Solvent effects on the two-photon absorption of distyrylbenzene chromophores

Article abstract of DOI:10.1021/ja052906g

A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA) fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and characterized. The central core was systematically substituted to modu

Full text of DOI:10.1021/ja052906g

Published on Web 09/30/2005
Solvent Effects on the Two-Photon Absorption of
Distyrylbenzene Chromophores
Han Young Woo, Bin Liu, Bernhard Kohler, Dmitry Korystov,
Alexander Mikhailovsky, and Guillermo C. Bazan*
Contribution from the Mitsubishi Chemical Center for AdVanced Materials, Department of
Materials, Institute for Polymers and Organic Solids, UniVersity of California,
Santa Barbara, California 93106
Received May 3, 2005; E-mail: bazan@chem.ucsb.edu
Abstract: A series of organic- and water-soluble distyrylbenzene-based two-photon absorption (TPA)
fluorophores containing dialkylamino donor groups at the termini was designed, synthesized, and
characterized. The central core was systematically substituted to modulate intramolecular charge transfer
(ICT). These molecules allow an examination of solvent effects on the TPA cross section (δ) and on the
TPA action cross section. In toluene, the δ values follow the order of ICT strength. The effect of solvent on
δ is nonmonotonic: maximum δ was measured in an intermediate polarity solvent (THF) and was lowest
in water. We failed to find a correlation between the observed solvent effect and previous theoretical
predictions. Hydrogen bonding to the donor groups and aggregation of the optical units in water, which are
not included in calculational analysis, may be responsible for the discrepancies between experimental results
and theory.
Introduction
donor or acceptor groups, respectively, and π represents a
π-conjugated bridge, have shown exceptional performance.
Emerging technologies such as memory storage, photody-
namic therapy, microfabrication techniques, and multiphoton
microscopy, have stimulated substantial research activity on
organic molecules that can simultaneously absorb two (or more)
photons.1-6 Structure-property relationships have emerged for
the molecular design of efficient two-photon absorption (TPA)
chromophores. The role of several factors, including the
properties of the π-conjugated segment, the strength of donor
and/or acceptor substituents, molecular symmetry, and the
molecular dimensionality, have been examined.7-9 In one
particularly successful strategy, molecules containing quasilinear
D-π-D or D-π-A-π-D structures, where D and A are
Their TPA cross sections (δ, expressed in GM ) 1 × 10-50
cm4‚s‚photon-1‚molecule-1), which reflect the probability of
absorption, have been reported to be greater than 1000 GM.
The magnitudes of δ depend on the degree of intramolecular
charge transfer (ICT) upon excitation.10,11
In contrast to the extensive investigation of molecular design
strategies11 and the successful application of the resulting
chromophores,1-6 the effect of the surrounding medium and
specifically the role of solvent on TPA has not been fully
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9
10.1021/ja052906g CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 14721-14729
14721

A R T I C L E S
Woo et al.
elucidated.12-14 Since solvent polarity influences the magnitude
of ICT, it is expected to change δ in D-π-D and D-π-A-
π-D chromophores.
state polarization by imposing an external field, directed to favor
stabilization of charge transfer from the donor to the acceptor.
In this work, the TPA peak (δmax) and the difference (∆µge) in
dipole moments between the ground state (S0) and the lowest
singlet excited state (S1) were observed at an intermediate value
of bond order alternation (BOA)24 between the neutral structure
and the cyanine limit. The BOA can be considered to be related
to the degree of ground-state polarization in media of different
polarity. According to this work, it is necessary to optimize the
combination between donor-acceptor strength, solvent polarity,
and π-conjugation linker to maximize δ for a given structure.
The solvent effect on the TPA of the push-pull polyene
structure NO2(C2H2)2NH2 was studied.13a In this set of calcula-
tions, δ displayed a strong dependence on the geometrical
changes that take place as a function of the polarity of the
medium. The calculated δ showed an initial pseudolinear
dependence on solvent polarity, with higher values in more polar
solvents, ultimately reaching saturation at the highest dielectric
constants.
Understanding how aqueous media influence the linear
spectroscopy and TPA15-17 of π-conjugated systems is of
importance for fully optimizing fluorescent tags used in two-
photon fluorescence microscopy (TPM)18 of biological samples,
such as living cells.19 For optimum TPM signal-to-noise and
photostability, the two-photon action cross section, defined as
ηδ (where η is the fluorescence quantum yield), should be
maximized. The majority of fluorophores currently used are
those previously developed for one-photon excitation methods
and display ηδ values in the range of 1-50 GM.16,20 We note
that water not only has a high dielectric constant, which
influences the ICT, but is capable of very effective hydrogen
bonding and can therefore interact with the donor groups in
the ground and the excited states. Perturbations on the optical
properties of π-conjugated systems by water are poorly under-
stood, relative to organic solvents, in part because there have
been fewer molecular candidates available for study. Recent
interest in using water-soluble conjugated polymers for the
optical amplification of fluorescent biosensors has improved the
synthetic methodologies for accessing these materials.21,22
Although extensive work (theoretical and experimental) has
been devoted to understand the solvent dependence of linear
spectroscopy,23 theoretical efforts to address solvent effects on
the TPA of conjugated molecules have been limited. Kogej et
al. studied donor-acceptor stilbene derivatives using quantum-
chemical calculations at the semiempirical level.12 The TPA
evolution was evaluated as a function of the degree of ground-
Solvent effects on the conformational changes of a pyridinium
N-phenolate betaine dye and the resulting impact on δ were
calculated using the semiempirical GRINDOL method.13b The
TPA cross section in the gas phase is enhanced several times
at an interplanar angle (between the pyridinium and the
phenolate rings) close to 80°, as compared to the planar
structure. Moreover, the δ values in water were found to be
lower than in the gas phase, and the dependence on the
interplanar angle was less pronounced. A decrease of oscillator
strength and an increase of the transition energy to the charge
transfer state were given as the cause for the decrease of δ in
water.
(12) Kogej, T.; Beljonne, D.; Meyers, F.; Perry, J. W.; Marder, S. R.; Bre´das,
J.-L. Chem. Phys. Lett. 1998, 298, 1.
