Establishing trends based on solvent system changes in cocrystals containing pyrogallol [ 4 ] arenes and fluorescent probes rhodamine B and pyronin Y †

Cocrystal systems containing pyrogallol[4]arene molecules of various aliphatic tail lengths, the fluorescent probe rhodamine B, and a variety of solvents are examined and discussed. Additionally, two pyronin Y cocrystals of C-methylpyrogallol[4]arene in methanol and C-ethylpyrogallolij4]arene in ethanol are crystallized and analyzed. For both the pyronin Y and rhodamine B cocrystals, solvent and aliphatic chain length have an effect on the packing, bowl shape of the pyrogallol[4]arene, probe complexation, and hydrogen bonding schemes. With pyronin Y cocrystals of C-methylpyrogallolij4]arene form a tube-like structure, with pyronin Y molecules in the centre of the tube while cocrystals of C-ethylpryogallol[4]arene form a bilayer-type structure.


Introduction
Pyrogallol [4]arenes are important building blocks for supramolecular complexes.The molecule consists of four aromatic groups arranged into a bowl shape and adorned with twelve hydroxyl groups on the upper-rim.Four aliphatic tail groups reside on the bridging aldehyde residues.Pyrogallol[4]arenes have the ability to form numerous architectures such as dimers, hexamers, nanotubes, and nanocages.Such structures have potential applications as catalysts and molecular transporters. 1Dimeric and hexameric pyrogallol [4]arenes may be formed by hydrogen bonding or by metal ions such as Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , or Zn 2+ . 2 Additionally, because of their ability to act as a host, various types of guest molecules such as small inorganic molecules, pharmaceutical molecules, and ionic liquids have been cocrystallized with pyrogallol[4]arenes. [3][4][5] Fluorescent probe molecules have also been cocrystallized with pyrogallol[4]arenes. 6 C-hexylpyrogallolĳ4]arene (PgC 6 ) has been cocrystallized with both pyrene butyric acid (PBA) and 1-Ĳ9-anthryl)-3-Ĳ4-dimethylaniline) propane (ADMA) in acetonitrile.Additionally, C-propan-3-ol-pyrogallolĳ4]arene (PgC 3 OH) has been cocrystallized with the fluorescent probe acenaphthlene in methanol.There exists a need to study cocrystallizations with more types of fluorescent probes with a variety of pyrogallol [4]arenes and solvents, not just PgC 6 and PgC 3 OH in methanol and acetonitrile.Cocrystals with fluorescent probes can lead to insight into the chemical environment inside and outside the bowl of the pyrogallol[4]arene, intermolecular and intramolecular interactions, and the formation of novel supramolecular architectures. 7ven though there have been studies with pyrogallol [4]arenes and fluorescent probes, information is lacking regarding the factors influencing chemical environment and interactions. 8t has been shown that structural properties can be "finetuned" by changing the shape, size, and chemical composition of the components. 9More extensive studies are needed in order to learn more about the influence of characteristics such as aliphatic tail length and solvent systems on structural properties.In this study we examine cocrystals of rhodamine B and of pyronin Y with pyrogallol[4]arenes of different aliphatic chain length and in various solvent.
This journal is © The Royal Society of Chemistry 2015 synthesized using the previously reported method by Gerkensmeier et al., using acetaldehyde, propionaldehyde, and butyraldehyde as the respective aldehydes. 10ocrystal 1 was crystallized by mixing PgC 1 and pyronin Y in a 1 : 1 molar ratio (0.1 : 0.0497 g) in 15 mL of methanol.The mixture was then sonicated for 30 minutes and allowed to slowly evaporate until crystallization of pink, prism-shaped crystals formed.In the same manner, cocrystal 2 was crystallized by mixing PgC 2 and pyronin Y in a 1 : 1 molar ratio (0.05 : 0.0228 g) in 10 mL of ethanol and allowed to slowly evaporate until crystallization of pink, plate-shaped crystals.
Cocrystals 3-8 were all crystallized by dissolving PgC x and rhodamine B in a 1 : 1 molar ratio (0.1 : 0.0788 g for PgC 1 , 0.1 : 0.0721 g for PgC 2 , and 0.1 : 0.0665 g for PgC 3 ) in a given amount of solvent (see Table 1).The mixture was then allowed to slowly evaporate.All crystallizations resulted in pink, prism-or plate-shaped crystals.
Single-crystal X-ray diffraction data for cocrystals 1 and 5 were collected with a Bruker Apex II CCD diffractometer at 173 K using Cu Kα radiation (λ = 1.54178Å).All single-crystal X-ray diffraction data for the other cocrystals was obtained with a Bruker Apex II CCD diffractometer at 173 K using Mo Kα radiation (λ = 0.71073 Å).

