Probing the Photoexcited States of Rhodium Corroles by Time-Resolved Q-Band EPR. Observation of Strong Spin-Orbit Coupling Effects V. Rozenshtein, † L. Wagnert, † A. Berg, † E. Stavitski, †,§ T. Berthold, ‡ G. Kothe, ‡ I. Saltsman, # Z. Gross, # and H. Levanon* ,† Department of Physical Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel, Department of Physical Chemistry, UniVersity of Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany, and Department of Chemistry and Institute of Catalysis Science and Technology, Technion - Israel Institute of Technology, Haifa 32000, Israel ReceiVed: February 18, 2008; ReVised Manuscript ReceiVed: March 31, 2008 The photoexcited states of two 5,10,15-tris(pentafluorophenyl)corroles (tpfc), hosting Rh(III) in their core, namely Rh(pyr)(PPh 3 )(tpfc) and Rh(PPh 3 )(tpfc), have been studied by time-resolved electron paramagnetic resonance (TREPR) combined with pulsed laser excitation. Using the transient nutation technique, the spin polarized spectra are assigned to photoexcited triplet states. The spectral widths observed for the two Rh(III) corroles crucially depend on the axial ligands at the Rh(III) metal ion. In case of Rh(PPh 3 )(tpfc), the TREPR spectra are found to extend over 200 mT, which exceeds the spectral width of non-transition-metal corroles by more than a factor of 3. Moreover, the EPR lines of the Rh(III) corroles are less symmetric than those of the non-transition-metal corrroles. The peculiarities in the TREPR spectra of the Rh(III) corroles can be rationalized in terms of strong spin-orbit coupling (SOC) associated with the transition-metal character of the Rh(III) ion. It is assumed that SOC in the photoexcited Rh(III) corroles effectively admixes metal centered 3 dd-states to the corrole centered 3 ππ*-states detected in the TREPR experiments. This admixture leads to an increased zero-field splitting and a large g-tensor anisotropy as manifested by the excited Rh(III) corroles. Introduction The development of a simple and efficient procedure for the synthesis of polypyrrolic macrocycles has initiated numerous studies of the chemical and physical properties of porphyrin analogues, such as the corroles. 1–12 However, in contrast to the metalloporphyrins, 13–16 the studies of the photoexcited states of metallocorroles are scarce. 17,18 As a matter of fact, direct studies of electronically excited corrole complexes involving transition- metal ions do not exist. It is well-known that fully conjugated corrole or porphyrin rings each contain 18 delocalized π-electrons. 19,20 As a result, the corroles are structurally similar to the porhyrins but differ in the charge of the deprotonated macrocycle, which is -3 for corroles and -2 for porphyrins. Trianionic corrole macrocycles provide four pyrrole nitrogens as ligands for the central metal ion. Thus, two axial ligands can still be added. Generally, these axial ligands modify the energy of the metal centered states and can be used to “tune” the photochemical and photophysical properties of the transition-metal complex. 21 Although chemi- cally and spectroscopically (in the UV-visible region) corroles are closely related to porphyrins, 22 the smaller cavity and reduced symmetry of the corroles (C 2ν as compared to D 4h ) lead to distinct photophysical properties, as will be shown in the following. In this paper we present the first time-resolved electron paramagnetic resonance (TREPR) study of two newly synthe- sized metallocorroles involving Rh(III) as transition-metal ion. The ground states of these complexes are diamagnetic and, thus, EPR silent. Our objective is a detailed spectroscopic charac- terization of the photoexcited states of the Rh(III) corroles using TR Q-band (34 GHz) EPR combined with pulsed laser excita- tion. Particular emphasis is given to the exploration of the zero- field splitting (ZFS) observable in the TREPR spectra. The two studied Rh(III) corroles contain 5,10,15-tris(pentafluorophenyl) corrole (tpfc) as macrocycle and possess different axial ligands. Figure 1 shows the molecular structure of the complexes Rh(pyr)- (PPh 3 )(tpfc), 2, and Rh(PPh 3 )(tpfc), 3, together with the structure of Sn(Cl)(tpfc), 1, included in the study for comparison. The specific choice of these metallocorroles allows for the identifica- tion of strong spin-orbit coupling (SOC) effects which can be related to the transition-metal character of the Rh(III) ion. Materials and Methods Sample Preparation. The two Rh(III) corroles, Rh(pyr)(PPh 3 )- (tpfc) and Rh(PPh 3 )(tpfc) (see Figure 1), coordinated by different axial ligands, were synthesized as described elsewhere. 23,24 The EPR spectroscopic properties of the Sn(IV) corrole, namely, Sn(Cl)(tpfc) (see Figure 1), have been published earlier. 17 The Rh(III) complexes were dissolved in the liquid crystal (LC) E-7 (Merck Ltd.). The concentrations used were ∼5 × 10 -4 mol/L. The LC E-7 was used without further purification. The Rh(III) corroles were first dissolved in toluene, which was then evap- orated and the LC was introduced into the quartz tube. The samples were degassed by several freeze-pump-thaw cycles on a vacuum line and sealed under vacuum. 17 The LC E-7 shows a clearing temperature of 333 K and a melting point of 263 K. In general, however, E-7 avoids crystallization and, thus, exhibits a nematic phase between 333 K and the melting point and a “nematic glass” below 263 K, which was found to be stable down to very low temperatures. * Corresponding author. E-mail: levanon@chem.ch.huji.ac.il. † The Hebrew University of Jerusalem. § Current address: Inorganic Chemistry and Catalysis Group, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands. ‡ University of Freiburg. # Technion - Israel Institute of Technology. J. Phys. Chem. A 2008, 112, 5338–5343 5338 10.1021/jp801425d CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008