Synthesis of Viologens with Extended π-Conjugation and Their Photochromic Behavior on Near-IR Absorption

Full text of DOI:10.1021/jo990911v

J . Org. Chem. 2000, 65, 593-595
593
Syn th esis of Viologen s w ith Exten d ed
π-Con ju ga tion a n d Th eir P h otoch r om ic
Beh a vior on Nea r -IR Absor p tion
Masato Nanasawa,* Makio Miwa, Michiko Hirai, and
Tetsuo Kuwabara
Department of Applied Chemistry and Biotechnology,
Yamanashi University, Takeda 4, Kofu 400-8511, J apan
Received J une 4, 1999
Colorless viologens composed of diquaternary salts of
4,4′-bipyridine turn to colored radical cations with reduc-
ing agents1 or by electrochemical reaction,2 whose colored
species spontaneously disappear in the liquid phase,
because the rate of back electron transfer is rapid and
accelerated by the existence of oxygen in the system.
When viologens were dispersed in appropriate aprotic
matrixes such as poly(N-vinylpyrrolidone) (PVP) or lay-
ered as organized solid films, photogenerated radical
cations were stabilized by the surrounding solid matrix
through restriction of both thermal reverse electron
transfer and air oxidation. The resulting colored species
exist with visible lifetime (redox photochromism).3,4 The
absorption of viologen radical cation, its photosensitivity,
and the bleaching rate depend on the kind of viologens,
the polymer matrixes employed, and film preparations.
Photochromism involving an IR absorption maximum is
required for some purpose, e.g., IR reading technology.5,6
Described herein is the synthesis of two groups of
viologens with extended π-conjugation (Scheme 1) and
their peculiar photocolor development in the near-IR
region.
F igu r e 1. Cyclic voltammetry of (a) 4, (b) 5, and (c) 6 in DMF
(electrolyte Bu4NCl (0.1 M), glassy carbon electrode, scan rate
500 mV s-1, versus Ag/AgCl, at 25 °C).
Sch em e 1
N-Aryl viologens (1-3) were synthesized by the method
adopted for N-phenylpyridinium chloride:7 N,N′-quater-
nization of 4,4′-bipyridine with 2,4-dinitrochlorobenzene
and subsequent amine-exchange reaction with aryl-
amines. Although unstable polydienylideniminium in-
termediates might be formed during amine-exchange
reaction, compounds 1-3 could be obtained in relatively
high yield. Compound 5 was synthesized by the Wittig
reaction with 4-bromomethylpyridine and glycosal, fol-
lowed by the Menschutkin reaction with benzyl bromide.
The vinylene protons in 4,4′-di(4-pyridyl)butadiene present
an AA′BB′ system in the 1H NMR spectrum, and the
compound exists as the E,E-form. The proton signals of
conjugated double bonds shift markedly to lower field by
the quaternization of both pyridine rings (0.7 ppm for
compound 4; 1.11 and 0.45 ppm for HR and Hâ in 5),
indicating positive charge on nitrogen atoms to be delo-
calized throughout π-conjugated system.
mide (6); however, the first redox potential shifts ca-
thodically by 170 mV relative to that of 6, and 5 exhibits
the single reduction potential (E1/2 ) -0.57 V, Figure 1).
The radical cation in 6 is stabilized by delocalization
within the π-conjugated system of vicinal pyridinium
rings, while the ethylene bond between two rings might
reduce the electron-withdrawing character against an-
other ring, and the stability of the radical cation through-
out two pyridinium rings resulted in the shift of the first
reduction potential to the negative direction.8
The absorption maxima of 1-3 shift to longer wave-
length compared with that of 6 (Table 1); however, the
shift decreased with N-substitutions with large molecular
size despite an increase in the contribution of the
resonance structures. The bulky aryl group reduces the
planarity with the pyridinium ring, which hinders the
The shape of the cyclic voltammogram of 4 is quite
similar to that of N,N′-dibenzyl-4,4′-bipyridinium dibro-
* To whom correspondence should be addressed. Fax: 81-552-
208772. E-mail: nanasawa@ab11.yamanashi.ac.jp.
(1) Kosower, E. M.; Cotter, J . L. J . Am. Chem. Soc. 1964, 86, 5524.
(2) Watanabe, T.; Honda, K. J . Phys. Chem. 1982, 86, 2617.
(3) Kamogawa, H.; Masui, T.; Nanasawa, M. Chem. Lett. 1980, 1145.
(4) Nanasawa, M.; Matsukawa, Y.; J in, J .-J .; Haramoto, Y. J .
Photochem. Photobiol., A 1997, 109, 35.
(5) Irie, M.; Miyatake, O.; Sumiya, R.; Hanazawa, M.; Horikawa,
Y.; Uchida, K. Mol. Cryst. Liq. Cryst. 1994, 246, 155.
(6) Yokoyama, Y.; Tanaka, T.; Kurita, Y. Chem. Lett. 1991, 1125.
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92, 5641.
(8) Kunitake, M.; Nasu, K.; Manabe, O.; Nakashima, N. Bull. Chem.
