-9,10-ethenoanthracene-11,12-bis(diphenylmethanol) (1) forms stable cr
ystalline complexes with ethanol, acetone and toluene. In solution, di
ol 1 undergoes the di-pi-methane photorearrangement, but irradiation o
f the complexes in the crystalline state leads to a novel dehydration
reaction. X-ray crystallography reveals the probable source of this di
fference in behavior: pre-organization of the hydroxyl groups in the s
olid state in a hydrogen-bonded arrangement that is particularly favor
able for dehydration.