Info

The UV absorption of the dendrimers 1 in spincoat films is somewhat broader as in solution, and the maximum is shifted by about 10 nm to longer wavelengths. Both effects point to an increased intermolecular interaction. Prolonged irradiation of the films with À = 350 nm leads in all cases to a complete or almost complete disappearance of the long-wavelength band. The simultaneous FT-IR measurement reveals the disappearance of the band at 955 m-1, which corresponds to the typical out-of-plane deformation of trans

Figure 7. Photodegradation of 1b (R1 = R2 = OC12H25) in a 4 x 10-6 M solution in cyclohexane with monochromatic light (A = 366 nm). Reprinted with permission from [58], R. Petermann et al., J. Inf. Recording 25, 259 (2000).

configured stilbene building blocks. On the whole, the reaction is very similar to the solution photolysis [59].

A detailed study was made for the deuterated compounds 1a (R1 = R2 = OCD2-C11H23) and 1b (R1 =R2 = OCD2-C11H23) [60]. The absence of OCH2 groups allows an integration of the 1H NMR signals of the CH groups formed in the photolysis. The deuterated system 1a is photostable (A = 350 nm) in the crystalline phases. [Compare Table 2,1a (r1 = R2 = OC12H25).] The enhanced mobility in the Colhd phase at 50 °C permits a photooligomerization because it enables an interaction of the olefinic centers within the average singlet lifetime. The generation of the oligomers, proven by MALDI-TOF mass spectroscopy, is accelerated by raising the temperature to 67 °C, and further to 77 °C, where an isotropic phase is present. Figure 8 shows the irradiation through a grating. Small amounts of oligomer formation (<5%) lead to a reversible degradation of the LC phase. Below this limit, the LC phase can be thermally regenerated; the unirradiated zones crystallize at room temperature, whereas the texture of the LC phase is maintained at 23 ° C.

According to the 1H NMR spectrum, the neat compound 1b (R1 = R2 = OCD2-C11H23) already shows a photoreac-tivity at room temperature because it exists there in a mobile Colhd phase. The reaction rate of the monochromatic irradiation (A = 350 nm) is enhanced at 88 °C in the Colob phase and still higher at 108 °C in the isotropic phase. DSC measurements of the starting compound doped with increasing amounts of (before isolated) photooligomers revealed that, already at 4% oligomer contents, the phase transition temperatures are somewhat lowered; even more pronounced is the effect on the phase transition enthalpies. The AH value for the transition Colhd ^ Colob is strongly increased when 4-7% of oligomers are present.

The scaffold of 2 and the other stilbenoid dendrimers exhibit similar photochemical CC bond formations, unless an efficient energy transfer (Section 7) occurs. The main difference in the photochemical behavior between (E)-stilbene and (E,E)-1,4-distyrylbenzene building blocks consists of high quantum yields for E/Z isomerization reactions of the first and very low quantum yields for the latter system. This

Figure 8. Short-term irradiation (A = 350 nm) of the Colhd phase of 1a (R1 = R2 = OC12H25) through a grating. Top: irradiation at 72 ° C (homeotropic orientation); middle: regeneration of an LC phase (in a nonhomeotropic orientation) in the irradiated areas on cooling to 50 °C; bottom: starting crystallization of the unirradiated areas at 23 °C.

Figure 8. Short-term irradiation (A = 350 nm) of the Colhd phase of 1a (R1 = R2 = OC12H25) through a grating. Top: irradiation at 72 ° C (homeotropic orientation); middle: regeneration of an LC phase (in a nonhomeotropic orientation) in the irradiated areas on cooling to 50 °C; bottom: starting crystallization of the unirradiated areas at 23 °C.

effect results from the extended conjugation, which influences the n bond orders in the first excited singlet state S1 [50]. The low intersystem crossing rates S1 ^ T1 render the E/Z isomerization ineffective in the triplet state, too, unless a high triplet population is reached by the energy transfer from a suitable triplet sensitizer. The model of le (R1 = R2 = OC6H13), shown in Figure 1, demonstrates that there are many cavities close to the core in these size- and shape-persistent particles. In dendrimers with less rigid dendrons, the backfolding of the arms prevents the formation of such cavities in the center. The inclusion of guest molecules in the cavities enables a guest-host chemistry; particularly guests as ketones and 1,2-diketones, which can act as triplet sensitizers, provoke an efficient E/Z isomerization [61]. The incorporation of guest molecules works in the solid state (X-ray crystal structure analysis), in LC phases, and in some cases, even in the vacuum of vapor phases [20].

Finally, it should be mentioned that the stilbenoid compounds are principally sensitive toward oxidation and pho-tooxidation processes [23].

0 0

Post a comment