Electronic Transitions Absorption And Emission

The (_E)-stilbene chromophore shows a strong absorption for the energy-lowest ww* transition. Because of the cross conjugation, the influence of the generation of stilbenoid dendrimers on the absorption maxima is very low [20, 23]. The compounds of series 1a-e (R1 = R2 = OC6H13), for example, have long-wavelength maxima in dichloromethane at A = (330 ± 3) nm. The emax values increase linearly with the number of stilbene units; the average value per stilbene unit amounts to (2.76 ± 0.30) x 107 cm2 • mol-1. The fluorescence maxima in this series were found at A = (429 ± 3) nm. The (£,£)-1,4-distyrylbenzene units in 2a-c provoke a bathochromic shift in comparison to series 1. The absorption and emission maxima were observed at A = (375 ± 2) nm and (445 ± 2) nm, respectively. The average emax value per distyrylbenzene unit amounts to (4.8 ± 0.6) x 107 cm2 • mol-1. The Stokes shift in series 2 is considerably smaller than in series 1 [23, 39].

The dendrimer series with special cores like 4-8 have typical absorptions of the stilbenoid dendrons (as in 1 and 3) and bands at larger wavelengths for the excitation of the cores [42, 44, 45]. The same dual-absorption effect can be observed for compounds with chromophores (like fullerene)

Table 2. Formation of thermotropic LC phases of stilbenoid dendrimers 1a, b.

Compound R1, R2

Phase transitions1 with T/\Hb

1a 1a 1b 1b

0 0

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