Final Report Summary - MINT (Mechanically Interlocked Carbon Nanotubes)
Among 1D nanomaterials, single-walled carbon nanotubes (SWNTs) are adorned with a particularly attractive set of physical properties. The combination of high electric and thermal conductivity, outstanding strength and stiffness, and a finite band-gap, make them suitable for a wealth of applications, from electronics to biology. At a time where SWNTs are past their hype, it is worth reminding that the advances in synthesis and purification technologies allow working with samples purer than ever. In Pulickel Ajayan’s words “now is the most interesting time to work on carbon nanotubes” (Chem. Eng. News 2015, 93, 10-15).
Mechanically interlocked molecules (MIMs): MIMs consist of two or more separate components which are not connected by chemical (i.e. covalent) bonds. Examples of MIMs are rotaxanes, where one or more macrocycles are trapped onto a linear component (thread) by bulky substituents at its ends (stoppers) that prevent dissociation, and catenanes, where two or more macrocycles are interlocked as links in a chain. These structures are true molecules, as each component is intrinsically linked to the other through a mechanical bond, which prevents dissociation without cleavage of one or more covalent bonds. They are therefore fundamentally different from supramolecular species, where equilibrium between bound and unbound states always takes place.
In this project, We have developed a general strategy for the synthesis of rotaxane-type derivatives of carbon nanotubes, the first example of mechanically interlocked derivatives of SWNTs which we call MINTs (Angew. Chem. Int. Ed., 2014, 53, 5394-5400, Highlighted in Chem&Eng News, 2014, 92, 31; ChemPlusChem, 2015, 80, 1153–1157; Chem. Commun. 2015, 51, 5421-5424.; Chem. Sci. 2017, 8, 1927-1935). In the key rotaxane-forming step, we employ U-shaped macrocycle precursors equipped with two recognition units (i.e. two molecular fragments that like to “stick” to the nanotube) and terminated with bisalkenes that are closed around the nanotubes through ring-closing metathesis (RCM). Because the SWNTs are too long for the macrocycle to find their tips and “fall off”, the mechanical link in MINTs is as kinetically stable as a covalent bond, but without disrupting the native structure of the SWNTs.
Besides the MINT’s interesting molecular architecture, we have proven they might also be useful. For example, we know that MINTs are better at reinforcing polymers than pristine SWNTs, and way better than classic supramolecular complexes with the exact same chemical composition! (ACS Nano 2016, 10, 8012-8018). And we know why: the macrocycles around the SWNTs force the polymers to adopt an elongated conformation, which helps them profit from the extraordinary mechanical properties of the nanotubes.
We also know that the electronic properties of the SWNTs are affected differently than in a classical supramolecular modification (Nanoscale, 2016, 8, 9254-9264).
A deep understanding of weak noncovalent interactions between small molecules and SWNTS was of key importance for MINT. So we have also developed tools that help us understand how molecules interact with nanomaterials.
To study supramolecular interactions, chemists measure association constants, basically, a number that indicates how strong the noncoalent interacitons are, so it allows us to compare different interactions. Despite more than a decade of research in supramolecular chemistry of SWNTs, there was no standard method for the quantification of their noncovalent chemistry in solution/suspension. We managed to solve it by realizing that you actually don’t need to know the molar concentration of SWNTs to extract accurate association constants. It is enough to know the concentrations of free and bound host. To test the method, we measured binding constants between five different hosts and two types of SWNTs in four solvents. We determined numeric values of Ka from 1 M-1 (very very small) to 104 M-1 (rather large) Chem. Sci., 2015, 6, 7008-7014.
Mechanically interlocked molecules (MIMs): MIMs consist of two or more separate components which are not connected by chemical (i.e. covalent) bonds. Examples of MIMs are rotaxanes, where one or more macrocycles are trapped onto a linear component (thread) by bulky substituents at its ends (stoppers) that prevent dissociation, and catenanes, where two or more macrocycles are interlocked as links in a chain. These structures are true molecules, as each component is intrinsically linked to the other through a mechanical bond, which prevents dissociation without cleavage of one or more covalent bonds. They are therefore fundamentally different from supramolecular species, where equilibrium between bound and unbound states always takes place.
In this project, We have developed a general strategy for the synthesis of rotaxane-type derivatives of carbon nanotubes, the first example of mechanically interlocked derivatives of SWNTs which we call MINTs (Angew. Chem. Int. Ed., 2014, 53, 5394-5400, Highlighted in Chem&Eng News, 2014, 92, 31; ChemPlusChem, 2015, 80, 1153–1157; Chem. Commun. 2015, 51, 5421-5424.; Chem. Sci. 2017, 8, 1927-1935). In the key rotaxane-forming step, we employ U-shaped macrocycle precursors equipped with two recognition units (i.e. two molecular fragments that like to “stick” to the nanotube) and terminated with bisalkenes that are closed around the nanotubes through ring-closing metathesis (RCM). Because the SWNTs are too long for the macrocycle to find their tips and “fall off”, the mechanical link in MINTs is as kinetically stable as a covalent bond, but without disrupting the native structure of the SWNTs.
Besides the MINT’s interesting molecular architecture, we have proven they might also be useful. For example, we know that MINTs are better at reinforcing polymers than pristine SWNTs, and way better than classic supramolecular complexes with the exact same chemical composition! (ACS Nano 2016, 10, 8012-8018). And we know why: the macrocycles around the SWNTs force the polymers to adopt an elongated conformation, which helps them profit from the extraordinary mechanical properties of the nanotubes.
We also know that the electronic properties of the SWNTs are affected differently than in a classical supramolecular modification (Nanoscale, 2016, 8, 9254-9264).
A deep understanding of weak noncovalent interactions between small molecules and SWNTS was of key importance for MINT. So we have also developed tools that help us understand how molecules interact with nanomaterials.
To study supramolecular interactions, chemists measure association constants, basically, a number that indicates how strong the noncoalent interacitons are, so it allows us to compare different interactions. Despite more than a decade of research in supramolecular chemistry of SWNTs, there was no standard method for the quantification of their noncovalent chemistry in solution/suspension. We managed to solve it by realizing that you actually don’t need to know the molar concentration of SWNTs to extract accurate association constants. It is enough to know the concentrations of free and bound host. To test the method, we measured binding constants between five different hosts and two types of SWNTs in four solvents. We determined numeric values of Ka from 1 M-1 (very very small) to 104 M-1 (rather large) Chem. Sci., 2015, 6, 7008-7014.