Polarised emission

Figure 5. The P-polarised emission spectrum obtained for hole injection by the thianthrene radical cation into Au(lll) for emission angle = 30° and excitation energy Eex = 3.0eV. (NB.

Polymer LEDs offer further interesting properties. Orientation of the polymer chains in the plane of the diode results in polarised emission since, like the absorption, the principal component of the emission is polarised parallel to the... 

Circularly polarised PL and EL emission has been obtained from polymers, e.g., 22 and 23 (Scheme 8), bearing chiral side-chains [42-45]. The copolymer 24 also produced circularly polarised emission, but the degree of dissymmetry was much reduced [42 - 44]. This was attributed to the dioctylfluorene units preferring a planar conformation of the backbone while the chiral-substituted units adopt a helical structure. A combination of a helical backbone conformation and liquid crystallinity is thought essential for obtaining a high degree of circular polarisation. 

Fig. 8 a Narrow emission spectrum (linewidth 0.7 meV) shifted by the 0-0 transition energy, exhibiting linearly polarised emission (P = 0.98) as shown in (c). b The broad emission spectrum (linewidth 2.40 meV) of a different single chain is only weakly linearly polarised (P = 0.33), (d). The dotted lines in c and d indicate the fluorescence background. Reprinted from [28]... 

Polarisation. In the event that e.e.m. takes place then the emission from the trap, excimer in the case of pure polymers, should be completely depolarised. Some time ago data was published on the emission of excimer for both polystyrene and poly(a-methyl styrene) indicating polarised emission (8) and therefore little e.e.m. More recently Phillips questioned the validity of the data and reported mesurements which suggest the excimer emission is depolarised (9 ). An experimental difference between the two sets of data is apparent - Phillips solutions were more dilute than these used in ref. 8. The range of concentrations has subsequently been extended with the results shown in Figure 1. A possible explanation for the effect of concentrations and molecular weights on the extent of polarisation of excimer emission is that the rate of rotational relaxation of this bulky entity becomes slower than the emission life-time as these two parameters increase. It is difficult to reconcile the concept of e.e.m. with polarised excimer emission. 

The number of studies on luminescence of metallomesogens in the liquid crystal state is still limited, but considerable progress is to be expected in the near future. New strategies for the design of room-temperature mesophases exhibited by metal complexes will make more types of metallomesogens available for photophysical studies. The possibility to obtain polarised emission via these systems and the use of these compounds in luminescent devices can be an additional drive for further exploration of this research field. ° The most promising metal ions to be included in luminescent metallomesogens are Eu, Tb, Sm, Ru, ... 

If, however, such a sample is excited with a directional laser beam, and particularly a polarised laser beam, the vectorial nature of the interaction between the molecule and the light will usually give rise to an anisotropic excited state distribution. Subsequent fluorescence will then result in polarised emission. Greene and Zare (1982,1983) have shown how this polarisation can be related to the moments of the angular momentum distribution, with the dominant contribution given by the second moment/ 2) (the alignment) when using linearly polarised one-photon excitation, where for each Ji... 

Scheme 14 PPPs from which polarised emission has been obtained...

Circularly polarised emission is possible from polymers containing chiral groups. Scherf and coworkers have prepared a cyclophane-substituted PPP by the Suzuki route using the dibromocyclophane 35 and the corresponding bis-boronic acid 36 (Scheme 15) [98,99]. If racemic monomers were used the resulting polymer 37 was not chiral with the cyclophanes randomly distributed on either face of the polymer (atactic). If resolved enantio-pure monomers were used, then the stereoregular isotactic 38 or syndiotactic 39 polymers could be obtained depending upon which enantiomer of each monomer was used. The isotactic polymer is chiral and both enantiomers have been prepared. 

Scheme 16 Other PPPs producing circularly polarised emission...

Organic Light-Emitting Diodes (OLEDs) with Polarised Emission... 

One of the first reports on the orientation and subsequent crosslinking of a reactive mesogen for OLED applications with polarised emission has been published by Bacher et al. [63], who synthesized the conjugated bis-stilbene with two polymerizable acrylate groups shown in Fig. 7.17. The reactive mesogen was oriented by heating into the liquid crystalline phase and subsequently thermally crosslinked at 175°C. 

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