Synthesis and photophysical studies of porphyrin arrays

Abstract

Discovering porphyrin compounds capable of harvesting the light had enhanced the research to mimic the energy and electron transfer process in the artificial photosynthetic systems that can be applied in the development of optoelectronic devices. The well-organized porphyrin arrays as electron transfer centers with attached chromophores are important to ensure the ability of controlled energy and electron flow in the porphyrin building blocks which in turn become great challenges. In this study, newly reported porphyrins with appended fluorenone as the light harvesting antennas were synthesized through Adler Longo and Lindsey’s condensation reactions, and further being assembled into porphyrin dimer, trimer and pentamer arrays via copper-free Sonogashira coupling reaction. Subsequently, the bathochromic shifts in absorption and emission spectra of fluorenone based porphyrins were compared to the reference porphyrin. Afterwards, metallo-porphyrin system with a zinc(II) ion was introduced to influence the electron donating-withdrawing properties by observing the emission population of free base porphyrin. The effect of naphthalene diimide (NDI) in the porphyrin trimer system was also investigated in order to increase the electron push-pull effect and π electron conjugation which caused the quenching of fluorescence quantum yields in more polar solvents. Diphenylacetylene linkage had been used to ensure the overlapping of π electrons between the host porphyrins and promoting the energy transfer in the porphyrin arrays. The roles of the linkages were investigated through comparison of the luminescence spectra of covalent and non-covalently linked compounds. Towards this end, a novel synthetic strategy for alkenyl type porphyrins was developed by using the Tebbe and Petasis reagents, which paved a way to prepare conjugated porphyrins such as vinyl linked porphyrin dimer and bis-fluorenylidene porphyrins. These porphyrin compounds were characterized using 1H and 13C NMR, FTIR, UV-VIS and MALDI-TOF for the structural confirmation. Luminescence properties and fluorescence quantum yields of the targeted compounds were compared with their non-fluorenone and fluorenone porphyrin references. The energy transfer of appended fluorenone into porphyrin system was confirmed by the fluorescence measurement. It was also found that the quantum yields of the porphyrin and NDI building blocks were quenched proposing that electron charge transfer processes occurred in the systems. Overall, a series of porphyrins with appended fluorenone, and the porphyrin dimers, trimers, pentamer and porphyrin-NDI systems newly reported herein were successfully synthesized. The obtained photophysical properties indicated their potential application as materials for photonic devices.