870 Bull. Chem. Soc. Jpn. Vol. 81, No. 7 (2008)
Structures and Degree of CT of D–ꢀ–A Compounds
ꢁꢃ
electron-donors.19 ꢁþ–ꢀ–A molecules having long alkyl or
D
thioacetylalkyl groups form Langmuir–Blodgett (LB) films1
and self-assembled monolayers,5 and unimolecular electrical
rectification has been observed in these films.1–4 Hybrid LB
films composed of I1–3CNQ-R compounds (R = pristine
and (MeO)2) and clay have also been studied for second har-
monic generation.20 Determination of the molecular structure,
crystal packing, and ꢁ value is essential to explain the micro-
scopic electronic structure of the solids and films for the re-
ꢁꢃ
search of functional materials based on Dꢁþ–ꢀ–A type uni-
molecular organic molecules; however, structural analyses of
ꢁꢃ
D
ꢁþ–ꢀ–A molecules with long alkyl groups have rarely
been reported.10b
Scheme 1. Synthetic procedures for In–3CNQ.
In this study, we prepared a series of In–3CNQ derivatives
with a wide range of alkyl chain lengths (n ¼ 1{8, 10, 14, 16,
18, 20, and 22, R ¼ H) and investigated their thermal, redox,
optical, and structural properties. To elucidate the functionali-
ties of Dꢁþ–ꢀ–Aꢁꢃ compounds, a comprehensive discussion of
not only the electronic structure of a single molecule but also
the molecular arrangement in the solid state is crucial. In this
context, we discuss (1) the crystal packing of In–3CNQ deriv-
atives with respect to the cooperation between dipole–dipole
and/or face-to-face interactions of the D–ꢀ–A skeletons and
self-aggregation of the long alkyl chains, and (2) the estima-
tion of ꢁ values of In–3CNQ-R molecules in the solid state.
As for the estimation of ꢁ values, we utilized the intramolecu-
lar bond lengths in crystal structure analysis and compared the
estimated ꢁ values to those evaluated from molecular orbital
calculations. The effect of molecular conformations on these
values is also discussed.
piperidine, and then in situ generated indoline III reacted with
TCNQ under reflux giving In–3CNQ (Scheme 1, Method 1).
The reaction using isolated III, which was purified by extraction
after neutralization, also gave In–3CNQ (Scheme 1, Method 2).
Solvents were dried and distilled under of nitrogen prior to use,
and the reactions were performed in a nitrogen atmosphere. The
appearance, melting point, and optical properties of In–3CNQ
derivatives are summarized in Table 1.
Typical Procedures for the Preparation. Method 1: Prep-
aration of I4–3CNQ: 2,3,3-Trimethyl-3H-indole (I) (3.18 mL,
20.0 mmol) and 1-iodobutane (2.3 mL, 20 mmol) were refluxed
in ethyl acetate (10 mL) for 20 h. After cooling, ether (70 mL)
was added to the reaction mixture and stirred; the ether layer thus
formed was removed by decantation. The oily residue was washed
with a mixture of ethyl acetate and ether. The product was dried
under reduced pressure to give 1-butyl-2,3,3-trimethyl-3H-indo-
lium iodide (II) (3.18 g, 47%) in the form of a soft red powder.
Salt II was used for the next reaction without further purification.
Salt II (1.76 g, 5.13 mmol) was dissolved in PhCl (50 mL), piperi-
dine (0.52 mL, 5.26 mmol) was then added to the solution. To this
III solution, TCNQ (1.09 g, 5.34 mmol) dissolved in PhCl (80 mL)
was added, and the mixture was refluxed for 1 h. After evaporation
of the solvent, the crude product was purified by column chroma-
tography (CHCl3/ethyl acetate = 4:1, silica gel) and then recrys-
tallized from MeCN to give green platelet crystals of I4–3CNQ (4)
(304 mg, 15%).
Method 2: Preparation of I5–3CNQ: 1-Pentyl-2,3,3-trimeth-
yl-3H-indolium iodide salt II was prepared by the same procedure
as Method 1 (31% yield). Salt II (1.80 g, 5.03 mmol) was added to
a KOH aqueous solution (85%, 20 mL), and the mixture was stir-
red for 1.5 h. The yellow oily product was extracted with ether
(20 mL), and the organic extract was washed with a saturated
NaCl aqueous solution (20 mL ꢄ 3), and then dried over MgSO4
and concentrated under reduced pressure, to give neutral III
(89%). TCNQ (912 mg, 4.47 mmol) dissolved in PhCl (70 mL)
and crude III (1.00 g, 4.37 mmol) dissolved in PhCl (10 mL) were
refluxed for 7 h. After evaporation of the solvent, the crude prod-
uct was extracted with CHCl3, the solvent was then removed by
evaporation under reduced pressure. Green platelet crystals of
I5–3CNQ (5) (700 mg, 39%) were obtained by recrystallization
from MeCN (100 mL).
Experimental
Measurements and Calculations.
1H NMR spectra were
measured at 400 MHz on a JEOL JNM-FX400 spectrometer using
CDCl3 or DMSO-d6 as a solvent and tetramethylsilane as an inter-
nal standard. Elemental analyses were performed at the Center for
Organic Elemental Microanalysis, Kyoto University. Crystalliza-
tion, melting, and decomposition temperatures were measured us-
ing differential scanning calorimetry (DSC) thermograms (1 or
10 K minꢃ1 cooling/heating rate) on a Shimadzu-60 instrument,
equipped with nitrogen cryostatic cooling. Cyclic voltammetry
was measured in acetonitrile (MeCN) containing 0.1 M (1 M =
1 mol dmꢃ3) of n-Bu4NþBF4ꢃ with Pt electrodes vs. SCE (saturat-
ed calomel electrode) at a scan rate of 50 mV sꢃ1 using an ALS/
chi Electrochemical Analyzer model 650A, operated at room tem-
perature. Ultraviolet–visible (UV–vis) spectra were measured on a
Shimadzu UV-3100 spectrometer in dry solvents (chlorobenzene
(PhCl), acetone, MeCN, and methanol (MeOH)), KBr pellets, or
on quartz plates. Infrared (IR) spectra were measured using a
Perkin-Elmer Paragon 1000 in KBr pellets or on quartz plates (res-
olution of 2 or 4 cmꢃ1). Semi-empirical molecular orbital calcula-
tions were performed using MOS/F V4 with INDO/S parameter-
ization coupled with a 20 dimension CI matrix, which are
sufficient to give approximately invariant dipole moments. Geo-
metrical parameters were extracted from the crystal structures.
X-ray Crystal Structure Analyses. The crystal structures of
In–3CNQ derivatives, I2–3CNQ (2), (I3–3CNQ)(MeCN) (3a),
(I3–3CNQ)(C6H6)0:5 (3b), I4–3CNQ (4), I5–3CNQ (5), I6–
3CNQ (6a and 6b), (I6–3CNQ)(MeCN) (6c), I7–3CNQ (7a and
7b), (I8–3CNQ)(C6H6)0:5 (8), I10–3CNQ (10), and I20–3CNQ
(20), were determined. The intensity data of the structural analy-
Preparation of In–3CNQs.
In–3CNQ derivatives were
ꢁꢃ
prepared by methods used for 3CNQ-type Dꢁþ–ꢀ–A
sys-
tems.1,10b,11 2,3,3-Trimethyl-3H-indole (I) and alkylhalide
(CnH2nþ1X, X ¼ Br or I) were reacted to produce 1-alkyl-2,3,3-
trimethyl-3H-indolium halide II. Salt II was neutralized with