Effects of dendronization on the linear and third-order nonlinear optical properties of bis(thiopyrylium) polymethine dyes in solution and the solid state

Article abstract of DOI:10.1021/cm3002139

Effects of the size and attachment position of benzyl aryl ether dendrons covalently attached to bis(thiopyrylium) penta- and heptamethines on the optical properties of these dyes in solution and in solid films have been investigated. In dilute solution the low-energy absorption bands of some of the dendronized species differ from those of the parent compounds in having much smaller transition dipole moments, this effect possibly due to differences in ion pairing, while at higher concentrations, dye-dye interactions lead to a decrease in the transition dipole moments of the nondendronized species, but not of the dendronized ones. Consequently, in the high concentration range, dendronized and nondendronized species exhibit similar values of the real part of the microscopic third-order polarizability at 1550 nm. Solid-state film absorption spectra suggest that the dendrons significantly disrupt the chromophore- chromophore interactions seen for the nondendronized species, reducing, but not eliminating, linear absorption losses in the near-IR, and suppressing absorption peaks that are hypsochromically shifted from the solution spectra maximum: centrally placed dendrons have a larger effect than terminal dendronization, so that the corresponding thin-film spectra more closely resemble those seen in solution with increasing generations of dendronization. Z-scan measurements at 1550 nm indicate that the third-order susceptibility of dendronized heptamethine guest-host films depend approximately linearly on doping ratio of dyes and are in reasonable agreement with values extrapolated from solution-derived third-order polarizabilities; in contrast, the susceptibilities of films highly doped with an undendronized analogue fall short of values expected from solution polarizabilities.

Full text of DOI:10.1021/cm3002139

Article
pubs.acs.org/cm
Effects of Dendronization on the Linear and Third-Order Nonlinear
Optical Properties of Bis(thiopyrylium) Polymethine Dyes in Solution
and the Solid State
Annabelle Scarpaci,‡ Arpornrat Nantalaksakul,‡ Joel M. Hales, Jonathan D. Matichak, Stephen Barlow,
Mariacristina Rumi, Joseph W. Perry,* and Seth R. Marder*
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901
Atlantic Drive, Atlanta, Georgia 30332-0400, United States
S
* Supporting Information
ABSTRACT: Effects of the size and attachment position of benzyl aryl ether dendrons covalently attached to bis(thiopyrylium)
penta- and heptamethines on the optical properties of these dyes in solution and in solid films have been investigated. In dilute
solution the low-energy absorption bands of some of the dendronized species differ from those of the parent compounds in
having much smaller transition dipole moments, this effect possibly due to differences in ion pairing, while at higher
concentrations, dye−dye interactions lead to a decrease in the transition dipole moments of the nondendronized species, but not
of the dendronized ones. Consequently, in the high concentration range, dendronized and nondendronized species exhibit
similar values of the real part of the microscopic third-order polarizability at 1550 nm. Solid-state film absorption spectra suggest
that the dendrons significantly disrupt the chromophore−chromophore interactions seen for the nondendronized species,
reducing, but not eliminating, linear absorption losses in the near-IR, and suppressing absorption peaks that are hypsochromically
shifted from the solution spectra maximum: centrally placed dendrons have a larger effect than terminal dendronization, so that
the corresponding thin-film spectra more closely resemble those seen in solution with increasing generations of dendronization.
Z-scan measurements at 1550 nm indicate that the third-order susceptibility of dendronized heptamethine guest−host films
depend approximately linearly on doping ratio of dyes and are in reasonable agreement with values extrapolated from solution-
derived third-order polarizabilities; in contrast, the susceptibilities of films highly doped with an undendronized analogue fall
short of values expected from solution polarizabilities.
KEYWORDS: polymethines, dendrons, aggregation, optical properties, third order nonlinear optical properties
polymethines, this is dominated by a term in which the
transition dipole moment, Mge, and transition energy, Ege,
between the ground- and first-excited state appear, cyanine-like
species exhibiting large Mge values and small Ege values:
INTRODUCTION
■
Materials for all-optical signal processing (AOSP) applications
must exhibit large ultrafast macroscopic third-order suscepti-
bilities (χ(3)) and low linear and nonlinear optical loss at the
wavelength(s) of interesttypically in the telecommunications
wavelength range (1300−1550 nm)as well as good
processability. Cyanine-like polymethines are particularly
promising organic dyes for AOSP applications due to their
large (negative) nonresonant microscopic third-order polar-
izabilities, γ,1−6 compared to many other conjugated materials
of comparable size.7−12 The origin of these large values can be
understood using the sum-over-states expression for the static
value of the real part of γ, Re(γ0). In the case of cyanine-like
3
Re(γ ) ∝ −Mge4/Ege
(1)
0
Moreover, due to the similarity of the ground and excited-
state geometries, the lowest energy absorption band of
chromophores of this class is typically characterized by a
Received: January 19, 2012
Revised: March 20, 2012
Published: April 17, 2012
© 2012 American Chemical Society
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sharp low-energy onset, allowing one to use compounds with
low-lying excited states and, therefore, large Re(γ0), and to take
advantage of near-resonance enhancement of γ without
incurring unacceptable linear loss. We reported on a series of
bis(chalcogenopyrylium)-terminated polymethines with an
