Fluorene substituted thieno[3, 2-b]thiophene – a new electrochromic conjugated polymer

A new electrochromic conjugated polymer, poly 2-(9,9-dicotyl-9 H-fluorene-2-yl)-3,6-bis(5-hexylthiophen-2-yl)thieno[3,2- b]thiophene), containing fluorene (F8-DT6) substituted thieno[3, 2-b]thiophene has been designed by Stille and Suzuki coupling reactions and then polymerized at different potentials through electrochemical polymerization. The as-synthesized polymer is characterized by nuclear magnetic resonance (NMR) spectroscopy, cyclic-voltammetry (CV), ultraviolet-visible spectroscopy (UV-vis), and atomic force microscopy (AFM). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels are −2.67 eV and −5.21 eV respectively, as determined by cyclic voltammetry. As the film, P(F8-DT6-TT) exhibits UV-vis at 502 nm, the bandgap values from electrochemical and optical measurements are in excellent agreement. The CIE colour coordinates L^*, a^*, and b^* of neutral and oxidized states are (76.81, −3.04, 67.46) and (68.90, −10.92, 32.04) respectively giving vivid orange colour in the bleached/oxidation state and a very light-yellow colour in the reduced state. P(F8-DT6-TT) exhibits better stability between −0.1 and 2 V and the stability pattern moves towards a stable position by increasing the number of cycles. AFM images reveal a higher root-mean-square value (51.83 nm) that is useful for providing a convenient optical response and film stability. Effective colour tuning by substituting fluorene (F8-DT6) in thieno[3,2-b] thiophene units is a notable property of P(F8-DT6-TT), which renders it an appropriate electrochromic material.