Beyond the cyanine limit: Peierls distortion and symmetry collapse in a polymethine dye

Article abstract of DOI:10.1021/ja9626953

Theory predicts that cyanine dyes and related linear systems undergo symmetry collapse and bond localization at long chain lengths. Beyond this 'cyanine limit', the properties of these systems do not extrapolate from their shorter counterparts. To test this prediction, dipyridocyanines have been synthesized and shown to undergo such symmetry collapse with chain lengths as short as 13.

Full text of DOI:10.1021/ja9626953

J. Am. Chem. Soc. 1997, 119, 3253-3258
3253
Beyond the Cyanine Limit: Peierls Distortion and Symmetry
Collapse in a Polymethine Dye
Laren M. Tolbert* and Xiaodong Zhao
Contribution from the School of Chemistry and Biochemistry, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400
ReceiVed August 2, 1996. ReVised Manuscript ReceiVed February 6, 1997X
Abstract: Theory predicts that cyanine dyes and related linear systems undergo symmetry collapse and bond
localization at long chain lengths. Beyond this “cyanine limit”, the properties of these systems do not extrapolate
from their shorter counterparts. To test this prediction, dipyridocyanines have been synthesized and shown to undergo
such symmetry collapse with chain lengths as short as 13.
Introduction
polymethines should be non-zero and, rather, should depend
on end gap effects.6 More recently, using a free electron model,
Kuhn determined that simple cyanines with 30 carbon atoms
should exhibit asymmetric ground states.7 Hush and co-workers
have confirmed this prediction at a higher calculational level.8
Together, these predictions relate the presence of bond alterna-
tion with the formation of a non-zero band gap. Thus, as a
result of our continuing interest in the chemistry and physics
of “molecular wires”, we embarked on an examination of this
phenomenon by both calculation and experiment.
In order to maintain a systematic approach to the investigation
of solitonic behavior in molecules, we investigated molecules
isoelectronic with the subjects of our earlier studies, the
diphenylpolyenyl anions DPN,9 where DP represents diphenyl
and N represents the number of chain atoms up to 17. In the
present case, the para carbon of each phenyl was replaced by
nitrogen, i.e., phenyl became 4-pyridinium (acronym DPyN).
Optical band gap and bond alternation are two intimately
related characteristics of linear polyenes which control a number
of their properties, including optical polarizability and electrical
conductivity. At one extreme lies polyacetylene. In the absence
of bond alternation, the band gap would vanish and polyacety-
lene would be a true metal. However, a pseudo Jahn-Teller
effect known as the Peierls distortion intervenes.1 The resulting
“dimerization” produces both the well-known bond alternation
and the non-zero band gap. “Doping” of the polymer introduces
charged defects (“solitons”) which ameliorate bond alternation
within a finite domain.2 At the other extreme lie cyanine dyes.
As a result of the two dominant resonance forms with positive
charge at opposite ends of a polymethine chain, each carbon-
carbon bond is represented by a double bond in one form and
a single in the other, leading to an average “bond and a half”,
i.e., non-alternating, structure. Several crystal structures of
cyanines of various types confirm the lack of bond alternation
in cyanines.3 In between are a host of intermediate forms,
especially the merocyanines, for which bond alternation is a
complex function of chain length, end groups, and solvent
dielectric. Marder has pointed out a curious relationship
between bond alternation and optical hyperpolarizability in linear
polyenes4 and has invoked Da¨hne’s notion of the “cyanine limit”
as representing molecules with vanishing bond alternation.5
The absence of bond alternation in symmetrical cyanines
presents a conundrum. On the one hand, within this simple
prediction of the Da¨hne-Redaglia model,5 the band gap for a
cyanine of infinite length should vanish and the molecule should
exhibit metallic behavior. On the other hand, such a molecule
is within an end gap effect of polyacetylene, the prototypical
(and insulating) bond-alternant molecule. A partial answer
resolution to this conundrum is provided by early work of
Tyutyulkov, who predicted that the limiting band gap of
For experimental studies, it was necessary to quaternize the
terminal pyridines with a methyl group. The anions DPN were
characterized by charge distribution placing the highest charge
density at the center of the polyene chain. Although the cationic
species DPyN are isoelectronic with the parent anionic DPN,
we would expect that electrostatic interactions would cause the
negative charge localized in the center of this moiety to migrate
to one end or the other to annihilate one of the positive charges.
Thus, each resulting structure corresponded to one of two
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
(1) Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Phys. ReV. B. 1980, 22,
2099.
(2) (a) Tolbert, L. M.; Zhao, X. Proceedings, NATO Workshop on
Modular Chemistry,; Michl, J., Ed.; Kluwer Academic Publishers, in press.
(b) Tolbert, L. M. Acc. Chem. Res. 1992, 25, 561.
(6) Tyutyulkov, N.; Dietz, F.; Favian, J.; Melhorn, A.; Tadjer, A. Theor.
Chim. Acta 1981, 60, 185.
(7) Kuhn, C. Synth. Met. 1991, 43, 3681.
(3) (a) Potenza, J. A.; Zyontz, L.; Borowski, W. Acta. Crystallogr. 1978,
B34, 193, (b) Kulpe, S.; Kuban, R.-J.; Schulz, B.; Da¨hne, S. Cryst. Res.
Technol. 1987, 22. (c) Sieber, K.; Kutschabsky, L.; Kulpe, S. Krist. Tech.
1974, 9, 1111. (d) Zimer, V. B.; Kulpe, S. J. Prakt. Chem. 1975, 317 185.
(4) (a) Marder, S. R.; Perry, J. W.; Tiemann, B. G.; Gorman, C. B.;
Gilmour, S.; Biddle, S. L.; Bourhill, G. J. Am. Chem. Soc. 1993, 115, 2524.
