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

Abstract

Picosecond time-resolved Raman spectroscopy has been used to study the ultrafast relaxation dynamics of aromatic cation radicals following two-photon ionization. In acetonitrile, integrated Raman intensities due to the cation radicals rise in tens of picoseconds, and reach their maxima at a delay time of 40-60 ps from the pump pulse. Such a slow-rise component is observed in all the cation radicals treated (biphenyl, trans-stilbene and naphthalene), suggesting that the picosecond relaxation process increasing the cation Raman intensities occurs after the photoionization of aromatic molecules. In weak polar solvents such as ethyl acetate, on the other hand, only an instrumental-limited rise (<5 ps) is observed. The rise time of the cation Raman intensity does not correlate with the dielectric relaxation time but depends on the polarity of the solvent. This result indicates that the picosecond relaxation process is not controlled by the dielectric solvent relaxation alone. The positional changes and the band narrowings of the cation Raman bands occur on a 10-20 ps time scale. These are associated with intermolecular vibrational relaxation of the cation radical toward a thermal equilibrium with solvents. The time scale of the intermolecular vibrational relaxation is the same as that of the rise component of the cation Raman intensity. From these observations, it is suggested that the thermal excitation of the solvent shell disturbs the solvation structure of the cation radical, which causes the observed picosecond change in the cation Raman intensity.

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.…”

Excited-State Double-Proton Transfer in the 7-Azaindole Dimer in the Gas Phase. Resolution of the Stepwise versus Concerted Mechanism Controversy and a New Paradigm

“…Our experimental system for picosecond time-resolved study is described in several references. 45,63,64 Figure 4 displays the principle of picosecond time-resolved resonance-enhanced multiphoton ionization (REMPI) spectroscopy to measure the decays of the excited normal 7AI 2 dimer. The pump laser excites 7AI 2 in a specific single vibronic state of the normal dimer, subsequently the probe laser ionizes the normal dimer.…”

Dynamics of electron ejection on photoionization of trans-stilbene and biphenyl in acetonitrile as observed with femtosecond time-resolved near-IR absorption spectroscopy

“…Photoionization in solution has been drawing attention as a fundamental but complex process in chemical reactions, as temporal behaviors of a solute, an ejected electron and surrounding solvent molecules need to be elucidated for explaining its mechanism. [1][2][3][4][5][6][7] When an electrically neutral molecule is photoexcited to a highly excited state above the ionization potential, an electron can be ejected from the molecule, leaving the parent molecule as the radical cation. The highly excited state can be an excited singlet state 8,9 and a Rydberg state.…”

“…The highly excited state can be an excited singlet state 8,9 and a Rydberg state. 10 The excess energy, or difference between the excitation energy and the ionization potential, will be distributed to the translational motion of the electron and the radical cation 11,12 as well as to the rotational, vibrational 5 and electronic 13 degrees of freedom of the radical cation. It has not been clearly understood, however, how the photoionization proceeds, or how an electron leaves the parent molecule in solution where the process should be critically affected by the solvent molecules.…”