Quantum Chemical Study of the OLED Materials Tris[4'-(1"-phenylbenzimidazol-2"-yl)phenyl] Derivatives of Amine and Benzene

Abstract

Since its discovery by Tang and Van Slyke and by Burroughes and co-workers, organic light-emitting diode (OLED) is under intensive study as a very promising material for low-cost large-area light-emitting display technology. Several types of OLED, including conjugated polymers, metal complexes, and dyes have demonstrated desirable optoelectronic properties, such the emitting wavelength and efficiency, thermal stability, and amorphous film formation property. Synthetic aspects of the OLED investigation have been largely on a trial and error basis, mostly depending on empirical rules, such as the prediction of emitting wavelengths based on HOMO and LUMO energy levels. If the relationship between the optoelectronic properties and molecular structures of OLED is systematically understood, designing of the light-emitting materials would be more facile and economical. Quantum chemical investigations that have tremendously affected synthetic chemistry through studies of structureproperty relationship and underlying mechanism seem to have been much less helpful for studying OLED materials in this respect, probably due to the large size of molecules. Recent progress in quantum chemical methods, especially the advent of density functional theory (DFT) and timedependent DFT (TDDFT), now allows systematic calculations for the structure and properties of OLED materials both in electronically ground and excited states to provide invaluable knowledge for photoabsorption and emission, and charge carrier mobility. In the present work, we study the tris[4'-(1''-phenylbenzimidazol-2''-yl)phenyl] derivatives of amine (TPBPA: tri[4(1'-phenylbenzimidazol-2'-yl)phenyl]amine) and benzene (TPBPB: 1,3,5-tris[4'-(1''-phenylbenzimidazol-2''-yl)phenyl]benzene) that were observed to have very useful emission properties. We describe their geometry and the electronic spectra, finding that the calculated electronic absorption and emission spectra of the two OLED materials agree very well with experimental observations. Structures of the electronic ground states are obtained by using the B3LYP (Becke’s three parameter hybrid method and the correlation functional by Lee, Yang and Parr) method with the 6-31g(d) basis set implemented in Gaussian 03 set of programs. The energy of the electronic excited states is calculated by using the TDDFT (B3LYP) and single-excitation CIS (CI-Single excitations) with the 631g(d) basis set. Stationary structures are obtained by verifying that all the harmonic frequencies are real. Default options are employed for all optimizations. No symmetry constraints are imposed during the optimizations.