Experimental and Theoretical Study of a Cadmium Coordination

Mar 6, 2017 - Inorganic Chemistry Department, Science Faculty, University of Granada, 18071 Granada, Spain. § Nuclear Magnetic Resonance Platform, CI...
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Experimental and Theoretical Study of a Cadmium Coordination Polymer Based on Aminonicotinate with Second-Timescale Blue/ Green Photoluminescent Emission Jose M. Seco,† Antonio Rodríguez-Diéguez,‡ Daniel Padro,§ Jose A. García,∥ Jesus M. Ugalde,⊥ Eider San Sebastian,*,† and Javier Cepeda*,†,⊥ †

Applied Chemistry Department, Chemistry Faculty, University of the Basque Country (UPV/EHU), 20080 Donostia, Spain Inorganic Chemistry Department, Science Faculty, University of Granada, 18071 Granada, Spain § Nuclear Magnetic Resonance Platform, CIC BiomaGUNE, 20080 Donostia, Spain ∥ Applied Physics Department, Science and Technology Faculty, University of the Basque Country (UPV/EHU), 48940 Bilbao, Spain ⊥ Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center, P.K. 1072, 20080 Donostia, Euskadi, Spain ‡

S Supporting Information *

ZnII-based counterpart.9 The electronic mechanisms responsible for the experimentally observed photoluminescent behavior have been identified by means of time-dependent density functional analysis (TD-DFT). In this line, although it has been well documented that charge-transfer excitations are normally poorly described by hybrid approximate exchange-correlation functionals,11 we found by an explicit comparison with our experimental data that PL of both ZnII and CdII CPs is accurately described by hybrid functionals. This fortuitous likely cancellation of errors is herein confirmed to take place, in line with similar conclusions reported recently12 for the photoinduced intra-molecular electron transfer from zinc and cadmium sulfide clusters to organic ligands attached to their surfaces. Finally, evidence is shown supporting that PL of both ZnII and CdII CPs follows Kasha’s rule;13 namely, the phosphorescent emission originates from the lowest-lying triplet spin state. The computed vertical transition energies (λvert-phosp) from the optimized lowest-lying triplet state to the ground state provide a highly accurate description of the experimentally observed luminescent properties of both compounds. The mixture of H2O/methanol solutions containing 6aminonicotinic acid (H6ani) and Cd(NO3)2·4H2O lead to a white polycrystalline precipitate of the formula [Cd(μ-6ani)2]n (1), as confirmed by elemental (EA), thermogravimetric (TGA/ DTA), and infrared (FTIR) analyses (see the Supporting Information). As aforementioned, 1 is isostructural to the previously reported Zn6ani,9 as revealed by powder X-ray diffraction (PXRD) data analysis (Table S1). On the basis of the latter results, the structure of 1 was simulated by dispersioncorrected periodic density functional theory (DFT) geometry optimization10 starting from crystallographic data of Zn6ani [Figures S1 and S2]. The crystal structure consists of stacked 2D neutral layers built up from the linkage among the bridging μ6ani ligands to the CdII atoms. The central cadmium atom exhibits a N2O4 donor set due to its coordination to four equivalent μ-6ani ligands through two pyridine nitrogen atoms

ABSTRACT: A new cadmium/6-aminonicotinate-based coordination polymer (CP) with an unprecedented multicolored and long-lasting emission is reported. This material shows a blue fluorescence which rapidly turns to green persistent phosphorescence with a lifetime of nearly 1 s. Time-dependent density functional theory calculations revealed that electronic transitions arising from both first excited singlet and triplet states involving ligand-centered and ligand-to-metal charge-transfer mechanisms are responsible for such behavior.

S

olid-state light-emitting materials are key to the development of light emitting, display sensing and/or optical devices, among others.1 In this line, persistent luminescence, long-lasting phosphorescence, and afterglow are largely desired phenomena since they enlarge the scope of these materials and bring easier deployment in the industry.2 Organic luminescent compounds have emerged as an improved alternative to the lanthanide-based codopants used traditionally in inorganic matrixes, and are promoting a rapid evolution of efficient organic light-emitting diodes.3 In this regard, metal−organic frameworks (MOFs) have proven to be very promising as multifunctional luminescent materials because of their metal−organic hybrid nature.4−6 The widely studied noble-metal- (platinum, iridium, and ruthenium) or lanthanide (europium and terbium) ionsbased photoluminescent (PL) MOFs typically present photoemission lifetimes from hundreds of microseconds to a few milliseconds derived from ligand-to-metal (LMCT) or metal-toligand charge-transfer and metal-centered emission mechanisms.7 However, these decay times are below the desired threshold of tens of milliseconds that yields the persistent luminescence required for those devices where human eye sensitivity intervenes. Recently, attention has been shifted over closed-shell d 10 metal ions, for they exhibit improved phosphorescence and longer afterglow times.8 Herein, we report on the PL emission of a CdII-based CP, which shows brighter and bathochromically shifted emission compared to the reported © 2017 American Chemical Society

