Photochemical Single-Crystal-to-Single-Crystal (SCSC) Reactions of

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Photochemical Single-Crystal-to-Single-Crystal (SCSC) Reactions of Anthraphane to Dianthraphane and Poly anthraphane 1D

Marco Servalli, Michael Solar, Nils Trapp, Michael Wörle, and Dieter Schlüter Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01184 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Crystal Growth & Design

Photochemical Single-Crystal-to-Single-Crystal (SCSC) Reactions of Anthraphane to Dianthraphane and Poly1Danthraphane Marco Servalli†*, Michael Solar‡, Nils Trapp‡, Michael Wörle‡, A. Dieter Schlüter† †

Laboratory of Polymer Chemistry, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland.

‡

Laboratory of Inorganic Chemistry, Small Molecule Crystallography Center, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland.

ABSTRACT: This work describes the search for reactive crystal packings of anthraphane, a D3hsymmetric cyclophane with three vertically arranged anthracene units. Packings in which anthracene units of neighboring anthraphanes assume a face-to-face stacked geometry were selected from the available library of crystal structures of this compound. The corresponding single crystals were irradiated at 465 nm in the temperature range of -100 °C to 25°C and the dimerization between the anthracene units was followed and quantified by XRD. This was possible because all these reactions proceeded in a single-crystal-to-single-crystal (SCSC) fashion. While all findings comply with the critical distance between reactive units postulated by Schmidt, significant reactivity differences were observed among reactive anthracene pairs. Qualitative explanations for the different reactivities will be provided using as geometrical descriptors the Schmidt distance and the offsets between the anthracene pairs, and also considering the degree of structural rearrangement required during reaction. All reactive packings afforded dimers and in a particular case, a two-step polymerization afforded the novel ladder polymer poly1Danthraphane, the first case of linear macromolecule synthesized in single crystals using anthracene dimerization as growth reaction. An investigation into the temperature range required to induce retro-cycloaddition reactions in the dimers formed upon photoirradiation is also presented.

* Marco Servalli ETH Zurich Institut für Polymere Vladimir-Prelog-Weg 1-5/10 8093 Zürich Switzerland

[email protected]

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Photochemical Single-Crystal-to-Single-Crystal (SCSC) Reactions of Anthraphane to Dianthraphane and Poly1Danthraphane Marco Servalli†*, Michael Solar‡, Nils Trapp‡, Michael Wörle‡, A. Dieter Schlüter† †

Laboratory of Polymer Chemistry, Department of Materials, ETH Zurich, Vladimir-Prelog-

Weg 5, 8093 Zurich, Switzerland.

*E-mail: [email protected] ‡

Laboratory of Inorganic Chemistry, Small Molecule Crystallography Center, Department of

Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland.

ABSTRACT:

This work describes the search for reactive crystal packings of anthraphane, a D3h-symmetric cyclophane with three vertically arranged anthracene units. Packings in which anthracene units of neighboring anthraphanes assume a face-to-face stacked geometry were selected from the available library of crystal structures of this compound. The corresponding single crystals were irradiated at 465 nm in the temperature range of -100 °C to 25°C and the dimerization between the anthracene units was quantified by XRD. This was possible because all these reactions proceeded in a singlecrystal-to-single-crystal (SCSC) fashion. While all findings comply with the critical distance between reactive units postulated by Schmidt, significant reactivity differences were observed among anthracene pairs. Qualitative explanations for the different reactivities will be provided using as geometrical descriptors the Schmidt distance and the offsets between the anthracene pairs, and also considering the degree of structural rearrangement required during reaction. All reactive packings afforded dimers and in a particular case, a two-step polymerization afforded the novel ladder polymer poly1Danthraphane, the first case of linear macromolecule synthesized in single crystals using anthracene dimerization as growth reaction. An investigation into the temperature range


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required to induce retro-cycloaddition reactions in the dimers formed upon photoirradiation is also presented.

Introduction Since the pioneering work by Schmidt and Cohen[1–3], topochemical reactions in the single crystal have been developed into a mature field of science[4]. They are known to be powerful tools for selectively obtaining target molecules which are normally not accessible in solution synthesis, especially in terms of regioselectivity[4,5]. Typical examples are the well-known dimerization of cinnamic acid and stilbene derivatives [1,2]. For reactions to occur in crystals, reactive packings are needed. In these, the reactive units of neighboring compounds must meet certain distance and orientation criteria, which ensure that there is sufficient orbital overlap to bring about the reaction upon external stimulation, typically by light irradiation or heat. Topochemistry has also been successfully applied to linear[6–9] and, more recently, to two-dimensional polymerizations[10–14].

We have been engaged for quite a while in exploring the reactive packings of anthracene-based compounds such as the anthraphane 1[15] (Figure 1). This compound is potentially interesting for topochemistry because it combines the option to perform photoinduced [4+4]-cycloadditions between anthracene units[16–18] with a trifunctionality, opening up the fascinating opportunity to synthesize dimers, linear polymers and 2D polymers depending upon which reactive packing can be realized. Supposed the anthracene blades of neighboring anthraphanes can be brought into the respective face-to-face (ftf)-stacked geometry required for topochemistry, these products can be accessed in virtually quantitative conversion and high regioselectivity.



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Figure 1. Chemical structure of anthraphane 1 and its top view, as used when discussing its different photoreactive crystal packings. Anthracene units which are in a parallel relationship with their corresponding neighbor’s anthracenes units are represented in blue and pink color: blue anthracene pairs are generally close to each other and are expected to dimerize topochemically via [4+4]-cycloaddition, whereas pink pairs despite the parallel alignment, are generally more offset and might not react. The remaining gray anthracene units are never engaged in parallel relationships and are therefore not expected to react topochemically.

Motivated by this opportunity the packing behavior of compound 1 was recently investigated using 30 different solvents for crystallization which led to the discovery of seven different packing motifs[19], in which anthraphane is always arranged in layers. In the packings the anthracene units can be either engaged in exclusive edge-to-face (etf) relationships (etf packing 1 and 2), in both etf and ftf relationships (etf/ftf packings 1, 2, 3 and 4) or in no particular relationship. In the three etf/ftf packings 1, 2 and 4, which are the only ones that could possibly be used for topochemistry, the anthracene units of compound

1 assume ftf-stacks with different offsets, rendering it difficult to predict whether the dimerization reaction can be brought about (Figure 2). While no packing that could be used for a two-dimensional polymerization was found (exclusive ftf packing), the aforementioned three packings ought to provide access to dimers and potentially even to a novel hydrocarbon linear ladder polymer[20,21]. Packing 1 can be obtained from 15 different crystallization solvents, whereas packings 2 and 4 are obtained exclusively from isophorone and ethyl 2-oxocyclohexanecarboxylate, respectively. In the present study it was discovered in addition to the previous findings18, that isophorone as solvent results in polymorphism 4 ACS Paragon Plus Environment

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(Figure S2 and S3), yielding prismatic plates corresponding to packing 1 (major form) and needles corresponding to packing 2 (minor form).

Figure 2. The three potentially photoreactive packings of 1 discovered in the previous crystallization study with the corresponding crystallization solvents. Packing numbering is chosen in accordance with Ref. 18. Anthracene units are displayed in red color.

