Aromaticity-controlled thermal stability of photochromic systems based on a six-membered ring as ethene bridges: photochemical and kinetic studies.

Three photochromic compounds--2-butyl-5,6-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-yl]-1 H-benzo[de]isoquinoline-1,3(2 H)-dione (BTE-NA), 4,5-bis[5-(4-methoxyphenyl)-2-methylthiophen-3-yl]benzo[c][1,2,5]thiadiazole (BTA), and BTTA, which contain naphthalimide, benzothiadiazole, and benzobisthiadiazole as six-membered ethene bridges with different aromaticities--were systematically studied in solution, sol-gel, and single-crystal states. They exhibit typical photochromic performance with considerably high cyclization quantum yields. BTE-NA, BTA, and BTTA form a typical donor-π-acceptor (D-π-A) system with significant intramolecular charge transfer (ICT) between HOMO and LUMO upon excitation, thus realizing the fluorescence modulation by both photochromism and solvatochromism. The three ethene bridges with different degrees of aromaticity can provide a systematic comparison of the thermal stability evolution for their corresponding closed forms (c-BTE-NA, c-BTA, and c-BTTA). c-BTE-NA shows first-order decay in various solvents from cyclohexane to acetonitrile. c-BTA only shows first-order decay in polar solvents such as chloroform, whereas it is stable in nonpolar solvents like toluene. In contrast, the less aromatic property of BTTA gives rise to its unprecedented thermal stability in various solvents even at elevated temperatures in toluene (328 K). Moreover, the small energy barrier between the parallel and antiparallel conformers allows the full conversion from BTTA to c-BTTA. In well-ordered crystal states, all three compounds adopt a parallel conformation. Interestingly, BTTA forms a twin crystal of asymmetric nature with interactions between the electron-rich oxygen atom of the methoxy group and the carbon atom of the electron-deficient benzobisthiadiazole moiety. This work contributes to the understanding of aromaticity-controlled thermal stability of photochromic systems based on a six-membered ring as an ethene bridge, and a broadening of the novel building blocks for photochromic bisthienylethene systems.