(b) Marder, S. R.; Gorman, C. B.; Tiemann, B. G.; Cheng L.-T. J. Am.
Chem. Soc. 1993, 115, 3007.
(8) (a) Reimers, J. R.; Craw, J. S.; Wong, A.; Bacskay, G. B.; Hush, N.
S. Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. A 1993, 234, 51. (b) Reimers,
J. R.; Craw, J. S.; Hush, N. S. Proceedings, U. S. Engineering Foundation
Conference on Molecular Electronics: Science and Technology, St. Thomas,
U.S. V.I., 1991. (c) Craw, J. S.; Reimers, J. R.; Bacskay, G. B.; Wong, A.
T.; Hush, N. S. Chem. Phys. 1993, 176, 407-20.
(9) (a) Tolbert, L. M.; Ogle, M. E. Mol. Cryst. Liq. Cryst. 1990, 189,
279. (b) Tolbert, L. M. Ogle, M. E. J. Am. Chem. Soc. 1990, 112, 9519. (c)
Tolbert, L. M.; Ogle, M. E. J. Am. Chem. Soc. 1989, 111, 5958.
(5) Da¨hne, S.; Radeglia, R. Tetrahedron 1971, 27, 3673.
S0002-7863(96)02695-9 CCC: $14.00 © 1997 American Chemical Society

3254 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Tolbert and Zhao
Figure 3. Synthesis of cyanines DPyN, N ) 5, 9, 13.
Figure 1. Charge migration in dipyridocyanines DPyN.
Figure 4. Spectra of DPyN cations in Me2SO.
corresponding to the classical resonance picture of cyanines,
while DPy13 does not.
Synthesis and Spectroscopy of Cyanines. In order to
confirm this phenomenon, we synthesized cyanines of type
DPyN, specifically, DPy1, DPy3, DPy5, DPy9, and DPy13.
Syntheses of DPy1 and DPy3 were accomplished following
literature procedures. The remaining molecules were synthe-
sized following a modification of our procedures for the
diphenylpolyenyl anions using conventional Wittig methodol-
ogy11 (see Figure 3). The cyanines were generated by depro-
tonation of the intermediate bis(pyridinium)polyenes, producing
colors ranging from deep red to green to blue. Cyanine DPy13
gave several early indications of uniqueness, including extreme
oxygen sensitivity, requiring inert atmosphere handling, and
incomplete deprotonation, or, for strong bases, decomposition.
Optimum conditions for generating DPy13 involved use of
quinuclidine base in Me2SO, which nevertheless gave incom-
plete deprotonation. The absorption spectra of the dipyridocya-
nines following deprotonation are shown in Figure 4. Particu-
larly noteworthy is the absorption spectrum of DPy13, which
is broad and featureless, in contrast to the shorter members of
the series. At higher concentrations, the shape of the absorption
of DPy13 at ca. 13 500 cm-1 emerges and shows a remarkable
solvent dependence to both maximum and peak width (see
Figure 5). Finally, the infrared absorption spectrum of DPy13
exhibits a new absorption at 1729 cm-1 which is absent in the
earlier members of the series (see Figure 6).
Figure 2. Bond alternation in (a) DPy9 (b) DPy13.
equivalent resonance forms of a cyanine dye (see Figure 1).
Again, at sufficient length the finite soliton width required that
the two forms no longer be equivalent due to bond alternation
induced by Peierls distortion. The electron density should
localize at one end or the other of the polyene, but the ends
could not communicate except through a solitonic (time-
dependent) motion.
Results
Calculations. Preliminary calculations at the AM1 level on
cyanines constrained to planarity predict the formation of an
asymmetric ground state as early as 13 carbon centers.10 This
phenomenon is visualized most readily by an examination of
the bond alternation as a function of chain length. The
dipyridylnonatetraenyl cation (DPy9) has a symmetric ground
state, with minimal bond alternation in the vicinity of the chain
center (see Figure 2a, in which the difference in adjacent bond
lengths, ∆r, was plotted versus the position of the atom). In
contrast, for the dipyridyltridecahexaenyl cation (DPy13), C2V
symmetry is unstable relative to a Peierls distortion. A lifting
of the symmetry to produce the asymmetric ground state is
shown in Figure 2b. Thus, DPy9 clearly exhibits C2V symmetry
Recently, following the efforts of Bre´das, Silbey, and others,12
based on the early work of Kuhn,13 we have adopted an
approach based on the particle in a box model, which originates
from modified free electron theory. This model predicts that
the absorption behavior of one-dimensional conjugated systems
can be described by eq 1:
(11) Wittig, G.; Eggers, H.; Duffner, P. Annalen 1958, 619, 10.
(12) Bre´das, J.-L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am.
Chem. Soc. 1983, 105, 6555.
(13) (a) Kuhn, H. HelV. Chim. Acta 1948, 31, 1441. (b) Kuhn, H. HelV.
Chim. Acta 1949, 32, 2247.
(10) Tolbert, L. M.; Zhao, X. Synth. Met. 1993, 55-57, 4788.

Beyond the Cyanine Limit
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3255
Figure 7. Energy gap ∆E vs (Neff)-1 for cyanines DPyN.
Similar phenomena in other cyanine systems were first reported
by Brooker et al. in their studies of thiocyanine series.14 They
attributed these behaviors to cis-trans isomerism, in which cis
isomers, presumably, have shorter effective chromophoric chain
Figure 5. Absorption spectrum of DPy13 in different solvents.
1
lengths. However, the coupling constants evident in the H-
NMR spectrum of DPy13 are in the same 12-14 Hz range
characteristic of the other cyanines and are consistent with the
all-trans configuration (see Experimental Section).
