Excimer

About: Excimer is a research topic. Over the lifetime, 3725 publications have been published within this topic receiving 75104 citations.

Exciton delocalization and dynamics in helical π-stacks of self-assembled perylene bisimides

Abstract: Whilst the excitonic properties of J-aggregates have been investigated in great detail, those of H-aggregates have not been systematically investigated yet. In this regard, we have explored the exciton dynamics and excited-species formation processes in columnar H-aggregates of planar PBI dyes that are stacked in a helical fashion by various spectroscopic techniques such as time correlated single-photon counting and femtosecond pump–probe measurements with anisotropy changes. The outcome of this study is that photogenerated excitons in helically stacked PBI dyes experience complicated relaxation processes that involve excited-state interactions such as exciton delocalization and excimer formation. To scrutinize the exciton dynamics in the helically stacked aggregates, we have also included distorted bay-substituted PBI dyes as reference molecules that exhibit either no or only relatively small-sized dimeric aggregate structures. The comparative study revealed that the excited-state interactions in the large-sized helically stacked aggregates extend beyond two PBI units, leading to a final excimer (here, excimer means not only an “excited dimer” but an “excited multimer”) trap state within ∼50 ps. Although in competition with this relaxation path into the excimeric trap state, exciton diffusion has been revealed by exciton–exciton annihilation processes, occurring at high excitation power. Whilst the excimer formation process interrupts the direct observation of exciton diffusion in these columnar PBI aggregates, the exciton migration distance could be estimated by the incorporation of non-fluorescent PBI quencher molecules. From this analysis we can conclude that the exciton diffusion can reach a length of about 10 monomer units. Although this value appears to be shorter than those values observed for J-aggregates, this result shows that columnar PBI stacks might still be useful for optoelectronic applications if the relaxation process leading to excimer traps is prevented, e.g. by structural modifications of the molecules.

Excimer formation in cofacial and slip-stacked perylene-3,4:9,10-bis(dicarboximide) dimers on a redox-inactive triptycene scaffold

Abstract: Excitation energy transfer in perylene-3,4:9,10-bis(dicarboximide) (PDI) aggregates is of interest for light-harvesting applications of this strongly absorbing and π–π stacking chromophore. Here we report the synthesis and characterization of two PDI dimers in which the chromophores are covalently linked by a redox-inactive triptycene bridge in orientations that are cofacial (1) and slip-stacked along their N–N axes (2). Femtosecond transient absorption experiments on 1 and 2 reveal rapid exciton delocalization resulting excimer formation. Cofacial π–π stacked dimer 1 forms a low-energy excimer state absorption (λmax = 1666 nm) in τ = ∼2 ps after photoexcitation. Inserting a phenyl spacer on the bridge to generate a slip-stacked PDI–PDI geometry in 2 results in a less stable excimer state (λmax = 1430 nm), which forms in τ = ∼12 ps due to decreased electronic coupling. The near-infrared (NIR) excimer absorption of cofacial dimer 1 is ∼120 meV lower in energy than that of slip-stacked dimer 2, further highlighting electronic differences between these states.

Bifunctional fluorescent calix[4]arene chemosensor for both a cation and an anion.

Abstract: A new fluorogenic cone calix[4]triazacrown-5 (1) bearing two pyrene amide groups and its structural analogue 2 have been prepared. Excited at 343 nm, 1 and 2 reveal excimer emissions at 448 and 472 nm, respectively. When heavy metal ions such as Pb2+ and Co2+ are bound to 1, the fluorescence intensities of both monomer and excimer are quenched whereas H bonding-assisted F- binding to 1 gives rise to a quenched monomer emission with little excimer emission change.

A Doubly Alkynylpyrene-Threaded [4]Rotaxane That Exhibits Strong Circularly Polarized Luminescence from the Spatially Restricted Excimer

Abstract: The Sonogashira coupling of γ-CD-encapsulated alkynylpyrenes with terphenyl-type stopper molecules gave a doubly alkynylpyrene-threaded [4]rotaxane. The rotaxane showed only excimer emission, with a high fluorescence quantum yield of Φf=0.37, arising from the spatially restricted excimer within the cavity of the γ-CD. The excimer emission suffered little from self-quenching up to a concentration of 1.5×10−5 M and was circularly polarized with a high glum value of −1.5×10−2. The strong circularly polarized luminescence may result from the two stacked pyrenes existing in the rotaxane in an asymmetrically twisted manner.