Fig. 5. Relationship between π–π distance and emission property through crystal analysis and theory calculation. (A) Scheme of proposed mechanism: upon the incorporation of solvents, stimulation of heat, isotropic pressure and mechanical force, different π–π distances determined by the competition between anthracene–anthracene (An–An) interactions and anthracene–solvent (An–solvent) interactions, resulting in excimer or monomer emission of X2A crystals. (B) Relationship between dihedral angles of anthryl with xanthene and distance of anthryl moieties. (C) Quantitative relationship between distances and interaction energies of anthryl moieties in X2A. (D) Scatter diagram for RDG versus sign (λ₂)ρ of the X2A-B-ClCH₂CH₂Cl crystal. (E) Electrostatic potential (ESP) analysis of the X2A-B-ClCH₂CH₂Cl crystal. (F) Calculated potential energy surfaces (PES) for the ground state and singlet excited state of X2A, with varied distances between two anthryl moieties (defined by distances of two C₁₀ atoms of anthryl moieties). Changes in molecular conformations with decreased distances of anthryl moieties can be divided into two sections: rotation of anthryl moieties and swing of anthryl moieties. (G) Scatter diagram for RDG versus sign (λ₂)ρ of the X2A-G crystal. (H) Electrostatic potential (ESP) analysis of the X2A-G crystal.