Another possible explanation of these observations involves
dye aggregation. While it is generally accepted that cyanines
do not aggregate in organic solvents, these dyes are known to
form aggregates in aqueous solution.14 It was observed that, for
many cyanines in aqueous solutions, the bands of longest
wavelength absorptions in dilute solution become weaker as the
concentration is increased and new absorption bands appear.
To exclude the possibility of dye aggregation for DPy13,
concentration dependence studies were conducted in organic
solvents. Solutions of DPy13 followed Beer’s law at least in
the range below 10-4 M. The absorption features, including
the relative intensity of the peaks and the positions and the
shapes of the absorption bands, did not change with concentra-
tion. No new absorption bands were observed as the concentra-
tion increased. The proton chemical shifts of this molecule also
showed no concentration-dependent changes associated with
significant aggregation of DPy13.15 Thus, no significant dye
aggregation of DPy13 was in evidence in organic solvents.16 If
the broad absorption in DPy13 is not attributable to aggregation,
what can account for this appearance? These absorptions are
most reminiscent of those associated with intervalence electron
transfer absorption in weakly coupled binuclear transition metal
complexes. A common feature of these absorptions is their large
band half-width, typically17 around 5000 cm-1. One might ask
if the broad absorption in the spectrum of DPy13 reflects an
analogous optical electron transfer. The theory for these
transitions has been developed extensively by Hush.18 Using
this approach, and for the temperature limit defined by hυ ,
kT and T ) 300 K, a simple relationship has been developed
for intervalence charge transfer absorptions:19
Figure 6. Infrared absorption spectra of (a) DPy9 and (b) DPy13.
∆E ) hc/λ ) E∞ + k/(Nπ + C) ) E∞ + k/Neff
(1)
In eq 1, Nπ is the number of π-electrons involved in the
conjugation, C is a function of the terminal groups; the
combination of Nπ + C corresponds to effective conjugation
length, Neff. The intercept E∞ corresponds to the band gap at
infinite chain length. For DPyN or DPN, use of the more
convenient absorption maximum energies and C ) 4 per phenyl
or pyridine provided a linear relationship, with a limiting
solitonic intercept of ca. 0.6 eV for dyes DPN (N ) 1-13) and
a zero intercept for dyes DPy1 through DPy9. In contrast,
DPy13 exhibits a remarkable hypsochromic shift relative to
DPy9 (see Figure 7).
Discussion
The spectral properties, as well as anecdotal chemical
stability, of DPy13 are clearly in accord with a profound shift
in electronic structure for this species, which we interpret to be
the result of symmetry collapse. This evidence includes the
apparent onset of an optical electron transfer band, the solva-
tochromicity, and the onset of a new infrared absorption band
consistent with a polarized CdN band.
Optical Electron Transfer in the DPy13 Absorption
Spectrum. The structureless shape of the broad absorption band
in DPy13 (Figure 5) differs radically from the sharp peak
absorptions of the cyanine dyes, although DPy9 appears to
exhibit a broadening of the strongly allowed π-π absorption.
∆ ) (2310υIT)1/2
(2)
(14) Brooker, L. G. S. In Theory of the Photographic Process, 3rd ed.;
Hillson, P. J., Sutherns, E. A., Eds., Macmillan: New York, 1991; p 204.
(15) Graves, R. E.; Rose, P. I. J. Phys. Chem. 1975, 79, 746.
(16) Aggregation was evident for some of the shorter cyanines in water.
This aggregation could be defeated by addition of cyclodextrins.
(17) Day, P. NATO, ASI Ser., Series C. 1975, 20, 1.
(18) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Hush, N. S.
Prog. Inorg. Chem. 1982, 30, 210. (c) Hush, N. S. Trans. Faraday Soc.
1961, 57, 12.
(19) Ludi, A. Chimia 1972, 26, 647. Callahan, R. W.; Keene, F. R.;
Meyer, T. J.; Salmon, D. J. J. Am. Chem. Soc. 1977, 99, 1964.

3256 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Tolbert and Zhao
Table 1. Observed and Calculated Band Half-Widths for DPy13
a
b
c
solvent
νIT
∆
calcd
∆
obsd
acetone
Me2SO
DMF
13 800
13 500
13 700
12 300
5650
5600
5630
5330
5840
5720
5840
5300
pyridine
b
a Optical transition energies are in cm-1
.
∆
is the calculated
calcd
c
band half-width in cm-1 from eq 2.
∆
obsd
is the observed ((20%)
band half-width in cm-1
.
Figure 9. Solvatochromic behavior for DPy3, DPy9, and DPy13.
in Figure 5 are plotted vs (1/n2 - 1/D) (see Figure 9), the slope
obtained is comparable to that of the merocyanines, which
clearly suggests that the electronic structure of DPy13 is closer
to that of merocyanines rather than that of classical cyanines.
Finally, the additional infrared band exhibited by DPy13 at 1739
cm-1 suggests the onset of double-bond polarization to form
the -HCdCHCHdN+R moiety absent from the symmetrical
DPy9 (see Figure 6). Thus, this observation is also in accord
with a bond-alternating structure and suggests that the onset of
the Peierls distortion may occur at a shorter chain length than
previously anticipated.
Figure 8. Representations of lowest unoccupied and highest occupied
molecular orbitals of DPy13.
For Equation 2, υIT is the energy, in cm-1, of the intervalence
absorption band and ∆ is the absorption half-width. One might
ask if this relationship holds for optical electron transfer bands
associated with symmetry-collapsed cyanines. The long-
wavelength absorption of DPy13 has the typical intervalence
band half-width, ca. 5000-6000 cm-1. In Table 1 are shown
the calculated band half-widths ∆ from eq 2 and the observed
band half-widths in different solvents from Figure 5 obtained
by triangulation of the low-frequency half-band. The agreement,
ca. 15%, between ∆obsd and ∆calcd suggests that the Hush model
for intervalence transfer in binuclear complexes is a satisfactory
description of the absorptive behaviors of DPy13 and verifies
the charge transfer character of this absorption band.
