Influence of Molecular Structure on the Optical Property of POSS: a DFT Calculation Based Quantum Chemistry Calculation

Abstract

Polymorphic structure of polyhedral oligomeric silsesquioxanes (POSS) derived from hydrolytic condensation of vinyltrimethoxysilane was validated by the spectral characterization, such as FTIR, UV-MALDI-TOF MS, etc. The models of 3D-amorphous cubic unit cells of different numbers of Si atom and hydroxyls generated during the hydrolytic condensation and the length of the organic side chain (T6, T7, T8, T9, T10, T8(OH)2and T8(OH)4cells), as models were established with a materials visualizer module of Material Studio(MS) software. The cells were optimized the configuration and calculated energy band structure and absorption coefficient (a) with Castep module of MS which is the quantum chemistry program based on density functional theory (DFT). The calculation results showed that the band gap was reduced with the number of Si atoms in POSS increased, but the presence of hydroxyl could make a significant reduction of them. The reduction of band gap would lower the frequency of the incident light that was absorbed by the POSS and heighten the wavelength of the absorbed incident light, which was proved by the calculation results of absorption coefficient. Such reduction was discussed in comparison with the vinyltri- methoxysilane hybrid systems.