Charge Resonance and Charge Transfer Interactions of Photogenerated Dicyanovinylstyrylpyridinyl Radicals in Solutions at Room Temperature

Article abstract of DOI:10.1021/jp9536108

Charge resonance and charge transfer interactions of dicyanovinylstyrylpyridinyl radicals were observed at room temperature in organic solvents by steady photolysis with absorption and ESR spectroscopy.Excitation of dicyanovinylstyrylpyridinium salts with

Full text of DOI:10.1021/jp9536108

9370
J. Phys. Chem. 1996, 100, 9370-9376
Charge Resonance and Charge Transfer Interactions of Photogenerated
Dicyanovinylstyrylpyridinyl Radicals in Solutions at Room Temperature
Toshihiko Nagamura,*,†,‡ Takashi Ichihara,† and Hideki Kawai‡,§
Crystalline Films Laboratory, Research Institute of Electronics, and Department of
Electronic Materials Science, Graduate School of Electronic Science and Technology,
Shizuoka UniVersity, 3-5-1 Johoku, Hamamatsu 432, Japan
ReceiVed: December 6, 1995; In Final Form: March 19, 1996X
Charge resonance and charge transfer interactions of dicyanovinylstyrylpyridinyl radicals were observed at
room temperature in organic solvents by steady photolysis with absorption and ESR spectroscopy. Excitation
of dicyanovinylstyrylpyridinium salts with tetraphenylborate in less polar solvents resulted in photoinduced
electron transfer reactions to form fairly stable dicyanovinylstyrylpyridinyl radicals. In addition to the
remarkable color changes due to the characteristic absorption of dicyanovinylstyrylpyridinyl radicals in the
visible region, a charge resonance absorption band (CR band) of dimer radical cations and a charge transfer
band of radical dimers were observed in the near-infrared region with peaks at 1050 and 1120 nm, respectively.
The rise of the CR band was observed in the pulse width of a nanosecond laser, which was as fast as that of
the formation of radical itself.
1. Introduction
absorption changes following prolonged irradiation and/or
storage in the dark after irradiation.24
We have been studying photochemical and photophysical
properties of ion-pair charge-transfer (IPCT) complexes in
various microenvironments to photochemically control both their
linear and nonlinear optical properties, as potential candidates
for future photonics devices.1-21 By the use of tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate anions as counterions of 4,4′-
bipyridinium ions, we have achieved novel photochromism
(photoinduced electrochromism) due only to the photoinduced
electron transfer and thermal reverse reactions in organic
solutions and in polymer or Langmuir-Blodgett films.1-13,15-21
Very stable 4,4′-bipyridinium radical cations formed by a
photoinduced electron transfer reaction, as seen by its charac-
teristic blue color in an inert atmosphere. The ultrafast color
changes, in less than 1 ps upon IPCT excitation, were observed
by femtosecond laser photolysis.21 The lifetime of the blue state
was controlled by altering the solution counterions, the mi-
croenvironment, and the temperature, over a very wide range
from 1.9 ps to almost infinity.12,15,21 Novel second harmonic
generation from IPCT complexes and its photochemical control
via photoinduced electron transfer were achieved using substi-
tuted styrylpyridinium salts.14
Recently we have reported, for the first time, the existence
of a charge resonance (CR) band as a broad absorption with a
peak at 950 nm upon steady photoexcitation of 1-alkyl-4-(4-
nitrostyryl)pyridinium (NS+) tetraphenylborate (TPB-) in 1,2-
dimethoxyethane (DME) solution with concentrations higher
than 10 µM at room temperature as a result of electronic
interaction between a photogenerated nitrostyrylpyridinyl radical
and a parent nitrostyrylpyridinium cation.22 In the much more
polar solvent, acetonitrile, the CR band of NS+TPB- was very
weak and showed a peak at 1166 nm.23 Neither the CR band
nor radical formation were observed in trifluoromethyl- or
cyano-substituted styrylpyridinium salts upon steady photoly-
sis.23 The strong tendency for self-association of nitrostyrylpy-
ridinyl radicals was also indicated from the observed complex
The CR band is characterized by its absorption spectrum in
the near-IR region, due to the splitting of the energy levels of
both the ground and the excited states resulting from the
electronic interaction between the two chromophores of the
dimer radical cation. Dimer radical cations have been studied
