The photophysical properties of organic solid-state materials are inextricably linked to the molecular structures and packing modes. However, it is still a challenge to accurately tune the fluorescence color or on–off regulation of an organic material at the molecular level. Here we demonstrate a crystal engineering strategy to regulate luminescence on and off based on the control of the packing structure for photodimerization. A cyanostilbene crystal, (Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(4-(pyridin-3-yl)-phenyl)acrylonitrile (1M), shows a luminescence “off” state because the packing of olefins facilitates the solid-state [2+2] photodimerization. However, the cyanostilbene cocrystal (1M–I2), consisting of 1M and 1,4-diiodotetrafluorobenzene (I2), displays a luminescence “on” state because halogen bond-induced repacking of olefins inhibits the photodimerization. A two-component cocrystal can be formed by grinding, while the rare transformation from a cocrystal to a crystal is realized by heating. With the invertible uptake and escape of halogen-bonded donors, a reversible luminescence regulation is accomplished in situ. Thus, we provide a simple and clarified way to accurately achieve a crystal-to-cocrystal phase transition and control the optical properties.