Structure and optoelectronic properties of helical pyridine-furan, pyridine-pyrrole and pyridine-thiophene oligomers.

Density functional theory based calculations have been carried out to systematically investigate the structural and optoelectronic properties of pyridine-furan, pyridine-pyrrole and pyridine-thiophene oligomers. Comparison of results obtained at B3LYP/6-31G(d) and B3LYP-D3/6-31G(d) levels of theories reveals that the inclusion of dispersion correction with the B3LYP functional has a major impact on ground state structures and stabilities of the most stable conformers, which are helical for our studied systems. Calculation of stabilization energies, gained due to non-bonding interaction between adjacent helical turns, shows that stabilities of helical oligomers increase with an increase in the chain length. Ground state dipole moment values of these helical oligomers fluctuate between a certain range and these values depend on the number of repeating units (n) and the number of repeating units needed to complete one helical turn (u) of a helix. To obtain vertical excitation energies, oscillator strengths and absorption spectra of each oligomer, time dependent density functional theory single point calculations were carried out at the B3LYP-D3/6-31G(d) level using optimized geometries obtained at the same level. The absorption spectrum of a helical oligomer is composed of multiple electronic transitions having significant oscillator strengths and the transition with the largest oscillator strength is blue shifted with an increase in the size of the oligomer. Furthermore, for the most important electronic transition (S0→ Sm) of oligomers with n > u, m increases with increasing n. For these helices, excitations involving molecular orbitals other than frontier molecular orbitals significantly contribute to major electronic transitions.