Solvatochromism of distyrylbenzene pairs bound together by [2.2]paracyclophane: Evidence for a polarizable "through-space" delocalized state

Article abstract of DOI:10.1021/ja044326+

A series of compounds were designed and synthesized to examine how through-space and through-bond electron delocalization respond to solvent effects. The general strategy involves the study of "dimers" of the distyrylbenzene chromophore held in close prox

Full text of DOI:10.1021/ja044326+

Published on Web 04/27/2005
Solvatochromism of Distyrylbenzene Pairs Bound Together
by [2.2]Paracyclophane: Evidence for a Polarizable
“Through-Space” Delocalized State
Janice W. Hong, Han Young Woo, Bin Liu,* and Guillermo C. Bazan*
Contribution from the Institute for Polymers and Organic Solids, Departments of Materials and
Chemistry & Biochemistry, UniVersity of California at Santa Barbara,
Santa Barbara, California 93106
Received September 17, 2004; E-mail: bazan@chem.ucsb.edu; bliu@chem.ucsb.edu
Abstract: A series of compounds were designed and synthesized to examine how through-space and
through-bond electron delocalization respond to solvent effects. The general strategy involves the study of
“dimers” of the distyrylbenzene chromophore held in close proximity by the [2.2]paracyclophane core and
a systematic dissection of the chromophore into components with through-space and through-bond electronic
delocalization. Steady state and time-resolved fluorescence spectroscopy in a range of solvents reveals a
red-shift in emission and an increase in the intrinsic fluorescence lifetime for the emitting state in polar
solvents when donor substituents are absent. We propose that through-space delocalization across the
[2.2]paracyclophane core is more polarizable in the excited state, relative to the through-bond (distyryl-
benzene based) excited state. When strong donors are attached to the distyrylbenzene chromophore, the
charge transfer character of the distyrylbenzene-based excited state dominates fluorescence properties.
Introduction
collective response often rests on an arrangement of optical units
with no long-range order. Defects often dominate emission, since
Complex noncovalent assemblies of organic chromophores
provide a challenge to chemical synthesis, the control of
supramolecular arrangements, and our understanding of elec-
tronic coupling in collections of optical units. These assemblies
are important in biology and in the function of organic
semiconducting materials. A classic example of a well-defined
self-organized array of optical units can be found in photosyn-
thetic systems such as that of purple bacteria, where light
absorption by antennae units, followed by energy migration
between donor and acceptor groups, leads to charge separation
and the conversion to chemical energy.1 Electron and hole
transport along duplex DNA, thought to proceed through
π-stacked base pairs, is of interest due to the connection between
charge recombination in oxidative damage and repair mecha-
nisms in DNA2 and constitutes an area of substantial debate.3
In amorphous organic semiconductors, structure/property
correlations can be more difficult to understand because the
facile fluorescence resonance energy transfer (FRET)4 channels
excitations to low-energy sites,5 which may be poorly defined
or too low in concentration to be fully characterized.6 The history
of conjugated polymers is replete with examples that show how
interchain orientation depends on solvent history and how this
spatial disposition influences bulk performance in light-emitting
diodes7-9 and thin film transistors.10 The role of interchro-
mophore orientation is also well appreciated in chromophore
aggregates.11,12 For example, H- and J-aggregates have been
known for a long time,13 have different optical properties, and
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J. AM. CHEM. SOC. 2005, 127, 7435-7443
7435

A R T I C L E S
Hong et al.
differ with respect to the orientation of the molecular dipole
moments.14-16
the paracyclophane core have also been designed to probe
intramolecular charge transfer (ICT) across the transannular
gap30 and thus the properties of donor/acceptor groups separated
through space.31 Multiple substitution on the 4,7,12,15-sites32
provides for nonlinear chromophores with a strong octupolar
component.33,34 It is also possible to incorporate [2.2]paracy-
clophane structures within the backbone of polymers to yield
structures with both π- and through-space- conjugation.35
The unique structure and optical properties of paracyclophane
molecules with prebuilt interchromophore delocalization sug-
gested that this molecular platform could be useful for designing
water-soluble optical reporters with decreased sensitivity to
optical perturbations brought about by aggregation phenomena.36
For example, the water-soluble compound octa(tetrabutyl-
ammonium) 4,7,12,15-tetrakis(3′,5′-bis(butoxysulfonate))styryl-
[2.2]paracyclophane (pCp-O-, shown in Scheme 1) shows
decreased perturbation with the addition of surfactant, relative
to linear anionic polymers and oligomers with phenylenevi-
nylene repeat units.37 This decreased sensitivity to changes by
the environment was attributed to a reduced tendency for
Ambiguities in the local structure within bulk materials and
the importance of correlating geometry with optical properties
for gauging the theoretical description of dipole-dipole coupling
and/or electron exchange17 has prompted efforts toward control-
ling the assembly of chromophores by techniques such as
Langmuir-Blodgett monolayer deposition,18 solvent-dependent
dimerization,19 DNA-assisted spatial control,20 and incorporation
into functionalized phospholipids,21 among others.22 Another
approach involves the synthesis of molecules that constitute
precisely defined chromophore pairs. Characterization of these
molecules and comparison against the monomeric building
blocks yield information on pairwise electronic communication,
which may be extrapolated to larger multichromophoric systems.
