Picosecond transient absorption spectroscopy of trans-4-R-4′-nitrostilbenes with R: OMe, NH2 and NMe2

The spectroscopic properties of the trans isomer of four 4-R-4′-nitrostilbenes (I–R, R: H, OMe, NH2, NMe2) in solution at 25°C were studied by time-resolved absorption techniques. An absorption increase within 10 ps and the subsequent decay in the visible range are attributed to the excited singlet state (S1→Sn transition). The lifetime for this decay (τS) varies for trans-I–NMe2 from 1–3 ns in solvents of low polarity to ≅10 ps in acetonitrile. Also for trans-I–NH2 and trans-I–OMe, τS is sensitive to the solvent polarity. In those cases where the quantum yield of intersystem crossing is substantial, the transient absorption due to the T1→Tn transition at an appropriate wavelength, increases with almost the same lifetime and then remains constant.