by electron spin resonance (ESR) and optical absorption. The
CR band has been observed for dimer radical cations formed
by γ-radiolysis and pulse radiolysis in rigid matrices at or near
77 K , by chemical oxidation in solutions at low temperatures,
and by nanosecond (ns) laser flash photolysis and nanosecond
pulse radiolysis at room temperature in solution.25-39 All of
these dimer radical cations, except those found by us in
nitrostyrylpyridinium salts22-24 and that in ref 27, were formed
between neutral parent molecules (M) and radical cations (M+)
generated from M. Ishitani and Nagakura reported the appear-
ance of a CR band from paracyclophane anions chemically
produced in solutions at low temperatures.27 Itoh reported the
intramolecular association of the pyridinyl radical and the
pyridinium cation in the radical cations of bis(4-methoxycar-
bonylpyridinium iodides), connected with tri- and tetramethyl-
enes, in solution at room temperature.40 A broad new absorption
was observed at 430-450 nm, but it was not assigned to a CR
band.40 In the case of dimer radical cations, the same number
of groups of equivalent protons (and other nuclei with nonzero
nuclear spin) were observed as for the corresponding monomer
radicals, but each group contains twice the number of protons
and only half the value of the hyperfine coupling constants.28-30
A near-IR absorption with a peak at 870 nm in addition to a
strong visible absorption was reported upon chemical reaction
of 1-methyl-4-cyanopyridinium ion or electrochemical reduction
of methylviologen V2+ in solutions at room temperature.41 It
was not a CR band, but was assigned to an allowed transition
in the methylviologen radical cation dimer, (V+•)2, a diradical.41
We now report near-IR absorption and ESR studies on
specific CR and charge transfer (CT) interactions of styrylpy-
ridinyl radicals formed by photoinduced electron transfer
reactions of newly synthesized 1-alkyl-4-[4-(dicyanovinyl)styryl]-
pyridinium tetraphenylborate in solution at room temperature.
† Crystalline Films Laboratory.
‡ Department of Electronic Materials Science.
§ Research Fellow of the Japan Society for the Promotion of Science.
X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03610-0 CCC: $12.00 © 1996 American Chemical Society

CR and CT Interactions of Dicyanovinylstyrylpyridinyl
J. Phys. Chem., Vol. 100, No. 22, 1996 9371
Figure 2. Visible absorption spectra of 1-hexadecyl-4-[4-(dicyanovi-
nyl)styryl]pyridinium (DCS+C16) salts with tetraphenylborate (TPB-)
counterions: 0.1 mM, in (a) DME, (b) methanol, (c) ACN, and (d)
Na+TPB- in methanol.
Figure 1. Synthetic route for the formation of 1-alkyl-4-[4-(dicy-
anovinyl)styryl]pyridinium tetraphenylborate salts.
Figure 3. Fluorescence and excitation spectra of DCS+C16TPB-: 0.1
mM, in (a) DME, (b) DME, (c) methanol, and (d) DME. λex ) (a)
424, (b) 530, and (c) 418 nm; λem ) (d) 650 nm.
2. Experimental Section
to yield 2 as a white powder (71%). Compound 2 and 4-picoline
were heated in acetic anhydride at 100 °C in nitrogen atmo-
sphere for 3 h. Purification by silica column (eluent: ethyl
acetate) yielded 3 as a white powder (11%). Compound 3
dissolved in acetone was reacted with methyl iodide or
hexadecyl iodide at room temperature for 2 days or 3 weeks,
respectively, to yield 4 (90%) or 5 (58%). The counterions of
these styrylpyridinium iodide salts were exchanged with tet-
raphenylborate in methanol to give 6 or 7 as yellow powders.
The purity was confirmed by NMR, TLC, elemental analysis,
and fluorescence spectroscopy.
NMR spectra were recorded on a JEOL EX-90 spectrometer,
using CDCl3 as a solvent. Fluorescence spectra were recorded
on a Shimadzu RF-5000 fluorescence spectrometer.
Sample solutions for both absorption and ESR measurements
were degassed in a branch of each cell under repeated freeze-
pump-thaw cycles using high-purity Ar. The electronic
absorption spectra were recorded with a Hitachi U-3500 UV-
vis-near-IR spectrophotometer before and after irradiation in
an inert atmosphere using a 150 W Hg-Xe lamp through an
L-39 filter (>365 nm) and an IR cut filter. Sample solutions
were stirred with a magnetic stirrer during irradiation unless
otherwise indicated. The ESR spectra during and after irradia-
tion by the same photoexcitation system without stirring were
measured at room temperature with a JEOL JES-RE1X spec-
trometer (100 kHz field modulation).
