Molecular Engineering of Nonplanar Porphyrin and Carbon Nanotube Assemblies: A Linear and Nonlinear Spectroscopic and Modeling Study

Abstract

The importance of molecular conformation to the nature and strength of noncovalent interactions existing between a series of increasingly nonplanar tetraphenylporphyrin (TPP) derivatives and carbon nanotubes was systematically investigated experimentally in solution using a range of linear and nonlinear optical techniques. Additional complementary molecular dynamics studies were found to support the experimental observations. Convincing evidence of binding between single walled nanotubes (SWNTs) and some of these porphyrins was discovered, and a nonplanar macrocycle conformation was found to increase the likelihood of noncovalent binding onto nanotubes. Nonlinear optical studies showed that the optical limiting behavior of the TPP derivatives deteriorated with increasing porphyrin nonplanarity, but that formation of nanotube composites dramatically improved the optical limiting properties of all molecules studied. It was also found that the significant photoluminescence quenching behavior reported in the literature for such porphyrin/SWNT composites is at least partly caused by photoluminescence and excitation self-absorption and is, therefore, an artifact of the system.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

“…Except this example, interaction of non-planar porphyrins with carbon allotropes such as carbon nanotubes has been also studied with phosphorescence-quenching experiments. 308 Hydrogen-bonding supramolecular assembly with a POM. In contrast to the cases of [{Mo V 72(O)} 2 (H n XW 12 O 40 )] (X = B, Si, P, in which terminal oxo ligands directly bind to the Mo(V) centers to afford 'porphyrin hamburgers', 290,292 Thus, the formation of 235 is an entropy-controlled process.…”

“…Dodecaarylporphyrins, 81 such as those reported by Kojima and co-workers, 189 result in saddle-shaped porphyrins in only some cases; these more conformationally flexible examples adopt saddle, ruffle or wave conformations, dependent on the intermolecular packing arrangement of the crystal structure. 134,166 b Structures of dodecaarylporphyrins have been reported with M = 2H, 81,190 3H, 191 4H, 189 b V IV , 189 a Mn III , 192 Fe III , 193 Co II , 81 Ni II , 148,190 Cu II , 81 Zn II , 194 Mo V , 195 Sn II , 142 Sn IV , 196 Rh, 197 Ru 198 and Os. 198…”

“…Except this example, interaction of non-planar porphyrins with carbon allotropes such as carbon nanotubes has been also studied with phosphorescence-quenching experiments. 308…”

“…(Metallo)porphyrins on surfaces can achieve nonplanarity related to their crystalline counterparts, with additional deformations imposed by the proximal surface. 130,134…”