One example involves molecules with the general structure
naphthalene-bridge-naphthalene, where the pairs of naphtha-
lene chromophores are set in place by a rigid linker of variable
length.23
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core25 have been reported. The parent core may be viewed as a
pair of strongly interacting benzene rings and displays spec-
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conjugation length, and contact site determine the lowest energy
state for the pair of chromophores. Chromophores containing
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7436 J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005

Solvatochromism of Distyrylbenzene Pairs
A R T I C L E S
Scheme 1. Molecular Structures and Abbreviations Used in
Previous and Current Studies
blocks of the dimer structures. Compounds 1,4-bis(4′-(N,N-bis-
(6′′-iodohexyl)amino)styryl)benzene (DSB-N), 1,4-bis(4′-(N,N-
bis(6′′-(N,N,N-trimethylammonium)hexyl)amino)styryl)-
benzene tetraiodide (DSB-N+), 1,4-bis(3′,5′-bis(6′′-bromohexyl)-
styryl)benzene (DSB-C), and 1,4-bis(3′,5′-bis(6′′-(N,N,N-tri-
methylammonium)hexyl)styryl)benzene tetrabromide (DSB-C+)
are the neutral and charged chromophore building blocks
(Scheme 1). Pseudo-p-bis(6-bromohexyl)-[2.2]paracyclophane
(pCp) and pseudo-p-bis(6-(N,N,N-trimethylammonium)hexyl)-
[2.2]paracyclophane dibromide (pCp+) provide derivatives of
[2.2]paracyclophane soluble in organic solvents and water,
respectively.
Results
Synthesis. Two important building blocks for the synthesis
of the target compounds are 4,7,12,15-tetra(diethylphos-
phonatemethyl)[2.2]paracyclophane (1) and 1,4-bis(diethylphos-
phonatemethyl)benzene (2).33a,39 Reaction of 1 or 2 with the
appropriately substituted benzaldehydes via the Horner-Em-
mons-Wittig reaction yields the DSB and pCp molecules in
Scheme 1. The synthesis of 1 is available in the literature.33
Briefly, it involves tetrabromomethylation of [2.2]paracyclo-
phane under ultrasonic conditions, followed by treatment with
triethyl phosphite. The overall yield of 1 from [2.2]paracyclo-
phane is approximately 20%.
aggregation in water and to the intramolecular delocalization,
which diminishes perturbations by adjacent molecules in the
aggregate.
During studies of pCp-O-, it was noted that the natural
lifetime of pCp-O- (26 ns) in water was considerably longer
than that of the neutral precursor 4,7,12,15-tetrakis(3′,5′-bis(tert-
butyldimethylsiloxy)styryl)-[2.2]paracyclophane, pCp-O, in or-
ganic solvents such as toluene (4 ns). Furthermore, the parent
water-soluble distyrylbenzene chromophore DSB-O- shows a
lifetime that is similar to those of previously characterized
distyrylbenzene chromophores.38 These differences suggested
that the energy of the though-space state is influenced more
strongly by the solvent.
In this contribution we detail the synthesis and optical studies
of [2.2]paracyclophane molecules that are specifically designed
to examine how the solvent perturbs the photophysics of
pairs of distyrylbenzene molecules (Scheme 1). Specifically,
the molecules prepared and studied are 4,7,12,15-(N,N-bis(6′′-
iodohexyl-4′-aminostyryl)-[2.2]paracyclophane (pCp-N), 4,7,-
12,15-tetra(N,N-bis(6′′-(N,N,N-trimethylammonium)hexyl)-
4′-aminostyryl)-[2.2]paracyclophane octaiodide (pCp-N+),
4,7,12,15-tetra(3′,5′-bis(6′′-bromohexyl)styryl)-[2.2]paracyclo-
phane (pCp-C), and 4,7,12,15-tetra(3′,5′-bis(6′′-(N,N,N-tri-
methylammonium)hexyl)styryl)-[2.2]paracyclophane octabro-
mide (pCp-C+), where C/N corresponds to the substituents at
the termini of the chromophores (C for alkyl and N for
dialkylamine) and C/C+ or N/N+ stands for the chromophores
in their neutral or charged states.
The synthetic routes to the benzaldehyde intermediates are
shown in Scheme 2. 3,5-Bis(6′-bromohexyl)benzaldehyde was
synthesized in two steps, starting from 3,5-dibromobenzalde-
hyde. After protection with pinacol, the resulting 2-(3′,5′-
dibromophenyl)-4,4,5,5-tetramethyl-[1.2]dioxolane (3) under-
went two consecutive steps involving a lithium-halogen
exchange achieved with t-BuLi and quenching with an excess
of anhydrous 1,6-dibromohexane in THF. Deprotection via
stirring at 60 °C in 10% HBr in a 2:1 mixture of water and
THF affords 3,5-bis(6′-bromohexyl)benzaldehyde (4) in 60%
yield. Attempts to use n-BuLi in the dual lithium-halogen
exchange procedure proved unsuccessful and led to a mixture
of products. The synthesis of N,N-bis(6′-iodohexyl)-4-ami-
nobenzaldehyde (7) starts with the reaction of aniline with
6-chlorohexanol in butanol, which provides N,N-dihexanol-
aniline (5) in 70% yield. Treatment of 5 with phosphorus
oxychloride in DMF produces N,N-bis(6′-chlorohexyl)-4-ami-
nobenzaldehyde (6) in 75% yield. Chloride/iodide exchange via
Finkelstein exchange under refluxing conditions in dry acetone
affords 7 in 85% yield. The aldehyde products 4, 6, and 7 were
purified by chromatography using a silica column, and the
structures were confirmed by HR-MS and NMR spectroscopy.
