A Rational Approach to Tetra‐Functional Photo‐Switches

Abstract

α,ω‐Bis(1,8‐dichloroanthracen‐10‐yl)alkanes with (CH₂)_n‐linker units (n=1–4) were synthesized starting from 1,8‐dichloroanthracen‐10(9H)‐one. This was transformed into anthracenes with allyl, bromomethyl and propargyl substituents in position 10; these were converted in various C−C‐bond formation reactions (plus hydrogenation), leading to two anthracene units flexibly linked by α,ω‐alkandiyl groups. 1,2‐Ethandiyl‐ and 1,3‐propandiyl‐linked derivatives were functionalized with ethynyl groups in positions 1, 8, 1’ and 8’, and these terminally functionalized by Me₃Sn groups using Me₂NSnMe₃. All linked bisanthracenes were subjected to UV light induced cyclomerization and a series of 9,10 : 9’,10’‐photo‐cyclomers were obtained. Their thermal cycloreversion and (repeated) switchability was demonstrated. 1,3‐Bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10‐yl}propane served as model compound for photo‐switchable acceptor molecules and its open and closed forms were characterized by NMR and DOSY experiments.

Light on the switch: Linked bisanthracenes bearing four (partly metal‐containing) substituents undergo photo‐cyclomerization that enables the parallelization of all four functions and can be thermally reopened by cycloreversion which allows repeated switchability.

Introduction

Functionalized anthracenes provide well‐defined distances and orientations of the substituents. Thus, they are frequently used for organic and organometallic applications where defined geometries of several functions are needed.1,2 Photo‐dimerization of these rigid frameworks doubles the number of directed functions. In most cases, irradiation with UV light induces a [4π+4π] cycloaddition reaction and 9,10 : 9’,10’‐photo‐dimers are formed, but usually more than one photo‐isomer is obtained.3 In the case of 1,8‐substituted anthracenes the formation of two isomers [head‐to‐head‐ (A) and head‐to‐tail‐photo‐dimer (B)] is observed (Scheme 1). Due to steric repulsion and symmetry considerations in electronic effects, the head‐to‐tail‐isomer is generally favored and formed in excess.2e,4

Scheme 1.

The photo‐dimers A (head‐to‐head) and B (head‐to‐tail) are obtained upon UV irradiation of 1,8‐substituted anthracenes. Compounds C–E are examples for bichromophoric anthracenes connected by flexible linker units.8, 9, 10

Different approaches are known to control this regioselectivity, for instance by dimerization of anthracene derivatives in the ordered solid state5 or in preorganized supramolecular assemblies, e. g. inside cyclodextrins6 or within nanoflasks.7 Furthermore, a common strategy to avoid head‐to‐tail‐photodimerization is to introduce a short linker unit between two anthracene subunits, usually in positions 9/9’. As shown in Scheme 1, compounds C−E are interesting examples for these bichromphoric systems.8, 9, 10

Our present search for new selectively complexing poly‐Lewis acids makes it necessary to decorate rigid and donor‐free frameworks with Lewis acid functions. In this context, systems based on flexibly bridged bisanthracenes could be of special interest for the reversible complexation of guest molecules by switching from open to closed isomers. The synthesis of the required organic frameworks is the main element of this contribution. Recently, we synthesized a series of 1,8‐dichlorinated anthracene derivatives, with two such units flexibly bridged with −Me₂Si−(CH₂)_n−SiMe₂− linkers. For these systems we found the bulky dimethylsilyl groups to inhibit any cyclomerizing.11 In contrast, bisanthracenyl compounds with short α,ω‐alkandiyl chains are known to be well‐suited for 9,10 : 9’,10’‐photo‐cyclomerization,3,8 at least for the unsubstituted derivatives. For our purpose, the use of substituted analogues could be a suitable access to photo‐switchable poly‐Lewis acids.

Herein, we report the syntheses of molecules with two 1,8‐substituted anthracene units, connected by methylene, 1,2‐ethandiyl, 1,3‐propandiyl and 1,4‐butandiyl chains in positions 10/10’. For comparison bisanthracenes with even longer linker units were synthesized. All of these compounds were tested for UV‐induced [4π+4π] cycloaddition and the resulting photo‐cyclomers for thermal cycloreversion. Moreover, fourfold (trimethylstannyl)ethynyl‐functionalized bisanthracenes were synthesized as model compounds for photo‐switchable acceptor molecules.

Results and Discussion

There are several literature protocols for the synthesis of α,ω‐bis(anthracen‐9‐yl)alkanes, usually starting from anthrone, anthraldehyde or dihydroanthracene.12 An efficient synthesis was recently reported by Klaper and Linker. They converted anthracene in a Birch reduction, followed by alkylation with α,ω‐dibromoalkanes.13 Since we are interested in chloro‐substituted anthracenes, which can be further modified with rigid alkynyl‐spacers in Kumada cross‐coupling reactions, completely new strategies for the synthesis of α,ω‐bis(1,8‐dichloroanthracen‐10‐yl)alkanes had to be developed in this work.

Initially, 1,8‐dichloroanthracenes bearing different functional groups in position 10 were required. They were subsequently subjected to C−C‐bond formation reactions, in order to interconnect two anthracene building blocks. As shown in Scheme 2, the methyl‐, vinyl‐ and allyl‐substituted derivatives 2, 3 and 4 were synthesized by reacting the corresponding Grignard reagent with 1,8‐dichloroanthracen‐10(9H)‐one (1). The latter is easily available by reduction of 1,8‐dichloroanthraquinone with sodium dithionite in DMF.14 After aqueous workup, rearomatisation and purification by column chromatography, the substituted anthracenes were obtained as bright yellow crystals.

Scheme 2.

Syntheses of the 1,8‐10‐substituted anthracenes 2–7. Reagents and conditions: a) RMgBr, THF, r.t., 16 h, 71 % (2), 33 % (3), 85 % (4); b) NBS, AIBN, benzene, reflux, 2 h, 95 %; c) Me₃Si−C≡C−MgBr, CuI, THF, 0 °C to r.t., 16 h, 91 %; d) AgNO₃, H₂O, acetone, 4 d, 85 %.

Bromination of 2 in benzylic position with N‐bromosuccinimide (NBS) gave 10‐(bromomethyl)‐1,8‐dichloro‐anthracene (5) in a very good yield of 95 %. 5 was further converted with (trimethylsilyl)‐ethinylmagnesium bromide in the presence of copper(I) iodide, forming the propargyl‐substituted anthracene derivative 6. Cleavage of the trimethylsilyl protecting groups had to be carried out under mild conditions using silver nitrate and water in acetone. The use of potassium carbonate in methanol did not afford the desired propargyl‐substituted anthracene 7, but quantitatively 1,8‐dichloro‐10‐(prop‐1‐yn‐1‐yl)‐anthracene (7 b) (Scheme S2, SI). All new compounds were characterized by multinuclear NMR spectroscopy and high resolution mass spectrometry. In case of vinylanthracene 3, bromomethylanthracene 5 and propargylanthracene 7 the molecular structures in the crystalline state were determined by X‐ray diffraction (Figures S32–S34, SI).

The methylene‐ and ethylene‐linked dichloroanthracenes 8 and 9 were synthesized each in one step from bromomethylanthracene 5 (Scheme 3). Therefore, compound 5 was lithiated with n‐butyllithium at −78 °C. For the preparation of compound 8 the lithiated species was used for a nucleophilic addition reaction with anthrone 1. The elimination of water and the subsequent aromatization was achieved by addition of phosphorus pentoxide and keeping the mixture at 80 °C for three hours. In order to form the ethylene linked bisanthracene 9, the lithiated species was converted in a salt elimination reaction with a second equivalent of 5 (Scheme 3). Due to very low solubility, bis‐(anthracen‐10‐yl)ethane 9 was solely characterized by ¹H NMR spectroscopy and high resolution mass spectrometry.

Scheme 3.

Syntheses of the bisanthracenylalkanes 8 and 9. Reagents and conditions: a) n‐butyllithium (1 eq), THF, −78 °C, 0.5 h, 1,8‐dichloroanthracen‐10(9H)‐one (1), −78 °C to r.t., 16 h, 25 %; b) n‐butyllithium (0.5 eq), THF, −78 °C to r.t., 16 h, 84 %. Tetrachlorolepidopterene 9 b is observed as a by‐product of bisanthracenylethane 9.

A byproduct, generally observed in the preparation of bis‐(anthracen‐10‐yl)ethanes, is the corresponding tetrabenzotetracyclotetradecatetraene, also referred to as lepidopterene.15 In our case, this species is the peritetrachlorolepidopterene 9 b, first reported by Becker and co‐workers.16 Actually, we found the formation of compounds 9 and 9 b in competition to each other. However, we were able to avoid the formation of 9 b by heating the reactive solution to room temperature immediately after adding n‐butyllithium and thus obtaining the precipitated product 9 in 84 % yield. Stirring the mixture at −78 °C for several hours instead, lepidopterene 9 b was found to be the main product. However, in larger scales, the formation of 9 b cannot be avoided. Its solid state structure was determined in X‐ray diffraction experiments (Figure S35, SI).

1,3‐Propandiyl‐ and 1,4‐butandiyl‐linked anthracenes 11 and 13 were both obtained in two‐step syntheses (Scheme 4). The three‐membered spacer was introduced by a Sonogashira‐Hagihara cross‐coupling reaction of the propargyl‐substituted anthracene 7 with 10‐bromo‐1,8‐dichloroanthracene,17 yielding compound 10. The 1,4‐but‐2‐endiyl‐linked bisanthracene 12 was obtained in nearly quantitative yield by subjecting the allyl‐functionalized anthracene 4 to olefin metathesis using Grubbs I catalyst. Note, that no reaction was observed when 1,8‐dichloro‐10‐vinylanthracene (3) was treated under the same conditions. The unsaturated linker units of compounds 10 and 12 were reduced by use of hydrogen at room temperature in the presence of palladium/carbon, yielding the desired compounds 11 and 13.

Scheme 4.

Syntheses of the bisanthracenylalkanes 11 and 13. Reagents and conditions: a) 10‐bromo‐1,8‐dichloroanthracene, PdCl₂(PPh₃)₂, CuI, diisopropylamine, reflux, 3d, 33 %; b) H₂, Pd/C, CH₂Cl₂, 2d, 87 %; c) Grubbs’ I cat. (5 mol%), CH₂Cl₂, 40 °C, 16 h, 95 %; d) H₂, Pd/C, CH₂Cl₂, 1d, 63 %.

Compounds 11–13 were characterized by X‐ray diffraction. The molecular structures of 11 and 13 in the crystalline state are shown in Figure 1 for comparison (see Figure S34, SI for solid state structure of 12). All structural parameters of 11 and 13 are in good agreement with the values of the non‐chlorinated derivatives, crystallized by Arslan and co‐workers.18,19 Compared to each other, no differences within experimental error can be observed for the anthracene skeletons, solely for the C(10)−CH₂−CH₂ bond angle [115.2(2)° for 11 and 113.9(1)° for 13]. In general, the bond angles of the carbon atoms in the linker units show higher diversity for propane 11 [110.3(2)–115.2(2)], than for butane 13 [113.1(2)–113.9(1)].

Figure 1.

Molecular structures of 1,3‐bis(1,8‐dichloranthracen‐10‐yl)propane (11, above) and 1,4‐bis(1,8‐dichloroanthracen‐10‐yl)butane (13, below) in the crystalline state. Displacement ellipsoids are drawn at 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°] for 11/13: C(1)−C(2) 1.433(4)/1.424(2), C(1)−C(14) 1.353(4)/1.354(2), C(1)−Cl(1) 1.750(3)/1.743(2), C(2)−C(3) 1.394(4)/1.387(2), C(2)−C(11) 1.440(4)/1.438(2), C(10)−C(11) 1.413(4)/1.405(2), C(10)−C(29/15) 1.514(4)/1.513(2), C(15)−C(16)−/1.524(2), C(16)−C(16’)−/1.516(3), C(29)−C(30) 1.537(4)/−; C(1)−C(2) −C(3) 122.6(3)/122.8(2), C(1)−C(2)−C(11) 117.6(3)/117.6(1), C(2)−C(1)−Cl(1) 118.4(2)/118.8(1), C(2)−C(3)−C(4) 120.9(3)/121.5(2), C(2)−C(11)−C(10) 119.6(3)/119.7(2), C(10)−CH₂−CH₂ 115.2(2)/113.9(1), C(11)−C(10)−CH₂ 120.3(3)/120.1(2), C(15)−C(16)−C(16’)−/113.1(2), C(29)−C(30)−C(31) 110.3(2)/−.

In order to investigate the influence of the chain length in terms of the ability to form photo‐cyclomers, polymethylene‐linked bisanthracenes with more than four members would be interesting. However, the formation of such compounds might require more sophisticated synthetic strategies. Assuming that dimethylsilyl groups within the bridging moiety should not significantly affect the photochemistry of the corresponding bisanthracenes, we synthesized derivatives 15 and 16; they are linked by five and ten‐membered chains containing one and two dimethylsilyl groups, respectively. As shown in Scheme 5, 10‐allyl‐1,8‐dichloroanthracene (4) and 1,8‐dichloro‐10‐ethynylanthracene (14) were used as starting materials and converted in one or two steps (for experimental details and solid state structures, see SI).

Scheme 5.

Syntheses of bis[(1,8‐dichloroanthracene‐10‐yl)ethyl]dimethylsilane (15) and 1,2‐bis{[3‐(1,8‐dichloroanthracen‐10‐yl)propyl]dimethylsilyl}ethane (16). For experimental details see SI.

Photo‐Cyclomerizing Reactions

The UV/VIS‐absorption spectra of compunds 8, 9, 11 and 13 (see Figures S27 and S28, SI) are highly consistent with characteristic p absorption bands located at 320–420 nm, as it is observed for 1,8‐dichloroanthracene.20 These four bisanthracenes were investigated in UV light induced photo‐cyclomerization experiments (Scheme 6). Therefore, small amounts of the yellow solids were dissolved or suspended in degassed deuterated chloroform in an NMR tube and the mixtures were irradiated with UV light (365 nm) for a certain period of time. The conversion was indicated by a rapid change of the sample's colour from yellow to colorless. ¹H NMR spectroscopic experiments demonstrated the quantitative formation of the photo‐cyclomers 8_PC, 9_PC and 11_PC. Compounds 8_PC and 9_PC were found to be much better soluble than their non‐irradiated precursors 8 and 9. However, the NMR signals of compound 8_PC rapidly disappeared after a few minutes and a colorless solid precipitated, which could not be dissolved in any common organic solvent. Irradiating butane 13, a mixture of different species was obtained. Besides cyclomer 13_PC, which could be explicitly identified, a colorless precipitate is obtained. We suppose that hardly soluble photo‐dimers and oligomers were formed due to intermolecular cycloaddition, indicated by additional characteristic resonances in the ¹H NMR spectrum. Evidence to this assumption was further provided by the entire recovery of 13, if the sample is heated to 220 °C for 1 h.

Scheme 6.

UV light irradiation of compounds 8, 9, 11 and 13 and formation of the corresponding [4π+4π] photo‐cyclomers; a) CDCl₃, UV light (365 nm), 1–3 h. The photo‐cyclomers 8_PC, 9_PC and 11_PC are formed quantitatively. In addition to cyclomer 13_PC, photo‐oligomers were formed by intermolecular cycloaddition.

The cyclomers 8_PC, 9_PC, 11_PC and 13_PC were further characterized by ¹³C NMR spectroscopy and by (high) resolution mass spectrometry. Their molecular structures in the crystalline state were determined by X‐ray diffraction studies and are depicted in Figure 2 (for details see SI). To the best of our knowledge, these structures are the first known examples for substituted 9,10 : 9’,10’‐photo‐cyclomers with four functional groups, pointing in the same direction. The structural parameters of all four compounds are well comparable to each other, except for the corresponding linker units.

Figure 2.

Angled views (above) and side views (below) of the molecular structures of photo‐cyclomers 8_PC, 9_PC, 11_PC and 13_PC (from left to right) in the crystalline state. Displacement ellipsoids are drawn at 50 % probability level. Hydrogen atoms are omitted for clarity. The intramolecular distances between the chlorine substituents are found to be 4.6–4.8 Å (shown horizontally) and 3.7–3.9 Å (shown vertically). Selected bond lengths and angles are listed in Table S1 (SI).

In contrast to the α,ω‐alkandiyl‐based bisanthracenes, the irradiation of the SiMe₂‐containing bisanthracenes 15 and 16 did not lead to intramolecular photo‐cyclomers. ¹H NOESY NMR experiments indicate the formation of intermolecular photo‐oligomers by revealing the proximity of the linker protons to those in position 9 of the anthracene dimer units (Figures S22–S25, SI). These results are consistent with previous observations that typically no photo‐cyclomers are obtained for bisanthracenes bearing more than four‐membered (polymethylene) linker units.3

Since compounds 9 and 11 were found to be most suitable for photo‐cyclomerization, only these two bisanthracenes were used for further modifications. In order to expand the aromatic systems with rigid alkynyl substituents, 9 and 11 were functionalized in quadruple Kumada cross‐coupling reactions with (trimethylsilyl)ethynyl‐substituents (Scheme 7). A subsequent cleavage of the trimethylsilyl protecting groups using potassium carbonate in methanol afforded the ethynyl‐substituted derivatives 19 and 20.

Scheme 7.

Syntheses of the bisanthracenylalkanes 17–22. Reagents and conditions: a) Me₃Si−C≡C−MgBr, Ni(acac)₂, PPh₃, THF, reflux, 67 % (17), 49 % (18); b) K₂CO₃, MeOH, 16 h, quant. (19/20); c) Me₃Sn−NMe₂, THF, 60 °C, 2–4 h, quant. (21/22).

According to a literature protocol by Wrackmeyer et al.21 the terminal alkynes 19 and 20 were reacted with (dimethylamino)trimethylstannane in amine elimination reactions, to afford the tetrastannanes 21 and 22, respectively, in quantitative yield. They were characterized by multinuclear NMR spectroscopy, elemental analysis and X‐ray diffraction experiments. Their molecular structures are depicted in Figure 3. Both compounds crystallize in stretched configurations. The intramolecular distances between Sn(1) and Sn(2) as well as Sn(3) and Sn(4) range from 5.4 to 6.0 Å. The coordination sphere of the tin atoms is distorted from local threefold symmetry, most significantly demonstrated for compound 22 by the bond angles C(16)−Sn(1)−C(17) [101.6(1)°] and C(17)−Sn(1)−C(19) [117.0(1)°]. Repulsive interactions between the bulky Me₃Sn‐groups are indicated by the torsion angles Sn(1)−C(1)−C(5)−Sn(2) and Sn(3)−C(25)−C(29)−Sn(4) ranging from ±11° to 13° and by a considerable bending of the alkynyl‐groups.

Figure 3.

Molecular structures of 1,2‐bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10yl}ethane (21, above) and 1,3‐bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10yl}propane (22, below) in the crystalline state. Displacement ellipsoids are drawn at 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°] for 21/22: C(1)−C(2) 1.447(3)/1.443(3), C(1)−C(15) 1.443(4)/1.439(3), C(2)−C(3) 1.388(3)/1.392(2), C(2)−C(11) 1.442(3)/1.432(2), C(10)−C(11) 1.413(3)/1.415(3), C(10)−C(49) 1.514(3)/1.511(2), C(15)−C(16) 1.205(4)/1.207(3), C(16)−Sn(1) 2.108(3)/2.118(2), C(17)−Sn(1) 2.131(3)/2.136(2), C(49)−C(50) 1.545(3)/1.534(2), C(50)−C(51)−/1.537(3); C(1)−C(2)−C(3) 121.5(2)/120.8(2), C(1)−C(2)−C(11) 118.9(2)/119.5(2), C(1)−C(15)−C(16) 177.0(3)/174.6(2), C(2)−C(3)−C(4) 121.5(2)/121.2(2), C(2)−C(11)−C(10) 119.6(2)/120.0(2), C(10)−C(49)−C(50) 111.7(2)/112.9(2), C(11)−C(10)−C(49) 120.4(2)/118.4(2), C(15)−C(16)−Sn(1) 177.0(3)/165.1(2), C(16)−Sn(1)−C(17) 110.0(1)/101.6(1), C(17)−Sn(1)−C(19) 109.0(1)/117.0(1), C(49)−C(50)−C(51)−/111.3(2).

Combining the attributes of flexibly linked anthracene backbones and fourfold directed metal‐containing substituents, tetrastannanes 21 and 22 were tested as model compounds for photo‐switchable acceptor molecules.

Initially, appropriate investigations were carried out for the unsubstituted tetraalkynes 19 and 20 according to the previously described experiments. Due to the more extended π‐systems, the UV/VIS‐absorption spectra of 19 and 20 (see Figure S30, SI) show bathochromic shifts of about 20 nm for the p absorption bands (340–440 nm), compared to the analogous chloro‐substituted derivatives 9 and 11. The maxima are located at 423 nm for 19 and 420 nm for 20, which is considerably red‐shifted compared to simple 1,8‐diethynylanthracene at 405 nm.20 As for the tetrachlorides 9 and 11, the products of intramolecular 9,10 : 9’,10’‐cycloaddition 19_PC and 20_PC were obtained upon UV‐light induced photo‐cyclomerization (Scheme 8). In the case of propane 20, traces of an unknown photo‐product (probably an intermolecular formed photo‐dimer) were observed in its ¹H NMR spectrum.

Scheme 8.

UV light induced photo‐cyclomerization of the bisanthracenylalkanes 9, 11, 19 and 20 and thermal cycloreversion: a) CDCl₃, UV light (365 nm); b) 1,2‐dichlorobenzene‐d₄, 160–220 °C.

Aside from NMR spectroscopic investigations, absorption spectroscopy is a well suited tool to monitor the photoreactions. This was exemplarily performed for bisanthracenylethane 19 (Figure 4). In case of a low concentrated solution (2.5×10⁻⁵ mol L⁻¹), upon UV irradiation the absorption bands of 19 rapidly decrease. After only four minutes, the maxima at 261 nm (β band) and 423 nm (p band) disappeared and a new hypsochromically shifted band arises at about 230–250 nm. These observations result from the loss of the larger anthracene π‐system in favor of multiple non‐conjugated benzene rings of the cyclomer.

Figure 4.

UV/VIS spectrum of bisanthracene 19 (2.5×10⁻⁵ mol L⁻¹) and the formed photo‐cyclomer 19_PC as a function of UV irradiation duration.

Bergmark and Jones reported the cycloreversion of photo‐cyclomers by thermal dissociation.22 In case of bisanthracenylalkanes they used the high boiling 1,2‐dichlorobenzene as solvent, since temperatures far above 100 °C are required. In order to reobtain the bisanthracenylethanes and ‐propanes, analogous experiments concerning the thermal dissociation were performed for cyclomers 9_PC, 11_PC, 19_PC and 20_PC, respectively (Scheme 8). An entire reversibility of the cyclomerization, monitored by ¹H NMR spectroscopy, could be demonstrated for 9_PC, 11_PC and 19_PC by heating 1,2‐dichlorobenzene‐d₄ solutions up to 160–200 °C (for details see Experimental Section). Compound 20_PC could be partially dissociated as well at 220 °C, however, the corresponding isomer 20 was not completely reobtained, even after heating for several days. Instead, the loss in intensity of the signals indicates slow decomposition. In general, lower temperatures (160–180 °C) and faster reaction times are required for the cycloreversion of the ethylene‐linked compounds, compared to the analogous propanes (200–220 °C). This might be due to the higher ring strain, caused by the cyclobutane unit. By alternating irradiating and heating the NMR samples of 9/9_PC, 11/11_PC and 19/19_PC under the above mentioned conditions photo‐cylomerization and cycloreversion could be repeated several times for these compounds (four cycles could be carried out effortlessly), proving their ability to be used as photo‐switches. However, the high temperatures required for ring opening could be a disadvantage for later applications.

The solid state structures of 19 and 20 and their corresponding photo‐cyclomers 19_PC and 20_PC were determined by X‐ray diffraction experiments. Exemplarily, those of 20 and 20_PC are depicted in Figure 5. Expectedly, the bond angles C(2)−C(3)−C(4) [121.2(2) for 20 and 107.4(1) for 20_PC] as well as C(9)−C(10)−C(11) [119.6(2) for 20 and 106.3(1) for 20_PC] indicate the change from trigonal planar to tetrahedral geometry at the bridgehead atoms. Furthermore, the bonds C(2)−C(3), C(3)−C(4), C(9)−C(10) and C(10)−C(11) are increased from about 1.4 Å to approximately 1.5 Å. No or only slight differences are found for the outer benzene rings and the ethynyl‐functions. As a characteristic for photo‐dimers and photo‐cyclomers, the new bonds formed by cycloaddition, C(3)−C(21) [1.606(1) Å] and C(10)−C(28) [1.662(2)], are very long compared to the standard C−C single bond with 1.52 Å.23 They are in the range of very long C−C bonds like those in diamantoid dimers and longer than the central bond in diamantyl‐diamantane [solid state, XRD: 1.647(4), gas‐phase, GED: 1.630(5) Å].24

Figure 5.

Molecular structures of 1,3‐bis(1,8‐diethynylanthracen‐10‐yl)propane (20, above) and its 9,10 : 9’,10’‐photo‐cyclomer 20_PC (below) in the crystalline state. Displacement ellipsoids are drawn at 50 % probability level. Hydrogen atoms and a half disordered benzene solvent molecule in 20 are omitted for clarity. Selected bond lengths [Å] and angles [°] for 20/20_PC: C(1)−C(2) 1.441(3)/1.405(2), C(1)−C(15) 1.436(3)/1.436(2), C(2)−C(3) 1.395(3)/1.511(1), C(2)−C(11) 1.438(3)/1.400(2), C(3)−C(21) −/1.606(1), C(10)−C(11) 1.407(3)/1.536(2), C(10)−C(28) −/1.662(2), C(10)−C(37) 1.514(3)/1.545(2), C(15)−C(16) 1.181(3)/1.193(2), C(37)−C(38) 1.536(3)/1.521(2); C(1)−C(2)−C(3) 121.1(2)/122.0(1), C(1)−C(15)−C(16) 177.8(2)/178.7.(1), C(2)−C(3)−C(4) 121.2(2)/107.4(1), C(2)−C(3)−C(21) −/112.8(1), C(2)−C(11)−C(10) 120.0(2)/117.4(1), C(9)−C(10)−C(11) 119.6(2)/106.3(1), C(10)−C(37)−C(38) 110.6(2)/105.5(1), C(11)−C(10)−C(28) −/110.0(1), C(37)−C(38)−C(39) 114.4(2)/100.1(1).

No cyclomerization reaction takes place in case of the tin‐functionalized ethylene derivate 21. After several hours of UV irradiation, slow decomposition can be observed in the ¹H NMR spectrum. In contrast, the propylene‐linked bisanthracene 22 is converted completely into its 9,10 : 9’,10’‐photo‐cyclomer 22_PC (Scheme 9) under these conditions at very low concentrations.

Scheme 9.

UV light induced photo‐reactions of the bisanthracenylpropane 22, forming the intramolecular photo‐cyclomer 22_PC and the intermolecular photo‐dimer 22₂, respectively: a) CDCl₃, UV light (365 nm), high dilution; b) CDCl₃, UV light (365 nm), low dilution.

The head‐to‐tail photo‐dimer 22₂ is obtained beside 22_PC if more concentrated solutions of 22 are irradiated. The ratio of both photo‐products, depending on the concentration of 22 in differrent (NMR‐)solvents is given in Table 1.

Table 1.

Ratio between photo‐cyclomer 22_PC and photo‐dimer 22₂, formed upon UV irradiation (365 nm) depending on the concentration of the reactant 22 in different solvents.

solvent	[22]/10−4 mol L−1	22PC [%]	222 [%]
CDCl3	9.7	100	0
o‐Cl2C6D4	26.2	100	0
C6D6	61.2	73	27
CDCl3	157.1	63	37
CDCl3	201.6	53	47

Both compounds can be easily differentiated by means of multinuclear and two‐dimensional NMR spectroscopy. Diffusion ordered NMR experiments (DOSY) were performed to determine the diffusion coefficient D of the reaction products. D correlates inversely to the molecular size.25, 26, 27 The diffusion coefficient of the non‐reacted bisanthracene 22 has also been determined and was used as reference (Table 2). The results clearly demonstrate the significantly different molecular sizes of 22_PC and 22₂. While the diffusion coefficients of 22 and of 22_PC indicate similar molecular sizes the diffusion coefficient of 22₂ was found to be considerably smaller as it is expected for a bulkier molecule [see Figure S26 (SI) for a DOSY plot of 22_PC and 22₂]. The assembly of a photo‐polymer, containing more than two monomeric units can be excluded. Thus, the results from ordinary NMR spectroscopy and DOSY NMR studies are in good agreement.

Table 2.

Diffusion coefficients D [10⁻¹⁰ m² s⁻¹] of compounds 22, 22_PC and 22₂ in CDCl₃ solution, determined by means of DOSY NMR measurements.

	22	22PC	222
D	6.03	6.62	4.42

As for the previous bisanthracenes, the thermal dissociation of compound 22_PC was attempted. However, no indications for a successful cycloreversion were obtained under the tested conditions. At temperatures higher than 80 °C, 22_PC is starting to decompose. Notwithstanding, 22 is rebuild from a mixture of 22_PC and 22₂ at 160 °C, illustrating that photo‐dimer 22₂ can be dissociated without any difficulties.

Conclusions

First steps on the way to photo‐switchable poly‐Lewis acids have been made. In this context, different “anthracene monomers”, bearing functional groups at the central ring were used to synthesize α,ω‐bis(1,8‐dichloroanthracen‐10‐yl)alkanes and some derivatives. UV irradiation experiments were performed for these bisanthracenes, obtaining their corresponding 9,10 : 9’,10’‐photo‐cyclomers or intermolecular photo‐oligomers, respectively. The ethylene‐ and propylene‐linked derivatives were found to be optimal for photo‐cylomerization and thus the modification by replacing the chloro‐substituents with rigid ethynyl‐groups was performed. The capability as reversible photo‐switches was proven by multiple cycles of photo‐cylomerization and thermal retro‐Diels‐Alder cycloreversion. The terminal alkynes were functionalized with trimethylstannyl groups, obtaining quadruply metalated bis‐anthracenes. DOSY NMR experiments were used to differentiate between intramolecular cylomers and intermolecular dimers, proving the shape of the formed photo‐products. All in all, the presented results are an important step on the way to generate systems that are capable of reversible guest complexation by photo‐switchable poly‐Lewis acids.

Experimental Section

General: 1,8‐Dichloroanthracene‐10‐(9H)‐one (1),14 1,8‐bis[(trimethylsilyl)ethynyl]‐10‐methylanthracene (2 b),28 bis[(1,8‐dichloroanthracene‐10‐yl)ethynyl]dimethylsilane (14 b)29 and 10‐bromo‐1,8‐dichloroanthracene17 were synthesized according to literature protocols. For the syntheses of compounds 2–7, 10, 12, 15, 16, 1,8‐bis[(trimethylsilyl)ethynyl]‐10‐(bromomethyl)anthracene (2 c), 1,8‐dichloro‐10‐(prop‐1‐yn‐1‐yl)anthracene (7 b), 10,10′‐(prop‐1‐ene‐1,3‐diyl)bis(1,8‐dichloroanthracene) (11 b), its photo‐cyclomer 11b_PC and lepidopterene 17 b see Supporting Information. Methylmagnesium bromide (3.0 m in Et₂O), vinylmagnesium bromide (1.0 m in THF), allylmagnesium bromide (1.0 m in Et₂O), ethylmagnesium bromide (3.0 m in Et₂O), n‐butyllithium (1.6 m in n‐hexane; all purchased from sigma aldrich), N‐bromosuccinimide, copper(I) iodide, nickel(II) acetylacetonate (from acros organics), dichlorodimethylsilane (from alfa aesar), phosphorus pentoxide, palladium/charcoal activated (10 % Pd, oxidic form), AIBN, diisopropylamine (from merck), Karstedt catalyst (2 % Pt in xylol, from abcr), bis(triphenylphosphine)‐palladium(II)dichloride, 1,2‐bis(dimethylsilyl)ethane, trimethylsilylacetylene, (from fluorochem) and PPh₃ (from fluka) were used without further purification. First generation Grubbs I catalyst was purchased from strem chemicals and hydrogen from linde. (Dimethylamino)trimethylstannane (from abcr) was used after condensation. All reactions using metal organic reagents, as well as Kumada cross‐coupling reactions were carried out under an anhydrous, inert atmosphere of nitrogen using standard Schlenk techniques in dry solvents (THF dried over potassium, Et₂O dried over LiAlH₄, both freshly distilled before use). All photo‐cyclomerization reactions were performed using a UVGL‐25 Compact UV Lamp (365 nm, 4 W, 230 V, 0.16 A). The distance between UV lamp and NMR tube/flask was about 1 cm. Column chromatography was performed on silica gel 60 (0.04–0.063 mm). NMR spectra were recorded on a Bruker Avance III 300, a Bruker DRX 500, a Bruker Avance III 500, a Bruker Avance III 500 HD and a Bruker Avance 600 at ambient temperature (298 K). The chemical shifts (δ) were measured in ppm with respect to the solvent [CDCl₃: ¹H NMR δ=7.26 ppm, ¹³C NMR δ=77.16 ppm; C₆D₆: ¹H NMR δ=7.16 ppm, ¹³C NMR δ=128.06 ppm; o‐Cl₂C₆D₄ (from deutero): ¹H NMR δ=6.93 and 7.19 ppm] or referenced externally (²⁹Si: SiMe₄, ¹¹⁹Sn: SnMe₄). EI mass spectra were recorded using an Autospec X magnetic sector mass spectrometer with EBE geometry (vacuum generators, Manchester, UK) equipped with a standard EI source. MALDI experiments were performed using a Q‐IMS‐TOF mass spectrometer Synapt G2Si (waters, Manchester, UK) in resolution mode, interfaced to a MALDI ion source or with an Ultraflex MALDI‐TOF/TOF mass spectrometer (bruker daltonik, Bremen, Germany) operated in reflectron mode (Thermo Scientific TSQ 7000, thermofinnigen). No HRMS could be measured for a few compounds e. g. non‐isolated photo‐cyclomers, or if the intensity of the signal was too low due to fragmentation or H/D exchange of the acid ethynyl protons during the UV irradiation. Elemental analyses were performed with CHNS elemental analyzer hekatech EURO EA or by Mikroanalytisches Labor Kolbe (Oberhausen). UV/VIS spectra were recorded on a thermo evolution 300 UV‐Visible spectrophotometer using Hellma Analytics QX absorption cells with PTFE stopper (10.00 mm, 200–3500 nm). The numbering scheme for NMR assignments of the anthracenes (Scheme 10) is based on IUPAC guidelines.

Scheme 10.

Numbering scheme for NMR spectroscopic assignments.

Bis(1,8‐dichloroanthracen‐10‐yl)methane (8): A solution of n‐butyllithium (1.6 m in n‐hexane, 0.62 mL, 0.99 mmol) was added dropwise to a solution of 10‐(bromomethyl)‐1,8‐dichloroanthracene (5, 326 mg, 1.00 mmol) in THF (14 mL) at −78 °C. After stirring for 0.5 hours, a solution of 1,8‐dichloroanthracen‐10(9H)‐one (1, 315 mg, 1.20 mmol) in THF (24 mL) was added at −78 °C and the mixture was allowed to warm to ambient temperature. After stirring for 16 h and hydrolyzing with an aqueous saturated solution of NH₄Cl (30 mL), the aqueous layer was extracted with dichloromethane (4×20 mL) and the combined organic phases were dried over MgSO₄. The solvent was evaporated and the residue was dissolved in dry toluene (40 mL). P₂O₅ (two tips of a spatula) was added and the mixture was heated to 80 °C for 3 h. The hot mixture was filtered and the solid was washed with toluene (30 mL). After removing the solvent, the residue was washed with chloroform (3×10 mL). Air‐drying afforded 8 as a yellow solid. Yield: 126 mg (25 %). ¹H NMR (500 MHz, CDCl₃): δ=9.40, (s, 2H, H9), 8.14 (d, ³ J _H,H=9.1 Hz, 4H, H4/H5), 7.57 (d, ³ J _H,H=7.2 Hz, 4H, H2/H7), 7.20 (dd, ³ J _H,H=9.1, 7.2 Hz, 4H, H3/H6), 5.97 (s, 2H, ArCH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=133.8, 131.6, 129.6, 126.4 (C3/C6), 125.9 (C2/C7), 123.6 (C4/C5), 121.5 (C9), 30.2 [solely observable by HMQC (ArCH₂)] ppm. One signal missing due to low solubility. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=504.0 [M]⁺, 469.0 [M−Cl]⁺. HRMS (MALDI‐TOF): calculated for C₂₉H₁₆Cl₄ ⁺: 504.00006; measured: 503.9995, dev. [ppm]: 1.11, dev. [mmu]: 0.56.

