Synthesis of B <- N embedded indacenodithiophene chromophores and effects of bromine atoms on photophysical properties and energy levels

Developing novel fused pi-conjugated chromophores has been an energetic research realm and knowing how to adjust their photophysical properties and energy levels through structure tailoring is of pivotal importance. Herein, based on the ladder-type pi-conjugated indacenodithiophene (IDT) moiety, four B <- N embedded IDT structures, namely, BNIDT, BNIDT-2Br, BNIDT-4Br, and BNIDT-6Br are synthesized and fully characterized. The influences of B <- N unit embedded in the IDT backbone and Br atoms anchoring at the periphery on the photophysical properties and energy levels are discussed systematically. From IDT to BNIDT, a new intra-molecular charge transfer (ICT) transition band appears at lower energy (400-600 nm) in the absorption spectra with reduced optical bandgaps (E-g) from 3.25 eV to 2.11 eV and the fluorescence emission peaks red-shift from 390 nm to 565 nm along with remarkably extended fluorescence lifetimes from 1.2 ns to 12.4 ns due to the introduction of electron-deficient B <- N into the backbone. Further anchoring Br atoms at the periphery of the backbone gives rise to depressed optical bandgaps, decreased fluorescence quantum yields (Phi), and shortened fluorescence lifetimes (tau) from BNIDT (E-g = 2.11 eV, Phi = 0.46, tau = 12.4 ns), BNIDT-2Br (E-g = 2.08 eV, Phi = 0.18, tau = 4.9 ns), BNIDT-4Br (E-g= 1.67 eV, non-emission) to BNIDT-6Br (E-g = 1.61 eV, non-emission). The HOMO and LUMO levels estimated from ultraviolet photoelectron spectroscopy (UPS) and optical bandgaps also experience synergetic lowering from IDT to BNIDT-6Br. This work indicates that backbone modification with electron-deficient B <- N unit and side groups tailoring with halogen atoms are powerful to manipulate the optical properties and energy levels of fused pi-conjugated chromophores. (C) 2018 Elsevier Ltd. All rights reserved.