Calculation Driven Synthesis of an Excellent Dihydropyrene Negative Photochrome and its Photochemical Properties

• Published in: Journal of the American Chemical Society 2011-03, Vol.133 (11), p.4040-4045
• Main Authors: Ayub, Khurshid, Li, Rui, Bohne, Cornelia, Williams, Richard Vaughan, Mitchell, Reginald H
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja1100596

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 °C with t 1/2 = 4.5 h, giving an extrapolated 20 °C t 1/2 of ∼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 °C. The ring-opening isomerization quantum yields (ϕopen) 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 (ϕopen) of 0.66 ± 0.02 for any known dihydropyrene, ∼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 ∼2 y at 20 °C.