exceptional combination of microscopic nonlinear optical
(NLO) properties at telecommunications wavelengths, that is,
large real components of γ (Re(γ) is related to nonlinear
refraction) and small imaginary components of γ (Im(γ) is
related to nonlinear loss, specifically two-photon absorption),
which could be promising for AOSP.2
dendrons on the linear and NLO properties of bis-
(chalcogenopyrylium) polymethines (Charts 1 and 2) in both
Chart 1. Structures of Pentamethines Studied in This Paper
However, while the relationship between chemical structure
and the magnitude of γ is relatively well-understood,1,2,9,13−17
an important challenge for realizing AOSP applications is to
translate these promising microscopic nonlinearities into high
chromophore volume-fraction materials with the correspond-
ingly large bulk nonlinearities, χ(3), needed for device operation
at acceptable optical intensities while maintaining relatively
short interaction lengths. It is well-known that cyanine
chromophores are inherently highly polarizable and are,
therefore, prone to aggregation at high concentrations, leading
to the formation of dimers or other aggregates with various
relative orientations of molecules, that is, H- or J-aggregates
(with parallel long axes and a face-to-face arrangement or end-
to-end arrangement, respectively) or oblique arrangements of
chromophores. The intermolecular electronic interactions lead
to a mixing of the excited state wave functions that can result in
a splitting (for inclined or oblique dimers), or a hypsochromic
shift (for H-aggregates) or bathochromic shift (for J-
aggregates) of the electronic absorption band relative to what
is seen for isolated molecules in dilute solutions.6,18−22 Such
changes in the absorption spectrum could have adverse effects
on the macroscopic properties of materials derived from these
dyes. In particular, a hypsochromic shift could lead to decreased
χ(3) relative to that expected from the solution values of γ for
isolated molecules and a bathochromic shift, while potentially
augmenting the macroscopic nonlinearity, could lead to
increased linear losses at the operating wavelengths.
Considerable efforts have been made to control the
aggregation of cyanine and cyanine-like dyes in solution23−33
and in films.18,34−42 For example, polymeric carbohydrates such
as carboxymethylamylose have been used to promote the
formation of J-aggregates of mono- and trimethine dyes in
solution,29−33 while electrostatic self-assembly of layers of
trimethines alternating with layers of polymers or other dyes
bearing complementary charges has been shown to lead to the
formation of well-defined aggregates in films.35,40−42 However,
we are not aware of solid-state aggregation studies of
polymethine dyes with sufficiently extended conjugation to
exhibit promising molecular-level optical properties for AOSP.
In the case of bis(chalcogenopyrylium) polymethines, an ideal
situation would be retention of the dilute-solution optical
properties of the dyes in the solid state, that is, prevention of
aggregation phenomena. Previous studies of cyclodextrins43 and
dendrimer/cyanine host−guest systems20 suggest routes
favorable for aggregate suppression; however, the chromophore
loading densities employed were orders-of-magnitude lower
than the ≥10 wt % loadings required for AOSP. Covalently
attached dendrons have been used to prevent aggregation of a
variety of chromophores,44−52 but this strategy has not been
employed for polymethines in the context of third-order NLO
materials. Here, we systematically investigate the effect of size
and position of covalently attached benzyl-ether-based
solution and solid state. As discussed above, these types of
cyanine-like dyes exhibit very promising solution properties for
AOSP applications; in the present study, we specifically use
dyes with thiopyrylium end groups, since these have a similar
NLO response to their selenium analogues,2 but are expected
to be less toxic, and a heptamethine precursor having a readily
functionalizable position on the methine bridge is commercially
available.
EXPERIMENTAL SECTION
■
Starting materials were obtained from commercial sources and used
without further purification. Compounds 1,53 14,54 15,55 16a−c,56−58
59
C7,2 and NaBAr′4 were synthesized according to reported
procedures. The synthesis of the new compounds is shown in
Schemes 1−3 and is described in the Supporting Information, along
with characterizing data. UV-visible near infrared (UV-vis-NIR)
spectra were recorded in 1 cm cells using a Varian Cary 5E
spectrometer. Varian 300, Bruker 400, and Bruker 500 spectrometers
were used to record NMR spectra. Mass spectrometry was carried out
using a Micromass Quattro LC for ESI experiments, a Micromass
AutoSpec M for EI experiments and an Applied Biosystems 4700
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Chart 2. Structures of Heptamethines Studied in This Paper
Proteomics Analyzer for MALDI-TOF experiments. Elemental
analyses were performed by Atlantic Microlabs. Neat films for linear
absorption measurements were prepared by dissolving each dye (10
mg) in dichloromethane (0.5 mL) in a 5 mL vial; the solution was left
stirring for about 30 min and then filtered through a 0.45 μm PTFE
syringe filter before it was spin coated onto a glass coverslip at 700
rpm for 1 min. Blended films for linear and nonlinear measurements
were obtained by mixing a filtered dichloromethane solution of the dye
(30 mg in 0.4 mL) with a 1,1,2-trichloroethane solution of APC (30
mg poly(bisphenol A carbonate), from Sigma-Aldrich, 435120, in 0.4
mL) in the desired ratio and spin-coating the mixture onto a glass
coverslip at 700 rpm for 2 min (see Table S1 in Supporting
Information for thicknesses of the resultant films). The real and
imaginary components of the third-order polarizability, γ, and the
third-order susceptibility, χ(3), at 1550 nm were measured using the
femtosecond-pulsed Z-scan technique,60 in the same way as previously
described in refs 2, 19, and 61.
