Excimer interchain

Many studies on side-chain modifications in PF were initially based on the idea of excimer formation, resulting in the green emission during LED operation or in solid-state PL on annealing PF films. This resulted in several proposed strategies for the design of fluorene side-chain homopolymers, where bulky substituents at position 9 of the fluorene moiety should sterically prevent (hinder) interchain interaction and thus improve the stability of blue emission. 

Fluorescent hydrophobes (naphthyl and pyrenyl groups) incorporated into the polysulfobetaines are a powerful tool for studying the formation of intra-and interpolymer aggregates in aqueous and aqueous salt solutions [85,229-231]. Intermacromolecular hydrophobic association is observed as an increase in the excimer emission relative to that of the monomer emission, where h/Iu is the ratio of intensities of excimer and monomer fluorescence which reflects the extent of inter/intra macromolecular interactions. Intramolecular micellization is easily monitored by the quenching efficiency of the thallium ions. The decrease of h/Iu reflects the breaking of the intra- and interchain associations in aqueous salt solutions, leading to chain expansion. 

Hyperbranch also attracts a lot of interest for developing efficient electroluminescent devices. Since the hyperbranched and globular structure can depress the interchain interaction and excimer formation so as to make the materials form high-quality amorphous films, and consequently improve the stability, light-emitting efficiency, and emission color purity. Bo and coworkers reported the first example of hyperbranched polymers used as blue light-emitting materials in 2004 (Li and Bo, 2004). They adopted a novel ABa + AB ... 

Advincula et al. oxidised the polyionene 87 withiron(III) chloride to obtain an insoluble polyfluorene network 88 (Scheme 40) [171]. This shows violet PL ( max 410 nm), suggesting that the polyfluorene segments are rather short. Due to its insolubility this material cannot be used to make good-quality films for use in LEDs, but the incorporation of a conjugated backbone within a network is one possible way to obtain isolated chains and so avoid the problems associated with interchain interactions, e.g. excimer formation. 

Weak interchain electronic coupling. Interchain excited state complexes (e.g. excimers and exciplexes), which may quench luminescence. Dephasing of inter-molecular wavefunctions. 9... 

The charge-transfer component further stabilizes the excimer in two ways. First, the intermolecular Coulomb interaction induces an attraction between the electron-hole pair, resulting in a weakly bound charge-transfer exciton. Second, the interchain one-electron Hamiltonian couples the charge-transfer component with the exciton component. 

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