Effect of halogen atom on electron spin polarization studied by CIDEP,using halogen substituted aromatic acyl compounds

The reaction and spin dynamics of the photocleavage reaction of 2-chloro-2′-acetylnaphthalene were studied by time-resolved FT-EPR and transient absorption (TA) spectroscopy. The photocleavage reaction from both singlet and triplet states was observed by TA and EPR experiments, although the radical cleavage reaction in the excited triplet state is energetically unfavourable. This feature has been explained by the ionic cleavage reaction due to the electro-negativity of the chlorine atoms. The time-resolved FT-EPR spectra were similar to those observed in the bromine substituted compound, 2-BAN, reported in a previous paper. The origin of the electron spin polarization was assigned to the radical triplet pair mechanism (RTPM) and free radical pair mechanism (F-pair RPM) from analysis of the time profiles of the spin polarization.     

Photocleavage reaction of bromine substituted aromatic acyl compounds studied by CIDEP and transient absorption spectroscopy

The photocleavage of the CBr bond in bromoacetylnaphthalene is investigated by transient absorption and time resolved EPR spectroscopy. In the transient absorption of 2-bromo-2′-acetylnaphthalene, the absorption band observed at λ_max ~440 nm is assigned to the triplet state of the parent molecule. After decay of the triplet absorption, a long lived absorption band is observed at λ_max ~380 nm, which is assigned to naphthoylmethyl radical. The yield of this radical is not dependent on the concentration of oxygen even though the absorption band of the triplet state was quenched by addition of oxygen. Thus we conclude that the spin multiplicity of the precursor molecule is singlet. The CW time resolved EPR spectrum shows a typical E?/A CIDEP pattern of three hyperfine lines of the naphthoylmethyl radical. This result suggests some contribution from triplet precursor molecules. However, a careful analysis of the time profile of the CIDEP intensity observed by FT-EPR revealed that the polarization is generated from the radical pair mechanism (RPM) from the encountered pair of two free naphthoylmethyl radicals and the radical-triplet pair mechanism. RPM polarization by the geminate radical pair, formed by the Br atom and the naphthoylmethyl radical, is not observed. This fact indicates that large spin-orbit coupling (Δg and/or fast spin relaxation by g anisotropy) spoils the RPM polarization. The finding is in contrast to the recent observation of RPM polarization in the Cl cleavage reaction of 1-(chloromethyl)naphthalene.     

Magnetic field and spin effects from sequential p-type and d-type triplet mechanisms

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br- and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical ‘p-type’ triplet mechanism) and modification of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer (‘d-type’ triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V _d for the pure d-type and V _pd for the combined p- and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic field and heavy atom effects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters.