A novel Sn(IV) porphycene-ferrocene triad molecule, trans- bis(ferrocenecarboxylato)(2,3,6,7,12,13,16,17-octaethylporphycenato)tin(IV), [SnIV(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 functionallzed 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) ℙF = 0.094; 2, ℙF = 0.01 ). The electron transfer process from the ferrocene units to the excited Sn(IV) porphycene was directly observed by subplcosecond transient absorption spectroscopy in acetonltrile (polar solvent) and toluene (nonpolar solvent). In acetonltrile, 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 109 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 [SnIV(OEP)(FcCOO)2] (4), in which OEP denotes the 2,3,6,7,12,13,16,17-octaethylporphyrln ligand, was observed. However, no equilibrium between the charge separation and the triplet states was observed in both the acetonitrlle 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 (-ΔGCs) 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 (-ΔGCs) 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 π-electron framework. This result indicates that porphycenes are excellent candidates as an electron acceptor in photoinduced electron transfer systems.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry