Structure and properties of α-cyclo[N]thiophenes as potential electronic materials -: A theoretical study

Density functional theory (DFT) calculations of ring-shaped alpha-cyclo[N]thiophenes with N = 2 to N = 18 have been performed for ideal structures of high symmetry (point groups C-nv and D-nh) and for optimum energy structures of lower symmetry (D-2d, C-s, C-2v, C-i or C-1). Whereas the first three members of the series behave exceptionally the higher members are typical cyclothiophenes consisting of weakly interacting thiophene rings. In contrast to neutral compounds, cations and dications of cyclothiophenes with N >= 5 exhibit pronounced electron delocalizations along the carbon backbone. However, if the functional B3LYP is replaced by BH cations of large ring-size cations show polaron-type charge defects. According to broken symmetry DFT calculations dications with N = 14 and N = 18 have biradical character. These structures correspond to two-polaron-type structures rather than to dipolarons. The calculated vertical ionization energies of cyclo[N]thiophene are comparable with those of oligo[N]thiophenes of the same number of thiophene rings but the calculated absolute energies are probably too low at large ring size. Cyclothiophenes absorb light of lower energies than the related oligothiophenes. Cyclothiophenes belong to the strongly absorbing organic chromophores. In case of high molecular symmetry some of the excited states of cyclothiophenes are degenerate. The degeneracy is lifted with lower symmetries but the general absorption feature remains. The theoretical results are discussed with respect to recent experimental findings. Copyright (c) 2007 John Wiley & Sons, Ltd.