RESUMEN
Resonant electron capture by aza and diaza derivatives of phenanthrene (7,8-benzoquinoline and 1,10-phenanthroline) and anthracene (acridine and phenazine) at incident free electron energies (Ee) in the range of 0-15 eV was studied. All compounds except 7,8-benzoquinoline form long-lived molecular ions (M-) at thermal electron energies (Ee â¼ 0 eV). Acridine and phenazine also form such ions at epithermal electron energies up to Ee = 1.5-2.5 eV. The lifetimes (τa) of M- with respect to electron autodetachment are proportional to the extent of aza-substitution and increase on going from molecules with bent geometry of the fused rings (azaphenanthrenes) to linear isomers (azaanthracenes). These regularities are due to an increase in the adiabatic electron affinities (EAa) of the molecules. The EAa values of the molecules under study were comprehensively assessed based on a comparative analysis of the measured τa values using the Rice-Ramsperger-Kassel-Marcus theory, the electronic structure analysis using the molecular orbital approach, as well as the density functional calculations of the total energy differences between the molecules and anions. The only fragmentation channel of M- ions from the compounds studied is abstraction of hydrogen atoms. When studying [M-H]- ions, electron autodetachment processes were observed, the τa values were measured, and the appearance energies were determined. A comparative analysis of the gas-phase acidity of the molecules and the EAa values of the [M-H]· radicals revealed their proportionality to the EAa values of the parent molecules.
RESUMEN
Macrocyclic tetrapyrrolic compounds, such as naturally occurring or artificial porphyrins and phthalocyanines, have unique and highly attractive properties for applications in medicine and technology. The interaction of free-base phthalocyanine (H2Pc) and tetraphenylporphyrin (H2TPP) molecules with low-energy (0-15 eV) electrons was studied in vacuo by means of negative ion resonant electron capture mass spectrometry. Close similarities in formation and decay of negative ions of these compounds were revealed. Efficient formation of long-lived molecular negative ions (MNIs) was observed in the incident electron energy range of 0-8 eV, unprecedentedly wide for organic compounds and comparable to the range characteristic to carbon atomic clusters, fullerenes. Experiments testify to the strong persistence of MNIs of both compounds to dissociative decay, isomerization, and electron autodetachment. Lifetimes of MNIs as a function of incident electron energy were measured and it was concluded that the isolated anions may retain additional electrons in a time scale of up to hundreds of seconds at standard temperature due to the high adiabatic electron affinity of these large molecules. For the representatives of dyes and photochromic compounds comprehensively studied in terms of interaction with light, the present work highlights yet another unique property of these molecules, namely the capability to attach and durably retain an additional electron of low, pre-ionization energy.