Bioinspired Orientation of β-Substituents on Porphyrin Antenna

Feb 1, 2017 - Synopsis. We introduced a bioinspired approach by orientation of β-dilactone moieties on porphyrinates to modulate the energy transfer ...
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Bioinspired Orientation of β‑Substituents on Porphyrin Antenna Ligands Switches Ytterbium(III) NIR Emission with Thermosensitivity Yingying Ning,† Xian-Sheng Ke,† Ji-Yun Hu,† Yi-Wei Liu,† Fang Ma,‡ Hao-Ling Sun,‡ and Jun-Long Zhang*,† †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ Department of Chemistry, Beijing Normal University, Beijing 100875, P. R. China S Supporting Information *

ABSTRACT: “Configurational isomerism” is an important approach found in naturally occurring chlorophylls to modulate light harvesting function without significant structural changes; however, this feature has been seldom applied in design of antenna ligands for lanthanide (Ln) sensitization. In this work, we introduced a bioinspired approach by orientation of β-dilactone moieties on porphyrinates, namely cis-/trans-porphodilactones, to modulate the energy transfer process from the lowest triplet excited state of the ligand (T1) to the emitting level of ytterbium(III) (2F5/2, Yb*). Interestingly, near-infrared (NIR) emission of Yb(III) could be switched “on” by the cis-porphodilactone ligand, while the trans-isomer renders Yb(III) emission “off” and the ratio of quantum yields is ∼8. Analysis of the structure−photophysical properties relationship suggests that the significant emission difference is correlated to the energy gaps between T1 and Yb* (1152 cm−1 in the cis- vs −25 cm−1 in the transisomer). More interestingly, due to back energy transfer (BEnT), the Yb(III) complex of cis-porphodilactone exhibits NIR emission with high thermosensitivity (4.0%°C−1 in solution and 4.9%°C−1 in solid state), comparable to previously reported terbium (Tb) and europium (Eu) visible emitters, in contrast to the trivial emission changes of the trans-isomer and porphyrin and porpholactone analogues. This work opens up new access to design NIR emissive Ln complexes by bioinspired modification of antenna ligands.

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INTRODUCTION Design of NIR emissive lanthanides such as ytterbium (Yb), erbium (Er), and neodymium (Nd) attracts considerable attention for their potential applications in telecommunications, optical materials, and biological imaging.1 Exploring antenna ligands with the appropriate lowest triplet excited state to efficiently sensitize lanthanide emission plays a central role in overcoming the forbidden f−f transition of lanthanides.2 Thus, modulation of such a sensitization process by fine-tuning antenna ligands forms a chemical basis to Ln based optical functional materials. However, unlike extensively studied Eu and Tb visible emitters,3 how to precisely modulate the energy transfer process for an NIR emissive Ln(III) ion, due to the lack of systematic works on the structure-sensitization relationship, is still a difficult task. To address the issue, the natural light harvesting system sets a golden standard for harnessing sunlight power and tuning the energy transfer process (up or down conversion) with molecular precision.4 A prominent example is given by widespread chlorophylls b, d, and f in land plants, algae of ocean, and stromatolites, respectively, with gradually red-shifted Q bands by differing β-formyl substituent orientation (Figure 1a).5 In contrast to the tremendous modeling studies on deciphering how chlorophylls confer light harvesting function,6 much less has been reported regarding the application of such insights in © 2017 American Chemical Society

designing luminescence materials. Porphyrins and their derivatives are important antenna ligands to sensitize NIR emissive lanthanides, for their intense absorption, good coordination ability, and tunable triplet excited states.7 However, how to precisely predict the triplet excited state of the ligand is difficult for the relatively narrow absorbances in the Soret band (400− 420 nm) and Q-band (500−600 nm), which makes rational design of porphyrin antenna ligands to switch NIR emission of lanthanides challengeable. As synthetic models for natural chlorophylls, porpholactone showed intriguing photophysical properties between porphyrins and chlorins.8 Moreover, further β-dilactonization of porphyrin leads to porphodilactone, in which β-dioxazolone moieties replace the opposite pyrroles in porphyrin, allowing modulation of the excited states of porphyrinoids via subtle β-substituent orientation, in a similar manner to chlorophylls (Figure 1b).9 This provides an opportunity to circumvent the problems of designing NIR active porphyrinates such as stability, solubility, and intermolecular π−π stacking, arising from the traditional strategy based on “extended π-conjugation” in synthetic porphyrin chemistry.10 Previously, we reported the advantages of such modification in tuning NIR absorption,9a up-conversion function based Received: October 12, 2016 Published: February 1, 2017 1897