Here we describe the three potentially reactive packings 1, 2 and 4 in terms of distances between the C9 and C10 positions of neighboring anthracenes and off-sets between the ring centroids of the ftf-stacked anthracene. The reactivity of these stacked anthracene pairs will be investigated in terms of irradiation time and temperature. On the basis of these reactivity data, it will be explored inasmuch the above descriptors can be considered independently in qualitatively describing reaction rate. Because these transformations proceed in a single-crystal-to-single-crystal (SCSC) fashion, structure analysis by SC-XRD is particularly convenient. Of particular interest was to investigate whether these factors can have a direct bearing on reaction rate and whether less suited packings with large offsets in the ftf-stacks can nevertheless be made to react by, for instance, increasing the temperature during irradiation. For this purpose, packing 1 lends itself as interesting case study because it contains anthracene pairs with two different off-sets. We will first describe whether selective dimerization of better aligned anthracene pairs can be induced 5 ACS Paragon Plus Environment


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by an optimum choice between irradiation time (minutes to days) and temperature (-100 25°C). As next, the results of a prolonged irradiation time and an increased temperature (70 120°C) will be presented to see whether the anthracene pair with larger off-set can be forced into dimerization, a process that would furnish a linear ladder polymer. Finally, the thermal reversibility of the dimerization reaction of 1 will be investigated by solid-state NMR spectroscopy.

Results and discussion Crystallization and structural analysis of the photoreactive packings of anthraphane

Anthraphane 1 was synthesized according to the literature procedure and single crystals were obtained by dissolving the molecule at high temperatures in high boiling point solvents, followed by a controlled cooling to room temperature over 24 h[15]. For a pair of anthracenes to photodimerize by [4+4] cycloaddition, their frontier orbitals have to be properly aligned and in close proximity. This scenario is typical for anthracenes which are ftf stacked. The distance between their respective C9 and C10 positions (d9,10’ and d10,9’), where their electron density in the frontier orbitals is the highest and consequently bond formation occurs, is a common descriptor when analyzing the potential photoreactivity in crystal structures. As a rule of thumb for dimerization to occur, the distances should be below 4.2 Å, the so-called critical Schmidt distance[3,22]. In order to see whether this distance is the only useful criterion, we also analyze the structures in terms of off-set, by considering the shift of the centroids of the anthracenes engaged in pairs. These parameters will be taken into account for describing the photoreactive packings of 1 and to try to rationalize the observed 6 ACS Paragon Plus Environment

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reactivities. From the library of crystal structures of anthraphane, three packing motifs in which some anthracene units are in ftf relationship were considered: the etf/ftf packings 1, 2 and 4 (Figure 2). In the next paragraphs, these packings will be analyzed to assess their potential photoreactivity on the basis of the distances d9,10’ and d10,9’ and offset (shift) between the anthracenes[23]. Due to the coplanarity of the anthracene pairs and virtually perfect alignment of their molecular axes (angle between the normals of the two anthracene planes is virtually 0), the two d values result identical and therefore only one value per pair is given. In all the three packings considered here, two of the three anthracene units of anthraphane 1 are coplanar with that of the corresponding nearest neighbor, while the remaining unit does not assume a geometry potentially useful for dimerization (edge-toface). Anthracenes in a coplanar relationship are represented in blue and pink color: the blue pairs are characterized by short d9,10’ distances whereas the pink ones by longer values, being generally more offset. The same color codes will be retained in the packings to facilitate the discussion.

Etf/ftf Packing 1

In this packing there are two types of anthracene pairs engaged in ftf-stacking: the blue and pink pairs, indicated in Figure 3a for the specific case of the 1,3-dimethoxybenzene solvate. The distance between the 9 and 10 positions, d9’,10, of the anthracenes is approximately 3.697 Å, which is well in range for a possible dimerization. In addition, the pair is only slightly offset (1.465 Å shift) rendering a photodimerization likely without the need of important adjustments in the crystal structure. In all the other 14 solvates, d9’,10 ranges from 3.638 – 4.000 Å. On the other hand, the pink anthracene units are more offset (3.097 Å shift), with

d9’,10 of approximately 4.597 Å. In all the other solvates, the values are similarly large ranging 7 ACS Paragon Plus Environment


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from 4.267 - 4.539 Å. Dimerization between this pair is therefore unlikely, unless a serious displacement takes place in the crystal structure, for instance, in the form of increased thermal movement or due to the proximity to crystal defects[24,25]. Such a displacement is however not likely to be tolerated by the crystal, which can result in its disintegration. This packing is therefore expected to afford dimers. A unique case in which the pink anthracene pairs are less offset is the isophorone solvate, which can potentially result in linear polymers. This solvate will be discussed later in the SCSC photopolymerization section.

Etf/ftf packing 2

In this uncommon polymorph of isophorone (Figure 3b), similarly to the previous case, the blue anthracene pairs are only slightly offset, with d9’,10 of 3.827 Å and a shift of 1.722 Å, rendering photodimerization likely. On the other hand, the pink anthracene pairs are completely offset, with d9’,10 of 6.276 Å and shift of 5.915 Å. This packing is thus expected to afford dimers.

Etf/ftf packing 4

In this peculiar packing obtained exclusively by crystallization from ethyl 2oxocyclohexanecarboxylate (Figure 3c), the ftf-stacked blue pairs have a slightly more pronounced offset with d9’,10 = 3.948 Å and a shift of 1.904 Å, making these pair potentially photoreactive. The remaining coplanar pink anthracene units are too displaced to one another to potentially photoreact, having d9’,10 = 6.816 Å. The structural parameters for the coplanar anthracene pairs of the etf/ftf packings 1, 2 and 4 are summarised in Table 1, providing the d9,10’ values and the overall offsets (shift) decomposed in their x and y components (Δx and Δy) from an ideally coplanar superimposed 8 ACS Paragon Plus Environment

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anthracene pair, with the p-orbitals perfectly aligned; the distance between the anthracene planes in a pair is also provided.

Figure 3. The etf/ftf packings 1, 2 and 4 with detailed views of the ftf-stacked blue and pink anthracene pairs and their SCSC reactions. (a) 1,3-dimethoxybenzene solvate. d9’,10 = 3.697 Å (blue pair) and d9’,10 = 4.597 Å (pink pair). The SCSC dimerization involves a considerable mismatch between the unit cell parameters of monomer and dimer crystals. This can results 9 ACS Paragon Plus Environment


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in cracks in the crystals upon photoirradiation. (b) Isophorone solvate. d9’,10 = 3.827 Å (blue pair) and d9’,10 = 6.276 Å (pink pair). In this SCSC dimerization the mismatch between the unit cell parameters of the monomer crystal and the dimer crystal is minimal and the single crystals withstand the reaction perfectly without cracking. (c) Ethyl 2oxocyclohexanecarboxylate solvate. d9’,10 = 3.948 Å (blue pair) and d9’,10 = 6.816 Å (pink pair). Single crystals withstand the SCSC reaction perfectly without cracking.

