The photochromic properties of dihydropyrenes have been substantially improved by making use of density functional theory (DFT) activation barrier calculations, which suggested that the di-isobutenylcyclophanediene 15' should have a significant barrier to thermal isomerization to the dihydropyrene (DHP) 15, which itself should resist isomerization involving migration of the internal groups to the rearranged dihydropyrene 9 (X = -CH=C(Me)(2)). As a result of these calculations, the synthesis of the colorless cyclophanediene (CPD) 15' was undertaken and achieved from the dinitrile 28 in four steps in 37% overall yield %. The cyclophanediene 15' thermally isomerized to the dihydropyrene 15 at 100 degrees C with t(1/2) = 4.5 h, giving an extrapolated 20 degrees C t(1/2) of similar to 16 y, consistent with the DFT calculations. No evidence for [1,5]-sigmatropic rearrangement in to 9 (X = -CH=C(Me)(2)) was observed on heating to 130 degrees C. The ring-opening isomerization quantum yields (phi(open)) for DHP 15 in to CPD 15' were determined in cyclohexane to be 0.12 +/-0.01, which is three times greater than for the benzoDHP 1. Friedel-Crafts naphthoylation of 15 gave 70% of purple 32, which in toluene showed the largest photochemical ring-opening isomerization quantum yields (phi(open)) of 0.66 +/- 0.02 for any known dihydropyrene, similar to nine times greater than 1 in toluene. The thermal closing of 32' to 32, although faster than for 15', gave a useful extrapolated t(1/2) of similar to 2 y at 20 degrees C.