dyes B2, A4, D1, and C2 (left to
right) in chloroform upon excitation at 365 nm.
surprisingly, C1, bare of any substituent on the central ring,
showed the highest Tm and lowest solubility of all compounds
investigated here, preventing any characterization in solution.
The introduction of 3,7-dimethyloctyloxy groups (C2) had the
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do not lend themselves to excimer emission.21e In this context, the
2-ethylhexyloxy tails appear to be ideal, as they impart the new
B2 with low melting temperature and high solubility without
providing strong interactions that have a governing influence on
the dye’s solid state structure.
C2 displays a deep red solid-state emission (Table 1 and
Fig. 5c) centered around 618 nm, which is shifted by 65 nm
compared to the emission maximum in solution. The contrast
between solution and solid state for C2 is smaller than that of any
dye in the A or B-series, which is perhaps a result of the bulky
3,7-dimethyloctyloxy groups, which prevent compact crystal
packing, much in the same way as was suggested for the A and B-
series based upon the trend of higher energy emission for
increasing bulkiness seen for those dyes.
Molecules D1 and D2 are almost identical in structure and
their spectra are indeed very similar. The D series also has bulky
3,7-dimethyloctyloxy groups much like the C-series which can
frustrate excimer formation. Furthermore, the weakly electron
donating nitrogen accounts for much less ICT as the nitro
substituents in the C-series or the alkyloxy substituents in the A
or B-series, so it is not surprising that the D-series exhibits the
smallest contrast between emission of solution and solid state (49
and 53 nm for D1 and D2, respectively). Indeed the digital images
in Fig. 5 show the least impressive contrast between emission of
solution and solid state.
The excimer-forming propensity of the new chromophores is
useful for the design of stimuli-responsive, color-changing fluo-
rescent polymers, which are produced by incorporating small
amounts of excimer-forming dyes into a host polymer of
interest.21 The general approach relies on the stimulus-driven
self-assembly (e.g. upon exposure to heat, chemicals) or disso-
ciation (e.g. upon mechanical deformation, exposure to light) of
nanoscale aggregates of these dyes in a host polymer, which
results in a pronounced change in the fluorescence color. Some
representatives of the A and B series have already proven to be
very useful in this context. They have been shown to be stable
under melt-processing conditions at temperatures of over 250 ꢁC
in a variety of host polymers, which bodes well for the exploi-
tation of the new dyes in such a context.
Fig. 5 Fluorescence spectra (excitation at the wavelength of absorbance
maximum) and digital images (under excitation at 365 nm) of (a) A4, (b)
B2, (c) C2, and (d) D1 in chloroform and as a solid powder.
A and B series dyes, with the exception of A2, which displays
considerable charge-transfer interactions in the solid state.24
A4 continues the trend set by A3 vis a vis A1 and A2 in that
functionalization with bulky substituents (i.e. the combination of
octyloxy groups attached to the aromatic core and 2-ethyl-
hexyloxy groups in the peripheral positions) blue-shifts the solid
state spectra by another 25 nm in comparison to A3. The emis-
sion characteristics of B2 are identical to those of B1. In view of
the hypsochromic shift observed between A3 and A1 (vide supra)
and the fact that dyes of the B-series with long linear peripheral
alkyloxy groups (dodecyloxy and octadecyloxy) show blue
monomer emission,21e this finding is quite intriguing, as a low-
melting, yellow-blue color-changing excimer-forming cyano-
OPV has so far not been available. As was shown previously, the
formation of ground-state excimers in cyano-OPVs is usually the
result of crystal structures, in which adjacent molecules are
shifted with respect to one another so that the electron-poor
cyano-group overlaps with the electron-donating central ring of
its neighbor.21e In the case of B-series derivatives with long linear
peripheral alkyloxy groups, interactions between the latter
prevail over p–p interactions, imparting crystal structures that
4. Conclusions
In conclusion, we have measured large TPA cross-sections in the
range of 400–850 GM for several CN00-OPVs, a class of mole-
cules whose optical nonlinearities have previously little been
studied. The functionalization with bulky alkyloxy substituents
has afforded derivatives that display a melting point as low as
84 ꢁC and a solubility in chloroform as high as 2 ꢃ 10ꢂ1 M while
exhibiting a s of ꢀ630 GM. These CN00-OPVs have the
propensity to form excimers and exhibit a wide array of
absorption/emission colors, which are dependent on the func-
tionalization of the OPV motif with electron accepting and
donation groups.
The combination of significant optical nonlinearity, low melt-
processing temperature, high thermal stability, and the propen-
sity for excimer-formation makes the new CN00-OPVs interesting
candidates for a range of applications, in particular optical data
storage systems that function on the basis of TPA-induced
switching of the aggregation state and fluorescence color.6 The
This journal is ª The Royal Society of Chemistry 2012
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14 P. W. Nolte, D. Pergrande, S. L. Schweizer, M. Geuss, R. Salzer,
B. T. Makowski, M. Steinhart, P. Mack, D. Herrmann, K. Busch,
C. Weder and R. B. Wehrspohn, Adv. Mater., 2010, 22, 4731.
15 H. Song, K. D. Singer, J. Lott, Y. Wu, J. Zhou, J. H. Andrews,
E. Baer, A. Hiltner and C. Weder, J. Mater. Chem., 2009, 19, 7520.
16 H. Y. Woo, B. Liu, B. Kohler, D. Korystov, A. Mikhailovsky and
G. C. Bazan, J. Am. Chem. Soc., 2005, 127, 14721.
availability of such dyes which cover a range of emission colors
makes the design of complex systems with a multitude of
switching states for each voxel possible.
Acknowledgements
17 H. M. Kim and B. R. Cho, Chem. Commun., 2009, 153.
18 S. J. K. Pond, M. Rumi, M. D. Levin, T. C. Parker, D. Beljonne,
This work is based on research supported by the US National
Science Foundation (NSF) under grant no DMR-0602767
(Materials World Network) and DMR-0423914 (Center for
Layered Polymer Systems at Case Western Reserve University).
We also acknowledge financial support from the Adolphe Mer-
kle Foundation and thank P.W. Nolte, D. Pergande, M. Geuss,
D. Langhe, M. Steinhart, K. Busch, and R.B. Wehrspohn for
stimulating discussions. We are grateful to the research group of
John D. Protasiewicz for help with the determination of crystal
structures.
ꢀ
M. W. Day, J. L. Bredas, S. Marder and J. W. Perry, J. Phys.
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21 See for example: (a) B. R. Crenshaw and C. Weder, Chem. Mater.,
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