Soc. J pn. 1994, 67, 375.
10.1021/jo990911v CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/29/1999

594 J . Org. Chem., Vol. 65, No. 2, 2000
Ta ble 1. Absor p tion Ma xim a a n d P h otoin d u ced Color Develop m en t of Con ju ga ted Viologen s
Notes
viologen
λ
max, nm (ꢀ) (H2O)
λmax nm, (abs), immediately after irradiationa (PVP matrix)
ú
1/2,b min
1
2
3
4
5
6
337 (1.84), 296 (1.20), 250 (1.48)
277sh (3.80), 271 (3.87)
291 (1.0), 246 (2.08)
324 (3.85), 339sh (2.50)
382 (4.49), 365 (5.41)
259 (2.96)
445 (1.84), 600 (0.89), 650 (1.02)
390 (1.79), 441 (1.51), 646 (1.36)
418sh (3.11), 585 (1.71), 642 (1.60)
524 (1.54), 716 (0.59), 796 (0.97)
631 (1.96), 944 (0.99
42
31
39
18
41
34
400 (1.95), 569sh (0.90), 605 (1.07)
a
b
3 min of irradiation. Numbers in parentheses are the absorbance normalized to 0.01 mm thickness. Half-life of the highest near-IR
absorption peak at 25 °C.
transfer with the counteranion radical and air oxidation
is largely restricted in the solid state, because the
reductive anion radical might be oxidized by surrounding
polymer functions to some extent,12,13 and the later air
oxidation proceeds with diffusion control in the solid
state; thus, the colored species exist for a long time. The
absorption peaks of radical cations of N-aryl viologens
shift about 40 nm to longer wavelength compared with
that of 6, presumably due to participation of the N-aryl
group in delocalization of the radical cation. The red shift
decreases upon increasing N-aryl conjugation from tolyl
to naphthyl and biphenyl as mentioned above, whereas
the absorption peaks of 4 and 5 radical cations appear
in the near-IR region, and the shifts of 5 amount to 230
nm for the absorption peak at 400 nm and up to 340 nm
for the 605 nm peak of a conventional viologen radical
F igu r e 2. Typical visible absorption spectra developed by
cation. The shift value per ethylene bond is much greater
light for PVP films. The measure of absorbance is arbitrary
than those of ionic polyeneazomethines (100 nm),10
only to show the shapes of the absorption spectra.
indicating that the delocalization of the radical is ex-
tremely large in the ionic π-conjugated system. Although
a bleaching process proceeds by the reverse electron
transfer to the counteranion radical or by air oxidation,
there is no relationship between E1/2 and the half-life
(ú1/2), suggesting that the bleaching proceeds predomi-
nantly by air oxidation, with diffusion control in the
polymer matrix.
Exp er im en ta l Section
All reagents were of commercial quality. 1H NMR spectra were
recorded at 60 and 400 MHz. Cyclic voltammetry was conducted
with a glassy carbon working electrode, Ag/AgCl reference
electrode, and Pt-wire auxiliary electrode in DMF containing 0.1
M tetrabutylammonium chloride.
Gen er a l P r oced u r e for N,N′-Dia r yl-4,4′-bip yr id in iu m
Dich lor id e (1-3). The solution of 4,4′-bipyridine (3.2 g, 20
mmol) and 2,4-dinitrochlorobenzene (14.0 g, 70 mmol) in anhy-
F igu r e 3. Bleaching of the absorption spectrum developed
by light for a PVP film containing compound 5. Numbers in
the diagram denote the bleaching time at 25 °C.
drous acetonitrile (MeCN, 60 mL) was heated to reflux for 72 h
with stirring, and then diluted with MeCN (60 mL). The
resulting yellow precipitate was filtered, and washed with ethyl
ether to afford N,N′-bis(2,4-dinitrophenyl)-4,4′-bipyridinium
dichloride in 97% yield. To the solution of the dinitrophenylbi-
pyridinium dichloride (3.3 g, 6 mmol) in 50% aqueous ethanol
(EtOH,100 mL) was added arylamine (27 mmol) in EtOH (150
mL) dropwise with stirring, the reaction mixture being stirred
at room temperature for 25 h, and then concentrated in vacuo.
The residue was washed with ethyl ether and dissolved in water
(50 mL). The reaction mixture was heated at 100 °C for 24 h,
an insoluble byproduct was filtered off, and then the filtrate was
lyophilized. The residue was recrystallized from EtOH-ethyl
acetate (1:2) to give a pale yellow powder.
π-conjugation into N-aryl groups.9 While the red shift of
vinylene viologens (4, 5) increased with increasing num-
ber of ethylene bonds, the shift per ethylene bond is
estimated to be an intermediate value between neutral
R,ω-diphenylpolyenes and ionic cyanine dyes.10
Transparent PVP films of 1-6 colored varying from
blue to green upon near-UV irradiation (Figure 2, Table
1), whose color gradually bleached, reached the original
one within a couple of hours (Figure 3). A photoinduced
coloration in the polymer matrix is considered to proceed
in a way similar to that in solution:11 the viologen radical
cation might be mainly formed by intramolecular electron
transfer from the counteranion.3,12 The reverse electron
1: yield 83%; mp 290 °C dec; IR (KBr) 1620 (CdC), 1430
(CdN) cm-1. Anal. Calcd for C24H22N2Cl2: C, 70.41; H, 5.42; N,
6.84%. Found: C, 70.27; H, 5.57; N, 6.67%.
