| Краткое содержание: | Introduction
Tetrathiafulvalene (TTF) chemistry has a long history since
the first discovery of its conductivity in the 1970s.1 Interests in
the related molecules have spread into various fields conce
ed
with electroactive materials because of their potential for the
electronic devices such as field effect transistors (FETs),2 and
photovoltaic cells.3 Carrier generation is the primary conce
in order to obtain electroactive organic materials. The carriers
in molecular conductors4 are usually generated based on a
charge-transfer mechanism between the electron donor and
acceptor molecules, a mechanism that was proposed by Mulliken
in the 1950s.5 This mechanism is the underlying concept for
carrier generation in the current molecular conductors, and
thereafter has been extensively adopted for single-component
molecular conductors.6
On the other hand, protonation of aromatic compounds by a
Brønsted acid giving rise to radicals has been intensively studied
since the 1990s.7 This phenomenon also takes place with a
typical donor molecule, TTF, because of the reactivity of the
central carbon-carbon double bond toward a proton.8 In 1994,
Giffard et al. spectroscopically investigated the protonation of
TTF with Brønsted acids in solution and found the generation
† Waseda Institute for Advanced Study (WIAS), Waseda University.
‡ The University of Tokyo.
§ RIKEN.
# Consolidated Research Institute for Advanced Science and Medical
Care (ASMeW), Waseda University.
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10.1021/ja809425b CCC: $40.75 XXXX American Chemical Society J. AM. CHEM. SOC. XXXX, xxx, 000 9 A
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Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/ja809425b
of a radical species, TTF•+ from TTFH+. They proposed that
TTF•+ is generated via an electron transfer between protonated
TTFH+ and neutral TTF species,8b as shown in Scheme 1.
Furthermore, they have also reported that TTF crystals treated
with a Brønsted acid such as HBF4, exhibit a small electronic
conductivity of 10-10 S/cm, suggesting that TTF•+ produced via
the protonation of the TTF molecule could be the origin of the
conductivity. However, neither the chemical properties of TTF•+
nor its electronic state associated with the open-shell electron
in the crystalline state is as yet well-understood,8d because of
the lack of structural and molecular orbital information about
this radical species in the crystal.
We have studied functional crystalline organic salts composed
of ammonium and carboxylate ions, where formation of a onedimensional
hydrogen-bonding network plays a decisive role
in determining their physical properties.9 In the course of related
research, we have serendipitously found that an ammonium salt
of an acidic derivative of TTF, that is, ammonium tetrathiafulvalene-
2-carboxylate (TTFCOO-NH4
+), exhibits conductivity.
TTFCOO-NH4
+ is not a charge-transfer complex because NH4
+
has no electron-accepting property. Moreover, it is obvious that
TTFCOO-NH4
+ is not an ion-radical salt because the stoichiometry
of the ammonium and anionic TTF moieties are
equivalent in this salt. Although the protonation-triggered radical
generation of TTF originally proposed by Giffard et al. seems
a possible mechanism in this phenomenon, no Brønsted acid
was exte
ally added to the salt. Carrier generation in such
neutral closed-shell molecules is of particular interest and is
worth investigating in detail because of its remarkable potential
to produce unprecedented electroactive materials. In the present
paper, we report the chemical, structural, and electronic properties
of TTFCOO-NH4
+, together with molecular orbital characteristics,
focusing on the role of the hydrogen-bonding
interaction in the carrier generation. |