1,2‐Bis(1,8‐dichloroanthracen‐10‐yl)ethane (9): A solution of n‐butyllithium (1.6 m in n‐hexane, 0.20 mL, 0.32 mmol) was added dropwise to a solution of 10‐(bromomethyl)‐1,8‐dichloroanthracene (5, 224 mg, 0.66 mmol) in THF (18 mL) at −78 °C. After stirring for ten minutes the mixture was allowed to warm to ambient temperature, whereat a yellow solid precipitated. After stirring for 16 h and hydrolyzing with water (5 mL), the suspension was filtered and the residue washed with n‐pentane and dichloromethane (5 mL each). Air‐drying afforded 9 as a yellow solid. Yield: 142 mg (84 %). ¹H NMR (500 MHz, CDCl₃): δ=9.29, (s, 2H, H9), 8.00 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.60 (d, ³ J _H,H=7.1 Hz, 4H, H2/H7), 7.32 (dd, ³ J _H,H=8.9, 7.1 Hz, 4H, H3/H6), 4.07 (s, 4H, CH ₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=9.28, (s, 2H, H9), 8.11 (d, ³ J _H,H=9.0 Hz, 4H, H4/H5), 7.46 (d, ³ J _H,H=7.1 Hz, 4H, H2/H7), 7.28 (dd, ³ J _H,H=8.9, 7.2 Hz, 4H, H3/H6), 3.94 (s, 4H, CH ₂) ppm. Due to the extreme low solubility of the compound, a ¹³C{¹H} NMR spectrum could not be recorded. MS (EI, 70 eV): m/z [assignment]=518.3 [M]⁺, 259.0 [C₁₅H₉Cl₂]⁺, 225.0 [C₁₅H₉Cl]⁺, 189.0 [C₁₅H₉]⁺. HRMS (EI): calculated for C₃₀H₁₈Cl₄ ⁺: 518.01571; measured: 518.01459; dev. [ppm]: 2.16, dev. [mmu]: 1.12.

When the mixture was stirred for several hours at −78 °C before warming to ambient temperature, peritetrachlorolepidopterene 9 b was formed as the main product. Analytical data of 9 b: ¹H NMR (500 MHz, CDCl₃): δ=7.10, (dd, ³ J _H,H=8.0 Hz, 4H, H2/H7), 6.80 (t, ³ J _H,H=7.9 Hz, 4H, H3/H6), 6.69 (d, ³ J _H,H=7.8 Hz, 4H, H4/H5), 5.82 (t, ³ J _H,H=2.9 Hz, 2H, H9), 2.86 (d, ³ J _H,H=2.9 Hz, 4H, CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=145.2, 139.5, 129.8, 126.6 (C3/C6), 126.6 (C2/C7), 121.6 (C4/C5), 54.5 (C10), 37.3 (C9), 27.7 (CH₂) ppm. MS (EI, 70 eV): m/z [assignment]=517.9 [M]⁺, 258.9 [C₁₅H₉Cl₂]⁺, 189.0 [C₁₅H₉]⁺. Elemental analysis calcd (%) for C₃₀H₁₈Cl₄: C 69.26, H 3.49; found: C 68.84, H 3.67.

1,3‐Bis(1,8‐dichloroanthracen‐10‐yl)propane (11): A suspension of 10,10′‐(prop‐1‐yne‐1,3‐diyl)bis(1,8‐dichloroanthracene) (10, 250 mg, 0.47 mmol) and Pd/C (50 mg, 10 % Pd, oxidic form) in dichloromethane (50 mL) was stirred under hydrogen atmosphere (starting at p=1.6 bar) at ambient temperature. After each day the pressure was increased by 0.3 bar until 2.2 bar and finally stirred for further 4 h. The mixture was filtered, the residue washed with dichloromethane (5 mL) and the filtrate evaporated. Washing the residue with n‐pentane (10 mL) afforded 11 as a yellow solid. Yield: 218 mg (87 %). ¹H NMR (500 MHz, CDCl₃): δ=9.28, (s, 2H, H9), 8.02 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.61 (d, ³ J _H,H=7.2 Hz, 4H, H2/H7), 7.35 (dd, ³ J _H,H=8.9, 7.2 Hz, 4H, H3/H6), 3.77 (t, ³ J _H,H=7.9 Hz, 4H, ArCH ₂), 2.34 (p, ³ J _H,H=7.9 Hz, 2H, ArCH₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.9 (C10), 133.6, 130.9, 129.4, 125.9 (C3/C6), 125.8 (C2/C7), 123.5 (C4/C5), 120.4 (C9), 32.7 (ArCH₂ CH₂), 28.4 (ArCH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=9.25, (s, 2H, H9), 7.99 (d, ³ J _H,H=9.0 Hz, 4H, H4/H5), 7.45 (d, ³ J _H,H=7.1 Hz, 4H, H2/H7), 7.21 (dd, ³ J _H,H=8.9, 7.3 Hz, 4H, H3/H6), 3.69 (t, ³ J _H,H=8.0 Hz, 4H, ArCH ₂), 2.21 (p, ³ J _H,H=7.9 Hz, 2H, ArCH₂CH ₂) ppm. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=532.0 [M]⁺. HRMS (MALDI‐TOF): calculated for C₃₁H₁₂Cl₄ ⁺: 532.03136; measured: 532.0327; dev. [ppm]: 2.52, dev. [mmu]: 1.34. Elemental analysis calcd (%) for C₃₁H₂₀Cl₄: C 69.69, H 3.77; found: C 70.37, H 3.98.

1,4‐Bis(1,8‐dichloroanthracen‐10‐yl)butane (13): A suspension of 1,4‐Bis(1,8‐dichloroanthracene‐10‐yl)but‐2‐ene (12; 209 mg, 0.38 mmol) and Pd/C (12 mg, 10 % Pd, oxidic form) in dichloromethane (40 mL) was stirred under hydrogen atmosphere (p=5.0 bar) at ambient temperature for one day. After removing the solvent, the residue was treated with boiling toluene (25 mL) and the mixture was filtered. The solid was washed with small amounts of toluene. Cooling the solution to ambient temperature afforded 13 as yellow crystals. Yield: 133 mg (63 %). ¹H NMR (500 MHz, CDCl₃): δ=9.28 (s, 1H, H9), 8.16 (d, ³ J _H,H=8.9 Hz, H4/H5), 7.64 (d, ³ J _H,H=7.1 Hz, H2/H7), 7.43 (dd, ³ J _H,H=8.9, 7.1 Hz, H3/H6), 3.65–3.71 (m, 4H, ArCH ₂), 2.04–2.10 (m, 4H, ArCH₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=136.4 (C10), 133.6, 130.8, 129.5, 125.8 (C2/C3/C6/C7), 123.7 (C4/C5), 120.3 (C9), 31.9 (ArCH₂ CH₂), 28.7 (ArCH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=9.22 (s, 1H, H9), 8.09 (d, ³ J _H,H=8.9 Hz, H4/H5), 7.47 (d, ³ J _H,H=7.0 Hz, H2/H7), 7.20–7.29 (m, H3/H6), 3.51–3.64 (m, 4H, ArCH ₂), 1.98–2.10 (m, 4H, ArCH₂CH ₂) ppm. MS (EI, 70 eV): m/z [assignment]=546.0 [M]⁺, 259.0 [C₁₅H₉Cl₂]⁺. HRMS (EI): calculated for C₃₂H₂₂Cl₄ ⁺: 546.04701; measured: 546.04455; dev. [ppm]: 4.51, dev. [mmu]: 2.46. Elemental analysis calcd (%) for C₃₂H₂₂Cl₄: C 70.10, H 4.04; found: C 68.74, H 4.37.

General procedure for Kumada cross‐coupling reactions: [(Trimethylsilyl)ethynyl]magnesium bromide was obtained by adding trimethylsilylacetylene to a solution of ethylmagnesium bromide in THF at 0 °C. The mixture was stirred at room temperature for 16 h and gas evolution was observed. The freshly prepared [(trimethylsilyl)ethynyl]magnesium bromide (1 m in THF, ca. 12 eq.) was added to a solution of the bis(1,8‐dichloroanthracen‐10‐yl)alkane, Ni(acac)₂ and PPh₃ in THF at room temperature. The color of the solution changed from yellow to dark red and the mixture was heated to reflux for several days. After hydrolyzing with a saturated aqueous solution of NH₄Cl, the aqueous layer was extracted with dichloromethane for several times. The combined organic phases were dried over MgSO₄. The solvent was evaporated and the crude yellow brownish solid was purified by either washing with dichloromethane or column chromatography on silica gel.