Compounds 1, 2, 4, 6, and 7 provide for a modular preparation
of the compounds in Scheme 1 which contain distyrylbenzene
fragments (Scheme 3). For example, Horner-Emmons-Wittig
coupling between 1 and excess 4 using potassium tert-butoxide
We also include in these studies distyrylbenzene and [2.2]-
paracyclophane derivatives that correspond to the building
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A R T I C L E S
Hong et al.
Scheme 2. Synthesis of 3,5-Bis(6′-bromohexyl)benzaldehyde (4), N,N-Bis(6′-chlorohexyl)-4-aminobenzaldehyde (6), and
N,N-Bis(6′-iodohexyl)-4-aminobenzaldehyde (7)a
a Reaction conditions: (i) pinacol, trace TsOH, benzene, 48 h; (ii) t-BuLi, 1,6-dibromohexane, -78 °C to room temperature, 24 h; (iii) HBr, THF/H2O,
60 °C, 24 h; (iv) 6-chloro-1-hexanol, n-butanol, K2CO3, reflux, 4 days; (v) POCl3, DMF, 100 °C, 2 h; (vi) NaI, dry acetone, reflux, 2 days.
Scheme 3. Synthesis of PCp-C, PCp-C+, PCp-N, PCp-N+,
DSB-C, DSB-C+, DSB-N, and DSB-N+a
Table 1. Summary of Spectroscopic Data for DSB and
pCp-Based Chromophores in Different Solvents
λabs
(nm, eV)
λPL
(nm, eV)
τmeas
(ns)
τint
(ns)
compd/solvent
Φa
DSB-C/toluene
DSB-C/DMSO
DSB-C+/DMSO
DSB-C+/water
DSB-N/toluene
DSB-N/THF
DSB-N/DMSO
DSB-N+/DMSO
DSB-N+/water
pCp-C/toluene
pCp-C/THF
pCp-C/DMSO
pCp-C+/DMSO
pCp-C+/water
pCp-N/toluene
pCp-N/THF
pCp-N/DMSO
pCp-N+/DMSO
pCp-N+/water
pCp/hexanes
361, 3.43
365, 3.40
364, 3.41
356, 3.48
410, 3.02
410, 3.02
420, 2.95
420, 2.95
408, 3.04
396, 3.13
394, 3.15
399, 3.11
397, 3.12
392, 3.16
434, 2.86
436, 2.84
445, 2.79
445, 2.79
435, 2.85
200, 6.20
200, 6.20
389, 3.19b
393, 3.16b
391, 3.17b
387, 3.20b
450, 2.76
461, 2.69
506, 2.45
506, 2.45
561, 2.21
454, 2.73
458, 2.71
473, 2.62
471, 2.63
496, 2.50
486, 2.55
495, 2.51
536, 2.31
535, 2.32
553, 2.24
345, 3.59
352, 3.52
0.9
1.3
1.3
1.6
1.1
1.2
1.5
1.5
1.7
2.2
3.1
4.9
4.9
12.2
1.7
2.2
3.8
3.8
0.5
c
0.70
0.69
0.60
0.70
0.93
0.77
0.63
0.87
0.24
0.75
0.77
0.50
0.60
0.52
0.75
0.67
0.57
0.58
0.04
0.03
0.02
1.3
1.8
2.1
2.3
1.2
1.5
2.4
1.8
6.9
2.9
4.0
9.8
8.2
23.5
2.3
3.3
6.7
6.6
12.5
a Reaction conditions: (i) t-BuOK, dry THF, 0 °C, 6 h; (ii) NaI, dry
acetone, reflux; (iii) N(CH3)3, THF/H2O, RT, 24 h.
Scheme 4. Synthesis of PCp and PCp+a
a Reaction conditions: (i) t-BuLi, 1,6-dibromohexane, -78 °C to room
temperature, 24 h; (ii) N(CH3)3, THF/H2O, RT, 24 h.
in THF at 0 °C provides pCp-C in 67% yield. Formation of
the ionic water-soluble pCp-C+ is achieved via quaternization
with trimethylamine in a mixed solvent of THF and water with
a yield of 91%. The analogous procedures for pCp-N, pCp-
N+, DSB-C, DSB-C+, DSB-N, and DSB-N+ are shown in
Scheme 3. Full details can be found in the Supporting Informa-
tion.
pCp+/water
c
a The quantum yield was measured using fluorescein and 9,10-diphen-
ylanthracene as the references, with an estimated error of (5%. b The first
vibronic peak is reported rather than λmax.
c Laser source not available for
the fluorescence lifetime measurement.
The synthesis of the “core-only” derivatives pCp and pCp+
begins with the pseudo-para derivative 4,16-dibromoparacy-
clophane.40 As shown in Scheme 4, two-fold lithium-halogen
exchange of 4,16-dibromoparacyclophane using t-BuLi, fol-
lowed by quenching with a large excess of 1,6-dibromohexane,
provides pCp in 16% yield. Treatment of pCp with excess
trimethylamine in THF/H2O provides pCp+.
