Two-Component Supramolecular Organic Hosts as Colorimetric Indicators for Aromatic Guests: Visual...

Published: July 18, 2011

r 2011 American Chemical Society 4120 dx.doi.org/10.1021/cg200690c | Cryst. Growth Des. 2011, 11, 4120–4128

ARTICLE

pubs.acs.org/crystal

Two-Component Supramolecular Organic Hosts as ColorimetricIndicators for Aromatic Guests: Visual Molecular Recognition viaCation�π InteractionsSandipan Roy and Kumar Biradha*

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

bS Supporting Information

ABSTRACT:

Supramolecular two-component organic host systems composed of 2,20,6,60-tetracarboxybiphenyl (H41)/4,40-bipyridylethylene(bpe) and (H41)/4,40-bipyridine (bpy) were shown to recognize various aromatic guest molecules. The inclusion of the guestmolecules in the host systems is evident from the remarkable exhibition of different colors in their corresponding inclusioncomplexes. The host networks are formed by the self-assembly of two (H31) anions and dication (H2bpe or H2bpy) via O�H 3 3 3Oand N�H 3 3 3O hydrogen bonding. The pyridinium ions have exhibited greater tendency to interact with aromatic guest moleculesvia strong cation�π interactions. The host system (H31)2(H2bpe) was found to recognize anthracene, phenanthrene, and pyrene,while (H31)2(H2bpy) was found to recognize several aromatic guests such as phenanthrene, pyrene, phenol, o-cresol, m-cresol,p-cresol, and chlorobenzene. The crystal structures of all these materials were determined and analyzed in terms of hydrogen bondingbetween the two host components and host�guest interactions. The extent of cation�π interactions is reflected in the color of thesecomplexes. Solid-state diffusion reflectance spectral (DRS) studies have been carried out for all these different colored complexes.

’ INTRODUCTION

The design of supramolecular host systems has been one of theprimary focuses in the burgeoning field of crystal engineering1 andmolecular recognition2 due to their several applicative aspects suchas catalysis,3 chemical separation,4 gas-storage devices,5 and targeteddrug delivery.6 In the context of analytical chemistry, the develop-ment of host systems for colorimetric analysis is of primary focus fora supramolecular chemist because it is a simple technique for theidentification ofmolecules with the naked eye.7 To date,most of thevisualmolecular recognition systems that have been explored are forthe recognition of metal ions8 and anions,9 and very few examplesare reported for the recognition of organic guest molecules.7,10 Forsuch a purpose, the prerequisite is to design organic host systemsthat are capable of forming crystalline inclusion complexes withspecific colors upon inclusion of guest molecules. Organic ionichosts formed by two components are expected to fulfill such arequirement due to their ability to exert charge transfer cation�πinteractions11 with guest molecules. The mixing of colorless protondonor with colorless proton acceptor in the presence of aromaticguests is expected to result in different colored crystals due to theformation of a three-component charge transfer (CT) complex thatexhibits a large absorption band in the visible region.

Recently, the supramolecular synthons II and III were shown toform two-component host architectures for a variety of aromaticguest molecules.12 Previously we have reported that H41 with 4,

40-bipyridylethylene (bpe) forms cocrystals of [H41][bpe]2 fromconcentrated MeOH solution, while crystals of an ionic complexwere obtained from dilute MeOH solution.13 The cocrystals werefound to exhibit an 8-fold interpenetrated diamondoid network viasynthon-III, and the ionic complex exhibited layered networks viasynthons-I and -II. However, the use of 4,40-bipyridine (bpy) inplace of bpe resulted in crystals of three types of ionic architecturesdepending on the solvent of crystallization. One of the interestingaspects in the structures of salts is that the bpe/bpy is typicallyenough to deprotonate only one of the four �COOH groups ofH41. Among the other three�COOHgroups, one is involved in anintramolecular O�H 3 3 3Owith the deprotonated�COO�, whilethe other two �COOH groups assemble the H31 units in one ortwo dimensions via O�H 3 3 3O hydrogen bonds.