DOI: 10.1021/acs.inorgchem.6b02481 Inorg. Chem. 2017, 56, 1897−1905

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Inorganic Chemistry

Figure 1. Effect of orientation of β-substituents on NIR absorption for (a) chl b, d, f and (b) cis- and trans-F20TPPDL.

on triplet−triplet annihilation,9b and singlet oxygen sensitization,9c and trans-5,10,15,20-tetrapentafluorophenylporphodilactone (trans-F20TPPDL) is more efficient than the cis-isomer (cis-F20TPPDL). These previous studies demonstrated the perspective of porpholactone and their derivatives to sensitize NIR emissive lanthanides.11 The choice of appropriate Ln ion for this study is critical. The f-orbital energies of Gd(III) are generally higher than those of the ligand π orbitals, so that energy transfer from T1 to Gd(III) does not occur and only phosphorescence derived from T1 occurs. On the contrary, the Yb(III) 2F5/2 f-orbital is of lower or comparable energy, so that the emission could occur from the 2F5/2 → 2F7/2 transition of Yb(III) and also possibly from T1 in Yb(III) complexes. This provides an appropriate model to investigate the effect of β-dioxazolone orientation on tuning energy transfer (down conversion) from T1 to Yb*. In this work, we prepared the Yb(III) complexes of cis- and trans-porphodilactones (Yb-1: cis-F20TPPDL-Yb(III)-LOMe; Yb-2: trans-F20TPPDL-Yb(III)LOMe; LOMe: Kläui’s ligand [(η5-C5H5)Co{(H3CO)2PO}3]−). Interestingly, we found that cis-porphodilactone could switch the NIR emission of Yb(III) “on” and the trans-isomer leads to NIR emission “off” (the ratio of quantum yields is ∼8). Photophysical studies showed that the significant emission difference is correlated to the energy gaps (ΔEYb‑1: 1152 cm−1 vs ΔEYb‑2: −25 cm−1) and spectral overlaps between the triplet excited states of porphodilactone ligands and the emitting level of Yb(III). This clearly demonstrated the importance of subtle structural changes on designing NIR emissive lanthanides. More importantly, due to BEnT occurring, the Yb(III) complex of cis-porphodilactone exhibits a thermosensitive NIR emission, whereas other Yb analogues do not. The thermosensitivity (4.0%°C−1 in solution and 4.9%°C−1 in solid state) is comparable to previously reported Eu(III) and Tb(III) emitters in the visible region.12 As temperature dependent NIR emission is important for noninvasive and accurate measurement of temperature by a remote optical detection system without background interference from the visible region,13 this approach showed the potential to design thermosensitive NIR emissive lanthanides through precise modulation of the energy transfer process with minor structural changes.

(Figure 2a), respectively, with Kläui’s tripodal ligand as ancillary ligand were synthesized following the reported procedures.14 Yb-1 and Yb-2 were obtained in the yields of 70 and 75%, after purification by column chromatography and subsequent recrystallization. Their structures were characterized by highresolution electrospray ionization mass spectrometry, element analysis, and IR spectroscopies (detailed synthetic procedure and characterization are listed in the Experimental Section and Supporting Information). Single crystals of Yb-1 (CCDC: 1487066) and Yb-2 (CCDC: 1487065) were obtained by diffusion of hexane into the CHCl3 solution of Yb-1 and Yb-2, respectively. They both crystallized in the orthorhombic space group (Table S1). Crystal structures showed seven coordination environments around the ytterbium center (Figure 2b), which is sandwiched between the porphyrin ring and the tripodal Kläui ligand. The average Yb−N distances are similar (2.368− 2.380 Å) in the two complexes, and ytterbium(III) ion is located 1.215 and 1.209 Å above the N4 porphyrin mean plane for Yb-1 and Yb-2, respectively. Thus, combining the previously reported ytterbium porphyrin (Yb-3: 5,10,15,20tetrapentafluorophenylporphyrin-Yb(III)-LOMe) and porpholactone (Yb-4: 5,10,15,20-tetrapentafluorophenylporpholactoneYb(III)-LOMe) analogues,9c we could systematically investigate the effect of β-modification of porphyrinates on Yb(III) NIR emission. Photophysical Properties. The absorption spectra of Yb-1 and Yb-2 in dichloromethane solution at 298 K are shown in Figure 3a. They displayed an intense Soret band at ca. 428 nm and Q bands in the range 550−700 nm (Figure 3a). Compared to Yb-3 and Yb-4 (Figure S1),9c Yb-1 and Yb-2 presented typical red-shifted Qy(0,0) bands at ca. 636 and 666 nm, respectively, along with increasing extinction coefficients (Table 1 and Figure S1). Upon excitation at 425 nm, Yb-1 exhibited strong and characteristic emission bands at ca. 900−1200 nm derived from the 2F5/2 → 2F7/2 transition of Yb(III) with a monoexponential decay lifetime of 28.8 μs in degassed CH2Cl2 (Figure 3b), similar to Yb-3 and Yb-4 (Figure S2 and S3). However, Yb-2 displayed weak and broad emission (Figure 3b) with a biexponential decay lifetime of 6.5 μs (67%) and 25.8 μs (33%). Gaussian deconvolution of the NIR emission of Yb-2 at 77 K (Figure 3c) suggests that one contributed to the 3π−π* intraligand transition (ca. 982, 1050, and 1135 nm), similar to the Gd complex (ca. 978, 1047, and 1037 nm, Figure 3d), and another to the 2F5/2 → 2F7/2 transition of Yb(III) ion, different