Absorption and Fluorescence Spectroscopy. Table 1 sum-
marizes the spectroscopic data obtained for the compounds in
this study. We note that for the remainder of the paper the
spectra of neutral compounds are reported in toluene, while
those of the charged compounds are reported in water, unless
otherwise stated. The absorption and photoluminescence (PL)
spectra of DSB-C and pCp-C are shown in Figures 1 and 2,
respectively. The absorption maximum (λabs) of DSB-C is 361
nm and the PL maximum (λPL) is 389 nm. The PL of DSB-C
contains vibronic structure and displays a Stokes shift charac-
Figure 1. Normalized absorption (a) and PL (b) for DSB-C in toluene
(solid lines) and absorption (c) and PL (d) for DSB-C+ in water (dashed
lines). All fluorescence spectra were obtained by excitation at λabs
.
teristic of distyrylbenzene chromophores in nonpolar solvents.41
The most prominent differences in the absorbance of DSB-C
(41) Renak, M. L.; Bartholomew, G. P.; Wang, S.; Ricatto, P. J.; Lachicotte, R.
(40) Reich, H. J.; Cram, D. J. J. Am. Chem. Soc. 1969, 91, 3527.
J.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 7787.
9
7438 J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005

Solvatochromism of Distyrylbenzene Pairs
A R T I C L E S
Figure 2. Normalized absorption (a) and PL (b) for pCp-C in toluene
(solid lines) and absorption (c) and PL (d) for pCp-C+ in water (dashed
Figure 3. Normalized PL spectra for pCp-C (dashed line) and pCp-C+
(solid line) in DMSO. All fluorescence spectra were obtained by excitation
lines). All fluorescence spectra were obtained by excitation at λabs
.
at λabs
.
versus pCp-C is the red-shifted λabs of pCp-C (396 nm) and
the “double hump” appearance of the absorption for pCp-C in
the 300-450 nm range, which reflect the through-space
electronic exchange between the two DSB fragments.29 For a 1
× 10-6 M solution, the vibronic structure in the PL spectra is
better defined in DSB-C than in pCp-C, which is consistent
with the monomeric versus dimeric nature of the two structures.
The photoluminescence quantum efficiencies for DSB-C (70%)
and pCp-C (75%) are similar.
Figures 1 and 2 also contain the absorption and PL of DSB-
C+ and pCp-C+. The spectra of DSB-C+ are similar to those
of DSB-C, except for a broadening of the PL, which blurs the
vibronic fine structure (Gaussian peak shape analysis reveals
the first vibronic peak of DSB-C in toluene and DSB-C+ in
water to be 390 and 384 nm, respectively; see Supporting
Information). In the case of the pCp-C/pCp-C+ pair, there is
a negligible change in absorption; however a substantial red-
shift in PL is observed for pCp-C+. That the λPL of pCp-C+
is 42 nm (0.23 eV) red-shifted relative to pCp-C is the first
indication that the through-space, or “phane”, state localized
primarily on the [2.2]paracyclophane core shows a stronger
solvatochromic effect, relative to the distyrylbenzene excited
state.
Figure 4. Normalized absorption (a) and PL (b) for DSB-N in toluene
(solid lines) and absorption (c) and PL (d) for DSB-N+ in water (dashed
lines). All fluorescence spectra were obtained by excitation at λabs
.
As described previously,37 the structurally related pairs DSB-
O/DSB-O- and pCp-O/pCp-O- show similar trends to those
described for their DSB-C/DSB-C+ and pCp-C/pCp-C+
counterparts, indicating that the alkoxy substitution pattern at
the meta positions of the aromatic ring does not influence greatly
the ground state and excited state energies. Additionally, possible
interactions between the oxygen atoms and solvents, such as
hydrogen bonding, do not obviously perturb the optical proper-
ties of the “O” series. For both the “C” series and the “O” series,
there are no dramatic changes in quantum efficiencies with
variations of solvent polarity.
The PL of pCp-C and pCp-C+ are identical in DMSO with
λPL ) 472(1 nm (Figure 3). This solvent is of intermediate
polarity, between water and toluene, and can dissolve both the
neutral and charged molecules. Similar PL spectra in DMSO
(and DMF) were observed for each neutral/charged pair of
structures, i.e., DSB-C/DSB-C+, DSB-N/DSB-N+, and pCp-
N/pCp-N+. On the basis of this information, we attribute
changes in the PL spectra of structurally related chromophore
pairs (which differ only in the charge of the pendant groups) to
the interactions of the conjugated organic fragments with the
Figure 5. Normalized absorption (a) and PL (b) for pCp-N in toluene
(solid lines) and absorption (c) and PL (d) for pCp-N+ in water (dashed
lines). All fluorescence spectra were obtained by excitation at λabs
.
solvent medium. In other words, the charged ionic groups on
the periphery of the molecular structure necessary for solubility
in water do not perturb the excited state energies.42
Figures 4 and 5 show the absorption and PL spectra of DSB-N
and pCp-N. For DSB-N, λabs ) 410 nm and the emission
displays vibronic structure with λPL ) 450 nm. It is immediately
obvious that both the absorption and PL of DSB-N are red-
shifted relative to DSB-C. This difference is attributed to the
strong electron-donating properties of the amino group.43 For
(42) Pond, S. J. K.; Tsutsumi, O.; Rumi, M.; Kwon, O.; Zojer, E.; Bre´das, J.-
L.; Marder, S. R.; Perry, J. W. J. Am. Chem. Soc. 2004, 126, 9291.
