Among 1-pyrenylethylenes with an alkyl (t-butyl) or an aryl (phenyl) group on the β ethylenic carbon, 1-(3,3-dimethyl-1-butenyl)pyrene (BP) undergoes cis-to-trans one-way isomerization on benzil sensitization, whereas 1-styrylpyrene (SP) undergoes mutual isomerization between the cis and trans isomers to afford an extremely high photostationary state composition (98%) of the trans isomer. Transient spectroscopy and measurements of the isomerization quantum yields revealed that the trans triplet state is the most stable and populated conformation on the triplet energy surface for both BP and SP; however, a higher degree of stabilization of the trans conformation for BP than for SP compared to the perpendicularly twisted conformation makes BP and SP one-way and two-way isomerizing, respectively.
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Okamoto et al. (1990) studied this question.
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