First observation of monomer and dimer radical cation upon photoreduction of cyanopyridinium derivatives in solution by electron spin resonance and absorption spectroscopy

Article abstract of DOI:10.1021/jp025786x

The monomer radical and the dimer radical cation of cyanopyridinium derivatives have been studied by absorption and electron spin resonance (ESR) spectroscopies at room temperature in acetonitrile upon steady photolysis. The stable cyanopyridinyl radical

Full text of DOI:10.1021/jp025786x

J. Phys. Chem. A 2002, 106, 11987-11991
11987
First Observation of Monomer and Dimer Radical Cation upon Photoreduction of
Cyanopyridinium Derivatives in Solution by Electron Spin Resonance and Absorption
Spectroscopy
Sang Hyun Park,† Hideki Kawai,‡ and Toshihiko Nagamura*,†,‡
Department of Electronic Materials Science, The Graduate School of Electronic Science and Technology,
Shizuoka UniVersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan, and Molecular Photonics Laboratory,
Research Institute of Electronics, Shizuoka UniVersity, 3-5-1 Johoku, Hamamatsu 432-8011, Japan
ReceiVed: March 25, 2002; In Final Form: September 5, 2002
The monomer radical and the dimer radical cation of cyanopyridinium derivatives have been studied by
absorption and electron spin resonance (ESR) spectroscopies at room temperature in acetonitrile upon steady
photolysis. The stable cyanopyridinyl radical of 1,3-bis(4-cyanopyridiniumyl)propane (acceptor) formed by
one-electron transfer from tetraphenylborate anion (donor) interacted intramolecularly with a parent cation,
forming a dimer radical cation. A charge resonance band was observed at 1360 nm and assigned to the
formation of intramolecular dimer radical cations. The ESR spectra revealed that one unpaired electron in the
intramolecular dimer radical cation was completely shared between two chromophores of photoreduced 1,3-
bis(4-cyanopyridiniumyl)propane (1). On the other hand, the intermolecular dimer radical cation of 1-(4-
cyanopyrdiniumyl)hexadecane (2) as a monochromophoric compound was not formed because no charge
resonance band was observed in the near-infrared region. Clearly resolved ESR spectra of both the dimer
radical cation and the monomer radical were observed, for the first time, upon photoreduction of these
cyanopyridinium derivatives. The effect of the chromophore size on the charge resonance band in the near-
infrared region was also demonstrated by homo- and hetero-type bichromophoric compounds containing
4-cyano- and 4-nitrostyryl-pyridinium.
1. Introduction
of its CR band. A number of examples for CR bands have been
reported.7-9 To date, all dimer radical cations, except those
A large and very fast optical modulation of the refractive
index in the near-infrared (NIR) region is an essential require-
ment for optical data processing applications that are compatible
with present day light communication systems that operate in
the wavelength range of 1300-1600 nm. Organic materials
showing absorption changes in the visible (vis) to the NIR
region, brought on by various types of photochemical or
photophysical processes, can be used in molecular photonic
devices having a guided wave geometry to control the reflected
intensity or phase of the probe light in a parallel way.1-5
Materials processing large changes, upon excitation, in refractive
index and absorption in the 1300-1600 nm region and attractive
chemical and physical properties are ideal candidates for
consideration provided they also show charge resonance (CR)
interactions.
