Size Matters: [2 + 2] Photoreactivity In Macro- and Microcrystalline

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Size Matters: [2+2] Photoreactivity In Macro- and Micro-crystalline Salts of 4-Amino-Cinnamic Acid Simone D'Agostino, Elisa Boanini, Dario Braga, and Fabrizia Grepioni Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00109 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Crystal Growth & Design

Size Matters: [2+2] Photoreactivity In Macro- and Micro-crystalline Salts of 4-Amino-Cinnamic Acid Simone d'Agostino,†,* Elisa Boanini, † Dario Braga,† and Fabrizia Grepioni†,* †

Dipartimento di Chimica G. Ciamician, Università di Bologna, Via F. Selmi 2, 40126 Bologna,

Italy.

ABSTRACT

Solid-state photoreactivity of the 4-amino-cinnamic acid (1) and molecular salts [1H]ClO4 and [1H]HSO4 has been investigated, and structural changes have been monitored by X-ray diffraction techniques and FTIR spectroscopy. While the parent compound 1 and its perchlorate salt [1H]ClO4 are photoinerts, irradiation of [1H]HSO4 single crystals results into partial single crystal to single crystal (SCSC) [2+2] photodimerization, with formation of the {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 solid solution. If crystalline powders of [1H]HSO4 are irradiated, however, the formation of a monomer/dimer solid solution is followed by “extrusion” and recrystallization of both the phodimerization product [12H2]HSO4 and unreacted [1H]HSO4 as non-isomorphous crystal phases. The outcome of the UV irradiation on [1H]HSO4, therefore, is influenced by the crystal size of the reacting monomer, and changes dramatically if the irradiated sample is in the form of macrosingle crystals or micro-crystalline powder.

INTRODUCTION The description and rationalization of solid state [2+2] photoreactions in cinnamic acid derivatives dates back to the 70s with the pioneering works of Schmidt and Cohen.1-3 An important outcome of their work was the enunciation of the “topochemical rule”, which states that, in order to react, the double bonds must lie parallel and at the maximum separation of 4.2 Å. In his seminal studies ACS Paragon Plus Environment

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Schmidt foresighted the blossoming of a new discipline lying at the crossroad of chemistry, crystallography, and material sciences, namely crystal engineering.4-6 Since then, this class of reactions has stimulated increasing interest in the scientific community, being at the core of intense investigations focused on the control of the reactivity by means of (i) templating units interacting via hydrogen bond7-9 or halogen bond,10,11 (ii) coordination to metal centers,12-14 and (iii) salts formation.15-17 With the increase in the number of papers reporting on examples of solid state photoreactions, the number of positive or negative18exceptions to this rule also increased. This led to additional geometrical criteria,19 and alternative mechanisms affecting the solid state photoreactivity.20,21 Beyond purely theoretical reasons, there is also a considerable interest for utilitarian applications of such transformations, e.g. for the production of polymers synthesized from natural molecules,22-24 UV-filters in plastic materials,25 sunscreens,26,27 and more recently photochemical actuators displaying mechanical motion as a response to light absorption.28,29 Amongst all molecular systems prone to solid state photodimerization, cinnamic acid derivatives and cinnamates play a key role as model systems, especially when their transformations occur in a single-crystal-to-single-crystal (SCSC) fashion, because this allows to appreciate how the crystal phases evolve during irradiation, and to highlight subtle details, like the formation of intermediate crystal phases, by means of X-ray diffraction techniques. Recently, we reported in a series of papers15,16 on a clean and efficient strategy based on molecular salt formation as a means to activate the solid state photoreactivity of 4-amino-cinnamic acid (1). This strategy allowed us to obtain head-to-tail photodimers in a fast and quantitative way,15 and also afforded an unprecedented example of polymorphic transition triggered by light.16 In spite of these efforts, however, no photoreactive crystals of 4-amino-cinnamic acid derivatives have yet been discovered able to form head-to-head photodimerization products.

