Electron Spin Dynamics in Photoexcited Diamagnetic and Paramagnetic Corroles

Abstract

Three corroles, which differ by their cavity's core, namely, diamagnetic free-base tris(pentafluorophenyl)corrole and its gallium(III) complex and the paramagnetic oxo-chromium(V) complex, were studied by steady-state and time-resolved electron paramagnetic resonance (EPR) spectroscopy. The magnetic and orientational parameters of the corroles, oriented in a nematic liquid crystal, were determined and interpreted in terms of their structure, geometry, and excited states spin dynamics. It was shown that both diamagnetic corroles, photoexcited to their triplet states, exhibit similar EPR line shapes, which is characterized by a negative zero-field splitting parameter, D, whose origin is due to molecular "stretching". Photoexcited Cr(V)O-corrole exhibits polarized ground-state EPR spectrum in emission mode. This polarization stems from the sequence of photophysical and photochemical reactions, involving the formation of the trip-quartet/trip-doublet composite states and their selective quenching via a charge transfer state.

Exploring the photoexcited triplet states of aluminum and tin corroles by time-resolved Q-band EPR

“…Finally, the analysis yields information on the ordering of the molecular axes of the metallocorroles in the LC. Values for the orientational parameters cr£ and ore [27,31] the in-plane Y-axis parallel and the out-of-plane Z-axis perpendicular to the director (Fig. 2b).…”

“…However, with the previous assignment of D < 0 for free-base corrole and Ga(III) corrole [31], one can evaluate the dominant ISC path for the studied metallocorroles. Analysis reveals that in Sn(Cl)(tpfc) the in-plane triplet sublevels are preferentially populated, A x, A r 87 A z ( Table 1).…”

“…The compounds were first dissolved in toluene (concentrations used were about 5-10 -4 mol/l), which was then evaporated and the LC was introduced into a Pyrex tube. The samples were degassed by several freeze-pump-thaw cycles on a vacuum line and sealed under vacuum [31].…”

“…Generally, the high spectral and time resolution of this method enables one to characterize the paramagnetic intermediates, to monitor their spin dynamics and to determine the magnetic parameters over a wide range of temperatures [26][27][28][29][30]. In a recent paper we have reported on the photophysical properties of the triplet states of the free-base 5,10,15-tris(pentafluorophenyl)corrole (tpfc) and its Ga(III) complex, Ga(tpfc) [31]. An unusual polarization pattern observed in the triplet EPR spectra of these systems was rationalized in temas of a negative zerofield splitting (ZFS) parameter D, exceptional for planar tetrapyrrolic compounds.…”

Triplet line shape simulation in continuous wave electron paramagnetic resonance experiments

“…It presents the line shape of a triplet gallium-pentafluorophenylcorrole (Ga(tpfc)) in toluene. This is a unique example where the ZFS parameter D is negative as a result of molecular distortion and head-to-tail spin alignment (3,31).…”