Picosecond time-resolved fluorescence spectroscopy of (Z)-1-(2-anthryl)-2-phenylethene and its model compounds: Understanding the photochemistry by distinguishing between the s-cis and s-trans rotamers

The photochemical reactions (Z-E isomerization and photocyclization) of the s-cis and s-trans rotamers of (Z)-1-(2-anthryl)-2-phenylethene (Z2APE) were investigated. The absorption, steady-state and picosecond time-resolved fluorescences, and transient absorption spectra were measured in order to achieve a better understanding of the rotamer photochemistry. The spectra were compared with those of the model compounds ((Z)-1-[2-(1-methylanthryl)]-2-phenylethene: Z1Me2APE and (Z)-1-[2-(3-methylanthryl)]-2-phenylethene: Z3Me2APE). It is confirmed that the s-trans rotamer undergoes only Z→E adiabatic isomerization and that the s-cis rotamer mainly photocyclizes to give the dihydrophenanthrene-type intermediate with a much faster rate constant (k_cyc = 2.3 × 10¹⁰ s^-1). The reason Z2APE does not give an aromatized photocyclization product (1,2-naphtho[a]anthracene, NA) is that the return of the dihydrophenanthrene-type intermediate to the Z isomer is much faster than the oxidation to produce NA.