Excitation Energy Transfer Supported Amplified Charge-Transfer

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Excitation Energy Transfer Supported Amplified Charge-Transfer Emission in an Anthracenedicarboxylate- and BipyridophenazineBased Coordination Complex Komal Prasad,†,‡ Debabrata Samanta,†,‡ Ritesh Haldar,† and Tapas Kumar Maji*,†,§ †

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

coordination polymer.16 The work demonstrates the initial absorption of light by 2,6-naphthalenedicarboxylate, a linker of the coordination polymer, which transfers the excitation energy to a CT complex formed by another ligand, o-phenanthroline, and an encapsulated aromatic amine guest in the framework. However, it would be advantageous to have such energy-transfer phenomena in stable and simple coordination complexes for practical application in an optoelectronic device. To the best of our knowledge, an example of amplified CT emission by an energy-transfer process in discrete coordination complexes is yet to be documented (Scheme 1).

ABSTRACT: A highly luminescent tetrameric zinc(II) complex, {[Zn4(adc)3(bpz)6(HCOO)2]·2H2O} (1; adc = 9,10-anthracenedicarboxylate and bpz = bipyridophenazine), was synthesized by a solvothermal technique and characterized by single-crystal X-ray diffraction. The linear tetrameric units extend into three dimensions via π stacking of adc/bpz and bpz/bpz and multiple CH−π interactions. The compound shows strong red emission at 597 nm (λex = 480 nm), which is attributed to chargetransfer (CT) emission within an adc/bpz donor−acceptor pair. This is also supported by density functional theory computations. Interestingly, the CT emission is amplified by energy transfer from another adc linker that is not involved in the CT interaction.

Scheme 1. Schematic Representation of the Amplified CT Emission for the Present Work

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ature is a constant inspiration for chemists to build artificial tools by mimicking its sophisticated working principles. For instance, the photosynthetic system harvests light energy by absorbing sunlight, which is transferred to the catalytic reaction center and is finally stored as chemical energy.1 Over the decades, scientists have been utilizing energy-transfer phenomena in multichromophoric organic systems, metal−organic frameworks, organic polymers, and inorganic quantum dots to build artificial light-harvesting and optoelectronic materials.2−14 Coordination complexes are widely used as efficient lightharvesting materials because they can hold chromophores in close proximity inside the lattice with the highest degree of order.9−11,15−20 Moreover, coordination-driven spatial organization of multichromophoric ligands results in novel photophysical properties in coordination complexes.21 For example, coordination complexes, synthesized by combining electron-rich and -deficient chromophoric ligands, often produce donor− acceptor or charge-transfer (CT) complexes with exciting properties like exciplex or CT emission.22−30 Moreover, in a few cases, excitation energy-transfer phenomena were also observed from a neighboring chromophore to the exciplex or CT complex, leading to an enhanced and tunable emissive material.16,17 Significant spectral overlap between the donor emission and acceptor absorbance as well as an optimum donor− acceptor distance are the primary requirements to realize the energy-transfer process. Such materials provide the additional advantage of absorbing light from a wider spectral range. An example of such energy-transfer phenomena was reported by our group, utilizing a guest-encapsulated metal−organic porous © XXXX American Chemical Society

Because of the versatile optical applications of such materials, a novel coordination compound, [{Zn4(adc)3(bpz)6(HCOO)2}· 2H2O] (1), has been synthesized utilizing 9,10-anthracenedicarboxylate (adc, a donor) and bipyridophenazine (bpz, an acceptor) chromophores in the backbone. The formation of a CT complex, bpz/adc/bpz, and the energy-transfer phenomenon from a neighboring adc to the CT complex were observed. Moreover, the system showed tunable emission depending upon the solvent polarity. Complex 1 has been synthesized by the self-assembly of ZnII and bpz and adc linkers in dimethylformamide under solvothermal conditions at 90 °C, yielding block-shaped darkred single crystals (see the Supporting Information, SI). Singlecrystal structure determination revealed that compound 1 crystallizes in the monoclinic P21/n space group and the asymmetric unit contains two ZnII (Zn1 and Zn2) centers, one formate, and 1.5 adc (adc1 and adc2) linkers (Figure 1). Each octahedral Zn1 center is chelated to two bpz linkers through N1, N2 and N3, N4 atoms, and two other positions are filled by a chelated carboxylate group (O3 and O4) from adc1. Each Received: October 19, 2017

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DOI: 10.1021/acs.inorgchem.7b02698 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

complex 1 is stable up to 300 °C. The TGA profile also showed 1.2 wt % loss in the temperature window of 80−170 °C, and this corresponded to the loss of two guest water molecules from the crystal (calcd 1.23 wt %; Figure S3). We have further studied the photophysical properties of complex 1 in detail. The absorbance spectrum of 1, in the solid state, showed two partially overlapped bands at 340 and 380 nm for bpz and adc chromophores, respectively (Figure 3a, red).

Figure 1. Crystal structure of complex 1 showing the coordination environment of Zn1 and Zn2.

pentacoordinated Zn2 center is chelated to one bpz (N9 and N10) and three monodentate carboxylate O atoms (O1, O5, and O8) from two adc and one formate ligands. The Zn1 and Zn2 centers are bridged by adc1, and the resulting dimeric fragment Zn1Zn2(bpz)3(adc1)(HCOO) is further linked to its symmetryrelated counterpart by the adc2 linker, forming a tetranuclear complex [{Zn4(adc)3(bpz)6(HCOO)2}·2H2O]. In the complex, the Zn−N and Zn−O bond distances are in the ranges of 2.099(4)−2.279(5) and 1.945(4)−2.294(4) Å, respectively, and the separations between Zn1−Zn2 and Zn2−Zn2 bridged by adc1 and adc2 are 11.01 and 10.95 Å, respectively. Because the complex is composed of two different extended π-chromophoric linkers (adc and bpz), the crystal structure extends through π−π interactions. The tetramers are π-stacked via adc2/bpz in a zigzag fashion (Figure 2), which extends the structure in one dimension

Figure 3. (a) Solid-state UV−vis spectrum of 1 (red) and PL of adc (black). Inset: picture of 1 under visible light. (b) Solid-state PL of 1 upon excitation at 380 nm (black) and 480 nm (red). The excitation spectrum collected at 597 nm is presented in blue. Inset: picture of 1 under UV light. (c) PL of 1 in different solvents along with photographs. (d) Fluorescence lifetime spectrum of 1 monitored at 597 nm upon excitation with both 376 nm (blue) and 532 nm (red) laser.

Additionally, a red-shifted band was also observed in the visible region at 480 nm, which can be attributed to a ground-state CT interaction due to the cofacial arrangement of adc/bpz chromophores. Possibly, this interaction results in the red color of 1 (Figure 3a, inset). Complex 1 exhibited a single emission band at 597 nm (bright red) upon excitation at 480 nm, corresponding to the CT complex (Figure 3b). The solid-state excitation spectrum of 1 (λem = 597 nm) showed a band at 451 nm, along with two other bands at 403 and 335 nm, indicating the origin of the emission is the CT complex (Figure 3b). The spectrum clearly suggests the presence of adc/bpz CT interaction, where bpz acts as an acceptor and adc as a donor. The fluorescence lifetime was measured to be 6.72 ns upon monitoring at 597 nm (λex = 532 nm; Figure 3d). Such a long excited-state lifetime additionally supports an excited-state photophysical process with CT character. The absolute quantum yield of the solid sample was determined to be 8.6%. In order to obtain more insight about the origin of the CT band in the UV−vis spectrum and corresponding bright-red emission, we employed time-dependent density functional theory (TD-DFT) computations for three different model systems (models 1−3, Figure 4) and computed their excited states. Model 1 illustrates adc/bpz π stacking, model 2 shows bpz/bpz stacking, and model 3 considers the possibility of CT from nonstacked adc to a bpz chromophore. The CAM-B3LYP method,31 in combination with the 6-31+G* basis set, was utilized for computations in the gas phase, as implemented in Gaussian09.32 Because the gas-phase computations showed large deviations from the UV−vis/photoluminescence (PL) measured

Figure 2. Two-dimensional packing of complex 1 through adc2/bpz and bpz/bpz π stacking.

along the a axis. Notably, the π-stacking distance between adc2 and bpz is 3.59 Å. The one-dimensional chains are further assembled through bpz/bpz π stacking, which leads to twodimensional extension of the packing on the ac plane. The two different π stacks, adc2/bpz and bpz/bpz, are positioned in an alternative arrangement in the ac plane. Finally, the twodimensional layers are interdigitated by multiple CH−π interactions, resulting in a three-dimensional extended network. This has a small void space that hosts two water molecules, as realized from thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), and elemental analyses. The PXRD pattern of the as-synthesized complex 1 matched well with the simulated pattern of a single crystal, suggesting phase purity of the bulk product (Figure S2). TGA revealed that B

