Understanding the structure-determining solid fluorescence of an azaacene derivative

Tuning the solid-state light emission properties of a single azaacene derivative system to achieve a high emission efficiency without any further modification is highly desirable and probably has many potential applications. In this research, we prepared a new azaacene derivative (TBIDQ) without any aromatic rotors and found that it exhibited different fluorescence emission behaviors in its three different single crystal forms (Form I, Form II and Form III). The fluorescence quantum yields (f) of Form I, Form II and Form III are 30.2%, 26.0% and 14.6%, respectively. From single-crystal structural analysis and optical experimental results, we found that the mean distance of the interlayers and the radiative decay rate decreased from Form I to Form III, while the pi-pi stacking interactions and the nonradiative decay rate increased. Our results strongly suggest that planar molecules can display the loose stacking mode with weak pi-pi stacking interactions through careful crystal-engineering.