Results
All rhodamine B cocrystals are comprised of hydrogen bonded bilayer structures in which a rhodamine B molecule is endo to one PgC x molecule: one part of the rhodamine B molecule is inside the bowl of the PgC x molecule.Unless otherwise noted, the -NĲCH 2 CH 3 ) 2 group of the rhodamine B molecule is the endo part of the rhodamine B molecule in the bowl of the PgC x molecule and is oriented vertically (see Fig. 2).One important characteristic examined in both types of cocrystals is the C-H⋯π distance.These comprise interactions involving a calculated centroid (the π-electron density) of an aromatic group donating to a hydrogen atom bonded to a carbon atom.A second important structural feature is the cross-sectional distance.This is the distance between opposite middle hydroxyl groups on the rim of the PgC x molecule (see Fig. 3).When these distances differ by less than 0.75 Å, the bowl of the pyrogallol [4]arene is referred to as conical and when the distances differ by more than 0.75 Å the bowl of the pyrogallol [4]arene is referred to as pinched.Additionally, when discussing the oxygen atoms of the rhodamine B molecule, the oxygen atom in the ring system is not involved in any hydrogen bonding unless otherwise noted.

Cocrystal 1
There are one pyronin Y molecule, one PgC 1 molecule, five methanol molecules, and three water molecules in the asymmetric unit of cocrystal 1.Two of the water molecules are disordered; one is modeled at 50% occupancy and six are modeled at 25% occupancy.The PgC 1 bowl is nearly conical; the cross-sectional distances are 8.24 Å and 8.75 Å.The -NĲCH 3 ) 2 group is endo to the bowl of the PgC 1 .Three probes are stacked directly on top of each other in the center of four PgC 1 molecules (forming a square with the PgC 1 molecules at the corners and the upper-rim hydroxyl groups pointing towards the center of the square) (see Fig. 4).The PgC 1 molecules form a tube-like structure in which the pyronin Y molecules pack in the empty space of the tube (see Fig. 5).In total, the PgC 1 molecule participates in twelve hydrogen bonds (1.87-2.20 Å (H⋯A), 109.8-168.1°(O-H⋯A)).Three of the hydrogen bonds are intramolecular hydrogen bonds between the upper-rim hydroxyl groups and two of the hydrogen bonds are intermolecular hydrogen bonds to hydroxyl

Cocrystal 2
Cocrystal 2 has an asymmetric unit which contains one endopyronin Y molecule, one PgC 2 molecule, one ethanol molecule, and two chloride ions (see Fig. 6).The ethanol molecule is disordered over two positions and is modeled at 60% and 40%.One of the chloride ions is disordered over two positions and is modeled at 50% and the second chloride ion is disordered over five positions and is modeled at 30%, 20%, 20%, 15%, and 15%.The bowl of the pyrogallol[4]arene is pinched and has cross-sectional distances of 6.30 Å and 9.83 Å.Unlike cocrystal 1, the PgC 2 molecules pack in a typical bilayer arrangement and the endo pyronin Y molecule is arranged horizontally (aligned with the bowl).Throughout the crystal lattice, the hydroxyl groups of the PgC 2 molecule participate in fifteen hydrogen bonds (1.66-2.37Å (H⋯A), 109.1-162.2°(O-H⋯A))(see Fig. 7).Five of the hydrogen

Cocrystal 7
Contained in the asymmetric unit of cocrystal Cocrystal 8 In the asymmetric unit of cocrystal 8 there is one rhodamine B molecule, one PgC 3 molecule, two and one half water molecules, and half of an acetonitrile.One of the water molecules is disordered over two positions and is modelled at 90% and 10%.The acetonitrile molecule is disordered with two water molecules and the three molecules are modelled at 50%, 25%, and 25%, respectively.One of the aliphatic tail groups on the PgC 3 molecule is disordered over two positions and is modelled at 65% and 35%.The macrocycle is in a pinched conformation; with cross-sectional distances 7.77 Å and 9.09 Å.There is significantly less hydrogen bonding in cocrystal 8 than in the previous five cocrystals.There are only nine hydrogen bonds throughout the system (1.87-2.21Å (O-H⋯A), 116.1-177.1°(O-H⋯A)).In total there are three intramolecular bonds between the hydroxyl groups, two intermolecular bonds where the hydroxyl hydrogens donate to two adjacent PgC 3 hydroxyl groups, three hydrogen bonds in which the hydroxyl hydrogens donate to three water molecules, and one hydrogen bond where a hydroxyl hydrogen atom donates to one of the oxygen atoms on the