RESULTS AND DISCUSSION
■
Synthesis of Dyes with Dendronized Thiopyrylium
Termini. Polymethines with dendronized thiopyrylium termini
were prepared by Knoevenagel condensation of dendronized 4-
methyl-thiopyrylium cations with phenylaminium-terminated
tri- and pentamethines. To obtain dendronized thiopyrylium
units, dendrons with a terminal alkyne at the focal point are
required (6a−c). Scheme 1 shows the synthetic routes to
obtain these desired dendrons. Compound 6a is commercially
available, while compounds 6b and 6c were prepared from
dimethyl 5-(trifluoromethylsulfonyloxy)isophthalate, 1. Sono-
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Scheme 1. Synthesis of Dendrons with Alkyne Units as Focal Functionality
Scheme 2. Synthesis of Tetrafluoroborate Salts of Thiopyrylium Cations with Different Generations of Dendron and Their
Conversion to Penta- and Heptamethines
gashira coupling between 1 and ethynyltri(isopropyl)silane led
to the protected alkyne 2 in near-quantitative yield. Reduction
of the methyl ester functionalities using lithium aluminum
hydride gave the corresponding diol (3) in 77% yield; 3 was
then converted to the corresponding di(bromomethyl) species
(4) in 91% yield using N-bromosuccinimide (NBS) and
triphenylphosphine. Compound 5b was obtained by the
nucleophilic substitution of 4 with 3,5-di-tert-butylphenol in
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Scheme 3. Synthesis of Dendrons with a Benzene Thiol at the Focal Point and Conversion to the Corresponding Dendronized
Heptamethine Cyanines
the presence of potassium carbonate and 18-crown-6. Finally,
deprotection of the tri(isopropyl)silyl (TIPS) protecting group
using tetra-n-butylammonium fluoride led to compound 6b. To
synthesize a higher generation dendron with alkyne function-
ality (6c), the di(bromomethyl) intermediate 4 was reacted
with dimethyl 5-hydroxyisophthalate to obtain dendron 7, in
which the peripheral groups are methyl esters. Repetitive
synthetic procedures to install 3,5-di-tert-butylphenyl groups at
the peripheries and to deprotect the TIPS group were
performed under similar conditions to those described above
to afford the desired product, 6c, in a good overall yield.
The 2,6-diaryl-4H-chalcogenapyran-4-ones 12a−c were
prepared according to a literature procedure62 (Scheme 2):
the reaction of ethyl formate with 2 equiv of the Grignard
reagents derived from reaction of 6a−c and ethylmagnesium
bromide led to the hydroxylated dendrons 10a−c, which were
then oxidized by pyridinium chlorochromate (PCC) to give the
ketone derivatives 11a−c. These species were then reductively
cyclized with sulfur to afford compounds 12a−c. Finally, the
addition of 1 equiv of methylmagnesium bromide, followed by
treatment with tetrafluoroboric acid, gave the tetrafluoroborate
salts of different generations of dendronized 4-methylthiopyry-
lium cations (13a−c).
The nondendronized (C5) and dendronized (G1-C5 and
G2-C5) pentamethine dyes were synthesized by reacting 2
equiv of the appropriate 4-methylthiopyrylium salt (13a−c)
with trimethine salt 14 in acetic anhydride at 90 °C (Scheme
2). The progress of the reaction was tracked by the evolution of
a characteristic absorption band of bis(thiopyrylium) pentam-
ethines around 900 nm. When the evolution of this band had
ceased, the reaction was quenched with water to give a
precipitate, which was isolated by filtration. A counterion
exchange was then performed using sodium tetrakis(3,5-
bis(trifluoromethyl)phenyl)borate (NaBAr′4). The presence of
the BAr′4− counterion63 greatly increases the solubility and also
facilitates purification by column chromatography, especially for
the species with lower generation dendrons.
The nondendronized heptamethine dye (C7) was obtained
59,63
from counterion metathesis with NaBAr′4
of the corre-
sponding commercially available tetrafluoroborate salt (IR-
1061), as previously described,2 to increase the solubility of the
dye in common organic solvents and to allow direct
comparison of the aggregation behavior of undendronized
and dendronized dyes having the same counterions.
G1-C7 was prepared under conditions similar to those used
for its dendronized pentamethine analogs (Scheme 2). In this
case, 2 equiv of 13b was allowed to react with the amino-
terminated pentamethine salt (15) in acetic anhydride, and the
evolution of the characteristic band associated with the
heptamethine dye at ca. 1080 nm was monitored. The pure
compound was obtained in 40% yield after metathesis with
NaBAr′4.
Synthesis of Dyes with Dendronization on the Bridge.
Bis(thiopyrylium) heptamethine dyes with dendrons attached
to the middle of the polymethine bridge were obtained by
nucleophilic substitution of the chloro substituent in the center
of the bridge of the commercially available heptamethine dye
(IR-1061) by dendrons with a nucleophile at their focal point.
Benzenethiolate was chosen as the reactive group due to its
high nucleophilicity, thereby allowing the use of mild reaction
conditions to minimize the possibility of dye decomposition.