9
J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005 7439

A R T I C L E S
Hong et al.
Figure 7. Fluorescence intensity decays of (a) DSB-C in toluene, (b) DSB-
C+ in water, (c) pCp-C in toluene, and (d) pCp-C+ in water.
Figure 6. Log-scale absorption (a) and normalized PL spectra (b) for pCp
in hexanes and log-scale absorption (c) and normalized PL (d) for pCp+
in water. The PL spectra were obtained by excitation at 240 nm.
electronic transitions at 200 and 227 nm, a weakly allowed
transition at 255 nm, and forbidden transitions at 287 and 306
nm. These data are in good agreement with previously observed
spectra25,26c and theoretical analysis26b for the absorption spectra
of [2.2]paracyclophane.
Comparison of the absorption spectra of pCp/pCp+ shows
that the peaks are fairly insensitive to the solvent, indicating
that the electronic description of the ground states is similar.
However, for pCp in hexanes, λPL ) 345 nm, while in water,
the λPL for pCp+ occurs at 352 nm. The lower energy PL
frequency for the paracyclophane core in more polar media
provides further evidence of the solvatochromism of the
“through-space” state.
Fluorescence Lifetime Measurements. Fluorescence inten-
sity decay measurements provide information complementary
to the steady state data and give insight into excited state
dynamics.4 In this section, we compare the fluorescence decay
properties of the pCp derivatives with those of their parent DSB
counterparts in different solvents.
The emission intensity decays of DSB-C and DSB-C+, as
measured using single photon counting techniques, are shown
in Figure 7. Table 1 contains both the lifetime obtained from
decay measurements (τmeas) and the intrinsic, or natural, lifetime
(τint) for each chromophore. The τint is calculated using the
formula τint ) τmeas/Φ, where Φ is the quantum yield of the
chromophore. DSB-C has τint of 1.3 ( 0.2 ns, while τint ) 2.3
( 0.2 ns for DSB-C+. The difference for a distyrylbenzene
chromophore without strong acceptor or donor groups as a
function of solvent is therefore small. Fluorescence intensity
decay curves for pCp-C and pCp-C+ are also shown in Figure
7. Similar to DSB-C, pCp-C also has a relatively short τint of
2.9 ( 0.3 ns. However, pCp-C+ has a τint of 24 ( 2 ns. This
longer lifetime can be used to infer that the through-space state,
which has a low oscillator strength,28 participates more strongly
in the description of the excited state.
DSB-N, there is a progressive red-shift in λPL and decrease in
quantum yields with increased solvent polarity (Table 1). For
example, on going from toluene to DMSO, λPL shifts from 450
to 506 nm and the corresponding quantum efficiencies drop from
93% to 63%. Absorption maxima are less sensitive to solvent
polarity, indicating that the solute-solvent interactions are of
lesser importance in the ground state. This behavior is well
known and can be attributed to a solvent stabilization of an
excited state with charge transfer character.44,45
The absorption maximum of pCp-N is 24 nm red-shifted
relative to DSB-N and does not change to an appreciable extent
when measured in different solvents (Table 1).33a The PL of
pCp-N (λPL ) 486 nm) is noticeably red-shifted from that of
the parent DSB-N (λPL ) 450 nm), and the PL red-shifts for
both compounds with increasing solvent polarity. This trend is
different relative to pCp-C/DSB-C. As will be discussed later,
these data provide evidence that the excited state in pCp-N is
most likely localized on the distyrylbenzene arm.
Figures 4 and 5 also contain the absorption and PL of DSB-
N+ and pCp-N+. The absorption of DSB-N+ is similar to that
of DSB-N. However, there is a red-shift of 111 nm in the PL
for DSB-N+ (λPL ) 561 nm) and a lower quantum efficiency
(24%) as compared to DSB-N (93%). Similar to the DSB-N+/
DSB-N pair, a red-shift of 67 nm is observed in the λPL of pCp-
N+, as compared to pCp-N. The quantum efficiency changes
from 75% for pCp-N to 4% for pCp-N+. It is interesting to
note that both DSB-N+ and pCp-N+ have structureless
emission and very similar λPL. The more pronounced sensitivity
of pCp-N+ relative to pCp-C+ suggests that the pCp-N+
excited state contains more charge transfer character or that it
is substantially more sensitive to solvent polarity and polariz-
ability.
The absorption and PL spectra of pCp in hexanes and pCp+
in water are shown in Figure 6. Larger oscillator strengths denote
allowed transitions, and both pCp and pCp+ show allowed
The τints of DSB-N, pCp-N, DSB-N+, and pCp-N+ follow
a different trend. The τint value (1.2 ( 0.1 ns) of DSB-N is
similar to that of DSB-C, while DSB-N+ has a τint ) 6.9 (
0.7 ns. Similar to pCp-C, pCp-N displays a relatively short
(43) (a) Rumi, M.; Ehrlich, J. E.; Heikal, A. A.; Perry, J. W.; Barlow, S.; Hu,
Z.; McCord-Maughon, D.; Parker, T. C.; Ro¨ckel, H.; Thayumanavan, S.;
Marder, S. R.; Beljonne, D.; Bre´das, J.-L. J. Am. Chem. Soc. 2000, 122,
9500. (b) 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.