Another theoretical approach to study the solvent effects on
molecular geometry, electronic structures, and the TPA of
4-trans-[p-(N,N-dibutylamino)-p-stilbenylvinyl]pyridine was pub-
lished.14 A typical D-π-A molecular structure was used in
these calculations. The δ calculated from a two-state model and
the solvatochromism of the ICT state were found to be solvent-
dependent. A nonmonotonic behavior of δ with respect to the
polarity of the solvents was observed. The δ value increases
with respect to the solvent polarity and reaches a maximum at
ꢀ ) 20.7. It is noted that the maximum δ is approximately 2
times larger than in the gas phase and 1.3 times of the value in
the nonpolar solvent of cyclohexane. It was suggested that the
presence of solvent results in an increase of the charge separation
in both ground and ICT excited states and that the electronic
structure change by the solvent is the main reason behind the δ
increase.
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As shown by the different expectations in the above calcula-
tions, there is no general agreement. Moreover, there are few
experimental data available to guide theoretical analysis of
solvent effects.15,25 One exception concerns a study on the
molecular environment effects on TPA behavior of fluorene-
based heterocyclic chromophores using a nonlinear absorption
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4-(dimethylamino)-4′-formylstilbene.
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14722 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Distyrylbenzene-Based Two-Photon Absorption
A R T I C L E S
Scheme 1. Molecular Structures of Neutral and Ionic
Distyrylbenzene Chromophores
67% yields. Potassium tert-butoxide must be used for the
synthesis of 10N. The water-soluble chromophores 8C-11C
are obtained by treatment of 8N-11N with condensed tri-
methylamine in a THF/water mixture for 24 h. After addition
of trimethylamine, a precipitate gradually forms as the reaction
proceeds, consistent with the formation of ionic groups via
quaternization. To ensure that the reaction proceeds to comple-
tion, it is recommended that the precipitate be dissolved by
adding water and an excess of trimethylamine. Removal of
unreacted trimethylamine does not impose a problem, because
of its low boiling point.
Both the neutral precursors and the water-soluble chro-
mophores were characterized by mass spectrometry, elemental
analysis, and NMR spectroscopy. The conjugated framework
exhibits characteristic 1H NMR spectroscopy patterns: two
doublets for the outer phenyl rings, two doublets of half the
intensity for the vinyl protons, and a singlet (where appropriate)
for the central phenyl ring. The trans stereochemistry of the
olefin linkages was established by the coupling constant of the
method.25c,26 In this work, the measured effective TPA cross
section depends on the solvent used. The solvent dependence
of the effective TPA cross section proved to be associated with
absorption by a solvent sensitive excited state.
In this contribution, we provide a synthetic entry into
distyrylbenzene chromophores of general D-π-A-π-D struc-
tures, which are soluble across a range of solvents (Scheme 1).
Different substituents were chosen to allow the molecules to
be neutral, and soluble in nonpolar organic solvents, or charged,
and water-soluble. The internal ring was systematically substi-
tuted to modulate the electronic property of the core and the
overall ICT from the terminal donor groups. TPA spectra for
both the neutral and ionic chromophores in a range of solvents
are also presented and compared. The overall set of compounds
allows the examination of the solvent influence on δ and the
effect of ICT on ηδ.
1
vinylic protons in the H NMR spectra (J ) ∼16 Hz). After
quaternization, the signal characteristic for the -CH2I protons
(∼3.2 ppm in CDCl3 for 8N, 10N, and 11N; 2.7 ppm for 9N in
C6D6) disappeared, while new peaks appeared at ∼3.3 ppm
(-CH2N+(CH3)3I-) and ∼3.05 ppm (-CH2N+(CH3)3I-) for the
water-soluble chromophores, indicative of >99% quaternization.
Further evidence of the identity of 8C-11C was obtained by
electrospray ionization mass spectrometry (ESI-MS). Peaks
corresponding to the correct molecular mass were detected, and
the elemental results are in agreement with the proposed
structures.
Results and Discussion
Synthesis and Characterization. Scheme 2 provides the
synthetic entry into the molecules in Scheme 1. Using a modified
literature method, the reaction of aniline with 6-chlorohexanol
in n-butanol provides 1 in 60% yield.27 Further treatment of 1
with phosphorus oxychloride in DMF introduces the aldehyde
group and affords N,N-bis(6′-chlorohexyl)-4-aminobenzaldehyde
(2) in 75% yield. With the motivation of activating halogen/
amine substitution, chloride/iodide exchange is accomplished
via a Finkelstein-type reaction to afford N,N-bis(6′-iodohexyl)-
4-aminobenzaldehyde (3) in 80% yield. In the 1H NMR spectra,
the peak at ∼3.55 ppm from the methylene adjacent to chloride
disappeared and a new peak appeared at approximately 3.2 ppm
originating from the methylene adjacent to iodide. The core
phosphonates 4-7 are known in the literature28-31 and were
prepared by treatment of the corresponding bromomethyl
compounds with excess triethyl phosphite in DMF.
Linear Absorption and Photoluminescence Spectroscopy.
Table 1 summarizes the linear absorption and photolumines-
cence (PL) spectra for 8-10 in different solvents.
Absorption spectra of toluene solutions containing 11 showed
degradation under ambient illumination. For example, the
absorption maximum decreased by approximately 20% after
solutions were left standing on a lab bench for 30 min
(Supporting Information). This instability prevented us from
making careful measurements of photoluminescence quantum
yields and TPA. In nonpolar solvents such as toluene, the neutral
chromophores 8N-10N display maxima and peak shapes
(Figure 1) that are typical for this type of molecule.7a,32 The
absorption maxima range from 409 nm for 8N to 485 nm for
the cyano-substituted 10N, while the corresponding emission
changes from 449 nm for 8N to 526 nm for 10N. Introduction
of methoxy groups into the central core produces a red-shift of
18 nm in the absorption and 26 nm in the emission, relative to
8N. For each chromophore, the PL spectra show a progressive
red-shift and a loss of vibronic structure, together with a decrease
in η, as the solvent polarity increases, consistent with the strong
ICT character of the solvent relaxed emissive state (Table 1).33,34
Coupling 2 equiv of aldehyde 3 with phosphonates 4-7 under
Horner-Emmons-Wittig coupling conditions using sodium
hydride or potassium tert-butoxide in THF at room temperature
affords the precursor neutral chromophores 8N-11N in 35-
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A. G.; Vaia, R. A.; Tan, L.-S. J. Chem. Phys. 2004, 120, 5275. (b) He, G.
