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'-nitrostilbe nes (I-R, R: H, OMe, NH2, NMe2) in solution at 25 degrees C were studied by time-resolved absorption techniques. An absorption increase within 10 ps a nd the subsequent decay in the visible range are attributed to the excited singlet state (S-1 --> S-n transition). The lifetime for this decay (tau(S) ) varies for trans-I-NMe2 from 1-3 ns in solvents of low polarity to congru ent to 10 ps in acetonitrile. Also for trans-I-NH2 and trans-I-OMe, tau(S) is sensitive to the solvent polarity. In those cases where the quantum yiel d of intersystem crossing is substantial, the transient absorption due to t he T-1 --> T-n transition at an appropriate wavelength, increases with almo st the same lifetime and then remains constant. (C) 2000 Elsevier Science B .V. All rights reserved.