(44) Sarker, A. M.; Strehmel, B.; Neckers, D. C. Macromolecules 1999, 32,
7409.
(45) Strehmel, B.; Sarker, A. M.; Malpert, J. H.; Strehmel, V.; Seifert, H.;
Neckers, D. C. J. Am. Chem. Soc. 1999, 121, 1226.
τint (2.3 ( 0.2 ns). However, pCp-N+ has a natural lifetime of
τint ) 12 ( 1 ns, which is considerably shorter than that of
pCp-C+. In the Discussion section we show that these
differences in τint values for the different chromophores can be
used to gauge the relative contributions of through-bond and
through-space states in the description of the emissive state.
9
7440 J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005

Solvatochromism of Distyrylbenzene Pairs
A R T I C L E S
From Table 1, it can be seen that for each pair of related
structures pCp-C/pCp-C+ and pCp-N/pCp-N+, there is a clear
progression toward longer τint values with increasing solvent
polarity, with the “carbon only” pCp-C/pCp-C+ molecules
being more sensitive.
polarity. Since the λPL of pCp+ in water is 7 nm (0.07 eV)
red-shifted relative to pCp in hexanes, we infer a more
pronounced polarization or charge transfer component in the
emitting state relative to the ground state.
There is a strong similarity between the solvent effects on
pCp-C and pCp-C+, and the solvatochromism of the pyrene
and naphthalene excimers, which has been studied in detail.47
For these excimers in nonpolar solvents, different solvation
energies have been attributed primarily to dispersion interactions,
which result from fluctuations in the instantaneous dipole
moments of the solvent and the solute molecules, and the solute
transition dipole moment contribution. In polar solvents one
needs to also consider the solvent Stark effect, which describes
the motion of solvent dipoles around the emissive molecules.48
The solvent dependence of the emission frequency, in conjunc-
tion with the fact that excimers are nonpolar,49 indicates that
the spectral shifts result from differences in polarizability
between the excimer and the dissociative ground state.
Extension of the results from naphthalene and pyrene excimer
studies to the solvatochromism of pCp implies that the excited
state of pCp is more polarizable than the ground state (which
is not dissociative by virtue of its covalent structure). Similarly,
for DSB-C, the absence of measurable solvent dependence of
the fluorescence spectra implies that the ground and emissive
states have similar polarizabilities. We also note that the solvent
stabilization energy as determined by the energy difference in
the λPL of pCp and pCp+ in hexanes and water, respectively
(0.07 eV), is smaller than that observed in pCp-C/pCp-C+
(0.23 eV), which suggests that the polarizability of pCp-C/pCp-
C+ is further enhanced through its more extended π-conjugated
system. For pCp-O and pCp-O-, a similar explanation may be
offered.
A qualitative summary of the electronic structure of bichro-
mophoric [2.2]paracyclophane compounds, shown in Scheme
5, is necessary to understand the effect of solvent on τint.28c,29a
In Scheme 5, the characteristics of the S1 and S2 electronic states
of the dimer can be approximated and understood as a
combination of two states: one that is similar to that of the
parent chromophore (TB) and another which contains delocal-
ization between the two monomers and is largely localized on
the [2.2]paracyclophane core (TS). The TB abbreviation is used
to highlight typical through-bond π conjugation, while TS refers
to the through-space delocalization across the transannular gap.50
Three different structural situations have been encountered and
studied previously. Compound A contains two stilbene units in
close proximity. The main absorption band corresponds to that
of the stilbene framework. A lower energy state exists (S1),
which has a much weaker oscillator strength, is primarily located
on the internal core, and has mostly TS character. Absorption,
followed by internal conversion, populates the S1 state.
In situation B, two distyrylbenzene chromophores are con-
tacted via their terminal aromatic rings. Within this structure,
the excited state is more stable when localized along the
Discussion
Schemes 3 and 4 provide a straightforward modular access
to [2.2]paracyclophane derivatives that contain distyrylbenzene
chromophores on the top and bottom “decks” of the core
structure and that are soluble in solvents ranging from alkanes
to water. The key steps are the Horner-Emmons-Wittig
reaction of 1 with benzaldehydes 4 and 6, which ultimately yield
pCp-C and pCp-N. Ionic versions pCp-C+ and pCp-N+ are
produced in near quantitative yields by treatment of pCp-C and
pCp-N with excess trimethylamine. Removal of unreacted
trimethylamine does not pose a problem because of its low
boiling point. Modification of these steps, starting with 2,
provides for the parent distyrylbenzene chromophores. Com-
pound pCp is generated via double lithiation of 4,16-dibromo-
[2.2]paracyclophane, followed by quenching with 1,6-dibro-
mohexane. Finally, trimethylamine and pCp provide pCp+.
