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Luminescent Metal−Organic Complexes of Pyrene or Anthracene Chromophores: Energy Transfer Assisted Amplified Exciplex Emission and Al3+ Sensing Ritesh Haldar,† Komal Prasad,† Pralok Kumar Samanta,‡ Swapan Pati,†,‡ and Tapas Kumar Maji*,†,§ †

Molecular Materials Laboratory, New Chemistry Unit, §Chemistry and Physics of Materials Unit, and ‡Theoretical Science Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore, 560064, India S Supporting Information *

ABSTRACT: Using pyrene and anthracene monocarboxylate chromophores two metal−organic complexes, {[Cd(pma)2(o-phen)2]·2H2O·MeOH)}n (1) and {Cd2(μH2O)(amc)4(o-phen)2}n (2) (Hpma = pyrene monocarboxylic acid; Hamc = 9anthracene monocarboxylic acid; o-phen = orthophenanthroline) have been synthesized, respectively and characterized using a single crystal X-ray diffraction study. Compound 1 contains a seven-coordinated Cd2+ center connected by two o-phen and two pma, where one pma and o-phen pair stacks in a face-to-face fashion, and the other pma:o-phen pair is linked through C−H···π interaction. Compound 2 is a dimeric complex of Cd2+ reported previously, and it contains two pairs of face-to-face stacked amc:o-phen. Compound 1 shows a red-shifted bright cyan emission compared to pma monomer emission that can be attributed to pma:o-phen exciplex formation. This exciplex emission is further sensitized by another pma through a Förster resonance energy transfer (FRET) process. Similarly in the case of 2, amc:o-phen exciplex emission is sensitized through FRET from the other amc linker in the solid state, while in methanol such an energy transfer process is perturbed resulting in a dual emission related to a monomer of amc and exciplex of amc:o-phen. Interestingly, the blue emission of 2 dispersed in methanol changes to a bright cyan-green emission upon addition of Al3+ and remains almost unchanged or slightly affected with other metal ions leading to selective chemosensing of Al3+. Moreover, the density functional theory based calculations provide clear evidence of ground state interactions between pma:o-phen and amc:o-phen in 1 and 2, respectively.

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INTRODUCTION The design and synthesis of luminescent metal−organic complexes, polymers, or supramolecular frameworks are gaining interest as they find applications in molecular sensing, tunable emission for various light emitting devices, and light harvesting.1−11 The presence of organic as well as inorganic building blocks makes these hybrids thermally more robust and versatile in terms of functionality. In such hybrids, luminescence may originate from metal ions such as lanthanides (Tb3+, Sm3+, Eu3+, etc.) or from organic chromophoric ligands.12−15 Because of a wide variety of available organic chromphores, ligand based luminescence properties have been widely studied.16−18 Versatile photophysical properties such as charge transfer (CT), exciplex or excimer emission and energy transfer process can be realized with a suitable choice of donor- acceptor pairs in metal−organic system.19−25 Luminescence efficiency (based on CT and exciplex) or energy transfer processes are highly dependent on the spatial disposition of the organic chromophores in a metal−organic structure. As noncovalent interactions (C−H···π or π···π) govern such processes, chromophores with a large aromatic π surface or with an electron rich or deficient nature are more often used to construct such systems. Thus, formed donor−acceptor pairs are also sensitive to the microenvironment and hence can be used as a sensing tool for a targeted analyte.26,27 © XXXX American Chemical Society

