Electronic structure of light-induced lophyl radical derived from a novel hexaarylbiimidazole with π-conjugated chromophore

Article abstract of DOI:10.1039/b203814e

A novel photochromic hexaarylbiimidazole with a bithienyl group as an extended π-conjugation unit was synthesized and the light-induced lophyl radical was found to be stabilized due to the delocalization of an unpaired electron, and to strongly absorb near-infrared light.

Full text of DOI:10.1039/b203814e

Electronic structure of light-induced lophyl radical derived from a novel
hexaarylbiimidazole with p-conjugated chromophore
Azusa Kikuchi, Tomokazu Iyoda and Jiro Abe*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1
Minami-Ohsawa, Hachioji, Tokyo 192-0397, Japan. E-mail: jiro@ecomp.metro-u.ac.jp; Fax: +81 426 77
2821; Tel: +81 426 77 2833
Received (in Cambridge, UK) 19th April 2002, Accepted 23rd May 2002
First published as an Advance Article on the web 12th June 2002
A novel photochromic hexaarylbiimidazole with a bithienyl
group as an extended p-conjugation unit was synthesized
and the light-induced lophyl radical was found to be
stabilized due to the delocalization of an unpaired electron,
and to strongly absorb near-infrared light.
known procedure (Scheme 2).1 2T-HABI exhibited the ex-
pected reversible photochromic behavior, which showed low
sensitivity to the presence of oxygen, both in solution and in
solid state. The yellow benzene solution of 2T-HABI showed
the absorption at 393 nm corresponding to the bithienyl
chromophore. On irradiation of the benzene solution with 340
nm light, the yellow solution quickly turned to dark violet and
gave rise to two maxima at 534 nm and 862 nm (Fig. 1). The
absorption spectrum has a characteristic broad band in the near-
infrared region as well as a second band. This is the first
example of a lophyl radical absorbing light in the near-infrared
region. On standing in the dark, the dark violet color gradually
faded with the decrease in these absorbances showing isosbestic
points. Thermal reversion to the original spectrum was observed
over the course of approximately 2 hours at 293 K in the dark,
suggesting a relatively high stability for the lophyl radical even
Hexaarylbiimidazole (HABI) was first synthesized by Hayashi
and Maeda in 1960.1 HABIs are readily cleaved, both thermally
and photochemically, into a pair of triarylimidazolyl radicals
called lophyl radicals that can recombine to form the lophine
dimer. The high yield of lophyl radicals in solid matrices and
their low sensitivity to the presence of oxygen stimulated
industrial interest in the use of HABI as a photoinitiator and
photosensitizer for a wide variety of imaging materials and
resulted in extensive investigations of substituted HABIs.2
Especially, 2-chlorohexaarylbiimidazole (o-Cl-HABI) has been
used as a free radical initiator in photopolymer imaging for
more than three decades. Although the photochromic behavior
of HABI and its derivatives in solution and solid matrices has
been extensively studied by spectroscopic methods, there is
practically no knowledge of the molecular and electronic
structures of the lophyl radicals and the pair of lophyl radicals
with an extended p-conjugation unit.3 We have recently
reported an unprecedented example of the in situ direct
observation of a light-induced radical pair (RP) in a crystal of o-
Cl-HABI by X-ray diffraction,4 and described the first example
of the measurement of intermolecular exchange coupling for the
light-induced RP by ESR measurement.5 Moreover, we have
investigated the electronic structures for the RP by density
functional theory.5 The spin density of the lophyl radical is
found to be mainly delocalized over the imidazolyl ring. These
studies could reveal both the molecular and electronic structures
for the RP, and would lead to a better understanding of HABI
chemistry and radical-pair chemistry. In this report we describe
the synthesis of a family of a novel HABI with a bithienyl
chromophore as an extended p-conjugation unit, and investigate
the electronic structures of both the lophyl radical and the light-
induced RP. This represents the first photochemical properties
for the novel HABI system with a p-conjugated chromophore,
which provides a new type of lophyl radicals which strongly
absorb near-infrared light.
HABI derivative 2T-HABI was prepared by oxidation of the
corresponding lophine derivative, 2T-lophine, according to
Scheme 2 i. Pd(OAc)2, PPh3, N(C2H5)3–CH3CN, 120 °C, 7 days, yield
(30%); ii. benzyl, CH3COO2NH4+–CH3COOH, 130 °C, 1 day, (30%); iii.
K3[Fe(CN)6], KOHaq–benzene, rt, 1 day, (96%).
Scheme 1 Reversible photodissociation of HABI into two lophyl radi-
cals.
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Fig. 1 Absorption spectra of 2T-HABI in benzene (a) before irradiation (b)
immediately after irradiation with 340 nm light at room temperature.
Concentration of 2T-HABI: 2.50 3 1025 mol l21
.
Fig. 3 ESR spectra of a UV-irradiated toluene solution of 2T-HABI at 77
K. The top spectrum is the detection of a forbidden weak transition at half-
field. The dotted curve shows spectral simulation. The simulation used giso
= 2.003 and Gaussian line shapes with varying line widths.