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RESULTS AND DISCUSSION Synthesis and Characterization. Starting from cis- and trans-F20TPPDL, two Yb(III) complexes Yb-1 and Yb-2 1898

DOI: 10.1021/acs.inorgchem.6b02481 Inorg. Chem. 2017, 56, 1897−1905

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Inorganic Chemistry

Figure 2. (a) Chemical structures of Yb-1 to Yb-4; (b) Single crystal structures of Yb-1 and Yb-2 (50% probability). Hydrogen atoms and solvent molecules are omitted for clarity.

was calculated by the characteristic 2F7/2 → 2F5/2 absorption of Yb(III) (Figure S6) based on a modified Einstein equation.17 Therefore, ηsen values for Yb-1 and Yb-2 were obtained to be 84 and 12%, respectively, indicating that the orientation of β-dioxazolone moieties significantly affects the T1 → Yb* energy transfer process. Since the orientation of β-dioxazolone moieties on porphyrinates is known to tune the triplet excited state of the ligand according to our previous studies on Pt, Pd, and Gd complexes,9b,c we investigated the energy gap (ΔE) and spectral overlap between T1 and Yb*. Since the emitting energy levels of Gd(III) ion are high-lying compared to the ligand’s T1 state, the ligand centered phosphorescence could be observed due to the heavy atom effect of Gd(III).18 Therefore, we estimated the lowest triplet excited state of cis-/trans-porphodilactones by the phosphorescence spectra of Gd-1 (cis-F20TPPDL-Gd(III)-LOMe) and Gd-2 (transF20TPPDL-Gd(III)-LOMe) at 77 K. As shown in Figure 3d, broad and vibronic-structured emission bands with maxima at ca. 11402 and 10225 cm−1 were observed for Gd-1 and Gd-2, respectively. Thus, the T1 energy level for Yb-1 was calculated to be ca. 1152 cm−1 above the emitting level of Yb(III) (ca. 10,250 cm−1),19 in contrast to the small and negative ΔE (−25 cm−1) for Yb-2. This contributes to the lower T1 →Yb* energy transfer efficiency in Yb-2 than Yb-1, according to the energy gap law.20 On the other hand, we examined the spectral overlap between the