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Here we report on the solid-state photoreactivity of the salts [1H]HSO4 and [1H]ClO4 obtained by reaction of 4-amino-cinnamic acid (1) with H2SO4 and HClO4, and we discuss the different response to irradiation exhibited by single crystals and crystalline powder of the two salts. We also report the structural characterization from powder of the parent compound 1, which helps in elucidating its photostability upon UV irradiation. EXPERIMENTAL SECTION Synthesis All reagents were purchased from Sigma-Aldrich and used without further purification, with the exception of 4-amino-cinnamic acid (1), which was dissolved in boiling methanol and left to slowly evaporate at RT and in the dark. After complete evaporation of the solvent (ca. 5 d), small needlelike crystals suitable for XRD were obtained. The molecular salts [1H]ClO4 and [1H]HSO4 were obtained by placing 50-80 mg of 1 in 10% HClO4 and H2SO4, respectively. The resulting suspensions were sonicated until dissolution, boiled for a few minutes and left to slowly evaporate at RT in the dark. Needle-like and block-shaped crystals for [1H]ClO4 and [1H]HSO4, respectively, suitable for XRD were obtained over a period of 1 week. Single crystals of the solid solution {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 were obtained through the SCSC transformation (see below). X-ray Diffraction from Single Crystal Single-crystal data for compound 1 and its salts were collected at RT on an Oxford XCalibur S CCD diffractometer equipped with a graphite monochromator (Mo-Kα radiation, λ = 0.71073Å). Data collection and refinement details are listed in Table 1. All non-hydrogen atoms were refined anisotropically. HCH atoms for all compounds, were added in calculated positions and refined riding on their respective carbon atoms; HOH and HNH atoms were either directly located or added in calculated positions, with exception of {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 for which all H atoms were added in calculated positions. ACS Paragon Plus Environment


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In spite of many attempts of obtaining better single crystals for compound 1 and [1H]ClO4, crystalline specimen used for single crystal data collection were in general not of very good quality, as it can be seen from Table 2, where high wR2 factors are reported for both. Compound 1 was invariably obtained as tiny and weakly diffracting crystals, while [1H]ClO4 suffered its deliquescence also during data collection. Deliquescence have likely led to continuous dissolution and recrystallization affording a twinned specimen. SHELX9730 was used for structure solution and refinement on F2. The program Mercury31 was used to calculate intermolecular interactions and for molecular graphics. Crystal data can be obtained free of charge

via www.ccdc.cam.ac.uk/conts/retrieving.html (or from

the Cambridge

Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or e-mail: [email protected]). CCDC numbers 1817071-1817075.


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Table 1. Crystal data and refinement details for crystalline 1, [1H]ClO4, [1H]HSO4, {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64, and [12H2][HSO4]2. 1

[1H]ClO4

C9H9N1O2 C9H10ClNO6 Formula 163.18 263.63 fw Cryst. System Monoclinic Monoclinic P21/c C2/c Space group 8 8 Z 2 1 Z' 23.166(3) 30.390(6) a (Å) 3.8942(5) 10.095(3) b (Å) 19.377(3) 7.407(3) c (Å) 90 90 α (deg) 113.698(15) 96.40(3) β (deg) 90 90 γ (deg) 3 1600.7(3) 2258.1(12) V (Å ) 3 1.354 1.551 Dcalc (g/cm ) -1 0.097 0.355 µ (mm ) 10882 3707 Measd reflns 2824 1876 Indep reflns 2 0.1215 R1[on F0 , I>2σ(I)] 0.1178 0.4315 0.4447 wR2 (all data)

[1H]HSO4 C9H11NO6S 261.25 Triclinic P-1 2 1 5.4613(6) 7.6883(8) 13.7987(14) 85.320(8) 94.806(9) 106.851(9) 551.56(10) 1.573 0.311 4185 2488 0.0442 0.1103

{[1H]HSO4}0.36 [12H2][HSO4]2 ·{[12H2][HSO4]2}0.64 C9H11NO6S C18H22N2O12S2 261.25 522.50 Triclinic Orthorhombic P-1 Pca21 2 4 1 1 5.5055(6) 15.6663(14) 7.6405(8) 5.4952(6) 13.9481(17) 25.962(2) 92.034(9) 90 91.940(9) 90 105.941(10) 90 563.22(11) 2235.1(4) 1.540 1.553 0.304 0.307 3634 6017 1987 4000 0.0768 0.0711 0.2369 0.1558

