Excitons centres

With tlie development of femtosecond laser teclmology it has become possible to observe in resonance energy transfer some apparent manifestations of tire coupling between nuclear and electronic motions. For example in photosyntlietic preparations such as light-harvesting antennae and reaction centres [32, 46, 47 and 49] such observations are believed to result eitlier from oscillations between tire coupled excitonic levels of dimers (generally multimers), or tire nuclear motions of tire cliromophores. This is a subject tliat is still very much open to debate, and for extensive discussion we refer tire reader for example to [46, 47, 50, 51 and 55]. A simplified view of tire subject can nonetlieless be obtained from tire following semiclassical picture. 

In other models, the confined exciton was treated as one electron with the reduced mass p = (1 /m -I- 1/nih) which moves in the field of the walls, and a positive hole h fixed at the centre of the sphere. Curve a in Fig. 34 gives the lowest energy level (wavelength of transition to this level) as calculated in the semi-classical approxima-... 

A, Absorption chi, chlorophyll car, carotenoid EET, excitonic energy transfer EF, exoplasmic fracture face EM, electron microscopy FWHM, full width at half maximum lEF, Isoelectric Focusing, LD, linear dichroism LHC, light harvesting complex PAGE, polyacrylamide gel electophoresis PF, protoplasmic fracture face PS, photosystem RC, reaction centre SDS, sodium dodecyl sulphate SSTT, single step transfer time. 

The CD spectrum of the 1,2-dibenzoate 6 in 9 1 MeOH/dioxane showed a pair of typical exciton-split Cotton effects with opposite signs centred upon the UV absorption (227 nm) of the benzoate chromophore AE235 5 -15.9 and A 221 5 +6.66. The negative longer wavelength Cotton effect clearly defines the negative chirality between the two electric transition dipoles of the benzoate chromophores... 

Many researchers have been exciting samples grown along the [0001] direction on C-plane oriented substrates. This makes it easy to detect Tj dipole excitons allowed in a polarisation. Hoffinan and Eckey [39] decided to measure the energy of the f6 mode forbidden dipole at the zone centre. They used... 

Fig. 26.4. Calculated spectra (left) and exciton manifold right) for three qualitatively different BChl a arrangements, i.e., circular symmetric top), elliptic centre), and elliptic with a gap bottom). The black circles indicate the oscillator strength of the respective exciton states. For each geometry two spectra for mutually orthogonal polarizations black, gray) are shown. For the circular symmetric geometry the two spectra coincide. The upper black line corresponds to the sum of these spectra. Adapted from [55]...

Since the relaxation of the higher exciton states occurs on an ultrafast timescale of about 100 fs [23,26], the absorption spectrum for a closed structure, Fig. 26.4, top and centre, consists of either one or a few relatively broad spectral bands, respectively. For both cases, i.e., circular and elliptical arrangement, the transitions from the k = 1 exciton states are polarized perpendicular with respect to each other. Moreover, the lowest exciton state is optically forbidden, because a C2-type symmetry reduction alone, i.e., an ellipse, does not give rise to oscillator strength in the k = 0 state. This situation is reminescent to the electronically excited states of the B850 BChl a... 

That even low levels of defects can produce strong emission is exemplified by the case of Ph-LPPP (108). The PL emission from 108 is very similar to that from 106 with maxima at 460 and 490 nm. However, the EL spectrum shows an additional long wavelength band. This is not a broad featureless band as seen for the defect emission from 5 or 106, but one with well-resolved maxima at 600 and 650 nm. Photophysical investigation of this emission showed the feature at 600 nm to be emission from a triplet exciton (phosphorescence) with a vibronic shoulder at 650 nm [158]. Elemental analysis of the polymer showed it contained 80 ppm of palladium (cf. <2 ppm in 106). It was therefore proposed that residues of the palladium catalyst used to make the precursor polymer 103 reacted with the phenyllithium and the polymer to introduce covalently bound palladium centres onto the polymer chain. These then act as sites for phosphorescent emission. 

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