Isomerization is an essential chemical process that often evokes dramatic change of chemical, physical, or biological properties. For a long time, isomerization has been known as a transformation that is induced by certain external energy such as light, heat, or mechanical force. Herein, a new isomerization phenomenon is described, which does not require external energy but simply occurs during molecular packing. The proposed isomerization is demonstrated by a series of symmetric donor–acceptor–donor (D–A–D) molecules, the donor of which may adopt two different stereoisomeric forms. Based on the evidence of the asymmetric isomers in crystals, the occurrence of isomerization during molecular packing is proved. Moreover, the unique asymmetric geometry in the solid state favors the restriction of intramolecular motion, resulting in highly efficient organic solids with quantum yields approaching unity. A new isomerization phenomenon occurring during molecular packing is reported. Such a new type of isomerization tends to offer tight molecular packing with fewer voids in stacking, resulting in unique packing with abundant CH…π interactions. The unique packing in the solid state favors the restriction of intramolecular motion, resulting in highly efficient organic solids with quantum yields approaching unity.