Near-Infrared Photoluminescence and Reversible Trans-to-Cis Photoisomerization of Mononuclear and Binuclear Ytterbium(III) Complexes Functionalized by Azobenzene Groups.

Two mononuclear and one binuclear ytterbium complexes with dual near-infrared (NIR) photoluminescence and reversible trans-to-cis photoisomerization functions were synthesized and characterized. The central ytterbium(III) ion coordinates with two β-diketonate (4,4,4-trifluoro-1-phenylbutane-1,3-dionate (tfd)) ligands and one deprotonated azobenzene-containing tetradentate ligand [(E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (HL), (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or (E)-4,4'-N',N'-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (H2DL)] to form a neutral ternary complex ([Yb(tfd)2L], [Yb(tfd)2(NL)], or [Yb2(tfd)4(DL)], respectively), where the ytterbium(III) ion is eight-coordinated to N3O5 donor sets. X-ray crystallographic analysis shows that all three complexes form a trigonal dodecahedron geometry with similar -N=N- distances that are slightly longer than those of the pure azobenzene-containing ligands. The NIR luminescence properties of the Yb(III) complexes were determined at a wavelength of about 980 nm with quantum yields in the range of 0.4-0.6% in ethanol and acetonitrile solutions at room temperature, and trans-to-cis photoisomerization was determined with the quantum yields (Φt→c = 10-2) at the same level as their pure ligands. The trans-to-cis photoisomerization rates of the complexes (10-4 s-1) are slightly higher than those of the pure ligands and similar to azobenzene (10-5 to 10-4 s-1). From time-dependent density functional theory calculations of the energy levels of the first excited triplet states of the ligands, the energies of the lowest excited triplet states of all of the ligands are higher than the resonance level of Yb3+ (2F5/2, 1.2722 eV). We suggest that these azo-containing ligands may participate in energy transfer to the ytterbium ion, in addition to the main "antenna effect" ligand tfd. This is the first report of azobenzene group-functionalized ytterbium complexes with dual NIR luminescence and photoisomerization properties, indicating that azobenzene-containing lanthanide(III) complexes have potential applications as dual function materials in biological systems.

Introduction

It is well-known that typical organic photochromic compounds, such as diarylethene, fulgides, spiropyrans, naphthopyrans, and azo compounds, can be interconverted between two states with different spectroscopic properties using light, which makes them good candidates for realizing smart optical modulation.[1−7] Integration of photochromic units with luminescent lanthanide components has potential applications in functional optoelectronic systems and biological imaging without optical interference.[8,9] For example, the combination of lanthanide-doped upconverting nanoparticles and diarylethene photoswitches not only has optical memory applications but can also be modulated with near-infrared (NIR) light, offering new opportunities in photodynamic therapy.[10,11]

Among the typical organic photochromic compounds, azobenzene derivatives have been extensively studied both in solution and in polymer matrices, and they are promising for applications in a variety of areas, from biological science to information technology.[12−18] The photochromism mechanism of azobenzene derivatives is because of trans-to-cis isomerization upon irradiation with ultraviolet (UV) light and reverse isomerization by irradiation with visible light or heating.

In recent years, our group has put considerable effort into developing photochromism unit azobenzene-attached tris-β-diketonate lanthanide complexes to investigate the possibility of materials with dual photoluminescence and photoisomerization functions. Initially, we designed 4,4,5,5,6,6,6-heptafluoro-1-azobenzene-1,3-hexanedione and its derivative ligands.[19] We found that the coordinated Ln(III) ion enhances the reversible trans-to-cis isomerization properties of the ligand and remarkably increases its fatigue resistance without luminescence. We subsequently manipulated the ligands and designed (E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide lanthanide complexes.[20] We detected trans-to-cis photoisomerization and visible emission of 614 nm from Eu3+ in the complexes in solution. This is remarkable, although it is not very useful from a practical biological viewpoint, owing to the strong optical interference generated by the biological samples.[21]

