Experimental study of end-capped acceptors influencing the photo-physical,electrochemical and thermal parameters of A-D-π-D-A type small molecularelectron donors

We have designed A-D-π-D-A type three new small molecular donor materials denoted as FD-IND, FD-DCV andFD-NBR, which have been synthesised and their photo-physical, electrochemical and thermal properties explored.The synthesised materials have fluorene as “π-spacer,” dithieonopyrrole as “Donor” (D) and various “Acceptor” (A) unitsare 1,3-indanedione (IND), dicyanovinylene (DCV) and N-butyl rhodanine (NBR). All these three materials have goodsolubility (~30 mg/mL) in most common organic solvents and have relatively broad absorption in the visible regioncovering the range of 300-650 nm with two primary absorption bands. The absorption bands located at shorter wavelengthregion are ascribed to a localized aromatic π-π* transition of the A-D-π-D-A system, while the bands at longer wavelengthare mainly attributed to an ICT transition. All the compounds exhibit excellent thermal stabilities in the range of 260-373°C.Cyclic voltammetry data provided HOMO values as −5.26, −5.32 and −5.32 eV for FD-NBR, FD-DCV and FD-INDrespectively. LUMO values estimated from HOMO values and optical band gap are −3.27, −3.22 and −3.37 eV forFD-NBR, FD-DCV and FD-IND respectively. Therefore, these are potential molecules for organic solar cells.