X-ray Diffraction from Powder For phase identification and Rietveld refinement purposes X-ray powder diffractograms in the 2θ range 3–70° (step size, 0.026°; time/step, 200s; 0.02 rad soller; V x A 40 × 40) were collected on a Panalytical X’Pert PRO automated diffractometer operated in transmission mode (capillary spinner) and equipped with a Pixel detector. The program Mercury31 was used for simulation of X-ray powder patterns on the basis of single crystal data. Chemical and structural identity between bulk materials and single crystals was always verified by comparing experimental and simulated powder diffraction patterns. Powder diffraction data were analyzed with the software TOPAS4.1.32 A shifted Chebyshev function with 7 parameters and a Pseudo–Voigt function (TCHZ type) were used



to fit background and peak shape, respectively. A spherical harmonics model was used to describe the preferred orientation. An overall thermal parameter for the C, H, N, O, S atoms was adopted. Refinements converged with the following Rwp: 4.9%(0d), 9.7% (1d), 11.0% (2d), 9.6%(3d), 8.3%(4d), 8.2%(5d), 13.5%(6d) , 6.8%(7d) , 11.4%(8d) , 9.8%(9d) , 9.0%(10d). Solid State Photoreactions Single crystals and polycrystalline samples were irradiated using a UV-LED (Led Engin LZ110UV00-0000) with a wavelength centered at 365 nm and placed at a distance of 1-2 cm. Cross-Polarized Optical Microscopy Images were collected with the imaging software Visicam analyzers from an Olympus BX41 stereomicroscope equipped with polarizing filters. Scanning Electron Microscopy Morphological investigations of crystals were performed using a HITACHI S-2400 scanning electron microscope operating at 15 kV. The samples were observed as prepared and not sputter coated before examination. Sample preparation is as follows: single crystals were selected directly from the mother liquor and stuck on conductive adhesive tape (carbon) on stub. For this reason it is possible to see on the crystal surface the presence of smaller crystals or traces of polycrystalline samples. UV irradiation was performed in situ. FTIR spectroscopy. FTIR spectra were collected using a Bruker Alpha FTIR spectrometer. FTIR spectra in the range 2000-900 cm-1 were run on KBr pellets (sample amount: 1-2 mg, KBr amount: 200 mg), resolution was set to 2 cm-1, and 128 cycles for both background and measurement were collected. Spectra


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were run twice, before and after prolonged UV irradiation (3 d) on two different single crystal specimens.

RESULTS AND DISCUSSION Before discussing the photochemical behavior of the salts obtained by reaction of 4-amino-cinnamic acid (1) with H2SO4 and HClO4, we will describe the previously unknown crystal structure of the parent compound 1, and comment on the effect of the packing features on its photoresponse. Compound 1 crystallizes in the monoclinic space group P21/c with two molecules in the asymmetric unit (Z'=2). The carboxylic acid groups form the well known homomeric hydrogen bond dimers [OC=O…OOH = 2.614(7) - 2.622(6)Å] (see Figure 1), which are π-stacked in ribbons parallel to the b-axis direction. Adjacent ribbons are linked via hydrogen bonding interactions between the amino groups [NNH…NNH = 3.161(1) - 3.201(8) Å].

Figure 1. Infinite ribbons of hydrogen bonding dimers, π-stacked parallel to the b-axis direction, in crystalline 1. The ribbons interact with each other via N(H)···N hydrogen bonds involving the amino groups. HCH omitted for clarity. As a consequence of the hydrogen bonding features, all cations within the ribbons are arranged in a head-to-head fashion, with d = 3.894(9) Å, τ = 0° and α = 70.1(5)°, analogous to what observed in



the β-cinnamic acid,17,33,34 (see Scheme 1 and Figure 1). The values of the d, τ and α parameters19 all suggest a potential photoreactive behaviour for compound 1 in its solid state.

Scheme 1. Geometrical parameters§, used to evaluate the photoreactivity of the parent molecule 1 and the salts described in the present study.