The charge transfer character of the first excitation band of
DPy13 can be readily understood by an examination of the AM1
coefficients of the ground and first excited state molecular
orbitals of the molecule (see Figure 8). While the positive
charge is localized at one end in the ground state, it clearly
migrates to the other end in the first excited state. These results
suggest that DPy13 is a Robin-Day class II system with a
double minimum potential surface and a degenerate ground
state,20 i.e., a Peierls distorted molecule.
Solvatochromicity. While the visible absorption spectra of
symmetrical cyanines exhibit very limited solvatochromism,21
consistent with symmetrical ground and excited states, if the
longest wavelength absorption band of DPy13 is analogous to
the intervalence charge transfer (IT) for binuclear transition
metal complexes, a large solvatochromic shift should be
observed. With the assumption of a dielectric continuum model,
the Hush approach has been used to analyze the dependence of
transition energy EIT on solvent polarization (eq 3).22
Conclusions
The concept of resonance has been fundamental to our
understanding of reactivity in conjugated systems. Our work
with polyenyl anions has demonstrated, however, that a non-
bonding electron pair in an unsaturated system is confined within
a ca. 30 carbon atom framework, corresponding to the soliton
width. The fact that a cyanine such as DPy13 should exhibit
the onset of bond alternation at a chain length of 13 carbon
atoms would seem to shorten that confinement width. This
contradiction vanishes when one realizes that DPy13 has the
largest HOMO coefficient at the formal trivalent nitrogen. In
this case, delocalization can occur over only one-half the region
occurring when the highest electron density is in the center of
the chain. Beyond the “cyanine limit,” i.e., for bond-localized
cyanines for which optical electron transfer is weak, the optical
excitations reduce to the local ones predicted by Tyutyulkov.6
Thus, the delocalization width of 30 re-emerges as a fundamental
limit to delocalization in a polyene system. This fact alone
means that the possibility of achieving metallic, i.e., zero band
gap, behavior vanishes, leaving solitonic or polaronic time-
dependent processes available for long-range molecular level
conduction. In this context, it is important to realize that the
limiting rate for such processes is quite slow, i.e., approximately
three times the speed of sound.23
Experimental Section
Materials. For air-sensitive materials, acetone and acetonitrile were
distilled over CaH2 , then subjected to deoxygenation by several freeze-
pump-thaw cycles with argon purging, and stored under argon. For
vacuum line manipulations, argon (Holox, Atlanta, Ga) was dried by
passing through a series of columns containing Drierite and P2O5. For
inert atmosphere applications, argon was deoxygenated with a BASF
catalyst (R-311) drying train. Diethyl ether and tetrahydrofuran were
distilled under argon immediately before use from potassium/sodium
alloy containing benzophenone ketyl. Dimethyl sulfoxide (Me2SO,
Fisher) was purified according to the procedure of Bordwell.24 Me2-
1
1
1
1
1
D
EIT ) hυIT ) øi + e2
+
-
-
(3)
2
(
)(
)
2a1 2a2
d
n
Here (1/n2 - 1/D) is the solvent polarity defined in terms of
index of refraction n and solvent dielectric D, øi is the gas phase
absorption energy, and e2(1/(2a1) + 1/(2a2) - 1/d) describes
the absorption dipole geometry.22 When the absorptions shown
(20) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10,
247.
(21) West, W.; Geddes, A. L. J. Phys. Chem. 1964, 68, 837.
(22) For a discussion, see: (a) Powers, M. J.; Salmon, D. J.; Callahan,
R. W.; Meyer, T. J. J. Am. Chem. Soc. 1976, 98, 6731. (b) Tom, G. M.;
Creutz, C.; Taube, H. J. Am. Chem. Soc. 1974, 96, 7827
(23) Kuhn, C.; van Gunsteren, W. F.; Solid State Commun. 1993, 87,
203. (b) Kuhn, C. Synth. Met. 1993, 57, 4350.
(24) Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.;
Cornforth, F. J.; Drucher, G. E.; Margolin, Z.; McCallum, R. J.; McCollum,
G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006.

Beyond the Cyanine Limit
J. Am. Chem. Soc., Vol. 119, No. 14, 1997 3257
SO-d6 (Aldrich, 99.9% isotopic purity) was dried and deoxygenated in
the same fashion. Pyridine (Fisher, reagent) was deoxygenated by
several freeze-pump-thaw cycles under an argon atmosphere. The
deoxgenated pyridine was stored under argon in a glovebox.
Diisopropylamine (Aldrich, 99+%) was dried over CaH2 for at least
1 day and distilled over CaH2 immediately before use. cis-1-Methoxy-
1-buten-3-yne (Aldrich), received as a 50% w/w solution in methanol/
water (4:1), was purified according to the method of Krause and
Frazier.25 Piperidine (Aldrich, 99%) was distilled over molecular sieves
before use.
hydrochloride with aqueous ammonia, was added. The mixture was
heated at 100 °C for 10 min, neutralized with aqueous ammonia, and
extracted with ether (3 × 50 mL). The combined organic layers were
concentrated and distilled in Vacuo (bp 128-130 °C/2 mmHg) to yield
4-(phenylthio)pyridine as a colorless oil. The oil was treated with 2
equiv of MeI and heated to gentle reflux for 30 min. The resulting
solid was recrystallized (MeOH) to yield 7.12 g (75%) of N-methyl-
4-(phenylthio)pyridinium iodide as light yellow crystals (mp 172-174
°C, lit.34 174-176 °C). 1H NMR (Me2SO): δ 8.61 (2H, d, J ) 6.9
Hz), 7.71-7.65 (5H, m), 7.58 (2H, d, J ) 7.2 Hz), 4.15 (3H, s).