The dynamics of the photoreactions upon excitation with a
third harmonic (355 nm, 10 ns) of a Nd:YAG laser (Spectra-
Physics GCR-170, 10 Hz) and subsequent decay was studied
using a He-Ne laser (633 nm), a laser diode (830 nm), or a
CW YAG laser (1064 nm) as a probe. Transient signals
detected with a reverse-biased PIN photodiode (Hamamatsu
Photonics S2839) were digitized by a Sony Tektronix TDS540A
digital oscilloscope (50 Ω) through a fast preamplifier (Stanford
Research Systems, SR445).
4. Results and Discussion
4.1. Vis-Near-IR Absorption and Fluorescence Spectros-
copy. The absorption spectra of 1-hexadecyl-4-[4-(dicyanovi-
nyl)styryl]pyridinium (DCS+C16) salts with tetraphenylborate
(TPB-) in (a) DME, (b) methanol, and (c) acetonitrile (ACN)
are shown in Figure 2 together with that (d) of sodium
tetraphenylborate in methanol, which showed absorption below
about 290 nm. Only in DME was an extended tail in absorption
in the visible region observed above about 450 nm. Fluores-
cence from locally excited DCS+ was observed at 446 nm in
DME (a), at 443 nm in methanol (c), and at 440 nm in ACN.
In addition to local excitation (LE) fluorescence, a new
fluorescence with a peak at 650 nm was observed only in DME
as shown in Figure 3b. The excitation spectra of this new
fluorescence showed a peak at about 530 nm as shown in Figure
3d for λem ) 650 nm. This excitation spectrum most probably
corresponds with the extended tail absorption in DME, which
was overlapped with that of the LE band as shown in Figure 2.
3. Synthesis and Characterization
1-Alkyl-4-[4-(dicyanovinyl)styryl]pyridinium tetraphenylbo-
rate (6, 7) were prepared according to the scheme shown in
Figure 1. Terephthalaldehyde monodiethylacetal and malono-
nitrile were refluxed briefly in ethanol with a small amount of
piperidine. Purification by silica column (eluent: benzene)
yielded 1 as a pale yellow oil (71%). It was treated with
trifluoroacetic acid and water in chloroform at room temperature
The relative quantum yield of this new fluorescence with λex
530 nm was more than 250 times higher than that of LE
)
fluorescence with λex ) 424 nm. Very interestingly, the

9372 J. Phys. Chem., Vol. 100, No. 22, 1996
Nagamura et al.
Figure 4. Absorption spectra after excitation of DCS+C16TPB-, 0.1
mM, in DME irradiated through an L-39 filter (>365 nm) for 1 min.
Figure 5. Concentration dependence of absorption spectra of
DCS+C1TPB- in DME just after irradiation for 1 min: (a) 10, (b) 8,
and (c) 5 µM.
intensity of this new fluorescence band decreased upon irradia-
tion at 400-500 nm and recovered in the dark. These specific
absorption and fluorescence properties of DCS+C16TPB- salts
in DME were ascribed to the formation of IPCT complexes with
TPB- as a donor and DCS+C16 as an acceptor in a similar
manner as observed with 4,4′-bipyridinium salts,2,3,10,19 with 2,2′-
bipyridinium salts,20 and with the nitrostyrylpyridinium TPB-
salt.22 Reversible decay of IPCT fluorescence upon photoex-
citation and recovery in the dark were also observed with several
4,4′-bipyridinium salts.19 The distance between DCS+C16 and
TPB- in less polar solvents such as DME is expected to be
much shorter than that in polar solvents such as methanol or
ACN. The blue-shift of fluorescence from locally excited DCS+
with increasing solvent polarity is due to the weaker stabilization
of an excited state with a neutral character.
Figure 6. Absorption spectra of DCS+C16TPB-, 0.1 mM, in a 9:1
mixture of DME and ACN after irradiation for 1 min.
