Porphyrin and carbon nanotube assemblies in polar solvents

One of the chief obstacles to exploiting the useful electronic and materials properties of single-wall carbon nanotubes (SWCNTs) is their inclination to form ropes and bundles. Understanding the reaction paths involved in the transition from isolated SWCNTs to bundles in the presence of solvent is basic to controlling the process. Single- and multiple-wall carbon nanotubes (CNTs) in polar, or charged, solvents can also form aggregate assemblies and macromolecular complexes with porphyrin derivatives. This potential is of great interest, as the structural and optical properties of porphyrin derivatives and complexes can be easily engineered, a reality evident not just in the laboratory but also in nature. Indeed, in photosynthesis and other processes, the quantum mechanisms governing charge and energy transfer processes are fundamental to life.

Recent experiments investigating CNTs in amide solvents have led to the debatable conclusion that dispersion and partial debundling can be achieved at low nanotube concentrations with a variety of highly polar solvents possessing high surface tension.¹ Among these, N-methylpyrrolidone (NMP) is considered to be the most effective. In particular, it has been postulated that at very low concentrations, the equilibrium (stable) state is a debundled one. Moreover, whether dispersion occurs appears to depend strongly on the method of sample preparation. Taken together, these results suggest that the debundled state is, in fact, not in equilibrium but is metastable (transient though relatively long-lived).

We conducted a simulation study² in which we drastically simplified the problem at hand, reducing it to the study of two SWCNTs in a solvent. The order parameters then describe the relative distances and orientations of the pair (see Figure 1), whereas the free-energy dependence of these parameters is computed using so-called umbrella sampling. The energy barrier to drawing the SWCNTs together to form a bundle in the geometry shown in Figure 1 (left) is so large as to be essentially insurmountable. Moreover, four molecular layers of solvent between the two SWCNTs are visible.

Figure 1. Two identical uncapped parallel (left), identical perpendicular uncapped (center), and identical capped (right) SWCNTs in an NMP solvent. Yellow represents an NMP molecule selected to suggest the relative scale of the nanotubes compared with the solvent molecules. In the first two cases, the inter-SWCNT distance is varied, with the pair of tubes remaining parallel to a single side of the periodic box (left) and each SWCNT remaining parallel to different sides of the periodic box (center). In the third case (right), the torsion angle between the pair of SWCNTs is varied from 90 to 180 degrees.

The nature and strength of adsorption between a series of nonplanar conjugated tetraphenylporphyrin (TPP) derivatives and CNTs in a solution of dimethylformamide (DMF) was investigated using molecular simulation.^3,6 Convincing evidence of binding between SWCNTs and some of these porphyrins was discovered, and a nonplanar macrocycle conformation was found to increase the likelihood of adsorption onto CNTs. This was also observed experimentally through linear and nonlinear spectroscopy.⁴ The two extreme cases, diethyltetraphenylporphyrin (DETPP) and TPP, are presented in Figure 2, along with the starting conditions for the simulations, using hydroxyethyltetraphenylporphyrin (HETPP).

Figure 2. (a) Simulation initial conditions are shown for the HETPP-SWCNT-DMF system. Although there are 20 HETPP molecules, only 4 of them, a single DMF (yellow) molecule and the SWCNT, are actually rendered using effective van der Waals diameters (a measure of the molecules’ extent). (b) The pronounced tendency of DETPP to aggregate on and close to the nanotube is evident. (c) The TPP-CNT-DMF solvent system. TPP does not typically aggregate on or close to the CNT.

The results of these exercises, and the fact that distribution of SWCNT bundles in amide solvents without massive injection of energy has never been reported experimentally, support the following conclusion: CNT dispersions in NMP are not in equilibrium, as suggested recently, but are most likely metastable, separated from far more stable bundle states by a major free-energy barrier. The transition from a metastable dispersed state to one dominated by bundles is an activated process, and therefore should happen more quickly at higher temperatures. The reverse transition, however, from bundle to dispersion, is practically impossible without pumping substantial energy into the system, such as high-intensity ultrasound.

The possibility of engineering the macrocycle of TPP derivatives to favor their adsorption onto SWCNTs in solution, as demonstrated here, opens up the possibility of engineering the role of nonadiabatic nonradiative channels of decay through simulation⁵ and spectroscopy. These channels play an important role in ultrafast charge and energy transfer, photosynthesis, solar cells, and optical limiters. The remaining challenges are to control the lifetime of the metastable SWCNT state for long enough to enable processing, and to design porphyrin-SWCNT assemblies with specific structural and optical properties. The steps that we will take toward this end include modeling the behavior of an SWCNT pair in an NMP solvent under flow conditions (shear) and further modeling of porphyrin-SWCNT assemblies.