Organic chromophores, such as naphthalene, anthracene, pyrene, or perylene, show excellent photophysical properties. These all can form an excited state dimer (excimer), can act as a good donor chromophore to form an exciplex or CT complex, and can show high quantum yield and also a long fluorescence lifetime.28−32 Using these chromphores, particularly with anthracene and pyrene, very few metal−organic complexes have been reported to date.33−36 However, all these reports do not discuss any potential sensing applications, such as for biologically toxic metal ions (Al3+, Cu2+, etc.). Al3+ is one of the most abundant in the biosystem and known for its toxicity.37,38 Diseases such as anemia, speech problems, softening of bone, rickets, colic, and Alzheimer’s can be caused by extreme Al3+ exposure.39−41 Hence, developing an effective method to detect such toxic Al3+ is of paramount importance, and to serve this purpose various molecular probes have been developed.42−47 In most cases, a mechanistic pathway involves a charge transfer, electron transfer, or energy transfer phenomena.42−47 Apart from purely organic based probes, there are no reports of metal−organic hybrid systems for potential Al3+ sensing. We envisioned that sensing applications can be realized by modulating donor−acceptor assembly in metal−organic Received: July 12, 2015

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DOI: 10.1021/acs.cgd.5b01448 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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yellow color rod-shaped crystals of 1 were found at the walls of the tube. A good quality single crystal was picked up from the mother liquor and immediately covered with paraffin oil, and crystal data were collected at 293 K. To prepare the compound in bulk, ligand solution was directly mixed with metal solution and stirred for 48 h. The polycrystalline bulk compound was filtered off, washed several times with methanol, and dried under a vacuum. The PXRD pattern of the bulk powder matches well with simulated PXRD pattern of 1 indicating phase purity (Figure S1). Yield: 67%, relative to Cd2+. Anal. Calcd for C59H42CdN4O7: C, 71.78; H, 5.52; N, 3.22. Found: C, 71.11; H, 5.17; N, 3.01. FT-IR (KBr pellet, 4000−400 cm−1) (Figure S2): 3037(b), 1588(s), 1614(s), 1565(s), 1514(s), 1427(s), 1342(s), 1314(s), 1178(s), 1151(s), 1100(s), 1046(s), 841(s), 787(s), 723(s). Synthesis of {Cd2(μ-H2O)(amc)4(o-phen)2}n (2). 2 is synthesized following similar methodology as used for 1, except Hamc was used instead of pma. Light yellow color crystals were observed after 10−15 days, and bulk powder compound was prepared by direct mixing. PXRD patterns of the bulk polycrystalline compound and simulated patterns match well confirming phase purity of the bulk sample (Figure S3). Yield: 71%, relative to Cd2+. Anal. Calcd for C84H52Cd2N4O9: C, 67.88; H, 3.50; N, 3.77. Found: C, 68.10; H, 3.26; N, 3.36. FT-IR (KBr pellet, 4000−400 cm−1) (Figure S4): 3049(w), 2365(w), 1570(s), 1518(s), 1428(s), 1387(s), 1322(s), 1269(s), 1221(s), 843(s), 725(s), 668(s), 637(s). X-ray Crystallography. X-ray single crystal structural data of 1 was collected on a Bruker Smart-CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube X-ray source with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA. The program SAINT52 was used for integration of diffraction profiles and absorption correction was made with SADABS53 program. The structure was solved by SIR 9254 and refined by full matrix least-square method using SHELXL-97.55 All the hydrogen atoms were geometrically DFIX and placed in ideal positions. All crystallographic and structure refinement data of 1 are summarized in Table 1. Selected bond lengths and angles for 1 are given in Table 2. All calculations were carried out using SHELXL 97,56 PLATON,57 SHELXS 97, and WinGX system, Ver 1.70.01.

complex through binding of a specific metal ion that will lead to a subtle change in photophysical properties. It is worth mentioning that inorganic complexes based on Ru2+/Os2+ are known for sensing applications.48,49 Here, instead of constructing an extended polymeric structure, we sought to synthesize molecular complexes for ease of solution processability. For this purpose, mono carboxy-functional pyrene and anthracene chromophore based linkers, pyrene monocarboxylic acid (Hpma) and 9-anthracene monocarboxylic acid (Hamc), have been chosen. Additionally a chelating ligand with an extended aromatic π-surface orthophenanthroline (o-phen) was selected. Co-assembly of these monocarboxylic linkers (Hamc and Hpma) will not produce extended coordination polymers; rather a molecular complex can be achieved using these linkers. Cd2+ was chosen as a metal ion for its flexible coordination geometry and assembled with pma/amc and o-phen to obtain two metal−organic complexes {[Cd(pma)2(o-phen)2]·2H2O· MeOH)}n (1) and {Cd2(μ-H2O)(amc)4(o-phen)2}n (2). Although 2 has been synthesized previously,50 its photophysical properties have not been explored properly. 1 and 2 both show preassociated exciplex emission of pma:o-phen and amc:o-phen, respectively. Evidence of ground state interactions in pma:ophen and amc:o-phen is supported by density functional theory (DFT) based calculations. Interestingly, the exciplex emissions are sensitized through a Förster resonance energy transfer (FRET) process from pendent pma and amc subunits, in the case of 1 and 2, respectively. Furthermore, compound 2 dispersed in methanol shows a ratiometric fluorescence response for Al3+ among several other metal ions. The selective sensing of Al3+ can be attributed to the changes in spatial arrangement of the pendent amc linker that inhibits the monomer emission.

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EXPERIMENTAL SECTION

Materials. All the reagents employed were commercially available and used as provided without further purification. Cd(NO3)2·4H2O was obtained from Spectrochem, and pyrene monocarboxylic acid and orthophenanthroline were obtained from Sigma-Aldrich chemicals. 9Anthracene monocarboxylic acid was prepared following previously reported methods.51 Physical Measurements. Elemental analyses were carried out using a Thermo Fischer Flash 2000 elemental analyzer. FT-IR spectra were recorded on a Bruker IFS 66v/S spectrophotometer using KBr pellets in the region 4000−400 cm−1. Powder XRD patterns of the products were recorded by using Cu−Kα radiation (Bruker D8 Discover; 40 kV, 30 mA). Electronic absorption spectra were recorded on a PerkinElmer Lambda 900 UV−vis-NIR spectrometer, and emission spectra were recorded on a PerkinElmer Ls 55 luminescence spectrometer. UV−vis and emission spectra were recorded in a 1 mm path length cuvette. Fluorescence decay was recorded in a time correlated single-photon counting spectrometer of Horiba-Jobin Yvon with a 350−450 nm picosecond Ti-saphhire laser. The solution state UV−vis and emission spectra were recorded by dissolving compound 2 in methanol, and methanolic metal solution was added to check the metal ion sensing experiments. FT-IR and EDAX experiments of Al3+ bound compound 2 were performed in the solid state. About 30 mg of compound 2 was immersed in 2 mL of Al3+ methanolic solution for 1 day. The solid compound was filtered off and washed several times with methanol and then dried in a vacuum before the measurements. Synthesis of {[Cd(pma)2(o-phen)2]·2H2O·MeOH)}n (1). An aqueous solution (25 mL) of Hpma (1 mmol, 0.288 g) was mixed with a methanolic solution (25 mL) of o-phen (1 mmol, 0.184 g) and stirred for 30 min to mix well. Cd(NO3)2·4H2O (0.5 mmol, 0.151 g) was dissolved in 50 mL of water, and then ligand solution (2 mL) was layered above the metal solution (2 mL) and after 10−12 days light

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COMPUTATIONAL DETAILS

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RESULTS AND DISCUSSION

Electronic properties of each molecule are calculated with the help of ab initio density functional theory (DFT) combined with timedependent DFT (TD-DFT) as implemented in Gaussian 09 package.58 For the DFT and TD-DFT calculations, we use the 631g(d) basis set for all atoms except for Cd atoms for which LANL2DZ, which uses a widely used effective core potential (ECP)type basis set.59−61 For our calculations, we have considered the X-ray crystallographic structures and optimized only the H atoms position using DFT with B3LYP62−64 exchange and correlation functional. Then the optical properties are calculated using TD-DFT methods using B3LYP62−64 exchange correlation functional and with two different long-range corrected functional, CAM-B3LYP65 and B97XD.66,67 The results are tabulated in Table S1.