NIR luminescence of lanthanide complexes, such as Nd(III), Ho(III), Er(III), and Yb(III) complexes, is a well-established phenomenon with wide applications in solid-state laser materials, telecommunication, and biological systems.[22−28] Among these complexes, highly luminescent lanthanide-containing systems, especially Yb(III) complexes, can be used as probes and labels for a variety of chemical and biological applications because the Yb(III) ion exhibits a relatively high quantum efficiency with an emission wavelength near 1000 nm, which avoids optical interference from the background and enhances the signal-to-noise ratio in biological tissue applications.[10,24,26,29−31] Some Yb(III) complexes have been tested for photodynamic therapy of cancer and NIR luminescence localization of tumors.[10,32]

In our previous study,[19] we investigated azobenzene-containing Yb(III) complexes but did not capture NIR luminescence at room temperature, owing to the forbidden nature of their electronic transitions. In this study, we incorporated the aforementioned tetradentate ligand into β-diketonate ytterbium complexes to complete the coordination sphere around the Yb(III) ion and increase the luminescence intensity. We believe that the newly designed complexes could possess highly asymmetric eight-coordinate structures to overcome solvent coordination, allowing efficient energy transfer from the coordinated ligands to the Yb(III) ion. Mononuclear and binuclear Yb(III) complexes constructed from two β-diketonate ligands [4,4,4-trifluoro-1-phenylbutane-1,3-dionate (tfd)] and one deprotonated tetradentate ligand bearing an azobenzene group [(E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HL), (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or (E)-4, 4′-N′,N′-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (H2DL)] show NIR luminescence and photo- and thermoisomerization properties. These features and functions have possible applications in biological systems.

Results and Discussion

X-ray Crystallographic Analysis

Quality crystals of H2DL, [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] were obtained by the vapor diffusion method with N,N-dimethylformamide (DMF) and acetonitrile binary solutions. The molecular geometries of their crystal structures were determined by single-crystal X-ray crystallography. The crystallographic data are given in Table . Selected bond lengths and angles of H2DL, [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] are summarized in Table S1. The hydrogen bond parameters are summarized in Table S2.

Table 1

Crystallographic Data of the H2DL Ligand and [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] Complexes

crystal data	H2DL	[Yb(tfd)2L]	[Yb(tfd)2(NL)]	[Yb2(tfd)4(DL)]
CCDC number	1471579	1508963	1508962	1471580
empirical formula	C38H34N10O2, 2(H2O)	C45H33F6N7O5Yb, C3H7NO	C47H38F6N7O5Yb	C78H56F12N10O10Yb2, 4(C3H7NO)
fw	698.78	1097.91	1067.88	2159.79
temp (K)	203(2)	183(2)	203(2)	203(2)
wavelength (Å)	0.71073	0.71073	0.71073	0.71073
cryst syst	monoclinic	monoclinic	triclinic	triclinic
space group	P21/n	P21/c	P1̅	P1̅
a (Å)	6.414(2)	25.968(6)	8.728(3)	13.820(9)
b (Å)	32.891(12)	9.345(2)	15.386(6)	14.788(7)
c (Å)	8.516(3)	20.528(4)	17.749(7)	22.784(13)
α (deg)	90	90	69.465(7)	91.11(3)
β (deg)	107.395(7)	111.521(2)	83.487(7)	90.11(5)
γ (deg)	90	90	87.270(7)	97.78(4)
V (Å)	1714.3(11)	4634.29(18)	2217.6(14)	4613(4)
Z	2	4	2	2
calcd density (mg m–3)	1.354	1.574	1.599	1.5555
abs. coeff (mm–1)	0.092	2.099	2.189	2.109
θ range	2.48–52.00	3.0–50.50	2.82–52.00	0.89–50.00
no. of reflns collected	9515	14 465	12 276	23 254
data/restraints/parameters	3364/0/238	9759/1/615	8532/0/597	15862/2/1197
goodness of fit on F2	0.908	1.108	0.938	1.048
final R indexes [I ≥ 2σ(I)]	R1 = 0.0514	R1 = 0.0419	R1 = 0.0377	R1 = 0.0399
	wR2 = 0.1121	wR2 = 0.0669	wR2 = 0.0774	wR2 = 0.1097
R indexes [all data]	R1 = 0.0895	R1 = 0.0591	R1 = 0.0450	R1 = 0.0482
	wR2 = 0.1213	wR2 = 0.0717	wR2 = 0.0789	wR2 = 0.1131

