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Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self‐Assembly, and Photophysical Properties
Author(s) -
Chi Chunyan,
Im Chan,
Enkelmann Volker,
Ziegler Andreas,
Lieser Günter,
Wegner Gerhard
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200500275
Subject(s) - fluorenone , polyfluorene , materials science , photoluminescence , intermolecular force , photochemistry , fluorene , electroluminescence , crystallography , quenching (fluorescence) , trimer , pentamer , fluorescence , molecule , chemistry , polymer , nanotechnology , optoelectronics , dimer , organic chemistry , optics , layer (electronics) , biochemistry , physics , composite material
Abstract Oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center ( OF n K : n =3, 5, or 7) were synthesized and used as models to understand the origin of the low‐energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. All compounds form glasses with T g at 30 °C ( OF3 K ), 50 °C ( OF5 K ) and 57 °C ( OF7 K ). Oligomers OF5 K and OF7 K exhibit smectic liquid crystal phases that undergo transition to isotropic melts at 107 and 205 °C, respectively. Oligomer OF5 K could be obtained in form of single crystals. The X‐ray structure analysis revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit. The packing pattern precludes formation of excimers. Electrochemical properties were investigated by cyclic voltammetry. The ionization potential ( I p ) and electron affinity ( E a ) were calculated from these data. Studies of the photophysical properties of OF n K in solution and thin film by steady‐state and time‐resolved fluorescence spectroscopic measurements suggest efficient funneling of excitation energy from the photoexcited fluorene segments to the low‐energy fluorenone sites by both intra‐ and intermolecular hopping events whereby they give rise to green emission. Intermolecular energy transfer was investigated by using a model system composed of a highly defect free polyfluorene PF2/6 doped by OF n K . Förster‐type energy transfer takes place from PF2/6 to OF n K . The energy‐transfer efficiency increases predictably with increasing concentration of OF n K .