An irreversible phase transition in 1-n-butyl­indeno­[2,1-c]pyran-3,9-dione

At ambient temperature, the title compound, C₁₆H₁₄O₃, is triclinic, with the n-butyl side chain disordered in an out-of-plane orientation. On cooling below 240 K, it converts into a different triclinic phase with an ordered planar conformation and denser packing, which is retained on warming to room temperature. The transition (occasionally) proceeds from single crystal to single crystal.