Estimating the Donor–Acceptor Distance To Tune the Emission

Dec 22, 2016 - The influence of the donor–acceptor distance RL on the photophysical properties, including the emission quantum yield, of two europiu...
1 downloads 11 Views 677KB Size
Download PDF
Communication pubs.acs.org/IC

Estimating the Donor−Acceptor Distance To Tune the Emission Efficiency of Luminescent Lanthanide Compounds Jorge H. S. K. Monteiro,†,‡ Ana de Bettencourt-Dias,*,‡ and Fernando A. Sigoli*,† †

Institute of Chemistry, University of Campinas, Campinas, São Paulo, Brazil Department of Chemistry, University of Nevada, Reno, Nevada 89557, United States

‡

S Supporting Information *

ABSTRACT: The influence of the donor−acceptor distance RL on the photophysical properties, including the emission quantum yield, of two europium complexes with the same coordination number, and thus similar microsymmetries, was investigated by spectroscopic and computational methods. K3[Eu(dipicCbz)3] was synthesized using the new ligand dipicCbz and its photophysical properties compared to Cs3[Eu(dipic)3]. We found that a 50% increase in RL from 4.1 to 6.5 Å results in a substantial decrease in the emission efficiency from 24 to 1.8%.

Figure 1. [Eu(dipic)3]3− and [Eu(dipicCBZ)3]3−.

T

he synthesis of luminescent lanthanide compounds is an active field because of the wide range of applications such as displays, imaging devices, and sensors, due to the color purity of the emission and long emission lifetimes.1,2 Because the emission is due to parity-forbidden f−f transitions, an organic chromophore bound to the lanthanide is commonly used as a sensitizer; it absorbs energy to populate singlet and triplet states and then transfers it to the metal ion, which then emits light. The process is known as the antenna effect (Scheme 1).3−6 Substantial work has been done in pursuit of complexes with improved emission efficiency, and the properties of the ideal antenna have been the subject of several publications.3,7,8 Since for many applications excitation wavelengths of >350 nm are desirable, new complexes should absorb at low energy.9−13 Thus, the tuning of excited-state energies through ligand functionalization has been an active area of research.7,14−16 To achieve lowenergy excitation and an increased two-photon absorption cross section, Maury et al. described compounds with a substantial

Figure 2. Excitation and emission spectra of (a) K3[Eu(dipicCbz)3] obtained in a Tris/HCl-buffered water/dimethyl sulfoxide solution (pH ∼ 7.4) and (b) Cs3[Eu(dipic)3] in a Tris/HCl-buffered aqueous solution (pH ∼ 7.4). The inset shows the magnified 5D0 → 7F0 peak.

bathochromic shift in the excitation wavelength, achieved by increasing the electronic conjugation of the ligand.17−19 However, increased conjugation can also negatively impact the metal-centered emission efficiency, as reported by Andres and Chauvin20 and Andres and Borbas.8 The donor−acceptor distance RL also affects the efficiency of energy transfer. For a dipole−dipole exchange mechanism, the efficiency is inversely proportional to RL6.21 While the influence of this parameter is theoretically known, few studies8,20 address how it affects the emission efficiency. Here we show how we can use straightforward calculations21−24 to estimate triplet state energies and RL to correlate the electronic structure of the ligand with the rates of energy and back transfer and the emission efficiency ΦLEu of the complex. We synthesized K3[Eu(dipicCbz)3] [dipicCbz = 4-(9H-carbazol-9-yl)pyridine-2,6dicarboxylato; details are given in Scheme S1 and Figures S3 and S4] and compared it with the known analogue Cs3[Eu-

Scheme 1. Jablonski Diagram Illustrating the Antenna Effect for EuIIIa

A is absorption, F fluorescence, P phosphorescence, ISC intersystem crossing, IC internal conversion, NR nonradiative pathways, S states with singlet multiplicity, and T states with triplet multiplicity. a