Changes in absorption spectra upon excitation of DC-
S+C16TPB- in DME through an L-39 filter are shown in Figure
4. New absorption peaks were observed in the visible region
at 573 and 642 nm, which were red-shifted by about 120-70
nm as compared with those (450 and 570 nm) observed in
irradiated nitrostyrylpyridinium TPB- salts.22 Since the latter
has been assigned to nitrostyrylpyridinyl radicals,42,43 the
observed visible absorption is most likely due to dicyanovinyl-
styrylpyridinyl radicals formed by photoinduced electron transfer
reaction from TPB- to DCS+. An ESR spectrum was observed
upon irradiation of DCS+C16TPB- in DME as mentioned below,
which clearly indicated the formation of radicals. These
photogenerated styrylpyridinyl radicals are fairly stable mainly
because of oxidative decomposition of TPB- after photoinduced
electron transfer.44 The red-shift in the absorption spectra of
styrylpyridinyl radicals observed in the present system strongly
suggests that the dicyanovinyl group makes more of a contribu-
tion to the electron delocalization than the nitro group. In the
near-IR region a peak at 1120 nm and a shoulder at about 1000
nm were observed as shown in Figure 4, which was very
different from the CR band with a peak at 950 nm for irradiated
nitrostyrylpyridinium TPB- salts in DME.22 Both visible and
near-IR absorption were observed until about 1 h after irradiation
as shown in Figure 4. A new peak at about 532 nm gradually
increased in the dark with the decay of radicals. It strongly
suggested the formation of a new species via radicals, but it is
not assigned yet. Cis-trans isomerization of styrylpyridinium
ions was reported by Takagi et al. to be induced via a one-
electron reduction.45 Then one possible species might be cis-
DCS+ formed via radicals from trans-DCS+, though the
absorption spectrum of cis-NS+ reported by Go¨rner et al.42,46
showed a peak at about 370 nm in methanol. No effects of the
variation in length of alkyl groups were observed on the
absorption spectra in 0.1 mM DME solutions. In 1 mM DME
solutions, however, red-brown precipitates were observed for
1-methyl-4-[4-(dicyanovinyl)styryl]pyridinium (DCS+C1TPB-)
after 1 min of irradiation, whereas no precipitates were formed
in DCS+C16TPB- even after 5 min of irradiation. These results
indicate that long alkyl substituents increase the solubility of
photogenerated styrylpyridinyl radicals in DME.
The concentration dependence of the absorption spectra just
after irradiation for 1 min is shown in Figure 5 for 5-10 µM
DME solutions. The absorption spectrum in the visible region
was observed at all concentrations. However, the peak in the
near-IR region disappeared in the 5 µM solution. These results
strongly suggested that the near-IR absorption was caused by
some intermolecular interaction of photogenerated styrylpyridi-
nyl radicals. In irradiated nitrostyrylpyridinium TPB- salts, the
CR band was observed above 10 µM.22 The dicyanovinyl group
seemed to facilitate the intermolecular interaction of styrylpy-
ridinyl radicals as compared with the nitro group.
Irradiation of DCS+C1TPB- or DCS+C16TPB- in acetonitrile
solutions caused no absorption changes in the visible and near-
IR regions except for a slight decrease of a peak at 370 nm and
a slight increase near 300 nm. These results indicated that
photoinduced electron transfer from TPB- to DCS+ became
impossible due to the much increased interionic distance in polar
ACN (dielectric constant ꢀ ) 37.5) as compared with that in
the less polar DME (ꢀ ) 7.20). Interesting results were obtained
using a 9:1 mixture of DME and ACN as shown in Figure 6.
The absorption spectrum after irradiation of DCS+C16TPB- for
1 min in this mixture consisted of two peaks at 624 nm and
about 1000 nm. The absorbance of both peaks increased slightly
until 10 min after irradiation was stopped and then decreased
and gave a new peak at 514 nm. Similar spectra in the near-IR
region with a single peak at 1050 nm and similar changes after
irradiation as shown in Figure 6 were also observed for
DCS+C16TPB- in DME when the irradiation time was short
(10 s). These results indicate that the intermolecular interaction
of photogenerated styrylpyridinyl radicals, when the polarity

CR and CT Interactions of Dicyanovinylstyrylpyridinyl
J. Phys. Chem., Vol. 100, No. 22, 1996 9373
Figure 7. ESR spectra of DCS+C16TPB- in DME (0.5 mM) (a) during
and just after irradiation for 5 min and (b) after storage in the dark for
1.5 h.
