Book Description

The self-assembly of organic molecules has attracted much interest because of the special properties and possible technological applications of the mesoscopic materials which are intermediate materials between molecules and solids.1 Structural and spectroscopic studies on J- and H-aggregates present valuable information for understanding interactions at the molecular level in aggregation processes, and may also provide understanding that leads to alternative molecular devices. The synthesis and preliminary photophysical properties of two generations (G1-G2) of glutamic acid-based peptide dendrimers containing free-base or zinc porphyrin at the cores were performed. In this work, steady-state absorption and fluorescence experiments for the free-base dendrimers 59 and 63 were observed to have similar spectra relative H2TPP in DMAc, while the absorption and fluorescence spectra of zinc porphyrin dendrimers 60 and 64 were almost identical to that of ZnTPP in DMAc. These dendrimers tend to not to aggregate in polar solvents like DMAc, therefore, other nonpolar solvents will be sought for self-assembly these dendrimers. Second, two electon donor (D)- acceptor (A) dyads are composed of free-base porphyrin (Fb) and perylene bisimide (PDI) groups that are linked by leucine (65) or polyleucine (n=2, the dyad 66) were prepared in this work. Fb and PDI were chosen because of the useful photophysical properties each exhibits that will enable us to perform both energy and electron transfer within each dyad. The absorption spectra of the dyads indicate a close match with the sum of the component spectra, indicating little or no electronic coupling occurs between porphyrin and PDI groups in the ground state. Due to the overlap of the emission bands of porphyrin and perylene, fluorescence quantum yield analysis could not be determined with any accuracy. Fluorescence excitation spectra will therefore be performed in the near future. Intramolecular FRET experiments will also be achieved on these two dyads, whereas these measurements will allow us to precisely determine the donor-acceptor distance at any given time. After we measure the distance between the donor-acceptor dyad, we will investigate how the rate of the electron transfer varies with the distance, and what effect the dyad structure has in controlling this rate constants. The photophysical and electrochemical properties of covalently-linked arrays of N-confused porphyrins (NCPs) are of interest to a variety of supramolecular and biomimicking materials. While differing from the parent porphyrins by inversion of only two atoms, NCPs exhibit physical and chemical properties different than normal porphyrins. The synthesis and preliminary photophysical properties of three dimeric and trimeric N-confused porphyrin (NCP) arrays (75, 77 and 80) are presented. The absorption and emission maxima for the NCP dimer (75) and trimers (77 and 80) in DMAc were found to be broader and red-shifted relative to that of NC-TPP. The fluorescence quantum yield [Phi]Fl calculated for these NCP arrays were found to be (~ 91-97%) smaller than the monomer NC-TPP. This fluorescence quenching can be attributed to energy-transfer between NCP units. Time-resolved fluorescence experiments will therefore be performed on these NCP arrays in the near future. Here, I also report the synthesis and preliminary characterization of a donor-acceptor triad containing zinc N-confused porphyrin dimer (ZnNCP) 87 and pyridine-appended PDI 95, to generate triad 89. The formation of triad 89 was monitored using UV-visible absorption and fluorescence spectroscopy upon titrating PDI 95 to the ZnNCP dimer 87 in CH2Cl2. The absorption intensity at 462 nm that is attribiuted to the free monomer ZnNCP reached a maximum with the addition of equal amounts of PDI 95 to ZnNCP 87. Further additions of PDI did not result in appreciable changes in the absorption intensity of the Soret and Q bands, confirming the complete dissociation of ZnNCP dimer and formation of the triad complex 88. The fluorescence intensity of ZnNCP monomer reached the maximum upon addition of one equivalent of the PDI 95. The intensity of ZnNCP emission was diminished with addition of access of PDI. The emission quenching with excess PDI addition can be attributed to formation of the penta-coordinate ZnNCP-PDI complex 88. This complex formation will further be investigated by means of time-resolved fluorescence spectroscopy as well as mass spectrometry.