Received: January 18, 2017 Published: March 6, 2017 3149

DOI: 10.1021/acs.inorgchem.7b00110 Inorg. Chem. 2017, 56, 3149−3152

Communication

Inorganic Chemistry and the oxygen atoms of two chelating carboxylate groups (Figure 1 and Table S2). This feature constitutes one salient

Figure 1. View of a layer of compound 1 showing an excerpt of the coordination shell. Color code: carbon, gray; hydrogen, pink; nitrogen, blue; oxygen, red; cadmium, green.

difference with respect to the zinc counterpart (with a N2O4 set), which seems to obey the larger size of CdII. Despite that subtle structural feature, 6ani ligands remain quasi-planar (rotation angle of −COO− with respect to the ring of 16.1°) and act as bridges between Cd atoms (Cd···Cd distance of ca. 8 Å), in such a way that the resulting building unit can be referred to as a square-planar node from a topological point of view. This disposition comes from the intramolecular N−H···O hydrogenbonding interaction established between the amino and carboxylate groups (Table S3). Hereafter, the junction of the building units gives rise to corrugated 2D layers showing a squared grid (sql topology) that spreads out along the crystallographic ab plane. These layers are piled up along the c axis following an efficient ABAB packing by means of strong faceto-face π−π and weak hydrogen-bonding interactions, given that the 6ani ligands are inserted among the rings of neighboring layers (see the Supporting Information). The resulting high cohesivenes entails a high thermal stability (ca. 410 °C), as depicted from TGA/DTA. Compound 1 exhibits interesting PL behavior consisting of a multicolored emission. While it shows a strong and bright-blue emission under excitation with UV light, this emission rapidly turns to a pale-green afterglow that persists for a few seconds after removal of the excitation source, thus behaving as a longlasting phosphor. It is worth noting that this process can be seen by the naked human eye. Under excitation at 332 nm, 1 shows an intense and wide emission band with its maximum centered at 444 nm, in which other remarkable maxima are clearly distinguished at 397, 415, 459, and 482 nm and a shoulder at 515 nm (Figure 2). The presence of so many bands together with the large bathochromic shift compared with the free H6ani ligand (Figure S4) concurs with the existence of different sorts of transitions due to ligand coordination to the metal atom.14 On the other hand, excitation spectra monitored at the most intense emission peaks reveal that the aforementioned maxima derive from two main transitions (λex at 332 and 354 nm). In this sense, the opposite evolution for the relative intensity of these transitions according to the increase of the emission wavelength (see Figure S5) must be highlighted. The latter imposes that emissions below the global maximum (365−435 nm) occur preferentially upon excitation with the 354 nm line, whereas those in the 440−600 nm range need a higher excitation energy (λex = 332 nm). A similar dependence was found for the related [Cd3(SO4)2(μ3-6ani)2(H2O)4]n compound,15 for which phosphorescent behavior is not reported. With the aim of getting a deeper insight into the intriguing PL performance of 1, timedependent DFT (TD-DFT) calculations were carried out on a suitable model of 1, which consists of an excerpt containing the

Figure 2. (a) Micro-PL photographs of 1 representative of the timedependent emission. (b) Experimental (solid line) and TD-DFTcomputed (dashed line) PL spectra at 10 K. Red and blue captions stand for the main lines of the excitation (λem = 444 nm) and emission (λex = 332 nm) spectra, respectively.

cadmium atom and the four 6ani ligands coordinated to it (Figure S12). The calculated spectra reproduced fairly well the experimental ones (see Figure 2b). The two main S0 → S1 transitions, at 344 and 350 nm, are governed by HOMO → LUMO+6/HOMO−1 → LUMO+7 and HOMO−5 → LUMO +4/HOMO−4 → LUMO+5 electronic excitations, respectively. Given that HOMO−n orbitals lie on the carboxylate groups and that LUMO−n orbitals are extended over the aromatic rings and have clear π* character, it can be stated that the excitation takes place through σ → π* and π → π* transitions. Regarding the emission, a clear distinction can be made between transitions involved in the bands below/or close to and above the global maximum (λem at 444 nm). On the one hand, the bands at 397 and 415 nm mainly originated from HOMO−1 ← LUMO+6 and HOMO−4 ← LUMO+3 transitions (note that, for clarity, orbitals involved in the emission events are named according to their classification in the ground-state structure but that HOMO and LUMO are not strictly the “highest occupied” and “lowest unoccupied” molecular orbitals anymore in the excited state), thus suggesting that the initial blue emission seems to proceed via a ligand-centered charge-transfer (LCCT) mechanism. On the other hand, bands sited at 452, 482, and 515 nm (involved in the pale-green delayed emission) arise mainly from both the HOMO−1 ← LUMO+4 and HOMO−1 ← LUMO+5 singlet emissions. Taking into account that the electron density of the LUMO+4 orbital contains, in addition to π* character, a sizable contribution from the CdII atom, the phosphorescent emission 3150