From the single-crystal structure data (Table ), it can be seen that the H2DL ligand is a monoclinic system with the P21/n space group. The molecule is composed of two para-substituted bis(pyridin-2-ylmethyl)benzohydrazide groups and a −N=N– bridge (Figure S1). In the unit cell of H2DL, the asymmetric unit contains only half a molecule with an inversion center at the azo bridge and one solvent H2O molecule. Each molecule of H2DL forms hydrogen bonds with solvent H2O molecules. The N=N double-bond length is 1.221(3) Å, the C=O double-bond length is 1.209(2) Å, the C–N bond lengths are in the range 1.300–1.320 Å, and all of the bond lengths are very close to the parameters for similar known structures.

The [Yb(tfd)2L] complex crystallizes in the monoclinic P21/c space group with four molecules in the unit cell. The crystal of complex [Yb(tfd)2L] forms by chelation of two tfd and one deprotonated HL ligand anions. The tfd ligand chelates to the Yb(III) ion in a normal fashion through its two oxygen atoms. The L anion chelates to the Yb(III) ion in a tetradentate fashion by its oxygen atom and three nitrogen atoms from the bis(pyridin-2-ylmethyl)benzohydrazide group. The coordination sphere is an eight-coordinated Yb(III) center with a [YbN3O5] coordination environment, forming a trigonal dodecahedron geometry, as shown in Figure . The Yb–O bond lengths are in the range of 2.216(2)–2.272(3) Å and the Yb–N bond lengths are in the range of 2.490(3)–2.506(3) Å, which are in agreement with the typical values for previously reported Yb(III) complexes.[19,33] The N=N double-bond length in [Yb(tfd)2L] is 1.233(5) Å and the C=O double-bond length in L is 1.279(4) Å, which are slightly longer than the corresponding distances for the pure H2DL ligand.

Figure 1

Crystal structure of the [Yb(tfd)2L] complex (only the hydrogen atoms involved in hydrogen bonds are shown) including the (a) coordination polyhedron geometry of the central Yb(III) ion and (b) packing in the unit cell.

The [Yb(tfd)2(NL)] complex crystallizes in the triclinic P1̅ space group, with two molecules in the unit cell. The coordination sphere of [Yb(tfd)2(NL)] is the same as that of [Yb(tfd)2L], with two tfd ligands and one deprotonated HNL ligand chelating to the Yb(III) center in a trigonal dodecahedron geometry (Figure S2). The Yb–O bond lengths are in the range of 2.215(3)–2.288(3) Å and the Yb–N bond lengths vary from 2.492(3) to 2.530(3) Å, which are also reasonable ranges. The N=N double-bond length in [Yb(tfd)2(NL)] is 1.282(5) Å and the C=O double-bond length in L is 1.279(4) Å, which are also a little longer than the corresponding distances for the pure H2DL ligand.

The [Yb2(tfd)4(DL)] complex also crystallizes in the triclinic P1̅ space group. The asymmetric unit contains two complex molecules, with each molecule composed of two Yb(III) ions, four tfd anion ligands, and one DL anion ligand, and two DMF molecules. The coordination environment of each Yb(III) center is the same as those of [Yb(tfd)2L] and [Yb(tfd)2(NL)]. The coordination sphere of each Yb(III) ion in [Yb2(tfd)4(DL)] has a trigonal dodecahedron geometry (Figure ). The Yb–O bond lengths vary from 2.227(3) to 2.332(4) Å and the Yb–N bond lengths are in the range of 2.445(4)–2.490(4) Å. The N=N double-bond lengths of [Yb2(tfd)4(DL)] are 1.245(8) and 1.231(8) Å and C=O double-bond lengths of L are 1.269(6) and 1.282(6) Å, which are similar to those of the [Yb(tfd)2L] and [Yb(tfd)2(NL)] complexes. The similar ranges of the N=N and C=O distances are presumably because the ligands of [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] are the “same,” except for the p-dimethylamino substituent on the NL ligand.