Benzyl aryl ether dendrons containing the bromomethyl
functionality at the focal point (16a−c) were synthesized by
a convergent approach, following conditions reported in the
literature (Scheme 3).56−58 Reaction of the bromomethyl
dendrons with 3-bromobenzene thiol gave the bromophenyl
functionalized dendrons (17a−c) in excellent yields. Sub-
sequently, the bromoaryl dendrons and 2-ethylhexyl-3-
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mercaptopropionate were coupled using Pd2(dba)3 and 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos) as
the catalyst system to obtain dendrons with the 2-ethylhexyl-
3-mercaptopropionate group at their focal points in excellent
yields (18a−c).64 Finally, these alkylated benzene thiols were
cleaved using sodium ethoxide to give the corresponding aryl
thiol dendrons (19a−c), the formation of which could be
confirmed by the presence of the thiol resonance at around
Table 1. Linear Optical Properties of Bis(Thiopyrylium)
Polymethines in Dilute (< 10 μM) Chloroform Solutions
a
b
cmpd
C5
λmax (nm)
εmax (M−1 cm−1
)
Mge (D)
904
904
3.11 × 105
3.55 × 105
2.72 × 105
4.01 × 105
3.66 × 105
3.34 × 105
2.67 × 105
2.48 × 105
15.9
17.0
15.5
19.3
18.5
18.8
17.0
G1-C5
G2-C5
C7
899
1067
1079
1081
1081
1085
1
G1-C7
G1′-C7
G2′-C7
G3′-C7
3.40 ppm in the H NMR spectra. However, the yield in this
step is quite low for 19a and 19b dendrons (37% and 10%,
respectively). An attempt to purify G3′ benzene thiol (19c)
failed, perhaps due to instability of the compound, and
therefore, this compound was used directly in the next step
of the synthetic procedure without purification.
Dendrons were attached to the heptamethine bridge by the
reaction of dendron-functionalized benzene thiols 19a−c with
IR-1061 using triethylamine as a base in THF (Scheme 3). The
formation of the desired products was again monitored by UV-
vis-NIR spectrometry and was indicated by a 5−10 nm
bathochromic shift in the position of the absorption maximum
relative to that of the starting material and differences in the
vibronic structure of this absorption feature. Finally, the G1′-
G3′ dendronized dyes were subjected to metathesis with
NaBAr′4 to afford the desired products, G1′-C7, G2′-C7, and
G3′-C7.
16.3
a
b
Experimental uncertainty in εmax estimated to be 2%. Obtained
from Mge = 0.09584(∫ εdν/νmax)0.5, where Mge is in debye, D, ε is in
M
−1 cm−1, and νmax, the wavenumber corresponding to the absorption
maximum, is in cm−1, and the integral is over the lowest energy
absorption band.
formation is not responsible for the observed reduction in εmax
values. However, differences in ion pairing between the
dendronized species and the nondendronized model com-
pounds may play a role: for example, similar spectral
modifications for an indolinium-based heptamethine have
been attributed to counterion- and solvent-dependent ion-
pairing effects,66 and similar changes are found for the spectra
of C7 (IR1061) analogues possessing various counterions in
solvents with different polarity (see Supporting Information,
Figure S2, Table S2).
Linear Absorption Spectra in Dilute Solution. Figure 1
shows the UV-vis-NIR absorption spectra of bis(thiopyrylium)
Linear Absorption Spectra in Concentrated Solutions.
To investigate the efficacy of dendron units in the prevention of
aggregate formation, UV-vis-NIR spectra of dyes C7, G1-C7,
and G3′-C7 were recorded at concentrations up to three
orders-of-magnitude larger than those discussed in the previous
section (up to 3.9 × 10−3 M, 8.6 × 10−4 M, and 8.0 × 10−4 M,
respectively, see Figure 2). In the absence of the dendron unit,
an increase in dye concentration causes the normalized
absorption spectra of C7 to show increases in intensity of the
higher lying vibronic (E0−1) band relative to the lowest energy
band (E0−0). Furthermore, concomitant decreases in εmax and
Mge are observed. This deviation from Beer’s law is suggestive
of aggregate formation at these elevated concentrations,
although changes in the extent of ion-pairing may also
contribute. However, when dendron units are introduced at
the periphery (G1-C7) or at the center of the dye (G3′-C7),
the shape and εmax values of the spectra do not change
significantly when dye concentration is increased; that is,
dendronization is effective at suppressing concentration
dependence, perhaps by reducing chromophore−chromophore
interactions. An important consequence in the context of the
solution NLO measurements described below is that the high-
concentration solution values of Mge for C7 and for the
dendronized heptamethines are similar (see Supporting
Information, Table S3), apparently due to the similarity of
the effects of chromophore−chromophore interactions on the
spectrum of the former to those of dendronization on the
spectra of the latter.