DOI: 10.1021/acs.inorgchem.7b00110 Inorg. Chem. 2017, 56, 3149−3152

Communication

Inorganic Chemistry should tentatively be ascribed to a LMCT mechanism (see Scheme S1). Nonetheless, this assumption is in apparent contradiction with the dominant thought that considers the phosphorescent emission to be a consequence of a radiative spinforbidden T1 → S0 transition centered on the d10 metal atom.16,17 Accordingly, the geometry of the lowest-lying triplet excited state was optimized starting from model 1. Consequently, its vertical emission energy (λvert‑phosp) was estimated to be 501 nm. This value fits fairly well with the peaks involved in the delayed green emission. Therefore, upon irradiation at 332 nm (mimicking experimental conditions), this state may be populated even though it involves a forbidden transition (intersystem crossing), for it is highly likely to be enabled by the spin−orbit coupling facilitated by the heavy-metal ion. Therefore, the emission from the triplet state, whose HOMO and LUMO orbitals are centered on 6ani ligands (Figure S15), may be considered to be the responsible for the experimentally observed phosphorescence, thus following the well-established Kasha’s rule.13 Moreover, an equivalent calculation for the Zn6ani9 compound (λvert‑phosp vs longest experimental lifetime of 453 vs 461 nm) confirms the validity of the method to estimate the phosphoresencent emission. Decay curves were measured at different emission wavelengths (every 20 nm) in order to estimate the lifetimes of the multicolored emission of the material. As expected, the decay profiles are found to be not uniform with respect to the emission wavelength because they show a nonlinear curve that confirms the presence of several radiative processes of different lifetimes, so the multiexponential It = A0 + A1 exp(−t/τ1) + A2 exp(−t/τ2) + A3 exp(−t/τ3) expression was employed for fitting purposes (see the Supporting Information). Below 410 nm, only very short lifetimes of ca. 6 ns are found, whereas remarkably long-lived components of 0.56−0.83 s as well as intermediate lifetimes (0.22−0.49 s) characterize the emissions in the 415−600 nm range. These are, to the best of our knowledge, the longest PL lifetime values ever reported for cadmium-based metal−organic compounds. In particular, lifetimes rise slowly with an increase of the emission wavelength to reach a maximum (τ2 = 0.34 and τ3 = 0.83 s) at 482 nm peak, after which they decrease gradually. These data show fair agreement with the TD-DFT computations. A more accurate description of the time-dependent evolution of the emission was achieved through the timeresolved emission spectra (TRES) experiment (Figures S17− S19). As shown by the normalized spectra, the emission undergoes a rapid change because the initial structured band (t0) equivalent to that recorded at steady state gives way to a spectrum containing a wide band with a maximum at 460 nm and a shoulder at 521 nm, which is still appreciated at t > 2 s. Moreover, deconvolution of the spectra with regard to the CIE 1931 color system confirms the visually observed change from a bright blue to the persistent green emission (Figure 3). Finally, it is worth noting the remarkable high stability of the lifetimes associated with green phosphorescence with respect to the temperature, as revealed by the variable-temperature measurements up to 250 K (Table S7 and Figure S11). This suggests a weak-coupling-mediated nonradiative quenching of the material. In summary, we have synthesized a CdII-based CP that presents best-in-class PL properties. This material exhibits a multicolored emission, combining an intense-blue fluorescence that evolves to a green phosphorescence that persists a few seconds after removal of the excitation UV light. Electronic structure calculations have allowed us to unravel the complex emission pattern of this material. As revealed by TD-DFT calculations, singlet transitions, dictated by a LCCT mechanism,

Figure 3. Color evolution of the emission plotted on CIE 1931 chromaticity diagram extracted from TRES.

are mainly responsible for the fluorescence, whereas LMCT is involved in the delayed green emission. Notwithstanding the latter, DFT optimization of the first triplet excited state, which is populated by the spin−orbit coupling driven by intersystem crossings facilitated by the cadmium atom, and subsequent evaluation of the triplet to ground-state singlet decay energy at the triplet geometry confirm that the phosphorescent emission is governed by the lowest-lying triplet state.

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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.inorgchem.7b00110. Detailed synthesis, PXRD characterization, TGA/DTA, FTIR spectroscopy, PL characterization, and details on the periodic TD-DFT calculations (PDF) CIF file generated for the simulated structure of 1 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.S.S.). *E-mail: [email protected] (J.C.). ORCID

Javier Cepeda: 0000-0002-0147-1360 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Junta de Andalucı ́a (Grant FQM-1484), Red Guipuzcoana de Ciencia, Tecnolgıá e Innovación (Grant OF215/2016), and University of the Basque Country (Grants GIU14/01 and EHUA16/32). J.C. thanks the University of the Basque Country (UPV/EHU) for a postdoctoral fellowship. SGI/IZO-SGIker of UPV/EHU is gratefully acknowledged for a generous allocation of computational resources. 3151

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DOI: 10.1021/acs.inorgchem.7b00110 Inorg. Chem. 2017, 56, 3149−3152