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C. A.; Pond, S. J. K.; Marder, S, R.; Perry, J. W. J. Phys. Chem. B 2002,
106, 6853.
The PL of 10N is the most sensitive to solvent effects, with
the λem shifting from 526 nm in toluene to 636 nm in DMSO
(32) (a) Pond, S. J. K.; Rumi, M.; Levin, M. D.; Parker, T. C.; Beljonne, D.;
Day, M. W.; Bre´das, J.-L.; Marder, S. R.; Perry, J. W. J. Phys. Chem. A
2002, 106, 11470. (b) Renak, M. L.; Bartholomew, G. P.; Wang, S. J.;
Ricatto, P. J.; Lachicotte, R. J. Bazan, G. C. J. Am. Chem. Soc. 1999, 121,
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9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14723

A R T I C L E S
Woo et al.
Scheme 2. Synthetic Procedure for Accessing Neutral and Water-Soluble TPA Chromophoresa
a Reagents and conditions: (i) 6-chloro-1-hexanol/n-butanol, K2CO3, reflux, 4 days; (ii) POCl3/DMF, 100 °C, 2 h; (iii) NaI/dry acetone, reflux, 2 days;
(iv) NaH (or t-BuOK) in dry THF, room temperature; (v) NMe3, THF/H2O, room temperature, 24 h.
Table 1. Summary of Absorption and Photoluminescence Spectroscopies in Different Solvents
toluene
THF
DMSO
water
a
λabs
(ꢀmax
)
λem
(
η b
)
λabs
(ꢀmax
)
λem
(η)
λabs
(
ꢀmax
)
λem
(η
)
λabs
(ꢀmax
)
λem (η)
8N
8C
9N
9C
10N
10C
409 (7.5)
427 (6.5)
485 (6.5)
449 (0.93)
475 (0.92)
526 (0.58)
409 (8.6)
428 (6.5)
483 (7.0)
461 (0.77)
479 (0.92)
559 (0.63)
418 (7.8)
418 (6.2)
439 (6.3)
439 (5.3)
497 (6.5)
495 (4.9)
506 (0.63)
506 (0.59)
515 (0.52)
515 (0.55)
636 (0.19)
635 (0.22)
406 (3.9)
423 (4.9)
469 (3.8)
558 (0.27)
567 (0.40)
-
a Molar absorption coefficient at λabs (104‚mol-1‚cm-1). b Quantum yields were measured relative to 9,10-diphenylanthracene in cyclohexane and fluorescein
in water at pH ≈11.
(Table 1 and Figure 2). In these D-π-A-π-D systems, the
cyano functionality is the stronger acceptor group, gives rise to
more pronounced ICT in the excited state, and produces larger
changes by solvent stabilization.
The absorption and emission spectra of 8C-10C in water
are shown in Figure 3. Perturbations by the central ring acceptor
functionalities are similar to those observed for the N series.
The λabs values in water are slightly blue-shifted, relative to
those in toluene, in structurally related pairs [(in nm) 8N/8C,
409/406; 9N/9C, 427/ 423; 10N/10C, 485/469]. This blue-shift
may be attributed to a modified geometry with partially restricted
delocalization due to the hydrophobic nature of the conjugated
backbone. It is also possible that water can perturb the donor
strength of the nitrogen atom via hydrogen bonding and
ultimately the ICT process.33a,35 The PL spectra are broad and
(34) Absorption maxima are less sensitive to solvent polarity (Table 1). The
ICT excited state is polar, with a large dipole moment due to a substantial
charge redistribution, and is expected to be sensitive to solvent polarity.
Direct transition from the ground state to the ICT state may be forbidden.
The less sensitive absorption maxima to the solvent polarity suggest that
the ICT state is derived following relaxation of the initially formed Franck-
Condon excited state (ref 32a). See: (a) Leinhos, U.; Ku¨hnle, W.;
Zachariasse, K. A. J. Phys. Chem. 1991, 95, 2013. (b) Strehmel, B.; Sarker,
A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.; Neckers, D. C. J. Am.
Chem. Soc. 1999, 121, 1226.
9
14724 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Distyrylbenzene-Based Two-Photon Absorption
A R T I C L E S
Figure 1. Normalized absorption and emission spectra of 8N-10N in toluene. Emission scans were collected by exciting at the absorption maximum (λabs
)
of each sample.
respectively. With the cyano-substituted pair (10N/10C), one
observes a more dramatic change. For compound 10N in
toluene, η ) 0.58, but 10C shows no detectable emission in
water with our instrumentation.36 We also note that in a solvent
of intermediate polarity (DMSO), there are no measurable
differences in the linear spectroscopy for structurally related
neutral (N) and charged (C) pairs. The charged end groups do
not appear to be interacting with the electronic structure of the
chromophores and do not modify spectral properties (Supporting
Information).
Acceptor group strengths were probed through a study of the
oxidation potential of the chromophores using cyclic voltam-
+
metry (CV). The measured electrochemical EM
/
M are 240 mV
for 8N, 140 mV for 9N, and 410 mV for 10N, relative to a
Ag/AgNO3 reference (Figure 4). CV measurements were carried
out in 0.1 M n-Bu4NPF6 solution in THF at a scan rate of 100
mV/s. Coupling this information to the data in Table 1, we note
that η in water (0.27 for 8C, 0.40 for 9C and no emission for
10C) is inversely proportional to the strength of the acceptor
group (10C > 8C > 9C).
Figure 2. PL spectra of 10N in different solvents. The areas under the
spectra are proportional to the η in each solvent.
In Figure 5, the absorption and emission spectra of 8C in
water are given as a function of HCl concentration. Table 2
contains a summary of similar experiments for 9C and 10C.
