Isobenzothiadiazole-based dichroic dyes for smart windows

Six novel donor-acceptor-donor (D-A-D) dichroic dyes (1–6) based on isobenzothiadiazole (isoBTD) as acceptor units are designed and synthesized. Different groups (i.e., amyl, N,N-dibutyl, pentyloxy, fluorine atom, thiobutyl and acetylene bond) are introduced into the donors aiming at evaluating the influence of terminal groups on photophysical, photostability and dimming properties. Also, density functional theory (DFT) calculations are used to assist in the explanation. Interestingly, 1, 4 and 5 show liquid crystal phase with high clearing points. The absorption and emission spectra of all the dyes have varying degrees of redshift and blueshift by modifying the terminal groups. Among these six dyes, 2 has the smallest electrochemical band gap (2.13 eV) and theoretical band gap (2.68 eV) due to a high electron-rich ability of the N,N-dibutyl group, which is conducive to the redshift of the spectrum. Meanwhile, 5 bearing the thiobutyl group shows the highest dichroic ratio (D_A = 6.24), as well as the smallest color difference (ΔE = 26.69) under UV light (365 nm) irradiation for 2 h, while 3 containing the pentyloxy group gives the widest transmittance difference (ΔT = 22.44 %). These results provide useful references for the development of dichroic dyes for smart windows.