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Davydov splitting and singlet fission in excitonically coupled pentacene dimers
Singlet fission (SF) allows two charges to be generated from the absorption of a single photon and is, therefore, potentially transformative toward improving solar energy conversion. Key to the present study of SF is the design of pentacene dimers featuring a xanthene linker that strictly places two pentacene chromophores in a rigid arrangement and, in turn, enforces efficient, intramolecular π-overlap that mimics interactions typically found in condensed state (e.g., solids, films, etc.). Inter-chromophore communication ensures Davydov splitting, which plays an unprecedented role toward achieving SF in pentacene dimers. Transient absorption measurements document that intramolecular SF evolves upon excitation into the lower Davydov bands to form a correlated triplet pair at cryogenic temperature. At room temperature, the two spin-correlated triplets, one per pentacene moiety within the dimers, are electronically coupled to an excimer state. The presented results are transferable to a broad range of acene morphologies including aggregates, crystals, and films. © 2019 The Royal Society of Chemistry.
Rhodamine 6G and 800 intermolecular heteroaggregates embedded in PMMA for near-infrared wavelength shifting
The opto-electronic properties of small-molecules and functional dyes usually differ when incorporated into solid matrices with respect to their isolated form due to an aggregation phenomenon that alters their optical and fluorescent properties. These spectroscopic modifications are studied in the framework of the exciton theory of aggregates, which has been extensively applied in the literature for the study of molecular aggregates of the same type of molecules (homoaggregation). Despite the demonstrated potential of the control of the heteroaggregation process (aggregation of different types of molecules), most of the reported works are devoted to intramolecular aggregates, complex molecules formed by several chromophores attached by organic linkers. The intramolecular aggregates are specifically designed to hold a certain molecular structure that, on the basis of the exciton theory, modifies their optical and fluorescent properties with respect to the isolated chromophores that form the molecule. The present article describes in detail the incorporation of Rhodamine 6G (Rh6G) and 800 (Rh800) into polymeric matrices of poly-(methyl methacrylate), PMMA. The simultaneous incorporation of both dyes results in an enhanced fluorescent emission in the near-infrared (NIR), originating from the formation of ground-state Rh6G–Rh800 intermolecular heteroaggregates. The systematic control of the concentration of both rhodamines provides a model system for the elucidation of the heteroaggregate formation. The efficient energy transfer between Rh6G and Rh800 molecules can be used as wavelength shifters to convert effectively the light from visible to NIR, a very convenient wavelength range for many practical applications which make use of inexpensive commercial detectors and systems.
Thermally triggered optical tuning of π-conjugated graft copolymers based on reversible Diels–Alder reaction
In order to design a π-conjugated polymer film with tunable optical properties by thermally triggered activation of energy transfer after processing, two monodisperse phenylene ethynylene based oligomers with different optical properties were synthesized and attached to aliphatic polymers as π-conjugated side chains. Subsequently, the exchange of the side chain chromophores between the prepared donor and acceptor graft polymers in the solid state based on a reversible Diels–Alder reaction was studied in detail. The resulting donor–acceptor graft copolymer exhibits intra polymer energy transfer upon excitation of the donor moiety. The photophysical properties of the original and exchanged graft copolymers were investigated by means of absorption and emission spectroscopy. This novel concept opens the possibility for optical tuning of π-conjugated polymer films after processing as well as applications as thermally triggered sensor systems.