Highly broadband NLO response of acceptor-donor-acceptor materials with a planar conformation

Abstract

The third-order nonlinear optical properties of A-D-A structural molecules which are originally used as acceptors in organic photovoltaic (OPV) devices were investigated in solution and film to deeply understand the molecular conformation-property relationship. Firstly, a planar structure (O-IDTBR) bearing the indacenodithiophene (IDT) core and a twisted molecule (IDFBR) comprising the indenofluorene (IDF) core, which possess identical acceptor units, were measured by the Z-scan technique at the near-infrared region in toluene solution. O-IDTBR exhibits a more superior TPA cross section than IDFBR in the range of 800-1030 nm (ca. 1497-8811 GM). In particular, the TPA cross section of the planar configuration is 8811 GM, over 4 times that of the twisted molecule at 800 nm. Secondly, their corresponding spin-coating films are investigated via a 4f coherent imaging experiment at 532 nm. It is noticeable that the nonlinear refractive index of the planar molecule is up to 2.09-2.15 × 10-15 m2 W-1, which is nearly 5 times that of IDFBR. The dynamic processes and morphologies of films are systematically characterized by transient absorption spectroscopy and AFM. The investigations present a better understanding of the structure-property relationship of the A-D-A conjugated materials which are believed to have promising NLO application in the future.