1,2‐Bis{1,8‐bis[(trimethylsilyl)ethynyl]anthracen‐10‐yl}ethane (17): Synthesis according to the general procedure for Kumada cross‐coupling reactions using 1,2‐bis(1,8‐dichloroanthracen‐10‐yl)ethane (9, 205 mg, 0.39 mmol), PPh₃ and Ni(acac)₂ (one spatula tip of each compound), reflux for 6 d. Washing the crude product with n‐pentane/dichloromethane (2 : 1, 20 mL) and dichloromethane (5 mL) afforded 17 as bright yellow solid. Yield: 201 mg (67 %). ¹H NMR (500 MHz, CDCl₃): δ=9.35, (s, 2H, H9), 8.18 (d, ³ J _H,H=9.1 Hz, 4H, H4/H5), 7.77 (d, ³ J _H,H=6.7 Hz, 4H, H2/H7), 7.38 (dd, ³ J _H,H=8.5, 7.1 Hz, 4H, H3/H6), 4.03 (s, 4H, CH ₂), 0.39 (s, 36H, CH ₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.2 (C10), 132.2 (C2/C7), 131.2, 129.8, 125.3 (C4/C5), 125.2 (C3/C6), 123.7 (C9), 122.4, 104.0 (C≡CSi), 100.2 (C≡CSi), 29.6 (CH₂), 0.6 (CH₃) ppm. Due to the extreme low solubility of the compound, a ²⁹Si{¹H} NMR spectrum could not be recorded. MS (MALDI‐TOF, positive ions, ABS): m/z [assignment]=766.3 [M]⁺, 383.2 [C₂₅H₂₇Si₂]⁺. HRMS (MALDI‐TOF): calculated for C₅₀H₅₄Si₄ ⁺: 766.3303; measured: 766.3314, dev. [ppm]: 1.49, dev. [mmu]: 1.14. Elemental analysis calcd (%) for C₅₀H₅₄Si₄: C 78.27, H 7.09; found: C 77.89, H 7.25.

1,3‐Bis{1,8‐bis[(trimethylsilyl)ethynyl]anthracen‐10‐yl}propane (18): Synthesis according to the general procedure for Kumada cross‐coupling reactions using 1,3‐bis(1,8‐dichloroanthracen‐10‐yl)propane (11; 218 mg, 0.41 mmol), PPh₃ and Ni(acac)₂ (one spatula tip of each compound), reflux for 3 d. Column chromatography [∅=3 cm, l=9 cm, eluent: n‐pentane/dichloromethane (8 : 1)] afforded 18 as bright yellow solid. R _f=0.1 [n‐pentane/dichloromethane (8 : 1)]. Yield: 154 mg (49 %). ¹H NMR (500 MHz, CDCl₃): δ=9.33, (s, 2H, H9), 8.08 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.76 (d, ³ J _H,H=6.9 Hz, 4H, H2/H7), 7.36 (dd, ³ J _H,H=8.9, 6.9 Hz, 4H, H3/H6), 3.73 (t, ³ J _H,H=7.8 Hz, 4H, ArCH ₂), 2.31 (p, ³ J _H,H=7.8 Hz, 2H, ArCH₂CH ₂), 0.38 (s, 36H, CH ₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.5 (C10), 132.2 (C2/C7), 131.2, 129.5, 125.4 (C4/C5), 125.2 (C3/C6), 123.3 (C9), 122.3, 104.0 (C≡CSi), 100.1 (C≡CSi), 32.8 (ArCH₂ CH₂), 28.0 (ArCH₂), 0.6 (CH₃) ppm. ²⁹Si{¹H} NMR (99 MHz, CDCl₃): δ=−17.5 ppm. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=780.3 [M]⁺. HRMS (MALDI‐TOF): calculated for C₅₁H₅₆Si₄ ⁺: 780.34536; measured: 780.3468, dev. [ppm]: 1.85, dev. [mmu]: 1.44.

1,2‐Bis(1,8‐diethynylanthracen‐10‐yl)ethane (19): A suspension of 1,2‐bis{1,8‐bis[(trimethylsilyl)ethynyl]anthracen‐10‐yl}ethane (17, 171 mg, 0.22 mmol) and K₂CO₃ (300 mg, 2.17 mmol) in methanol (120 mml) was heated to reflux for 1d. After cooling to ambient temperature, the suspension was filtered. Washing the crude product with methanol (10 mL) and water (10 mL) afforded 19 as yellow solid. Yield: 107 mg (quantitative). ¹H NMR (500 MHz, CDCl₃): δ=9.48, (s, 2H, H9), 8.15 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.76 (d, ³ J _H,H=6.7 Hz, 4H, H2/H7), 7.38 (dd, ³ J _H,H=8.7, 7.1 Hz, 4H, H3/H6), 4.06 (s, 4H, CH ₂), 3.26 (s, 4H, C≡CH) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=133.1 (C10), 131.6, 131.3 (C2/C7), 129.8, 125.4 (C4/C5), 125.2 (C3/C6), 123.5 (C9), 121.4, 82.9 (C≡CH), 82.0 (C≡CH), 29.4 (ArCH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=9.59, (s, 2H, H9), 8.23 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.68 (d, ³ J _H,H=6.8 Hz, 4H, H2/H7), 7.34 (dd, ³ J _H,H=8.9, 7.0 Hz, 4H, H3/H6), 3.95 (s, 4H, CH ₂), 3.64 (s, 4H, C≡CH) ppm. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=478.3 [M]⁺. HRMS (MALDI‐TOF): calculated for C₃₈H₂₂ ⁺: 478.17160; measured: 478.1715, dev. [ppm]: 0.21, dev. [mmu]: 0.10. Elemental analysis calcd (%) for C₃₈H₂₂: C 95.37, H 4.63; found: C 89.67, H 4.59. Too low carbon values are known for hydrocarbons with terminal alkynes.

1,3‐Bis(1,8‐diethynylanthracen‐10‐yl)propane (20): A suspension of 1,3‐bis{1,8‐bis[(trimethylsilyl)ethynyl]anthracen‐10‐yl}propane (18, 140 mg, 0.18 mmol) in methanol (100 mL) was temporarily heated to reflux and then cooled to ambient temperature. After addition of K₂CO₃ (150 mg, 1.09 mmol), the mixture was stirred for 16 h. Evaporation of the solvent and filtration of the crude product over silica gel with dichloromethane afforded 20 as yellow solid. Yield: 88 mg (quantitative). ¹H NMR (500 MHz, CDCl₃): δ=9.46, (s, 2H, H9), 8.10 (d, ³ J _H,H=9.0 Hz, 4H, H4/H5), 7.75 (d, ³ J _H,H=6.8 Hz, 4H, H2/H7), 7.37 (dd, ³ J _H,H=9.0, 6.8 Hz, 4H, H3/H6), 3.75 (t, ³ J _H,H=7.9 Hz, 4H, ArCH ₂), 3.61 (s 4H, C≡CH), 2.32 (p, ³ J _H,H=7.9 Hz, 2H, ArCH₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.6 (C10), 131.6, 131.3 (C2/C7), 129.5, 125.6 (C4/C5), 125.3 (C3/C6), 123.2 (C9), 121.4, 82.9 (C≡CH), 82.0 (C≡CH), 32.9 (ArCH₂ CH₂), 27.9 (ArCH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=9.57, (s, 2H, H9), 8.07 (d, ³ J _H,H=9.0 Hz, 4H, H4/H5), 7.67 (d, ³ J _H,H=6.7 Hz, 4H, H2/H7), 7.26 (dd, ³ J _H,H=8.9, 6.9 Hz, 4H, H3/H6), 3.69 (t, ³ J _H,H=7.9 Hz, 4H, ArCH ₂), 3.60 (s 4H, C≡CH), 2.19 (p, ³ J _H,H=7.9 Hz, 2H, ArCH₂CH ₂) ppm. MS (EI, 70 eV): m/z [assignment]=492.3 [M]⁺. Elemental analysis calcd (%) for C₃₉H₂₄: C 95.09, H 4.70; found: C 87.91, H 4.70. Too low carbon values are known for hydrocarbons with terminal alkynes.

General procedure for the syntheses of the trimethylstannyl‐substituted compounds 21 and 22: The gas elimination reactions were carried out analogous to a procedure described by Wrackmeyer and co‐workers.20 For that purpose, (dimethylamino)trimethylstannane (ca. 6 eq) was added to a solution of the corresponding tetraalkyne in dry THF. The mixture was heated to 60 °C for 2–4 h. After removing all volatile compounds in vacuo, the fourfold (trimethylstannyl)ethynyl‐functionalized bisanthracenes were quantitatively obtained as yellow solids.

1,2‐Bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10yl}ethane (21): Synthesis according to the general procedure using 1,2‐bis(1,8‐diethynylanthracen‐10‐yl)ethane (19, 50 mg, 0.10 mmol), (dimethylamino)trimethylstannane (0.1 mL, 0.6 mmol) and THF (5 mL). ¹H NMR (500 MHz, CDCl₃): δ=9.47, (s, 2H, H9), 8.18 (d, ³ J _H,H=8.9 Hz, 4H, H4/H5), 7.75 (d, ³ J _H,H=6.8 Hz, 4H, H2/H7), 7.38 (dd, ³ J _H,H=8.9, 6.8 Hz, 4H, H3/H6), 4.02 (s, 4H, CH ₂), 0.47 (s, 36H, CH ₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.1 (C10), 131.7 (C2/C7), 131.4, 129.8, 125.3 (C3/C6), 124.8 (C4/C5), 124.0 (C9), 122.9, 107.8 (C≡CSn), 99.5 (C≡CSn), 29.6 (CH₂), −6.9 (CH₃) ppm. ¹¹⁹Sn{¹H} NMR (186 MHz, CDCl₃): δ=−63.9 ppm. Elemental analysis calcd (%) for C₅₀H₅₄Sn₄: C 53.15, H 4.82; found: C 53.22, H 4.96.

