The ultraviolet and visible absorption maximum for 9,10-dioxa-syn-(hydro,chloro)bimane (3,7-dichloro-1,5-diazabicyclo[3.3.0]octa-3,6-diene-2,8-dione, syn-(H,Cl)B) appears at much longer wavelengths in the crystal (458 nm) than in solution or the glassy state (389 nm). The shift occurs in stages (glassy (I) → II (416 nm) → crystal (III)). An X-ray crystal structure determination shows that the molecules of syn-(H,Cl)B are tightly packed (d = 1.863 g cm-3) in space group P1, associating via dipolar and C-H⋯O hydrogen bonds. The latter gives rise to a particularly strong C-H IR absorption at stage III. Electrostatic stabilization of an excited-state relative to the ground state is assessed by using the observed intermolecular distances and estimates for the charge distributions in ground and excited states. It is shown to account for most of the spectroscopic shift between the glassy (I) and crystalline (III) stages.