Table 1. Geometric parameters of the anthracene pairs for the three reactive etf/ftf packings 1,2 and 4 shown in Figure 3 and data on dimerization conversions when irradiating crystals at 465 nm at different temperatures. d9,10’ and d10,9’ are the distances between the respective 9 and 10 positions of the anthracene pairs. Due to the coplanarity of the pairs and virtually perfect alignment of their molecular axes, the values are identical and only one value per pair is therefore given. The overall offset (shift) describes the shift of the anthracene centroids from an ideally coplanar superimposed anthracene pair and is derived from the x and y components (Δx and Δy), also provided. The distance between the anthracene planes in a pair is also provided.

d9,10’ [Å] Shift [Å] Δx [Å] Δy [Å] Pln dist [Å]

3.697 1.465 1.10 0.97 3.352

4.597 3.097 2.86 1.19 3.323

3.827 1.722 1.17 1.26 3.419

6.276 5.915 5.78 1.25 2.081

3.948 1.904 1.32 1.37 3.473

2min (25°C) 2min (-0°C) 30min (25°C) 30min(-10°C) 4 h (-50°C) 4 h (-100°C) 8 h (80°C) 8 h (120°C)

Dimerization conversion with irradiation at 465 nma 100% 0% 100% 0% 100% 0% 100% 0% 100% 100% 0% 100% 0% 100% c 50% 0% 0% 0% b 100% 0% b 100% Crystal breaks -

6.816 5.477 5.44 0.63 4.050 0% 0% -

a

Conversion according to SC-XRD. bBlue pair dimerized at -10°C, followed by heating of the crystal to 80°C and 120°C in order to attempt dimerization of the pink pair. cExpressed as disorder in the crystal structure.


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SCSC Photodimerizations of Anthraphane Taking into account the structural considerations exposed previously, the etf/ftf packings 1, 2 and 4 were tested for their photoreactivity. Having each only one pair of anthracene units engaged in proper ftf π···π stacking (blue pair), photo-irradiation was expected to trigger the dimerization of these pairs, converting the monomer crystals into dimer crystals. Care was taken to ensure single-crystal-to-single-crystal (SCSC) reactions so that the structure of the products could be unambiguously characterized by SC-XRD. This involved irradiation into the tail-end of their UV/Vis absorption spectrum, a method pioneered by Enkelmann and coworkers in 1994[26]: irradiating a crystal with the lowest possible energy wavelength ensures a more homogeneous reaction throughout the entirety of the crystal. The solid-state UV/Vis absorption spectrum of anthraphane is displayed in Figure S4. The irradiation wavelength was chosen to be 465 nm corresponding to visible blue light in order to be sufficiently far out in the absorption tail. An in-house built cylindrical photoreactor equipped with 16 high power LEDs was used as irradiation source and a cooler helped to keep the temperature at the desired value during the photoreaction. A typical experimental setup is depicted in Figure S5. Single crystals were selected and mounted on the pin of a goniometer head and their unit cell and diffraction quality was checked by collecting enough frames on the diffractometer. The pin was subsequently detached from the goniometer head and put under the cooler in the middle of the photoreactor where irradiation was performed at constant temperature for the desired amount of time. After irradiation, the crystal was again checked by SC-XRD and if diffracting properly, a complete dataset was collected for structure determination. For 11 ACS Paragon Plus Environment


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details on the calculation of conversion ratio of SCSC reactions, please refer to the SI. Crystallographic data and refinement details for the photodimerizations are displayed in Table 2. Table 2. Crystallographic data and refinement details for the photodimerization reactions of the 1,3dimethoxybenzene, isophorone minor polymorph and ethyl-2-oxocyclohexanecarboxylate (EOCHC) co-crystals.

CCDC No.

1,3-Dimethoxybenzene 50% dianthraphane 1568161

1,3-Dimethoxybenzene 100% dianthraphane 1568168

Isophorone minor 100% dianthraphane 1505890

EOCHC 100% dianthraphane 1568202

Empirical formula

C132H60

C74H40O2

C177H130O5

C159H99O9

Formula weight

1645.80

961.06

2336.80

2153.38

Temperature/K

100.0(2)

100.0(2)

100.0(2)

200.0(2)

Crystal system

triclinic

triclinic

triclinic

monoclinic

Space group

P-1

P-1

P-1

P21/n

a/Å

13.2846(6)

14.6127(3)

13.4200(14)

18.6591(4)

b/Å

14.5856(7)

17.9127(3)

14.3444(17)

15.7411(3)

Co-crystal

c/Å

15.4094(7)

21.1999(4)

19.785(2)

19.8904(4)

α/°

91.592(3)

73.1650(10)

103.867(4)

90

β/°

103.646(3)

75.9990(10)

91.639(4)

90.758(2)

115.546(3)

79.5230(10)

117.634(3)

90

2589.1(2)

5116.07(17)

3232.2(6)

5841.6(2)

Z

1

4

1

2

ρcalcg/cm3

1.056

1.248

1.201

1.224

μ/mm-1

0.460

0.570

0.071

0.585

F(000)

852.0

2000.0

1232.0

2250.0

Radiation/Å

1.54178

γ/° Volume/Å

3

1.54178

0.71073

1.54178

2Θ range for data collection/° 5.968 to 133.424

4.446 to 133.71

5.068 to 55.186

6.452 to 133.372

Reflections collected

32775

66864

34160

40060

Independent reflections

8927

17900

14888

10245

Data/restraints/parameters

8927/416/695

17900 /0/1373

14888/75/874

10245/227/813

1.074

1.066

1.007

1.172

Goodness-of-fit on F

2

R1, wR2 [ I >= 2σ (I) ]

0.0726, 0.1959

0.0583, 0.1682

0.0600, 0.1249

0.0911, 0.2739

R1, wR2 [all data]

0.0894, 0.2057

0.0675, 0.1768

0.1204, 0.1499

0.1206, 0.3121

Largest diff. peak/hole/e Å-3

0.20/-0.24

0.77/-0.56

0.59/-0.30

1.00/-0.55

Dimers from the etf/ftf packing 1 Among the various solvates of packing 1, single crystals grown from 1,3-dimethoxybenzene were chosen for the irradiation experiment due to their particular high quality diffraction, resistance to solvent loss and reasonable size. The monomer crystal was irradiated for times ranging from 2 min to 8 h and at temperatures ranging from -100 °C to 120 °C. Except for the 12 ACS Paragon Plus Environment

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lowest temperatures[27], dimer crystals were obtained with full conversion (according to SCXRD). The results are summarized in Figure 3a and Table 1, with the same anthracene color code used previously. The striking feature of this topochemical dimerization is the pronounced molecular movement and the transformation of the unit cell associated with it: while the a-axis varies only slightly from 14.577(2) Å to 14.6127(3) Å (0.2% increase), the baxis lengthens considerably going from 15.067(2) Å to 17.9127(3) Å (5.3% increase) and the c-axis dramatically shortens from 24.253(4) to 21.1999(4) (12.6% decrease). The same happens with the angles of the unit cell which undergo variations ranging approximately from 3° to 9°. These considerable variations in the unit cell are reflected by cracks appearing on the irradiated single crystals, visible by the naked eye. However birefringence and crystallinity are mostly retained and the crystals still diffract properly, usually even better than prior to irradiation. The unit cell volume goes from 5124 to 5140 Å3 (0.3% expansion). It was found that keeping the temperature during irradiation between -20°C and -10°C limits crack formation within the crystals, whereas irradiating near and above room temperature results in their disintegration. The change in unit cell parameters was used to investigate the rate of the reaction: crystals were irradiated for a certain amount of time and the unit cell was then determined. It was found that at -10°C the topochemical reaction is already completed after 2 min of irradiation. As a general remark, apart from tail-end irradiation, in order to favor SCSC reactions and prevent crystal disintegration, two other factors are important: crystal size and rate of reaction[28]. Smaller crystals are able to relax more efficiently and better dissipate the stress and strain resulting from the mismatch between lattice metrics, densities and intermolecular interactions exerted during the topochemical reaction. Likewise, rapid reactions could lead to fast accumulation of strain favoring crystal cracking. Since irradiation of smaller fragments 13 ACS Paragon Plus Environment


