| Molecular assembly is a very important process since it could form high ordered and special functional supramolecular system. Assemblied molecules changed to some extent or even essentially in the electronic structure, geometry and performance, which are closely related to the band position, intensity and spectral shape, etc. in molecular vibrational spectroscopy and surface-enhanced Raman spectroscopy (SERS). Thus, the two spectra above provide valuable information for understanding mechanism of assemblies, shape and performance of substrates and variations of orientation on the substrate surface.It was found that 4,4'-bipyridine (BPy), 4, 4′-azopyridine (AZPY), trans-1,2-bis(4-pyridyl)-ethylene (t-BPE) have execellent Raman scattering signals and can form self-assemblies film on the silver foil substrate for SERS exploration. The molecular geometry and vibrational frequencies of BPy, AZPY and t-BPE in the ground state and Surface-enhanced Raman Scattering (SERS), were calculated using density functional theory (DFT) of B3LYP, BP86 and Pw91 method, potential energy distributions (PED) and Amsterdam Density Functionl (ADF). The results provides valuable information for understanding the vibrational spectra and SERS of BPy AZPY and t-BPE molecule.1. The molecular geometry and vibrational frequencies of BPy in the ground state were calculated using DFT methods (B3LYP) with 6-31++G(d,p) basis set. The results of the optimized molecular structure suggest no largerπ-bond conjugated system, but there areπ-bonds conjugated system andπ-electron delocalization in the pyridyl rings. Between pyridyl ring with D2 symmetry, BPy is skewed by about 37.8o with using density functional B3LYP/6-31++G(d,p) methods. On the basis of the DFT and PED computed results, FTIR, FT-Raman and NIR-SERS bands were assigned. It is shown that the calculated and observed frequencies are in good agreement with each other for FTIR and FT-Raman spectra. On the basis of the SERS surface selection rule, it was concluded that the orientation of BPy on the silver substrate is upright. It was concluded Raman activity that BPy is skewed by about 0, 10, 20, 30 and 37.8o. Some vibrational frequency which are sensitive to the planar or non-planar structure of BPy, and to the dihedral angle were concluded.The SERS spectra of BPy on silver foils were studied at different excitation wavelengths of 488, 514.5 and 1064 nm, respectively. The Raman shifts in the observed SERS spectra of BPy were simulated by DFT calculation using the B3LYP functional with Lanl2dz and 6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set for the Ag-BPy, Ag3-BPy and Ag4-BPy complex. The Raman bands of BPy were assigned on the basis of the calculation results of PED. The DFT calculated result also showed that the angles between two pyridyl rings for BPy, Ag-BPy, Ag3-BPy and Ag4-BPy were skewed by about 37.81o, 38.44o, 35.50o and 37.1o, respectively. The energy gap between the HOMO and LUMO is expected to occur at 415 to 912 nm for BPy-Ag complex. Which is far away from the 1064 nm laser line, we confirm that chemical enhancement mechanism give a clear influence in the SERS of BPy at 1064 nm excitation.The relative intensity of Raman bands observed with different excitation laser lines also supports the above conclusion. Therefore, the good agreement between the Raman spectra calculated for BPy-Ag and experimental results.2. In the second part, the molecular geometry and vibrational frequencies of AZPY were calculated using the density functional theory (DFT) methods. The results of the optimized molecular structure suggest a strong conjugation for AZPY with using B3LYP/6-31++G(d,p) level. It is shown that the calculated and observed frequencies are in good agreement with each other for FTIR and FT-Raman spectra. On the basis of the Gaussview, FTIR, FT-Raman and NIR-SERS bands were assigned. Surface selection rules were employed to infer the orientations