Simultaneous measurement of mechanical and fluorescence properties in a single chain of conjugated polyfluorene photodynamic attraction by aggregates within the polymer chain | Tokyo Tech News | Tokyo Institute of Technology

A research team led by Professor VACHA Martin, Department of Materials Science and Technology, Tokyo Institute of Technology, and a research team led by Professor Takeshi Nakajima, Department of Applied Chemistry, School of Materials Science and Technology, Tokyo Institute of Technology.Conjugated polymer^[用語1]We managed to measure the fluorescence and the mechanical properties of the chain at the same time.

Polyfluorene, which is a conjugated polymer, sometimes emits green light at the same time as normal blue light emission in solution, but the reason for this is not clear. The research team believed that this green emission could be explained by the formation of aggregates between molecular chains, and carried out an experiment. Specifically, one end of a single molecular chain is used as a substrate, and the other end is used as a substrate.Atomic force microscope (AFM)^[用語2]bound to the tip of the molecule, and the luminescence was measured by laser excitation while pulling the molecular chain with AFM. As a result, they discovered that the emission color changed from blue green to pure blue by extending the molecular chain. This is thought to be because the aggregate formed in the folded molecular chain emits green light, but this is stretched and dissociated, resulting in a change in the emission color of the entire molecular chain. In addition to the emission color change, we have also discovered that photodynamic attraction occurs in single molecular chains. This attraction is associated with the dissociation of aggregates andvan der waals force^[用語3]aexciton interaction^[用語4]derived from

The information obtained in this research will contribute greatly to the field of molecular photophysics. In addition, this research is a new example that shows the conversion of energy from light to power at the molecular level, and is key to the development of molecular motors.

This research was carried out by Professor VACHA Martin, Assistant Professor Shun Omagari, Department of Materials Science and Technology, Tokyo Institute of Technology, Professor Ken Nakajima, Assistant Professor Liang Xiaobin, Department of Applied Chemistry, School of Materials Science and Technology, Tokyo Institute of Technology .Nano ASC” published in.

molecular assembly^[用語5]is a phenomenon that has been known for almost a century, and its unique photophysical characteristics have fascinated many scientists. The specific physical properties of aggregates have also been used in the creation and development of new functional materials. Phenomena peculiar to molecular bonds are not limited to low molecular weight dyes. Even in conjugated polymers, spectral features appear due to interactions between constituent sites in dense structures and thin films, causing similar phenomena to those of low molecular weight aggregates. One example is polyfluorene, which exhibits high intensity blue emission in solution. This blue light emission may be accompanied by green light emission, and the cause has long been debated. Although it is now widely recognized that green emission is due to photo-oxidation, experimental evidence can also be explained by the formation of a link between molecular chains. In this study, by performing fluorescence microscopy, spectroscopy and force curve measurements on a single polyfluorene chain, we observed that the green emission band disappeared when the molecular chain was elongated. This is consistent with the concept of the dynamic development of linkage states in folded molecular chains.

Theoretically, in the formation of molecular bonds,ground state^[用語6]aagitated state^[用語7]In both, the attraction and its associated energy are attributed to the electronic interaction between molecules. However, at present, there is no direct observation of such attraction and energy in molecular bonds. The research group found that stretching a single polyfluorene molecular chain under UV light irradiation caused a new peak in the force curve. This new photodynamic force was discussed from the point of view of interactions through aggregation in single molecule chains.

In this study, we used poly(9,9-dioctylfluorene) (PFO), which has amino groups at both ends, in dynamic stretching experiments (force spectra) of single polyfluorene molecular chains. This amino group binds covalently to epoxy groups on silane-modified quartz substrates and silicon cantilever tips for atomic force microscopy (AFM) (Fig. 1a). Looking at an example of a force curve (extension force curve) shown in Fig. 1b, a large force peak reaches hundreds of pN. This means that the PFO molecular chain is stretched to the limit until the bond breaks. In the experiment, the fluorescence spectrum was measured by fluorescence microscopy while the PFO molecular chain was long under UV excitation light irradiation (375 nm). At the beginning of the molecular chain elongation, it exhibited a blue-green emission consisting of a green emission band and a blue emission band seen in common PFO (Fig. 1c). After the initial step, the green emission band disappeared as it was stretched, leaving only the blue emission band observed in normal PFO, and this emission was maintained even after bond dissociation (Fig. 1d). The disappearance of the green emission band due to the mechanical stretching of the molecular chains is direct evidence that the connections within the molecular chains (Fig. 1e, f) are the cause of the green emission band.

In some PFO molecular chains, a characteristic peak (Fig. 1b) occurs at the beginning of the force curve during stretching only when irradiated with UV light, suggesting that a new photodynamic force is at work. This force corresponds to the dissociation of the aggregate (Fig. 1e,f) and originates from interactions in the ground and excited states between molecules. By analyzing these peaks, the association energy can be determined. Statistics from multiple measurements of this energy revealed a broad distribution with an average of 0.55 eV (Fig. 1g). Further analysis also revealed that the attractive force of such associations works for molecules larger than 4.6 nm on average. These numbers are consistent with the photodynamic forces due to van der Waals forces and exciton interactions.

This research is an innovative example of the mechanical manipulation of dyes under light irradiation, and can be applied to molecules other than those dealt with in this research. Aggregates like this one and related phenomena can be developed as a new method of converting light into mechanical energy, like a molecular motor. In addition, exciton transport in polymer nanofibers and the mechanical properties of the molecular chains themselves can be controlled by nano-level mechanical manipulation.