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of the single crystals still resulted in sporadic crack formation we hypothesized that the extremely rapid reaction may be the main problem. In this regard, using a weaker light source might result in a more well-behaved SCSC transformation. Another interesting feature of this topochemical reaction is that the solvent 1,3-dimethoxybenzene goes from a disordered state in the monomer crystal to a fully ordered one in the dimer crystal. Initially it was not clear whether this reorientation of the solvent molecules was a consequence of the dimerization step (perhaps a readjustment of the dipole moment of the crystal) or whether it was needed for the dimerization to occur. In a preliminary experiment trying to find the optimal irradiation conditions, single crystals were irradiated at -100°C for several hours and shown not to undergo any dimerization at all; by irradiating the same single crystals at -50°C for 4 h, a 50% conversion to the dimers was found (expressed as disorder). Interestingly, the solvent was found to be still disordered. This not only suggests the reorientation to happen at the final stages of the reaction but also that reorientation is not a prerequisite for dimerization to occur. A natural readjustment of the solvent orientation after crystallization due to relaxation can be excluded as the monomer crystals were measured several weeks after being grown. What is puzzling about this reaction is the amount of molecular movement involved with it, which seems unnecessary since the reacting anthracene pairs are stacking nicely and within standard distances for dimerization. While we try to offer an explanation further below, these studies show the temperature dependence of topochemical reactions, in which, kinetically speaking, temperature is not associated with overcoming the activation energy as in conventional solution reactions but to provide the thermal bath for sufficient motion of the molecules around their lattice site[29] (see SI page S30). In this case, temperatures such as -100°C are not enough to allow molecular movement for the 14 ACS Paragon Plus Environment

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topochemical reaction to occur under the selected irradiation conditions; at -50°C the reaction proceeds at a slow rate whereas at -20°C and higher temperatures the reaction becomes very fast. In future works, more solvates of the etf/ftf packing 1 should be dimerized and compared with the 1,3-dimethoxybenzene for a better understanding of this SCSC transformation; in fact, in the next sections, in the etf/ftf packing 2 and 4 of the isophorone and ethyl 2-oxocyclohexanecarboxylate solvates, and in the isophorone solvate of the etf/ftf packing 1, it will be shown that the molecular movement associated with the topochemical reaction of the blue pairs is minimal. It should be noted that attempts to dimerize in a SCSC reaction the pink anthracene pairs in order to form 1D polymer chains were not successful. Prolonged irradiations of the dimer crystals even at 80°C did not show any further dimerization reaction, and exposure of the crystals to temperatures above 100°C (with and without irradiation) resulted in complete loss of crystallinity.

Dimers from the etf/ftf packing 2 The monomer crystals grown from isophorone were irradiated for 30 min at -10°C and at 25°C and in both cases the dimer crystals were obtained with full conversion (according to SC-XRD). Compared to the previous case, the SCSC reaction proceeds more smoothly without visible cracks in the crystals (Figure 3b), whose quality upon dimerization even increases (reduced mosaicity, better diffraction). The molecular movement associated with this reaction is minimal, which reflects a small mismatch between the unit cell parameters of monomer and dimer crystals: the space group remains unchanged and the a-axis increases from 13.2429(9) Å to 13.4200(14) Å (1.3%), the b-axis increases from 14.1163(9) Å to 15 ACS Paragon Plus Environment


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Page 16 of 46

14.3444(17) Å (1.6%) and the c-axis decreases from 19.8453(14) Å to 19.785(2) (0.3%). The unit cell volume goes from 3210 to 3232 Å3 (0.7% expansion). The disordered solvent molecule next to the dimerizing pair remains disordered in the dimer crystal. It can be assumed that due to the low degree of molecular re-orientation involved, the temperature dependence on the rate of the reaction is less pronounced compared to the previous case.

Dimers from the etf/ftf packing 4 The single crystals grown from ethyl 2-oxocyclohexanecarboxylate were irradiated for 30 min at -10°C and at 25°C resulting in dimer crystals with full conversion (according to SCXRD). The crystals diffract better after the reaction; the solvent molecules that had to be masked when solving the monomer crystal diffraction could now be fully modeled (Figure 3c). In line with the large crystal size and high reaction rate, cracks form sporadically. The fragments of the crystals still diffract and retain their birefringence. The space group does not change, the a-axis shortens from 18.8347(5) Å to 18.6591(4) Å (0.9%), the b-axis increases from 15.5816(4) Å to 15.7411(3) Å (1.0%) and the c-axis lengthens from 19.4211(6) Å to 19.8904 (4) Å (2.3%). The cell volume increases from 5699 to 5842 Å3 (2.4% expansion). The overall molecular movement associated with this SCSC transformation is again minimal. It has to be stated that prior to irradiation, the single crystals already displayed approximately 33% of dimerization (expressed as disorder), probably due to accidental exposure to light.

SCSC Photopolymerization of Anthraphane


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Crystals obtained from isophorone can give rise to two polymorphs, but predominantly result in the etf/ftf packing 1. Compared to the other solvates of this packing however, the pink anthracene pair is much less offset, with a shift from a perfectly superimposed anthracene pair of 2.005 Å and d9,10’ = 3.969 Å (compare: 1,3-dimethoxybenzene solvate with d9,10’ = 4.597 Å and 3.097 Å of shift), making it potentially photoreactive. This raised our

interest to use this particular case for a more in-depth study of the different reactivities of blue and pink anthracene pairs. Dimerization of both the blue and pink pair in this packing would result in a linear polymerization, forming a novel hydrocarbon linear ladder polymer. Table 3 compiles the different irradiation conditions applied. Similarly to previous cases, the blue anthracene pairs cleanly dimerize in a SCSC fashion at temperatures ranging from -10°C to 25°C, with full conversions achieved within minutes. Prolonged irradiations at -100°C did not result in any conversion, as already observed for the dimerization of the 1,3dimethoxybenzene solvate. After this expected initial outcome, we analysed how the dimerization of the blue pairs influenced the unit cell and the geometry of the unreacted pink pairs in the crystal. While the unit cell reorients, notable variations only occur for the angles, while the axes and its volume remain virtually unchanged (Figure 4): the a-axis increases by 1.5%, the b-axis increases by 0.3%, the c-axis decreases by 0.06% and the unit cell volume expands by 0.15%.



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Figure 4. Two step SCSC linear polymerization of the isophorone solvate in the etf/ftf packing 1. In the first step, dimers are formed. The mismatch between the unit cell parameters of the monomer crystal and the dimer crystal is minimal and the single crystals withstand the reaction perfectly without cracking even at room temperature. In the second step, when polymerization takes place, however, the crystals crack, but still without preventing XRD analysis.