Despite the suitable arrangement of double bonds, however, crystalline 1 was found to be inert under UV light, in either single crystals and polycrystalline samples, even after prolonged exposure (up to 3 days). The lack of photoreactivity was confirmed via X-ray powder diffraction and FTIR spectroscopy, since no appreciable changes could be detected in the diffraction patterns of samples measured before and after irradiation, and no peaks for the cyclobutane could be observed in the infrared spectra of the irradiated samples (see Figures SI-1 and SI-2). We attribute the photostability of crystalline 1 to the presence of the strong hydrogen bonding dimer between the carboxylic group (see Figure 1): formation of the cyclobutane fragment would cause a severe distortion of the hydrogen bonded ring, probably making the overall energetic balance unfavorable, or causing the crystal to break apart. We already observed and commented upon a similar behaviour, i.e. photostability in the presence of carboxylic acid dimers, in a series of salts of 4-amino-cinnamic acid.15,16 An additional obstacle to the reaction might be found in the inevitable deformation of the original “compact” packing, as the covalent pairing of molecules would generate “voids” around the dimer and, as a consequence, a loss of global crystal cohesion. We have recently shown that, while solid 4-amino-cinnamic acid is photostable, its sulfate,15 chloride15 and bromide16 salts undergo photocyclization reactions in the solid state. In the following


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section we report the surprising behavior of the hydrogen sulfate salt, [1H]HSO4, obtained by reaction of compound 1 with an aqueous solution of H2SO4; the perchlorate salt, [1H]ClO4, will be also be described, and its photostability explained in terms of crystal packing features. Scheme 1 shows how, depending on the type of anion, the cations arrangements in the salts are expected to (i) resemble those observed in the α-cinnamic acid (head-to-tail) or in the β-cinnamic acid (head-to-head), and (ii) generate α- or β-type cyclization products, respectively.

SCHEME 2. Possible mutual arrangements of the 4-ammonium-cinnamic acid cations in their crystalline salts, and the corresponding, expected photodimerization products. Anions not shown in the scheme. The molecular salt obtained by reaction of 4-amino-cinnamic acid (1) with H2SO4: [1H]HSO4 The molecular salt [1H]HSO4 crystallizes in the triclinic space group P-1. The presence of the HSO4- anion disrupts the hydrogen bonded rings formed by the carboxylic groups; in terms of geometry, the heteromeric dimer is analogous to the homomeric dimer [OS-O-…OOH = 2.616(3)Å, OSO-H…OC=O = 2.610(3)Å] (see Figure 2), but the hydrogen bond is now reinforced by the presence of a negative charge on the anion. The terminal oxygens on the hydrogen sulfate anions are free to ACS Paragon Plus Environment


interact via charge-assisted hydrogen bonds with the ammonium fragments of neighboring molecules [NN-H…OSO = 2.816(3) - 2.841(3) Å]. In this way both types of hydrogen bonds provide strong directionality, while the Coulombic forces acting between cations and anions are responsibe for the main cohesive energy.

Figure 2. Hydrogen bonding interactions in crystalline [1H]HSO4 leading to an infinitely parallel stacking of double bonds which makes the pairs of [1H]+ cationic units potentially photoreactive. HCH omitted for clarity. As a result of the new intermolecular bonds, the head-to-head arrangement observed in 1 has been converted into a head-to-tail arrangement in [1H]HSO4. Two slightly different but favorable sets of parameters [d = 3.694(1) Å, α = 87.4(1)°, τ = 0°; and d = 4.010(3)Å, α = 87.2(4)°, τ = 0°] between the C=C bonds of a cation and those of two adjacent [1H]+ units, make this salt potentially photoreactive under UV light. Thus, the same single crystal used for structure determination was exposed to UV radiation for 24h. Subsequent X-ray data collection on the irradiated sample showed small variations in the unit cell parameters and an expansion of the cell volume (+1.7%). New electron density peaks lying above and below the unreacted double bonds consistent with cyclobutane formation evolved in the Fourier-difference map. They were refined as three components of a disorder due to in-situ formation of the photoreaction product: it was evident from ACS Paragon Plus Environment

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the

structural

data

that

[1H]HSO4

had

been

converted

to

the

solid

solution

{[1H]HSO4}0.36·{[12H2][HSO4]2}0.64, as can be seen in Figure 3. The SCSC reaction was confirmed also by means of Crossed-Polarized Optical Microscopy (Figure 4) and SEM micrographs (Figure 5).

