Synthesis and properties of fluorene based small molecule acceptors containing aromatic malononitrile functionalities

Abstract

Four novel small molecule acceptors (SMA) with fluorene-based cores were synthesized with terminal phenyl, thienyl, and furyl malononitrile groups in good yields. The products were obtained from a two-step reaction of 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) with m- or p-bromobenzaldehyde, 5-bromo-2-thiophenecarboxaldehyde and 5-bromo-2-furaldehyde under Suzuki conditions. Subsequent reaction with malononitrile yielded 9,9-dioctylfluorene −2,7 (1-phenyl-3(or 4)-methylene malononitrile) (m-FPM or p-FPM), 9,9-dioctylfluorene −2,7 (2-thienyl-5-methylene malononitrile) (FTM) and 9,9-dioctylfluorene −2,7 (2-furyl-5-methylene malononitrile) (FFM) respectively. Structural characterization of the compounds was performed using 1H and 13C NMR, and FT-IR. Optical and electronic characterization were done using, UV–visible and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations. The compounds exhibit absorption bands ranging from 300 to 550 nm. m-FPM has the least coplanar structure while FFM displays the most coplanar structure, which increases the effective π-conjugation length resulting in a red shift of the absorption bands. The HOMO/LUMO levels were determined by CV and supported by DFT calculations, with optical band gaps of 2.70 eV and 2.41 eV for m- and p-FPM and 2.21 eV and 1.86 eV for FTM and FFM respectively. Fluorescence quenching was observed when poly-3-hexyl thiophene (P3HT) and each of the four SMAs were mixed. Films of P3HT and each SMA were cast in 1:1 and 1:4 ratios respectively. AFM indicated the formation of continuous films and the impact of small molecule structure on film morphology. These combined results indicated the capability of all of the new molecules to act as small molecule acceptors.