Figure 1. Absorption spectra of polymethine dyes in chloroform.
pentamethine and heptamethine dyes in chloroform at low
concentrations (<10 μM); absorption maxima (λmax), molar
absorptivities (εmax), and transition dipole moments (Mge) for
the low-energy transitions are summarized in Table 1.65
Dendronization only has a minor effect on the position of
the absorption maxima for either the penta- or heptamethines
( 155 cm−1), but for many of the dendronized species, εmax
and Mge are significantly decreased: in the pentamethine series,
the G2 species possesses a weaker transition than its G1 and
undendronized analogues, while in the heptamethines the
transitions are weaker for all the dendronized species than for
the undendronized reference compound and decrease in
strength with increasing generation of bridge-based dendroni-
zation. No deviation from Beer’s law was observed at the
concentrations used (<10 μM), suggesting that aggregate
Linear Absorption Spectra in Films. The absorption
spectrum of the bis(thiopyrylium) pentamethine dye without
the dendron (C5) as a neat film exhibits two prominent low-
energy maxima (Figure 3). The higher-energy of these maxima
is at 721 nm, hypsochromically shifted by ca. 2800 cm−1 from
the solution absorption maximum of C5, while a weaker feature
has a maximum at 917 nm, slightly bathochromically shifted
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Figure 2. Absorption spectra of C7 (top), G1-C7 (middle), and G3′-C7 (bottom) in chloroform at different concentrations. Left: Spectra graphed
on a normalized absorbance scale. Right: Spectra graphed on an ε scale.
relative to the solution peak. According to the two-level model
for γ of eq 1, the overall hyspochromic shift of the majority of
the transition oscillator strength is expected to lead to a
decreased magnitude of the third-order polarizability and
susceptibility. Moreover, the film spectrum also shows a tail
extending to much longer wavelengths than for the solution
spectrum, which may be due to either absorption or scattering,
and is potentially problematic from the point of view of
obtaining good linear transparency in the near-IR for AOSP
applications.
When the G1 dendron (G1-C5) is incorporated at the
thiopyrylium termini, the resulting low-energy absorption of
the neat film exhibits three distinct maxima, the strongest of
which appears at about 805 nm, that is, hypsochromically
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Figure 4. UV-vis-NIR absorption spectra of C7 in chloroform and
films of neat C7, 1:1 w/w C7 in APC, and neat G1-C7.
Figure 3. UV-vis-NIR absorption spectra of C5 in chloroform and C5
and its dendronized analogs as neat films.
shifted from that seen in solution, but less so than that of its
nondendronized analog (C5), with additional maxima at 879
and 954 nm. A qualitatively similar spectrum is seen for a neat
film of G2-C5, differing mainly in the relative absorbance at the
three maxima. The spectra of these dendronized pentamethine
systems also exhibit a sharper low-energy band edge than their
nondendronized counterparts: whereas the absorption/scatter-
ing tail of the C5 film extends to ca. 1500 nm, the onset of
extinction is ca. 1150 nm in the G1-C5 film. Thus, G1 or G2
dendronization at the termini evidently leads to modification of
the type of aggregation with respect to that found in neat films
of nondendronized chromophores; however, the spectra are
still very different from those observed in solution, suggesting
that the NLO properties may also deviate significantly from
those seen in solution. Nonetheless, the decrease in linear
optical loss throughout the telecommunications wavelength
range (1300−1550 nm) suggests that this approach may be
advantageous for AOSP applications.
The neat film of the parent bis(thiopyrylium) heptamethine
C7 shows an absorption peak at 785 nm (Figure 4),
substantially blue-shifted relative to the main absorption
maximum in solution, and a broad band (λmax = 1103 nm),
slightly red-shifted from the monomeric absorption peak; that
is, the spectrum is reminiscent of that of the neat C5 film
discussed above. The effect of terminal dendronization on the
spectrum is similar to that in the case of the pentamethine dyes:
the strongest absorption in the spectrum of a neat G1-C7 film
(λmax = 896 nm, Figure 4) is less blue-shifted than that of a C7
film and the extent of the absorption/scattering tail is reduced.
It should be noted here that dendronization dilutes the density
of the active chromophore in the films. To investigate whether
the observed spectral differences between films of non-
dendronized and dendronized systems are caused purely by
this dilution of dye content or by the architectural difference
between these two molecules, a film containing C7 and
amorphous polycarbonate (APC) in a 1:1 weight ratio was
prepared. This film contains 50% by weight of “chromophore”
(henceforth defined as the thiopyrylium polymethine molecule
and counterion excluding the dendron), similar to that of neat
G1-C7 (45%). The two absorption bands of the blended film
appear at nearly the same positions as those of the C7 neat film
but with a slight difference in their relative absorbances (Figure
4); thus, the effect of dendronization on the neat film spectrum
of G1-C7 cannot be fully attributed to a dilution effect,
suggesting that this type of dendronization does modify the
chromophore−chromophore interactions in the films.
Attaching dendrons to the middle of the heptamethine
bridge also leads to two maxima in the lower-energy part of the
neat-film spectra (Figure 5a). The higher energy of the two
maxima in each case (λmax = 918, 919, and 923 nm in G1′-C7,
G2′-C7, and G3′-C7, respectively) is blue-shifted compared to
the respective solution maximum, while a lower energy
maximum appears at around 1120 nm in all cases, the relative
absorbance (compared to the respective higher energy peak) of
which increases with increasing size of the dendron (Figure 5a).
Since the neat films of Gi′-C7 have different concentrations of
chromophores (77, 63, and 46 wt % for G1′-C7, G2′-C7, and
G3′-C7, respectively) APC-blended films of C7 and the Gi′-C7
series containing similar concentrations of the chromophore
(Figure 5b, 1:1 G3′-C7/APC, 3:7 G2′-C7/APC, 3:7 G1′-C7/
APC, and 3:7 C7/APC, containing 23, 19, 23, and 30%
chromophore by weight, respectively) were prepared to
compare once again the effect of dendrons on intermolecular
interactions. The trend in the relative absorbance of the two
main features can be mainly attributed to the specific effects of
the dendron, as opposed to a simple chromophore dilution
effect. In all cases, the thin-film spectra show absorption and/or
scattering tails that extend into the telecommunications region,
potentially adversely affecting the utility of these species for
AOSP.
