Toward Benzobis(thiadiazole)-based Diradicaloids.

We theoretically predicted that acetylene-bridged benzo[1,2-c;4,5-c']bis[1,2,5]thiadiazole (BBT) oligomers would show a quick increase of diradical character with the extension of chain length. To validate the hypothesis, six stable BBT-based diradicaloids were synthesized and fully characterized by X-ray crystallographic analysis and various spectroscopic measurements. Three of them showed prominent paramagnetic activity at elevated temperatures due to thermal population from the open-shell singlet ground state to triplet excited state. It was also found that substitution by electron-donating triphenylamine groups at the termini promoted the diradical character and reduced the singlet-triplet energy gap, and at the same time, resulted in intense near-infrared absorption.