Received: June 1, 2011Revised: July 14, 2011

4121 dx.doi.org/10.1021/cg200690c |Cryst. Growth Des. 2011, 11, 4120–4128

Crystal Growth & Design ARTICLE

Table1.

CrystallographicParam

etersfortheCrystalStructures

of2�

11

compound

23

45

67

89

1011

form

ula

C59H44N2O

17C59H44N2O

17C60H40N2O

16C56H38N2O

16C58H38N2O

16C48H36N2O

18C49H38N2O

18C49H38N2O

18C49H38N2O

18C48H33ClN

2O16

MW

1052.98

1052.98

1046.98

994.90

1018.93

928.80

942.83

942.83

942.83

929.23

T(K

)293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

293(2)

system

triclinic

triclinic

triclinic

monoclinic

monoclinic

triclinic

triclinic

triclinic

triclinic

triclinic

spacegroup

P1P1

P1P2

1/n

P21/n

P1P1

P1P1

P1

a(Å)

8.150(4)

8.154(3)

8.0203(11)

13.5901(12)

13.799(3)

8.1441(7)

8.103(3)

12.7869(10)

12.872(3)

8.0922(10)

b(Å)

12.611(6)

13.877(5)

12.7391(18)

7.9936(7)

8.2058(16)

12.4229(11)

12.474(8)

14.4399(12)

14.222(4)

12.4611(15)

c(Å)

13.746(7

22.904(9)

14.0636(19)

21.3350(19)

20.873(4)

12.5539(17)

12.498(5)

14.8308(12)

14.895(6)

12.587(2)

R(deg)

110.023(14)

82.541(11)

114.499(4)

90.00

90.00

113.319(3)

112.704(19)

116.417(2)

115.035(10)

113.496(5)

β(deg)

105.456(15)

87.688(12)

100.823(4)

90.121(3)

90.402(6)

96.264(3)

106.631(14)

96.957(2)

96.909(10)

96.338(5)

γ(deg)

98.199(15)

73.624(12)

98.386(4)

90.00

90.00

106.516(2)

96.29(2)

109.554(2)

110.169(7)

106.722(4)

vol(Å3 )

1236.0(11)

2465.4(16)

1243.4(3)

2317.7(4)

2363.4(8)

1082.2(2)

1081.4(9)

2189.5(3)

2202.6(12)

1077.5(3)

Z1

21

22

11

22

1

Dcalc(M

g/m

3 )1.413

1.417

1.396

1.426

1.432

1.425

1.448

1.430

1.381

1.432

R1(I>2σ

(I))

0.0710

0.0801

0.0568

0.0546

0.0758

0.0441

0.0467

0.0693

0.0697

0.0635

wR2(onF2,alldata)

0.1395

0.2014

0.1458

0.1676

0.1672

0.1519

0.1435

0.1665

0.1747

0.1455

Table 2. Hydrogen Bonding Parameters in the CrystalStructures of 2-11

Complexes Interaction

H 3 3 3A(Å)

D 3 3 3A(Å)

D-H 3 3 3A(deg)

2 N�H 3 3 3O 2.05 2.863(6) 158

2.48 2.985(6) 118

O�H 3 3 3O 1.93 2.716(5) 162

1.83 2.637(5) 168

O�H 3 3 3Oa 1.69 2.509(6) 174

3 N�H 3 3 3O 2.08(4) 2.834(5) 152(4)

1.84(4) 2.791(5) 161(4)

O�H 3 3 3O 1.63(4) 2.650(4) 170(4)

1.57(4) 2.619(4) 170(3)

1.98(5) 2.763(5) 168(5)

O�H 3 3 3Oa 1.82(4) 2.548(4) 172(5)

4 N�H 3 3 3O 1.88(3) 2.808(4) 163(3)

O�H 3 3 3O 1.62(4) 2.612(3) 169(2)

1.78(4) 2.604(4) 171(3)

O�H 3 3 3Oa 1.67(4) 2.572(4) 171(2)

