Fluorine atom effect in D-A-D type conjugated small molecules and polymers

• Published in: Polymer (Guilford) 2024-07, Vol.307, p.127322, Article 127322
• Main Authors: Boztaş, Yalçın, Çakal, Deniz, Önal, Ahmet M.
• Format: Article
• Language: English
• Subjects: Benzothiadiazole; Donor-acceptor-donor type monomer and polymers; Fluorine atom effect; ProDOT
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2024.127322

A new series of monomers in the donor-acceptor-donor array was synthesized by coupling of mono- and di-fluorinated benzothiadiazole (BT) acceptor units with didecyl-substituted 3,4-propylenedioxythiophene (ProDOT (P)) donor unit to obtain two novel monomers, namely 4,7-bis(3,3-didecyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-6-yl)-5 fluorobenzo[c][1,2,5] thiadiazole (Pr2BT-1F) and 4,7-bis(3,3-didecyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-6-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole (Pr2BT-2F), in order to investigate the effect of fluorine atom substitution on the electrochemical and optical properties of highly processable monomers and their corresponding polymers. In addition to these two novel monomers, non-fluorinated analog, namely 4,7-bis(3,3-didecyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepin-6-yl)benzo[c][1,2,5]thiadiazole (Pr2BT-0F), was also synthesized for comparison sake. All monomers exhibit an intramolecular charge transfer band at a longer wavelength along with a notable blue shift in their absorption and emission spectra upon fluorine atom substitution. The monomers also demonstrate solvatochromic and strong fluorescent characteristics, which make them promising candidates to be amenable as fluorescent probes in cell imaging applications. The oxidation potential of the monomers is found to exhibit an anodic shift with the increasing number of fluorine atom substitutions. As in the case of monomers, the fluorine atom substitution also resulted in an anodic shift in the oxidation potentials of the corresponding polymers, leading to a raising in the band gap energy from 1.48 eV to 1.70 eV as a result of the decreased level of planarity in the conjugated backbone due to steric hindrance induced by fluorination. Moreover, a further increase in fluorination along with bulky didecyl alkyl chains has an adverse effect on the electropolymerization process for di-fluorinated analog. Nonetheless, mono-fluorine substituted analog indicates reversible doping/dedoping with an explicit electrochromic response for potential device applications. [Display omitted] •Novel D-A-D type monomers bearing fluorinated benzothidiazole with external ProDOT units were synthesized and characterized.•Effect of fluorination on both electropolymerization and optoelectronic properties was investigated. f•The bright emission of fluorinated monomers makes them potential candidates to be used as fluorescent probes.•Electrochemically obtained polymers exhibit the electrochro