Morpholine acetate was afforded by a reaction of 1 equiv of
morpholine (Aldrich, 99%) and 1 equiv of acetate acid in ethanol. The
exothermic reaction yielded a large amount of white precipitate in 10
min at room temperature. Recrystallizations from methanol yielded
the pure compound as colorless crystals (mp 106-109 °C). Trimeth-
ylenebis(triphenylphosphonium) bromide was synthesized according to
a literature method26 and had mp 335-336 °C (lit.32 335-336 °C).
Manipulations. All glassware, including cannulas and syringes, was
dried at 135 °C for at least 24 h prior to use. Glassware was allowed
to cool under a flow of argon. Syringes and cannulas were flushed
with argon immediately before sample introduction. For anaerobic
reaction conditions, transfer of samples was conducted via stainless
steel cannulas or syringes. Inert conditions were maintained by an
atmosphere of argon.
Due to the extreme sensitivity of long-chain cyanines to oxygen, a
number of operations were conducted in a Vacuum Atmospheres
glovebox, containing an atmosphere of argon. The atmosphere inside
the glovebox was constantly deoxygenated with a dry train containing
a BASF catalyst (R-311). The dry train was maintained by routine
regeneration every 2 months. The glovebox was flushed with argon
(10-12 times of the volume of the dry box) prior to use if it had not
been used for a long period of time or large quantity of solvents were
used.
Instrumentation. Infrared (IR) spectra in the region of 2.5-25 µm
(4000-400 cm-1) were recorded on a Nicolet 520 FTIR equipment.
Solid samples were recorded in the potassium bromide (KBr) pellets.
Nuclear magnetic resonance (NMR) spectra were recorded on a Varian
Gemini 300 spectrometer in Me2SO-d6 (1H, 2.49 ppm; 13C, 39.5 ppm),
CDCl3 (1H, 7.24 ppm) or acetone-d6 (1H, 2.04 ppm). Chemical shifts
were reported in ppm with the solvent peaks as references.
For the air-sensitive samples, a special NMR tube connected with a
J. Young greaseless valve (Aldrich) was used. The valve consisted
of a borosilicate glass valve body and an O-ring-sealed PTFE piston
with an axial bore. A simple glass tube was used to connect the tube
to a vacuum apparatus.
UV-vis spectra were recorded on a Perkin-Elmer Lamda 19
spectrophotometer, which has a scan range of 190-2500 nm. Spectra
for air-sensitive compounds were taken with a specially constructed,
calibrated spectrophotometric cell.27
Mass spectra (MS) were recorded using a VG analytical 70-SE
spectrometer and are reported with mass-to-charge ratio and the intensity
indicated. High-resolution mass spectra were used to determine the
identity of the compounds.
Computer Calculations. Linear least-squares statistics were per-
formed using Excel 3.0 on a IBM 486 PC. Semiempirical (AM1)
calculations were performed on a Silicon Graphics Indigo work-
station using Spartan 3.0 from Wavefunction, Inc.
N,N-Dimethyl-4,4′-dipyridocyanine iodide (DPy1). DPy1 was
synthesized by a modified procedure of Sprague and Brooker.28,29
Benzenethiol (Aldrich, 4.70 g, 0.430 mol) was cooled to 0 °C, and
chloropyridine (3.34 g, 0.300 mol), obtained by neutralization of the
A mixture of N-methyl-4-picolinium toluenesulfonate (2.91 g, 10.0
mol), N-methyl-4-(phenylthio)pyridinium iodide (3.43 g, 10.0 mol),
triethylamine (3.30 g, 30.0 mol), and 1-propanol (99+%, Aldrich, 16
mL) was heated at reflux for 90 min. After cooling, the mixture was
added to ether (200 mL), and the resulting precipitate recrystallized
from ethanol to afford N,N,N-trimethyl-4,4′,4′′-tripyridocyanine as a
dark red solid mp 284-286 °C (dec)). The concentrated mother liquor
was chromatographed on alumina (9:1 acetone/methanol) to yield a
red solid. Recrystallization from methanol afforded 0.170 g of red
crystals (0.520 mmol, 5%), mp 187-189 °C, lit30 174-176 °C). 1H
NMR (Me2SO): δ 7.72 (4H, d, J ) 6.9 Hz), 7.03 (4H, broad), 5.42
(1H, s), 3.71 (6H, s). MS (m/z, intensity):31 320 (8), 199 (100, M+),
184 (7, M - CH3+•). FAB/HRMS: calcd for C13H15N2I 199.1235;
found 199.1252. IR (KBr pellet, cm-1): 1634.7, 1557.6, 1542.1,
1522.0, 1188.2. UV-vis (Me2SO): 486.0 nm (1.41 × 105 M-1 cm-1),
lit.35 483 nm (1.26 × 105 M-1 cm-1, in ethanol).
N,N-Dimethyl-4,4′-dipyridocarbocyanine Iodide (DPy3).32
A
mixture of N-methyl-4-picoline iodide (4.7 g, 20 mmol) and di-
ethoxymethyl acetate (99%, 1.6 g, 10 mmol) in 10 mL of piperidine
was heated to 120 °C for 3 h. The resulting blue precipitate was
collected and washed with ether. Alumina column chromatography
(10:90 CH3OH/acetone) and recrystallization from absolute ethanol
afforded small blue crystals (0.50 g, 14%, mp 209-210 °C (dec.)). 1H
NMR (Me2SO): δ 8.07 (1H, t, J ) 13.4 Hz), 7.70 (4H, d, J ) 7.15
Hz), 7.10 (4H, J ) 7.20 Hz, broad peak at room temperature, doublet
at 120 °C), 5.68 (2H, d, J ) 13.4 Hz), 3.70 (6H, s). MS (m/z):35 346
(adduct), 225 (M+), 210 (M - CH3+•). IR (KBr pellet, cm-1): 1637.1,
1558.4, 1530.8, 1485.3, 1465.8, 1419.0, 1290.9, 1143.5, 1010.7, 959.8.