© XXXX American Chemical Society

Received: November 1, 2016

A

DOI: 10.1021/acs.inorgchem.6b02637 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Table 1. Singlet (S) and Triplet (T) Energies, Judd−Ofelt Intensity Parameters (Ω2 and Ω4), Emission Lifetime (τ), Intrinsic Eu Quantum Yield (ΦEu L ), Quantum Yield of Sensitized Emission (ΦL ), Sensitization Efficiency (ηsens), and Ratio of the Integrated Phosphorescence and Fluorescence Bands (IP/IF) for the Europium(III) Complexes of dipicCbz and dipica

a

ligand

S/cm−1

T/cm−1

Ω2/10−20 cm2

Ω4/10−20 cm2

τ/ms

ΦEu Eu/%

ΦEu L /%

ηsens/%

IP/IF

dipicCbz dipic25,26

26030 ± 400 31460 ± 790

23460 ± 350 26740 ± 870

10.5 7.6

4.0 5.3

1.087 ± 0.005 1.593 ± 0.010

46 41

1.8 24

3.9 59

197 ± 10 244 ± 19

Details of spectroscopic measurements are summarized in the Supporting Information.

Figure 3. Ground-state geometries obtained with Sparkle/PM337 with RL indicated for (a) [Eu(dipicCbz)3]3− (6.5146 Å) and (b) [Eu(dipic)3]3− (4.1496 Å). The inset shows the coordination polyhedra.

Figure 6. Energy-transfer rates as a function of RL, as predicted by eqs S1−S3.21−24

symmetry around the EuIII ion, which was shown to be close to D3 in Cs3[Eu(dipic)3].26 The photophysical parameters of the two complexes are summarized and compared in Table 1. The close values of the phenomenological Judd−Ofelt intensity parameters27,28 Ω2 and Ω4, which are directly correlated with the geometry around the EuIII ion, confirm the similarity. The triplet energy level of dipicCbz, determined from the phosphorescence of the gadolinium(III) complex (Figure S5),29 is 23460 ± 350 cm−1, lower than the one reported for dipic (26740 ± 870 cm−1).26 The intersystem crossing efficiency, the ratio of the quantum yields of the phosphorescence and fluorescence emission (ΦP/ΦF),30 was estimated from the ratio of the integrated phosphorescence and fluorescence spectra (IP/ IF; Figure S6).30,31 The value of 197 ± 10 for dipicCbz differs only by ∼20% from the one for dipic (244 ± 19) and is in agreement with the values reported by Crosby et al. for [Gd(bzac)3].29 Because the IP/IS and microsymmetries are similar in both cases, we expected comparable values of ΦEu L and thus of the sensitization efficiency, ηsens. However, ΦEu L for K3[Eu(dipicCbz)3] is 1.8%, ∼20 times smaller than the 24% reported for Cs3[Eu(dipic)3].32,33 ηsens, the product of the efficiency of intersystem crossing and the efficiency of energy transfer, differs substantially for both complexes, at 3.9 and 59%, respectively. Similar results were reported for [Eu(LPhta)3]3− Phta (ΦEu = 4-[2-[2-[2-(1,3-dioxoisoindolin-2L = 15.3% and L yl)ethoxy]ethoxy]ethoxy]pyridine-2,6-dicarboxylato) 34 and for [EuL3]3− (ΦEu L = 4.5% and L = 4-(2,4,6-trimethoxyphenyl)pyridine-2,6-dicarboxylato).35 We therefore postulate that ΦEu L and ηsens are mostly affected by the differences in the rates of energy transfer, which, in turn, are caused by different conjugation lengths of both ligands and different values of RL.21 Using the software package LUMPAC,36 we estimated RL for both complexes (Figure 3). To assess the interdependency of these parameters, we calculated the energy-transfer rates using eqs S1−S422,24 as implemented in LUMPAC.36 At constant RL (Figure 4), the energy-transfer rate of ligand → EuIII is higher than the back energy transfer, if the triplet level is >20000 cm−1. The microsymmetry around EuIII also influences the emission

Figure 4. Calculated rates of energy transfer (T → 5D1,0) and back transfer (T ← 5D1,0) as a function of the different triplet energy levels, in the range of 16000−30000 cm−1. RL was fixed at 4.5 Å.