of AZPY on the silver foil substrate surface.The SERS spectra of AZPY on silver foils were studied at excitation wavelengths of 1064 nm. The Raman shifts in the observed SERS spectra of AZPY were simulated by DFT calculation using the B3LYP functional with 6-31++G(d,p) and 6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set for Ag-AZPY, Ag3-AZPY, Ag4-AZPY , Ag6-AZP and Ag7-AZPY complex with B3LYP/6-31++G(d,p)(C,H,N) /Lanl2dz(Ag) basis set. The Raman bands of AZPY were assigned on the basis of the calculation of PED. The DFT results showed that the angles between two pyridyl rings for Ag-AZPY, Ag3-AZPY, Ag4-AZPY Ag6-AZPY and Ag7-AZPY complex keep 0o. The energy gaps of the HOMO and LUMO from DFT were 363 to 1140 nm for AZPY-Ag complex. The calculated spectra of Ag4-AZPY and Ag6-AZPY complex were much closer to the experimental results of AZPY adsorbed on silver surface than that of Ag-AZPY complexes.3. To the best of our knowledge, this is the report on the vibrational spectra of t-BPE using the DFT with B3LYP/6-31++G(2d,p), BP86/6-31++G(2d,p), BPw91/ 6-31++G(2d,p) method. The vibrational frequencies obtained by DFT(B3LYP) are in good agreement with observed results. The results of the optimized molecular structure suggest a strong conjugation andπ-electron delocalization and a planar molecule for t-BPE with using B3LYP/6-31++G(2d,p) methods. According to the results of assignments on FTIR, FT-Raman spectra for t-BPE, the calculated and observed spectra of Raman and IR are in good agreement with each other. All FTIR, FT-Raman and NIR-SERS band of t-BPE were assigned on the basis of the Gaussview. On the basis of the SERS surface selection rules, it was concluded that the orientation of t-BPE on the silver substrate is upright.The SERS spectra of t-BPE on silver foils were studied at different excitation wavelengths of 514.5, 633, 785 and 1064 nm, respectively. The Raman shifts in the observed SERS spectra of t-BPE were simulated by DFT calculation using the B3LYP, BP86, BPw91 functional with Lanl2dz and 6-31++G(d,p)(C,H,N)/Lanl2dz(Ag) basis set for the Ag-t-BPE, Ag3-t-BPE, Ag4-t-BPE and Ag6-t-BPE complex. The B3LYP calculated spectra of t-BPE-Ag complexes were much closer to the experimental results of t-BPE adsorbed on silver surface than that of BP86 and Pw91. The Raman bands of t-BPE were assigned on the basis of the calculation results of PED. The DFT results showed that the angles between two pyridyl rings for Ag-t-BPE, Ag3-t-BPE, Ag4-t-BPE and Ag6-t-BPE complex keep 0o. The energy gap between the HOMO and LUMO is expected to occur at 415 to 912 nm for t-BPE-Ag complex. The relative intensity of Raman bands observed with different excitation laser lines was some differ. The ratio of chemical enhancement increase as the wavelength increases. We confirm that chemical enhancement mechanism give a clear influence in the SERS of t-BPE at 1064 nm excitation.4. Geometrical structure and Raman spectra of t-BPE-Ag complex have been calculated using the Amsterdam Density Functional (ADF) program package. The Becke-Perdew (BP86) XC-potential and a triple-ξpolarized slater type (TZP) basis set from the ADF basis set library have been used, with the 1s-4p core kept frozen for Ag. The vibrational frequencies and normal modes are calculated within the harmonic approximation. Full geometry optimization and frequency calculations for both isolated molecule, metal cluster, and molecule-metal complex have been performed prior to the polarizability calculations. The Raman spectra of the Ag-t-BPE, Ag4-t-BPE, Ag6-t-BPE and Ag10-t-BPE complex were discussed by ADF. The results indicates that Raman intensity depend on molecular chemical environment include the band position of metal and the wavelength excitation lines. That Raman spectra using ADF simulated excitation lines of 514.5, 633, 785 and 1064 nm and observed frequencies are in good agreement with each other.
|
PDF Full Text Request