Badger and Brocklehurst were the first to postulate the
existence of a CR band for various aromatic cation radicals.6
They attributed the appearance of new broad absorption bands
in the NIR region to a CR band produced from dimeric cation
radicals. In addition, absorption bands due to monomeric cation
radicals were observed in the vis region. The CR band is
identified with the transition between the split energy levels of
singly occupied molecular orbitals (SOMO). These energy levels
are formed from the electronic interaction between two chro-
mophores of a dimer radical cation. The stabilization energy of
a dimer radical cation is approximately equal to half the energy
reported by Hermolin and Kosower10 and Nagamura and co-
workers,11-23 are observed between cation radicals and their
neutral counterparts. Hermolin and Kosower reported the
appearance of a NIR absorption from 1,1-trimethylenebis(4-
carbomethoxy)pyridinium diiodide that had been partially
reduced either chemically or electrochemically.10
We reported, for the first time, the CR band of 1-alkyl-[4-
(4-nitrostyryl)]pyridinium (NS+) tetraphenylborate (TPB-),
which was formed by a photoreduction process in organic
solutions at room temperature.11 In this styrylpyridinium system,
the relative stability of the radical, formed by the electron
transfer from a counteranion upon photoexcitation, leads to a
strong electronic interaction between the radical and its parent
cation. The CR band produced by the dimer radical cation was
centered at 950 nm in 1,2-dimethoxyethane (DME). It has been
established that the behavior of CR bands in para-substituted
styrylpyridinium compounds is significantly affected by the
geometrical structure of the dimer, the length of the bridge
between the two chromophores making up the dimer, the nature
of electron-withdrawing group on the chromophores, the solvent,
and the temperature.12-23 Recently, we have also reported on
the unusual electronic absorption changes around 1550 nm in
meso-2,4-bis[4-(4-nitrostyryl)pyridiniumyl]pentane ditetraphen-
ylborates following photoreduction.22,23 The reason for this is
probably because of the strong electronic interactions that exist
between the styrylpyridinyl radical and the styrylpyridinium
cation, which together form to a long and planar structure that
allows for considerable delocalization. It has not yet been
possible to observe both the monomer styrylpyridinyl radical
* To whom correspondence should be addressed. E-mail: nagamura@
rie.shizuoka.ac.jp.
† The Graduate School of Electronic Science and Technology.
‡ Research Institute of Electronics.
10.1021/jp025786x CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/15/2002

11988 J. Phys. Chem. A, Vol. 106, No. 49, 2002
Park et al.
Figure 1. The structures and abbreviations of the compounds.
Figure 2. Difference absorption spectra in 280-2000 nm region after
irradiation of 1 (0.25 mM) and 2 (0.5 mM) in ACN through a band-
pass filter (365 nm) for 4 min. The inset shows the enlarged spectra in
the 280-600 nm.
and the dimer radical cation under the same experimental
conditions as were used for the styrylpyridinium derivatives.
In this study, we have used several cyanopyridinium com-
pounds to examine the formation of monomer radicals and dimer
radical cations in organic solution at room temperature. On the
basis of ultraviolet (UV)-NIR absorption and electron spin
resonance (ESR) spectra, it has now been clearly demonstrated,
for the first time, that both dimer radical cations and monomer
radicals are produced upon photoreduction of cyanopyridinium
derivatives.
2. Experimental Section
The bromide salts of 1,3-bis(4-cyanopyridiniumyl)propane (1)
and 1-(4-cyanopyridiniumyl)hexadecane (2) were synthesized
by quarternization of 4-cyanopyridine with corresponding 1,3-
dibromopropane and 1-hexadecylbromide, respectively. The
bromide salt of asymmetric 1-(4-cyanopyridiniumyl)-3-[(4-
nitrostyryl)pyridiniumyl]propane (3) was synthesized via a two-
step quarternization from 4-cyanopyridine and 1,3-dibromopro-
pane, followed by reaction with 4-(4-nitrostyryl)pyridine. 1,3-
Bis[(4-nitrostyryl)pyridiniumyl]propane (4) was synthesized
according to the method previously reported.15 Counterions of
all bromide salts were exchanged with sodium tetraphenylborate
in methanol. The purity of the compounds was confirmed by
1H NMR and HPLC. The structure and abbreviation of each
compound are shown in Figure 1. Absorption spectra were
recorded at room temperature by a Hitachi U-3500 UV-vis-
NIR spectrophotometer before and after irradiation in vacuo with
a 150 W Hg-Xe lamp through a band-pass filter (λex ) 365
nm). The ESR spectra were measured with a Bruker ESP 300E
spectrometer (100 kHz field modulation) at room temperature.
Figure 3. Difference absorption spectra in 280-2000 nm region after
irradiation of 2 in (a) 1.0 mM ACN and (b) 0.5 mM DME through a
band-pass filter (365 nm) for 4 min. The inset shows the enlarged
spectra in the 280-800 nm.
the basis of these results, the new characteristic NIR band of 1
can clearly be attributed to a CR interaction between a
photogenerated radical and a parent cation in the form of an
intramolecular dimer radical cation (Py+-(CH2)3-Py•). An
absorption at 1360 nm has been reported for 1,1-trimethylenebis-
(4-(carbomethoxy)pyridinium) diiodide that had been partially
(one-electron) reduced either chemically or electrochemically.10
The absorption was attributed to an intervalence transition within
a pyridinyl radical cation, but no direct evidence for it, such as
ESR spectra, was given. The UV bands were assigned to the
monomer radical or the local excitation (LE) band of a dimer
radical cation. It should be noted that no CR band that could
be assigned to an intermolecular dimer radical cation of 2, in
which only intermolecular interactions can be involved, was
observed in the NIR region.
