Emission luminescent conjugated polymers

An important difference between luminescent conjugated polymers and laser dyes is that the former still exhibit strong optical gain in the solid state. This property means that luminescent conjugated polymers can be used to produce stimulated emission in thin films of these materials. 

Yamaguchi and coworkers [709] have prepared luminescent silole polymers 615a-e. The blue emission of the homopolymers 615a,b can be shifted into the red region by copolymerization with other conjugated units (but for the price of lowered PLQY). Although no device studies have been reported yet, excellent electron-transport properties are expected from such materials [710]. 

Ley and Schanze have also examined the luminescence properties of the polymers Pq, Pio> P25> and P50 in solution at 298 K, and in a 2-methyltetrahydro-furan solvent glass at 77 K. These spectroscopic studies reveal that fluorescence from the 71,71" exciton state is observed at Amax=443 nm, 2.80 eV in the polymers P0-P50 at 298 and 77 K, but the intensity and lifetime of the fluorescence is quenched as the mole fraction of Re in the polymers is increased. This indicates that the metal chromophore quenches the 71,71" state. The quenching is inefficient even when the mole fraction is large, suggesting that interchain diffusion of the 71,71" exciton is slow compared to its lifetime [70]. Phosphorescence from the 71,71" state of the conjugated polymer backbone is observed at > max=b43 nm, 1.93 eV in P10-P50 at 77 K, and emission at Amax=690 nm, 1.8 eV is assigned to the d7i(Re) 7i oiy MLCT transition. 

Long-lived photoluminescence, at 826 nm, is reported (t 15 (is) for thin films of the processable, -conjugated polymer, poly(3-hexylthio-phene) (93JA8447). Excitation of the n—n transition with 518 nm light (So >S ) yields only very weak luminescence of 826-nm light, even at 18 K. The emission is enhanced, to point where it can be observed at room temperature, when the excitation wavelength is 250 nm, but it is completely quenched by oxygen. Prompt fluorescence decays within... 

Device motivation for interface studies, and Optical absorption and emission in conjugated oligomers and polymers. The principles of device physics of metal insulator field-effect transistors (MISFETs) and light emitting diodes (LEDs) are oudined mainly as motivation for the contents of the chapters which follow, but also to point out certain features relevant to developing an understanding of the nature of the polymer-metal interface (chapters 5 and 6). The basic principles of electro-luminescence are reviewed here, at the level consistent with the aims of this work. 

In this section, the basic features of light absorption and emission (luminescence) processes in conjugated systems are reviewed. The discussion will focus on poly(/>-phenylenevinylene), PPV, compounds, which provide typical examples of the physical phenomena to be highlighted in the context of polymer-based light emitting devices. 

Although the studies on OLEDs have achieved considerable success, it is still difficult to obtain pure emission colors from small organic molecules or conjugated polymers, because their emission spectra typically have a half peak width of about 100 nm. Lanthanide ions can exhibit spectrally narrow emission due to intra-atomic transitions within the 4f shell. Consequently, luminescent lanthanide complexes are good candidates as emitting materials in OLEDs. 

Figure 11.13 The PL spectra from CN-PPP and CN-PPP doped with 5 wt.% of the indicated Eu complexes. All of the films had the same thickness and absorbed approximately the same amount of light, so the emission spectra can be compared to each other to determine relative quantum yields [50]. (Reproduced with permission from M.D. McGehee et al., Narrow bandwidth luminescence from blends with energy transfer from semiconducting conjugated polymers to europium complexes, Advanced Materials, 1999, 11, 1349-1354. Wiley-VCH Verlag GmbH Co. KGaA.)...

Following photoexcitation from the ground state (lAg in the notation of molecular spectroscopy) to the lowest energy state with proper symmetry (1B ), recombination to the ground state can be either radiative (with the emission of light, i.e. luminescence) or non-radiative. Some families of conjugated polymers... 

---