Figure 9. Simulated ESR spectra of (A) styrylpyridinyl radical
monomer and (B) its dimer radical cation using a Lorentzian profile
with a component line width of 0.02 mT.
Figure 8. ESR spectrum of DCS+C16TPB- after DME solution (1 mM)
irradiated for 5 min was mixed with unirradiated solution.
of solvent is higher or the radical concentration is low, is
different from that observed for 1 min of irradiation in DME
as shown in Figures 4 and 5.
4.2. ESR Spectroscopy. From the vis-near-IR absorption
spectroscopy, it was strongly suggested that photoinduced
electron transfer occurs to form styrylpyridinyl radicals and that
the radicals partake in specific intermolecular interactions
depending on concentrations, polarity of solvents, or irradiation
time. ESR measurements were made to clarify the nature of
these interactions. The ESR spectrum of DCS+C16TPB- in
DME (0.5 mM) during and just after irradiation for 5 min is
shown in Figure 7 together with that 1.5 h after irradiation.
Broad singlet spectra were observed in all cases, while the
spectral width (∆Hmsl) slightly decreased from 0.87 mT (a)
during and just after irradiation to 0.62 mT (b) after 1.5 h in
the dark. These results strongly suggest that the exchange
broadening is due to the high local concentration of radicals
generated during irradiation without stirring. The extent of
exchange broadening increased during storage in the dark most
probably due to molecular association of less soluble styrylpy-
ridinyl radicals, which was confirmed by carrying out dilution
experiments. Figure 8 shows the ESR spectrum after the
irradiated DME solution, showing a broad singlet spectrum
similar to that in Figure 7, was mixed with unirradiated DME
stored in a branch under inert atmosphere. The observed
spectrum consisted of a sharp one with at least nine hyperfine
structures overlapped with a broad one. The intensity of this
spectrum, with hyperfine structure, slightly increased until 10
min after mixing and did not decay after 30 min.
Figure 10. Simulated spectrum as a sum of a broad singlet spectrum
(∆ ) 0.2 mT) and that of a dimer radical cation (∆ ) 0.02 mT) with
the intensity ratio of 1:4.
a(N) ) 0.312 mT. These values were obtained from best fits
with the observed sharp spectrum shown in Figure 8 for para-
substituted styrylpyridinyl radical monomers as mentioned
below. They are similar to those reported for 1-alkyl-4-
methoxycarbonylpyridinyl radicals,47 but are slightly smaller
than them. The hfc constants reflect the unpaired electron
density on C or N atoms of substituted pyridinyl radicals. The
electron-donating substituent at the 4-position will increase the
hfc constant as reported for the 1-methyl-4-alkylpyridinyl
radical.48 In the present case the nitro- or dicyanovinyl-
substituted styryl group is expected to be a stronger electron-
withdrawing group than the methoxycarbonyl group in ref 47,
which will decrease the hfc constants. The observed pattern of
a sharp spectrum with the highest intensity at the center in Figure
8 resembles that of the dimer radical cation in Figure 9B but
does not correspond to the radical monomer in Figure 9A. The
broad singlet spectrum shown in Figure 8 was also reproduced
using the same hyperfine coupling constants and a broader line
width (∆) of 0.2 mT. A simulated spectrum as the sum of broad
singlet (∆ ) 0.2 mT) and dimer radical cation (∆ ) 0.02 mT)
with the intensity ratio of 1:4 is shown in Figure 10. It coincides
almost perfectly with the observed one shown in Figure 8. These
results therefore indicate that the photogenerated dicyanovinyl-
Simulated spectra for styrylpyridinyl radical (A) and its dimer
radical cation (B) using a Lorentzian shape with a component
line width of 0.02 mT are shown in Figure 9. The following
hyperfine coupling (hfc) constants and their half-values were
used for radical monomers and dimer radical cations, respec-
tively; a(H)3,5 ) 0.01 mT, a(H)2,6 ) a(H)NCH ) a(H)4-CH
)
2

9374 J. Phys. Chem., Vol. 100, No. 22, 1996
Nagamura et al.
Figure 12. Schematic representation of the energy levels in a dimer
radical cation. ∆H is the stabilization energy, and CR is the CR band.