C�H 3 3 3O 2.31 3.004(5) 132

5 N�H 3 3 3O 1.77(2) 2.706(3) 160(2)

O�H 3 3 3O 1.81(3) 2.644(2) 171(3)

1.71(2) 2.628(3) 161(2)

O�H 3 3 3Oa 1.71(2) 2.578(2) 172(2)

C�H 3 3 3O 2.23 3.087(3) 152

6 N�H 3 3 3O 1.91(4) 2.671(6) 154(4)

O�H 3 3 3O 1.75(5) 2.661(5) 169(4)

1.74(5) 2.593(6) 167(4)

O�H 3 3 3Oa 1.60(4) 2.569(5) 172(4)

C�H 3 3 3O 2.54 3.242(6) 133

7 N�H 3 3 3O 2.40(4) 2.976(3) 109(2)

O�H 3 3 3O 1.77(2) 2.706(2) 172(2)

O�H 3 3 3Oa 1.460(18) 2.522(2) 176.1(18)

C�H 3 3 3O 2.42 3.204(3) 141

8 N�H 3 3 3O 2.51(2) 3.019(4) 112.3(19)

O�H 3 3 3O 1.66 2.631(3) 168

O�H 3 3 3Oa 1.59(3) 2.526(3) 179(3)

9 N�H 3 3 3O 1.89 2.744(5) 170

1.95(6) 2.781(5) 153(6)

O�H 3 3 3O 2.661(6)

2.649(4)

2.613(5)

2.624(6)

O�H 3 3 3Oa 2.547(4)

2.525(4)

C�H 3 3 3O 2.30 3.068(7) 139

10 N�H 3 3 3O 2.776(4)

2.736(4)

O�H 3 3 3O 2.601(4)

2.624(3)

2.641(3)

2.655(4)

O�H 3 3 3Oa 2.523(4)

2.556(3)

C�H 3 3 3O 2.44 3.279(5) 151

2.36 3.141(5) 142

11 N�H 3 3 3O 1.92(4) 2.763(5) 163(3)

2.55(4) 3.003(6) 114(3)

O�H 3 3 3Oa 1.51(3) 2.532(3) 175(3)

a Intramolecular.



These studies encouraged us to explore the visual guestrecognition nature of H41�bpe/bpy host systems. Here it isworthmentioning the studies of Stoddart’s group inwhich theCTinteractions between electron-deficient bpe cations and electron-rich aromatics have been extensively used to build mechanicallyinterlocked molecular machines such as rotaxanes andcatenanes.14 Further, Imai et al.7a have developed a two-compo-nent supramolecular host system that consists of 1,10-bis-2-napthol and p-benzoquinone for the selective visual recognitionof aromatic guest molecules via charge transfer π 3 3 3π interac-tions. In their studies, it was efficiently demonstrated that inclu-sion adducts can be obtained not only by conventional crystal-lization but also by simple solid-state dry grinding without use ofany solvents. Here we intend to utilize the aromatic guestrecognition ability of H2bpy or H2bpe cations in their complexeswith H41 via cation�π CT interactions. These host systemshave shown the capability of including several aromatic guestmolecules such as phenol, o-cresol,m-cresol, p-cresol, chlorobenzene,anthracene, phenanthrene, and pyrene.

’RESULTS AND DISCUSSION

The compound H41 was prepared by catalytic oxidation ofpyrene.15 In order to explore the guest inclusion behavior of

