A covalently linked trimer of 4,4-dimethyl-4H-cyclopenta[def]phenanthrene (MCPP), referred to as tri-MCPP, undergoes spontaneous self-assembly to form highly ordered colloidosomes in the absence of a template when subjected to reprecipitation from THF-water. The colloidosomes, and the oligomers in neat tetrahydrofuran have also been analyzed using optical methods. While the monomer, MCPP, was found to display a low fluorescence quantum yield (Φ F ∼ 0.03) and low molar absorptivity (ε ∼ 900 M -1 cm -1 ) for the S 1 ← S 0 transition due to symmetry restrictions, short oligomers formed via connection at the para phenylene positions displayed corresponding optical features that were found to increase gradually as a function of oligomer length. In the case of the largest species studied, the pentamer (penta-MCPP), the molar absorptivity was ca. 140 times larger than that of MCPP (ε = 126500) for the S 1 ← S 0 transition. The Φ F and radiative rates for the MCPP oligomers were found to be enhanced by a factor of ∼30 and ∼180, respectively, while going from monomer to pentamer, presumably because of the larger radiative size of the oligomers. Further, the colloidosomes derived from tri-MCPP displayed a fluorescence quantum yield (ca. 0.35 to ∼0.20) that was reduced by approximately 1.8 times relative to tri-MCPP. The fluorescence lifetime was likewise decreased by ∼10 times compared to tri-MCPP; presumably, this is the result of intermolecular interactions between the very closely organized oligomers. The fact that highly ordered structures are obtained in the case of tri-MCPP, but not the other species included in this study (monomer; other oligomers), is thought to reflect the mutually orthogonal orientation between the constituent monomeric subunits in the trimer, as well as an ability to stabilize favorable hydrogen bonding interactions with residual solvent entrapped under the conditions of THF-water induced precipitation. Specifically, it is proposed that, in the case of tri-MCPP, these interactions aid in the formation and subsequent stabilization of precise colloidosome superstructures. An analogous stabilization of a colloidosome superstructure was not found for the corresponding tetramer (tetra-MCPP) or pentamer (penta-MCPP), in spite of the fact that these species contain a greater number of monomeric MCPP subunits and would be expected to give rise to ostensibly similar structures. This leads us to suggest that specific structural requirements are required for MCPP-derived precursors to form self-assembled colloidosomes.
ASJC Scopus subject areas
- Materials Chemistry