UV-vis (Me2SO): 606 nm (1.60 × 105 M-1 cm-1).
N,N-Dimethyl-4,4′-dipyridodicarbocyanine Iodide (DPy5). 1,5-
Dipyridyl-1,3-pentadiene was prepared by a modification of the
procedure of Wittig et al.11 A solution of 1,3-propanediylidenebis-
(triphenylphosphorane) prepared from 4.54 g (6.25 mmol) of trimeth-
ylenebis(triphenylphosphonium) dibromide and 12.5 mmol of lithium
diisopropylamide in tetrahydrofuran was treated at -78 °C with
pyridinecarboxaldehyde (1.34 g, 12.5 mmol) in tetrahydrofuran (10 mL).
The mixture was kept at -78 °C for 3 h and quenched by addition of
1 mL of concentrated HCl in 5 mL of methanol. After evaporation of
solvent, sodium bicarbonate (1.3 g in 50 mL H2O) was added to adjust
to pH 8. The mixture was extracted with chloroform (3 × 50 mL).
The organic extracts were combined, dried, and concentrated. The
resultant red solid was subjected to silica gel column chromatography
(2% methanol/chloroform). 1,5-Dipyridyl-1,4-pentadiene was obtained
as a red oil (0.67 g, 49%) and was stored under an argon atmosphere
at -30 °C. 1H NMR (CDCl3): δ 8.56-8.50 (4H, m), 7.25-7.20 (2H,
m), 7.18-7.04 (2H, m), 6.96 (1H, dd, J ) 10.5 Hz, 15.6 Hz), 6.43
(1H, d, J ) 15.9 Hz), 6.30 (1H, dd, J ) 10.5 Hz, 14.4 Hz), 6.05 (1H,
dt, J ) 7.1 Hz, 14.6 Hz), 3.51 (2H, d, J ) 7.2 Hz).
A sample of 1,5-dipyridyl-1,3-pentadiene (48 mg, 0.22 mmol) was
dissolved in acetone (5 mL) and treated with methyl iodide (1.0 mL,
16 mmol). After 18 h at room temperature, the supernatant was
decanted, the red precipitate was dissolved in methanol, and the solution
was poured into ether. The resulting precipitate was filtered and washed
with ether to afford pentadienyl-1,5-bis(methyl-4-pyridinium) diidode
as a yellow powder (80 mg, 73%). 1H NMR (D2O): δ 8.51 (2H, d, J
) 6.6 Hz), 8.38 (2H, d, J ) 6.7 Hz), 7.75-7.78 (4H, m), 7.33 (1H,
(25) Kraus, G. A.; Frazier, K. Tetrahedron Lett. 1978, 3195.
(26) Horner, L.; Hoffman, H.; Klink, W.; Ertel, E.; Toscano, V. G. Chem.
Ber. 1962, 95, 581.
(27) Zhao, Xiaodong, Ph. D. Thesis, Georgia Institute of Technology,
Atlanta, GA 1995.
(28) Sprague, R. H.; Brooker, L. G. S. J. Am. Chem. Soc. 1937, 59,
2697.
(29) Leubner, I. H. J. Org. Chem. 1973, 38, 1098.
(30) Leubner, I. H. Nucl. Magn. Reson. 1974, 6, 253.
(31) Lemire, S. W.; Zhao, X.; Tolbert, L. M.; Busch, K. L. J. Am. Soc.
Mass Spectrom. 1994, 5, 1017.
(33) Leitis, L.; Rubina, K.; Goldberg, Yu. Sh.; Jansone, D.; Shimanskaya,
M. V. LatV. PSR Zinat. Akad. Vestis, Kim. Ser. 1980, 4, 469.
(34) Hagedorn, H.; Hohler, W. Angew. Chem., Int. Ed. Engl. 1975, 14,
486.
(32) Dent, S. G.; Brooker, L. G. S. U.S. Pat. 2,537,880 (Jan 9, 1951).
(35) Wollenberg, R. H. Tetrahedron Lett. 1978, 717.

3258 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Tolbert and Zhao
dd, J ) 10.3 Hz, 15.6 Hz), 6.59 (1H, d, J ) 15.5 Hz), 6.43 (1H, dd,
J ) 15.4 Hz, 10.4 Hz), 6.28 (1H, m), 4.63 (3H, s), 4.18 (3H, s), 3.73
(2H, d, J ) 6.6 Hz).
-78 °C under argon. After 30 min, pyridinecarboxaldehyde (2.98 g,
27.9 mmol) was added. The mixture was stirred at -78 °C for 3 h
and quenched with sodium bicarbonate/water/tetrahydrofuran (2.0 g/10
mL/10 mL). The solvent was removed, and the mixture was extracted
with ether (3 × 100 mL). The ether extracts were combined and
concentrated to yield a clear liquid containing a red gum. The clear
liquid (tetrabutyltin) was decanted, and the red gum was dissolved in
1:1 tetrahydrofuran/water. The pH was adjusted to 2 with 1 M aqueous
HCl. The solution was stirred at room temperature for 3 h, and the
pH was adjusted to 8.5 with aqueous ammonia. After reduction of the
volume under reduced pressure, the mixture was extracted with ether
(3 × 100 mL), and the combined organic fractions were concentrated
under reduced pressure. The yellow solid was subjected to chroma-
tography on silica gel (ethyl acetate). 5-(4-Pyridyl)-penta-2,4-dienal
was obtained as a light yellow solid (2.53 g, 57%, mp 55-57 °C). The
aldehyde was also prepared by oxidation of 5-(4-pyridyl)-2,4-penta-
dienol.26 1H NMR (CDCl3): δ 9.68 (2H, d, J ) 7.8 Hz), 8.64 (2H,
m), 7.35 (2H, m), 7.34-7.12 (2H, m), 6.95 (1H, d, J ) 14.9 Hz), 6.38
(1H, dd, J ) 8.0 Hz ; 15.2 Hz). IR (KBr pellet, cm-1): 1683.4, 1669.6,
1602.4, 1156.9, 1118.2, 995.7. MS (m/z, intensity): 159.1 (87.8, M+),
81.0 (50.8), 77.0 (40.3). FAB/HRMS: calcd for C10H9NO 159.0684;
found 159.0684.