Structural Description of {[Cd(pma)2(o-phen)2]·2H2O· MeOH)}n (1) and {Cd2(μ-H2O)(amc)4(o-phen)2}n (2). Compound 1 crystallizes in the triclinic P1̅ space group, and the asymmetric unit contains one Cd2+, two pma, two o-phen, guest water, and methanol molecules. The Cd2+ metal center is seven coordinated; two carboxylate oxygen atoms (O1 and O2) coordinate from one chelating pma (denoted as pma(I)), one oxygen atom (O3) from a carboxylate group of other pma (pma(II)), and the rest of the four coordination sites are occupied by the four nitrogen atoms (N1−N4) from two chelated o-phen (o-phen I and II) ligands (Figure 1). The Cd2+−O and Cd2+−N bond distances are in the range of 2.242(4)−2.569(6) Å and 2.359(3)−2.426(5) Å, respectively B



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Table 1. Crystal Data and Structure Refinement Parameters for Compound 1

a

parameters

1

empirical formula M crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T (K) λ (Mo Kα) Dc (g cm−3) μ (mm−1) θmax (deg) total data unique reflection Rint data [I > 2σ(I)] Ra Rwb GOF residual electron density e−/Å3

C59H42CdN4O7 1031.31 triclinic P1̅ 11.4286(8) 12.8633(10) 17.8410(13) 110.488(4) 104.772(4) 91.007(4) 2359.1(3) 4 293 0.71073 1.441 0.524 28.4 35336 9772 0.080 5708 0.0504 0.1419 0.88 0.735

Figure 1. Coordination environment of Cd2+ in complex 1 shown in a thermal ellipsoid model.

several hydrogen bonding intercations. The pendent free oxygen (O4) of the carboxylate group from pma(II) forms strong hydrogen bonds with guest O1W and O2W (2.741 and 2.959 Å, respectively) (Figure S5a). Several other hydrogen bonds between O3···O2W (2.987 Å), O1···O2W (2.983 Å) and O2···O1W (3.007 Å) hold two monomeric complexes together in the 2D layer (Figure S5b). The methanol molecule is also hydrogen bonded to the guest water O2W (O3W···O2W ≈ 2.724 Å) (Figure S5a). These 2D supramolecular layers stack along the c-direction via pma(I):pma(I) offset stacking (∼4.00 Å), thus forming a 3D supramolecular structure (Figure 3). Crystal structure of compound 2 is reported previously50 but is further elaborated here to highlight the important molecular interactions that play a pivotal role in its higher dimensional assembly and emission property. The asymmetric unit of 2 contains two Cd2+ centers, four amc, two o-phen, and one bridging water molecule (Figure 4a). Each Cd2+ center is in a distorted octahedral geometry; two Cd2+ centers are bridged by oxygen (μ-oxo) atoms from two different amc carboxylate groups and one water molecule to form a dimeric complex (Figure 4a). There are two types of amc ligands marked as amc(I) and amc(II); amc(I) acts as a monodentate ligand, whereas amc(II) acts as a bridging bidentate ligand and binds two Cd2+ through a μ-oxo bridge. Therefore, both amc linkers have one pendent carboxylate oxygen atom. Interestingly, the chelating o-phen and bridging amc(II) stacks in a face-to-face fashion in the dimer, and the monodentate amc(I) does not show any close contact in the dimer except hydrogen bonding between bridging water molecule (O2) and pendent carboxylate oxygen O4 (O2···O4−2.629 Å). One such dimer is connected to the neighboring dimer via several noncovalent interactions, such as the C32−H···π interaction between (∼2.8 Å) amc(II) aromatic hydrogen and amc(I) aromatic ring of other dimer and offset π···π stacking (∼3.56 Å) between o-phen ligands. In addition (C−H···O) hydrogen bonding between carboxylate oxygen (O3) from amc(I) and o-phen (C10−H···O ≈ 2.525 Å) also extends the supramolecular structure (Figure 4b). The π···π stacking and C32−H···π interaction extends the dimeric unit along the a and b axes, while the other C−H···O interaction operates along the c-axis to form a 3D supramolecular structure (Figure S6). Photophysical Properties of 1 and 2: Exciplex Emission and Energy Transfer. The absorption spectrum of 1 in the solid state shows a broad profile ranging from 300 to 425 nm (Figure S7). Excitation of 1 at 350 nm shows a broad featureless emission spectrum with maximum at 450 nm corresponding to cyan emission (Figure 5a). This red-shifted

