Photoswitchable Tetraethynylethene-Dihydroazulene Chromophores

Abstract

The synthesis, characterization, and photophysical as well as electrochemical properties of the photochromic hybrid systems 11 – 16 and 18, which contain photoswitchable tetraethynylethene (TEE; 3,4-diethynylhex-3-ene-1,5-diyne) and dihydroazulene (DHA) moieties, are presented. The molecular photoswitches were synthesized by a Sonogashira cross-coupling reaction between an appropriate TEE precursor (6 – 10 and 17) and an iodinated DHA 1 or its vinylheptafulvene (VHF) isomer (4) (Schemes 5 – 7). X-Ray crystal structures of five 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 adopts a clear boat conformation (Table 1). Presumably due to crystal-packing effects, the arylated TEE moieties in the hybrid systems show substantial distortions from planarity, with the dihedral angles between the planes of the central TEE core and the adjacent aryl substituents amounting to 44°. The switching properties were investigated by electronic absorption spectroscopy. Upon light absorption, DHAs 1, 12 – 16, and 18 underwent retro-electrocyclization in solution to give the corresponding VHFs (Figs. 6, 11, and 12). The reaction is thermally reversible, with half-lives τ1/2 between 3.9 and 5.8 h at 25° in CH2Cl2 (Figs. 7 and 13 and Table 3). A comparatively slower (E)→(Z) isomerization process about the central C=C bond of the TEE moiety was also observed. The N,N-dimethylanilino-(DMA) substituted TEE-DHA hybrid systems trans-11a and cis-11a did not react to the corresponding VHFs upon irradiation (Scheme 9). Instead, only the reversible (E)→(Z) photoisomerization of the TEE core occurred (Fig. 16 and Table 4). This process was further investigated for photofatigue by electronic-emission spectroscopy (Fig. 17). After protonation of the DMA group, the usual DHA→VHF photoreaction took place. Compound 11 represents a three-way chromophoric molecular switch with three addressable sub-units (TEE core, DHA/VHF moiety, and proton sensitive DMA group) that can undergo individual, reversible switching cycles (Scheme 9). A process modeling the function of an `AND' logic gate (Fig. 19) and three write/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 vs. Fc/Fc+ (ferrocene/ferricinium couple) (Table 5). Oxidations occur at ca. +1.10 V. After photoisomerization to the VHF derivatives, reduction steps at more positive and oxidation steps at more negative potentials were recorded. No DHA→VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).