Intramolecular charge-transfer characteristics of a series symmetric methoxy -substituted bi-1,3,4-oxadiazole derivatives with various substituted positions and quantities have been studied with a combination of experimental techniques and theoretical calculations to investigate the substituent effect. Different degrees of fluorescence red shift in polar solvents are observed in these compounds. The meta-substituted molecule (BOXD-m-OCH) exhibits a larger red shift (82 nm) than the other two monosubstituted molecules, BOXD-o-OCH (40 nm) and BOXD-p-OCH (37 nm); the polysubstituted molecules BOXD-D1 and BOXD-T1 show 80 and 104 nm red shifts, respectively, which are obviously larger than the monosubstituted molecules. The changes of molecular dipole moment between the ground state and charge transfer (CT) excited state are calculated to be on the same order with the degree of red shift (7.56 D in BOXD-o-OCH, 12.07 D in BOXD-m-OCH, 7.38 D in BOXD-p-OCH, 14.79 D in BOXD-D1, and 16.80 D in BOXD-T1). Theoretical calculations at the density functional theory level reveal that the first singlet excited state of all of these compounds shows both π-π* and CT characteristics and the charge has been proven to transfer from the terminal methoxy phenyl group to the central bioxadiazole group. The analysis of charge transfer based on electron density shows that the greater the amount substituent, the more charge would be involved in the intramolecular charge transfer. In addition, the negative barycenter has a tendency to locate close to the methoxy substituent, which would cause the difference in the charge-transferred distance. The transferred charge and CT distance work jointly and finally lead to differences in dipole moment variation. These findings could provide very good guidance for the design of molecules with intramolecular charge-transfer characteristics.
The
molecular packing, intermolecular interactions, and electron-transporting
and photophysical properties of a series symmetrical alkoxy-substituted
bis-1,3,4-oxadiazole derivatives (BOXD-o-OCH3, BOXD-m-OCH3, BOXD-D1, and BOXD-T1)
are carefully investigated through a combination of experimental techniques
and theoretical calculations here. The single-crystal structure analysis
reveals that all these single crystals exhibit similar layer structures,
while their molecular structures, displacement of the nearest adjacent
molecules in π-stacking, and molecular packing modes are effectively
tuned by changing the positions, amounts, or lengths of alkoxy groups
in these compounds. Careful analysis of intermolecular interactions
demonstrated that π–π intermolecular interactions
are the main forces in the formation of single crystal and that C–H···O
intermolecular interactions have a non-negligible contribution in
determining the distinguished packing modes of these single crystals.
BOXD-o-OCH3-α crystal exhibit a
maximum calculated electron mobilities value (5.2 cm2V–1S–1) among these selected crystals.
The absorption and emission maxima of these single crystals exhibit
different redshifts. In addition, these single crystals also exhibit
high fluorescence quantum efficiency. Our investigations would not
only provide an effective way to tuning crystal packing but also guide
the design of materials with excellent performance in a device.
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