R = Σ∥F0| − |Fc∥/Σ|F0|. bRw = [Σ{w(F02 − Fc2)2}/ Σ{w(F02)2}]1/2.

Table 2. Selected Bond Lengths (Å) and Angles (°) for {[Cd(pma)2(o-phen)2]·2H2O·MeOH)}n (1) Cd1−O1 Cd1−O3 Cd1−N2 Cd1−N4 O1−Cd1−O2 O1−Cd1−N1 O1−Cd1−N3 O2−Cd1−O3 O2−Cd1−N2 O2−Cd1−N4 O3−Cd1−N2 O3−Cd1−N4 N1−Cd1−N3 N2−Cd1−N3 N3−Cd1−N4

2.569(6) 2.242(4) 2.359(3) 2.426(5) 50.50(17) 155.70(16) 84.60(16) 102.43(15) 83.55(15) 116.46(17) 113.33(12) 86.22(13) 92.98(13) 89.94(12) 69.26(12)

Cd1−O2 Cd1−N1 Cd1−N3 O1−Cd1−O3 O1−Cd1−N2 O1−Cd1−N4 O2−Cd1−N1 O2−Cd1−N3 O3−Cd1−N1 O3−Cd1−N3 N1−Cd1−N2 N1−Cd1−N4 N2−Cd1−N4

2.475(6) 2.403(4) 2.386(3) 84.57(16) 133.85(17) 68.51(18) 153.72(14) 87.42(15) 87.93(13) 155.40(12) 70.18(13) 87.98(15) 149.27(14)

(Table 2). In the molecular complex unidentate pma(II) is stacked in a face-to-face (π···π ≈ 3.547 Å) fashion with ophen(II); the other pma(I) (chelated) undergoes C−H···π interaction (∼2.747 Å) with o-phen(I). Two such complexes are tethered through several supramolecular interactions. Figure 2a shows two neighboring complexes are attached via π···π (3.849 Å) interaction between two o-phen(II) and C−H···π (∼3.00 Å) interactions between o-phen(I) and o-phen(II). On the other hand, one o-phen(II) of one unit and pma(I) of other unit interacts via C−H···π interaction (2.627 Å) (Figure 2b). These C−H···π and π···π interactions operate along the a and b axes to form a 2D supramolecular layer along the ab plane (Figure 2c). The compound also contains noncoordinated solvent water and methanol molecules which are held through C



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Figure 2. Structural details of 1: (a) Two of the monomers interact through π···π and C−H···π interactions; (b) two monomers interact via C−H···π interaction; (c) 2D supramolecular layer formed in the ab plane (yellow part shows the dimeric unit shown in a).

Figure 3. π···π stacked 2D layers (shown in two different colors) of compound 1 to form a 3D supramolecular framework.

spectrum compared to a blue emission of pma monomer in methanol can be attributed to exciplex emission from a face-toface stacked o-phen(II):pma(II) dimer (Figure 5a). Evidently, the face-to-face stacked o-phen(II):pma(II) gives rise to a emissive state. The fluorescence lifetime monitored at 470 nm suggests a high lifetime of ∼3.7 ns compared to that of pma dissolved in methanol (∼1.6 ns) (Figure 5b). This high lifetime further indicates the presence of exciplex emission. The absolute quantum yield of 1 is ∼7.9%. Interestingly, the excitation spectrum monitored at 480 nm shows a high intensity peak at ∼435 nm and a weak intensity peak at ∼340 nm (Figure 5c). The distinct 435 nm peak in the visible region, red-shifted compared to the pma absorption, might be due to ground state interaction between o-phen(II):pma(II) indicating the preassociated nature of the exciplex. To gain insight into this preassociated complex ab initio DFT based calculations combined with TDDFT as implemented in Gaussian 09 program package were carried out for 1. Calculations suggest the presence of three low energy peaks at 314.4, 343.6, and