Figure 3. Crystal structure of the partially reacted salt {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64. Movement of carbon atoms, due to the photocyclization process, is evidenced by magenta and lightblue spheres, indicating the carbon atoms position before and after irradiation, respectively. HCH omitted for clarity.

Figure 4. Cross-polarized optical microscope pictures showing a single crystal of [1H]HSO4, used for structural determination, as grown from solution (a) and after 24h (b) and 72h (c) of UV irradiation. Both (b) and (c) correspond to the same solid solution of unreacted and reacted molecules, of formula {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 . ACS Paragon Plus Environment


Figure 5. SEM micrographs of a single crystal of [1H]HSO4 taken before (a) and after (b) UV irradiation corresponding to {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64, showing how crystal retains its overall integrity. Enlarged portions of the two images (c and d, respectively) clearly show how the crystal surface becomes only slightly damaged following irradiation. Invariably, only partial conversion took place, and the fraction of photoproduct could not be increased even after 3 days of irradiation. The reaction yield is therefore 64%; although this value is fairly lower than the theoretical value (82%) predicted for an infinite stack of double bonds;35,36 at the same time, the value is within the range experimentally observed for β-acids35 and much higher than the 30% reported by Schmidt3,35 for the β-form of the trans-cinnamic acid. In crystalline [1H]HSO4, however, the similarity with β-trans-cinnamic acid is limited to the infinite stacking of monomers, as all monomers, inter alia, are arranged in a head-to-tail fashion along the stacking direction (Figure 2).


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Incomplete photoconversion was confirmed also by FTIR spectroscopy, as evidenced by residual peaks at 980 (C-HC=C out-of-plane bending), 1640 (C=C stretching), and 1700 cm-1 (conjugated C=O stretching) present in the irradiated sample (see Figure SI-3);22-24,37 a shoulder at 1725 cm-1, due to the partial deconjugation of the carbonyl group, can also be detected in the spectrum. Recrystallization

of

a

dozen

of

crystals

corresponding

to

the

solid

solution

{[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 from acidic water resulted in a complete phase separation, and crystals of both pure reactant, [1H]HSO4, and photoproduct, [12H2][HSO4]2 were recovered. Interestingly, the two crystals are not isomorphous, as can be seen from Figure 6 and Table 1. The stacking sequence of monomers present in the parent solid is lost upon recrystallization of the dimers, and the hydrogen bonding pattern is profoundly changed. The hydrogen bonded rings formed between HSO4- anions and the carboxylic groups are replaced in solid [12H2][HSO4]2 by inter-anions interactions [O(H)S-OH···OS=O 2.631(9) Å], and the carboxylic groups are now involved in interactions with both the anion and the ammonium group O(H)COOH···OS=O 2.779(1) Å and OSOH…OS=O

2.911(1) Å. Direct hydrogen bonds of the N+(H)···OS=O type, present in the parent

compound, are maintained here [ONH···OS=O 2.631(9)-2.911(1) Å].



Figure 6. Crystal structure of [12H2][HSO4]2 viewed down the b-axis. HCH omitted for clarity. The non-isostructurality of solid [1H]HSO4 and of the pure photoproduct suggests that, for the photodimerization product [12H2][HSO4]2 to be “hosted” in the parent crystal and the photodimerazion process to be complete, a major rearrangement of crystalline [1H]HSO4 would be required. Indeed, complete conversion would imply loss of the reactant crystal symmetry and of favorable interactions around the cationic units. This can easily be visualized by looking at the Hirshfeld surfaces38 and the corresponding fingerprint plots39 for a [1H]+ pair and for a photodimer [12H2]2+ (see Figure SI-5). For these reasons, in agreement with Kitaigorodsky's condition of "isostructurality",40,41 the reaction cannot proceed to completion, and cannot afford solid solutions in the whole compositional range. Unexpectedly, the behavior of [1H]HSO4 upon irradiation changes dramatically depending on the size of its crystals. It is generally accepted that single-crystal-to-single-crystal photoreactions are favored when the particle size of the crystalline material decreases,42,43 and that small crystals experience less pressure, are less defective, therefore the photodimerization process can proceed towards completeness. In previous work15 we have shown that this is not necessarily true when ACS Paragon Plus Environment