Discussion
There are many similarities between the rhodamine B containing cocrystals.All the cocrystals have similar hydrogen bonding schemes, a 1 : 1 ratio of probe to pyrogallol[4]arene, a bilayer packing arrangement, and a rhodamine B endo to the pyrogallol[4]arene molecule.However, changes in solvent and aliphatic tail length leads to differences in C-H⋯π interactions and hydrogen bonding.Aliphatic tail length of the pyrogallol[4]arene molecules does seem to play a part in the C-H⋯π interactions and hydrogen bonding scheme of the cocrystals of rhodamine B. In regards to the C-H⋯π interactions, with an aliphatic tail length of one carbon atom (cocrystals 3 and 6) the amount of C-H⋯π interactions to endo and symmetry-generated rhodamine B molecules remains consistent with two C-H⋯π interactions to an endo rhodamine B. When aliphatic tail length is increased to two carbon atoms (cocrystals 4 and 7) the number of C-H⋯π interactions to symmetry-generated rhodamine B molecules increases from zero or one C-H⋯π interaction in cocrystals 6 and 3, respectively, (methanol cocrystals) to two C-H⋯π interactions (cocrystal 7) and three C-H⋯π interactions (cocrystals 4).
Changing the solvent has a small effect on the C-H⋯π interactions.When examining the PgC 1 cocrystals (cocrystals 3 and 4), when the solvent is changed from methanol to ethanol, the number of C-H⋯π interactions between the PgC 1 aromatic groups and the endo rhodamine B remain constant at two C-H⋯π interactions, but the number of C-H⋯π interactions involving the aromatic groups of the rhodamine B molecules and the interactions between the aromatic groups of the PgC 1 molecules and exo, symmetry-generated rhodamine B molecules are greater in the ethanol cocrystals.
Aliphatic tail length along with the solvent, influences the resulting hydrogen bonding network in the rhodamine B cocrystals (see Table 2).With acetonitrile/water (cocrystal 8) there are significantly less hydrogen bonds than with the other cocrystals.Cocrystal 8 has only nine hydrogen bonds whereas the others have at least thirteen hydrogen bonds.It has fewer intramolecular hydrogen bonds and hydrogen bonds to rhodamine B. Methanol as a solvent (cocrystals 3 and 6) maintains a constant hydrogen bonding scheme with different aliphatic tail lengths.Both cocrystals have the same number of intramolecular hydrogen bonds, intermolecular hydrogen bonds, and hydrogen bonds to rhodamine B. With ethanol (cocrystals 4, 5, and 7), the hydrogen bonding does not remain constant with changing aliphatic tail lengths.Cocrystals 4 and 5 (aliphatic tail lengths of one and two carbon atoms, respectively) have double the number of intramolecular hydrogen bonds than intermolecular hydrogen bonds.However, when the aliphatic tail length is increased to three carbon atoms (cocrystal 7), then there is the same number of intramolecular and intermolecular hydrogen bonds.Finally, the number of solvent hydrogen bonds decreases as the aliphatic tail length increases.This is likely due to the fact there is water in cocrystals 5 and 7 so hydrogen bonding has to be shared between the main solvent and the water molecules.
In terms of solvent change, in cocrystals containing PgC 1 molecules (cocrystals 3 and 4), the hydrogen bonding scheme remains consistent between the two cocrystals when the solvent changes from methanol to ethanol.However, in the cocrystals containing PgC 3 molecules (cocrystals 6, 7, and 8) the hydrogen bonding scheme changes in regards to total number of hydrogen bonds, hydrogen bonds to water molecules, and hydrogen bonding to solvent molecules.When the solvent changes from methanol (cocrystal 6) to ethanol (cocrystal 7) to acetonitrile (cocrystals 8), the number of hydrogen bonds to solvent molecules decreases from seven hydrogen bonds to three hydrogen bonds to zero hydrogen bonds.Furthermore, when the solvent changes from methanol to ethanol to acetonitrile, the total number of hydrogen bonds also decreases from seventeen hydrogen bonds to fifteen hydrogen bonds to nine hydrogen bonds.When the solvent changes from methanol to ethanol to acetonitrile the number of hydrogen bonds to water molecules increases from zero hydrogen bonds to two hydrogen bonds to three hydrogen bonds.
The only property that does not remain equivalent among the rhodamine B cocrystals is the shape of the pyrogallol[4]arene bowl.Cocrystals 4, 5, and 6 all have nearly conical bowls whereas cocrystals 3 and 7 have noticeably pinched cones (see Table 3).Cocrystals 3 and 7 are in different solvents and have different aliphatic tail lengths; thus there is no correlation between solvent, tail length, and bowl shape as of yet.One trend that seems to emerge is that rhodamine B prefers to be endo rather than exo to the pyrogallol[4]arene bowl.