With these minimal variations in the lattice metrics, it does not astound that the reaction proceeds without cracking the crystals. Regarding the geometry of the pink pair the small readjustments in the crystal structure caused by the dimerization of the blue pair actually increase the overlap of the pink pairs: d9,10’ is reduced to 3.895 Å and the shift to 1.853 Å. These values are similar to the previously reported blue pair of the ethyl 2oxocyclohexanecarboxylate solvate of Packing 4, which dimerizes fast. We thus expected to be able to bring about dimerization also here but were concerned about the anticipated variation of the lattice parameters associated with it. Moreover it was noticed, that the molecules in Packing 1 are more densely packed than in Packing 4 which could result in an aggravated re-adjustment and, thus, a lower reaction rate.

The differences in the dimerizations of the blue and the pink pair are nicely reflected by the temperature-dependant irradiation experiments in Table 3. While for short irradiation times (30 min), as already stated, the blue pairs are fully converted, the pink pairs do not 18 ACS Paragon Plus Environment

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show any reaction both at -10 °C and at 25°C. For it to dimerize at -10°C to a conversion of 100%, it takes 24 h. After dimerization, the crystal exhibits fine cracks visible to the naked eye. After 4, 8, and 21 h the conversions amount to only 10%, 20% and 50% respectively, with no cracks in the crystals. These data not only show a marked reactivity difference between the two pairs but also an astounding acceleration effect. While the first half of the conversion requires 21 h, the second half is finished within only 3 h more. This point will be further discussed below. If the irradiation is performed at higher temperatures such as 0°C, completeness of the dimerization of the pink pair is achieved already within 8 h with crack formation in the crystals. After 4 h the conversion amounts to only 20%, again pointing towards an acceleration at higher conversion. If the irradiations are carried out at higher temperatures (25°C, 75°C), the reactions are easily completed within 4 h furnishing cracked polymer crystals. The marked difference in reactivity between the blue and pink anthracene pairs effectively result in a controlled two-step polymerization, yielding poly1Danthraphane single crystals. Many factors can influence the rate of topochemical reactions. They include size, shape and quality of the crystal, the aspect of mosaicity which is associated with grain boundaries and the presence of micro-cracks or the formation thereof during conversion built-up. We therefore refrain from data interpretation, and will only refer to the observed acceleration. We consider this acceleration likely to be the reflection of favourable changes in the geometry of the pink pair with increasing conversion, possibly assisted by the formation of microcracks leading to stress relaxation. In this context the mismatch of the lattice parameters between dimer and polymer crystals is noteworthy: the a-axis shortens from 13.3332(2) Å to 9.7783(2) Å (26.7%), the b-axis shortens from 14.4791(3) Å to 13.3027(2) Å (8.1%), the c-axis increases from 16.4010(3) Å to 22.1100(5) Å (39.8%). Such 19 ACS Paragon Plus Environment


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Page 20 of 46

strong variations not surprisingly lead to considerable cracking of the crystals, which is accentuated at higher temperature. At lower temperatures such as 0°C and -10°C crack formation is less pronounced, indicating a better strain relaxation at lower reaction rates. Finally we note that the fragments of the cracked polymer crystals still retain their single crystallinity and that the monomer single crystals do not suffer from solvent loss. They can be stored dry for days. Exposure of the crystals to environmental light for 4 d (normal mercury-vapor lamps, no sunlight) causes no reaction. Crystallographic data for the photopolymerization of the isophorone solvate is displayed in Table 4. The polymer chains are running along the thickness of the crystals (Figure S23), which are usually between 10-50 µm thick, and for a hypothetical ideal single crystal, the chains could be composed by an impressive number of repeat units of up to 36000. The here described

photopolymerization

of

anthraphane

to

the

novel

ladder

polymer

poly1Danthraphane is the second ever synthesis of such complex double-stranded macromolecules based on anthracene growth chemistry, the first one being performed in solution[30], but to the best of our knowledge it is the first attempt in single crystals, while the use of photoreactive 1,8-diazaanthracenes is already present in the literature[8]. As a final note, attempts to isolate polymer chains from the crystals by solvent-induced exfoliation are currently being investigated. Additional characterization of the polymer and various dimer crystals such as IR and fluorescence spectra can be found in the SI. Table 3 Data of the SCSC polymerization and its relation with the structural parameters of the anthracene pairs in the isophorone co-crystal of the etf/ftf packing 1. d9,10 and d9’,10’ are the distances between the respective reactive 9 and 10 positions of the anthracene pairs; due to the coplanarity of the pairs (α = 0) and virtually perfect alignment of their molecular axes, the values are the same. The overall offset (shift) between the reactive positions is decomposed in its x and y components (Δx and Δy).


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d9,10’ [Å] Shift [Å]

3.695 1.354

3.969 2.005

3.895 1.853

Δx [Å]

1.20

1.50

1.42

Δy [Å]

0.62

1.33

1.19

Pln dist [Å]

3.448

3.435

3.441

Dimerization conversion with irradiation at 465 nm

a

30 min (25°C)

100%

0%

0%

30 min (-10°C)

100%

0%

0%

4 h (-100°C)

0%

0%

0%

4 h (-10°C)

100%

-

10%c

8 h (-10°C)

100%

-

25%c

21 h (-10°C)

100%

-

50%

24 h (-10°C)

100%

-

100% Crystal cracks

4 h (0°C)

100%

-

20%c

8 h (0°C)

100%

-

100% Crystal cracks

4 h (25°C)

100%

-

100% Crystal cracks

-

100% Crystal cracks

4 h (70°C)

b

100%

a

Conversion according to SC-XRD. bBlue pair first dimerized at 25°C for 15 min. cExpressed as disorder in the crystal structure. Table 4. Crystallographic data and refinement details for the photopolymerization reactions of the isophorone major polymorph co-crystal. Isophorone Major Polymorph Co-crystal CCDC No.

Non-irradiated crystal 1568203

Irradiation at -100°C, 4 h 0% conversion 1568206

Irradiation at -10°C, 4 h 100% dimer, 10% polymer 1568211

Irradiation at -10°C, 8 h 100% dimer, 25% polymer 1568214

Empirical formula

C74H30O

C66H30

C66H30

C66H30

Formula weight

934.98

822.90

822.90

822.90

Temperature/K

100.0(1)

100.0(1)

100.0(1)

100.0(1)

Crystal system

triclinic

triclinic

triclinic

triclinic

Space group

P-1

P-1

P-1

P-1

a/Å

13.1359(2)

13.1649(4)

13.3332(2)

13.2984(2)

b/Å

14.4358(2)

14.4453(4)

14.4791(3)

14.4068(2)

c/Å

16.4106(2)

16.3923(5)

16.4010(3)

16.6041(2)

α/°

91.3258(12)

91.037(2)

74.9840(10)

74.7700(10)

β/°

105.8864(14)

106.046(3)

72.1480(10)

71.5220(10)

γ/°

116.2111(17)

116.325(3)

62.6950(10)

62.5510(10)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Volume/Å

3

Z 3

ρcalcg/cm μ/mm

-1

Page 22 of 46

2647.59(8)

2649.14(15)

2651.60(9)

2651.65(7)

2

2

2

2

1.173

1.032

1.031

1.031

0.527

0.449

0.449

0.449

F(000)

964.0

852.0

852.0

852.0

Radiation/Å

1.54184

1.54184

1.54178

1.54184

2Θ range for data collection/°

6.922 to 158.584

18.944 to 133.19

6.938 to 133.386

6.982 to 158.406


146660

28389

34858

101535


11341

8992

9144

11307


11341/120/739

8992/0/595

9144/914/710

11307/521/740 1.074

2

1.074

1.072

1.056

R1, wR2 [ I >= 2σ (I) ]

0.0816, 0.2599

0.0607, 0.1620

0.0460, 0.1197

0.0645, 0.1755

R1, wR2 [all data]

0.0894, 0.2057

0.0876, 0.1778

0.0557, 0.1270

0.0823, 0.1875

0.20/-0.24

0.65/-0.28

0.48/-0.45

0.65/-0.28


Largest diff. peak/hole / e Å

-3

Isophorone Major Polymorph Co-crystal

Irradiation at -10°C, 21 h 100% dimer, 50% polymer

Irradiation at -10°C, 24 h 100% polymer

Irradiation at 0°C, 4 h 100% dimer, 20% polymer

Heating at 180°C, 48 h 100% polymer

CCDC No.

