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 448. 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 t 1/2 between 3.9 and 5.8 h at 258 in CH 2 Cl 2 (Figs. 7 and 13 and Table 3). A comparatively slower (E) 3 (Z) isomerization process about the central CC 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) 3 (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 3 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 CH 2 Cl 2 ( Bu 4 NPF 6 ) revealed the occurrence of characteristic first-reduction steps in the TEE-DHA hybrid systems between À 1.6 and À 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 3 VHF isomerization took place upon electrochemical oxidation or reduction (Fig. 20).

Deliberate Switching of Single Photochromic Triads

“…Such molecules can be reversibly transformed between two bistable forms by light, thereby changing their geometrical and electronic properties. Photochromic systems based on cis-trans isomerisation (stilbenes, azobenzenes) 10 11 or photocyclization reactions (fulgides, diarylalkenes, azulenes, spiropyrans) 6 12 13 14 15 16 have been studied extensively in the past. However, besides fascinating results also severe problems concerning weak contrast between the signals registered from the two states, low fatigue resistance, and/or crosstalk between read-out and switching have been reported.…”

Functionalization at C(1) of the Dihydroazulene/Vinylheptafulvene Photo‐/Thermoswitch – Establishing Structure−Property Relationship

“…NMR Spectra were acquired from either a Varian 300 MHz instrument or a Bruker 500 MHz instrument with a cryoprobe (allowing for two decimals on the 13 C-NMR data). The 1 H-NMR and 13 C-NMR spectra are referenced to the solvent residual peak (CDCl 3 δ(H) = 7.26 ppm, δ(C) = 77.16 ppm). 19 F-NMR spectra are referenced to trifluoroacetic acid (TFA) as internal standard (δ(F) = À 76.55 ppm).…”

“…[19] .03 (s, 1 H); 6.55 (dd, J = 11.0, 6.0, 1 H); 6.44-6.35 (m, 2 H); 6.25 (ddd, J = 10.1, 6.0, 2.1, 1 H); 6.03 (dd, J = 10.1, 4.0, 1 H); 3.43-3.41 (m, 1 H). 13…”

“…Advanced multimode switching can be Supporting information for this article is available on the WWW under https://doi.org/10.1002/hlca.201800153 obtained by linking DHA/VHF with redox-active [8][9][10][11] or other functional units. For example, Diederich and coworkers [12,13] reported an advanced three-way chromophoric molecular switch where DHA/VHF was combined with a tetraethynylene unit that can undergo cis-trans isomerization and with a pH-sensitive aniline unit.…”

Functional Dyes, and Some Hi‐Tech Applications

“…The most well-known organic photochromic switches are based on the ring opening/ring closure reactions in photoinduced electrocyclic reactions. There are six main groups of compounds that have found numerous applications as switching molecules: dihydroazulenes and vinylheptafulvenes [64,65], diarylethenes [55,66], spiropyrans [63], spiroxazines [63,67,68], naphthopyrans [69], dihydropyrenes [70], fulgides, fulgimides, and related compounds [71] ( Figure 10).…”