Reversible Metamorphosis of a Conjugated Polymer Induced by Photoexcitation

Optoelectronic properties of materials are frequently examined through spectroscopic tools based on the absorption and emission of (visible) photons. Typically, one assumes that these transient characterization processes do not cause observable change of these properties because of the low power of the employed light sources. However, for flexible conjugated polymers in a medium of low viscosity, it is decidedly unclear whether photoexcitation of electrons on the backbone of a long polymer chain has an influence on macromolecular conformations and the corresponding intermolecular interactions. Here, we provide clear experimental evidence that continuous absorption of photons progressively causes persistent changes in the spectroscopic properties of conjugated polymers, accompanied by microscopically observable changes in their spatial distribution within a low‐viscosity matrix. By contrast, all these changes were completely absent in nonilluminated parts of the same sample. Quantitative spectral analysis allowed to identify the fraction of molecules that underwent such metamorphosis, which increased with the dose of photoexcitation, that is, with the number of absorbed photons. Complementary experiments revealed the reversibility of this behavior and so demonstrated the integrity of the polymers. Our results depict new pathways for tailoring properties of conjugated polymers and widening their spectrum of advanced engineering applications.