Kinetic Control in the Alignment of Polar π-Conjugated Molecules inside Carbon Nanotubes

Dispersion-corrected density functional theory (DFT) calculations analyze potential energy surfaces (PESs) for the insertion of a certain polarized pi-conjugated molecule, p'(dimethylamino)-p'-nitrostilbene (DANS), inside an (m,m) carbon nanotube (nXDANS@(m,m)), where n is the number of inner guests. The current study considered two types of DANS alignments as the dimer structures: linear and stacked alignments, where their dipole moments are oriented in parallel. DFT calculations revealed that a single DANS insertion into a tube with a diameter of approximately 1.0 nm spontaneously proceeds through attractive host-guest interactions. From an end to the middle of a tube, various metastable states and a global minimum structure appear. In 2XDANS@(m,m), which is assumed to be generated by the insertion of another molecule into the tube that already contains one guest, significant roles of intermolecular interactions are found to distinguish their PESs from the 1XDANS@(m,m) case. In addition, the strength of the intermolecular interactions varies depending on the inner DANS dimer alignment and host tube diameters, and thus, striking contrasts were found in PESs for 2XDANS@(m,m). In the formation of linearly aligned DANS molecules inside a tube, the PES is almost identical to that in 1xDANS@(m,m) due to very weak intermolecular interactions. By contrast, intermolecular interactions between the parallel stacked DANS molecules are sufficiently stronger to distinguish their PESs from those in 1xDANS@(m,m); attractive (repulsive) intermolecular interactions within the (8,8) ((7,7)) tube stabilize (destabilize) PESs that are involved in their metastable states and global minimum structure. More importantly, in the stacked alignments, we found the activation energy for the transformation from a metastable state to a global minimum, for which the value decreases in the following order: 20 kcal/mol for 2xDANS@(7,7) > 13 kcal/mol for 2XDANS@(8,8). According to the DFT findings, one can kinetically control the alignments of the DANS molecules inside a thicker tube (i.e., (8,8) tube). In fact, through temperature optimization in heat treatments where the above-mentioned activation energy cannot be overcome, a one-dimensional array of DANS molecules can be formed inside a thicker tube despite its thermodynamic instability. Since linear DANS alignments are responsible for maximizing static hyperpolarizability, the kinetic control technique can expand the diameter ranges of tubes as hosts for DANS guest aggregates exhibiting a significant second-order nonlinear optical response.