1568216

1568222

1568225

1568227

Empirical formula

C74.19H42.74O0.91

C66H30

C66H30

C132H60

Formula weight

948.66

822.90

822.90

1645.80

Temperature/K

100.00(11)

100.0(1)

100.0(1)

100.0(1)

Crystal system

triclinic

triclinic

triclinic

triclinic

Space group

P-1

P-1

P-1

P-1

a/Å

13.2730(7)

9.76780(10)

13.3104(2)

9.7873(3)

b/Å

14.3413(4)

13.3114(3)

14.4435(2)

13.3110(3)

c/Å

16.7795(7)

22.1098(5)

16.5166(2)

22.1112(5)

α/°

74.512(3)

80.225(2)

74.8710(10)

80.204(2)

β/°

70.976(4)

77.884(2)

71.8180(10)

77.871(2)

62.437(4)

69.777(2)

62.632(2)

69.785(2)

2651.0(2)

2622.63(10)

2652.97(8)

2627.86(12)

γ/° Volume/Å

3


Page 23 of 46

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Z

2

2

2

1

1.188

1.042

1.030

1.040

0.526

0.454

0.449

0.453

F(000)

990.0

852.0

852.0

852.0

Radiation/Å

1.54184

1.54184

1.54184

1.54184

3

ρcalcg/cm μ/mm

-1

2Θ range for data collection/° 18.856 to 158.052

7.118 to 149.006

6.96 to 158.334

7.118 to 140.138


50594

102134

100881

49346


10696

10689

11317

9776


10696/0/584

10689/0/595

11317/498/749

9776/0/595

1.056

1.055

1.075

1.048

0.0891, 0.2341

0.0393, 0.0981

0.0603, 0.1611

0.0473, 0.1117


2

R1, wR2 [ I >= 2σ (I) ] R1, wR2 [all data] Largest diff. peak/hole / e Å

-3

0.1151, 0.2543

0.0509, 0.1029

0.0702, 0.1680

0.0715, 0.1212

0.45/-0.45

0.65/-0.28

0.36/-0.35

0.19/-0.20

Structural considerations for topochemical dimerization of anthracene pairs As a general observation, this study demonstrates the validity of the Schmidt’s critical distance, which has to be within 4.2 Å for topochemical reactions to occur. All the anthracene pairs that were dimerized had d9’,10 < 4.0 Å and for an anthracene pair with d9’,10 = 4.597 Å, dimerization was not observed. We further wanted to test the validity of the Schmidt distance on the L-carvone solvate of packing 1, where d9’,10 of the pink anthracene pairs is 4.274 Å. The crystals were first irradiated at -10°C to induce dimerization of the blue pair, which caused a rearrangement of the molecules in the crystals, resulting in a reduced d9’,10 of the pink anthracene of 3.936 Å (Table 5). Interestingly enough, the dimerization step despite involving a similar reorientation in the unit cell as in the isophorone solvate (Figure S28), and to a lower extent, the 1,3-dimethoxybenzene solvates of the same packing, proceeds more slowly; in fact, by irradiating 1 h at -10°C, only 75% conversion is observed. Another interesting observation is that the pink pairs, now having a better overlap and being within the Schmidt distance, do not react at all even after irradiating 20 h at -10°C. Crystallographic data for the photodimerizations of the L-carvone solvate can be found in Table 6. Table 5 Data on the SCSC dimerizations in relation with the structural parameters of the anthracene pairs in the L-carvone co-crystal of the etf/ftf packing 1. d9,10 and d9’,10’ are the distances between the respective reactive 9 and 10 positions of the anthracene pairs; due to the coplanarity of the pairs (α = 23 ACS Paragon Plus Environment


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Page 24 of 46

0) and virtually perfect alignment of their molecular axes, the values are the same. The overall offset (shift) between the reactive positions is decomposed in its x and y components (Δx and Δy).

d9’,10 [Å] Shift [Å]

3.705 1.492

4.274 2.596

3.936 2.009

Δx [Å]

1.09

2.05

1.81

Δy [Å]

1.02

1.54

0.87

Pln dist [Å]

3.394

3.440

3.390

Dimerization conversion with irradiation at 465 nm

a

1 h (-10°C)

75%b

-

0%

20 h (-10°C)

100%

-

0%

a

Conversion according to SC-XRD. bExpressed as disorder in the crystal structure.

Table 6. Crystallographic data and refinement details for the photodimerization reactions of the Lcarvone co-crystal. L-Carvone Co-crystal CCDC No.

Irradiation at -10°C, 1 h 75% dimer 1568244

Irradiation at -10°C, 20 h 100% dimer 1568245

Empirical formula

C66H30

C66H30

Formula weight

822.90

822.90

Temperature/K

100.0(1)

100.0(1)

Crystal system

triclinic

triclinic

Space group

P-1

P-1

a/Å

13.3489(2)

12.48770(10)

b/Å

14.6261(3)

13.29500(10)

c/Å

15.4842(2)

18.2627(2)

α/°

90.6640(10)

69.3730(10)


Page 25 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


β/° γ/° Volume/Å

3

Z 3

ρcalcg/cm μ/mm

-1

F(000)

104.6760(10)

79.6420(10)

116.230(2)

68.1810(10)

2596.66(8)

2630.22(5)

2

2

1.052

1.039

0.459

0.453

852.0

852.0

Radiation/Å

1.54184

1.54184

2Θ range for data collection/°

6.806 to 140.146

7.54 to 158.058


52493

100327


9740

11192


9740/387/710

11192/0/595

1.072

1.081


2

R1, wR2 [ I >= 2σ (I) ]

0.0599, 0.1682

0.0522, 0.1326

R1, wR2 [all data]

0.0735, 0.1764

0.0580, 0.1355

Largest diff. peak/hole / e Å-3

0.22/-0.25

0.18/-0.22

Another general observation is that, at the same irradiation conditions, larger d9’,10 and offsets between anthracene pairs, will result in slower reaction rates. This is nicely shown in the SCSC photopolymerization reaction, in which a difference of only 0.2 Å between the d9’,10 values (0.5 Å difference in shift) of the pink and blue anthracene pairs, can dramatically alter their reactivity, allowing to clearly separate the two reactions and resulting in a two-step photopolymerization. Very similar d9’,10 and shift values, can however also result in quite different reactivities as seen for instance for the pink pair in the isophorone polymorph of Packing 1 (3.895 Å), which react slowly, and the blue pairs in the isophorone polymorph of Packing 2 (3.827 Å) and the ethyl-2oxocyclohexanecarboxylate of Packing 4 (3.948 Å), which react very fast. Another example are the reactivity differences between the three packing 1 solvates: the blue pairs all have very similar d9’,10 25 ACS Paragon Plus Environment


