Synthesis and thermal, electrochemical, and photophysical investigation of carbazole/diphenyl benzothiadiazole-based fluorophores

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Herein, we describe the syntheses of symmetric and asymmetric benzothiadiazole (BTD)-based fluorophores via the Sonogashira coupling reaction, using 4,7-dibromobenzothiadiazole as the central core and propargyl carbazole and/or propargyl diphenylamine as the side groups. The synthesized fluorophores exhibited absorption bands in the ultraviolet region, with a maxima between 372 and 380 nm due to the π→π* electronic transitions. Although they presented very low fluorescence quantum yields in solution, they demonstrated typical aggregation-induced emission phenomenon. Their oxidation potentials ranged from 0.91 to 1.18 V vs. NHE and reduction potentials from −1.20 to 1.26 V vs. NHE. Their HOMO energies ranged from −5.35 to 5.62 eV electron affinity from −3.18 to 3.24 eV. Thus, the electrochemical bandgap of the BTD-based fluorophores was in the range of 2.16–2.38 eV. The symmetric BTD derivatives 3 and 5 exhibited good thermal stability, with decomposition temperatures at ~330 °C and 200 °C, respectively. BTD 7 showed lower thermal stability with a decomposition temperature of ~110 °C. BTD 3 and 5 showed glass transition temperature (Tg) values of ~75 °C (first heating) and 60 °C (first and second heating), respectively. BTD 7 did not show any Tg. The density functional theory (DFT) and time-dependent DFT calculations were performed at the CAM-B3LYP/6-31G** level of theory, which corroborated with the charge transfer characteristics of these compounds.