The Study On Synthesis Of Meso-Coumarin-Conjugated Porphyrins And Investigation Of Their Luminescence Properties

Polychromophoric molecules have attracted intense attention recently, as they can be exploited as artificial energy transfer systems with useful applications in a variety of areas such as organic light-emitting diodes (OLED), light harvesting systems, and chemosensors.In a recent communication, we described a series of coumarin-conjugated porphyrins (Eur. J. Org. Chem. 2007, 4301-4304.) exemplified by Meso-tetra(7-diethylaminocoumarin-4-yl)porphyrin in which the coumarin dye and porphyrin fluorophore function as the energy donor and acceptor, respectively. Upon excitation (at the absorption maximum of the coumarin dye), the excitation energy was absorbed by the coumarin donor and transferred to the porphyrin acceptor. In this coumarin-conjugated porphyrin system, although the energy transfer was efficient in solid film, the energy transfer in solution was relatively inefficient. Only around 73 % energy transfer efficiency was obtained in solution. We hypothesized that this inefficient energy transfer could be due to two main reasons. First, the spectral overlap between the emission spectrum of diethylaminocoumarin donor and the emission spectrum of porphyrin acceptor was rather limited, which is not favorable for the long-range F?rster type resonance energy transfer. Second, the relatively steric bulky coumarin moiety was directly connected to the poprhyrin core, which may force them to have a big torsion angle. This may reduce the conjugated extent of them and hamper the energy transfer through bonds.Building on our previous report, the aim of this work was to enhance the energy transfer efficiency of the coumarin-porphyrin dichromophoric system in solution. With this goal in mind, we constructed dichromophoric compounds based on the above hypothesis. We selected 7-butoxy-coumarin or unsubstituted coumarin as the energy donor, because the emission spectra of these two coumarin derivatives have a strong overlap with the absorption spectrum of the porphyrin unit. This may facilitate the F?rster type energy transfer. Furthermore, a phenyl spacer was incorporated between the coumarin dye and the porphyrin core to potentially decrease the torsion angle between them and thus to increase the conjugated extent. This may be beneficial for the energy transfer from the coumarin fluorophore to the porphyrin core through bonds.We designed and synthesized a series of new coumarin-conjugated porphyrins as novel artificial energy transfer systems. Indeed, as designed, in solution new energy transfer systems display an almost perfect energy transfer efficiency of 99.8, 97.7 and 96.1%, respectively, which is much higher than that of previous reported Meso-tetra(7-diethylaminocoumarin-4-yl)porphyrin. The improvement in energy transfer efficiency could be ascribed to the enhancement of the spectral overlap between the donor emission and the acceptor absorption and the decrease of the torsion angle between the donor and acceptor. In solid film, these coumarin-conjugated porphyrins still maintain high energy transfer efficiency. We believe that these novel energy transfer dyads should find interesting applications in many areas.