Och

OQ2H23 H Cri

C Cr1

OC6Hi3 H ColA,

C Colfti

OC12H2:

Col,

Col,

99/17

aCr: crystalline state, g: glassy state, Col,,: columnar hexagonal disordered phase, Colob: columnar oblique phase, I: isotropic phase. Measurement of the second heating process H and the first cooling process C.

bPhase transition temperatures (onset) T[° C]/transition enthalpies AH [kJ • mol-1].

cDue to very small AH values, this transition is only visible in X-ray small-angle scattering.

.Source: Reprinted with permission from [20], H. Meier et al., Chem. Eur. J. 6, 2462 (2000). © 2000, Wiley-VCH.

attached to the cores in the focal points of the dendrons (series 12). The influence of functional groups such as CHO, CN, Br, etc., in 9-12 decreases with increasing generation. The fluorescence quantum yield of stilbenoid dendrons is high [21]; nevertheless, their emission can be partly or completely quenched by substituents like Br40 or by an efficient energy

Figure 5. Model of the thermotropic Colhd phase of 1b (R1 = R2 = OC12H25) with a = 4.80 nm.

or charge transfer to the core. The latter effects are discussed in Section 7.

All A values mentioned above are related to measurements in diluted solutions. The aggregation phenomenon, described in Section 5, can have some impact on the absorption and fluorescence bands—particularly on the long-wavelength tailing of the bands [45]. The comparison of solution measurements and measurements in films, as well as the comparison of photoluminescence (PL) and electroluminescence (EL) led to some elucidation [42, 43]; however, more experiments are necessary for a satisfying understanding of the influence of ground state aggregation and excited state aggregation (excimer formation) on absorption and fluorescence bands.

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