2: yield 60%; mp 285 °C dec. Anal. Calcd for C30H22N2Cl2: C,
74.85; H, 4.61; N, 5.82. Found: C, 74.87; H, 4.73; N, 5.78.
3: yield 74%; mp 273 °C dec. Anal. Calcd for C34H26N2Cl2: C,
76.55; H, 4.88; N, 5.25. Found: C, 76.40; H, 5.11; N, 5.14.
(9) Elliot, C. M.; Freitag, R. A.; Blaney, D. D. J . Am. Chem. Soc.
1985, 107, 4647.
(10) Rao, C. N. R. Ultra-violet and Visible Spectroscopy, Chemical
Application, 2nd ed.; Butterworth & Co.: London, 1967.
(11) Ebbesen, T. W.; Ferraudl, G. J . Phys. Chem. 1983, 87, 3717.
(12) Vermenlen, L. A.; Thompson, M. E. Nature 1992, 358, 656.
(13) Kaneko, M.; Yamada, A. Makromol. Chem. 1981, 182, 1111.

Notes
1,2-Bis(N-ben zyl-4-p yr id in iu m )eth ylen e Dibr om id e (4).
J . Org. Chem., Vol. 65, No. 2, 2000 595
gas through the EtOH solution gave a yellow precipitate, which
was collected and then dissolved in H2O. The solution was
basified with Na2CO3 (pH > 10) and CH2Cl2 extraction followed
by solvent evaporation gave 1,4-di(4-pyridyl)butadiene. The
product was recrystallized from MeOH-H2O (1:1) to give color-
less needles: yield 18%; mp 160-161 °C (lit. mp 158-161 °C).
N-benzylation was conducted in a manner similar to that of 4,
and the product 5 was recrystallized from H2O: yield 30%; mp
185 °C dec; IR (KBr) 1645 (CdC) cm-1. Anal. Calcd for C28H26N2-
Br2: C, 61.10; H, 4.76; N, 5.09. Found: C, 61.04; H, 4.80; N,
5.19.
1,2-Di(4-pyridyl)ethylene was prepared according to the litera-
ture,15 and the E-form was isolated by recrystallization from
hexane-benzene. N-benzylation of 1,2-di(4-pyridyl)ethylene was
conducted by refluxing with benzyl bromide in MeCN, and the
product 4 was recrystallized from methanol (MeOH)-ethyl ether
(1:3): yield 80%; mp 245 °C dec. Anal. Calcd for C26H24N2Br2:
C, 59.56; H, 4.61; N, 5.34. Found: C, 59.34; H, 4.74; N, 5.19.
1,4-Bis(N-ben zyl-4-pyr idin iu m )bu tadien e Dibr om ide (5).
1,4-Di(4-pyridyl)butadiene was prepared by the method adapted
from Carsky:16 To the suspension of 4-pyridylmethyltriph-
enylphosphonium bromide (4.34 g, 10 mmol) in CH2Cl2 (15 mL),
prepared from triphenylphosphine and 4-pyridylmethyl bromide,
were added a 40% aqueous solution of glyoxal (0.32 mL, 5.5
mmol) and then 50% NaOH solution (5 mL) under nitrogen
atmosphere. The reaction mixture was stirred for 1 h at room
temperature, and then diluted with CH2Cl2 (25 mL) and H2O
(25 mL). The organic layer was concentrated in vacuo, and the
residue was dissolved in anhydrous EtOH (50 mL). Passing HCl
P r ep a r a tion of P olym er Th in F ilm s a n d P h otoch r om ic
Mea su r em en ts. The solution of viologens 1-6 (3.0 × 10-6 mol)
and PVP (Mn 20 000, 0.2 g) in water (0.2 mL) was spread
homogeneously on a glass plate (1.3 × 3.8 cm2), followed by
drying in a dark place to afford a transparent film ranging in
thickness from 6 to 9 µm, measured by a digital micrometer.
The film thus prepared was stored in a dark desiccator held at
58% relative humidity (saturated NaBr solution) at least
overnight before use, and then irradiated at a distance of 10 cm
with a 75 W mercury lamp for 3 min through a UV cutoff filter.
The resulting color development was determined by transmission.
(14) J in, J . J .; Haramoto, H.; Nanasawa, M. Macromol. Rapid
Commun. 1999, 20, 135.
(15) Katsumoto, T. Bull. Chem. Soc. J pn. 1960, 33, 1376.
(16) Carsky, P.; Hunig, S.; Stemmler, I.; Scheutzov, D. Liebigs Ann.
Chem. 1980, 291.
J O990911V