Green emission peaks in single polyfluorene chains: Understanding the process

Clusters of small molecules that are held together by relatively weak forces that are thought to originate from electronic interactions between the molecules are known as molecular aggregates. Due to their exceptional photophysical properties, molecular aggregates continue to be the focus of intense research and have unique applications in numerous technological fields. As a matter of fact, specialists have used sub-atomic totals to design different practical materials, biomedical instruments, and nanodevices.

However, molecular aggregation is not just a problem with small molecules. Individual polymer fragment chains can likewise collaborate with each other and bring about peculiarities that are basically the same as those tracked down in atomic totals. Polyfluorene, a conjugated polymer that, when excited by ultraviolet light or an appropriate electric field, produces a brilliant blue light in solution, is one such example.

“Our research provides concrete proof that the green emission band is caused by intra-chain PFO aggregation, which disintegrates upon mechanical unfolding and causes the green emission to vanish.”

 Professor Martin Vacha, 

The spectral peaks of polyfluorene have long been the subject of speculation, despite the fact that it is a well-studied polymer. It just so happens that, notwithstanding the blue discharge, it once in a while likewise radiates a green emanation, the reason for which has been bantered about by mainstream researchers. The occurrence of a photooxidation reaction is one plausible explanation for the green emission. Notwithstanding, there is likewise proof that inclines toward the interchain conglomeration of chain portions as a reason.

An innovative experimental protocol that aimed to close this knowledge gap was recently developed by a group of researchers from Japan’s Tokyo Institute of Technology under the direction of Professor Martin Vacha. In their review distributed in ACS Nano, the specialists joined two microscopy methods to perform exact photomechanical estimations in poly(9,9-dioctylfluorene) (PFO), a prototypical sort of polyfluorene.

First, a single PFO chain was chemically attached to a transparent substrate on one end and the tip of an atomic force microscope (AFM) on the other. The PFO chain was initially folded, and the position of the AFM tip was precisely controlled to slowly stretch it.

Accordingly, the AFM could continually check how much mechanical power was expected for the PFO chain to extend. Using a fluorescence microscope positioned beneath the substrate, the team simultaneously measured the spectra of the UV light emitted by PFO. They were able to study the mechanical stretching-induced changes in the PFO chain’s emission spectrum as a result of this.

During the initial phase of stretching, they noticed that the blue emission band was accompanied by a green emission band. As the folded chain was stretched and the intra-chain aggregates broke up, the AFM measurements confirmed that the green emission band eventually vanished.

Making sense of these perceptions, Prof. According to Vacha, “Our work presents an immediate proof that the green outflow band is seen due to intra-chain conglomeration of PFO, which goes through separation on mechanical unfurling and brings about the vanishing of the green emanation.”

Furthermore, the group likewise noticed extra otherworldly elements, proposing a novel optomechanical force following up on a portion of the PFO chains. They discovered that the breakup of intra-chain aggregate coupling upon UV illumination was the source of this force after conducting additional analysis. Exact analyses were used to determine the nature of this coupling.

“The force by which the aggregate was coupled and the length for which the coupling was effective revealed that this optomechanical force is the result of van der Waals interactions and excitonic coupling,” Prof. Vacha says.

These spearheading methods could track down charming applications in different fields, remembering energy change for atomic engines or the nano-mechanical control of sub-atomic properties in polymer chains and atomic nanofibers.

More information: Tomonori Nakamura et al, Simultaneous Force and Fluorescence Spectroscopy on Single Chains of Polyfluorene: Effect of Intra-Chain Aggregate Coupling, ACS Nano (2023). DOI: 10.1021/acsnano.2c09773

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