Synthesis and Properties ofN-Arylcarbazolophane-Paracyclophane Combined with Carbazoles

Abstract

S y n t h e s i s a n d P r o p e r t i e s o f N -A r y l c a r b a z o l o p h a n e -P a r a c y c l o p h a n e C o m b i n e d w i t h C a r b a z o l e s Abstract: The intramolecular etherification of precursor tetrol 7 afforded N-arylcarbazolophane 5 as a single isomer. Carbazolophane 5 is considered to be a paracyclophane combined with a carbazole, and possesses excimer-like fluorescence properties with l max = 423 nm.Carbazole is the constituent of poly(N-vinylcarbazole) which is well-known as a photoconductor, and the photophysical properties of its dimer model compounds 1 and polymers 2 have been extensively investigated. A cyclophane composed of carbazoles, namely carbazolophane, in which the relative arrangement of two carbazole chromophores is fixed more rigidly, is desirable for the elucidation of the relationship between the chromophore arrangement and fluorescence properties. Dioxa[3.3](3,6)carbazolophane, the first carbazolophane to be synthesized by Tani et al., was found to adopt an anticonformation, leading to small overlap between the carbazole rings and therefore, the absence of excimer fluorescence. 3 We have synthesized cyclobutane-fused [2.n]cyclophanes that contain various aromatic hydrocarbons by the intramolecular [2+2] photocycloadditions of bis(vinylaryl)alkanes. 4 This synthetic method is usually suitable for the preparation of syn-isomers; in some cases, syn-isomers are exclusively obtained. By using this method, [2.n](3,9)carbazolophanes 1 and 2 (n = 4, 5) were successfully prepared from the corresponding vinyl compounds (Figure 1). 5 These carbazolophanes displayed fluorescence spectra different from one another, depending on their structures: for instance, 1a produced broad emission assigned as sandwich excimer fluorescence, whereas 2a gave monomer fluorescence with vibrational structures. Recently, we have succeeded in the preparation of triplebridged [2.2.n](3,6,9)carbazolophanes 3 (n = 4, 5) by the photoreaction of a,w-bis(3,6-divinyl-N-carbazolyl)alkanes. 6 These carbazolophanes 3 adopted exclusively a syn-conformation and suffered from no isomerization into anti-conformation, since two carbazole nuclei are fixed more rigidly by two cyclobutane rings and an oligomethylene linkage. Carbazolophanes 3 were composed of three isomers based on the configuration of cyclobutane rings and their separation was extremely difficult.Introduction of aromatic rings at the N-position of the carbazole is expected to extend the carbazole-based p-conjugated system, leading to a red shift in both absorption and fluorescence spectra. Thus, we were stimulated to prepare novel triply-bridged N-arylcarbazolophane 4 by incorporating benzene rings into 3. This carbazolophane 4 is also regarded as a kind of paracyclophane that incorporates carbazole chromophores. Since it was likely that 4 would be obtained as a troublesome isomer mixture similar to 3, the synthesis of 5 was also undertaken, in which cyclobutane rings were replaced by -CH 2 OCH 2 -units. The latter bridging units were expected to ...