from the sharp emission peaks of other Yb(III) complexes (Figure S4). As the first component of the decay lifetime (6.5 μs) is also comparable to the phosphorescence decay lifetime of the Gd trans analogue,9c the emission of Yb-2 should be a mix of the ligand’s phosphorescence and the 2F5/2 → 2F7/2 transition of ytterbium(III). Importantly, Yb-1 exhibited ca. 8 times higher quantum yield (3.3%) than Yb-2 (0.4%) with the reference of 5,10,15,20-tetraphenylporphyrin-Yb(III)[(cyclopentadienyl)tris(di(ethyl)phosphito)cobaltate] (YbTPP(LOEt)), ΦΔ = 0.024),15 comparable to Yb-3 (2.5%) and Yb-4 (2.8%), in degassed CH2Cl2 (Table 1 and Figure S5). Thus, these results revealed a significant effect of the orientation of β-dioxazolone moieties on Yb(III) NIR emission. Nanosecond transient absorption of Yb-2 exhibited an appearance of strong characteristic absorbance from the ligand’s triplet states, accompanied by the absorption bleaching of the ground state, in contrast to the weak signal change ( 300 °C (dec.). Yield: 75%; IR (KBr): ν̃ = 1771 cm−1 (CO); HRMS (ESI+) m/z [M + H]+: calcd for C53H28CoF20N4O13P3Yb 1633.9267; found: 1633.9228; elemental analysis calcd (%) for C53H27CoF20N4O13P3Yb+C2H5OH: C 39.35; H 1.98; N 3.34; found: C 39.27, H 1.85, N: 3.40. Yb-3. Mp > 300 °C (dec.). Yield: 74%; HRMS (ESI+) m/z [M + H]+: calcd for C55H32CoF20N4O9P3Yb 1597.9783; found: 1597.9756; elemental analysis calcd (%) for C55H31CoF20N4O9P3Yb: C 41.37, H 1.96, N 3.51; found: C 41.21, H 1.80, N 3.50. Yb-4. mp >300 °C (dec.). Yield: 83%; IR (KBr): 1771 cm−1 (CO); HRMS (ESI+) m/z [M + H]+: calcd for C54H30CoF20N4O11P3Yb 1615.9525; found: 1615.9521; elemental analysis calcd (%) for C54H29CoF20N4O11P3Yb: C 40.17, H 1.81, N 3.47; found: C 40.05, H 1.82, N 3.23. X-ray Crystallography. Complete data sets for Yb-1 and Yb-2 were collected. Single crystals suitable for X-ray analysis were coated

EXPERIMENTAL SECTION

General Materials and Methods. Unless otherwise stated, all reactions were performed under an inert atmosphere of nitrogen. Free 5,10,15,20-tetrapentafluorophenylporphyrin (F20TPP), 5,10,15,20-tetrapentafluorophenylporpholactone (F20TPPL), and cis(cis-F20 TPPDL) and trans-5,10,15,20-tetrakispentafluorophenylporphodilactones (trans-F20TPPDL) were synthesized according to the literature procedures.9a,30 UV−vis spectra were recorded on an Agilent 8453 UV−vis spectrometer equipped with an Agilent 89090A thermostat (±0.1 °C) at 25 °C. NIR absorption spectra were recorded on a Shimadzu UV3600Plus. The emission spectrum and lifetime were recorded on an Edinburgh Analytical Instruments FLS920 lifetime and steady state spectrometer (450 W Xe lamp, 5 W microsecond flash lamp, PMT R928 for visible emission spectrum, HAMAMATSU R5509-73 PMT with C9940-02 cooler for NIR emission spectrum and luminescence lifetime). The emissions in the NIR region were all calibrated with the calibration curve (Figure S16). The temperaturedependent experiments were recorded on the Edinburgh Analytical Instruments FLS920 equipped with an Oxford Optistat DN2 cryostat. The transient absorption spectra and triplet excited state decay dynamics were recorded on an Edinburgh LP920 spectrometer combined with OPO laser excitation pulse (10 Hz, 2 mJ pulse−1). IR spectra were recorded on a Bruker VECTOR22 FT-IR spectrometer as KBr pellets. ESI mass spectra were recorded on a Bruker APEX IV FTICR mass spectrometer. Elemental analyses (C, H, N) were recorded on an ElementarAnalysensysteme GmbH vario EL Elemental Analyzer. For the optical measurements in liquid solution, spectroscopicgrade CH3OH, C2H5OH, and CH2Cl2 were used as purchased from Alfa-Aesar, and anhydrous CH2Cl2 was distilled from calcium hydride. For the optical measurements at 77 K, the CH3OH/C2H5OH = 1:1 glassing solvent was used. Synthetic Procedures of Yb-1 to Yb-4. Complexes Yb-1 to Yb-4 were synthesized according to previous procedures.7a,14,31 Generally, free base porphyrinoid ligand (0.05 mmol, cis-F20TPPDL, 1902