With increasing [HCl], the nitrogen atoms at the termini of the
chromophore are protonated and three different species are
expected: unprotonated, monoprotonated, and doubly proto-
nated. The absorption spectra of solutions of 8C with HCl at
intermediate levels of [HCl] were deconvoluted using a general
equilibrium analysis technique,20 and the absorption from the
monoprotonated species was observed to have λabs ) 370 nm
(Supporting Information). The PL spectra, however, contain
contributions from only the neutral and the doubly protonated
species. Indeed, the effective η of the solution decreases first
(36) The ICT often produces PL quenching by charge separation after excitation,
increasing the rates of the radiationless decay processes. Several mechanisms
for the ICT-related quenching were suggested. See: (a) Schuddeboom, W.;
Jonker, S. A.; Warman, J. M.; Leinhos, U.; Ku¨hnle, W.; Zachariasse, K.
A. J. Phys. Chem. 1992, 96, 10809. (b) Jager, W. F.; Volkers, A. A.;
Neckers, D. C. Macromolecules 1995, 28, 8153. (c) Lipinski, J.; Chojnacki,
H.; Grabowski, Z. R.; Rotkiewicz, K. Chem. Phys. Lett. 1980, 70, 449. (d)
Catala´n, J.; D´ıaz, C.; Lo´pez, V.; Pe´rez, P.; Claramunt, R. M. J. Phys. Chem.
1996, 100, 18392. (e) Baumann, W.; Bischof, H.; Fro¨hling, J.-C.; Brittinger,
C.; Rettig, W.; Rotkiewicz, K. J. Photochem. Photobiol. A: Chem. 1992,
63, 49. In addition, PL quenching by specific solute-solvent interaction
such as hydrogen bonding should be considered in water. The excited-
state complex via hydrogen bonding may activate radiationless depopulation
channels, for example, energy dissipation through the vibrations connected
with hydrogen bonding. See: (f) Herbich, J.; Waluk, J. Chem. Phys. 1994,
188, 247. (g) Kumar, C. V.; Tolosa, L. M. J. Photochem. Photobiol. A:
Chem. 1994, 78, 63.
Figure 3. Normalized absorption and PL spectra of 8C-10C in water.
Emission scans were collected by exciting at the λabs of each sample.
Compound 10C has no detectable emission in water.
featureless and are further red-shifted from the absorption spectra
in the charged compounds (∆νabs-em ) 6709 cm-1 for 8C, 6004
cm-1 for 9C), compared to their neutral counterparts in toluene
(∆νabs-em ) 2178 cm-1 for 8N, 2367 cm-1 for 9N). Compounds
8N and 8C have η values of 0.93 and 0.27 in toluene and water,
(35) Halpern, A. M.; Ruggles, C. J.; Zhang, X. J. Am. Chem. Soc. 1987, 109,
3748.
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14725

A R T I C L E S
Woo et al.
nm) 409 and 720 for 7N, 427 and 725 for 8N, 485 and 815 for
9N] and depends on the effective conjugation and the ICT over
the entire molecule.10,11 Increasing the acceptor strength of the
core results in an enhanced ICT from dialkyl-
amino donor units to the core, giving 10N the largest δ. The δ
value of 9N is lowest due to methoxy being the poorest acceptor.
Figure 7 shows the TPA spectra of 8N and 8C in different
solvents. The spectra display similar band shape and λTPA and
variations in δ [(in GM) 910 in toluene, 1540 in THF, 1200 in
acetone; ∼900 GM in DMSO, and 330 GM in water]. Plotting
δ vs dielectric constant shows a nonmonotonic dependence
(Figure 8). We note here that a similar trend was obtained for
[2.2]paracyclophene-based TPA chromophores.15,37
Figure 9 shows that, in water, the TPA of 8C and 9C show
similar λTPA and band structure, relative to the measurements
of the neutral counterparts in organic solvents. However, the δ
values are considerably lower (Table 3). The δ of 10C could
not be determined with our technique because 10C is not
fluorescent in water. We also note that in a solvent of
intermediate polarity (DMSO), the TPA spectra of the N and
C series are similar. The charged or neutral terminal groups
therefore are not a major consideration.
Figure 4. Cyclic voltammograms of 8N-10N (1 mg/mL concentration)
relative to Ag/AgNO3 in 0.1 M n-Bu4NPF6 in THF at a scan rate of 100
mV/s.
and subsequently increases with increasing [HCl], suggesting
that the monoprotonated species has a very low η or is
nonfluorescent. When [HCl] ) 10-2 M, conditions where only
the doubly protonated species of each chromophore is expected
to be present (i.e. 8CH2+, 9CH2+, and 10CH2+), one observes
blue-shifted λabs and λem values (Table 2) and an increase in η.
Analogous spectral shifts were made with metal ion sensing
two-photon fluorophores.20
Our measurements are best accommodated by the theoretical
calculations by Wang et al.,14 where a maximum δ was
anticipated in a solvent of intermediate polarity. The enhance-
ment of δ was explained in terms of increased charge separation
assisted by solvent, and they suggested that the resulting change
in electronic structure provides the main perturbation on δ.
However, there are still substantial deviations between the
measured quantities and these calculations, which suggest that
other factors need to be considered, such as solute-solvent
interactions and solubility differences. Low solubility and
hydrophobic interactions can lead to aggregation or structure
changes (for example, by distortion), with a corresponding
modification of linear and nonlinear optical responses.38,39
Fluorescence and near-field scanning optical microscopy (NSOM)
studies of conjugated polymer aggregates show different sol-
vatochromism, relative to amorphous polymer regions.40 Ad-
ditionally, strong enhancement (30-fold) of δ from a J aggregate
of a porphyrin-type molecule in water was also observed.17
Evidence for structural changes and aggregation in the molecular
systems described in this paper can be obtained from a
comparison of the linear absorption spectra of the N series in
toluene with those of the C series in water, which are broad,
lack vibronic structure, and have blue-shifted λabs values that
are concentration-dependent (Supporting Information).
