The electronic structures and spectra of conducting pentacene derivatives

Theoretical studies of pentacene and a series of its derivatives were performed using the PM3 and DFT methods. Based on B3LYP/6-31G(d) optimized geometries, the electronic, IR, and 13C NMR spectra of the derivatives were calculated using the INDO/CIS, PM3, and B3LYP/6-31G(d) methods, respectively. The energy gaps of the derivatives decreased as the chain length increased and electron-withdrawing substituents were introduced. The polymer based on pentacene, especially in the presence of trimethylsilylacetylene, offers promise as an excellent conducting polymer. The main absorption bands in the electronic spectra of the derivatives compared with those of pentacene were shifted to the red, whereas the IR frequencies for some of the C=C and C-H bonds were shifted to the blue. The 13C chemical shifts of the carbon atoms connected with electron-withdrawing substituents were shifted upfield, while those of the bridged carbon atoms in the middle part of the pentacene unit shifted downfield.