complexes of H41with bpe/bpy, the reactions of H41with bpe orbpy have been carried out in the presence of excess aromaticguest molecules (solid or liquid form) in MeOH or CH3CN.In the case of bpe, single crystals suitable for X-ray diffrac-tion studies were obtained for the complexes [H31]2[H2bpe] 3 -anthracene 3MeOH(2), [H31]2[H2bpe] 3 phenanthrene 3MeOH-(3), and [H31]2[H2bpe] 3 pyrene (4). In the case of bpy, singlecrystals are obtained for the complexes [H31]2[H2bpy] 3 phen-anthrene (5), [H31]2[H2bpy] 3 pyrene (6), [H31]2[H2bpy] 3 -phenol 3H2O (7), [H31]2[H2bpy] 3 o-cresol 3H2O (8), [H31]2-[H2bpy] 3m-cresol 3H2O (9), [H31]2[H2bpy] 3 p-cresol 3H2O (10),and [H31]2[H2bpy] 3Cl-benzene (11). Except the crystals of 11,all the crystals exhibited different colors depending on the typeof guest molecule present. It is interesting to note here that the[H31]2[H2bpe] host exhibits the capability to include onlysolid guest molecules, while the [H31]2[H2bpy] host systemhas shown more versatility for the inclusion of both solid andliquid guest molecules. All the crystal structures have beencharacterized and analyzed for understanding the aggregationsof ions and inclusion of guest molecules. The charge transfer(CT) properties of these materials were further examinedby solid-state diffuse reflectance spectra (DRS). The crystal-lographic parameters for all the crystal structures are given

Figure 1. Illustrations for the crystal structures of 2 and 3: (a) one-dimensional chain (chain B) of H31 units through O�H 3 3 3O hydrogen bonds;(b) corrugated layer formed by chains via C�H 3 3 3O and aromatic interactions; (c) joining of anionic layers by H2bpe units via synthon-II to form3D-network, guests colored orange; (d,e) cation�π interactions between (d) H2bpe and anthracene and (e) H2bpe and phenanthrene.



in Table 1, and hydrogen-bonding parameters are given inTable 2.H41 and bpe Hosts. All three complexes (2�4) exhibited

similar composition of H41, bpe, and guest. The geometry ofthe H31 moiety is also similar in all three cases because itforms intramolecular hydrogen bonds between the �COO�

and �COOH groups. However, the crystal structures are notisostructural because some important differences exist betweenthem. The crystal structures of 2 and 3 also contain a moleculeof MeOH in addition to one aromatic guest per host, while 4contains exclusively pyrene. However, the structures of 2 and 3also differ from each other in terms of asymmetric unit contents:3 contains two times the components of 2. As a result, theanthracene and bpe sit on an inversion center in the case of 2. Inall three cases, the monoanions assemble into one-dimensionalhydrogen bonding chains (A and B), but the hydrogen bondingpattern of 4 differs from that of 2 and 3. In 4, the anions exhibitthree connections to form a one-dimensional chain (chain A):one synthon-I and two charge-assisted O�H 3 3 3O hydrogenbonds, �COOH 3 3 3

�OOC� (Figure 2a). The anions in 2 and3 assemble via four O�H 3 3 3O hydrogen bonds and do notexhibit synthon-I to form chain B (Figure 1a). In both cases, thechains have zigzag geometry and further assemble into highlycorrugated two-dimensional layers via C�H 3 3 3O and aromaticinteractions (Figures 1b and 2b). These layers are connected into

three-dimensional networks by H2bpe units via synthon-II andother hydrogen-bonding interactions (Figures 1c and 2c). TheH2bpe units can best be described as pillars between the layers.The aromatic guests are sandwiched via cation�π interactionsbetween the pillars along the a-axis in all three cases (Figures 1d,eand 2d). Therefore, half of the length of the a-axis represents theplane to plane distance between H2bpe and aromatic guests(8.154 Å in 2 and 3 and 8.020 Å in 4). The distances between thecentroids of aromatic rings and N+ pyridinium are much shorterthan the distances between the planes of guest and pyridiniumions: 3.574, 3.452, and 3.281 Å in 2, 3, and 4 respectively. Thecrystals of complexes 2, 3, and 4 exhibit dark-red, orange, and

Figure 2. Illustrations for the crystal structure of 4: (a) one-dimensional chain (chain A) of H31 units through O�H 3 3 3O� hydrogen bonds and

synthon-I; (b) corrugated layer formed by the chains via C�H 3 3 3Ohydrogen bonds and aromatic interactions; (c) joining of anionic layers into a three-dimensional network by H2bpe units via synthon-II, guests colored green; (d) cation�π interaction of pyrene with H2bpe.

