Irradiation of 9-(arylmethoxymethyl)anthracenes 3 leads either to a cyclomer and cyclodimer mixture (3a Ǟ 4a,5a), to selectively formed dimers (3b Ǟ 5b), or a selectively formed cyclomer (3c Ǟ 4c). The [4π+4π]cyclodimerization is under the conditions used a regioselective head-to-tail process. In the crystals of the dimers 5a,b, the sidechains are attached

Dendrimers 6G(i) (i=1-4) consisting of an anthracene core and Fréchet dendrons which are attached via a CH(2)OCH(2) chain in the 9-position undergo quantitative and completely reversible intramolecular [4pi+4pi] cycloaddition. The process can be monitored by absorption and fluorescence measurements. The Fréchet dendrons act as an energy funnel that collects and focuses the photon energy but does not change the photostationary states, which for both directions are completely on the product side when the separate chromophores are selectively irradiated. The quantum yields of anthracene fluorescence and of singlet energy transfer from the dendrons to the core were studied as a function of dendrimer generation.

No abstract

No abstract

I n t r a -a n d I n t e r m o l e c u l a r P h o t o c y c l o a d d i t i o n o f 9 -S u b s t i t u t e d A n t h r a c e n e sAbstract: A number of 9-[(arylmethoxy)methyl]anthracenes were prepared and their photoreactivity was studied. The intramolecular [4p+4p]cycloaddition competes with the intermolecular head-totail [4p+4p] cyclodimerization. The aryl substituents control the selectivity; concentration only plays a role in selectivity in certain cases. The complete reversibility of the cycloaddition makes this process suitable as a molecular switch, provided that traces of acids are avoided; when acid is present the cyclomers pursue an irreversible enol ether cleavage route.Reversible photochemical reactions between compounds that have different absorption (and/or emission) are of great interest for optical switching and optical data storage processes. 1 Recently, we developed such a system, which is based on the intramolecular [4p+4p] cycloaddition of benzene and anthracene moieties. 2 A prerequisite condition for this reaction is that the two chromophores are linked by a three-atom chain, like CH 2 -O-CH 2 . Useful switching systems should have clean, uniform reactions in both directions. The [4p+4p] cyclodimerization is an alternative photoreaction of anthracenes and is an old and well-known process. 1e,3 Therefore, we have now studied the competition between the desired intramolecular [4p+4p] cycloaddition and possible intermolecular [4p+4p] cyclodimerizations. For this purpose, 9-[(arylmethoxy)methyl]anthracenes 3 were generated. Their preparation was based on the nucleophilic substitution of 1 and 2 (Williamson synthesis), whereby the role of nucleophile and electrophile can be exchanged (Table 1, Methods A and B). The best results were obtained by using phase-transfer conditions (KOH, TBAB).The possible photoproducts of 3a-n are shown in Scheme 1. 4-8 Apart from the cyclomers 4, head-to-tail dimers 5 and head-to-head dimers 5¢ can be expected. 9,10 Compared to 5, the dimers 5¢ are highly thermolabile. In the past such compounds were usually detected in solution by NMR measurements of the crude product mixture. Nevertheless, many head-to-head dimers 5¢ have been reported. 4,11,12 In the first approximation, the compounds 3a-n contain two separate chromophores. Therefore, selective irradiation (l ≥300 nm) into the anthracene chromophore is possible. This precondition is necessary in order to prevent the light-induced reverse reactions 4,5,5¢ → 3. Table 2 shows the product ratios 4/5 that were obtained in the irradiation experiments of 3a-h. The halogen-containing compounds 3i-k are highly light sensitive. However, their irradiation yields product mixtures in which dimers, but no cyclomers, can be detected by 1 H NMR and FD-MS measurements; 3k reacts even in daylight. Compound 3l was studied in another context. 7 For the compounds 3a and 3m, no cyclomers 4a and 4m were reported, but mixtures of dimers 5a/5¢a and 5m/5¢m, respectively, were found; 4 on irradiation compound 3n shows a quantitative intram...