Between the two pyronin Y structures, there are considerable differences between the supramolecular architecture, hydrogen bonding schemes, probe complexation, and bowl shape.Most notably, the supramolecular structure formed with PgC 1 molecules (cocrystal 1) is a tube-like structure whereas PgC 2 molecules (cocrystal 2) form a bilayer-type structure.Cocrystal 1 has more symmetry than cocrystal 2 resulting in different crystal unit parameters (see Table 4).Different packing motifs led to dissimilar hydrogen bonding schemes.With cocrystal 1 (PgC 1 ), there are a total of twelve hydrogen bonds, while with cocrystal 2 (PgC 2 ) there are a total of fifteen hydrogen bonds.Cocrystal 1 also has more hydrogen bonds with solvent molecules (seven hydrogen bonds) than intermolecular and intramolecular hydrogen bonds (two and three hydrogen bonds, respectively).Cocrystal 2 has more intermolecular and intramolecular hydrogen bonds (five hydrogen bonds each) than solvent hydrogen bonds (two hydrogen bonds).
Along with the different packing arrangements and hydrogen bonding schemes of the PgC x molecules, the probes have different orientations within the structure based on cocrystallization with PgC 1 or PgC 2 molecules.When cocrystallized with PgC 1 molecules, the pyronin Y molecule is oriented vertically in the bowl of the PgC 1 molecule.However, in cocrystal 2 (PgC 2 ) the pyronin Y molecule is oriented horizontally.Lastly, the bowl shape in the two cocrystals is dissimilar; cocrystal 1 (PgC 1 ) has a conical bowl (8.24, 8.75 Å) and cocrystal 2 (PgC 2 ) has a pinched bowl (6.30, 9.83 Å).
Although the two pyronin Y structures have different supramolecular architectures, hydrogen bonding schemes, probe orientation, and bowl shape they differ from their rhodamine B analogues in similar ways.First, both differ in their bowl shapes.In cocrystal 1 (PgC 1 , methanol, and pyronin Y), the bowl of the PgC 1 molecules is conical but in cocrystal 3 (PgC 1 , methanol, and rhodamine B) the bowl of the PgC 1 is pinched.Both PgC 2 analogues (cocrystal 2 (pyronin Y) and 5 (rhodamine B)) also differ in bowl shape; cocrystal 2 has a pinched bowl but cocrystal 5 has a conical bowl.In regards to C-H⋯π interactions (see Table 5), cocrystals 1 and 2 (pyronin Y) have one C-H⋯π interaction to an adjacent pyrogallol[4]arene molecule whereas cocrystals 3 and 5 (rhodamine B) do not have any C-H⋯π interactions to adjacent pyrogallol[4]arene molecules.Additionally when examining just the PgC 2 cocrystals, compared to cocrystal 5 (rhodamine B), the PgC 2 cocrystal of pyronin Y (cocrystal 2) has fewer C-H⋯π interactions overall (four interactions for cocrystal 2 and five interactions for cocrystal 5), C-H⋯π interactions involving the aromatic groups of the probe molecules (one for cocrystal 2 and two for cocrystal 5), and C-H⋯π interactions involving the aromatic groups of the pyrogallol[4]arene molecules and the hydrogen atoms of the probe molecules (two for cocrystal 2 and three for cocrystal 5).The trend is opposite in regards to the number of overall C-H⋯π interactions in cocrystals contacting PgC 1 molecules.The pyronin cocrystal (cocrystal 1) has more C-H⋯π interactions than the rhodamine B cocrystal (cocrystal 3).Unlike with C-H⋯π interaction, the hydrogen bonding schemes are similar in the analogues (see Table 6) except for hydrogen bonding to the probe molecules and intermolecular hydrogen bonding in analogues of PgC 2 .In both rhodamine B cocrystals (3 and 5),   there are two hydrogen bonds with two hydroxyl hydrogen atoms donating to two different oxygen atoms on the rhodamine B -COOH group.In the pyronin Y analogues, there are no hydrogen bonds to the pyronin molecule.In the PgC 1 cocrystals (cocrystals 1 and 3), there are double the number of intermolecular hydrogen bonds in the rhodamine B cocrystal (cocrystal 3) than in the pyronin Y cocrystal (cocrystal 1).Finally, the packing arrangement differs in the analogues of PgC 1 and PgC 2 cocrystals (see Fig. 12).In the PgC 1 cocrystals (cocrystals 1 and 3 ‡), the pyronin Y cocrystal has a tube-like architecture while the rhodamine B cocrystal has a bilayer structure.In both PgC 2 cocrystals (cocrystals 2 and 5), there is a bilayer structure; however, in the pyronin Y cocrystal (cocrystal 2) the probe molecules are aligned directly next to each other, but in the rhodamine B cocrystal (cocrystal 5) each end is aligned with the end of a different rhodamine B molecule.Even though both pyronin Y and rhodamine B have similar backbone structures, the addition of the carboxylic phenyl group of the rhodamine B changes the bowl shape, crystal packing, and hydrogen bonding schemes of the resulting cocrystals.cocrystal, horizontal for PgC 2 cocrystal), and hydrogen bonding schemes (more hydrogen bonds to solvent for PgC 1 cocrystal, more intermolecular and intramolecular hydrogen bonds for PgC 2 cocrystal).Thus, not only does solvent influence the final cocrystals but also the aliphatic chain length influences the final structure.The versatility of the cocrystals with solvents and pyrogallol[4]arene tail lengths, and the presence of multiple hydrogen bonding sites provides a good start to future insight into understanding the assembly of supramolecular architectures in the solid-state (i.e.intermolecular interactions) and the factors that affect particular types of assemblies.

