Vibrational spectra and DFT studies of conjugation in poly(phosphorhydrazone) dendrimers

The FTIR and FT Raman spectra of the 11 generations of poly(phosphorhydrazone) dendrimers built from thiophosphoryl core with P−Cl (Gi) and oxybenzaldehyde (G'i) terminal groups have been studied. Spectral pattern of dendrimers depends on the ratio of the number of end groups and repeating units. This ratio tends to r-1 (r- functionality of the repeating unit) and becomes constant when the generation number exceeds 3. The experimental IR and Raman spectra of the different generations of dendrimers are very similar to each other in the agreement with the theory. Optimization of the structure and the analysis of normal vibrations was made for G'0 and G1 using the density functional theory (DFT). Calculations showed that the molecules of the first generation dendrimer have the form of a propeller with three blades and flat repeating units. Bond lengths and angles in the optimized geometry obtained by DFT show good agreement with experiment. Consideration of the molecular orbitals shows that in the repeating units and oxybenzaldehyde end groups of poly(phosphorhydrazone) dendrimers conjugation is realized. The oxybenzaldehyde terminal groups are characterized by a band 1702 cm−1 in the experimental IR spectra of G'i. The line in the Raman spectra of Gi and G'i at 1576 cm−1 refers to the vibrations of the repeating units.