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inorganic salts are employed, and quantitative conversion is observed also in crystals of macroscopic size. In the case of [1H]HSO4 the effect of the crystal size was also investigated: a polycrystalline sample with particles of microscopic size was irradiated and powder data were collected at intervals of 1 day. In order to investigate whether pure reactant phase [1H]HSO4 would directly

convert

to

[12H2][HSO4]2

or

would

pass

through

the

formation

of

{[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 and if in this latter case the concentration of the solid solution would remain constant during the process, we carried out Rietveld refinements on the collected data. Figure SI-6 reports the results of the refinements on the starting material and on the same sample after 10 days of UV exposure (see Figure SI-7 for the others). Figure 7 shows the relative amounts extracted from each powder XRD pattern vs irradiation time. The plot clearly shows some "specular" fluctuations between the amounts of reactant and solid solution phases, while the amount of the photoproduct grows steadily but slowly as the irradiation time increases.

Figure 7. Composition (%) of crystalline powder vs. irradiation time: monomer [1H]HSO4 (blue), solid solution {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 (green), and photoproduct [12H2][HSO4]2 (black). ACS Paragon Plus Environment


Therefore, as a result of UV irradiation, photoproduct molecules accumulate within the reactant crystals, and a solid solution is formed; once the concentration limit is reached, however, nucleation and growth of the pure solids [12H2][HSO4]2 and [1H]HSO4 occurs, resulting in their extrusion from the solid solution; at this point a second photoreaction “cycle” begins. While the first process takes place in 1 day, it can be seen from Figure 7 that subsequent cycles span a period of ca. 4 days. Upon time a slow but steady increase in the amount of the pure photoproduct can be observed. According to Kitaigorodsky,39 the degradation of the solid solution in polycrystalline samples may be due to differences in crystal grains dimensions, or, most likely, to the presence of defect sites in the powder grains, that inevitably result from mechanical grinding. This mechanism is summarized in Figure 8, and it is in stark contrast with the photoreaction [1H]Cl → [12H2]Cl2 previously investigated in polycrystalline powders,15 for which a continuous and complete transformation from monomers to dimers, via solid solutions formation, was observed.

Figure 8. Schematic representation of the solid state photoreaction “loop” in polycrystalline [1H]HSO4: solid solution formation is always followed by recrystallization of pure photoproduct and pure monomer, which undergoes a new cycle.


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The molecular salt obtained by reaction of 4-amino-cinnamic acid (1) with HClO4: [1H]ClO4 The molecular salt [1H]ClO4 crystallizes in the monoclinic space group C2/c, with one [1H]+ cation and one ClO4- anion in the asymmetric unit. Crystalline [1H]ClO4 is isostructural with the previously reported [1H]BF4 salt Form I (PACHAF),15 and is characterized by the presence of homomeric hydrogen bonds between the carboxylic acid groups [OC=O…OOH = 2.651(1)Å], with the ClO4- anion bridging the -NH3+ ammonium groups via charge assisted hydrogen bonds ON(H)…OClO = 2.909(9)- 2.982(9) Å (see Figure 9).

Figure 9. Hydrogen bonding and electrostatic interactions and detail of the unsuitable orientation of the cationic units [1H]+ in crystalline [1H]ClO4. HCH omitted for clarity. ACS Paragon Plus Environment


