350
H. Katagiri
17.4.3 Crystal Structures and Optical Properties
of the Terazulene Isomers
Single crystals of the above-mentioned terazulenes obtained by sublimation were
subjected to X-ray crystallography, which revealed that TAz1 and TAz2 are planar,
while TAz3 and TAz4 have structures that are twisted about their 6,6
-bonds by about
50° (Fig. 17.8). The 6,6
-bond was also found to be twisted by about 50° in an isolated
molecule structurally optimized by computational methods; this twist is believed to be
caused by steric hindrance. On the other hand, the optimized structure of an isolated
molecule connected through a 2,6
-bond was twisted by about 30°, which suggests
that the planar structures observed in the crystals of TAz1 and TAz2 are due to
intermolecular interactions in the single crystals. In addition, the optimized structure
of the 2,2
-connected biazulene is planar, as observed in the crystal structures of
TAz2 and TAz3. All terazulene isomers formed edge-to-face herringbone packing
structures and were found to be packed in a layered manner along each molecular long
axis. These structures also adopt the typical packing structure observed in molecules
with large aspect ratios.
The optical properties of the terazulene isomers are summarized in Table 17.1.
The IPs of films obtained by atmospheric photoelectron spectroscopy ranged from
−5.45 to −5.56 eV; these values are not significantly different from the oxidation
potential of azulene itself (E 1/2 = 0.54 V vs. Fc/Fc
+ ) [30, 31] determined by cyclic
voltammetry and the HOMO energy (−5.18 eV) determined by quantum chemical
calculations. Therefore, the HOMO energy is largely unaffected by oligomerization.
Furthermore, the energy gaps calculated from the absorption spectra were 1.56–
1.63 eV, with LUMO energies calculated on the basis of these energies ranging from
−3.83 to −3.93 eV. Consequently, lower LUMO energies were determined to be
responsible for the smaller energy gaps; we conclude that this outcome is a major
feature of azulenes structurally expanded at their 2- and 6-positions.
17.4.4 OFET Properties and Thin-Film Structures
of the Terazulene Isomers
The FET characteristics of TAz1 are shown in Fig. 17.9. Interestingly, the TAz1 topcontact FET device showed only n-type semiconductor characteristics with a charge
mobility of 0.29 cm
2 /Vs, while the TAz2 and TAz3 devices exhibited ambipolar
characteristics, with both n-type and p-type polarities, and TAz4 showed only n-type
characteristics with a charge mobility of 0.15 cm
2 /Vs. These results clearly reveal that
the polarity of the semiconductor depends on the azulene orientation. In particular,
TAz2 showed ambipolar character when an Al electrode was used, but only p-type
character when an Au electrode was used, with a hole mobility of 1.32 cm
2 /Vs. The
unipolar p-type behavior of TAz2 is attributable to the higher LUMO energy of TAz2
compared to those of the other isomers.
H. Katagiri
17.4.3 Crystal Structures and Optical Properties
of the Terazulene Isomers
Single crystals of the above-mentioned terazulenes obtained by sublimation were
subjected to X-ray crystallography, which revealed that TAz1 and TAz2 are planar,
while TAz3 and TAz4 have structures that are twisted about their 6,6
-bonds by about
50° (Fig. 17.8). The 6,6
-bond was also found to be twisted by about 50° in an isolated
molecule structurally optimized by computational methods; this twist is believed to be
caused by steric hindrance. On the other hand, the optimized structure of an isolated
molecule connected through a 2,6
-bond was twisted by about 30°, which suggests
that the planar structures observed in the crystals of TAz1 and TAz2 are due to
intermolecular interactions in the single crystals. In addition, the optimized structure
of the 2,2
-connected biazulene is planar, as observed in the crystal structures of
TAz2 and TAz3. All terazulene isomers formed edge-to-face herringbone packing
structures and were found to be packed in a layered manner along each molecular long
axis. These structures also adopt the typical packing structure observed in molecules
with large aspect ratios.
The optical properties of the terazulene isomers are summarized in Table 17.1.
The IPs of films obtained by atmospheric photoelectron spectroscopy ranged from
−5.45 to −5.56 eV; these values are not significantly different from the oxidation
potential of azulene itself (E 1/2 = 0.54 V vs. Fc/Fc
+ ) [30, 31] determined by cyclic
voltammetry and the HOMO energy (−5.18 eV) determined by quantum chemical
calculations. Therefore, the HOMO energy is largely unaffected by oligomerization.
Furthermore, the energy gaps calculated from the absorption spectra were 1.56–
1.63 eV, with LUMO energies calculated on the basis of these energies ranging from
−3.83 to −3.93 eV. Consequently, lower LUMO energies were determined to be
responsible for the smaller energy gaps; we conclude that this outcome is a major
feature of azulenes structurally expanded at their 2- and 6-positions.
17.4.4 OFET Properties and Thin-Film Structures
of the Terazulene Isomers
The FET characteristics of TAz1 are shown in Fig. 17.9. Interestingly, the TAz1 topcontact FET device showed only n-type semiconductor characteristics with a charge
mobility of 0.29 cm
2 /Vs, while the TAz2 and TAz3 devices exhibited ambipolar
characteristics, with both n-type and p-type polarities, and TAz4 showed only n-type
characteristics with a charge mobility of 0.15 cm
2 /Vs. These results clearly reveal that
the polarity of the semiconductor depends on the azulene orientation. In particular,
TAz2 showed ambipolar character when an Al electrode was used, but only p-type
character when an Au electrode was used, with a hole mobility of 1.32 cm
2 /Vs. The
unipolar p-type behavior of TAz2 is attributable to the higher LUMO energy of TAz2
compared to those of the other isomers.