Synthesis of regioisomerically pure piperidine substituted perylenebisimide NIR dyes: A comparative study of spectroscopic, electrochemical and crystalline properties

New regioisomerically pure green and blue dyes i.e. 1,7 and 1,6 piperidine substituted perylenebisimides (1A, 1B, 2A, 2B) were synthesized. Photophysical properties were compared with unsubstituted and brominated perylenebisimide analogues (1, 1PBr(2), 2, 2PBr(2)). Piperidine was substituted on the bay region after imidization of dibromoperylene-3,4,9,10-tetracaboxylic dianhydride which induces strong bathochromic shifts. Large bathochromic shifts lower the optical band gaps which is the key feature of NIR absorbing fluorescent dyes with improved photovoltaic efficiencies. All synthesized fluorophores were characterized by NMR and Mass spectrometry. Photophysical properties were evaluated by UV-Vis, fluorimetry, and cyclic voltammetry. Fluorescence quantum yields were estimated by adopting a gradient method. The well-defined sharp peaks in X-ray diffraction pattern confirmed the crystalline nature of 1A and 1B. In contrast 2A and 2B were found non-crystalline. Thermal analysis by TG-DSC further explored the stability and phase transitions in dyes. HOMO and LUMO molecular orbitals were depicted by using density functional theory (DFT) at B3LYP/6-31(d,p). Electrochemical behaviors probed by cyclic voltammogram revealed dominant reversible oxidation reactions for 1,7 regioisomers (1A, and 2A) while both reversible oxidations and reductions were observed in 1,6 isomers (1B and 2B). Other structural analogues 1, 1PBr(2), 2, 2PBr(2) displayed reversible reductions. Thus a regular trend of changes in optical and electrical properties was observed on bromine and piperidine substitution. The observed outstanding properties clearly justify that our compounds might be promising candidates for various future applications such as biosensors, biomarkers, semiconductors, charge transporters, and stable fluorescent dyes. (C) 2017 Elsevier Ltd. All rights reserved.