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The C=C bonds of two nearest [1H]+ cationic units are slightly far [d = 5.309(1) Å], but more relevant is that the hydrogen bonded dimers are stacked in a criss-cross fashion [τ = 29.4(1)°], as can be seen in Figure 9. As a result, single crystals of [1H]ClO4 are photostable under UV light, and no changes in the unit cell and crystal structure were noticed, even after prolonged UV exposure (up to 3 days).⸷ Unfortunately, no further studies on polycrystalline samples were possible, due to their deliquescent nature. CONCLUSIONS In this work we have reported on the compound 4-amino-cinnamic acid (1) and on its molecular salts [1H]HSO4 and [1H]ClO4. All crystalline materials have been structurally characterized via single crystal and powder X-ray diffraction, and their photochemical behavior investigated by means of FTIR spectroscopy. Compound 1 is photostable, both as single crystals and polycrystalline material, even after prolonged UV exposure, in spite of the favorable, relative arrangements of the potentially reactive double bonds within the crystal. In its perchlorate salt [1H]ClO4 the double bonds are stacked perpendicular to each other in projection, and, as a consequence, also this solid is photostable. In the hydrogen sulfate salt [1H][HSO4] an α-cinnamic acid-like arrangement was found. Its photochemical behavior resulted quite striking since single crystals and polycrystalline materials behaved differently under irradiation. Single crystals of [1H]HSO4 converted partially via SCSC into the solid solution {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 upon irradiation (1d). This composition represents the concentration limit since it was not possible to raise the photoproduct fraction, even after prolonged UV exposure (3d). Following recrystallization, the two components completely separated in SCs of reactant and photoproduct [12H2][HSO4]2. On the other hand, UV irradiation of polycrystalline powders of [1H]HSO4 yielded a

mixture of reactant,

solid solution

{[1H]HSO4}0.36·{[12H2][HSO4]2}0.64 and photoproduct [12H2][HSO4]2 phases. Insights into the


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mixture composition vs. irradiation time (up to 10d) were obtained from Rietveld analysis. Indeed, diffraction data proved how photoproduct phase came from the "degradation" of the solid solution. This is, to the best of the authors' knowledge, the first observation for a solid state [2+2] photoreaction behaving differently, depending on the sample type, namely single crystals or polycrystalline powders. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. X-ray crystallographic data for compound 1, [1H]ClO4, [1H]HSO4, {[1H]HSO4}0.36·{[12H2][HSO4]2}0.64,

and [12H2][HSO4]2

(CIF). Additional information

concerning PXRD, Rietveld Refinement Plots, Hirshfeld Surface analyses, FingerPrint Plots, and FTIR spectra. AUTHOR INFORMATION Corresponding Author * (S. d. A.) E-mail: [email protected] * (F. G.) E-mail: [email protected] Orcid Simone d'Agostino 0000-0003-3065-5860 Elisa Boanini 0000-0003-3754-0273 Fabrizia Grepioni: 0000-0003-3895-0979



Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial support from MIUR and the University of Bologna is acknowledged. Notes §

d = centre-to-centre bond separation between carbon atoms of C=C bonds in adjacent molecules

(Schmidt’s rule). Upper limit is 4.2Å. τ = torsion angle C=C···C=C formed by the adjacent molecules. The ideal value is 0°. α = shift of one C=C bond along the second C=C. The ideal value is 90°. ⸷

Caution! Perchlorate salts are potentially explosive, please pay attention when irradiating them.

Do not irradiate more than few mg nor grind any perchlorate salt. The authors declare no competing financial interest. REFERENCES (1) Cohen, M. D. ; Schmidt, G. M. J. Topochemistry. Part I. A survey. J. Chem. Soc. 1964, 0, 19962000. (2) Heller, E.; Schmidt, G. M. J. Topochemistry. XXXIII. Solid-state photochemistry of some anthracene derivatives. Isr. J. Chem. 1971, 9, 449-462. (3) Cohen, M. D.; Schmidt, G. M. J.; Sonntag F. I. Topochemistry. Part II. The photochemistry of trans-cinnamic acids. J. Chem. Soc. 1964, 2000-2013.


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SYNOPSIS Here we report on the solid state photoreactivity of 4-amino-cinnamic acid (1) and of the salts [1H]ClO4, and [1H]HSO4. While 1 and [1H]ClO4 are photostables, [1H]HSO4 photoreacts in an SCSC fashion, affording the corresponding solid solution {[1H]HSO4}0.36·{[12H2][HSO4]2}. Irradiation of polycrystalline samples, surprisingly, triggers the extrusion of [1H]HSO4 and [12H2][HSO4]2 from the solid solution as highlighted by Rietveld analysis.

Graphic for Table of Contents and abstract

Size Matters: [2+2] Photoreactivity In Macro- and Microcrystalline Salts of 4-Amino-Cinnamic Acid Simone d'Agostino, Elisa Boanini, Dario Braga, and Fabrizia Grepioni


Size Matters: [2 + 2] Photoreactivity In Macro- and Microcrystalline

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