Microwave-assisted and conventional synthesis, characterisation, and DFT-based property prediction of BT-based (D–A) n and (A–D–A) n π-conjugated oligomers

Benzothiadiazole (BT)-based donor–acceptor oligomers with (D-A)n and (A-D-A)n architectures were synthesised through direct arylation polymerisation under both conventional and microwave-assisted conditions. The conventional heating method produced oligomers with higher degrees of polymerisation and more significant bathochromic shifts in their absorption spectra. Conversely, the microwave approach yielded lower molecular weights but provided faster reaction times, lower energy consumption, and reduced solvent use. NMR and IR spectroscopy confirmed the structures, while GPC, UV-Vis, and cyclic voltammetry revealed clear relationships between chain length, optical band gap (1.6–2.0 eV), and electronic structure. TD-DFT calculations accurately reproduced the experimental trends and demonstrated delocalisation of frontier orbitals and efficient intramolecular charge transfer. Global reactivity descriptors affirmed the donor-dominated character of these systems, while nonlinear optical (NLO) calculations indicated that polarizability and hyperpolarizability values were two to three orders of magnitude higher than those of urea. These findings suggest that short oligomers (DP = 3–4) already mimic the optoelectronic and NLO properties of longer chains, offering a strategic approach for designing π-conjugated materials for optoelectronic and photonic applications.