Vibrational spectra study of phosphorus dendrimer containing azobenzene units on the surface

摘要

The FTIR and FT Raman spectra of the first generation dendrimers, possessing oxybenzaldehyde (G_1) or oxyphenylazobenzaldehyde (G _2) terminal groups and sodium 4-[4-oxyphenyl)azo]-benzaldehyde (SOAB) were studied. The structural optimization and normal mode analysis were performed for dendrimer G_2 on the basis of the density functional theory (DFT). These calculations gave the frequencies of vibrations, infrared intensities and Raman scattering activities for the E- and Z-forms of azobenzene unit. The energy differences between the E- and Z-forms are 12.62 and 25.16 kcal/mol for SOAB and G_2. The calculated in gas phase dipole moments for the E- and Z-forms are equal to 20.86, 18.28 D (SOAB) and 7.56, 8.88 D (G_2). The calculated geometrical parameters and harmonic vibrational frequencies are predicted in a good agreement with the experimental data. It was found that dendrimer G_2 molecule has a concave lens structure with planar OC_6H_4CHNN(CH3)PS and OC _6H_4NNC_6H_4CHO fragments and slightly non-planar cyclotriphosphazene core. The experimental IR and Raman spectra of dendrimer G_2 were interpreted by means of potential energy distributions. Relying on DFT calculations a complete vibrational assignment is proposed. The strong band 1598 cm~(-1) in the IR spectra show marked changes of the optical density in dependence of substituents in the aromatic ring. The differences in the IR and Raman spectra of SOAB and G_2 for the E- and Z-forms of azobenzene units were cleared up. During structural isomerization of azobenzene units, redistribution of band intensities appears to a much higher extent than frequency shifts.