Figure 3. Monitoring hydrophilic/hydrophobic
surface. The fluorescence spectra
of membrane (A) before and (B) after modification of PVDF membrane
staining by HBT-DPI ,
inset: diagram of contact angle measurement (left) and colored membrane
(right). (C) Stabilized packing mode of HBT-DPI adsorption on
the original PVDF, blue background: solvents (DCM). (D) Stabilized
packing mode of HBT-DPI adsorption on the modified PVDF, blue
background: solvents (DCM). (E) Topological parameters of the
interaction between PVDF (green background) or modified PVDF (yellow
background) and HBT-DPI . (F) HBT-DPI adsorbed on the
modified PVDF surface and the parameter of the N-H···O H-bond. Color
code: white, H; sky blue, C; yellow, S; blue, N; red, O; Orange, F.
Further theoretical calculations explored the molecular behaviors ofHBT-DPI on the model surfaces to explain the changed
fluorescent emissions (details of theoretical calculations are available
in section 14 of the supplemental information). Since the original PVDF
membrane lacks efficient HBAs, only weak H-bonds (C-H···F) formed
between HBT-DPI molecules and the membrane (Figure 3E), leading
to parallelly piled HBT-DPI molecules which emit green
fluorescence on the membrane surface (Figures 3C and S15A-D, Video S1).
In contrast, the abundant oxygen (Tables S1 and S2) on the modified PVDF
surface leads to the formation of abundant H-bonds (C-H···F, C-H···O and
N-H···O) with HBT-DPI molecules, resulting in much stronger
affinity between the modified surface and HBT-DPI (Figures 3E,
S16 and S17). Especially, the bond energy and length of the N-H···O bond
is calculated as -4.27kcal/mol and 2.06 Å (Figures 3F and S18, Table
S3), providing dominant interactions to trigger the cross-stacking mode
of molecular assembly (Figures 3D and S15E-H, Video S2), which
corresponds to a yellow fluorescence according to the aforementioned
SXRD analysis (Figure 2E).