10468
J. Am. Chem. Soc. 2001, 123, 10468-10474
Proton Relay in a One-Dimensional Hydrogen-Bonded Chain
Composed of Water Molecules and a Squaric Acid Derivative
Hiroshi Terao,† Tadashi Sugawara,*,† Yasuo Kita,‡ Naoki Sato,‡ Eriko Kaho,§ and
Sadamu Takeda§
Contribution from the Department of Basic Science, Graduate School of Arts and Sciences,
The UniVersity of Tokyo, Meguro, Tokyo 153-8902, Japan, Institute for Chemical Research,
Kyoto UniVersity, Uji, Kyoto 611-0011, Japan, and DiVision of Chemistry, Graduate School of Science,
Hokkaido UniVersity, Sapporo 060-0810, Japan
ReceiVed February 26, 2001. ReVised Manuscript ReceiVed June 26, 2001
Abstract: The crystal structure of a hydrated crystal of bis(squaryl)biphenyl (BSQB‚4H2O), in which two
squaric acid moieties are connected with a 4,4′-biphenyl unit, was characterized by the presence of a one-
dimensional hydrogen-bonded chain composed of BSQB and water molecules. X-ray crystallographic analysis
showed that BSQB exists in a dianion form and that, on average, two of the four water molecules are protonated.
The enhanced temperature dependence of the thermal parameters of the oxygen atoms of the water molecules
suggested dynamic disorder of the water molecules. The solid-state magic angle spinning deuterium NMR
spectrum of BSQB‚4D2O revealed that deuterons are exchanged between heavy water molecules and oxonium
ions with an exchange rate of ca. 700 Hz around 250 K and that deuterons start to migrate in a hydrogen-
bonded cluster of water molecules. Ac dielectric measurements were also used to examine the dynamic process
in the hydrated crystal. The dielectric permittivity of the crystal dramatically increased above 250 K with a
distinct frequency dependence (ꢀ′ ) 4.7 × 104 at 340 K and 1 kHz). The frequency dependence of tan δ at
290 K exhibited a maximum at 3.0 kHz, and this maximum shifted to lower frequencies when the temperature
of the crystal decreased. These experimental results suggested that in the one-dimensional hydrogen-bonded
chain of BSQB‚4H2O a proton relay between oxonium ions and water molecules occurred within a cluster of
four water molecules and that the relay was transmitted to the adjacent cluster mediated by the modulation of
the negative charge distribution of the BSQB dianion. These phenomena were interpreted as the solitonic
migration of the charged domain boundaries along the one-dimensional hydrogen-bonded chain.
Introduction
transport. To understand the intrinsic nature of proton transport,
it is important to carry out model studies, but no such
investigations have been reported because of the difficulty of
constructing a 1D hydrogen-bonded chain composed of water
molecules.
Proton transfer along a one-dimensional (1D) hydrogen-
bonded chain of water molecules has drawn much attention not
only from the theoretical aspect of solitonic charge transport1
but also from the viewpoint of biological functions such as active
proton transport in membrane proteins, e.g., cytochrome oxidase2
and bacteriorhodopsin.3 A recent high-resolution X-ray crystal-
lographic analysis4 revealed that a number of water pools exist
between the amino acid residues of cytochrome c oxidase; these
clusters of water molecules play a crucial role in active proton
Along these lines, we are interested in a 1D hybrid hydrogen-
bonded chain composed of water molecules and a highly acidic
â-hydroxyenone, which can transport a proton from its hydroxyl
group to a carbonyl group of the adjacent molecule. Squaric
acid, which has two orthogonally located â-hydroxyenones, is
the sole example of a hydrogen-bonded crystal, which exhibits
intermolecular proton-transfer coupled with a polarity inversion
of the π-conjugated electronic system, even though the crystal
does not contain any water molecules. The crystal exhibits a
pronounced dielectric response above 371 K as revealed by
correlated polarity inversion.5 Expecting to observe a cooperative
intermolecular proton transfer in 1D hydrogen-bonded â-hy-
droxyenones involving water molecules, we synthesized a new
squaric acid derivative, bis(squaryl)biphenyl (BSQB), in which
two squaryl groups are separated by the 4,4′-biphenyl unit. For
BSQB to exhibit a 1D character in a hydrogen-bonded system,
the electronic coupling between the two squaryl groups in BSQB
* To whom correspondence should be addressed.
† The University of Tokyo.
‡ Kyoto University.
§ Hokkaido University.
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10.1021/ja010519a CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/05/2001