Charge-transfer complex study by chemical force spectroscopy: a dynamic force spectroscopic approach

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Charge-transfer interaction between poly(9-vinylcarbazole) and 3,5-dinitrobenzamido group or 3-nitrobenzamido group

We studied the charge-transfer (CT) interaction between poly(9-vinylcarbazole) (PVK) and 3,5-dinitrobenzamido (DBA) group or 3-nitrobenzamido (NBA) group. The complexation equilibrium constant of PVK and N-butyl-3,5-dinitrobenzamide was found to be larger than that of PVK and N-butyl-3-nitrobenzamide using UV-vis spectroscopy. The desorption process of PVK from a DBA-modified substrate was studied with single molecule force spectroscopy. A lot of force-extension curves with a plateau were observed, which indicated the existence of trainlike conformation. The average desorption force of a PVK chain from the DBA-modified substrate was 28 pN. For comparison, the desorption process of PVK from an NBA-modified substrate was also studied, and an average desorption force of 18 pN was obtained. The apparent interaction energy of a CT complex was calculated on the basis of the desorption force and the surface density of electron acceptors. The obtained apparent interaction energies were 14 and 8 kJ x mol(-1) for the carbazolyl-DBA complex and carbazolyl-NBA complex, respectively. On the basis of the energy of the frontier molecular orbital of carbazole, 3,5-dinitrobenzamide, and 3-nitrobenzamide, the difference in strength between the interactions was interpreted as the influence of the substituent group on the energy of the frontier molecular orbital.

Direct measurements of the interaction between pyrene and graphite in aqueous media by single molecule force spectroscopy: understanding the pi-pi interactions

Pyrene derivatives can absorb onto the surface of carbon nanotubes and graphite particles through pi-pi interactions to functionalize these inorganic building blocks with organic surface moieties. Using single molecule force spectroscopy, we have demonstrated the first direct measurement of the interaction between pyrene and a graphite surface. In particular, we have connected a pyrene molecule onto an AFM tip via a flexible poly(ethylene glycol) (PEG) chain to ensure the formation of a molecular bridge. The pi-pi interaction between pyrene and graphite is thus indicated to be approximately 55 pN with no hysteresis between the desorption and adhesion forces.