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

Abstract

The photoexcited triplet states of three 5,10,15-tris(pentafluorophenyl)corroles (tpfc), hosting Sn(IV) and AI(III) in their core, namely, Sn(Cl)(tpfc), Al(pyr)2(tpfc ) and Al(pyr)2(tpfc-Brs), were studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy in the nematic liquid crystal E7. Only two of these metallocorroles, namely, Sn(C1)(tpfc) and Al(pyr)2(tpfc-Br8), exhibit TREPR spectra following pulsed laser excitation. This result is rationalized in terms of a very low quantum yield of triplet formation in Al(pyr)2(tpfc ). Analysis of the spin polarized Q-band (34 GHz) EPR spectra of Sn(Cl)(tpfc) and Al(pyr)2(tpfc-Brs) provides detailed information on the magnetic and kinetic parameters of the triplet states as well as on the molecular ordering of the complexes in the liquid crystal. With the assignment of the zero-field splitting parameter D < 0 for the Sn(C1)(tpfc) and Al(pyr)2(tpfc-Brs), one can evaluate the dominant intersystem crossing path for these metallocorroles. Analysis reveals that in Sn(C1)(tpfc) the in-plane triplet sublevels are preferentially populated, i.e., A x, A r >> A z. This can be rationalized in terms of weak electronic interactions between the Sn(IV) ion and the corrole n-system, consistent with the domed structure of Sn(Cl)(tpfc). In Al(pyr)2(tpfc-Brs), however, the out-of-plane triplet sublevel is predominantly populated, i.e., A z > A x, Ar, which is attributed to a large increase in the spin-orbit coupling strength arising from the peripheral bromine atoms on the corrole skeleton.

Photoexcited Triplet State Properties of Brominated and Nonbrominated Ga(III)-Corroles as Studied by Time-Resolved Electron Paramagnetic Resonance

“…We believe that in the present case (contrary to the case of nonbrominated and brominated Al-corroles), 36 when both the corroles exhibit TREPR spectra, the analysis of their lineshapes could allow us not only to characterize excited state properties of the new corrole and to reveal bromination effects but also to further validate our conclusion concerning the sign of the D parameter in these complexes.…”

Ultrafast electronic and vibrational dynamics in brominated aluminum corroles: Energy relaxation and triplet formation

“…We attribute this remaining positive signal to triplet absorption. Triplet generation in Br 8 Al(tpfc)(py) 2 was previously observed by the time resolved EPR studies 5 . Importantly, the spectral shape of singlet excited state absorption and triplet absorption are strongly overlapping, and thus difficult to separate.…”

“…The system response was better than 100 fs (typically ∼80 fs). The Al(tpfc-Br 8 )(py) 2 was synthesized as reported previously 5,49 . Al(tpfc-Br 8 )(py) 2 samples of about 0.008 mol/l were prepared with a maximal absorption of about 1 OD in the Q-band at 640 nm in d 8 -toluene.…”

“…The simple and efficient procedure of corroles synthesis combined with readily tuned physical and chemical characteristics by varying the peripheral substituents, 3–6 central metal, 5,7 and axial ligands 8,9 has revived substantial interest in employing these contracted porphyrinoids in various fields. Examples for application of the corroles include dye-sensitized solar cells, 7,10 photodynamic therapy, 11–13 photodynamic detection, 14 photodynamic inactivation of mold fungi and green algae, 15,16 regular and sophisticated optical imaging, 17,18 formation of singlet oxygen for catalysis, 19–21 and corrole-based electron and energy transfer systems 22–24 .…”

Coordination chemistry of corroles with focus on main group elements

“…Other publications showed that the free-base corrole and the corrole complexes containing the ions Al III , Sn IV , Sb III , and Sb V display the same spectroscopic polarization patterns [19,66]. That means there is no indication of a heavy atom effect, i.e., changes in the ISC parameters due to the insertion of a heavy element into the coordination core of the molecule.…”