The absorption spectra of 2 at 1.0 mM in ACN and 0.5 mM
in 1,2-dimethoxyethane (DME), a less polar solvent, are shown
in Figure 3. The cyanopyridinyl radical showed a very strong
absorption in DME, almost 15 times greater than that in ACN
(Figure 2). This enhanced absorption in DME is attributed to a
higher probability of photoinduced electron transfer due to a
decreased interionic distance between the donor-acceptor
couple. However, no CR bands in the NIR were detected in
Figure 3 for either solvent system. These results clearly indicate
that when a cyanopyridinyl radical forms in a monochromo-
phoric compound it has difficulty interacting intermolecularly
3. Results and Discussion
The difference absorption spectra of compounds 1 and 2 after
photoirradiation through a band-pass filter (λex ) 365 nm) for
4 min at the same chromophore concentration in acetonitrile
(ACN) at room temperature are shown in Figure 2. The main
absorption bands of 1 and 2 before irradiation were observed
below 250 nm. The use of the band-pass filter limited the extent
of excitation into the extended tail absorptions of the com-
pounds, which was effective in prohibiting re-excitation of a
photogenerated radical or dimer radical cation.
A 0.25 mM solution of 1 in ACN showed a broad NIR
absorption with λmax at 1360 nm in addition to two sharp peaks
at 297 and 348 nm, as can be seen in Figure 2. A 0.5 mM
solution of 2 in ACN yielded only weak absorptions around
300 and 350 nm and no new absorption in the NIR region. On

Observation of Monomer and Dimer Radical Cation
J. Phys. Chem. A, Vol. 106, No. 49, 2002 11989
405 nm for SPPr) in 0.25 mM ACN solutions at room
temperature. Compound 4 showed two characteristic absorption
peaks at 950 and 1742 nm, which has been previously reported.15
The absorption bands were assigned to sandwich and partially
overlapped intramolecular dimer radical cations, respectively.15
The NIR absorption of 3 showed a peak at 1063 nm, which is
different from that of the intermolecular CR band observed at
1150 nm in monochromophoric N-alkyl-4-(4-nitrostyryl)pyri-
dinium tetraphenylborate in ACN solutions.12,15,17 The intramo-
lecular CR band at 1360 nm in 1 is considerably red-shifted by
about 297 and 410 nm as compared with those of 3 and 4,
respectively. The stabilization energies of the intramolecular
dimer radical cation were evaluated as ∆H ) 43. 9 kJ mol-1
for 1, 56.1 kJ mol-1 for 3, and 62.8 kJ mol-1 for 4. The
absorbance of the NIR band increased in the order of 1, 3, and
4 (at 950 nm). The order of stabilization energies reflects the
degree of overlap between two chromophores in a sandwich
configuration. The CR bands observed in the NIR region shifted
to longer wavelength on decreasing the size of the functional
group attached to the pyridinium rings, from 4-nitrostyrylpy-
ridinium to 4-cyanopyridinium. These results clearly indicate
that the extent of electron delocalization with the chromophores
also affects the stabilization of the dimer radical cation and hence
the CR band in the NIR region.
Figure 5 shows the proposed structural models of the
sandwich intramolecular dimer radical cation formed upon
photoexcitation of 1, 3, and 4 and partially overlapped one for
4. Two chromophores can be arranged parallel to each other in
the sandwich form, permitting interaction. The absorption
spectra observed in the NIR region as shown in Figure 4 most
probably stem from the sandwich form of various dimer radical
cations. The exception is the absorption at 1742 nm of 4, which
has been attributed to the dimer radical cation in its partially
overlapped form.15,23 Absorption bands of 1 and 3 at longer
wavelengths should not be assumed to arise from sandwich
structures because the extent of electron delocalization in the
cyanopyridinium derivatives is too small to form partially
overlapped structures such as those that occur with 4-nitro-
styrylpyridinium.
