Tuning Emission Properties via Aromatic Guest Inclusion in Organic

Dec 29, 2017 - An organic salt composed of 4,4'-dinitro-2,2',6,6'-tetracarboxybiphenyl and acridine has been shown to act as host to ac-commodate larg...
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Tuning Emission Properties via Aromatic Guest Inclusion in Organic Salts Composed of 4,4’-dinitro-2,2’,6,6’-tetracarboxybiphenyl and Acridine Abhijit Garai, Subhrajit Mukherjee, Samit K. Ray, and Kumar Biradha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01530 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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Crystal Growth & Design

Tuning Emission Properties via Aromatic Guest Inclusion in Organic Salts Composed of 4,4’-dinitro-2,2’,6,6’-tetracarboxybiphenyl and Acridine Abhijit Garai,† Subhrajit Mukherjee,‡ Samit K. Ray,§ and Kumar Biradha*,† †



Department of Chemistry and Advanced Technology Development Centre, Indian Institute of Technology, Kharagpur 721302, India. §

Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Kolkata-700106, India, and Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721302, India. KEYWORDS. Host-guest complexes, Aromatic stacks, Charge-transfer interactions, Photoluminescence ABSTRACT: An organic salt composed of 4,4’-dinitro-2,2’,6,6’-tetracarboxybiphenyl and acridine has been shown to act as host to accommodate large poly-aromatic guest molecules. Carboxylate-pyridinium hydrogen bonds, π-π and cation···π interactions are found to play pivotal role in these host-guest complexes. All the complexes are found to exhibit distinct colors and photoluminescence which correlate with the charge-transfer interactions between acridinium ion and guest molecules, and ionization potential or donor ability of the guest molecules.

Cocrystals and salts are composed of two or more organic molecules and are indispensable part of crystal engineering and supramolecular chemistry1-2 with their extensive applications in pharmaceuticals,3-5 solid state reactions,6-10 sensors,11 optoelectronics,12-16 optical waveguides17 etc. Although the prediction of the crystal structures and resultant properties of these materials is difficult, a considerable progress has been made in rationalizing their crystal structures in terms of hydrogen bonded supramolecular synthons.18-26 In case of molecular complexes containing acid and pyridine groups, the prediction of salt or cocrystal formation even more difficult as often the result is dependent on several factors such as pKa values of acidbase, concentration of solution, the process of crystallization, temperature, supramolecular synthon variation etc.27-31 The inclusion of aromatic guest molecules by acidpyridine cocrystals or salts is of interest given their modularity and ease of formation. Several two-component host systems have been explored using polycarboxylic acids and polypyridine molecular components.32-38 The hosts can generate large honeycomb, brickwall and diamondoid networks with a capability for including polyaromatics in their cavities/channels.37,39 In addition to acid-pyridine hydrogen bond, the guest included cocrystals/salts are useful for the exploration of photo-physical properties as there exist extended cation···π and π-π interactions.40 Accordingly, the cocrystal or salt based materials were shown to exhibit coformer or guest dependent tunable emission properties.41-50 The luminescence properties of the materials can be fine tuned by altering the extent of donor-acceptor

interactions such as cation···π and π-π interactions. This can be accomplished by systematically selecting the polyaromatic guest molecules with varied ionization potential or donor ability.51-53 Yan et al. have shown that optical properties of stilbene-type organic solid-state materials can be tuned using a cocrystal strategy.54 Imai et al. developed a two-component supramolecular host system composed of 1,1’-bis-2-napthol and p-benzoquinone for the selective visual recognition of aromatic guest molecules via charge transfer interactions.55 Ono et al. have demonstrated that electron donor-acceptor interactions effectively form multicomponent crystals which can show emission color variation.56 From our group, it was shown earlier that tetra carboxylic acid, namely 2,2’,6,6’-tetracarboxybiphenyl (tcbp), with di-basic N-heterocycles form supramolecular host systems which have a potential to include aromatic guest molecules via cation···π interactions.57 These systems were found to act as colorimetric indicators for the visual recognition of the aromatic molecules.58 The inclusion of electron rich aromatics resulted in the darker colors with different shades reflecting their extent of electron transfer while inclusion of electron deficient molecules resulted in the colorless materials indicating poor electron transfer. Further, tcbp with dipyridyl ligand containing cross conjugation was shown to form cation···π complex that exhibits photochromism and photoswitchable semiconducting behavior.59 The above results encouraged us to consider 4,4’dinitro-2,2’,6,6’-tetracarboxybiphenyl (H4A) in place of tcbp as it is more electron deficient due to the presence of

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–NO2 group and also ensures the formation of salts with pyridine containing molecules as it is more acidic than tcbp. The basic ligand considered for the complexation is acridine (acr) as it is more elctron rich mono-basic ligand. Polycyclic aromatic molecules such as anthracene (ant), phenanthrene (phe) and 9-anthraldehyde (9-ald) are considered in the complexation reactions for guest inclusion studies. These studies resulted in the crystals of four organic salts with distinct structures and photoluminescence properties which are correlated to the ionization potential of the guest molecules and extent of chargetransfer/cation-π interactions between the host and guest components.