381.5 nm (Figure 6). The transition at 314.4 nm is located on the C−H···π tethered pma(I), and the other red-shifted transition (343.6 nm) corresponds to pma(II) involved in exciplex formation. These absorption bands are attributed to π−π* transitions, whereas the low energy band at 381.5 nm clearly suggests a ground state interaction between pma(II) and o-phen(II). This evidently indicates the preassociated nature of the exciplex. Surprisingly, any spectroscopic signature of pma(I) remains absent in the emission spectrum. As there are no other quenching paths available, it can be concluded that exciplex emission is sensitized by the pma(I) emission through an energy transfer process. This is also supported by the good overlap between pma (monomer) emission and exciplex excitation spectra, and also the high fluorescence lifetime value observed for the exciplex (Figure 5a). Such sensitization of exciplex emission through energy transfer is a rare observation and yet to be properly studied.68 The photophysical characteristics of 2 in the solid as well as in the solvent dispersed state were examined. The absorption D



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Figure 4. (a) Coordination environment of Cd2+ in dimeric complex 2; (b) view of the packing showing different noncovalent interactions linked two dimers of 2.

Figure 5. (a) Emission spectrum of 1 in the solid state and pma in methanol upon excitation at 350 nm; (b) fluorescence decay profile of 1 in the solid state and pma dissolved in methanol observed at 480 and 430 nm, respectively; (c) excitation spectrum of 1 monitored at 500 nm; all the measurements are carried out at 25 °C.

one observed in the case of 1, although the donor is different. The fluorescence lifetime observed for the exciplex is sufficiently high (∼4.1 ns) compared to monomer amc (∼1.5 ns), indicating amc(II):o-phen excited state dimer formation (Figure 8a). The absolute quantum yield for 2 is found to be ∼8.5%. The excitation spectrum monitored at 510 nm shows three distinct intensities at ∼320, 370, and 390 nm, as has been observed for 1 (Figure 8b). Here also DFT calculations of the

spectrum of 2 in the solid state shows a relatively broad profile with a λmax ≈ 400 nm and a small shoulder at ∼430 nm (Figure S8). Compound 2 when excited at 350 nm shows a broad featureless spectrum with a maximum at 450 nm corresponding to cyan emission which is red-shifted compared to amc monomer emission (∼400 nm) observed in methanol (Figure 7). This characteristic emission feature suggests an exciplex emission attributed to the amc(II):o-phen dimer, similar to the E



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Figure 6. DFT calculations of 1: Frontier molecular orbitals of 1 and corresponding transition wavelengths. HOMO and LUMO are highest occupied molecular orbital and lowest unoccupied orbital, respectively.

complex were carried out in the gas phase, similarly as in the case of 1. Three low energy transitions at 356.7, 333.2, and 323.6 nm are observed; among these three transitions 333.2 and 323.6 nm absorption bands are due to π−π* transitions (Figure 9). The red-shifted transition at 356.7 nm was found to be due to amc(II):o-phen ground state interaction and thus indicates the preassociated nature of exciplex, similar to that observed in 1. In the emission spectrum of 2, no signature of amc(I) was observed. This is probably due to the energy transfer from amc(I) to the exciplex of amc(II):o-phen, and a similar energy transfer process is observed in compound 1. The emission spectrum changes clearly when compound 2 is dispersed in methanol. In addition to the exciplex emission at ∼450 nm, a distinct peak of lower intensity at 416 nm suggests emission from the monomer amc linker (Figure 10a). Such dual emission suggests subtle perturbation in the energy transfer process from pendent amc(I) to the amc(II):o-phen exciplex. As compound 2 contains several uncoordinated and coordinated carboxylate oxygens, this dimeric compound can

Figure 7. Emission spectrum of 2 in solid state and amc in methanol upon excitation at 350 nm; inset: photograph of solid crystalline 2 under UV light; all the measurements are carried out at 25 °C.