Figure 2

Crystal structure of the [Yb2(tfd)4DL] complex (only the hydrogen atoms involved in hydrogen bonds are shown) including the (a) coordination polyhedron geometry of the central Yb(III) ion and (b) packing in the unit cell.

Thermal Analysis of the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] Complexes

Thermogravimetric analysis (TGA) of the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] complexes was performed under a nitrogen atmosphere in the temperature range of 30–870 °C for powder samples. As seen in Figure , the TGA plots show that the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] complexes exhibit a similar thermal decomposition process. Below 260 °C, the three complexes do not decompose. The first decomposition stage occurs in the temperature range of 260–280 °C with mass losses of 21.04, 19.25, and 22.23% for the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] complexes, respectively. The mass loss percentages are in conformity with the material decomposition and elimination of one tfd ligand for complexes [Yb(tfd)2L] (theoretically calculated value 20.99%) and [Yb(tfd)2(NL)] (20.14%) and two tfd ligands for the complex [Yb2(tfd)4(DL)] (23.04%). Upon further heating, the complexes continue decomposing until 380 °C. The complexes then gradually reach constant masses at 870 °C. From the thermal analysis, it can be seen that the tfd ligand is more easily dropped down from the complexes than the L, NL, and DL ligands, confirming that the designed HL, HNL, and H2DL ligands are excellent coordinating tetradentate ligands for lanthanide ions (Scheme ).

Figure 3

TGA curves of the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] complexes.

Scheme 1

Synthetic Procedures for Preparation of the Ligands and Complexes

Photoisomerization Behavior of H2DL in Ethanol and Acetonitrile Solutions

The photoisomerization behavior of the HL and HNL ligands has been investigated in detail by our group. The results show that the photoisomerization quantum yield (Φt→c) of HL is less than that of substituted azobenzenes, such as 4-methoxy-4′-nitroazobenzene and 2,4,6-trimethylazobenzene. The para electron-donating N,N-dimethylamino group in HNL produces a drastic change in absorption, and overall, it is unfavorable for the photoisomerization properties of azobenzene in the molecule. We are curious about the photoisomerization properties of azobenzene that are affected by two para-substituted bis(pyridin-2-ylmethyl)benzohydrazide groups. Hence, the symmetric aromatic azo compound H2DL was synthesized by oxidative dimerization of the aromatic amines by tert-butylhypoiodite at room temperature according to the Minakata method.[34] The photoisomerization properties of H2DL were investigated and compared with those of HL and HNL.