Pentadienyl-1,5-bis(methyl-4-pyridinium) diiodide (120 mg, 0.24
mmol) was dissolved in 3 mL of 2:1 Me2SO/CH2Cl2. Piperidine (0.5
mL, 5.1 mmol) was added, and the solution was stirred at room
temperature for 2 min and poured into ether (150 mL). The dark blue
precipitate was collected and subjected to alumina column chroma-
tography (15% methanol/acetone). Recrystallization from ethanol of
the first blue band produced a green solid (42 mg, 47%, mp > 320
°C). 1H NMR (Me2SO-d6): δ 7.71 (4H, d, J ) 7.1 Hz), 7.52 (2H, t,
J ) 13.0 Hz), 6.98 (4H, d, J ) 6.5 Hz), 5.98 (1H, t, J ) 12.1 Hz),
5.75 (2H, d, J ) 13.3 Hz), 3.71 (6H, s). IR (KBr pellet, cm-1): 1545.5,
1545.7, 1505.8, 1482.4, 1414.6, 1111.1, 1017.4. MS (m/z):35 372
(adduct), 251 (M+), 236 (M - CH3+•). UV-vis (Me2SO): 707 nm
(1.74 × 105 M-1 cm-1).
N,N-Dimethyl-4,4′-dipyridotetracarbocyanine Iodide (DPy9). 3-(4-
Pyridyl)-2-propenal was prepared from 4-pyridinecarboxaldehyde (1.6
g, 0.015 mol), acetaldehyde (0.66 g, 0.015 mol), and morpholine acetate
(0.22 g, 1.5 mmol). Silica gel column chromatography (ethyl acetate)
afforded 3-(4-pyridyl)-prop-2-enal as a light yellow solid. Sublimation
(70 °C/0.5 Torr) yielded 1.48 g (74%) of light yellow crystals (mp
36-38 °C, lit. 35-36 °C). 1H NMR (CDCl3): δ 9.79 (1H, d, J ) 7.5
Hz), 8.73 (2H, d, J ) 6.0 Hz), 7.45 (1H, d, J ) 15.6 Hz), 7.43 (2H, d);
6.85 (1H, dd, J ) 7.5 Hz, 16.2 Hz). MS (m/z, intensity): 133.1 (17.2),
104.0 (14.0), 58.0 (22.7), 43.0 (100.0). FAB/HRMS: calcd for C8H7-
NO 133.05276; found 133.05278.
A 50 mL tetrahydrofuran solution of 1,3-propanediylidenebis-
(triphenylphosphorane) produced from 15 mmol of lithium diisopro-
pylamide and 5.5 g (7.5 mmol) of trimethylenebis(triphenylphos-
phonium) dibromide (5.5 g, 7.5 mmol was added to a solution of 3-(4-
pyridyl)-2-propenal (2.0 g, 15 mmol) in 50 mL of tetrahydrofuran (ca.
50 mL) at -78 °C. After 1 h at -78 °C for 1 h and 2 h at -50 °C,
the mixture was quenched with 1 mL of acetic acid in 5 mL of water.
The volume was reduced under reduced pressure, and the resultant
aqueous solution was neutralized with aqueous ammonia and extracted
with chloroform (3 × 50 mL). The combined organic extracts were
dried over MgSO4 and concentrated to afford a red tar. Silica gel
chromatography (2% methanol/chloroform) afforded afforded 1,9-
dipyridylnona-1,3,6,8-tetraene as a light red oil (0.60 g, 29%). 1H NMR
(CDCl3): δ 8.52 (4H, d, J ) 5.8 Hz), 7.35 (2H, dd), 7.28 (4H, d), 6.52
(2H, d, J ) 15.4 Hz), 6.26 (2H, t, J ) 10.8 Hz), 5.70 (2H, m), 3.33
(2H, t, J ) 8.0 Hz).
A sample of 1,5-dipyridylnona-1,3,6,8-tetraene (50 mg, 0.18 mmol)
was dissolved in acetone (3 mL) and treated with methyl iodide (1.0
mL, 16 mmol). After 18 h at room temperature, the supernatant solution
was decanted and the red precipitate was dissolved in Me2SO/methylene
chloride (3:1). The solution was poured into ether, and nona-1,3,6,8-
tetraenyl-1,5-bis(N-methyl-4-pyridinium) diidode (DPy9-H) precipitated
as a yellow solid and was collected by filtration (62 mg, 62%, mp
127-135 °C (dec.)). 1H NMR (Me2SO): δ 8.88 (2H, d, J ) 6.0 Hz),
8.76 (2H, d, J ) 6.6 Hz), 8.08 (2H, d, J ) 6.9 Hz), 7.98 (2H, d, J )
6.3 Hz), 7.71 (1H, dd, J ) 10.5 Hz, 15.6 Hz), 6.83-6.75 (2H, m),
6.67-6.34 (4H, m), 6.08 (1H, m), 4.29 (3H, s), 4.20 (3H, s), 3.79 (2H,
d, J ) 7.2 Hz).