DOI: 10.1021/acs.inorgchem.6b02481 Inorg. Chem. 2017, 56, 1897−1905

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with Paratone-N oil, suspended in a small fiber loop, and placed in a gaseous nitrogen stream on a Bruker D8 VENTURE X-ray diffractometer at 296(2) K and 180(10) K for Yb-1 and Yb-2, respectively. Diffraction intensities were measured using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection, indexing, initial cell refinements, frame integration, and final cell refinements were accomplished using the program APEX2.32 Absorption corrections were applied using SADABS.33 Scattering factors and anomalous dispersion corrections were taken from the International Tables for X-ray Crystallography. The structure was solved by direct methods using SHELXS34 and refined against F2 on all data by full-matrix leastsquares with SHELXL-2014 following established refinement strategies.35 Yb-1 crystallized in orthorhombic space group Pmmn with one cocrystallized methanol solvent molecule. Yb-2 also crystallized in orthorhombic space group Pmmn, with one cocrystallized n-pentane solvent molecule. Disorder of the atoms in the axial [(cyclopentadienyl)tris(di(methyl)phosphito)cobaltate] ligand and the dioxazolone moieties were observed, but no improvement was observed after further refinements or recrystallization in different solvents or at low temperature. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms binding to carbon were included into the model at geometrically calculated positions and refined using a riding model. Details of the data quality and a summary of the residual values of the refinements are listed in Table S1. Quantum Yields Determination. Quantum yields in solution were determined using a comparative method36 and the equation: Φs/Φr = (Gs/Gr)*(ηs2/ηr2), where the subscripts r and s denote reference and sample, respectively, Φ is the quantum yield, G is the slope from the plot of integrated emission intensity vs absorbance, and η is the refractive index of the solvent. The reference was YbTPP(LOEt) in CH2Cl2 (Φr = 0.024, λex = 425 nm).15 For the quantum yield measured in air-saturated CH2Cl2, YbTPP(LOEt) and solutions of Yb-1 to Yb-4 with four different concentrations were first prepared in anhydrous CH2Cl2. The absorbances of all the solutions at 425 nm were recorded on an Agilent 8453 UV−vis spectrometer equipped with an Agilent 89090A thermostat (±0.1 °C), and NIR emissions were recorded on an Edinburgh Analytical Instruments FLS920 lifetime and steady state spectrometer with a liquid N2 cooled Ge detector with the excitation wavelength at 425 nm under an identical condition. According to the ratio of the slope GYb‑1/Gr, GYb‑2/ Gr, GYb‑3/Gr, and GYb‑4/Gr, the relative quantum yields of Yb-1 to Yb-4 could be obtained according to the equation described above. The absorbances of all the samples and references are below 0.15, and absorbance values undergo background correction by subtracting the average over the range from 800 to 820 nm. The integrated emission intensity which integrates from 880 to 1150 nm has been subtracting the blank (integrated emission intensity with the only pure CH2Cl2 under an identical condition). For the quantum yield measured in degassed CH2Cl2, solutions of Yb-1 to Yb-4 with four different concentrations were prepared in anhydrous CH2Cl2 and degassed via five freeze−pump−thaw cycles. Then the measurement and calculation were the same as above. Determination of Rate Constants for Oxygen Quenching of NIR Emission of Yb-1 (ksv). Solutions of Yb-1 in CH2Cl2 were prepared to have absorption of 0.1 at 425 nm. The emission intensity in the absence of quencher (O2) was measured with the sample, which was degassed via five freeze−pump−thaw cycles. Then digital flow meters were used to mix different ratios of N2 and O2. These solutions were placed in sealed quartz cuvettes and bubbled with corresponding mixed gas (60 mL/min) for 5 min. The concentration of oxygen dissolved in the solutions was calculated using Henry’s Law. The mole fraction solubility of oxygen in liquid dichloromethane with 1 atm of oxygen is 0.000561.37 Henry’s Law Constant was calculated as 0.00863 M/atm for oxygen in CH2Cl2.38 Multiplying the mole fraction of oxygen in the gas mixture by 0.00863 yields the concentration of oxygen in the solution. ksv values were obtained from the slopes of Stern−Volmer plots (slope = ksv × τ0).

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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.6b02481. Detailed characterization and photophysical and crystallographic data (PDF) X-ray crystallographic data of Yb-1 in CIF format (CIF) X-ray crystallographic data of Yb-2 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun-Long Zhang: 0000-0002-5731-7354 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support from National Key Basic Research Support Foundation of China (NKBRSFC) (2015CB856301) and NSFC (grants no. 21571007, 21271013, and 21321001).

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DOI: 10.1021/acs.inorgchem.6b02481 Inorg. Chem. 2017, 56, 1897−1905

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DOI: 10.1021/acs.inorgchem.6b02481 Inorg. Chem. 2017, 56, 1897−1905