Figure 8. (a) Fluorescence decay profile of 2 in solid state (λmon = 470 nm) and amc in methanol (λmon = 410 nm); (b) excitation spectrum of 2 monitored at 510 nm; all the measurements are carried out at 25 °C. F



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Figure 9. DFT calculations of 2: Frontier molecular orbitals of 2 and corresponding transition wavelengths. HOMO and LUMO are highest occupied molecular orbital and lowest unoccupied orbital, respectively.

Figure 10. (a) Emission spectrum of 2 dispersed in methanol upon excitation at 350 nm; (b) excitation spectrum monitored at 550 nm for 2 dispersed in methanol; all the measurements are carried out at 25 °C.

blue color of the solution. Moreover, 2 contain noncoordinated free oxygens from the carboxylate groups, and hence these are available for interaction with different cations. We envisaged that metal ions of suitable size and charge would interact with those pendent oxygens from carboxylate groups, and corresponding emission characteristics might alter. Therefore, the emission properties of 2 upon addition of several metal ions such as Zn2+, Cd2+, Cu2+, Ni2+, Co2+, Fe3+, Mn2+, Hg2+, Cr3+, Mg2+, Ca2+, Pb2+, Li+, Na+, K+, and Eu3+ were studied (Figure 11a,b). After addition of 100 μM of different metal ions to the methanolic solution of 2 (100 μM), the emission band at 416 nm remains unchanged, while slight changes in the intensities of exciplex emission at ∼450 nm were observed (Figure 12a). Therefore, in all cases dual emission characteristics with

be readily solvated by a polar solvent like methanol. This solvation might lead to a rotation of the anthracene ring due to interaction of the carboxylate groups with methanol leading to a change in the dipole direction and thus energy transfer process. The compound can remain dispersed in methanol for many days without disintegrating to individual components as realized from the excitation spectrum monitored at 550 nm and also from exciplex emission which remains almost similar as observed in the solid state (Figure 10b). Emission Properties with Different Metal Ions: Selectivity toward Al3+. The emission properties of compound 1 change drastically in solvents like methanol due to disintegration of the compound (Figure S9), while dispersed compound 2 in methanol shows a dual emission that leads to a G



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Figure 11. (a, b) Emission spectra of 2 (100 μM) in methanol before and after addition of 1 equiv of various metal ions (λex = 350 nm) at 25 °C.

Figure 12. (a) Fluorescence response of 2 upon addition of different metal ions (100 μM): plotted intensities are observed at 411 and 470 nm (λex = 350 nm); (b) emission spectra of 2 with titration of Al3+ (0.1 mL of 6 M Al3+ in methanol) upon excitation at 350 nm; (c) fluorescence response of 2 upon addition of 100 μM Al3+, Ga3+, and In3+ (λex = 350 nm); all the measurements are carried out at 25 °C.

slightly low compared to that in the case of Al3+ (Figure 12c). The I470/I411 vs Al3+ shows a sigmoidal type curve upon titration with Al3+ (0.1 mL of 6 M Al3+ in methanol) in methanol solution of 2 (30 μM), and this observation suggests that both dynamic and static quenching mechanisms operate in this system (Figure S10).69 Interestingly, we observed that after addition of a total 60 μM methanolic solution of Al3+ to a 30 μM methanolic solution of 2, the fluorescence intensity saturates indicating each complex 2 can accommodate a maximum of two Al3+ ions. This quantification is further supported by the mass spectra, EDAX, and elemental analysis. Mass spectrum shows molecular ion peaks for compound 2 +

