Local Ionization in 2-Phenylethyl-N,N-dimethylamine:  Charge Transfer and Dissociation Directly after Ionization

Abstract

The 2-phenylethyl-N,N-dimethylamine (PENNA) radical cation offers two functional groups for a positive charge to reside, the benzyl ring and the substituted amine group. Previously published HeI photoelectron spectra (PES) and our B3LYP data of the cation ground state agree that the amine site has the lowest ionization energy (IE). In this work we present evidence that by resonant laser multiphoton ionization the electron is removed from the phenyl site. B3LYP calculations of the neutral molecule predict that by far the most stable structure is the nonsymmetric unfolded anti conformer and that no other conformer should be significantly populated. This is confirmed by the S0−S1 resonant two-photon ionization (R2PI) spectrum in which one conformer is found to be predominant. The presence of the vibrational fingerprint of the phenyl chromophore and the absolute energetic position of the R2PI spectrum clearly show the local character of the first photoexcitation. Surprisingly, the R2PI mass spectra taken via S1 resonances show strong fragmentation. The parent-to-fragment-ion ratio is about 1:10 and mostly independent of laser intensity. The metastable character of the decay excludes a fragmentation caused by cation photoabsorption. The strong dissociation directly after ionization is explained by (i) a local ionization at the phenyl chromophore, (ii) a fast charge transfer (CT) to the lower-energetic amine site, and (iii) a subsequent metastable dissociation. A first detailed analysis of the ionization process indicates that intersite R2PI ionization between local states is a one-photon two-electron process which is expected to be improbable and that a failure of local ionization only happens in cases of mixed intermediate S1 states or mixed cation states. PENNA with two possible charge sites spaced by a −CH2−CH2− bridge is an ideal model system to study the dynamics of a downhill charge transfer after local ionization to the first excited cation state as presented in a forthcoming paper.

Theoretical description of charge migration with a single Slater-determinant and beyond

“…[9][10][11][12][13][14] It has been shown in great detail that the many-body effects, i.e., the electron correlation and electron relaxation are the driving forces of charge migration. [18][19][20] Questions of whether charge migration still occurs on ultrafast time scales in larger molecules over larger distances and whether it remains relevant are of general interest. Recent time-resolved experiments by Schlag and Weinkauf on oligopeptide radical ions triggered interest to study ultrafast charge migration in these systems and, in general, in larger molecules.…”

Homolytic CαCβ Bond Cleavage in a Chiral Alkylarene Radical Cation: Effects of Asymmetric Microsolvation

“…[5,6] Mass-resolved resonant twophoton ionization spectroscopy (R2PI-TOF) on a supersonically expanded molecular beam represents a highly accurate tool for determining the energetics of radical ions and their microsolvated derivatives. The energetics of solvation at the microscopic level has been obtained by measuring vibronic spectra, bond-dissociation energies, and IPs (ionization potentials) of various molecular systems.…”

Molecule-Based Photonically Switched Half and Full Adder

“…6 Thus, all molecules in the interaction volume can be addressed by one resonant wavelength. This isomer is stretched, which allows for local excitation and ionization (see iii below).…”