Quinacridone (QA) possesses excellent color fastness and weather resistance, making it largely contribute to industrial coatings, pigments, and printing inks. Additionally, QA derivatives have been applied in the field of organic solar cells due to their strong absorption in the visible regions. The hyperconjugative planar structure of QA derivative molecules effectively facilitates efficient carrier transport in organic field-effect transistors (OFETs). Combined with their excellent optical, thermal, and electrochemical stability, QA derivatives hold potential as materials for organic light-emitting diodes (OLEDs). However, the planar QA, renowned for its intense fluorescence emission in solution due to strong conjugation effects, encounters fluorescence quenching in its solid state due to robustly intermolecular π-π stacking interactions between molecules. This quenching significantly constrains its applications in fluorescent dyes, displays, sensors, and optoelectronic devices. To address this limitation, the introduction of trifluorobenzene functional groups (-PhCF₃) with spatially extended orientation into the QA framework has been explored. The resulting compound, QA-CF₃, was synthesized and its photophysical and photochemical properties were compared to QA using UV-Vis absorption and fluorescence spectroscopies. In solution, both QA and QA-CF₃ exhibited strong fluorescence emission, with QA-CF₃ displaying a blue shift in its UV absorption and emission wavelengths compared to QA. Specifically, the maximum emission wavelengths of QA and QA-CF₃ are 547 and 536 nm in dimethyl sulfoxide, respectively, representing a blue shift of 11 nm. In contrast to the fluorescence quenching of solid QA, solid QA-CF₃ exhibits intense fluorescent emission, with the maximum emission wavelength red-shifted to 631.4 nm. Time-dependent density functional theoretical (TD-DFT) calculations reveal that the -PhCF₃ plane is nearly perpendicular to the QA plane, effectively mitigating the intermolecular π-π stacking interactions. This spatial orientation facilitates irradiative transitions, leading to the enhancement of fluorescence for QA-CF₃. Additionally, the increase in transition energies of QA-CF₃ accounts for the observed blue shift in its maximum absorption and emission bands compared to QA. By incorporating trifluorobenzene functional group, QA-CF₃ overcame the fluorescence quenching limitation of QA framework, displaying the excellent fluorescence performance. Not only did QA-CF₃ exhibit a blue-shifted fluorescence emission in solution, but also it showed an intense fluorescence in both solution and solid state. This work not only offers an updated design strategy of the fluorescent materials, but also provides an insightful perspective on understanding and manipulating intermolecular interactions. By elucidating the mechanism behind fluorescence quenching in solid QA, we propose a QA-based strategy to enhance fluorescence quantum yield, significantly broadening its applications in optical and electronic materials, including fluorescent dyes, sensors, and displays.

Key words: quinacridones, π-π stacking interaction, fluorescence enhancement, UV-Vis absorption spectrum, fluorescence spectrum