ABSTRACT
We present results of steady-state and transient photoluminescence studies of molecularly doped poly(fluorene) films. We study blends with increasing content of the triplet emitter (2,3,7,8,12,13,17,18-octaethyl-porphyrinato)Pt(II) (PtOEP) when dispersed in the polymeric poly(fluorene) matrix of the poly[9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl] (PF26) derivative. We carry out a unified study of the photophysical reactions that are involved in the energy transfer processes in this system by probing the three luminescence processes of a) PF26 fluorescence, b) triplet-triplet annihilation (TTA) induced up-converted PF26 delayed fluorescence and c) PtOEP phosphorescence. With increasing PtOEP content, the process of photon energy recycling in the PF26:PtOEP system is manifested from the quenching of the TTA-induced up-converted PF26 delayed fluorescence and it is rationalized with the use of Forster theory of resonant energy transfer. Based on the combined results of the photophysical and the transmission electron microscopy characterization of the as-spun PF26:PtOEP films, we determine the onset of PtOEP aggregation at 2-3 wt % PtOEP content. The analysis of the photophysical data is based on the use of modified Stern-Volmer photokinetic models that are appropriate for the solid state. A static component in the PL quenching of PF26 is revealed for PtOEP contents below 2 wt %. The modified Stern-Volmer kinetic scheme further suggests that co-aggregation effects between PF26 and PtOEP are operative with an association constant of ground state complex formation k(bind) approximately 15-17 M(-1). The involvement of the ground state heterospecies in the TTA-mediated PF26 up-converted luminescence is discussed. The participation of an electron-exchange step, in the excited state energy transfer pathway between PtOEP and PF26, is proposed for the activation mechanism of the PF26 up-converted fluorescence.
ABSTRACT
Lipid monolayers were prepared by the Langmuir-Blodgett-Kuhn technique (LBK) as free-standing films spanning a diameter of up to 1 micron. These films were investigated by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). The free-standing monolayer is shown to be in a transient state in which an aging process is proceeding: after storage for two weeks in air at room temperature the films tear off the edges of the perforated supporting film. Ca2+ ions induced lateral phase separation in these films prepared from a 50:50 mixture of lecithin/glycerol could be visualized by means of ESI, i.e., by comparing micrographs below and above the Ca absorption edge in the EEL spectrum. The domain sizes of the demixed phases were determined to vary between 30 and 60 nm. In addition it was shown that the counter ion of the negatively charged glycerol in these films is Ca2+ and not Na+.