Comparison of the data in Tables 1 and 2 shows that there is
a difference in the order of the absorption and emission maxima
for the doubly protonated chromophores, relative to the unpro-
tonated counterparts. In the doubly protonated species, the
internal ring becomes the electron rich fragment, and we expect
a net charge transfer to the outer aromatic fragments bearing
the ammonium groups. The donating ability of the groups is
MeO > H > CN, which corresponds to the order in λabs
.
Furthermore, 10C is not fluorescent in water, but strong emission
2+
from 10CH2 is observed. Overall, linear optical properties,
such as λabs, λem, and η, are strongly perturbed by protonation
on the terminal nitrogen atoms, which may be considered an
extreme case of hydrogen bonding in water. As will be discussed
later, hydrogen bonding can also be expected to modify overall
ICT, leading to a change of TPA.
Two-Photon Absorption Spectroscopy. TPA spectra were
obtained by using the two-photon induced fluorescence (TPIF)
measurement technique with femtosecond laser pulses, as
described previously in the literature.7a,18a Complete details can
be found in the Supporting Information. The TPA spectra of
8N-10N in toluene are presented in Figure 6 and are sum-
marized in Table 3. The TPA maximum wavelength (λTPA) and
the peak δ for 8N in toluene (720 nm and 910 GM, respectively)
are similar to those previously reported for 1,4-bis(4′-(N,N-
dibutylamino)styryl)benzene (730 nm and 995 GM),7a which
indicates negligible influence by the terminal iodide groups. The
TPA spectra of 9N and 10N are also in good agreement with
previous measurements of the corresponding structures bearing
dibutylamino substituents.7a,32
Water can also influence the electronic structure of D-π-
A-π-D chromophores via hydrogen bonding and a corre-
(37) We observed a similar trend on the solvent dependence of the measured
TPA cross section of the structure that contains two fragments of 8 held
together via the [2.2]paracyclophane core: 1290 GM in toluene, 2180 GM
in THF, 1300 GM in DMSO, and 370 GM in water. For the synthesis of
these compounds see: (a) Bartholomew, G. P.; Rumi, M.; Pond, S. J. K.;
Perry J. W.; Tretiak, S.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 11529.
(b) Bartholomew, G. P.; Bazan, G. C. J. Am. Chem. Soc. 2002, 124, 5183.
(38) (a) Siddiqui, S.; Spano, F. C. Chem. Phys. Lett. 1999, 308, 99. (b) Song,
Q.; Evans, C. E.; Bohn, P. W. J. Phys. Chem. 1993, 97, 13736. (c) Evans,
C. E.; Song, Q.; Bohn, P. W. J. Phys. Chem. 1993, 97, 12302. (d) Manas,
E. S.; Spano, F. C. J. Chem. Phys. 1998, 109, 8087.
(39) The absorption of 8N in toluene is invariant in the 10-6-10-4 M range.
For 8C in water, one observes a blue-shift in λabs with increasing [8C],
which indicates different levels of aggregation (See Supporting Information).
(40) (a) Nguyen, T.-Q.; Martel, R.; Avouris, P.; Bushey, M. L.; Brus, L.;
Nuckolls, C. J. Am. Chem. Soc. 2004, 126, 5234. (b) Nguyen, T.-Q.;
Schwartz, B. J.; Schaller, R. D.; Johnson, J. C.; Lee, L. F.; Haber, L. H.;
Saykally, R. J. J. Phys. Chem. B 2001, 105, 5153.
It is interesting to note the effect of the core substituents (8N,
-H; 9N, -OMe; 10N, -CN) on λTPA and δ. In toluene, the
maximum δ observed are 910 GM at 720 nm for 8N, 890 GM
at 725 nm for 9N, and 1710 GM at 815 nm for 10N. The trend
in λTPA values (toluene) is similar to that observed for λabs [(in
9
14726 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Distyrylbenzene-Based Two-Photon Absorption
A R T I C L E S
Figure 5. Absorption and emission spectra of 8C ([8C] ) 2 × 10-6 M) as a function of HCl concentration in water. The areas under the spectra are
proportional to η.
Table 2. Summary of Absorption and Photoluminescence Spectra
at [HCl] ) 10-2 M and [8C-10C] ) 10-6
M
λ
abs (nm)
λem (nm)
η
2+
2+
8CH2
9CH2
355
379
346
406
461
420
0.36
0.57
0.45
2+
10CH2
Figure 7. Solvent dependence of the measured TPA spectra of 8 (8N in
toluene, THF, acetone, and DMSO and 8C in DMSO and water).
Figure 6. Two-photon absorption spectra of 8N-10N in toluene.
Table 3. Summary of TPA Spectroscopy
solvent
λ
abs (nm)
λTPAa (nm)
δ
max (GM)b
ηδ (GM)c
8N
8C
9N
9C
10N
10C
toluene
water
toluene
water
toluene
water
409
406
427
423
485
469
720
720
725
730
815
910
330
890
360
1710
846
89
819
144
992
a Wavelength of two-photon absorption maximum. b TPA cross section
at λTPA
c Two-photon absorption action cross section.
.
sponding decrease of the ICT. Complexation via hydrogen
bonding between an electronically excited amine and water is
well-known and has been mentioned previously.33a,35 The exact
mechanism for the substantial drop of δ in water is, however,
not clear at present stage and awaits detailed theoretical
treatments that consider a comprehensive set of variables
involved in the experimental measurements.
Figure 8. Maximum TPA cross section of 8 vs solvent polarity (8N in
toluene, THF, acetone, and DMSO and 8C in DMSO and water).
mention molecular design guidelines for the efficient TPM
fluorophores for biological systems. The combined ηδ values
are 89 GM for 8C and 144 GM for 9C, respectively. However,
the molecular framework of 10C, which has the highest δ in
toluene, is not a useful structure to consider for TPM applica-
Despite the medium effect uncertainties, it is valuable to
9
J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14727

A R T I C L E S
Woo et al.
previously.41 Mass spectrometry and elemental analysis were performed
by UC Santa Barbara Mass Spectrometry Laboratory and Elemental
Analysis Center. 1,4-Bis(diethylphosphonatemethyl)benzene,28 1,4-bis-
(diethylphosphonatemethyl)-2,5-dimethoxybenzene,29 1,4-bis(dieth-
ylphosphonatemethyl)-2,5-dicyanobenzene,30 and 1,4-bis(diethylphos-
phonatemethyl)-2,3,5,6-tetrafluorobenzene31 were synthesized according
to literature precedent.