Figure 3. DRS for 2 (green line), 3 (red line), and 4 (black line).



bright red colors, respectively. The solid-state diffuse reflectancespectra (DRS) for the samples 2�4 indicate different absorptionedges justifying the different colors observed (Figure 3). Theabsorption edges of 2, 3, and 4 are located at around 506, 400,and 487 nm, respectively.H41 and bpy Hosts. Previously, we have reported that the

reactions of H41 with bpy in MeOH, t-butanol, and 2-propanolresult in single crystals of salts [H31][Hbpy] 3MeOH, [H31]-[Hbipy] 3 2H2O, and [H31]4[Hbpy]2[H2bpy] 3 4H2O, respec-tively.13 All three structures exhibited different compositions ofthe components and solvents. Change of concentration or

component ratios was found to have no effect on the formationof these crystals. These results and above-described results withbpe encouraged us to study these complexation reactions furtherin the presence of different aromatic guest molecules. Unlikethe above-described [H31]2[H2bpe] hosts, the [H31]2[H2bpy]hosts were found to include both solid and liquid guest moleculesto yield crystals of complexes 5�11.Solid Guest Molecules. Although several reactions were tried

in the presence of different solid guest molecules, single crystalssuitable for X-ray diffraction studies were obtained only in case ofphenanthrene (5) and pyrene (6). It was found that in both cases,one of the four�COOH groups was deprotonated similar to theone observed in 2�4. The crystal structures of 5 and 6 areisostructural, but they differ from those of complexes 2�4. Theanions assemble into a corrugated 2D-layer via synthon-I andother O�H 3 3 3O hydrogen bonds (Figure 4a). In the 2D-layer,each anion exhibits three-connectivity, one through synthon-Iand two through�COO�

3 3 3HOOC� charge-assisted hydrogenbonds. These layers are connected by H2bpy units via synthon-II(Figure 4c). Similar to the complexes 2�4, the phenanthrene andpyrene guest molecules are sandwiched between H2bpy unitsalong the b-axis (Figure 4d,e). The plane to plane distances ofguest molecules and H2bpy units are 3.997 and 4.103 Å in 5and 6, respectively. The crystals that include phenanthrene (5)exhibit a yellowish green color, which is different from the color ofcrystal 3, which also contains phenanthrene. The crystals includ-ing pyrene (6) exhibit dark red color similar to those of 2, whichcontains anthracene. The distances between the cationic N-atom

Figure 4. Illustrations for the crystal structures of 5 and 6: (a) assembling of anions (H31) to form a corrugated layer via synthon-I and otherO�H 3 3 3O hydrogen bonds; (b) side view of the layer; (c) three-dimensional network via joining of anionic layers by H2bpy using charge-assistedN�H 3 3 3Ohydrogen bonds (synthon-II); (d,e) cation�π interactions (guests colored violet) between (d) H2bpy and phenanthrene (disordered) and(e) H2bpy and pyrene.

Figure 5. DRS for 5 (red line) and 6 (black line).



of H2bpy and the centroid of the closest C6-ring of the aromaticguest molecules are 3.401 and 3.327 Å in 5 and 6, respectively.The solid-state DRS spectra indicate that the absorption edgesfor 5 and 6 are located at around 400 and 487 nm, respectively(Figure 5).Liquids (Phenols and Isomeric Cresols) as Guest Mol-

ecules. The host system of [H31]2[H2bpy] included phenol,o-cresol, m-cresol, and p-cresol to yield single crystals of com-plexes 7, 8, 9, and 10, respectively. Interestingly all were found toinclude one each of guest andH2Omolecules per host. However,the crystal structures differ significantly. The hydrogen bondingpatterns of host components in 7 and 8 (Figure 6) are similar tothose observed in [H31]2[H2bpe] host with anthracene (2) orphenanthrene (3) guest molecules. It is interesting to note herethat 2 and 3 also contain onemolecule ofMeOH each in additionto the aromatic guest molecule. The smaller size of bpy comparedwith bpe accounts for the accommodation of smaller aromaticssuch as phenol and o-cresol in 7 and 8 although they have similarhydrogen bonding patterns to 2 and 3 (chain B) (Figure 6a). Theguest molecules in 7 and 8 are heavily disordered.The crystal structures of complexes 9 and 10 are isostructural