variable intensities were observed, and the emission color remained unchanged. Surprisingly for Al3+, in a similar condition the emission peak at 416 nm slowly quenched and the exciplex emission shifted further to 466 nm with distinct enhancement (Figure 12b). Hence, the dual emission nature changes and a bright cyan-green emission can be observed upon Al3+ addition. The quenching of emission intensity at 416 nm corresponds to the quenching of anthracene monomer emission. From these observations we conjecture that Al3+ addition renders efficient energy transfer from amc(I) to the exciplex. The other metal ions of this group Ga3+ and In3+ show a similar trend, but change in intensity of emission remains H



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Scheme 1. Schematic of a Metal−Organic Complex with Exciplex Emission Amplified through Energy Transfer and Its Al3+ Sensing Properties

2Al3+, compound 2 + 2Al3+ + H2O and compound 2 + 2Al3+ + 2H2O (Figure S11). EDAX analysis suggests the presence of Al3+ along with Cd2+ in a 1:1 ratio (Figure S12). The FT-IR spectrum of Al3+ incorporated compound shows change in the carboxylate binding mode; νasym(COO) stretching frequency shifts to a lower wavenumber (1572−1543 cm−1)70,71 indicating binding toward Al3+; second the peak at around 1483 cm−1 suggests the presence of -NO3− ions which is essential for charge neutralization of the complex (Figure S13). This made us believe that hard Al3+ interacts with the complex probably through the hard pendent oxygen coordination and does not disintegrate the complex. A probable scheme of coordination with of Al3+ in 2 is given in Figure S14. The excitation spectrum monitored at 500 nm after addition of Al3+ does not show significant change compared to that of 2 in methanol (Figure S15). This reiterates that the complex does not involve in any ground state interaction with the metal ion and Al3+ is only enhancing the exciplex emission. The plausible reason for selectivity toward the Al3+ ion is due to the specific arrangement of the pendent oxygen atoms in the dimeric unit. Even in the presence of other metal ions (tested for Na+, Cu2+, and Ga3+) with Al3+ complex 2 shows a similar change in emission as that in the case of only Al3+ (Figure S16). Although, other metal ions such as Fe3+, Eu3+. Na+, Ga3+, Mn2+, Cu2+, and Zn2+ also coordinate to the pendent oxygen atoms of compound 2, as realized from the ICP analysis of the metal ion added compound 2, but they do not orient the amc linker in a favorable position for efficient energy transfer leading to dual emission (Table S2). To find out whether Cd2+ is exchanged with Al3+ ion, we have monitored the Cd2+ concentrations in compound 2 and Al3+ compound 2 through ICP analysis (Table S3). No distinct change of Cd 2+ concentration suggests no metal exchange or leaching.

process both play an important role to achieve such highly selective and easy-readout sensing performance. On the basis of our approach, further novel metal−organic complexes can be designed for specific metal ion sensing by changing the organic chromophores and controlling their metallophilicity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01448. FT-IR spectra, packing diagram of the crystal structures, HRMS spectrum, EDAX data and schematic of the possible Al3+ binding mode to the complex (PDF) Crystallographic information file (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 91-80 22082766. Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS In conclusion, we have synthesized two metal−organic complexes, using pyrene and anthracene chromophores as the building unit. Coordination driven supramolecular assembly of electron rich chromophores and acceptor chromophore o-phen gives rise to exciplex bright cyan emissions in compounds 1 and 2. Interestingly, in 1 and 2, exciplex emissions are further sensitized through FRET from pma and amc monomers, respectively. Such photophysical processes are unique in metal−organic structures. Further, the emission characteristic of 2 changes (cyan to blue) in solvent medium like methanol due to inhibition of energy transfer through a change in the spatial arrangement of the linkers. Such emission characteristic further changes selectively upon addition of Al3+ leading to a ratiometric fluorescence response. Such sort of efficient sensing in micromolar concentration using a metal−organic probe is unprecedented. Here, exciplex emission and energy transfer I



Article

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