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).