Photoswitchable tetraethynylethene-dihydroazulene chromophores

The synthesis. characterization, and photophysical as well as electrochemic al properties of the photochromic hybrid systems 11-16 and 18, which contai n photoswitchable tetraethynylethene (TEE: 3,4-diethynylhex-3-ene-1,5-diyne ) and dihydroazulene (DHA) moieties, are presented. The molecular photoswit ches were synthesized by a Sonogashira cross-coupling reaction between an a ppropriate TEE precursor (6-10 and 17) and an iodinated DHA 1 or its vinylh eptafulvene (VHF) isomer (4) (Schemes 5-7). X-Ray crystal structures of fiv e DHA derivatives (1. trans-11a, cis-11a, 12, and 13) are discussed (Figs. 2-5). In all compounds, the cyclohexatriene moiety of the DHA chromophore a dopts a clear boat conformation (Table I). presumably due to crystal-packin g effects. the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the plants of the central TEE core and thr adjacent aryl substituents amounting to 43. T he switching properties were investigated by electronic absorption spectros copy. Upon light absorption. DHAs 1. 12-16. and 18 underwent retro-electroc yclization in solution to give the corresponding VHFs( Figs h. II. and 12). The reaction is thermally reversible, with half-lives tau (1,2) between 3. 9 and 5.8 h at 2.5 in CH2Cl2 (Figs. 7 and 13 and Table 3). A comparatively slower (E) - (Z) isomerization process about the central C=C bond of the TE E moiety was also observed. The N,N-dimethylanilino-(DMA) substituted TEE-D HA hybrid systems trans-11a and cis-11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead. only the reversible (E) - (Z) ph otoisomerization of the TEE core occurred (Fig. 10 and Table S). This proce ss was further investigated for photofatigue bq electronic-emission spectro scopy (Fig. 17). After protonation of the DMA group, the usual DHA - VHF ph otoreaction took place. Compound 11 represents a three-way chromophoric mol ecular switch with three addressable sub-units (TEE core, DHA/VHF moiety, a nd proton sensitive DMA group) that can undergo individual. reversible swit ching cycles (Scheme 9). A process modeling the function of an 'AND' logic gate (Fig. 19) and three writc/erase processes could be performed with this system. Cyclic and linear sweep-voltammetry studies in CH2Cl2 (+ Bu4NPF6) revealed the occurrence of characteristic first-reduction steps in the TEE- DHA hybrid systems between - 1.6 and - 1.8 V L'S. Fc/Fc. (ferrocene/ferrici nium couple) (Table 5). Oxidations occur at cn. +1.10 V. After photoisomeri zation to the VHF derivatives. reduction steps at more positive and oxidati on steps at mt,re negative potentials H ere recorded. No DHA - VHF isomeriz ation took place upon electrochemical oxidation or reduction (Fig. 20).