DOI: 10.1021/acs.inorgchem.7b02698 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

at the CT absorption maximum. The fluorescence lifetime was found to be very low (7.75 ps) upon monitoring the emission at 420 nm, whereas the value is higher (0.29 ns) in the monomeric state (solution). In order to further cross-check the energytransfer process, we attempted to break the adc/bpz π-stacked supramolecular association by dissolving complex 1 in various solvents (Figure 3c). Indeed, in dichloromethane, the emission band at 597 nm completely vanishes and, consequently, an emission band appears at 400−440 nm, corresponding to the adc chromophore. The liquid chromatography−mass spectrometery of the solution showed two distinct peaks at 472.5 and 708.9 Da for [1 + 6H]6+ and [1 + 4H]4+, respectively (Figure S4). This indicates that complex 1 remains stable but disaggregates in solution, resulting in breakage of the CT complex and the disappearance of the CT emission. The energy-transfer efficiency was calculated to be 29.3% with an energy-transfer rate constant of 3.37 × 107 s−1 (SI). The above results suggest that adc1 acts as an energy-transfer antenna for amplifying the CT emission. In summary, we have synthesized a highly luminescent threedimensional supramolecular metal−organic complex, [{Zn4(adc)3(bpz)6(HCO2)2}·2H2O] (1), which showed a CT transition at 480 nm due to adc/bpz π stacking in the solid state and a corresponding emission in the red region at 597 nm. TDDFT computations on the adc/bpz π-stacked model system reveals that an electron transition occurs from the adc to bpz chromophore. Most interestingly, this study represents a classic example where the CT emission is amplified by energy transfer from the surrounding nonstacked anthracene moiety. Our results will help in the design of a new type of light-harvesting hybrid material by assembling multichromophoric ligands through coordinative interaction.

Figure 4. Models 1−3 represented with computed molecular orbital diagrams that have contributed to the lowest-energy electronic transition.

in the solid phase, we analyzed the origin of the CT band and emission by comparing experimental and computational Stokes shifts to cancel out the error due to the phase difference. The computed electronic transition of model 3 displayed the lowest energetic transition from HOMO (π orbital of adc) to LUMO+1 (π orbital of carboxylate) at 370 nm (f = 0.41), whereas the emission appeared at 444 nm with a Stokes shift of 74 nm (Figure 4). There was no electronic transition from adc to bpz observed in this model. Model 2 showed an electronic transition having a major contribution from HOMO to LUMO at 307 nm ( f = 0.59), but the emission band corresponding to the transition appeared at 319 nm with a Stokes shift of 12 nm only. Therefore, bpz/bpz-stacked model 2 and linear model 3 could not explain such a huge Stokes shift of 117 nm obtained in the experiment. As expected, model 1 with adc/bpz stacking showed a CT transition from HOMO (π orbital of adc) to LUMO+2 (π orbital of bpz) at 374 nm and corresponding emission at 508 nm with a Stokes shift of 134 nm matching well with the experiment. Therefore, the dark-red emission was generated as a result of excitation at the adc/bpz CT complex. Furthermore, a closer inspection of the crystal structure shows that adc1 moieties are not involved in the CT complex (Figure 2), and, hence, a separate emission band should be expected in the region of 400−440 nm for the adc1 chromophore upon excitation at 380 nm. Strikingly, no emission band was observed in the region upon excitation at several wavelengths in the range of 340−400 nm in the solid state. The absence of adc1 emission may be attributed to complete excitation energy transfer from the adc1 chromophore to the adc/bpz CT complex. This is feasible because the adc emission is completely overlapped with the CT band, as observed in UV−vis spectrum of 1 (Figure 3a). Interestingly, the emission intensity at 597 nm is higher when excited at 380 nm than that of 480 nm, indicating that the CT emission is amplified by energy transfer from adc1. This is further supported by the excitation spectrum collected at 597 nm, which shows the highest intensity at 403 nm, which is the absorption of the adc linker. The fluorescence lifetime was also measured to support the energy-transfer process (Figure 3d). In the solid state, the fluorescence lifetime was found to be 8.69 ns upon excitation at 376 nm (at adc) and monitoring the emission at 597 nm (CT emission), whereas the lifetime is 6.72 ns when excited

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02698. Experimental details and additional results (PDF) Accession Codes

CCDC 1581938 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tapas Kumar Maji: 0000-0002-7700-1146 Author Contributions ‡

K.P. and D.S. contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.K.M. and D.S. acknowledge DST, India (MR-2015/001019 and TRC DST/C.14.10/16-2724, JNCASR), and JNCASR for funding. C

DOI: 10.1021/acs.inorgchem.7b02698 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.7b02698 Inorg. Chem. XXXX, XXX, XXX−XXX