N,N-Bis(6-hydroxyhexyl)aniline (1). A mixture of 9.3 g (100 mmol)
of freshly distilled aniline, 30.0 g (220 mmol) of 6-chloro-1-hexanol,
and 30.4 g (220 mmol) potassium carbonate was heated in 50 mL of
n-butanol under reflux for 4 days. After cooling, the remaining solids
were filtered off, and the solvent was removed under reduced pressure
to afford the crude product. Purification by silica gel chromatography
(ethyl acetate/hexane ) 2:1) yielded 17.6 g (60%) of 1 as a colorless
oil. 1H NMR (400 MHz, CDCl3): δ 7.18 (m, 2H), 6.61 (m, 3H), 3.91
(t, J ) 6.4 Hz, 4H), 3.67 (t, J ) 6.4 Hz, 4H), 1.90-1.77 (m, 4H),
1.70-1.53 (m, 4H), 1.50-1.31 (m, 8H). 13C NMR (100 MHz,
CDCl3): δ 148.19, 129.31, 115.33, 111.86, 62.79, 51.06, 32.78, 27.30,
27.05, 25.76. HRMS (EI): m/z ) 293.2360 (M+), ∆ ) 1.7 ppm.
Figure 9. Two-photon absorption spectra of 8C and 9C in water.
N,N-Bis(6′-chlorohexyl)-4-aminobenzaldehyde (2). Phosphorus
oxychloride (17.5 g, 114 mmol) was added dropwise to 40 mL of dry
DMF at 0 °C. After 30 min, 11.1 g (38 mmol) of 1 in 20 mL of DMF
was added to the above solution. The resulting mixture was heated to
100 °C for 2 h. After cooling to 40 °C, the resulting mixture was poured
into 200 mL of ice water. The pH of the mixture was adjusted to 7 by
addition of saturated potassium acetate aqueous solution. The mixture
was extracted with dichloromethane, and the combined organic phase
was washed with water and dried over MgSO4. The solvent was
evaporated and the crude product was purified by silica gel chroma-
tography (chloroform/hexane ) 3:2) to afford 2 (10.2 g, 75%) as a
light yellow oil. 1H NMR (400 MHz, CDCl3): δ 9.71 (s, 1H), 7.71 (d,
J ) 8.4 Hz, 2H), 6.65 (d, J ) 8.4 Hz, 2H), 3.55 (t, J ) 6.6 Hz, 4H),
3.36 (t, J ) 7.2 Hz, 4H), 1.89-1.81 (m, 4H), 1.64-1.60 (m, 4H), 1.48-
1.36 (m, 8H). 13C NMR (100 MHz, CDCl3): δ 190.18, 152.66, 132.46,
124.87, 110.90, 51.14, 45.17, 32.73, 27.23, 26.88, 26.51. HRMS (EI):
m/z ) 357.1617 (M+), ∆ ) 2.6 ppm. Anal. Calcd for C19H29Cl2NO:
C, 63.68; H, 8.16; N, 3.91. Found: C, 63.33; H, 8.05; N, 3.97.
tions in water, because of its negligible η. As previously
demonstrated with [2.2]paracyclophane TPA chromophores,15
acceptors and/or donors of intermediate strength are more likely
to lead to enhanced ηδ.
Conclusion
In summary, we provide a synthetic entry to a series of
organic-soluble and water-soluble TPA chromophores based on
the distyrylbenzene framework. Introduction of alkyl halide
terminal units in neutral precursors and subsequent quaterniza-
tion using trimethylamine affords water-soluble TPM fluoro-
phores. The neutral and charged molecules allowed us to
measure the effect of the solvent polarity on δ as a function of
acceptor strength. We observed a maximum δ in THF, which
is of intermediate polarity. In water, δ shows a substantial
decrease. The solvent dependence of TPA in the D-π-A-
π-D distyrylbenzene structure is nonmonotonic when plotted
against the solvent dielectric constant, and we failed to find an
exact match against previous theoretical expectations. Closest
agreement was with the predictions by Wang.14 We suggest that
future theoretical work should consider specific solute-solvent
interactions, such as hydrogen bonding, which can substantially
modify the ICT states. The effect of protonation, an extreme
case of hydrogen bonding, on absorption and photoluminescence
spectra highlights chemical effects. Additionally, changes in the
chromophore geometry and/or multichromophore aggregation
are likely of importance. Finally, we note that to obtain optimum
two-photon action cross sections in water, the ICT strength must
be moderated to retain high quantum efficiencies.
N,N-Bis(6′-iodohexyl)-4-aminobenzaldehyde (3). A mixture of 2
(8.9 g, 14.7 mmol) and 37.5 g (250 mmol) of sodium iodide was
dissolved in 200 mL of dry acetone and heated under reflux for 2 days.
The solvent was evaporated and the resulting mixture was diluted with
300 mL of dichloromethane and washed with water several times. The
organic phase was dried over MgSO4 and filtered. After removal of
the solvent, the crude product was purified by silica gel chromatography
(chloroform/hexane ) 3:2) to afford 3 (10.7 g, 80%) as a light brown
oil. 1H NMR (400 MHz, CDCl3): δ 9.70 (s, 1H), 7.69 (d, J ) 8.4 Hz,
2H), 6.63 (d, J ) 8.4 Hz, 2H), 3.34 (t, J ) 7.6 Hz, 4H), 3.19 (t, J )