Figure 4. Difference absorption spectra in 800-2000 nm region after
irradiation of 1, 3, and 4 in ACN (0.25 mM) for 4 min through band-
pass filters, λex ) 365 nm for 1 and 3 and λex ) 405 nm for 4.
with a parent cation even at higher concentrations or in a low-
polarity solvent. It is clear that a cyanopyridinyl radical that
has little electron delocalization can only electronically interact
with a cyanopyridinium cation under confined conditions to form
a dimer radical cation such as in a bichromophoric compound
that is linked by a propane bridge (1).
In the systems being examined, a higher stability of the
pyridinyl radical has been achieved by the introduction of an
electron-withdrawing group at the para-position and by inhibit-
ing the reverse electron-transfer reaction following the oxidation
of the anion 5 as a consequence of its decomposition following
its one-electron oxidation.11-23 The presence of a trimethylene
bridge between two chromophores, as shown first by Hiraya-
ma,24 is known to be effective in taking on a favorable
conformation for intramolecular interaction of chromophores,
such as excimer formation. From the above reasons, we can
conclude that the relatively stable dimer radical cation is only
created from compound 1 in ACN at room temperature. Its
presence gives rise to the absorption band around 1360 nm that
is assigned to a CR band. This band persisted for several hours
in the dark after irradiation.
Figures 6 and 7 show the ESR spectra observed in ACN
solutions of bichromophoric 1 (1.0 mM) and monochromophoric
2 (1.0 mM) immediately following 5 min of irradiation through
a band-pass filter (λex ) 365 nm) at room temperature. These
Figure 4 shows the absorption spectra in the 800-2000 nm
region observed for compounds 1, 3, and 4 irradiated for 4 min
through a band-pass filter (λex ) 365 nm for 1 and 3 and λex
)
Figure 5. Proposed models of their sandwich intramolecular dimer radical cations 1, 3, and 4 and partially overlapped one of 4 formed by electron
transfer from anion 5. Counteranions (5) and protons are omitted for simplicity.

11990 J. Phys. Chem. A, Vol. 106, No. 49, 2002
Park et al.
Figure 6. The ESR spectrum of 1 (A) observed after irradiation in
ACN (1.0 mM) through a band-pass filter (365 nm) for 5 min and (B)
the simulated one.
Figure 7. The ESR spectrum of 2 (A) observed after irradiation in
ACN (1.0 mM) through a band-pass filter (365 nm) for 5 min and (B)
the simulated one.
TABLE 1: Hyperfine Splitting Constants of 1 and 2 in ACN
at Room Temperature
two well-resolved ESR spectra show totally different hyperfine
structures and total spectral width. The observed ESR spectra
of 1 and 2 shown in Figures 6A and 7A consist of sharp
hyperfine structures with at least 44 lines overlapping a broad
spectrum and those with at least 17 lines, respectively. A
computer simulation of these spectra was made for the dimer
radical cation and for the monomer radical using the WinSIM
software (Brukers). The simulation results are shown in Figures
6B and 7B. Its can be seen that these simulated spectra almost
coincided with the observed spectra. The hyperfine splitting (hfs)
constants for the best fits are summarized in Table 1. Itoh and
Nagakura studied the ESR spectra of 1-methyl-4-substituted
pyridinyl radicals with carbomethoxy, carbamide, acetyl, and
cyano groups upon chemical reduction.25 The hfs constants of
the 4-cyano derivative reported by them corresponded closely
to those for 2 given in Table 1. The hfs constants of radical 1
obtained from the spectrum in Figure 6 were approximately one-
half of those of the monomer radicals of 2 in Figure 7. The
reason for this reduction in the unpaired electron density of
radicals of 1 is most likely because of a decrease in the electron
delocalization between the two interacting chromophores. These
results clearly indicate that radicals in 1 formed by the
photoinduced electron transfer from counteranions were almost
quantitatively converted into intramolecular dimer radical
cations. In contrast, the photogenerated radicals of 2 persisted
as monomers without forming significant intermolecular interac-
tions. These results are consistent with the features of the
absorption spectra as shown in Figures 2 and 3.
