Investigation of Fluorine Atom Effect on Benzothiadiazole Acceptor Unit in Donor Acceptor Donor Systems

Abstract

Synthesis of two new π-conjugated monomers containing fluorine substituted benzothiadiazoles as acceptor units and 3,4-ethylenedioxythiophene (EDOT) as donor unit was accomplished by facilitating Stille cross-coupling reactions (4,7-bis(2,3-dihydrothieno[3,4-b][1,4] dioxin-5-yl)-5-fluorobenzo[c][1,2,5]thiadiazole (E 2 BTD-F) and 4,7-bis(2,3-dihydrothieno [3,4-b][1,4]dioxin-5-yl)-5,6-difluorobenzo[c][1,2,5] thiadiazole (E 2 BTD-2F)). NMR and HRMS spectra of newly synthesized compounds are in agreement with the structures. To compare fluorine substitution effects, non-fluorinated analogue (4,7-bis(2,3dihydrothieno[3,4-b][1,4]dioxin-5-yl)benzo[c][1,2,5]thiadiazole (E 2 BTD)) was also synthesized. Besides a blue shift in their absorption spectra, an anodic shift was also noted in the oxidation potentials of both monomers with fluorine atom substitution on the acceptor unit. This was further confirmed by DFT calculations such that upon fluorine substitution EDOT donor units deviate from planarity. Electropolymerization of the monomers were achieved successfully via potentiodynamic polymerization method. A similar trend in optoelectronic properties was also observed for the polymer films as for the monomers.

Effect of the donor units on the properties of fluorinated acceptor based systems

“…It was noted that both monomers and polymers undergo hyposchromic shift in their electronic absorption spectra upon fluorination of BT acceptor unit. This observation was explained in terms of deviation from planarity upon fluorination and also supported by theoretical calculations [26]. In this regard, in order to highlight heteroatom effect in the donor unit on fluorinated BT unit in terms of dihedral angle and planarity, two new D-A-D type monomers utilizing 5-fluorobenzo[c] [1,2,5]-thiadiazole as the acceptor unit and furan (F 2 BT-F) and selenophene (S 2 BT-F) with different heteroatom size as the donor unit have been synthesized.…”

“…The HOMO/LUMO energy levels of F 2 BT-F, T 2 BT-F, and S 2 BT-F were calculated as − 5.88 eV/-3.60 eV, − 5.94 eV/-3.61 eV and − 5.91 eV/-3.62 eV, utilizing the equations [26] given below, respectively. It can be seen that while LUMO energy levels do not change by changing the donor unit, HOMO energy levels show slight variations depending on the donor unit.…”

“…Although most of these articles are based on solar cell applications due to decreased highest occupied molecular orbital (HOMO) energy level with the effect of fluorination, there are limited number of studies investigating electrochromic properties of polymers containing fluorinated BT unit [14,[22][23][24][25]. In our previous work, the effect of fluorination was investigated on the BT moiety in terms of electrochromic features in D-A-D type monomer and polymer systems involving 3,4-ethylenedioxythiophene (EDOT) and thiophene donor units [26,27]. It was noted that both monomers and polymers undergo hyposchromic shift in their electronic absorption spectra upon fluorination of BT acceptor unit.…”

Benzothiadiazole vs. iso-Benzothiadiazole: Synthesis, Electrochemical and Optical Properties of D–A–D Conjugated Molecules Based on Them

“…The success of the BTD fragment as a building block for donor-acceptor materials lies in its high electron affinity, which determines the energy of the lowest unoccupied molecular orbital (E LUMO ). Various methods are known to decrease the LUMO energy of the acceptors by adding electronegative groups, such as fluorine [11,12]; by using conjugated electron-deficient heterocycles [13,14]; and by replacing carbon atoms in aromatic systems with more electronegative atoms, such as nitrogen [15,16]. Figure 1 shows that application of the above described approaches leads to a reduction of E LUMO from −0.71 eV for BTD to −0.89 eV for 5,6-difluorobenzo[c][1, 2,5]thiadiazole and −1.99 eV for benzo[1,2-c:4,5c ]bis[1, 2,5]thiadiazole.…”

Toward High-Performance Electrochromic Conjugated Polymers: Influence of Local Chemical Environment and Side-Chain Engineering

“…[ 32 , 33 ]. Önal et al reported 5-fluoro-2,1,3-benzothiadiazole-based ECPs [ 34 , 35 , 36 ], and analyzed the influence of a fluorine atom on the optical and electrical properties of a conjugated polymer. As we all know, alkyl side chains influence the redox behavior and electrochromic properties of the electrodeposited polymers [ 24 , 37 ].…”