Study of spectral and photophysical properties of covalently linked porphyrin dimers

摘要

Abstract: Unusual absorption and photophysical properties are revealed for a novel fixed-distance symmetric ethylene-bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbis $EQ OEP in short), investigated by the methods of absorption and fluorescence steady- state spectroscopies as well as picosecond fluorescence and femtosecond transient absorption time-resolved spectroscopies. It is found that in solutions of organic solvents, tbis $EQ OEP exists in two isomeric forms: an isomer P, and an isomer U (ca. 85% and 15%, respectively, in toluene). It was found that lifetimes of the lowest excited singlet states of both isomers are strongly dependent on solvent viscosity, the isomer U displaying the stronger dependence. Particularly, the fluorescence lifetime of the isomer U was found to increase to approximately 0.5 ns in polymer films at room temperature or in rigid glasses of organic solvents at 77 K. To the best of our knowledge, this is the first example of such viscosity-dependent photophysics for porphyrins. A qualitative explanation of these observations is proposed. !3