in solution at room temperature. The absorption bands of HABI
lie in the UV region at 278 nm in benzene, while the absorption
bands of the lophyl radical are in the visible region at 351 nm
and 574 nm.3 Thus, the extensively p-conjugated HABI system
shows a large bathochromic shift of the absorption band,
indicating an increase in the degree of delocalization of an
unpaired electron of the lophyl radical derived from 2T-HABI.
As shown in Fig. 2, the spin-density distributions calculated by
the DFT unrestricted Becke3LYP/6-31G(d) method clearly
indicate the penetration of the unpaired electron over the
bithienyl chromophore.†
Slow recombination of the lophyl radical favors the observa-
tion of radical pairs by ESR measurement under moderate
experimental conditions. As described before, we have esti-
mated the zero-field splitting (zfs) parameters for the light-
induced RP of known structure in a crystal of o-Cl-HABI.5 The
zfs parameters for a triplet RP are an extremely sensitive probe
of RPs since they are inversely proportional to the cube of the
distance separating the radical centers. Therefore, the ESR
spectrum for the light-induced RP was measured in order to
investigate the effect of the p-conjugated chromophore on the
electronic and molecular arrangement for the RP derived from
2T-HABI. The ESR spectrum of the UV-irradiated toluene
solution of 2T-HABI at 77 K shows a randomly oriented triplet
pattern with an overlapping doublet signal derived from a trace
of free radicals at g = 2.003 (Fig. 3). The detection of a
forbidden weak transition (DMs = 2) at half field indicates the
presence of a triplet-state species, which can be assigned to the
light-induced triplet RP. The zfs parameters for the triplet RP
are estimated to be ¡D/hc¡ = 0.0099 cm21 and ¡E/hc¡ = 0.0011
cm21 by simulation of the spectral line shape by the eigenfield
method.6 These values are comparable to the values estimated
for the light-induced RP in a crystal of o-Cl-HABI, ¡D/hc¡ =
0.0103 cm21 and ¡E/hc¡ = 0.0007 cm21. The average distances
between the two radical centers of the light-induced RP derived
from 2T-HABI and o-Cl-HABI, as derived from the point
dipole approximation, correspond to 6.40 Å and 6.26 Å,
respectively. These analyses suggest a similarity in the
molecular arrangement between them, and could give valuable
information about the effect of the p-conjugated chromophore.
Though the p-conjugated chromophore enlarges the region of
spin distribution, this effect is too small to induce the change in
the molecular interaction between the radicals.
The present result clearly demonstrates that HABI deriva-
tives with a p-conjugated chromophore show photochromic
behavior affording stable lophyl radicals with a large bath-
ochromic shift of the absorption band. Sensitization by
introducing an ambient p-conjugated chromophore could be
developed to increase the photospeed and extend the sensitivity
range of the HABI initiator system in free-radical-initiated
photoimaging systems.
This work was financially supported by a Grand-in-Aid for
Scientific Research (13022259) on Priority Areas, ‘Molecular
Synchronization for Design of New Materials System’ from the
Ministry of ESSC, Japan.
Notes and references
† Preparation of 2T-lophine: A mixture of 1.0 g (3.37 mmol) of
4-(2-[2,2A]bithienyl-5-ylvinyl)benzaldehyde, 708 mg (3.37 mmol) of benzyl
and 2.60 g (33.7 mmol) of ammonium acetate in acetic acid (40 ml) was
stirred. After stirring, the mixture was refluxed for an additional 1 day. After
this reaction period, aqueous ammonia was added until the solution was
neutral. After filtering, the residue was washed with hexane and purified by
column chromatography packed with silica gel in chloroform eluent and
recrystallized from chloroform to give the product as yellow powder in 30%
yield.
1 T. Hayashi and K. Maeda, Bull. Chem. Soc., 1960, 33, 565.
2 D. M. White and J. Sonnenberg, J. Am. Chem. Soc., 1966, 88, 3825; R.
L. Cohen, J. Org. Chem., 1971, 36, 2280; L. A. Cescon, G. R. Coraor, R.
Dessauer, E. F. Silversmith and E. J. Urban, J. Org. Chem., 1971, 36,
2262; B. M. Monroe and G. C. Weed, Chem. Rev., 1993, 93, 435; R.
Dessauer and C. Looney, Photogr. Sci. Eng., 1979, 23, 287; Y. Weidong,
Y. Yongyuan, W. Junshen, Z. Cunlin and Y. Meiwen, J. Photopolym. Sci.
Technol., 1994, 7, 187.
3 T. Hayashi and K. Maeda, Bull. Chem. Soc. Jpn., 1970, 43, 429; X.-Z.
Qin, A. Liu, A. D. Trifunac and V. V. Krongauz, J. Phys. Chem., 1991,
95, 5822; A. Liu, A. D. Trifunac and V. V. Krongauz, J. Phys. Chem.,
1992, 96, 207; J. V. Caspar, I. V. Khudyakov, N. J. Turro and G. C. Weed,
Macromolecules, 1995, 28, 636; D. Lavabre, G. Levy, J. P. Laplante and
J. C. Micheau, J. Phys. Chem., 1998, 92, 16; Y. Lin, A. Liu, A. D.
Trifunac and V. V. Krongauz, Chem. Phys. Lett, 1992, 198, 200.
4 M. Kawano, T. Sano, J. Abe and Y. Ohashi, J. Am. Chem. Soc., 1999,
121, 8106.
5 J. Abe, T. Sano, M. Kawano, Y. Ohashi, M. M. Matsushita and T. Iyoda,
Angew. Chem., Int. Ed., 2001, 40, 580.
Fig. 2 Spin-density distribution calculated by the DFT B3LYP/6-31G(d)
method.
6 Y. Teki, I. Fujita, T. Takui, T. Kinoshita and K. Itoh, J. Am. Chem. Soc.,
1994, 116, 11499.
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