with that of 4. Both 9 and 10 contain a one-dimensional chain Asimilar to the one observed in 4 (Figure 7a). The p-cresol in 10exhibits disorder, while m-cresol in 9 exhibits perfect order. Allfour structures form anionic layers, which are pillared by H2bpyunits viaN�H 3 3 3OandC�H 3 3 3Ohydrogen bonds (Figures 6cand 7c). The phenolic guests are sandwiched via cation�πinteractions between the pillars along the a-axis in 7 and 8 andalong the c-axis in 9 and 10 (Figures 6d,e and 7d,e). Therefore,half of the length of the a-axis represents the plane to planedistance between H2bpe and aromatic guests in 7 (8.144 Å) and

8 (8.103 Å), while that distance is one fourth of the c-axis in9 (14.831 Å) and 10 (14.895 Å). The crystals of complexes 7 and8 exhibit a similar yellow color, while those of 9 and 10 exhibitdark yellow and dark red colors, respectively.The solid-state diffuse reflectance spectra (DRS) for samples

7�10 show absorption edges of 375, 395, 405, and 415 nm,respectively (Figure 8). It is interesting to note here that the[H31]2[H2bpy] host also includes a chlorobenzene guest mole-cule to form crystals of 11. The crystal structure of 11 was foundto be isostructural with those of 9 and 10, but the crystals werefound to be colorless; this could be due to the presence of a highlyelectronegative atom such as Cl, which makes the ring electron-deficient thereby reducing the cation�π charge transfer interac-tions. Accordingly, the DRS spectrum of 11 shows no absorptionedge in the visible region (Figure 8). The chlorbenzene moleculein 11 exhibits disorder, which has been successfully modeled.Importance of H41 in Obtaining Cation�π Interactions in

Solid-State Complexes. In order to establish the role of organicanion, similar reactions have been carried out by taking HCl inplace of H41. All such reactions initially resulted in coloredsolutions that have the similar shades to the corresponding solidsthat are observed with H41 reactions. However, all these reac-tions were found to produce colorless crystals that correspond toHCl salts of bpe/bpy or guest molecules. These reactions clearlyindicate the importance of H41 for the inclusion of guestmolecules in the solid state and therefore retaining cation�πinteractions in the solid state.Mechanochemical Grinding and Precipitation Reactions.

In recent days, mechanochemical dry or wet grinding has beenfrequently used to prepare cocrystals or organic salts.16 Howeverin the present study, the dry grinding of the H41, bpe/bpy, and

Figure 6. Illustrations for the crystal structure of 7 and 8: (a) one-dimensional chain (chain B) of H31 units through O�H 3 3 3O hydrogen bonds;(b) corrugated two-dimensional layers via C�H 3 3 3O and aromatic interactions; (c) the two-dimensional anionic layers are connected into a three-dimensional network by H2bpy units via synthon-II, guest colored pink; (d,e) cation�π interactions between (d) H2bpy and o-cresol (disordered) and(e) H2bpy and phenol (disordered).



aromatic guest molecule was found to be ineffective in producingany coloredmaterials. Similarly, solvent drop assisted grinding wasalso found to be ineffective in producing similar colored materialsas the conventional crystallization reactions. Further, repetition ofthe reactions in the concentrated solutions was found to producesimilar colored materials as precipitates within a short period.

Through this type of precipitation reaction, we have exploredsome more guest molecules such as naphthalene, biphenyl, R-naphthol, β-naphthol, 9-anthryl alcohol, 9-anthraldehyde, pery-lene, resorcinol, phloroglucinol, and hydroquinone. The informa-tion about these reactions and colors of thesematerials are given inthe Supporting Information.