compd
N1
H2,6
Hâ
Hγ
H3,5
N(CN)
1
2
3.17
6.52
2.19
4.02
1.735
3.47
0.13
0.24
0.31
0.45
0.915
1.83
of compounds containing the same chromophores, although their
existence had been predicted. Itoh reported that ESR spectra of
bis(4-carbomethoxy)pyridinium with tri-, tetra-, and pentan-
methylene briges partially (one-electron) reduced chemically
were unexpectedly very similar to those of the corresponding
diradicals.26 They interpreted their results to indicate the
formation of an open-structured dimer in which there was
insignificant intramolecular interactions between a pyridinium
cation and a pyridinyl radical. They did not observe a CR band,
which would be present at a wavelength greater than 1400 nm.26
Tripathi et al. reported ESR spectra of the neutral and cationic
forms of reduced isonicotinamide, 4-carbamylpyridinyl radicals
generated by photolysis in aqueous solution containing acetone
and isopropyl alcohol.27 Howarth and Fraenkel28 reported that
the ESR spectrum of the naphthalene dimer radical cation,
generated by two sets of eight equivalent protons, was observed
upon chemical oxidation at low temperature. The hfs constants
obtained were approximately one-half the hyperfine splitting
constants of the naphthalene negative ion radical. Recently,
Kochi et al.9 also reported that the molecular association of the
octamethylbiphenylene cation radical (OMB+•) with its neutral
counterpart produced a dimeric cation radical ((OMB)2+•). The
ESR spectra showed at least 15 lines having approximately one-
half the hfs constants of the protons from the monomeric OMB+•
in dichloromethane at -80 °C.9
To date, no clear ESR identification of the monomer radical
or the dimer radical cation has been made for pyridinium
derivatives upon chemical, electrochemical, or photoreduction

Observation of Monomer and Dimer Radical Cation
J. Phys. Chem. A, Vol. 106, No. 49, 2002 11991
The ESR spectrum of the styrylpyridinyl radical at room
temperature was reported in our previous papers.14,19,23 The ESR
spectrum of the 1-hexadecyl-4-[4-(dicyanovinyl)styryl]pyridinyl
(DSC+C16) radical, as a monochromophoric compound in DME,
which was formed by photoinduced electron transfer after
irradiation for 5 min, showed a broad singlet due probably to
the high local concentration of radicals or their strong interac-
tion. The ESR spectrum showed a dramatic change after mixing
the irradiated solution with an unirradiated sample. A sharp
spectrum was observed with at least nine hyperfine peaks
overlapping with a broad singlet.14,19 The final spectrum was
assigned to intermolecular dimer radical cations that had formed
from the interaction of photogenerated radicals and unirradiated
cations. In this structure, one unpaired electron is shared between
two chromophores on the basis of a comparison with a simulated
ESR spectrum. The meso-2,4-bis[4-(4-nitrostyryl)pyridiniumyl]-
pentane (m-SPPe) radical, as a bichromophoric compound in
ACN solution, also produced a broad singlet ESR spectra.23
However, the shape of the ESR spectrum was not changed when
mixed with an unirradiated sample. The broadening of the ESR
spectrum observed in m-SPPe was attributed to a strong
intramolecular electron exchange reaction between a photo-
generated styrylpyridinyl radical and a styrylpyridinium cation.23
Acknowledgment. This work was partly supported by
the Grants-in-Aid for Scientific Research on Priority Areas
“Molecular Synchronization for Design of New Materials
System” (No. 13022230), “Photofunctional Interface” (No.
14050049), and the Grants-in-Aid for Scientific Research for
Encouragement of Young Scientists (A) (No. 13750757). The
authors thank Dr. T. Arimura of AIST Tsukuba Central 5 for
NMR measurements.
References and Notes
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4. Conclusions
Dimer radical cations and monomer radicals formed by the
photoreduction of cyanopyridinium derivatives were clearly
identified, for the first time, by absorption and ESR spec-
troscopies. The stable intramolecular dimer radical cation of 1
formed by a photoinduced electron transfer from an anion 5 to
a pyridinium cation in ACN at room-temperature results in the
formation of a broad charge resonance band with a peak at 1360
nm. On the other hand, photogenerated radicals of monochro-
mophoric 2 did not lead to an intermolecular interaction with a
parent cation. The hyperfine splitting constants of dimer radical
cation of 1 and monomer radical of 2 were evaluated from ESR
simulation spectra. The dimer radical cation of 1, with at least
44 sharp lines overlapping a broad signal, has one-half the
hyperfine splitting constant of the monomer spectrum that has
at least 17 lines. These results clearly indicated, for the first
time, the formation of monomer radicals and dimer radical
cations from the same cyanopyridinium cations by photoinduced
electron transfer in ACN at room temperature. The effect of
electronic delocalization in chromophores on the CR band was
also clearly demonstrated for three bichromophoric compounds.
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