1,3‐Bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10 yl}propane (22): Synthesis according to the general procedure using 1,3‐bis(1,8‐diethynylanthracen‐10‐yl)propane (20, 36 mg, 0.07 mmol), (dimethylamino)trimethylstannane (0.07 mL, 0.4 mmol) and THF (5 mL). ¹H NMR (500 MHz, CDCl₃): δ=9.44, (s, 2H, H9), 8.06 (d, ³ J _H,H=9.0 Hz, 4H, H4/H5), 7.73 (d, ³ J _H,H=6.8 Hz, 4H, H2/H7), 7.34 (dd, ³ J _H,H=8.7, 7.2 Hz, 4H, H3/H6), 3.73 (t, ³ J _H,H=7.8 Hz, 4H, ArCH ₂), 2.32 (p, ³ J _H,H=8.1 Hz, 2H, ArCH₂CH ₂), 0.45 (s, 36H, CH ₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=135.3 (C10), 131.7 (C2/C7), 131.3, 129.6, 125.2 (C3/C6), 124.9 (C4/C5), 123.7 (C9), 122.8, 107.8 (C≡CSn), 99.4 (C≡CSn), 32.8 (ArCH₂ CH₂), 28.1 (ArCH₂), −7.0 (CH₃) ppm. ¹¹⁹Sn{¹H} NMR (186 MHz, CDCl₃): δ=−64.1 ppm. Elemental analysis calcd (%) for C₅₁H₅₆Sn₄: C 53.55, H 4.93; found: C 53.87, H 4.84.

General procedure for photo‐cyclomerization reactions: In an NMR tube small amounts (3–5 mg) of the particular bisanthracene were dissolved or suspended in dry and degassed CDCl₃ (0.5 mL). The mixtures were irradiated with UV light (365 nm) until no more signals of the reactant could be observed in the ¹H NMR spectrum (1–3 h), respectively.

Photo‐cyclomer of bis(1,8‐dichloroanthracen‐10‐yl)methane (8_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. ¹H NMR (500 MHz, CDCl₃): δ=6.95, (dd, ³ J _H,H=7.7 Hz, ⁴ J _H,H=1.5 Hz, 4H, H2/H7), 6.80 (t, ³ J _H,H=7.5 Hz, 4H, H3/H6), 6.77 (dd, ³ J _H,H=7.4 Hz, ⁴ J _H,H=1.5 Hz, 4H, H4/H5), 5.98 (s, 2H, H9), 2.75 (s, 2H, CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=133.1, 127.3 (C3/C6), 127.0 (C2/C7), 122.4 (C4/C5), 44.2 (C9), 14.3 [solely observable by HMQC, (CH₂)] ppm. Three signals missing due to low solubility. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=504.0 [M]⁺. HRMS (MALDI‐TOF): calculated for C₂₉H₁₆Cl₄ ⁺: 504.00006; measured: 504.0006, dev. [ppm]: 1.07, dev. [mmu]: 0.54.

Photo‐cyclomer of 1,2‐bis(1,8‐dichloroanthracen‐10‐yl)ethane (9_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. For the synthesis of 9_PC on a preparative scale see the following protocol: A suspension of 1,2‐bis(1,8‐dichloroanthracen‐10yl)ethane (9, 83 mg, 0.16 mmol) in oxygen‐free dichloromethane (20 mL) was irradiated with UV light (365 nm) for one day. Removing the solvent afforded 9_PC as a colourless solid. Yield: Quantitative. ¹H NMR (500 MHz, CDCl₃): δ=7.03, (d, ³ J _H,H=7.7 Hz, 4H, H4/H5), 7.00 (d, ³ J _H,H=7.7 Hz, 4H, H2/H7), 6.88 (t, ³ J _H,H=7.7 Hz, 4H, H3/H6), 5.99 (s, 2H, H9), 3.00 (s, 4H, CH ₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=145.3, 139.4, 133.5, 127.1 (C2/C3/C6/C7), 121.8 (C4/C5), 62.6 (C10), 44.2 (C9), 17.0 (CH₂ CH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=6.87, (d, ³ J _H,H=7.6 Hz, 4H, H4/H5), 6.83 (d, ³ J _H,H=7.9 Hz, 4H, H2/H7), 6.73 (t, ³ J _H,H=7.8 Hz, 4H, H3/H6), 6.03 (s, 2H, H9), 2.80 (s, 4H, CH ₂CH ₂) ppm. MS (EI, 70 eV): m/z [assignment]=518.3 [M]⁺, 259.2 [C₁₅H₉Cl₂]⁺. Elemental analysis calcd (%) for C₃₀H₁₈Cl₄: C 69.26, H 3.49; found: C 68.00, H 3.53.

Photo‐cyclomer of 1,3‐bis(1,8‐dichloroanthracen‐10‐yl)propane (11_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. ¹H NMR (500 MHz, CDCl₃): δ=7.07 (d, ³ J _H,H=7.9 Hz, 4H, H4/H5), 7.00 (d, ³ J _H,H=7.9 Hz, 4H, H2/H7), 6.82 (t, ³ J _H,H=7.9 Hz, 4H, H3/H6), 6.09 (s, 2H, H9), 2.70 (t, ³ J _H,H=6.1 Hz, 4H, CH ₂CH₂CH ₂), 2.42 (p, ³ J _H,H=6.1 Hz, 2H, CH₂CH ₂CH₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=147.0, 140.3, 133.2, 127.1 (C2/C7), 126.8 (C3/C6), 122.6 (C4/C5), 64.3 (C10), 44.1 (C9), 35.3 (CH₂CH₂ CH₂), 26.9 (CH₂ CH₂CH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=6.90 (d, ³ J _H,H=7.9 Hz, 4H, H4/H5), 6.83 (d, ³ J _H,H=7.9 Hz, 4H, H2/H7), 6.68 (t, ³ J _H,H=7.9 Hz, 4H, H3/H6), 6.12 (s, 2H, H9), 2.48 (t, ³ J _H,H=5.9 Hz, 4H, CH ₂CH₂CH ₂), 2.23 (p, ³ J _H,H=5.9 Hz, 2H, CH₂CH ₂CH₂) ppm. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=532.0 [M]⁺. HRMS (MALDI‐TOF): calculated for C₃₁H₂₀Cl₄ ⁺: 532.03136; measured: 532.0322, dev. [ppm]: 1.58, dev. [mmu]: 0.84.

Photo‐cyclomer of 1,4‐bis(1,8‐dichloroanthracen‐10‐yl)butane (13_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. ¹H NMR (500 MHz, CDCl₃): δ=7.22 (d, ³ J _H,H=7.9 Hz, 4H, H4/H5), 7.01 (d, ³ J _H,H=7.9 Hz, 4H, H2/H7), 6.82 (t, ³ J _H,H=7.9 Hz, 4H, H3/H6), 6.13 (s, 2H, H9), 2.54–2.60 (m, 4H, ArCH ₂), 2.33–2.38 (m, 4H, ArCH₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=149.0, 140.8, 133.0, 127.1 (C2/C7), 126.7 (C3/C6), 123.3 (C4/C5) 57.3 (C10), 44.5 (C9), 29.5 (ArCH₂), 20.8 (ArCH₂ CH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=7.07 (d, ³ J _H,H=8.0 Hz, 4H, H4/H5), 6.84 (d, ³ J _H,H=7.7 Hz, 4H, H2/H7), 6.69 (t, ³ J _H,H=7.8 Hz, 4H, H3/H6), 6.18 (s, 2H, H9), 2.33–2.39 (m, 4H, ArCH ₂), 2.16–2.22 (m, 4H, ArCH₂CH ₂) ppm. MS (EI, 70 eV): m/z [assignment]=546.0 [M]⁺, 259.0 [C₁₅H₉Cl₂]⁺.

Photo‐cyclomer of 1,2‐bis(1,8‐diethynylanthracen‐10‐yl)ethane (19_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. ¹H NMR (500 MHz, CDCl₃): δ=7.08–7.12 (m, 8H, H2/H4/H5/H7), 6.89 (t, ³ J _H,H=7.7 Hz, 4H, H3/H6), 6.15 (s, 2H, H9), 3.31 (s, 4H, C=CH), 2.99 (s, 4H, CH ₂CH ₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=145.1, 143.7, 130.1 (C2/C7), 125.6 (C3/C6), 123.7 (C4/C5), 121.1, 82.0 (C≡CH), 81.5 (C≡CH), 61.6 (C10), 47.1 (C9), 16.4 (CH₂ CH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=6.94–7.01 (m, 8H, H2/H4/H5/H7), 6.77 (t, ³ J _H,H=7.7 Hz, 4H, H3/H6), 6.33 (s, 2H, H9), 3.40 (s, 4H, C≡CH), 2.83 (s, 4H, CH ₂CH ₂) ppm. MS (MALDI‐TOF, positive ions, DCTB): m/z [assignment]=478.2 [M]⁺. HRMS (MALDI‐TOF): calculated for C₃₈H₂₂ ⁺: 478.1716; measured: 478.1707, dev. [ppm]: 1.88, dev. [mmu]: 0.90.