6.8 Hz, 4H), 1.86-1.81 (m, 4H), 1.64-1.60 (m, 4H), 1.47-1.33 (m,
8H). 13C NMR (100 MHz, CDCl3): δ 190.07, 152.54, 132.32, 124.73,
110.82, 51.03, 33.38, 30.34, 27.06, 26.04, 7.19. EI-MS: 541 (M+),
344 (M+-C5H10I). Anal. Calcd for C19H29I2NO: C, 42.16; H, 5.40; N,
2.59. Found: C, 41.91; H, 5.23; N, 2.63.
General Procedure for the Synthesis of the Neutral Chro-
mophores. A solution containing 1 mmol of the phophonate (4, 5, 6,
or 7) and 2.5 mmol of the aldehyde 3 was prepared in dry THF. To
the above mixture was added 2 mmol of base (sodium hydride or
potassium tert-butoxide). The resulting suspension was stirred overnight
at room temperature. The resulting mixture was diluted with 200 mL
of dichloromethane and washed with brine. The organic layer was dried
over MgSO4, and the solvent was evaporated. The crude material was
Experimental Section
General Details. Chemicals were purchased from Aldrich Co.,
except 2,5-dimethylterephthalonitrile, which was purchased from TCI
America and were used without further purification. 1H- and 13C NMR
spectra were collected on a Varian Unity 400 MHz (or 200 MHz)
spectrometer. The UV-vis absorption spectra were recorded on a
Shimadzu UV-2401 PC diode-array spectrometer. Photoluminescence
spectra were obtained on a PTI Quantum Master fluorometer equipped
with a Xenon lamp excitation source. Fluorescence quantum yields were
measured relative to fluorescein (η ) 0.92) and 9,10-diphenylanthracene
(η ) 0.91) and verified using the referenceless technique described
(41) (a) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.;
Kessener, Y. A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Chem.
Phys. Lett. 1995, 241, 89. (b) Maciejewski, A.; Steer, R. P. J. Photochem.
1986, 35, 59. (c) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193.
9
14728 J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005

Distyrylbenzene-Based Two-Photon Absorption
A R T I C L E S
purified by silica gel chromatography (dichloromethane/hexane ) 2:1)
to afford the precursor chromophores 8N-11N in yields in the range
of 35-67%.
dried under vacuum to afford the final water-soluble chromophores
8C-11C. The product yields are close to 90%.
1,4-Bis(4′-(N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)amino)-
styryl)benzene tetraiodide (8C) was obtained as a yellow solid
1,4-Bis(4′-(N,N-bis(6′′-iodohexyl)amino)styryl)benzene (8N). The
1
1
pure 8N was obtained in 39% yield as a yellow solid. H NMR (400
(yield: 92%). H NMR (400 MHz, DMSO-d6): δ 7.48 (s, 4H), 7.39
MHz, CDCl3): δ 7.45 (s, 4H), 7.39 (d, J ) 8.8 Hz, 4H), 7.04 (d, J )
15.6 Hz, 2H), 6.89 (d, J ) 15.6 Hz, 2H), 6.63 (d, J ) 8.8 Hz, 4H),
3.30 (t, J ) 7.6 Hz, 8H), 3.22 (t, J ) 6.4 Hz, 8H), 1.89-1.82 (m, 8H),
1.64-1.57 (m, 8H), 1.49-1.43 (m, 8H), 1.41-1.35 (m, 8H). 13C NMR
(100 MHz, CDCl3): δ 147.73, 136.87, 128.32, 127.97, 126.41, 125.14,
123.89, 111.97, 51.15, 33.66, 30.59, 27.39, 26.32, 7.41. Anal. Calcd
for C46H64I4N2: C, 47.93; H, 5.60; N, 2.43. Found: C, 48.15; H, 5.47;
N, 2.60.
1,4-Bis(4′-(N,N-bis(6′′-iodohexyl)amino)styryl)-2,5-dimethoxyben-
zene (9N). The pure 9N was obtained in 46% yield as a yellow solid.
1H NMR (400 MHz, C6D6): δ 7.92 (d, J ) 16.0 Hz, 2H), 7.65 (d, J )
8.8 Hz, 4H), 7.42 (d, J ) 16.0 Hz, 2H), 7.28 (s, 2H), 6.65 (d, J ) 8.8
Hz, 4H), 3.50 (s, 6H), 3.01 (t, J ) 7.4 Hz, 8H), 2.73 (t, J ) 6.8 Hz,
8H), 1.43 (m, 8H), 1.36 (m, 8H), 1.07 (m, 8H), 0.96 (m, 8H). 13C NMR
(100 MHz, C6D6): δ 152.53, 148.50, 129.69, 129.01, 127.77, 127.30,
120.19, 113.16, 109.50, 56.39, 51.67, 34.19, 31.06, 28.02, 26.78, 7.32.
Anal. Calcd for C48H68I4N2O2: C, 47.54; H, 5.65; N, 2.31. Found: C,
47.24; H, 5.75; N, 2.41.
1,4-Bis(4′-(N,N-bis(6′′-iodohexyl)amino)styryl)-2,5-dicyanoben-
zene (10N). The pure 10N was obtained in 35% yield as a red solid.
1H NMR (400 MHz, CDCl3): δ 7.96 (s, 2H), 7.45 (d, J ) 8.8 Hz,
4H), 7.20 (d, J ) 16.0 Hz, 2H), 7.11 (d, J ) 16.0 Hz, 2H), 6.63 (d, J
) 8.8 Hz, 4H), 3.33 (t, J ) 7.6 Hz, 8H), 3.22 (t, J ) 6.8 Hz, 8H), 1.84
(m, 8H), 1.63 (m, 8H), 1.46 (m, 8H), 1.39 (m, 8H). 13C NMR (100
MHz, CDCl3): δ 140.04, 138.83, 129.27, 129.13, 123.21, 117.50,
116.86, 114.24, 111.86, 51.17, 33.65, 30.60, 27.40, 26.32, 7.31. Anal.
Calcd for C48H62I4N4: C, 47.94; H, 5.20; N, 4.66. Found: C, 48.37;
H, 5.30; N, 4.72.
