Time-dependent density functional theory study of electronic spectrum property for carbazolyl-pyridinyl alternating copolymers

Three A-B-type fluorescent copolymers comprised of alternating carbazolyl and pyridinyl units, poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(3,5-pyridinyl))](PEHCP-35), poly[(2,7-(N-(2-ethylhexyl)carbazolyl)-alt-(2,6-pyridinyl))] (PEHCP-26) and poly[(2,7-(N-(2ethyl-hexyl)carbazolyl)-alt-(2,5-pyridinyl))] (PEHCP-25), are studied by means of the density functional theory (DFT/B3LYP/6-31G). Based on the optimized geometries, the optical properties are calculated by employing time-dependent density functional theory (TD-DFT). The bandgaps and optical properties are saturated quickly in PEHCP-35 and PEHCP-26. It is known from experiment that PEHCP-25 is actually an oligomer with a polymerization degree of 4. So the tetramers of PEHCP-35, PEHCP-26, and PEHCP-25 are adopted to study the electronic and optical properties, and the calculated results are in close agreement with experiment. The calculated bandgaps of copolymers obtained from two ways, i.e., HOMO-LUMO gaps and the lowest excitation energies, decrease in the following order PEHCP-35 > PEHCP-26 > PEHCP-25, the same trend as the data obtained from the edge of the electric band but different from the electrochemically obtained data from experiment (PEHCP-25 > PEHCP-26 > PEHCP-35). The outcomes showed that, when excited, a charge transfer from carbazolyl unit to pyridinyl unit occurs, and the lumophor is mainly carbazolyl units. The UV absorption and emission wavelengths both exhibit bathochromic shifts: PEHCP-35 < PEHCP-26 < PEHCP-25. (c) 2006 Wiley Periodicals, Inc.