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COO−···HOOC hydrogen bonds (2.6255(2) Å). Within the chain, the molecules are related by translational symmetry. The acridinium ions are attached on both sides of 1D-networks [H2A]n via N+−H···−OOC hydrogen bonds (2.606(2) Å) (Figure 1a,b). The 1D-chains with pendents lead to the formation of 2D-layers via cation-π and π···π interactions between acridinium ions of adjacent chains (Figure 1c). The H2A dianions are also found to form 2D layer along ab-plane via weak O···N interactions (O···N: 3.006 Å) between two nitro groups of two H2A units (Figure 1d). (a)

(b)

Scheme1. Structures of Host (H4A and acr) and Guest Molecules (ant, phe and 9-ald) in the complexes 1-4 NO2

[H2A][Hacr]2, 1

[H2A][Hacr]2[phe], 3

HOOC

COOH

HOOC

COOH

H4A

phe CHO

(c)

(d)

NO2

ant N

[H2A][Hacr]2[ant], 2

acr

9-ald [H2A][Hacr]2[9-ald]2, 4

The compound H4A was synthesized by nitration reaction of 2,2’,6,6’-biphenyltetracarboxylic acid using HNO3 and H2SO4. Slow evaporation of methanolic solution (4 mL) of H4A (10.50 mg, 1eq) and acridine (18.00 mg, 4 eq) resulted in the formation of the single crystals of organic salt [H2A][Hacr]2, 1. The repeat of the same reaction in the presence of aromatic guest molecules such as anthracene, phenanthrene and 9-anthraldehyde (2 eq) in MeOHCHCl3 (4 mL MeOH and 2mL CHCl3) solvent mixture resulted in single crystals of complexes [H2A][Hacr]2[ant], 2, [H2A][Hacr]2[phe], 3, and [H2A][Hacr]2[9-ald]2, 4, respectively. The single crystals of complexes 1-4 exhibit greenish yellow, red, yellow and orange colors respectively. We would like to note here that the single crystals suitable for X-ray diffraction were not obtained in the presence of other solid aromatic guest molecules, despite of several trials. Further, the solvent (MeOH) assisted mechanochemical grinding was found to produce complexes 1, 3 and 4 but not the complex 2 (Figure S5-S8). The photophysical properties of the complexes 1-4 have been explored by diffuse reflectance spectroscopy and fluorescence spectroscopy. The complex 1 crystallizes in Pbcn space group and asymmetric unit is constituted by a half unit of H2A and one unit of acridinium ion. In this complex, two phenyl rings of H2A exhibit a torsion of 84.70° and interestingly, the two deprotonated–COOH groups come from the same phenyl group due to inherent symmetry of H2A. The C-O distances (1.250(2) Å) of deprotonated –COOH are found to differ significantly from those of –COOH (1.306(2) and 1.203(2) Å). The H2A units form onedimensional network of [H2A]n along a-axis via

Figure 1. Illustrations for the crystal structure of 1 : (a) pendent acridinium ions are connected with the hydrogen bonded 1D-chains of H2A; (b) top view of the 1D-chains of H2A with hanging acridinium ions; (c) formation of 2D-layers via acridinium···acridinium stacking interactions; (d) nitro-nitro interactions between H2A dianions.

Complexes 2 and 3 crystallize in P-1 space group and asymmetric units of these two complexes consist of one unit each of H2A and corresponding guest, and two units of Hacr. In H2A, torsion angle between two phenyl rings is 72.39° and similar to complex 1, two –COOH groups are deprotonated from the same phenyl ring. The hydrogen bonding (-COOH···-OOC-) (2.575(3) Å and 2.550(2) Å for 2 and 2.567(2) Å and 2.528(2) Å for 3) is also same as the one that is observed in 1 albeit with a difference in the symmetry of the chain. The acridinium ions are attached to the 1D chains of H2A as pendents via N+−H···−OOC hydrogen bonds (2.647(3), 2.784(3) Å for 2, and 2.659(2), 2.766(2) Å for 3) (Figure 2a,b). The molecules in the 1Dchain are related to each other by inversion center and it propagates along c-axis. The anions in 2 and 3 are spaced with somewhat longer distances (7.276, 7.831 Å for 2 and 7.144, 7.724 Å for 3) compared to those in 1 (6.879 Å). Overall [H2A.Hacr] can be seen as a pseudo tetrahedral node capable of forming two π···π interactions (arm length: 12.757, 12.886 Å for 2 and 13.007, 13.020 Å for 3) and two charge assisted hydrogen bonds (arm length: 7.276, 7.831 Å for 2 and 7.144, 7.724 Å for 3) to lead to the formation of diamondoid network with two distinctive

