Synthesis of a Novel Sn(IV) Porphycene−Ferrocene Triad Linked by Axial Coordination and Solvent Polarity Effect in Photoinduced Charge Separation Process

Abstract

A novel Sn(IV) porphycene-ferrocene triad molecule, trans-bis(ferrocenecarboxylato)(2,3,6,7,12,13,16,17-octaethylporphycenato)tin(IV), [Sn(IV)(OEPc)(FcCOO)(2)] (2), was synthesized and fully characterized by various spectroscopic methods. This is the first example of a metalloporphycene triad linked by the axial coordination of two functionalized units to a metallocenter. The steady-state fluorescence measurement indicated the efficient fluorescence quenching by coordination of the ferrocenecarboxylic acid in comparison to the corresponding dihydroxy-Sn(IV) porphycene, [Sn(IV)(OEPc)(OH)(2)] (1) (1, Phi(F) = 0.094; 2, Phi(F) = 0.01). The electron transfer process from the ferrocene units to the excited Sn(IV) porphycene was directly observed by subpicosecond transient absorption spectroscopy in acetonitrile (polar solvent) and toluene (nonpolar solvent). In acetonitrile, the transient species attributed to the Sn(IV) porphycene radical anion was observed at 750 and 850 nm within 1 ps after the excitation, and then the generated charge separation state disappeared with a value of 6.9 x 10(11) s(-1) for the time constant. On the other hand, the generated charge separation state decayed with two components, 3.9 x 10(11) and 9.6 x 10(9) s(-1) time constants, in toluene. For the observed two-component decay in toluene, a significant equilibrium between the charge separation state and the triplet state was proposed because these energy levels are close to each other. Therefore, the solvent-polarity-dependent long-lived charge separation state was obtained in the Sn(IV) porphycene-ferrocene triad system. The electron transfer upon excitation of the Sn(IV) porphyrin of [Sn(IV)(OEP)(FcCOO)(2)] (4), in which OEP denotes the 2,3,6,7,12,13,16,17-octaethylporphyrin ligand, was observed. However, no equilibrium between the charge separation and the triplet states was observed in both the acetonitrile and toluene. The difference in the charge recombination processes of the Sn(IV)-porphycene and -porphyrin is due to the small HOMO-LUMO gap and the large driving force (-DeltaG(CS)) of 2 compared to that of 4, which resulted in the energy level of the charge separation state close to the triplet state in toluene. Furthermore, the large driving force (-DeltaG(CS)) of 2 compared to that of 4 is attributed to the significant stabilization of the LUMO energy level caused by a decrease in the molecular symmetry and a large porphycene pi-electron framework. This result indicates that porphycenes are excellent candidates as an electron acceptor in photoinduced electron transfer systems.