Figure 6 compares the absorption spectra of G1-C7 and G3′-
C7 dyes having dendrons attached at the different positions in
the neat films but with similar chromophore content (45% and
46%, respectively). The prominent absorption peaks appear at
similar wavelengths for these two compounds, suggesting
chromophore−chromophore interactions in the two films to be
similar. However, the low-energy absorption of G1-C7 is
weaker than the higher energy feature, while the converse is
true for G3′-C7. This suggests that, for a similar chromophore
concentration, placing a bulkier dendron in the middle of the
bridge is more efficient at reducing the prominence of the
aggregate-induced higher-energy band than when multiple
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Figure 5. UV-vis-NIR absorption spectra of nondendronized and dendronized heptamethine dyes in (a) neat films and (b) blended films having
similar chromophore densities. The solution spectrum of C7 is included for comparison.
Table 2. Microscopic Third-Order Nonlinear Optical
Properties of Bis(Thiopyrylium) Polymethines in
a
Chloroform Solution
cmpd
C5
Re(γ) (esu)
|Re(γ)/Im(γ)|
−1.7 × 10−32
−2.2 × 10−32
−1.7 × 10−32
−3.6 × 10−32
−5.1 × 10−32
−3.9 × 10−32
−3.9 × 10−32
−4.4 × 10−32
3.2
4.7
3.2
52
G1-C5
G2-C5
C7
G1-C7
G1′-C7
G2′-C7
G3′-C7
27
22
16
20
a
Measurements performed using the femtosecond-pulsed Z-scan
technique at λexc = 1550 nm. Experimental uncertainties were
estimated to be 10% in Re(γ) and 14% in |Re(γ)/Im(γ)|.
previously for a vinylogous series of bis(selenopyrylium)
polymethines.2
Figure 6. Neat film absorption spectra of heptamethine dyes having
dendrons at the periphery (G1-C7) and at the center (G3′-C7) of the
polymethine bridge with ca. 45% of chromophore. The solution
spectrum of C7 is included for comparison.
The values for Re(γ) of the dendronized polymethines are
quite similar to those of their parent molecules. For the
heptamethine series, this similarity is the result of the
aforementioned effects of intermolecular interactions on the
concentrated-solution spectra. For the elevated concentrations
(0.8−4.0 mM) at which Z-scan measurements were performed,
the values of Mge were found to be comparable between
dendronized and nondendronized species (see Supporting
Information, Table S3). As such, the simplified two-level model
for γ given by eq 1 was adequate in describing the third-order
NLO response of these compounds (see Supporting
Information, Table S3). It is worth noting that |Re(γ)/Im(γ)|
values (Table 2) for the dendronized heptamethines are all
lower than that of C7; given the similarity of the Re(γ) values
for all these systems, the changes are attributed to a larger
magnitude of Im(γ) in the dendronized compounds, which is
indicative of stronger 2PA at this wavelength. This may be due
to differences in the shape and/or strength of the low-lying 2PA
bands, either the vibronically assisted excitation into the main
one-photon state67,68 or a higher lying 2PA-allowed state;
however, a more detailed understanding would require
broadband 2PA spectroscopy, which is beyond the scope of
this study.
smaller dendrons are placed at the termini of the
chromophores.
Microscopic Third-Order Nonlinear Optical Properties
in Solution. The real and imaginary components of the third-
order polarizability, γ, at 1550 nm were measured for the dyes
dissolved in chloroform and the results are summarized in
Table 2. The values of Re(γ) (Table 2) are negative in sign and
large in magnitude, as expected for third-order nonlinearities of
cyanine-like polymethines in the NIR spectral region.2,4,5,19,61
The heptamethines exhibit the expected marked increase in
Re(γ) compared to the pentamethines2 and are quite similar in
magnitude to those of selenopyrylium heptamethines reported
at the same excitation wavelength.2 Experimentally determined
values of |Re(γ)/Im(γ)| are also shown in Table 2; this
parameter defines the molecular two-photon figure-of-merit for
all-optical switching,2 and the values are quite large, particularly
for the heptamethines, owing to favorably spaced two-photon
absorption (2PA) resonances, consistent with results reported
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Table 3. Macroscopic Third-Order Nonlinear Optical Properties of Bis(thiopyrylium) Heptamethines in Neat and Blended
Films
a
b
c
d
APC/dye (wt %)
chromophore (wt %)
Reχ(3) measured (esu)
Reχ(3) neat (esu)
Reχ(3) extrapol. (esu)
10:90 (C7)
90
50
10
45
41
22
4
−4.3 × 10−11
−2.7 × 10−11
−0.8 × 10−11
e
−3.6 × 10−11
−2.9 × 10−11
e
−8.3 × 10−11
−2.6 × 10−11
−5.1 × 10−11
−2.7 × 10−11
−1.6 × 10−11
−5.7 × 10−11
−2.1 × 10−11
−4.8 × 10−11
−5.4 × 10−11
−8.0 × 10−11
e
−4.0 × 10−11
−5.8 × 10−11
e
−17 × 10−11
−8.7 × 10−11
−5.7 × 10−11
−5.4 × 10−11
−5.3 × 10−11
−6.3 × 10−11
−4.2 × 10−11
−7.5 × 10−11
50:50 (C7)
90:10 (C7)
0:100 (G1-C7)
10:90 (G1-C7)
50:50 (G1-C7)
90:10 (G1-C7)
50:50 (G1′-C7)
70:30 (G1′-C7)
10:90 (G2′-C7)
50:50 (G2′-C7)
70:30 (G2′-C7)
10:90 (G3′-C7)
50:50 (G3′-C7)
−4.9 × 10−11
38
23
57
32
19
42
23
−6.4 × 10−11
−5.2 × 10−11
−4.3 × 10−11
a
Chromophore wt % is determined by the amount of thiopyrylium polymethine molecule excluding the dendron, but including the counterion.