Figure 5. Theoretical emission quantum yield (Φ) as a function of the triplet energy level for different symmetries.

(dipic)3] (dipic = dipicolinato; Figure 1).25 Their excitation and emission spectra are shown in Figure 2. The absorption (Figure S8) and excitation (Figure 2) spectra of [Eu(dipicCbz)3]3− are red-shifted compared with those of Eu(dipic)3]3− because of increased electronic conjugation caused by the carbazole unit. This lowers the energy of the singlet excited state (Table 1) from 31470 cm−1 for dipic to 26030 cm−1 for dipicCbz. Both emission spectra (Figure 2) show the characteristic EuIII 5 D0 → 7FJ (J = 0−4) transitions. The peak splitting patterns are similar for both complexes,25 indicative of a similar microB

DOI: 10.1021/acs.inorgchem.6b02637 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Table 2. Rates of Energy and Back Transfer and RL Calculated for Complexes K3[Eu(dipicCbz)3] and Cs3[Eu(dipic)3] complex

T → 5D1/s−1

K3[Eu(dipicCbz)3] Cs3[Eu(dipic)3]

9.2 × 10 7.3 × 108 3

T ← 5D1/s−1 −8

1.1 × 10 1.9 × 10−2

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02637. Experimental details and descriptions of the equations (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ana de Bettencourt-Dias: 0000-0001-5162-2393 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge CAPES for a postdoctoral fellowship (Grant 159249/2014-7 to J.H.S.K.M.), CNPq (Grant 400214/ 2014-8 to F.S.M.), CAPES and FAPESP (Grant 2013/22127-2 to F.S.M.), and NSF (Grant CHE 1363325 to A.d.B.D.) for financial support.

