CL-150036
Received: January 15, 2015 | Accepted: February 18, 2015 | Web Released: February 25, 2015
Structure Determination from Powder X-ray Diffraction Data
of Black Azo (Hydrazone) Pigments
1
1
2
2
Junji Otani, Michio Matsumura, Kotaro Fujii,* and Hidehiro Uekusa
1
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2
Department of Chemistry and Materials Science, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
(
E-mail: kfujii@cms.titech.ac.jp)
Crystal structures of two black monoazo (hydrazone)
pigments have been determined from powder X-ray diffraction
data combined with DFT calculations. The DFT calculations
suggested the molecules are in hydrazone forms but not in azo
forms in both crystal structures. The molecular arrangements in
the crystal structures suggested Davydov splitting may occur by
excitonic interactions and it causes the red absorption band shift
in the crystalline state leading to the characteristic black colors.
crystal structures, in addition to their broadly extended π-
conjugation systems inducing narrowing of the HOMOLUMO
gap which provides a red shift of light absorption and hopefully
allowing a variety of electron transitions. However, unfortu-
nately, we failed to prepare single crystals of these compounds
because of their low solubility and no sublimation property,
and we cannot clarify their crystal structures. In such case,
techniques for carrying out structure determination directly from
PXRD data (ab initio PXRD analysis) are clearly essential to
reveal crystal structures without preparation of single crystals.
Ab initio PXRD analysis for organic molecular materials has
Azo compounds are widely used as organic dyes and
1
35
pigments which usually show yellow to red color. Character-
been developed in recent years. In the present letter, we report
istic colors of azo compounds come from a large π-electronic
conjugation system which is extended by an N=N with
conjugated polyene moieties. In solid state, colors of organic
pigments are known to be altered by their crystal structures
the crystal structures of 1 and 2 which have been determined
by ab initio PXRD analysis combined with density functional
theory (DFT) calculation.
The monoazo-type pigments 1 and 2 were synthesized
according to the procedures described in the Supporting
Information. Synchrotron X-ray powder diffraction data of
black powdery products of 1 and 2 were recorded under ambient
conditions on beamline 4B2 (Multiple Detector System) at
the Photon Factory, Tsukuba, Japan, with a wavelength of
1.196734(3) ¡. The samples were introduced into 2 mm di-
ameter borosilicate glass capillaries and were used for the
measurements. The data collection time was 7 h.
(
i.e. molecular arrangement). For example, diketopyrrolopyrrole
derivatives are known to change their color from vivid red to
yellowish red depending on the molecular arrangement in their
crystal structures, even though they show no spectral difference
2
in solutions. Such color change by crystallization is attributed
to the excitonic interaction. In this background, crystal structure
is very important for understanding the nature of the color of the
pigments in the solid state.
Recently, we have succeeded in preparing new black mono-
azo pigments, 1-(4-dimethylaminophenylazo)-3-(naphtho[1,2-
d]thiazol-2-yl)naphthalen-2-ol (1) and 1-(4-dimethylaminophen-
ylazo)-3,6-bis(naphtho[1,2-d]thiazol-2-yl)naphthalen-2-ol (2),
which are black in the solid state (Figure 1). Monoazo pigments
are usually yellow to red, and characteristic black colors of these
compounds would be due to the molecular arrangements in their
Indexing of the powder X-ray diffraction data was carried
6
7
out using the program X-cell for 1 and DICVOL04 for 2.
The structure solution calculations were carried out using the
8
9
program DASH followed by the Rietveld structural refinement
1
0
by the GSAS program. In the structural refinement, standard
restraints were applied to the bond lengths and bond angles, and
a global isotropic displacement parameter was used. There are
two possible tautomeric forms for 1 and 2 (azo and hydrazone
forms) and it is difficult to distinguish the difference from the
powder X-ray diffraction data. Also, the IR spectra of 1 and 2
did not give a clear answer to the tautomeric forms (Figure S1
in the Supporting Information). Therefore, DFT calculations
(
B3LYP with a 6-31G* basis set) were carried out using the
program SPARTAN’08 (Wavefunction, Inc.) to estimate the
energy difference of both tautomeric forms. The energy dif-
¹
1
ferences were calculated to be 3.92 kcal mol
3
for 1 and
¹
1
.93 kcal mol for 2 and the hydrazone forms have lower
energies than the azo forms in both molecules. The detailed
investigation using single-crystal X-ray diffraction combined
with DFT calculation on the azohydrazone tautomerization
of similar compounds suggested the hydrazone forms were
11
generally more favorable than azo forms. In addition, the
literature reported that the energetic difference more than ca.
1 kcal mol gave complete ordering of the tautomer. In addition,
the calculated UVvis spectra of the hydrazone forms (green
Figure 1. Molecular structures of 1 and 2 in azo forms (upper
right) and hydrazone forms (upper left). The pictures show
powdery solids of these compounds.
¹
1
© 2015 The Chemical Society of Japan