Polarization in emission

Analogous g-values may be defined for the degree of circular polarization in emission [or circularly polarized photoluminescence (CPPL)] and circularly polarized electroluminescence (CPEL), eg. gCppL = 2(JL - 1R)/(1L + 1r), where IL and IR denote the intensity of left- and right-handed circularly polarized emission, respectively. CPPL should not be confused with fluorescence-detected CD. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

The measurement of steady-state anisotropy r is simple and needs two polarizers, one in excitation and the other in emission beams. When the sample is excited... 

The same Suzuki methodology was used to synthesize a similar copolymer 446 [548], The polymer showed a solvent-dependent green-yellow emission (from 545 nm in THF to 565 nm in chloroform) as often observed for polar chromophores. The PL QE also varied with the solvent (from 11% in THF to 21% in decalin) but, in contrast to copolymer 445, no strong decrease in emission efficiency was observed in the solid state (4>p1 n= 13%) that could be attributed to the effects of substituents at the thiophene ring. LEDs based on 446 showed, for an ITO/PEDOT/446/Ca/Al architecture, a turn-on voltage of ca. 10 V with a maximum brightness of 340cd/m2 at 22 V and appreciable el = 0.14%. 

As previously outlined, the emission correction factors must be recorded with an emission polarizer in a defined orientation (preferably at the magic angle), and this orientation must be kept unchanged for recording emission or excitation spectra irrespective of whether or not the fluorescence is polarized. 

In some cases, filters are used instead of the emission monochromator. In principle, no G factor is then considered, but in practice, effects may be due to the sensitivity of the photomultipliers to polarization (in particular, photomultipliers with side-on photocathodes). 

Distortion of the fluorescence response measured by the detection system (monochromator + detector) arises when the emitted fluorescence is partially polarized. As explained in the Appendix, a response proportional to the total fluorescence intensity can be observed by using two polarizers an excitation polarizer in the vertical position, and an emission polarizer set at the magic angle (54.7°) with respect to the vertical, or vice versa (see the configurations in Figure 6.3). 

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