From a structural point of view, the pCp-X (where X ) O,
C, N) molecular series can be thought of as two DSB-X
chromophores held in close proximity by virtue of the [2.2]-
paracyclophane core.28 Differences in optical properties between
structurally related monomers and dimers stem from the through-
space electronic delocalization, or electron exchange, across the
center ring of the DSB-X structure.29 As shown in Figure 3
and Table 1, we find that there are no measurable differences
in the absorption or fluorescence spectra when one compares
the neutral version of a molecule to its charged counterpart (i.e.,
pCp-C vs. pCp-C+), in the same solvent. The cationic
quaternary groups and anionic sulfonate functionalities, together
with their corresponding counterions, do not play an important
role in determining the energies of the excited states.
DSB-C and DSB-C+ are the most basic distyrylbenzene
structures in this study. Their PL maxima (λPL) are not
influenced by the properties of the solvent. The absence of a
solvatochromic effect is consistent with the similar charge
distribution of the ground state and the emitting state.41 The
values calculated for the intrinsic lifetimes (τint) are also quite
similar (1.3 ns for DSB-C and 2.3 ns for DSB-C+). In contrast,
for pCp-C and pCp-C+, the spectra change as a function of
solvent. On going from toluene to water the λPL red-shifts by
approximately 42 nm (0.23 eV) and the τint increases from ∼3
to ∼24 ns. Therefore, by creating the possibility of delocalization
across the [2.2]paracyclophane bridge, one observes an emitting
state that is more sensitive to solvent polarity46 and has a lower
oscillator strength.
Figure 6 shows that the PL spectra of pCp and pCp+, which
are essentially alkyl-substituted [2.2]paracyclophane cores,
display a solvatochromic effect. Like pCp-C and pCp-C+,
neither pCp nor pCp+ has functionalities available for hydrogen
bonding, which strongly suggests dependence on the solvent
(47) (a) Castanheira, E. M. S.; Martinho, J. M. G. Chem. Phys. Lett. 1991, 185,
319. (b) Castanheira, E. M. S.; Martinho, J. M. G. J. Photochem. Photo-
biol. A: Chem. 1994, 80, 151.
(46) The fluorescence spectrum of pCp-C in dodecane is identical to that
measured in hexanes (see Supporting Information). Since the polarizability
of dodecane is significantly larger (R ) 22.8 Å3) than that of hexanes (R
) 11.8 Å3), we propose that the dipole of the solvent plays the major role
in the solvatochromic shifts. See: (a) Bosque, R.; Sales, J. J. Chem. Inf.
Comput. Sci. 2002, 42, 1154. (b) Bo¨ttcher, C. J. F. Theory of Electric
Polarization; Elsevier Scientific Publishing Co.: Amsterdam, 1973.
(48) Suppan, P.; Ghoneim, N. SolVatochromism; The Royal Society of Chem-
istry: Cambridge, U.K., 1997.
(49) Barltrop, J. A.; Coyle, J. D. Principles of Photochemistry; John Wiley and
Sons: Chichester, England, 1978.
(50) The TS state localized on the [2.2]paracyclophane core contains a degree
of π-σ-π delocalization via the ethylene bridges; however it arises
primarily due to cofacial π-π delocalization. See refs 28c and 29a.
9
J. AM. CHEM. SOC. VOL. 127, NO. 20, 2005 7441

A R T I C L E S
Hong et al.
Scheme 5. Qualitative Electronic Description of Bichromophoric
Scheme 7. Qualitative Description of the Effect of Polar Solvents
on the PCp-N/PCp-N+ Emissive State
[2.2]Paracyclophane Moleculesa
We now turn our attention to the nitrogen-containing com-
pounds. For DSB-N and DSB-N+, there is a strong solvato-
chromic effect. The red-shifted PL with increased solvent
polarity,51 as well as the loss in fluorescence quantum yields,
reflects the intramolecular charge transfer character of these
compounds52 due to the electron-rich terminal amino groups.
That vibronic structure appears in the fluorescence spectra of
both DSB-N and pCp-N, together with the fact that DSB-N+
and pCp-N+ have similar PL maxima and τint, indicates that
S1 is mainly described by TB, which is located on the donor-
substituted distyrylbenzene fragment (Scheme 7).
a (a) In A, absorption occurs via the stilbene fragment, as shown by
excitation from S0 to S2. Internal conversion populates S1, which is primarily
TS, and emission occurs from there. (b) In B, TB (chromophore) is of
lower energy, relative to TS. (c) The regiochemistry of the interchromophore
contact in C results in an S1 state with substantial mixing of TB and TS.
Conclusion
The series of compounds pCp-C/pCp-C+ and pCp-N/pCp-
N+ and the constituent structures pCp/pCp+, DSB-C/DSB-
C+, and DSB-N/DSB-N+ allow for a dissection of the problem
of how solvents of different polarities influence through-space
and through-bond delocalized fragments. We find that for a
given neutral/charged pair, the PL properties are identical when
in the same solvent, which implies that the ionic groups are
not responsible for spectral differences. The pCp-C system
shows a solvent-induced red-shift in PL and an increase in
fluorescence lifetime, which do not occur in the parent DSB-C
chromophore. That information, in conjunction with the solva-
tochromic effect in pCp/pCp+, indicates that the through-space
state created by electron exchange across the [2.2]paracyclo-
phane core is susceptible to solvent polarity.46 Analogy to
excimer solvatochromism suggests that this solvent effect is not
due to a charge transfer component in the excited state, but rather
to a more polarizable electronic structure. The solvent Stark
effect is further enhanced with increased conjugation, which is
known to increase polarizability. The lowering of the TS energy
by water makes its participation in the S1 description for pCp-
C+ more pronounced, which decreases the oscillator strength
and increases the fluorescence lifetime. A more complete
analysis for the reason of the increased polarizability awaits a
formal theoretical treatment. Similar PL properties are observed
for the DSB-N/DSB-N+ and pCp-N/pCp-N+ pairs, suggesting
the solvatochromic behavior is dominated by the charge transfer
component of the distyrylbenzene chromophore.
