Tuning of Sensitivity in Thermochromic Luminescence by Regulating Molecular Rotation Based on Triphenylamine-Substituted o-Carboranes

This study shows tuning of sensitivity in thermochromic luminescence from triphenylamine-substituted o-carboranes by regulating rotation barriers with a substituent effect. A series of o-carborane dyads with various numbers of methyl substituents were prepared, and their optical properties were compared. Initially, significant dual-emission properties consistent of the locally excited (LE) state and the induced charge transfer (ICT) state were observed from the solid samples of all dyads. In particular, diverse responses were obtained by cooling from the dyads. The pristine and mono-substituted dyads presented thermally independent emission during temperature changes, whereas the detected thermochromic luminescence with drastic color change from red to blue originated from the alteration of intensity ratios between LE and CT emission bands. From mechanistic studies (experimental and theoretical), it was clearly shown that the difference in the rotation barrier of the o-carborane unit caused by the substituent effect should play a key role in variable sensitivity in luminescent chromic behaviors in temperature change.