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Structure–Property Study on Two New D–A Type Materials Comprising Pyridazine Moiety and the OLED Application as Host
In
this paper, two new pyridazine based donor–acceptor type
materials, i.e., 3CzPyaPy: 9,9′-(3-(6-(9H-carbazol-9-yl)Âpyridazin-3-yl)Âpyridine-2,6-diyl)ÂbisÂ(9H-carbazole)
and 4CzPyPyaPy: 3,6-bisÂ(2,6-diÂ(9H-carbazol-9-yl)Âpyridin-3-yl)Âpyridazine,
were synthesized with high yields. These two materials exhibited strong
absorption/emission with high molar extinction coefficients and moderate
photoluminescence quantum yield. The glass transition temperature
of 3CzPyaPy was detected to be as high as 131 °C, showing its
high thermal stability. Although the absorption energies and oxidation/reduction
behaviors of the two materials were similar, the emission from 4CzPyPyaPy
with longer effective-conjugation length presented hypsochromic shift
both in films and in dilute solutions, contradicting to the common
sense. The single crystal structure study disclosed their different
space stretching and packing: 3CzPyaPy was twisted in larger angles
and adopted dimerlike packing, while 4CzPyPyaPy showed smaller torsion
angles and exhibited slipped herringbone packing. The dimerlike packing
in 3CzPyaPy is responsible for its bathochromic shift of emission
in solid state, while its unsymmetrical molecular structure accounts
for that in solution. We believe that the unsymmetrical molecular
structure of 3CzPyaPy is partially responsible for its high thermal-stability
and also responsible for its HOMO dispersion which renders it slightly
more difficult to oxidize. 3CzPyaPy was proved to be a bipolar-transport
material and when served as a phosphor host, a green phosphorescent
device achieved maximum efficiencies of 54.0 cd A<sup>–1</sup>, 42.4 lm W<sup>–1</sup>, and 17.7%, which are among the best
with nonoptimized device structure, demonstrating its great potential
for optoelectronic application. Furthermore, the new synthesized pyridazine
derivatives and the corresponding structural and molecular-packing
influences on material properties give a new insight into molecule
tailoring