Scheme 6. Qualitative Description of the Effect of Polar Solvents
on the PCp-C/PCp-C+ Emissive State
distyrylbenzene framework, with TS being higher in energy.
The properties of B are thus similar to those of the parent
distyrylbenzene framework, with little perturbation of the
emission frequency and fluorescence lifetimes. Finally, in
situation C, two distyrylbenzene chromophores are contacted
via their internal ring. This geometry causes mixing of TB and
TS and gives rise to an electronic structure that is delocalized
throughout the entire molecule.
On the basis of the solvatochromism observed in Figures 1-7,
the previous characterization of solvent effects on aromatic
excimers,47 the insensitivity toward solvent polarizability,46 and
the electronic structures described in Scheme 5, it is possible
to account for the dependence of τint of pCp-C and pCp-C+
on solvent polarity, as shown in Scheme 6. In a nonpolar solvent,
the description is similar to that in Scheme 5c, with the emitting
S1 state containing substantial TB and TS mixing. Increasing
solvent polarity leaves TB relatively unperturbed (recall the lack
of solvatochromism for DSB-C/DSB-C+), but lowers the TS
energy level (as in the case of pCp/pCp+). As the energy
difference of these states increases, the S1 state becomes more
TS-like and it acquires a smaller oscillator strength. Therefore,
the longer τint value of pCp-C+ in water, relative to pCp-C in
toluene, is due to the stabilization of TS and its greater
participation in the description of S1. When going from a
nonpolar to a polar solvent, the electronic structure of the
molecule becomes more like situation A in Scheme 5a. Similar
arguments can be put forth for pCp-O/pCp-O-.
Experimental Section
Optical Properties. All spectroscopic measurements were performed
using spectral grade solvents from Aldrich. UV-visible absorption
spectra were recorded on a Shimadzu UV-2401 PC dual beam
(51) The possible participation of solvent polarizability has not been evaluated
at this point.
(52) (a) Albota, M.; Beljonne, D.; Bre´das, J. L.; Ehrlich, J. E.; Fu, J.-Y.; Heikal,
A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-
Maughon, D.; Perry, J. W.; Ro¨ckel, H.; Rumi, M.; Subramaniam, C.; Webb,
W. W.; Wu, X.-L.; Xu, C. Science 1998, 281, 1653. (b) de Souza, M. M.;
Rumbles, G.; Samuel, I. D. W.; Moratti, S. C.; Holmes, A. B. Synth. Met.
1999, 101, 631.
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Solvatochromism of Distyrylbenzene Pairs
A R T I C L E S
spectrometer. Photoluminescence spectra were obtained on a PTI
QuantaMaster fluorometer at a detection angle of 90° relative to the
excitation source. The solutions had concentrations between 1 × 10-6
and 1 × 10-5 M. Quantum efficiencies were determined relative to
9,10-diphenylanthracene in cyclohexane53 and freshly prepared fluo-
rescein (in aqueous 0.1 M NaOH),54 with quantum efficiencies of 91%
and 92%, respectively. The error associated with this technique is
estimated to be (5%. Fluorescent lifetime measurements were per-
formed using a time-correlated single-photon-counting technique
(TCSPC).55 The PL was excited by laser pulses with a duration of nearly
120 fs, produced via the second-harmonic generation process from the
output of an ultrafast Ti:sapphire regenerative amplifier. The lumines-
cence was dispersed in a spectrometer and detected by a micro channel-
plate photomultiplier tube (MCP PMT; Hamamatsu R3809U-51). MCP
PMT output and trigging signal from a fast photodiode were connected
to a SPC-300 TCSPC board (Beker & Hickl), which performed the
statistical analysis of the photon flux and restored the fluorescence
transients. TCSPC data were filtered numerically to remove electrical
ringing noise and were deconvoluted from the instrument response.
Acknowledgment. The authors are grateful to the NSF
(DMR-0097611) and the Mitsubishi Chemical Center for
Advanced Materials for financial support of this work.
Supporting Information Available: Detailed synthetic pro-
cedures and characterization for all compounds, along with the
fluorescence spectra of pCp-C in dodecane and hexanes and
Gaussian curve fitting of the fluorescence of DSB-C and DSB-
C+ in various solvents. This material is available free of charge
via the Internet at http://pubs.acs.org.
(53) Maciejewski, A.; Steer, R. D. J. Photochem. 1986, 35, 59.
(54) Magde, D.; Wong, R.; Seybold, P. G. Photochem. Photobiol. 2002, 75,
327.
(55) O’Connor, D. V.; Phillips, D. Time Correlated Single Photon Counting;
Academic Press: London, U.K., 1984.
JA044326+
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