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Page 26 of 46

values, yet the L-carvone solvate reacts more slowly than the other two; this could be rationalized due to the slightly larger offset. It seems therefore that the shift values are as important geometrical descriptors as the more common distances d9’,10. In the dimer crystals of the isophorone packing 1 and L-carvone solvates, the pink pairs have d9’,10 of 3.895 Å and 3.936 Å respectively (only 0.041 Å difference), and shifts of 1.853 Å and 2.009 Å respectively. The pronounced 0.156 Å difference in shift values strongly affects the reactivity of these pairs making the ones with more overlap reactive, while the others do not react at all at the same irradiation conditions (despite being within the Schmidt distance range). In general, in the few cases and irradiation conditions analyzed in this study, shift values above 2.000 Å seem to result in unreactive anthracene pairs. Packing effects also come into play, namely, how easy it is for one packing to accommodate the molecular rearrangements associated with the topochemical reactions. Generally speaking, at the same irradiation conditions, large molecular rearrangements, should result in a lower reaction rate. This statement seems to hold, as all the blue pairs dimerize fast and do not result in important lattice mismatches between mother and product crystals (except for the 1,3-dimethoxybenzene solvate); conversely the pink pair in the polymerization reaction involves important molecular rearrangements and react more slowly. For dimerization to occur, large shift values obviously require more molecular movement in order to increase the overlap between the anthracene units. In this regard, it was also shown that dimerization can be completely inhibited by decreasing the irradiation temperatures to -100°C, effectively preventing the anthracene pairs to reduce their offset through thermally induced molecular movement. When discussing topochemical reactions in single crystals, not only pure geometrical factors such as d9’,10 and shift values should be considered, but also how the reactive units are effectively packed in the crystal structure. In this study, the anthracene units are tethered to a cyclophane framework through a somewhat flexible acetylenic unit and it is possible that for more rigid molecular systems or “free” anthracene molecules, quite different results would be observed.


Page 27 of 46

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Thermal stability of the anthraphane dimer and polymer Due to the thermal reversibility of the anthracene dimerization reaction, the stability of the anthraphane dimers was investigated using the dimers of the 1,3-dimethoxybenzene solvate as representative. Unfortunately the back-reaction could not be followed by SC-XRD, as the dimer crystals were found to mechanically disintegrate at temperatures above 100°C. Therefore, solid-state

13

C NMR spectroscopy was chosen as alternative analytical method.

For that purpose the dimer crystals were heated in a rotor at 180°C for different time intervals. After every heating cycle, a CP-MAS

13

C NMR spectrum was recorded and the

peaks assigned to the bridgehead carbons were compared with the peaks of the internal standard 1,3-dimethoxybenzene. During the experiments, the parameters on the spectrometer were kept constant, in order to better quantify the conversion of the backreaction. The results of the thermal stability of the anthraphane dimers are displayed in Figures 5 and 6.



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Page 28 of 46

Figure 5. Thermally induced back-reaction of the dimer crystals at 180°C followed by CPMAS13 C NMR spectroscopy. Conversion was estimated by the ratio of the integrals of the 1,3dimethoxybenzene signals with the anthracene dimer bridge signals.

Figure 6. Back-reaction of the dimer crystals at 180°C. After 72 h, the crystals still contain approximately 20% of dimers.


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The anthracene dimers started to progressively back-react at 180°C: after 4 h, 20% of backreaction was estimated and after 72 h, approximately 80% conversion to monomers was found. The thermal stability of the anthraphane dimer is in range with the thermal stability of an anthracene-based two-dimensional polymer reported in the literature[13]. The stability of the dimer crystals was also investigated in solution by 1H-NMR spectroscopy in order to find out whether the dimers could be recrystallized into another packing without suffering back-reaction. The dimers were found to be stable over days between 150°C-160°C, but unfortunately, no solvent could be found to completely dissolve the dimers crystals at that temperatures. (see Supplementary Information).

Interestingly, the polymer crystals displayed a higher thermal stability with respect to the dimer crystals, in fact, heating the crystals at 180°C for 48 h did not show any sign of back-reaction upon SC-XRD analysis; however, heating the polymer crystals at 200°C for 48 h resulted in loss of crystallinity, probably indicating the onset of the depolymerization reaction. This was confirmed by differential scanning calorimetry (DSC) analysis, in which the onset of the depolymerization reaction was characterized by an exotherm starting at around 200°C (see Figure S36).

Conclusions In conclusion, we demonstrated the photoreactivity of anthraphane in the single crystals by [4+4]-cycloaddition of its anthracene units. Four different reactive packings afforded dianthraphane dimers and another reactive packing resulted in a novel ladder polymer, poly1Danthraphane; such targets would not be possible to synthesize in solution (see Figure 29 ACS Paragon Plus Environment


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S47). All these reactions proceeded in a SCSC fashion and in some cases full conversion was achieved in minutes. The reactivity of the different ftf-stacked anthracene pairs in these packings was further investigated by time and temperature dependent irradiations experiments. In agreement with Schmidt’s critical distance of 4.2 Å, all the anthracene pairs that dimerized had distances d9’,10 < 4.0 Å. The focus was placed in the SCSC reactions of the major polymorph of the isophorone solvate, in which the pink and blue anthracene pairs have a difference of only 0.2 Å between their respective d9’,10 values and a considerable shift difference of 0.5 Å, making their reactivity dramatically different and allowing to clearly separate

the

two

dimerization

reactions,

effectively

resulting

in

a

two-step

photopolymerization, furnishing the novel linear ladder polymer poly1Danthraphane. At the same irradiation conditions and despite the many factors potentially influencing reaction rates in single crystals, larger d9’,10 and offset values resulted in lower reaction rates. In particular, the offset between the anthracene pairs resulted to be the most relevant geometrical descriptor for reactivity: with similar d9’,10 values, anthracene pairs with offset values below 2.000 Å resulted reactive while offset above that value did not result in dimerization. The offset can be correlated to the molecular movement in the crystal necessary to reach the proper overlap between the anthracene which then allows dimerization to take place. The larger the offset, the more molecular rearrangements will be needed. In this regard, we were able to demonstrate that low temperatures such as -100°C can inhibit dimerization even in good overlapping anthracene pairs. Geometrical parameters alone are not sufficient to describe reactivity, as packing effects come also into play, namely, how easy it is for one packing to accommodate the molecular rearrangements associated with the topochemical reactions.


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This study disclosed another interesting finding which concerns the temperature difference between the thermally induced retro-cycloadditions of dimers and linear polymer. While the former undergo this reaction back to monomer at 180 °C (48 h, 20% of dimer left) for the polymer under the same conditions nothing happens. This suggests that the individual retro-reaction steps in the case of the polymer are not completely independent from one another as they have to ‘work against’ a linear thread. This may have implications on similar reactions involving 2D polymers, where one then would not only have to work against a thread but a network.

Associated Content Supporting Information NMR data of compound 1. Details on crystallization procedure and experimental setup for SCSC reactions. Solid state UV/Vis absorption spectrum. Crystallographic data for every cocrystal, with optical micrographs and additional information on the structures. ATR-FTIR spectra and UV/Vis emission spectra of the crystals. NMR data on thermally-induced retrocycloadditions. NMR data of the dimers. DSC measurements.