■

RL/Å

3.8 × 10 3.5 × 108

9.2 × 10−13 1.9 × 10−6

6.5146 4.1496

(5) Bünzli, J.-C. G.; Eliseeva, S. V. In Lanthanide Luminescence: Photophysical, Analytical and Biological Aspects; Hänninen, P., Härmä, H., Eds.; Springer: Berlin, 2011; Chapter 1, pp 1−46. (6) Bünzli, J.-C. G.; Piguet, C. Taking Advantage of Luminescent Lanthanide Ions. Chem. Soc. Rev. 2005, 34, 1048−1077. (7) de Bettencourt-Dias, A.; Barber, P. S.; Viswanathan, S.; de Lill, D. T.; Rollett, A.; Ling, G.; Altun, S. Para-Derivatized Pybox Ligands as Sensitizers in Highly Luminescent Ln(III) Complexes. Inorg. Chem. 2010, 49, 8848−8861. (8) Andres, J.; Borbas, K. E. Expanding the Versatility of DipicolinateBased Luminescent Lanthanide Complexes: A Fast Method for Antenna Testing. Inorg. Chem. 2015, 54 (17), 8174−8176. (9) Rajendran, M.; Yapici, E.; Miller, L. W. Lanthanide-Based Imaging of Protein-Protein Interactions in Live Cells. Inorg. Chem. 2014, 53, 1839−1853. (10) Maindron, N.; Poupart, S.; Hamon, M.; Langlois, J. B.; Ple, N.; Jean, L.; Romieu, A.; Renard, P. Y. S Synthesis and Luminescence Properties of New Red-Shifted Absorption Lanthanide(III) Chelates Suitable for Peptide and Protein Labelling. Org. Biomol. Chem. 2011, 9, 2357−2370. (11) Tanner, P. A. In Lanthanide Luminescence: Photophysical, Analytical and Biological Aspects; Hänninen, P., Härmä, H., Eds.; Springer: Berlin, 2011; Chapter 7, pp 183−233. (12) Kalyani, N. T.; Dhoble, S. J. Organic Light Emitting Diodes: Energy Saving Lighting Technology-a Review. Renewable Sustainable Energy Rev. 2012, 16, 2696−2723. (13) Reineke, S.; Thomschke, M.; Luessem, B.; Leo, K. White Organic Light-Emitting Diodes: Status and Perspective. Rev. Mod. Phys. 2013, 85, 1245−1293. (14) Shavaleev, N. M.; Eliseeva, S. V.; Scopelliti, R.; Bunzli, J. C. G. NAryl Chromophore Ligands for Bright Europium Luminescence. Inorg. Chem. 2010, 49 (8), 3927−3936. (15) Shavaleev, N. M.; Eliseeva, S. V.; Scopelliti, R.; Bunzli, J. C. G. Designing Simple Tridentate Ligands for Highly Luminescent Europium Complexes. Chem. - Eur. J. 2009, 15, 10790−10802. (16) Samuel, A. P. S.; Xu, J. D.; Raymond, K. N. Predicting Efficient Antenna Ligands for Tb(III) Emission. Inorg. Chem. 2009, 48 (2), 687− 698. (17) Picot, A.; Feuvrie, C.; Barsu, C.; Malvolti, F.; Le Guennic, B.; Le Bozec, H.; Andraud, C.; Toupet, L.; Maury, O. Synthesis, Structures, Optical Properties, and TD-DFT Studies of Donor-π-Conjugated Dipicolinic Acid/Ester/Amide Ligands. Tetrahedron 2008, 64, 399− 411. (18) Picot, A.; D’Aleo, A.; Baldeck, P. L.; Grichine, A.; Duperray, A.; Andraud, C.; Maury, O. Long-Lived Two-Photon Excited Luminescence of Water-Soluble Europium Complex: Applications in Biological Imaging Using Two-Photon Scanning Microscopy. J. Am. Chem. Soc. 2008, 130, 1532−1533. (19) D’Aleo, A.; Picot, A.; Baldeck, P. L.; Andraud, C.; Maury, O. Design of Dipicolinic Acid Ligands for the Two-Photon Sensitized Luminescence of Europium Complexes with Optimized Cross-Sections. Inorg. Chem. 2008, 47 (22), 10269−10279. (20) Andres, J.; Chauvin, A.-S. Energy Transfer in CoumarinSensitised Lanthanide Luminescence: Investigation of the Nature of the Sensitiser and its Distance to the Lanthanide Ion. Phys. Chem. Chem. Phys. 2013, 15, 15981−15994. (21) de Sá, G. F.; Malta, O. L.; de Mello Donegá, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F. Spectroscopic Properties and Design of Highly Luminescent Lanthanide Coordination Complexes. Coord. Chem. Rev. 2000, 196, 165−195. (22) Malta, O. L. LigandRare-Earth Ion Energy Transfer in Coordination Compounds. A Theoretical Approach. J. Lumin. 1997, 71, 229−236.

S Supporting Information *

■

T ← 5D0/s−1

3

efficiency, and we found that a decrease in the symmetry leads to higher radiative emission rates (Arad; see eq S5 and ref 21) and higher emission quantum yield values for triplet energy levels >20000 cm−1 (Figure 5). In addition, the dependence of the energy-transfer rate of ligand → EuIII on RL (Figure 6) indicates that the highest rates are obtained for the shortest distances. To confirm that RL is the parameter that mostly influences ΦEu L , the rates of energy and back transfer for both complexes were calculated using LUMPAC36 (Table 2). The data show that a 50% increase in RL leads to a decrease of 5 orders of magnitude in the energy-transfer rate of ligand → EuIII, which thus accounts for the drastic decrease in ΦEu L . In summary, we investigated the parameters of symmetry, energy- and back-transfer rates, and RL that influence ΦEu L and provide an indication of the magnitudes that lead to the highest efficiencies to aid in the development of complexes with improved luminescence. We found that a change in the ligand conjugation changes RL, and this is the key influence on the large difference in the emission efficiencies of two complexes with otherwise comparable photophysical and structural properties.