(d, J ) 9.2 Hz, 4H), 7.09 (d, J ) 16.4 Hz, 2H), 6.90 (d, J ) 16.4 Hz,
2H), 6.64 (d, J ) 9.2 Hz, 4H), 3.28 (m, 16H), 3.04 (s, 36H) 1.68 (m,
8H), 1.55 (m, 8H), 1.34 (m, 16H). 13C NMR (100 MHz, DMSO-d6):
δ 147.35, 136.22, 128.10, 127.70, 124.06, 112.75, 111.51, 65.23, 52.16,
49.93, 26.68, 25.96, 25.77, 22.10. Anal. Calcd for C58H100I4N6: C,
50.15; H, 7.26; N, 6.05. Found: C, 50.07; H, 7.17; N, 6.14.
1,4-Bis(4′-(N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)amino)-
styryl)-2,5-dimethoxybenzene tetraiodide (9C) was obtained as a
1
yellow solid (yield: 89%). H NMR (400 MHz, DMSO-d6): δ 7.37
(d, J ) 8.8 Hz, 4H), 7.21 (s, 2H), 7.15 (d, J ) 16.4 Hz, 2H), 7.10 (d,
J ) 16.4 Hz, 2H), 6.65 (d, J ) 8.8 Hz, 4H), 3.87 (s, 6H), 3.29 (m,
16H), 3.05 (s, 36H), 1.68 (m, 8H), 1.55 (m, 8H), 1.34 (m, 16H). 13C
NMR (100 MHz, DMSO-d6): δ 150.52, 147.31, 140.27, 128.61, 127.64,
125.55, 124.54, 117.19, 111.56, 65.24, 56.19, 52.16, 49.93, 26.68, 25.96,
25.76, 22.10. Anal. Calcd for C60H104I4N6O2: C, 49.73; H, 7.23; N,
5.80. Found: C, 50.10; H, 7.12; N, 5.68.
1,4-Bis(4′-(N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)amino)-
styryl)-2,5-dicyanobenzene tetraiodide (10C) was obtained as a dark
1
red solid (yield: 87%). H NMR (400 MHz, DMSO-d6): δ 8.41 (s,
2H), 7.58 (d, J ) 16.4 Hz, 2H), 7.43 (d, J ) 8.8 Hz, 4H), 6.97 (d, J
) 16.4 Hz, 2H), 6.71 (d, J ) 8.8 Hz, 4H), 3.33 (m, 16H), 3.07 (s,
36H), 1.68 (m, 8H), 1.59 (m, 8H), 1.34 (m, 16H). 13C NMR (100 MHz,
DMSO-d6): δ 148.57, 137.98, 135.10, 128.89, 122.52, 117.10, 115.47,
113.01, 111.55, 65.21, 52.16, 49.91, 44.23, 26.64, 25.89, 25.73, 22.08.
Anal. Calcd for C60H98I4N8: C, 50.08; H, 6.86; N, 7.79. Found: C,
49.42; H, 6.95; N, 7.56.
1,4-Bis(4′-(N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)amino)-
styryl)-2,3,5,6-tetrafluorobenzene tetraiodide (11C) was obtained as
an orange solid (yield: 90%). 1H NMR (400 MHz, DMSO-d6): δ 7.45
(d, J ) 8.8 Hz, 4H), 7.33 (d, J ) 16.8 Hz, 2H), 6.79 (d, J ) 16.8 Hz,
2H), 6.67 (d, J ) 8.8 Hz, 4H), 3.31 (m, 16H), 3.05 (s, 36H), 1.67 (m,
8H), 1.55 (m, 8H), 1.33 (m, 16H). 13C NMR (100 MHz, DMSO-d6):
δ 148.44, 143.73 (d), 137.01, 128.69, 112.92, 114.15, 111.45, 107.44,
65.21, 52.16, 26.64, 25.90, 25.73, 22.08. Anal. Calcd for C58H96-
F4I4N6: C, 47.68; H, 6.62; N, 5.75. Found: C, 47.53; H, 6.30; N, 5.58.
1,4-Bis(4′-(N,N-bis(6′′-iodohexyl)amino)styryl)-2,3,5,6-tetrafluo-
robenzene (11N). Compound 11N was obtained in 67% yield as an
1
orange solid. H NMR (400 MHz, CDCl3): δ 7.42 (d, J ) 8.8 Hz,
4H), 7.40 (d, J ) 16.4 Hz, 2H), 6.85 (d, J ) 16.4 Hz, 2H), 6.62 (d, J
) 8.8 Hz, 4H), 3.32 (t, J ) 7.6 Hz, 8H), 3.21 (t, J ) 6.8 Hz, 8H),
1.89-1.82 (m, 8H), 1.64-1.57 (m, 8H), 1.49-1.43 (m, 8H), 1.41-
1.35 (m, 8H). 13C NMR (100 MHz, CDCl3): δ 148.56, 144.74 (d),
136.59, 128.61, 124.52, 114.79 (m), 111.82, 109.32, 51.14, 33.65, 30.58,
27.38, 26.31, 7.33. Anal. Calcd for C46H60F4I4N2: C, 45.12; H, 4.94;
N, 2.29. Found: C, 45.74; H, 4.96; N, 2.32.
General Procedure for the Synthesis of the Water-Soluble
Chromophores. The neutral precursor chromophores (8N-11N, 200
mg) were dissolved in 30 mL of THF and the solution was cooled to
-78 °C with dry ice/acetone bath. A large excess of condensed
trimethylamine (∼2 mL) was added to the solution. The solution was
allowed to warm to room temperature for 12 h and 20 mL of water
was added to dissolve the precipitated compounds. Another portion
(∼2 mL) of condensed trimethylamine was added at -78 °C, and the
mixture was stirred for another 12 h for the complete quaternization.
After removal of the solvents and an excess of trimethylamine under
vacuum, the residue was dissolved in water and filtered. Evaporation
of the water afforded the products, which were washed in acetone and
Acknowledgment. The authors are grateful to Mitsubishi
Chemical Center for Advanced Materials (MC-CAM), the NIH
(GM62958-01), and the NSF (DMR-0097611) for financial
support.
Supporting Information Available: Additional UV/vis and
PL spectra (11N, 8N/8C, 8C/8CH+/8CH22+), TPA spectra
measurements, concentration dependence of absorption in
toluene (8N) and in water (8C), and complete refs 1b and 11.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA052906G
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J. AM. CHEM. SOC. VOL. 127, NO. 42, 2005 14729