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

Abstract

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 π-conjugated effect enhances, OPA maximum wavelength (λ⁽¹⁾_max) shows a significant red-shift with increasing oscillator strength (ƒ_0k), and the TPA cross-section value (δ_max) also increases, the δ_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 δ_max such as transition dipole moments (M_0k and M_kn), the product of oscillator strengths (ƒ_0k × ƒ_kn), and the number of π 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.