Author Information Corresponding Author *E-mail: [email protected]

ORCID Marco Servalli: 0000-0001-8520-7868

Author Contributions 31 ACS Paragon Plus Environment


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the ETH Zürich, Switzerland (grant number ETH-26 10-2).

Notes The authors declare no competing financial interest.

Acknowledgement

We thank Dr. Kirill Feldman and Prof. Jan Vermant (Laboratory of Polymer Technology, ETH Zürich) for access to the optical microscopy equipment; many thanks go to Dr. Thomas Schweizer (Institute of Polymer Chemistry, ETH Zürich) for providing the PID-controller and crystallization apparatus, for building the LED-photoreactor and for the help with the DSC measurements. The help of Dr. René Verel (Laboratory of Inorganic Chemistry, ETH Zürich) for the solid-state NMR measurements is greatly acknowledged.

References

[1]

B. M. D. Cohen, G. M. J. Schmidt, J. Chem. Soc. 1964, 1996–2000.

[2]

M. D. Cohen, G. M. J. Schmidt, F. I. Sonntag, J. Chem. Soc. 1964, 2000–2013.

[3]

G. M. J. Schmidt, Solid State Photochemistry, Verlag Chemie, Weinheim, 1976.

[4]

K. Biradha, R. Santra, Chem. Soc. Rev. 2013, 42, 950–967.

[5]

V. Ramamurthy, K. Venkatesan, Chem. Rev. 1987, 87, 433–481.

[6]

M. Hasegawa, Y. Suzuki, J. Polym. Sci. Part B Polym. Lett. 1967, 5, 813–815. 32 ACS Paragon Plus Environment

Page 33 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[7]

G. Wegner, Zeitschrift für Naturforsch. 1969, 24 B, 824–832.

[8]

M. Li, A. D. Schlüter, J. Sakamoto, J. Am. Chem. Soc. 2012, 134, 11721–11725.

[9]

L. W. Dilling, Chem. Rev. 1983, 83, 1–47.

[10]

P. Kissel, R. Erni, W. B. Schweizer, M. D. Rossell, B. T. King, T. Bauer, S. Götzinger, a. D. Schlüter, J. Sakamoto, Nat. Chem. 2012, 4, 287–291.

[11]

R. Bhola, P. Payamyar, D. J. Murray, B. Kumar, A. J. Teator, M. U. Schmidt, S. M. Hammer, A. Saha, J. Sakamoto, A. D. Schlüter, et al., J. Am. Chem. Soc. 2013, 135, 14134–14141.

[12]

P. Kissel, D. J. Murray, W. J. Wulftange, V. J. Catalano, B. T. King, Nat. Chem. 2014, 6, 774–778.

[13]

M. J. Kory, M. Wörle, T. Weber, P. Payamyar, S. W. van de Poll, J. Dshemuchadse, N. Trapp, A. D. Schlüter, Nat. Chem. 2014, 6, 779–784.

[14]

R. Z. Lange, G. Hofer, T. Weber, A. D. Schlüter, J. Am. Chem. Soc. 2017, 139, 2053– 2059.

[15]

M. Servalli, N. Trapp, M. Wörle, F. G. Klärner, J. Org. Chem. 2016, 81, 2572–2580.

[16]

H. Bouas-Laurent, J.-P. Desvergne, A. Castellan, R. Lapouyade, Chem. Soc. Rev. 2000, 29, 43–55.

[17]

H. Bouas-Laurent, J.-P. Desvergne, A. Castellan, R. Lapouyade, Chem. Soc. Rev. 2001, 30, 248–263.

[18]

I. Zouev, D. Cao, T. V. Sreevidya, M. Telzhensky, M. Botoshansky, M. Kaftory, CrystEngComm 2011, 13, 4376–4381.

[19]

M. Servalli, N. Trapp, M. Solar, a. D. Schlüter, Cryst. Growth Des. 2017, acs.cgd.7b00367.

[20]

A. D. Schlüter, Adv. Mater. 1991, 3, 282–291.

[21]

A. D. Schlüter, M. Löffler, V. Enkelmann, Nature 1994, 368, 831–834.

[22]

V. Ramamurthy, Tetrahedron 1986, 42, 5753–5839.

[23]

H. Ihmels, D. Leusser, M. Pfeiffer, D. Stalke, Tetrahedron 2000, 56, 6867–6875.

[24]

M. D. Cohen, Angew. Chemie Int. Ed. English 1975, 14, 386–393.

[25]

J. O. Williams, J. M. Thomas, Mol. Cryst. Liq. Cryst. 1972, 16, 371–375.

[26]

V. Enkelmann, Mol. Cryst. Liq. Cryst. Sci. Technol. 1998, 313, 15–23. 33 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[27]

J. Ferguson, A. W.-H. Mau, Mol. Phys. 1974, 27, 377–387.

[28]

I. Halasz, Cryst. Growth Des. 2010, 10, 2817–2823.

[29]

Y. Ohashi, Reactivity in Molecular Crystals, John Wiley & Sons, 2008.

[30]

V. R. Sastri, R. Schulman, D. C. Roberts, Macromolecules 1982, 15, 939–947.

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For Table of Contents Use Only

SYNOPSIS The photoreactivity of “anthraphane” was investigated by selecting potentially photoreactive packings from the available library of crystal structures for this compound. Packings in which the anthracene units of anthraphane are face-to-face stacked were irradiated at 465 nm, yielding via [4+4]-cycloaddition dianthraphane and poly1Danthraphane in quantitative yields. The photoreactivity was investigated by varying irradiation time and temperature. The thermally-induced retro-cycloaddition was also demonstrated.


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etf /ftf packing 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Nitrobenzene 2-Cyanopyridine g-Butyrolactone o-Cresol Benzonitrile Isophorone L-Carvone Quinoline

1,2,4-Trichlorobenzene ε-Caprolactone 1,2,3-trichloropropane 1,2-Dimethoxybenzene 1,3-Dimethoxybenzene 1-Methylnaphtalene 2,4,6-Collidine

etf /ftf packing 2

Isophorone


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etf /ftf packing 4

Ethyl 2-oxocyclohexanecarboxylate

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465 nm, -10°C 120 s

Unit Cell Parameters a 14.577(2) Å b 15.067(2) Å c 24.253(4) Å

𝑷𝟏 α 76.592(4)° β 82.727(4)° γ 88.507(4)°

Unit Cell Parameters a 14.6127(3) Å b 17.9127(3) Å c 21.1999(4) Å ACS Paragon Plus Environment

𝑷𝟏 α 73.1650(10)° β 75.9990(10)° γ 79.5230(10)°


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465 nm, 25°C 30 min


𝑷𝟏 α 102.531(4)° β 94.499(4)° γ 115.374(4)°


𝑷𝟏 α 103.867(4)° β 91.639(4)° γ 117.634(4)°

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465 nm, 25°C 30 min


𝑷𝟐𝟏 /𝒏 α 90.000° β 90.418(2)° γ 90.000°


𝑷𝟐𝟏 /𝒏 α 90.000° β 90.758(2)° γ 90.000°


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Photochemical Single-Crystal-to-Single-Crystal (SCSC) Reactions of

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