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Crystal Growth & Design interactions (Figure 2c,d,e). The voids in the network are filled by the anthracene/phenathracene molecules via cation-π and π···π interactions. Complex 4 crystallizes in P21/c space group and its asymmetric unit is constituted by one unit of H2A and two units each of Hacr and 9-ald. The torsion angle between two phenyl rings of H2A is 89.36° which indicates that two phenyl rings remain almost perpendicular to each other unlike in the complexes 1-3. (a)

(b)

(c) (e)

From crystal structure analyses of the complexes 1-4, it is observed that there are three types of stacks possible: infinite stacks of acridinium ions (1), stacks with sandwiching of guest molecules between acridinium dimers (2 and 3), stacks with alternate arrangement of acridinium and guest molecules (4) (Figure 4a,b,c,d). In 2 and 3, the cation dimer was formed via π···π interactions between peripheral rings, cation-π and π···π interactions between central rings. The sandwiching of anthracene or phenanthrene by the cationic dimer also occurs through the combination of cation-π and π···π interactions. The distances between cationic N-atom of acridine and centroids of closest C6 of anthracene/9-anthraldehyde are in the range of 3.726 to 3.996 Å in 2 and 4, while the corresponding distance is found to be much shorter (3.554 Å) in 3. In case of π-π interactions, the centroid to centroid distances found to be in the range of 3.630 to 4.039 Å in all three complexes indicating strong π···π interactions (Figure S10S12). (a)

(b)

(d)

(c) (d)

Figure 2. Illustrations for the crystal structure of 2: (a) pendent acridinium ions are connected with the hydrogen bonded 1D-chains of H2A; (b) top view of the self-assembled 1Dchains of H2A with hanging acridinium ions; (c) pseudo tetrahedral node with two arms for COOH··· OOC hydrogen bonds and two arms for stacking interactions between two acridinium ions which are hydrogen-bonded with H2A units; (d) portion of diamondoid network with depiction of interactions involved; (e) diamondoid network showing the node (centroid of C-C of biphenyl) and node connections (hydrogen bonds and π···π interactions).

Further, in the complexes 2 and 3, the host-guest ratio is 1:1 while it is 1:2 in 4. However, complex 4 also exhibits similar 1D-chains via -COOH···-OOC- charge-assisted hydrogen bonding (2.519(2) Å and 2.555(2) Å) with acridinium ions as pendents (N-H···O: 2.621(2) and 2.633(2) Å) (Figure 3a,b). The acridinum ions of the 1D-chains do not have self-stacking interactions with neighboring chains, like the ones observed in 1-3, as it contains two equivalents of 9-ald. As a result, the 9-ald units join the adjacent chains to form a 3D-network via cation-π and π···π interactions with acridinium ions (Figure 3c,d).

Figure 3. Illustrations for the crystal structures of 4: (a) pendent acridinium ions connected with the hydrogen bonded 1D-chains of H2A; (b) top view of the 1D-chains of H2A with hanging acridinium ions; (c) view of a crystal lattice along aaxis; (d) 2D-layer of columnar stacked Hacr with 9-ald guest molecules.

The presence of these varied stacks with differences in their colors of the crystals prompted us to investigate their photophysical properties. Multicomponent crystals exhibit enhanced optoelectronic properties which cannot be exhibited by their individual components. Distinct colors of the guest-included complexes 2 (red), 3 (yellow) and 4 (orange) indicate that the host complex 1 (greenish yellow) can serve as colorimetric indicator to recognize aromatic guests such as anthracene, phenanthrene and 9anthraldehyde (Figure 5a,b). In order to understand their absorption behaviors, diffuse reflectance spectroscopic (DRS) studies were carried out for the powdered materials of the complexes 1-4. From DRS studies, it has been ob-