Monitoring the photo‐cyclomerization of 19 to 19_PC by UV/VIS spectroscopy was performed by irradiating a solution of 19 in dichloromethane (2.5×10⁻⁵ mol L⁻¹) in an absorption cell with PTFE stopper (10.00 mm, 200–3500 nm) and recording absorption spectra in defined time intervals (0, 0.25, 0.5, 1, 2 and 4 minutes).

Photo‐cyclomer of 1,3‐bis(1,8‐diethynylanthracen‐10‐yl)propane (20_PC): Synthesis according to the general procedure for photo‐cyclomerization reactions. ¹H NMR (500 MHz, CDCl₃): δ=7.13 (dd, ³ J _H,H=7.9 Hz, ⁴ J _H,H=0.9 Hz, 4H, H4/H5), 7.10 (dd, ³ J _H,H=7.7 Hz, ⁴ J _H,H=0.9 Hz, 4H, H2/H7), 6.83 (t, ³ J _H,H=7.8 Hz, 4H, H3/H6), 6.27 (s, 2H, H9), 3.30 (s, 4H, C≡CH), 2.69 (t, ³ J _H,H=6.1 Hz, 4H, CH ₂CH₂CH ₂), 2.41 (p, ³ J _H,H=6.1 Hz, 2H, CH₂CH ₂CH₂) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=146.0, 145.4, 130.2 (C2/C7), 125.4 (C3/C6), 124.5 (C4/C5), 120.8, 82.2 (C≡CH), 81.4 (C≡CH), 63.4 (C10), 47.2 (C9), 34.8 (CH₂CH₂ CH₂), 26.9 (CH₂ CH₂CH₂) ppm. ¹H NMR (500 MHz, o‐Cl₂C₆D₄): δ=6.98–7.01 (m, 8H, H2/H4/H5/H7), 6.72 (t, ³ J _H,H=7.8 Hz, 4H, H3/H6), 6.43 (s, 2H, H9), 3.41 (s, 4H, C≡CH), 2.49 (t, ³ J _H,H=6.0 Hz, 4H, CH ₂CH₂CH ₂), 2.25 (p, ³ J _H,H=6.0 Hz, 2H, CH₂CH ₂CH₂) ppm. MS (EI, 70 eV): m/z [assignment]=492.3 [M]⁺.

Photo‐cyclomer 22_PC and Photo‐dimer 22₂ of 1,3‐bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10 yl}propane: Differently concentrated solutions of 1,3‐bis{1,8‐bis[(trimethylstannyl)ethynyl]anthracen‐10yl}propane (22) in dry and degassed CDCl₃, benzene‐d₆ or 1,2‐dichlorobenzene‐d₄ were irradiated with UV light (365 nm) until no more signals of the reactant could be observed in the ¹H NMR spectrum, respectively. Depending on the concentration, photo‐cyclomer 22_PC was formed exclusively or in a mixture with the photo‐dimer 22₂ (see Table 1, main text).

NMR data for photo‐cyclomer 22_PC: ¹H NMR (500 MHz, CDCl₃): δ=7.03–7.09, (m, 8H, H2/H4/H5/H7), 6.76 (t, ³ J _H,H=7.7 Hz, 4H, H3/H6), 6.01 (s, 2H, H9), 2.65 (t, ³ J _H,H=6.1 Hz, 4H, CH ₂CH₂CH ₂), 2.39 (p, ³ J _H,H=6.1 Hz, 2H, CH₂CH ₂CH₂), 0.39 (s, 36H, CH₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=145.5, 145.4, 131.8 (C2/C7), 124.9 (C3/C6), 123.5 (C4/C5), 122.6, 109.1 (C≡CSn), 97.3 (C≡CSn), 63.0 (C10), 48.2 (C9), 35.1 (CH₂CH₂ CH₂), 27.0 (CH₂ CH₂CH₂), −6.5 (CH₃) ppm. ¹¹⁹Sn{¹H} NMR (186 MHz, CDCl₃): δ=−63.7 ppm.

NMR data for photo‐dimer 22₂: ¹H NMR (500 MHz, CDCl₃): δ=9.33 (s, 2H, H9_a), 8.21 (d, ³ J _H,H=9.0 Hz, 4H, H4_a/H5_a), 7.69 (d, ³ J _H,H=6.8 Hz, 4H, H2_a/H7_a), 7.34–7.39 (m, 8H, H3_a/H6_a/H4_b/H5_b), 7.02 (d, ³ J _H,H=7.7 Hz, 4H, H2_b/H7_b), 6.86 (t, ³ J _H,H=7.8 Hz, 4H, H3_b/H6_b), 5.43 (s, 2H, H9_b), 3.72–3.79 (m, 4H, Ar_aCH ₂), 3.55–3.62 (m, 4H, Ar_bCH ₂), 1.65–1.74 (m, 4H, CH₂−CH ₂−CH₂), 0.55 (s, 32H, CH ₃), 0.42 (s, 32H, CH ₃) ppm. ¹³C{¹H} NMR (125 MHz, CDCl₃): δ=144.8, 143.3, 135.5, 131.5 (C2_a/C7_a), 131.2, 130.9 (C2_b/C7_b), 129.2, 125.1 (C3_a/C6_a/C4_b/C5_b), 125.0 (C3_b/C6_b), 124.8 (C4_a/C5_a), 123.5(C9_a), 122.8, 122.4, 109.7 (C≡CSn), 107.8 (C≡CSn), 99.2 (C≡CSn), 97.1 (C≡CSn), 58.8 (C10), 57.2 (C9), 37.5 (Ar_b CH₂), 30.2 (Ar_a CH₂), 28.1 (CH₂ CH₂CH₂), −6.8 (CH₃), −7.0 (CH₃) ppm. ¹¹⁹Sn{¹H} NMR (186 MHz, CDCl₃): δ=−64.2, −66.4 ppm.

General procedure for thermal cyclo‐reversion reactions: In a Young‐NMR tube small amounts (3–5 mg) of the particular photo‐cyclomer were dissolved in degassed 1,2‐dichlorobenzene‐d₄ (0.5 mL). The solutions were heated in a sand bath to different temperatures until the cycloreversion, monitored by ¹H NMR spectroscopy, was completed (9_PC, 11_PC and 19_PC) or no more progress in the reaction was observed (20_PC). Reaction conditions for 9_PC: 1 h at 180 °C or 1 d at 170 °C; 11_PC: 3 d at 200 °C; 19_PC: 0.5 h at 180 °C or 2 h at 160 °C; 20_PC: 1 d at 220 °C (no complete conversion).

In the special case of compound 13_PC, the entire cyclo‐reversion of this intramolecular species and the additionally obtained intermolecular photoproducts was performed by heating the mixture to 220 °C for 1 h.

Reversibility of the photo‐switches: Switching (at least four times) between the open bisanthracenes (9, 11, and 19) and their closed photo‐cyclomers (9_PC, 11_PC and 19_PC) could be performed by treating NMR samples of these compounds (in 1,2‐dichlorobenzene‐d₄ 0.5 mL) in accordance to the general protocols for photo‐cyclomerization and thermal cyclo‐reversion reactions, alternately.

Crystal Structure Determination: Suitable crystals of the compounds 2 c, 3, 5, 7, 9 b, 11, 11 b, 12, 13, 15, 16, 17, 17 b, 19, 20, 21, 22, 8_PC, 9_PC, 11_PC, 11b_PC, 13_PC, 19_PC and 20_PC were obtained by slow evaporation of saturated solutions of chloroform (2 c, 5, 11 b, 21, 22, 9_PC, 20_PC), n‐pentane (3, 11b_PC), dichloromethane (7, 15), n‐pentane/dichloromethane (4 : 1) mixture (9 b), benzene (11, 12, 16, 17, 20, 11_PC, 13_PC, 19_PC), p‐xylene (13), toluene (17 b, 8_PC) and 1,2‐dichlorobenzene (19). They were selected, coated with paratone‐N oil, mounted on a glass fibre and transferred onto the goniometer of the diffractometer into a nitrogen gas cold stream solidifying the oil. Data collection was performed on a rigaku SuperNova diffractometer, a SuperNova, Dual, Cu at zero, Atlas diffractometer, a SuperNova, Single Source at Offset, Eos diffractometer, a nonius KappaCCD diffractometer and a bruker AXS X8 Prospector Ultra with APEX II diffractometer. Using Olex230 the structures were solved by direct methods and refined by full‐matrix least‐squares cycles (program SHELX).31 Crystal and refinement details, as well as CCDC numbers are provided in Tables S2–S4 (SI). CCDC 1872908 (2), 1003191 (3), 1872909–1872914 (5, 7, 9 b, 11, 11 b, 12), 1003193 (13), 1872915–1872922 (15, 16, 17, 17 b, 19, 20, 21, 22) and 1872923–1872929 (8_PC, 9_PC, 11_PC, 11b_PC, 13_PC, 19_PC, 20_PC) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.ac.uk/data_request/cif.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

P. Niermeier, J.-H. Lamm, A. Mix, B. Neumann, H.-G. Stammler, N. W. Mitzel, ChemistryOpen 2019, 8, 304.