b
Measurements performed using the femtosecond-pulsed Z-scan technique at λexc = 1550 nm. Experimental uncertainties were estimated to be
c
15%. Unfortunately reliable values of Imχ(3) could not be obtained due to saturable absorption (see text). Nonlinearity for an effective neat film
d
determined by dividing the measured Reχ(3) value by the doping wt % of dye. Extrapolated neat values of Reχ(3) were determined using the
following equation, χ(3) = γ·N·L4, where the values of γ were taken from Table 2, N is the density of dyes in the film (assuming a film density of 1 g/
e
cm3) and L is the Lorentz field factor assuming a film refractive index of 1.6. Certain films did not give a measurable closed-aperture Z-scan signal,
likely due to insufficiently thick films and/or too low dye doping levels, and therefore, a Reχ(3) value is not reported.
Macroscopic Third-Order Nonlinear Optical Proper-
ties of Dyes in Films. We also investigated the third-order
NLO properties of the heptamethine dyes in the solid state
using the femtosecond-pulsed Z-scan technique at 1550 nm.
The measured macroscopic nonlinearities (Reχ(3)) are
summarized in Table 3 (third column) for the films with
varying doping ratios of dye with respect to the host polymer. It
is instructive to examine at the values of Reχ(3) for the films of
the C7 parent molecule: first, the macroscopic nonlinearities
are negative in sign and large in magnitude, in accordance with
the microscopic nonlinearities discussed above, and similar to
other polymethine-based films measured in the NIR spectral
region;19 second, the magnitude of Reχ(3) increases with the
dye wt %, although this increase is sublinear. In the absence of
intermolecular interactions, the nonlinearity should scale
linearly with the concentration of the cyanine dye. By dividing
the measured Reχ(3) values of the various films by their
respective doping wt % to give neat-film effective values of
Figure 7. Absorption spectra of C7:APC blends at different dye
Reχ(3) (see fourth column in Table 3), the impact of dye−dye
content.
interactions can clearly be seen: the effective neat nonlinearities
exhibit a monotonic decrease of |Reχ(3)| with increasing dye wt
films exhibit spectra in which this blue-shifted absorption is
%.
increasingly prominent (see Figure 7). With an appreciable
amount of oscillator strength present in these hypsochromically
The effects of intermolecular interactions in these C7 guest−
host films can be further studied by comparing the effective
shifted bands, a corresponding reduction in the magnitude of
neat Reχ(3) values with extrapolated values of Reχ(3) based on
the solution Re(γ) values given in Table 2 (see last column in
Table 3).69 It is interesting to note that the extrapolated Reχ(3)
value (from Re(γ)) is very similar to the neat value of Reχ(3) for
a film containing 10 wt % of C7, consistent with the similarity
of the main absorption band of C7 in this low wt % film (Figure
7, 10 wt % dye) to that seen in solution. Nonetheless, there are
clear differences between this film spectrum and that of even
the most concentrated solutions (Figures 2 and S2, Supporting
Information): in addition to increased relative absorbance in
the vicinity of the vibronic shoulder of the main absorption
band, there is a higher energy feature similar to the aggregation-
induced band seen in neat films (Figure 4). The higher wt %
the effective neat Reχ(3) should be expected according to eq 1;
this is indeed observed in the values in Table 3.
It should be noted that there are few reliable studies
involving nonresonant third-order nonlinearities of cyanine-like
dyes in the solid-state. One previous study investigated
quinoline-based heptamethines in guest−host films via third-
harmonic generation in the 1.85−2.15 μm spectral range.70
A
50 wt % film was found to give a |χ(3)| value of 2 × 10−11 esu at
the peak of what was likely a three-photon resonance. The
deviation from a linear relationship between |χ(3)| and the wt %
of dye in the film at higher dye concentrations was attributed to
aggregation effects, the existence of which was inferred from the
linear absorption spectra. A dioxaborine-based nonamethine
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dye has also been shown to exhibit significant excitonic splitting
in spectra of neat films.19 However, in this case, the observed
Reχ(3) values agreed well with the values extrapolated from the
solution Re(γ), likely due to a distribution of the oscillator
strength between the hypsochromically shifted and bath-
ochromically shifted bands in the solid film spectrum such
that the decreased and increased contributions to Reχ(3) for
those two bands roughly balanced one another. Consequently,
while the Re(γ) values at 1550 nm for the dioxaborine
nonamethine dye of ref 19 and the thiopyrylium heptamethine
dye C7 discussed here are comparable, the nonamethine dye
possesses a neat Reχ(3) value nearly four times that of the
heptamethine (using the results for the 50 or 90 wt % films).