’CONCLUSIONS

Two new supramolecular two-component organic host sys-tems, [H31]2[H2bpe] and [H31]2[H2bpy], with the capability toinclude aromatic guest molecules have been developed. Themixing of three colorless components was found to generate

Figure 7. Illustrations for the crystal structures of 9 and 10: (a) one-dimensional chain (chain A) of H31 units through O�H 3 3 3O hydrogen bond andsynthon-I; (b) corrugated two-dimensional layer formed by the chains via C�H 3 3 3O and aromatic interactions; (c) joining of anionic layers by H2bpyunits via synthon-II to form a 3D-network, guests colored pink; (d,e) cation�π interactions between (d) H2bpy and p-cresol (disordered) and (e)H2bpy and m-cresol.

Figure 8. DRS for 7 (black line), 8 (green line), 9 (blue line), 10 (redline), and 11 (pink line).

Scheme 1. Model of Disorder for Phenathrene in 5 andChlorobenzene in 11



various colored crystals due to the formation cation�π interac-tions between the pyridinium ions and aromatic guest molecules.Crystal structure analyses revealed that [H31] can self-assemblein three ways via strong hydrogen bonding: two types of zigzagchains and one two-dimensional layer. The geometry of theanion was found to be similar in all the structures analyzed. Thesynthons I and II were found to play an important role in theaggregation besides charge-assisted O�H 3 3 3O hydrogen bonds.The N+ to centroid of the guest molecules varies from 3.281 to3.854 Å, while the absorption edge varies from 375 to 506 nm.Further, the [H31]2[H2bpe] host system was found to includeonly solid aromatic guest molecules, while the [H31]2[H2bpy]host system was found to include both solid and liquid aromaticguests, in particular those containing phenolic moieties. Thedifferent colors observed in these complexes indicate that thesehosts may serve as colorimetric indicators for various aromaticmoieties in the solid state.

’EXPERIMENTAL SECTION

FTIR spectra were recorded with a Perkin-Elmer instrument, Spec-trum Rx, serial no. 73713. Powder XRD data were recorded with aPHILIPSHolland PW-171 defractometer. The diffuse reflectance spectra(DRS) of the inclusion crystals were recorded with a Cary model 5000UV�visible�NIR spectrophotometer. Melting point measurementswere carried out using Fisher Scientific instrument Cat. No. 12-144-1.Preparation of Crystals of Materials 2�11. [H31]2[H2bpe] 3 -

Anthracene 3MeOH, 2.H41 (0.01 g, 0.030mmol), bpe (0.011 g, 0.060mmol),and 3 equiv of anthracene (0.016 g) were dissolved in methanol (8 mL)with warming. The resultant solution was allowed to evaporate slowly atroom temperature; deep red block-like single crystals suitable for X-raydiffraction were obtained in about 65�70% yield within 2 days. Thecrystals were separated from the mother liquor by filtration, washed withhexane, and dried under vacuum, Mp > 300 �C. Elemental analysis (%)calcd for C59H44N2O17: C, 67.30; H, 4.21; N, 2.66. Found: C, 66.96; H,3.89; N, 2.50. Similar procedure was employed for the preparation ofcrystals 3�6 by taking the corresponding components.[H31]2[H2bpe] 3 Phenanthrene 3MeOH, 3. Yield 60�65%, Mp >

300 �C. Elemental analysis (%) calcd for C59H44N2O17: C, 67.30; H,4.21; N, 2.66. Found: C, 66.96; H, 4.06; N, 2.78.[H31]2[H2bpe] 3 Pyrene, 4. Yield 65�70%, Mp > 300 �C. Elemental

analysis (%) calcd for C60H40N2O16: C, 68.96; H, 3.86; N, 2.68. Found:C, 68.75; H, 3.77; N, 2.84.[H31]2[H2bpy] 3 phenanthrene, 5. Yield 65�70%, Mp > 300 �C.