N,N-Dimethyl-4,4′-dipyridohexacarbocyanine Iodide (DPy13). A
200 mL tetrahydrofuran solution of 1,3-propanediylidenebis(triph-
enylphosphorane) prepared from trimethylenebis(triphenylphosphonium)
bromide (5.00 g, 6.80 mmol) and 13.8 mmol n-butyllithium was added
to a solution of 5-(4-pyridyl)-penta-2,4-dienal (1.31 g, 13.7 mmol) in
15 mL of tetrahydrofuran at -78 °C. After 3 h at -78 °C and 1 h at
-50 °C, 1 mL of acetic acid and 5 mL of water were added. The
volume of solvent was reduced evaporated under reduced pressure, and
the pH of the solution was adjusted to >8 with aqueous ammonia.
After extraction with CHCl3 (3 × 50 mL), the combined organic extracts
were dried over MgSO4 and concentrated to afford a red tar. Silica
gel column chromatography (25:75 tetrahydrofuran/CH2Cl2) yielded 1,-
13-dipyridyltrideca-1,3,5,8,10,12-hexaene as a light red oil (0.40 g,
18%). 1H NMR (CDCl3): 8.54 (4H, m), 7.26 (4H, m), 7.08-6.96 (2H,
m), 6.73 (1H, dd, J ) 15.2 Hz, 9.8 Hz), 6.55-6.16 (7H, m), 5.86 (1H,
m), 5.61 (1H, dd), 3.11 (2H, t, J ) 7.8 Hz).
1,13-Dipyridyltrideca-1,3,5,8,10,12-hexaene (115 mg, 0.350 mmol)
dissolved in 5 mL of acetone was treated with methyl iodide (1.0 mL,
16 mmol). After 18 h at room temperature, the supernatant solution
was decanted and the remaining red precipitate was dissolved in Me2-
SO/CH2Cl2 (3:1). The solution was poured into ether, and the resulting
precipitate was isolated by centrifugation to yield trideca-1,3,5,8,10,-
12-hexaenyl-1,13-bis(N-methyl-4-pyridinium)diidode (DPy13-H) as a
yellow solid (182 mg, 85%). 1H NMR (Me2SO): 8.87 (2H, d, J )
6.6 Hz), 8.75 (2H, d, J ) 6.8 Hz), 7.72 (1H, dd, J ) 15.4 Hz; 11.0
Hz), 6.88-6.33 (10H, m), 5.98 (1H, m), 4.27 (3H, s), 4.19 (3H, s),
3.76 (2H, d, J ) 7.1 Hz). IR (KBr pellet, cm-1): 2966.7, 1650.5,
1597.8, 1104.2, 1031.8, 808.0.
Trideca-1,3,5,8,10,12-hexaenyl-1,13-bis(N-methyl-4-pyridinium)dii-
dode (120 mg, 0.200 mmol) was dissolved in 3 mL of 2:1 Me2SO/
CH2Cl2. 1,4-Diazabicyclo[2.2.2]octane (0.34 g, 3.0 mmol) in Me2SO
(2 mL) was added dropwise. The deep blue-green solution was stirred
at room temperature for 2 min and then poured into ether with vigorous
stirring. The dark precipitate was filtrated and washed with ether.
Attempts to purify this dark solid using chromatography failed due to
the extreme sensitivity of the compound to oxygen. Spectral data for
DPy13 were as obtained. 1H NMR (Me2SO-d6): δ 7.69 (4H, d, J )
6.2 Hz), 7.21 (1H, t, J ) 12.4 Hz), 7.01 (4H, d, J ) 6.2 Hz), 6.70-
6.55 (4H, m), 6.30-6.10 (6H, m), 5.90 (2H, d, J ) 13.4 Hz), 3.68
(6H, s). IR (KBr pellet, cm-1): 2966.7, 2927.2, 1729.4, 1643.9, 1485.9,
The following procedure was conducted in a drybox: Nona-1,3,6,8-
tetraenyl-1,9-bis(N-methyl-4-pyridinium) diiodide (72 mg, 0.13 mmol)
was dissolved in Me2SO/methylene chloride (2:1, 2 mL). Piperidine
(0.30 mL, 3.1 mmol) in Me2SO (2 mL) was added. The green-blue
solution was stirred for 2 min and poured into ether under vigorous
stirring. The dark blue precipitate was filtered and subjected to alumina
column chromatography (15:85 methanol/acetone). The green band
was extracted with acetone to yield approximately 30 mg of N,N-
dimethyl-4,4′-dipyridotetracarbocyanine iodide (DPy9) as a dark green
solid. 1H NMR (Me2SO-d6): 7.70 (4H, d, J ) 7.3 Hz), 7.32 (1H, t, J
) 13.1 Hz), 7.03 (4H, d, J ) 7.4 Hz), 6,76 (2H, t, J ) 12.7 Hz),
6.19-6.05 (4H, m), 5.85 (2H, d, J ) 13.9 Hz), 3.70 (6H, s). IR (KBr
pellet, cm-1): 1636.4, 1456.7, 1404.8, 1101.2, 1011.4. UV-vis (Me2-
SO): 920 nm (ca. 4.0 × 104 M-1 cm-1).
1466.2, 1110.8. UV-vis (DMSO): 738 nm (ca. 1.7 × 104 M-1 cm-1
)
Acknowledgment. Support of this research by the U.S.
Department of Energy, Office of Basic Energy Sciences, is
gratefully acknowledged.
5-(4-Pyridyl)-penta-2,4-dienal. To a solution of 1-tri-n-butylstan-
nyl-4-methoxybuta-1,3-diene35 (10.4 g, 27.9 mmol) in tetrahydrofuran
(100 mL) was added butyllithium (12.2 mL of 2.5 M in hexanes) at
JA9626953