■

T → 5D0/s−1

REFERENCES

(1) Lanthanide Luminescence: Photophysical, Analytical and Biological Aspects; Springer: Berlin, 2011; p 385. (2) In Luminescence of Lanthanide Ions in Coordination Compounds and Nanomaterials; de Bettencourt-Dias, A., Ed.; John Wiley and Sons: New York, 2014. (3) Bünzli, J.-C. G. On the Design of Highly Luminescent Lanthanide Complexes. Coord. Chem. Rev. 2015, 293−294, 19−47. (4) de Bettencourt-Dias, A. In Luminescence of Lanthanide Ions in Coordination Compounds and Nanomaterials; de Bettencourt-Dias, A., Ed.; John Wiley and Sons: New York, 2014. C

DOI: 10.1021/acs.inorgchem.6b02637 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry (23) de Silva, F. R. G.; Malta, O. L. Calculation of the Ligand− Lanthanide Ion Energy Transfer Rate in Coordination Compounds: Contributions of Exchange Interactions. J. Alloys Compd. 1997, 250, 427−430. (24) Malta, O. L. Mechanisms of Non-Radiative Energy Transfer Involving Lanthanide Ions Revisited. J. Non-Cryst. Solids 2008, 354, 4770−4776. (25) Aebischer, A.; Gumy, F.; Bünzli, J.-C. G. Intrinsic Quantum Yields and Radiative Lifetimes of Lanthanide tris(Dipicolinates). Phys. Chem. Chem. Phys. 2009, 11, 1346−1353. (26) Brayshaw, P. A.; Bünzli, J.-C. G.; Froidevaux, P.; Harrowfield, J. M.; Kim, Y.; Sobolev, A. N. Synthetic, Structural, and Spectroscopic Studies on Solids Containing tris(Dipicolinato) Rare-Earth Anions and Transition or Main-Group Metal-Cations. Inorg. Chem. 1995, 34, 2068− 2076. (27) Judd, B. R. Optical Absorption Intensities of Rare-Earth Ions. Phys. Rev. 1962, 127, 750−761. (28) Ofelt, G. S. Intensities of Crystal Spectra of Rare-Earth Ions. J. Chem. Phys. 1962, 37, 511−520. (29) Crosby, G. A.; Whan, R. E.; Alire, R. M. Intramolecular Energy Transfer in Rare Earth Chelates. Role of the Triplet State. J. Chem. Phys. 1961, 34, 743−748. (30) Whan, R. E.; Crosby, G. A. Luminescence Studies of Rare Earth Complexes: Benzoylacetonate and Dibenzoylmethide Chelates. J. Mol. Spectrosc. 1962, 8, 315−327. (31) Agbo, P.; Abergel, R. J. Ligand-Sensitized Lanthanide Nanocrystals: Merging Solid-State Photophysics and Molecular Solution Chemistry. Inorg. Chem. 2016, 55, 9973−9980. (32) Chauvin, A. S.; Gumy, F.; Imbert, D.; Bünzli, J.-C. G. Europium and Terbium tris(Dipicolinates) as Secondary Standards for Quantum Yield Determination. Spectrosc. Lett. 2004, 37, 517−532. (33) Chauvin, A. S.; Gumy, F.; Imbert, D.; Bünzli, J.-C. G. Erratum. Spectrosc. Lett. 2007, 40, 193−193. (34) Gassner, A.-L.; Duhot, C.; Bünzli, J.-C. G.; Chauvin, A.-S. Remarkable Tuning of the Photophysical Properties of Bifunctional Lanthanide Tris(dipicolinates) and its Consequence on the Design of Bioprobes. Inorg. Chem. 2008, 47 (17), 7802−7812. (35) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; RodriguezUbis, J. C.; Kankare, J. Correlation Between the Lowest Triplet State Energy Level of the Ligand and Lanthanide(III) Luminescence Quantum Yield. J. Lumin. 1997, 75 (2), 149−169. (36) Dutra, J. D. L.; Bispo, T. D.; Freire, R. O. LUMPAC Lanthanide Luminescence Software: Efficient and User Friendly. J. Comput. Chem. 2014, 35, 772−775. (37) Freire, R. O.; Rocha, G. B.; Simas, A. M. Sparkle/PM3 for the Modeling of Europium(III), Gadolinium(III), and Terbium(III) Complexes. J. Braz. Chem. J. Braz. Chem. Soc. 2009, 20 (9), 1638−1645.

D

DOI: 10.1021/acs.inorgchem.6b02637 Inorg. Chem. XXXX, XXX, XXX−XXX