Organic Fluorescent Molecule with High Solid State Luminescent Efficiency
maximum recovered to 441 nm (Figure S4b). Further-
more, a pH titration method was used to tell the re-
sponse in different pH value (Figure S4). It can be found
that with increasing [HCl], the absorption maximum is
slightly red-shifted, together with the enhancement and
red-shift in the emission spectrum.
Conclusions
In summary, we present a novel fluorescent mole-
cule BDP2VB which emits faintly in solution, but be-
comes highly emissive in the crystalline state, demon-
strating its AIE property. According to single-crystal
X-ray diffraction, the molecule adopts a twisted con-
formation due to the steric effect, indicating that there
are no face-to-face π-π interactions and the J-type ag-
gregation is formed, which is beneficial to induce high
crystal fluorescence efficiency. Besides, its emission can
be switched by protonation of acid-stimuli. Theoretical
calculations suggest that the change in electron-with-
drawing ability of the pyridinyl moiety and molecular
conformation upon protonation is responsible for the
bathochromic shift in the fluorescence emission. Be-
cause of the remarkable color-changing properties,
BDP2VB may be a potential candidate for applications
in sensing and optical recording.
The Highest Occupied Molecular Orbital (HOMO)
and the Lowest Unoccupied Molecular Orbital (LUMO)
of BDP2VB are calculated based on B3LYP/6-31G**
and the results are shown in Figure 5. It reveals that the
HOMO is mainly located on the divinyl benzene, while
the LUMO of BDP2VB is partially distributed over the
molecule. The delocalization of electron cloud helps to
stabilize the molecule in the excited state.[15] Since the
protonation of nitrogen atoms enhance the electron-
withdrawing ability of pyridine, the calculated results of
one protonated molecule with H+ show clearly that the
LUMO is spread over the whole molecule, and the elec-
tron cloud density on pyridine is increased. The single-
crystal structure of BDP2VB provided in Figure 2b
shows that the molecule adopts a twisted conformation
due to the steric effect between the aryl rings, and the
vinyl twist from the phenyl core by 44.4° and 21.0°.
Whereas, after protonation, its conformation becomes
more planar, as obtained by theoretical calculation, the
torsional angles become smaller (4.8° and 2.8°). Previ-
ous study shows that molecule with a more flat confor-
mation also contributes to the delocalization. To sum up,
the delocalization of electron cloud contributes to the
bathochromic shift in the emission of protonated
BDP2VB.
Acknowledgement
This work was supported by 973 Program (No.
2013CB 834702), the National Natural Science Founda-
tion of China (Nos. 21074045, 21204027, 21221063),
the Research Fund for the Doctoral Program of Higher
Education of China (No. 20120061120016) and the
Project of Jilin Province (No. 20100704).
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Figure 5 The theoretical calculated frontier orbitals contribu-
tion of BDP2VB and protonated BDP2VB.
Chin. J. Chem. 2013, 31, 1418—1422
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