Figure 11. Spectral changes during DCS+C16TPB- in DME (0.5 mM)
upon irradiation in a 1 mm cell for 2 min without stirring (solid line)
and subsequent mixing with an unirradiated one at 18 min after
irradiation (broken lines).
and dicyanovinylstyrylpyridinyl dimer radical cations, respec-
tively. These values are larger than those (8-10 kcal/mol) of
fused aromatic dimer radical cations such as pyrene25,26,31-39
and are comparable to that (15.3 kcal/mol) of the benzene dimer
radical cation.26,30 The stabilization energy of the dimer radical
cation is determined by the attractive charge transfer interaction
together with electronic and steric repulsive forces. The
molecular structures of both the nitro- and dicyanovinyl-
substituted styrylpyridinium cation and their reduced forms are
planar on the basis of MOPAC calculations and a molecular
model. Higher stabilization of these systems can be expected
from such long and planar structures with extensive charge
delocalization which favor the electronic interaction. Hunter54
reported that so-called π-π interactions are dominated by
electrostatics, which means that the most favorable interaction
of π-systems is found when the substituents are both strongly
electron-withdrawing. Nitro- and dicyanovinyl-substituted
styrylpyridinium ions have two strong electron-withdrawing
groups, and their styrylpyridinyl radicals also have a strong
electron-withdrawing nitro- or dicyanovinyl group. These
substituent groups most probably facilitated the formation of
dimer radical cations producing the CR band observed at very
low concentrations.
4.3. Laser Flash Photolysis. The transient absorptions at both
633 and 1064 nm, corresponding to the radical monomer or
local excitation (LE) of dimer radical cations and the CR band,
as schematically represented in Figure 12, are shown in parts
A and B, respectively, of Figure 13 for 0.1 mM DCS+TPB- in
DME solutions excited by a nanosecond laser at 355 nm. Both
absorptions show a very fast rise within the pulse width of the
nanosecond laser. A small gradual rise after overshooting in
Figure 13B is most probably due to a thermal effect in DME,
which showed an absorption at 1064 nm. This explanation for
a gradual “rise” is supported by the following results. From
steady photolysis it was reported that only the CR band showed
an absorption for NS+TPB- in DME at both 830 and 1064 nm.22
The CR band monitored at 830 nm in Figure 14A, where DME
has no absorption, for 0.5 mM NS+TPB- in DME did not show
such a slow rise. However, the same CR band monitored at
1064 nm in Figure 14B did show a similar slow rise as observed
in Figure 13B for DCS+TPB-. Therefore, the apparent rise
observed at 1064 nm is not due to the real growth of transient
species.
styrylpyridinyl radicals are associated most probably as radical
dimers during and after irradiation, which caused the exchange
broadening. When they were mixed with unirradiated dicy-
anovinylstyrylpyridinium cations, the radical dimers were diluted
and most of photogenerated radicals interacted with the parent
cations to form dimer radical cations showing the CR band and
characteristic ESR spectrum. Some associated radical dimers
seemed to remain even after dilution.
To compare vis-near-IR absorption spectra under similar
conditions as in ESR measurements, DCS+C16TPB- in DME
(0.5 mM) was irradiated in a 1 mm cell for 2 min without
stirring. The irradiated sample was mixed with the unirradiated
one in a branch at 18 min after irradiation. Spectral changes
during these processes are shown in Figure 11. The irradiated
sample before mixing showed an absorption spectrum with
peaks at 642 and 1120 nm and a shoulder at about 1000 nm.