In light of the impacts of aggregation on the NLO response
of cyanine-like dyes in the solid-state, and of the efficacy of
dendronization in changing their aggregation behavior, as
gauged from the absorption spectra of films of the compounds,
it is now critical to assess if the third order susceptibility is also
affected by the presence of dendrons. The Reχ(3) values of the
films containing the dendronized heptamethines (see Table 3)
show similar behavior to that of the nondendronized analogue
C7: large magnitude, negative sign, and increased magnitude
with increased cyanine wt %. However, in most cases, the
calculated neat values of Reχ(3) seem to show a smaller
variability with doping wt % and closer agreement with the
solution-extrapolated values of Reχ(3). This can be rationalized
by noting that in the linear absorption spectra the relative
contributions from the higher energy bands compared to the
lower energy bands are reduced and the hypsochromic shifts
are smaller than those seen in the nondendronized films
(Figures 4 and 5). Thus, the presence of dendrons serves to
reduce the type of aggregation observed in the C7 films thereby
suppressing some of the deleterious impacts on Reχ(3) observed
in the latter films. This impact is further emphasized if one
compares the nonlinearities of the films with similar
chromophore wt % (second column, Table 3): the dendronized
films consistently exhibit larger values of Reχ(3) compared to
their nondendronized counterparts. While results from linear
absorption studies suggested the centrally placed dendrons may
be more effective at preventing aggregation than terminal
dendron susbtitution, the NLO results are inconclusive.
extinction coefficient) of the dendronized heptamethine were
found to be independent of concentration into the mM range, a
regime where concentration effects are evident in the parent
compound, suggesting that dendronization affects interchro-
mophore interactions. The dendronization significantly affects
the spectra of high-number density films with centrally
substituted dendrons seemingly reducing aggregation to a
greater degree than terminally substituted dendrons. The third-
order polarizabilities of the dendronized dyes in solution at mM
concentrations were found to be similar to those of their
nondendronized analogues. Conversely, the real part of the
third-order susceptibility (Reχ(3)) in the solid state was
significantly affected by the presence of the dendrons:
macroscopic nonlinearities in high-chromophore density films
approached values extrapolated from microscopic nonlinear-
ities, which was not the case for films of the parent
polymethines at comparable dye loading, suggesting that
dendronization is successful in reducing the negative effects
of aggregation on Reχ(3). However, the dendronization is
insufficient to give completely solution-like optical properties in
the solid-state, with linear absorption at telecommunication
wavelengths still present in films of the dendronized
compounds, although reduced relative to that seen for films
of the nondendronized compounds; presumably this is due to
the intrinsic flexibility of the dendrons, which may still allow the
conjugated cyanine cores of neighboring molecules to interact
with one another to some extent. Moreover, the use of the
higher generation dendrons, which are the most effective at
minimizing the adverse effects of aggregation, is inevitably
accompanied by dilution effects. Consequently, the covalent
attachment of other more rigid bulky moieties might prove a
more effective approach at disrupting aggregation.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis and characterization of new compounds, tables
showing film thicknesses, effects of solvent on linear spectra,
and values of Re(γ) estimated using the two-level model and
Figures showing solvent-dependent spectra and spectra of C7
in solution and APC at the same concentration. This material is
available free of charge via the Internet at http://pubs.acs.org.
Finally, we should mention that values of |Reχ(3)/Imχ(3)|
could not be determined for films. The presence of non-
negligible linear absorption at 1550 nm led to open-aperture Z-
scan signals that gave signatures typically associated with
saturable absorption. Since these signals are opposite in sign
compared to those associated with 2PA, which provides the
relevant value of Imχ(3) for AOSP, the two effects partially
cancel one another and a meaningful value of Imχ(3) could not
be obtained from the experimental results.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: seth.marder@chemistry.gatech.edu; joe.perry@
chemistry.gatech.edu.
Author Contributions
‡These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
CONCLUSION
■
A series of thiopyrylium-terminated polymethines functional-
ized with flexible benzyl aryl ether dendrons have been
synthesized to examine whether the dendrons can disrupt
interchromophore interactions that lead to undesirable effects
on the linear and nonlinear optical properties of high number-
density films at telecommunication wavelengths. In dilute
solution, the low-energy linear absorption bands of the
dendronized polymethines differ from those of their non-
dendronized analogues, showing significantly lower transition
dipole moments, perhaps due to ion-pairing effects. However,
the linear absorption properties (band position, spectral shape,
ACKNOWLEDGMENTS
■
This work was supported in part by the Science and
Technology Center Program of the National Science
Foundation (Agreement No. DMR-0120967) and by the Air
Force Office of Scientific Research through the COMAS MURI
program (Agreement No. FA9550-10-1-0558).
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current polymer host environment, extrapolation of solution-
determined γ values to χ(3) values in films is appropriate. To estimate
the impact of the host environment, APC/C7 blends with the same wt
% of dye as the solution concentrations used in the femtosecond Z-
scan measurements were fabricated (i.e., 3.9 mM corresponds to 0.4
wt % dye). The absorption spectra of this solution and film are
remarkably similar (see Supporting Information, Figure S3),
supporting the validity of the aforementioned extrapolation. We
note that this does not address possible reduction in the extinction
coefficient or transition dipole moment in the APC host environment.
(70) Matsumoto, S.; Kubodera, K.; Kurihara, T.; Kaino, T. Opt.
Commum. 1990, 76, 147.
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