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COMMUNICATION
RSC Advances
DOI: 10.103J9o/Cu4rRnAa1l0N30a7mF e
triflate 1 (77%) that was subsequently treated with ethylenediamine
in the presence of Pd(PPh3)4 in DMF to form the desired 5H- Fig. S6 shows UV/vis absorption and solution photoluminescence
Pyrazino[2,3-b]indole 2 (70%). This transformation was supported (PL) spectra of CzPhPz in various solvents under room temperature.
(1H and 13C NMR) by the appearance of peaks at 8.40, 8.48 ppm and The UV/vis absorption spectra in 1,3-Dioxane, CH2Cl2, and DMSO
141.6, 141.8 ppm (protons and carbons of C2 and C3 position), solvents are almost irrespective of solvent polarity. It implies that the
respectively. Synthesis of 5-(3-(9H-carbazol-9-yl)phenyl)-5H- Franck-Condon excited-state is subject to a rather small dipolar
pyrazino[2,3-b]indole (CzPhPz; 4) began with commercially change with respect to the ground state. However, the peaks in the
available 9H-carbazole, which was N-arylated with 1,3- solution PL spectra moved significantly to longer wavelength
dibromobenzene using Ullmann coupling reaction to give 3 (47%) according to increasing the solvent polarity. This phenomenon
that was subsequently treated with 5H-Pyrazino[2,3-b]indole to form should be explained by a mechanism involving rapid photoinduced
the desired host material, CzPhPz (33%). The synthesis of the electron transfer between the carbazole donor and the pyrazino
1
expected host materials was confirmed by H and 13C NMR, mass indole acceptor units, resulting in a large change in the dipole
data. (See Fig. S1 and S2 in ESI†)
moment in the excited state; a subsequent solvent relaxation process
To compare the electronic character of building blocks used in this leads to the solvent polarity-dependent emission.8 Plotting the
study, the molecular simulation of the carbazole and pyrazino indole emission peak frequencies of CzPhPz in various organic solvents as
moieties were carried out. (See in Fig. S3 and Table S1 in ESI†) As
a function of solvent polarities, we obtained almost linear
shown in Fig. S3, the LUMO distribution of pyrazino indole was relationship together with a slope as steep as -4179 cm-1, consistent
more dispersed than that of the carbazole unit. This means that the with our assignment of a charge-transfer emission (see Fig. S7 in
LUMO of pyrazino indole is stabilized compared to that of carbazole ESI†).
unit due to electron withdrawing character of symmetric unsaturated Photophysical properties of CzPhPz was estimated by using UV/vis
imine nitrogen (C=N) structures in the heterocycle. Furthermore, the and PL spectrometer. UV/vis absorption, solution PL and low
increased intramolecular charge transfer (ICT) characters of temperature PL emission spectra of CzPhPz are shown in Fig. 2. The
pyrazino indole unit make the energy of HOMO and LUMO UV/vis absorption spectrum of CzPhPz shows strong absorption
stabilized and the energy gaps between HOMO and LUMO decrease, peaks corresponding to π–π* absorption of the carbazole linked
which would make the photoluminescence spectrum red shifted. pyrazino indole backbone appeared below 300 nm. Also, the host
(See in Fig. S4 in ESI†) It can be presumed from the molecular material exhibits weak absorption bands between 310 nm and 370
simulation results that the electron accepting properties of pyrazino nm assigned to n–π* transition of the carbazole and pyrazino indole
indole moiety is better than that of carbazole moiety.
moiety.9 The triplet energy of CzPhPz was 2.83 eV, which could be
The thermal properties of CzPhPz were investigated by means of calculated from the first phosphorescent emission peak of low
DSC under a nitrogen atmosphere. The glass transition temperature temperature (77 K) PL spectrum at 438 nm. The triplet energy was
(Tg) of the host material was obtained from the second heating scan high enough to use the host material as a blue phosphorescent dopant.
of the cooled glassy sample after the first up to their melting The triplet energy of the bis[2-(4,6-difluorophenyl)pyridinato-
temperature (Tm). Its Tg and Tm were found to be 83 oC and 188 oC, C2,N](picolinato)iridium(III) (FIrpic) dopant is 2.62 eV10 and
respectively. (See in Fig. S5 in ESI†) The Tg is higher than that of the effective energy transfer from synthesized host material to FIrpic
N,N'-dicarbazolyl-3,5-benzene (55 oC)7 due to an enhanced dopant is expected.
intermolecular interaction between electron-rich carbazole and On the basis of the evaluated electrochemical oxidation onset
electron-deficient heteroaromatic moiety. It is even higher than Tg of potential, the HOMO level of CzPhPz was estimated as -6.20 eV. Its
o
9-(3-(9H-carbazol-9-yl)phenyl)-9H-pyrido[2,3-b]indole (75 C) with LUMO level was -2.94 eV, calculated from the HOMO level and
pyridine instead of pyrazine.5(c) One additional nitrogen of pyrazine energy bandgap determined from the UV/vis absorption threshold.
strengthens the intermolecular interaction and increases the Tg of the
host material.
Fig. 1 shows spatial distributions of the HOMO and LUMO of the
CzPhPz that calculated with the Gaussian 03 program at the
B3LYP/6-31G(d) level, using the density function theory (DFT) for
the geometry optimizations. Triplet energy was calculated as the
energy difference between the first excited triplet state and the
ground singlet state. The calculated HOMO energy of CzPhPz was
estimated to be -5.45 eV, and the LUMO energy was -1.65 eV. We
also calculated the triplet energy level to be 2.94 eV using the same
basis set, suggesting an effective triplet exciton confinement of
phosphorescent blue emitters in CzPhPz. As shown in Fig. 1, the
Fig. 2 UV/vis, solution PL and low temp. PL spectra of CzPhPz
(emission was excited at 340 nm).
HOMO of CzPhPz was localized on the carbazole unit, while the
LUMO of CzPhPz was dispersed over the pyrazino indole unit. The
HOMO and LUMO of CzPhPz were separated due to the hole
transport character of carbazole and electron transport character of
pyrazino indole unit.
CzPhPz-based hole- and electron-only devices were fabricated to
compare the hole and electron density in the host material. Fig. 3
shows the current density-voltage curves of hole- and electron-only
devices bearing CzPhPz. The electron current density of CzPhPz was
much higher than the hole current density, suggesting electron
transport character of the CzPhPz host material. Strong electron
deficiency of pyrazine unit of 5H-pyrazino[2,3-b]indole moiety
contributed to the electron transport character of CzPhPz. Although
CzPhPz was developed as a bipolar host material, electron transport
character was stronger than hole transport character.
Fig. 1 HOMO and LUMO distribution of CzPhPz.
2 | J. Name., 2012, 00, 1-3
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