Such a spectrum disappeared following the mixing of the
irradiated sample with the unirradiated one. A new absorption
spectrum with peaks at 534 nm, about 630 nm, and about 1050
nm appeared after mixing. These absorption bands gradually
increased until about 4 h after mixing and then decreased. From
these observations and ESR results, it can be concluded that
the broad absorption with a peak at about 1050 nm can be
attributed to the CR band of the dicyanovinylstyrylpyridinyl
dimer radical cations. Meanwhile, the near-IR absorption with
a peak at 1120 nm is most probably attributed to the CT
absorption of dicyanovinylstyrylpyridinyl radical dimers. A
gradual increase of the CR band suggests that the mixing was
not sufficient at the molecular level. The CT band, characteristic
of the radical dimers which correspond to the transition from
the singlet ground state to the antisymmetric CT state, was
observed in the vis-near-IR region for radical anion dimers,
radical cation dimers, and neutral radical dimers.49-53 1-Alkyl-
4-carbomethoxypyridinyl radical dimers showed a CT band with
a peak at 650-670 nm only at low temperatures (ca. 100 K).50
The considerably red-shifted CT band with a peak at 1120 nm
observed in the present study most probably indicates an
increased electron delocalization due to a dicyanovinylstyryl
group attached at the 4-position of the pyridinyl radical instead
of a carbomethoxy group. It resembles the CT absorption at
1140 nm for aromatic imide radical anion dimers prepared by
electrochemical reduction in N,N-dimethylformamide (DMF).53
The very fast rise in the nanosecond laser pulse width strongly
suggests that the electronic interaction between a photogenerated
dicyanovinylstyrylpyridinyl radical and a parent dicyanovinyl-
styrylpyridinium ion occurs without a diffusion process. A
similar very fast rise was observed for the nitro-substituted
styrylpyridinium salts.23 No suggestion of aggregation was
noted for the dicyanovinyl- or nitro-substituted styrylpyridinium
ions as might be indicated by spectral shift or broadening in
these solutions before excitation. MM2 calculations of two pairs
The stabilization energy (∆H) of the dimer radical cation is
roughly equal to half the energy of the observed CR transition
as schematically shown in Figure 12. A longer wavelength shift
of the CR band from 950 to 1050 nm indicates a slight decrease
of ∆H from 15.0 to 13.6 kcal/mol for the nitrostyrylpyridinyl

CR and CT Interactions of Dicyanovinylstyrylpyridinyl
J. Phys. Chem., Vol. 100, No. 22, 1996 9375
Figure 15. Optimized geometry of two pairs of DCS+C1TPB- based
on MM2 calculations.
pairs, in a nonpolar solution.55 Further studies using femto-
second laser flash photolysis on the CR band formation and on
the effects of other electron-withdrawing substituents are in
progress.
Figure 13. Time profiles of transient absorptions at (A) 633 and (B)
1064 nm for 0.1 mM DME solutions of DCS+C16TPB- excited by the
third harmonic of a Nd:YAG laser.
Conclusion
The intermolecular interaction of 1-alkyl-4-[4-(dicyanovinyl)-
styryl]pyridinyl radicals formed by a photoinduced interionic
electron transfer reaction produced a specific CR band at 1050
nm and a CT band at 1120 nm. Using nanosecond laser flash
photolysis, the CR interaction was found to occur without
diffusion in solution. The present study will contribute to the
development of a new field of physical chemistry involving
stable radicals and their possible applications of photoinduced
electron transfer reactions to optical data processing.
Acknowledgment. We thank Prof. J. Kobayashi of Shizuoka
Institute of Science and Technology for the use of an ESR
spectrometer. This work was partly supported by a Grant-in-
Aid for Scientific Research on Priority Areas “Molecular
SuperstructuressDesign and Creation” (No. 07241102) and a
Grant-in-Aid for Scientific Research (No. 05453118) from the
Ministry of Education, Science, Sports and Culture, Japan.
References and Notes
(1) Nagamura, T.; Sakai, K. J. Chem. Soc., Chem. Commun. 1986,
810-812.
Figure 14. Time profiles of transient absorptions of a CR band
monitored at (A) 830 and (B) 1064 nm for 0.5 mM DME solutions of
NS+C16TPB- excited by the third harmonic of a Nd:YAG laser.
(2) Nagamura, T.; Sakai, K. Chem. Phys. Lett. 1987, 141, 553-557.
(3) Nagamura, T.; Sakai, K. Ber. Bunsen-Ges. Phys. Chem. 1988, 92,
707-710.
(4) Nagamura, T.; Sakai, K. J. Chem. Soc., Faraday Trans. 1 1988,
84, 3529-3537.
of DCS+C1TPB-, which were optimized by MM2 and MOPAC,
gave the geometry shown in Figure 15. Two DCS+C1 ions are
located within about 0.57 nm of each other through a TPB-
counterion. Electronic interactions are hardly expected at such
distance as indicated by the absence of any spectral shifts.
However, once electron transfer has occurred from TPB- to
DCS+C1, generating CT interactions, and TPB- is oxidatively
decomposed. The mutual separation distance will then decrease,
since a styrylpyridinyl radical will become an electron donor
to a styrylpyridinium ion. These situations will give a very
fast rise to the CR band without diffusion. Diffusionless
intermolecular electron transfer was reported to occur from the
butyl tris(p-tert-butylphenyl)borate anion to an excited N,N′-
diethyl-2,2′-cyanine cation, which were self-associated as ion
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