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Crystal Growth & Design served that all the complexes exhibit distinct absorption bands (1: 445 nm,2: 545 nm, 3: 451 nm, 4: 496 nm)(Figure 5d). The emission behaviors of these materials 1-4 were explored by solid-state photoluminescence and their lifetime studies. In the emission spectra of 1-4, distinguishable emission bands with λemmax at 534, 646, 561 and 585 nm, respectively, were observed upon excitation with a laser of 380 nm (Figure 5e). From the lifetime measurements, the complex 3 was found to have longer lifetime (τav: 12.68 ns for 3, and 3.83, 5.47 and 5.11 ns for 1, 2 and 4 respectively) than other complexes (Figure 5f and SI table 1). (a)

are found at (0.57, 0.40), (0.47, 0.51) and (0.54, 0.44) respectively which clearly indicate the distinguishable colors (red, yellow and orange) of the guest-included complexes. The biphenylic tetracarboxylicacid molecule H4A with acridine was shown to form single crystals of salt [H2A][Hacr]2 that has 1D-chains with acridinum ions as pendents. These 1D-chains have shown capability for the inclusion of polyaromatic guests via π-π and cation···π interactions with the pendent acridinium ions. (a) (b)

(b) 1

3

2

(c)

4 Normalized Reflectance

(d)

(c) (d)

1.0 0.8 0.6

1 2 3 4

0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm)

Figure 4. Illustrations for the three types of stacking interactions: (a) infinite stacks of acridinium ions in 1; stacks with sandwiching of guest molecules between acridinium dimers (b) in 2 and (c) in 3; (d) stacks with alternate arrangement of acridinium and 9-anthraldehyde in 4.

Observed emission behaviors of the guest-included complexes are found to correlate with extent of charge transfer or cation-π interactions between the guest molecules and acridinium ions. The extent of charge transfer generally depends on the geometry of the stacks of donoracceptors as well as the ability of the donor to donate electrons (ionization potential). Interestingly, the luminescence or emission properties of these complexes are found to linearly correlate with the ionization potentials of guest molecules (SI table 1). The guest with the lowest ionization potential (2, anthracene, 7.44 eV) value is found to exhibit emission at higher wavelength (646 nm) compared to the one with highest IP value (3, phenanthrene, 7.89 eV, 561 nm); whereas the complex 1 without any guest is found to exhibit the emission band at lower wavelengths (534 nm). The emission colors of the complexes 1−4 were analyzed by the Commission Internationale de L’Eclairage (CIE) chromaticity diagram which clearly illustrated their wide range of colors (Figure 5c). The color-coordinate values for the complexes 2, 3 and 4

Normalized Intensity

(e) 1.0

1 2 3 4

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength(nm) (f)

Normalized Intensity

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

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1 2 3 4

1.0

0.8

0.6

0.4

0.2

0.0

0

20

40

60

80

100

Time(ns) Figure 5. Photographs of the complexes 1-4: (a) crystals, (b) grounded samples; (c) CIE coordinates, (d) DRS, (e) emission spectra and (f) fluorescence decay of the complexes 1-4.

The variations in the geometry of 1D-chains lead to the differences in their packing. The packing of the 1D-chains with coplanar pendents observed in 1 lead to the formation of 2D-layers with no cavities. Whereas in 2 and 3, the arrangement of pendents in three-dimensions along the 1D-chain resulted in the diamondoid networks via

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Crystal Growth & Design cation-π and π···π interactions with the cavities for the inclusion of guest molecules. In 4, no interactions between acridinium ions are observed as it includes two equivalents of guest molecules per host [H2A][Hacr]2. The charge transfer interactions between the various π-stacks present in the complexes 1-4 are found to be responsible for the differences in their colors. These materials have a potential to serve as colorimetric indicators for polyaromatics given the differences in the resultant colors depending on the presence of guest molecules. Further, 1-4 are also found to exhibit certain differences in their absorption, luminescence or emission properties. These properties are correlated to the observed structures and donor abilities (ionization potentials) of the guest molecules.

ASSOCIATED CONTENT Supporting Information: Experimental details, FTIR, PXRD, structural analyses, crystallographic table, hydrogen bonding parameters and photoluminescence data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +91-3222282252. Tel.: +91-3222-283346.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge SERB, New Delhi, India, for financial support, DST-FIST for the single crystal X-ray diffractometer, and A.G. acknowledges IIT-KGP for a research fellowship.

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Crystal Growth & Design

Tuning Emission Properties via Aromatic Guest Inclusion in Organic Salts Composed of 4,4’-dinitro-2,2’,6,6’-tetracarboxybiphenyl and Acridine Abhijit Garai, Subhrajit Mukherjee, Samit K. Ray, and Kumar Biradha*

An organic salt composed of 4,4’-dinitro-2,2’,6,6’-tetracarboxybiphenyl and acridine was shown to act as a host system for inclusion of large aromatic guest molecules via the combination of hydrogen bonding and stacking interactions. The host complex and the guest-included complexes exhibit distinguishable colors and photoluminescence properties.

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