Elemental analysis (%) calcd for C56H38N2O16: C, 67.60; H, 3.85; N,2.82. Found: C, 67.24; H, 3.69; N, 2.84.[H31]2[H2bpy] 3 Pyrene, 6. Yield 65�70%, Mp > 300 �C. Elemental

analysis (%) calcd for C58H38N2O16: C, 68.37; H, 3.76; N, 2.75. Found:C, 67.56; H, 3.28; N, 2.89.[H31]2[H2bpy] 3 Phenol 3H2O, 7. H41 (0.01 g, 0.030 mmol) and bpy

(0.0094 g, 0.060 mmol) were dissolved in acetonitrile/methanol(3:1, 4 mL) with warming, then phenol (1 mL) was added and mixedwell. The resultant solution was allowed to evaporate slowly at roomtemperature, light green prismatic like single crystals were formedovernight in about 55�60%, and the crystals were filtered, washed witha little hexane, and dried under vacuum.Mp > 300 �C. Elemental analysis(%) calcd for C48H36N2O18: C, 62.07;H, 3.91; N, 3.02. Found: C, 61.29;H, 3.45; N, 2.84. A similar procedure was employed for the preparationof crystals 8�11 by taking the corresponding components.[H31]2[H2bpy] 3 o-Cresol 3H2O, 8. Yield 55�60% Mp > 300 �C.

Elemental analysis (%) calcd for C49H38N2O18: C, 62.42; H, 4.06; N,2.97. Found: C, 61.84; H, 3.71; N, 3.15.

[H31]2[H2bpy] 3m-Cresol 3H2O, 9. Yield 45�50%, Mp > 300 �C.Elemental analysis (%) calcd for C49H38N2O18: C, 62.42; H, 4.06; N,2.97. Found: C, 61.84; H, 3.71; N, 3.15.

[H31]2[H2bpy] 3 p-Cresol 3H2O, 10. Yield 45�50%, Mp > 300 �C.Elemental analysis (%) calcd for C49H38N2O18: C, 62.42; H, 4.06; N,2.97. Found: C, 61.84; H, 3.71; N, 3.15.

[H31]2[H2bpy] 3 Cl-benzene, 11. Yield 45�50%, Mp > 300 �C. Ele-mental analysis (%) calcd for C48H33ClN2O16: C, 62.04; H, 3.58; N,3.01. Found: C, 61.56; H, 2.99; N, 2.85.Crystal Structure Determination. All of the single-crystal data

was collected on a Bruker-APEX-II CCD X-ray diffractometer that usesgraphite monochromated Mo KR radiation (μ = 0.71073 Å) at roomtemperature (293 K) by the hemisphere method. The structures weresolved by direct methods and refined by least-squares methods on F2

using SHELX-97.17 Non-hydrogen atoms were refined anisotropically,and hydrogen atoms were fixed at calculated positions and refined usinga riding model. The H-atoms attached to O- or N-atoms are locatedwherever possible and refined using the ridging model. Pertinent crystal-lographic details are given in Table 1, and hydrogen bonding parametersare given in Table 2. The MeOH molecules in 2 and 3 could not belocated, and aromatic guests as well as water molecules in 7, 8, and 10could not be modeled and refined. Therefore Platon squeeze option wasused in the final refinement of these structures.18 The guest-occupiedvolumes in the crystal structures of 2�11 are 30.5%, 30.3%, 28.2%,26.1%, 27.6%, 22.2%, 22%, 20.9%, 21.2%, and 21.3%, respectively, of thecrystal volumes. The disorder of phenanthrene guest in 5 and disorder ofchlorobenzene in 11 were modeled as shown in Scheme 1.

’ASSOCIATED CONTENT

bS Supporting Information. IR-spectra, X-ray powder pat-terns, information on colored solutions obtained with HCl with-outH41, information for the formation of colored complexes withother guest molecules via precipitation reactions, and crystal-lographic information for complexes 2�11 (CIF). This materialis available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Fax:+91-3222-282252.Tel: +91-3222-283346.

’ACKNOWLEDGMENT

We gratefully acknowledge financial support from DST andDST-FIST for the single-crystal X-ray facility. S.R. thanks IIT-KGP for a research fellowship.

’REFERENCES

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