Conformational dimorphism of the spiropyran 1',3',3'-trimethylspiro[2H-1-benzopyran-2,2'-indoline]-6-carbaldehyde).

The title compound, C20H19NO2, crystallizes from an acetone–heptane solution as two dimorphs in the space groups C2/c and Cc. Each dimorph has two mol­ecules in the asymmetric unit. The four mol­ecules adopt slightly different conformations expressed by the degree of bending around a vector connecting the O and C atoms attached to the bridgehead C atom of the pyran ring. Due to the fact that all four mol­ecules are chemically identical, the difference in bending is attributed to packing forces. This is evident from the close contacts of neighbouring mol­ecules perpendicular to the plane of the benzopyran moiety observed in the Cc structure and not in the C2/c structure. These observations provide a unique example that shows how packing forces can affect the conformation of a specific mol­ecule.