Fig. 2
Fig. 2 Vertical arrangement of the rhodamine B guest.Solvent molecules and hydrogen atoms are omitted for clarity.

Fig. 4
Fig. 4 Arrangement of PgC 1 molecules with the guest molecule pyronin Y in (a) stick representantion and (b) with space-filled pyronin Y molecules.Solvent molecules and hydrogen atoms are omitted for clarity.Fig. 5 Packing of the PgC 1 molecules and pyronin Y molecules along the (a) b-axis and (b) the a-axis.The pyronin Y molecules are in green and in space-filled representation.Solvent molecules and hydrogen atoms are removed for clarity.

Fig. 7
Fig. 7 Bonding present in cocrystal 2. (a) Hydrogen bonding: intermolecular hydrogen bonding in dashed green bonds, intramolecular hydrogen bonding in dashed pink bonds, solvent hydrogen bonding in blue dashed bonds.(b) C-H⋯π bonding (orange dashed bonds).All other solvent molecules and hydrogen atoms are omitted for clarity.

Fig. 8 Fig. 9
Fig. 8 Asymmetric unit of cocrystal 3, C 32 H 32 O 12 •C 28 H 30 O 3 N 2 •5CH 3 OH, in (a) stick representation, (b) stick representation of the PgC 1 molecule and space-filled representation of the rhodamine B molecule, and (c) space-filled representation.All solvent and hydrogen atoms are omitted for clarity.

Fig. 10
Fig. 10 Types of hydrogen bonds found in cocrystal 5. Intermolecular hydrogen bonds are green dashed bonds, intramolecular hydrogen bonds are pink dashed bonds, solvent hydrogen bonds with the PgC 2 molecule are blue dashed lines, and solvent hydrogen bonds to rhodamine B molecules are yellow dashed lines.Other solvent molecules and hydrogen bonds are removed for clarity.

Fig. 11
Fig.11The C-H⋯π interactions (orange dashed bonds) present in cocrystal 6.All solvent molecules and hydrogen atoms are omitted for clarity.
Cocrystals of rhodamine B with various aliphatic chain lengths and solvent systems produce cocrystals that pack in a bilayer arrangement with one rhodamine B endo to the pyrogallol[4]arene bowl.Solvent does have an effect on the hydrogen bonding scheme.On the other hand, the two pyronin Y cocrystals (two different aliphatic chains and solvent systems) have significantly different supramolecular architectures (tube-like for PgC 1 cocrystal, bilayer for PgC 2 cocrystal), bowl shapes (conical for PgC 1 cocrystal, pinched for PgC 2 cocrystal), probe orientation (vertical for PgC 1

Table 1
Type and amount of solvent used in cocrystallizations

Table 2
Types and number of hydrogen bonds in the rhodamine B cocrystals

Table 3
Comparison of the cross-sectional distances of cocrystals of rhodamine B

Table 4
Comparison of the cocrystals of pyronin Y

Table 5
Comparison of C-H⋯π interactions in cocrystal analogues of rhodamine B and pyronin Y

Table 6
Comparison of the hydrogen bonding in cocrystal analogues of rhodamine B and pyronin Y