Picosecond CARS and Transient Absorption Studies of 1,4-Diphenylbutadiene and trans-Stilbene:  A Study of Photoinduced Formation of a Radical Cation

Abstract

We report the direct observation of the formation and stabilization of radical cations of trans-stilbene and 1,4-diphenylbutadiene (DPB) in polar solvents (acetonitrile, linear alcohols) by picosecond "pump-probe" and polarization sensitive resonant CARS experiments. For the first time we have observed the formation and stabilization rate of the DPB radical cation, which occurs on a 10-20 ps time scale. We confirm that the formation process is mainly monophotonic and depends on the polarity of the solvent but found no correlation between the formation and stabilization rate and the solvent dielectric relaxation time.

Time-resolved studies on the photoisomerization of a phenylene–silylene–vinylene type compound in its first singlet excited state

“…1c). This effect has been observed for radical cations of other aromatic compounds [27,28]. Assignment of the 445 nm band as being due to the radical cation is supported also by recent pulse radiolysis experiments on trans-ST in acetonitrile and dichloromethane [29].…”

“…Assignment of the 445 nm band as being due to the radical cation is supported also by recent pulse radiolysis experiments on trans-ST in acetonitrile and dichloromethane [29]. The radical cation's absorption band observed in the ultrafast laser experiment in acetonitrile does not decay in the 50-2000 ps time window, indicating the absence of geminate recombination of the electrons with their parent radical cations [27,28]. In hexane a relatively smaller signal, ascribed to the radical cation, was observed.…”

“…Trans-ST in the first singlet excited state has a lifetime of 13.272.0 ps in hexane, which is significantly shorter than that of trans-stilbene (50 75 ps in the same solvent [27]). Based on the low fluorescence quantum yield (F F ¼0.008 in dichloromethane [6]) of trans-ST, this behavior can be rationalized by invoking an efficient radiationless transition deactivating trans-ST in its S 1 state.…”

“…In hexane a relatively smaller signal, ascribed to the radical cation, was observed. This can be rationalized since the ejected electron would not be stabilized effectively in hexane, and the electron and the radical cation would recombine on the femtosecond time scale [27,30].…”

Time-Resolved Raman Studies of Photoionization of Aromatic Compounds in Polar Solvents: Picosecond Relaxation Dynamics of Aromatic Cation Radicals

“…36 The rise of the cation Raman intensity is slow, which is well described with a single-exponential function with a time constant of 17 ps, while the S 1 Raman intensity rises instantaneously (<5 ps) and decays with the S 1 lifetime of 40 ps. 26 The slower rise of the cation Raman intensity in acetonitrile is similar to the result of the cation Raman intensity of biphenyl in the same solvent. Figure 10 shows the picosecond time-resolved resonance Raman spectra of naphthalene in acetonitrile.…”