A theoretical study of the ring size effect on one- and two-photon absorption properties of macrocyclic thiophene derivatives

The equilibrium geometries, electronic structures, one-and two-photon absorption (OPA and TPA) properties of macrocyclic thiophene derivatives [C[6T_A_E](n)-Me (n = 1-3)] with different ring sizes have been investigated by quantum chemistry calculation. The results indicate that as the unit number increases, the p-conjugated effect enhances, OPA maximum wavelength (lambda((1))(max)) shows a significant red-shift with increasing oscillator strength (f(0k)), and the TPA cross-section value (delta(max)) also increases, the delta(max) value of C[6T_A_E](3)-Me is twentyfold larger than that of the repeat unit C[6T_A_E]-Me. We have analyzed the internal factors influencing delta(max) such as transition dipole moments (M-0k and M-kn), the product of oscillator strengths (f(0k) x f(kn)), and the number of pi electrons (Ne) in detail. In addition, it is demonstrated that intramolecular charge transfer (ICT) is one of the intrinsic factors, but not a dominant one for the increase in TPA cross-section with regard to this system.