Fig. 1. Molecular design of intramolecular dimer model and in situ crystal structures under various external stimuli. (A) Molecular structures of rigid intramolecular dimer (X2R) models with R group varying from benzene (B), naphthalene (N) and anthracene (A), and the corresponding monomer model (X1R). (B) Decreased distances of R moieties from benzene to anthracene. (C) Monomer (X1R) model with only one R substituent and angles between 9,9-dimethylxanthene and R moieties increased from benzene to anthracene. (D) Precise adjustment of π–π distances of X2A crystals from 4.340 Å to 3.698 Å with emission colors changing from blue to green.