Figure shows the changes in the absorption spectrum of H2DL upon UV irradiation at 365 nm and visible-light irradiation at 450 nm with time in ethanol solution. From Figure , the UV–visible (UV–vis) absorption spectrum of the ligand H2DL shows a broad strong π–π* transition band at 332 nm, with a molar extinction coefficient (ε) of 3.48 × 104 L mol–1 cm–1 and two other intense absorption bands at 261 (ε = 2.09 × 104 L mol–1 cm–1) and 265 nm (ε = 1.78 × 104 L mol–1 cm–1). When the spectrum is enlarged (Figure S3), there is a very weak n−π* transition band of the azo group in the range of 400–500 nm. When the solution is irradiated by 365 nm UV light, the intensity of the π–π* absorption band at 332 nm gradually decreases with increasing irradiation time, whereas the absorbance bands at 261 and 265 nm increase with an isosbestic point at 289 nm. Because of the extremely weak n−π* transition band of the azo group, it is very difficult to observe the change in the absorption of the weak n−π* forbidden transition. The spectral change can still indicate transformation of the trans form of H2DL to the cis form. A photostable state is reached when the solution is irradiated for 10 min. With subsequent irradiation of the solution with 450 nm visible light, the intensity of the broad π–π* absorption band sharply increases in the first 5 s and remains unchanged after 2 min, finally leading to 75% recovery of the absorption spectrum. Owing to the special symmetrical structure of H2DL, the cis form of H2DL is very unstable, and it can rapidly transform to the stable trans form upon exposure to visible light. This spectral change reversibly and repeatedly occurs. The absorption intensity of the peak at 332 nm did not change when 10 cycles of irradiation with 365 nm UV light and 450 nm visible light were performed (Figure S4). The photoisomerization reaction of H2DL in acetonitrile solution was also investigated, and the reaction was similar to that in ethanol solution (Figures S5 and S6). The first-order rate constants (ks) of the photoisomerization reaction of H2DL in ethanol and acetonitrile solutions at 365 nm irradiation wavelength were calculated to be 2.0 × 10–3 and 1.3 × 10–3 s–1, respectively, using a previously reported method.[19] These data are at the same level as those of HL and HNL and slightly higher than those of azobenzene and its derivatives (10–5 to 10–4 s–1). The estimated photoreaction quantum yields (Φt→c) are 0.015 in ethanol and 0.012 in acetonitrile, which are slightly less than those of HL and HNL and less than those of substituted azobenzenes, such as 4-methoxy-4′-nitroazobenzene and 2,4,6-trimethylazobenzene.[20,35,36] These data are given in Table .

Figure 4

UV–vis spectral changes of H2DL in ethanol solution (2.0 × 10–5 mol/L) upon irradiation at 365 nm and recoverable irradiation at 450 nm as a function of time.

Table 2

Quantum Yields (Φt-c) and Photoisomerization Rate Constants (s–1) of H2DL, [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] in Different Solvents

	ethanol		acetonitrile
compounds	102Φt→c	104kiso	102Φt→c	104kiso
H2DL	1.5 ± 0.1	20.0 ± 1.1	1.2 ± 0.1	13.0 ± 0.6
[Yb(tfd)2L]	4.6 ± 0.4	38.0 ± 1.8	3.5 ± 0.3	45.0 ± 2.1
[Yb(tfd)2(NL)]	2.0 ± 0.3	7.8 ± 0.5	0.9 ± 0.2	1.5 ± 0.3
[Yb2(tfd)4(DL)]	0.6 ± 0.1	30.0 ± 1.4	0.2 ± 0.05	60.0 ± 2.5

Photoisomerization Behavior of the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] Complexes in Ethanol and Acetonitrile Solutions

The [Yb(tfd)2L] complex shows similar spectral changes in ethanol and acetonitrile solutions upon irradiation with 365 or 450 nm light. Figure shows the typical spectral change of the [Yb(tfd)2L] complex in ethanol solution upon irradiation with 365 nm UV light and its recoverable spectrum after irradiation with 450 nm visible light. The π–π* absorption band of the [Yb(tfd)2L] complex in ethanol solution has a peak at 329 nm and the weak n−π* absorption band has a peak at 449 nm (Figure ). When the ethanol solution of the [Yb(tfd)2L] complex is irradiated with 365 nm UV light, a band appears at 449 nm at the expense of the original band at 329 nm, which has two isosbestic points located at 283 and 389 nm. This spectral change indicates that the azo group of the [Yb(tfd)2L] complex undergoes reversible photoisomerization in solution. The [Yb(tfd)2L] complex reaches a photostable state after 30 min of irradiation at 365 nm, and 61% of the compound returns to the trans form when the UV irradiation is changed to 450 nm visible light for 4 s. The recovery percentage of the trans form is lower than that of the pure HL ligand but similar to that of other HL-functionalized lanthanide complexes.[20] The stability of trans–cis–trans photoisomerization was then investigated. Ten cycles were performed by irradiating the [Yb(tfd)2L] complex in ethanol solution with 365 nm light for 30 min followed by 450 nm light for 2 min. As shown in Figure , there are no signs of photodegradation. This indicates that the [Yb(tfd)2L] complex is stable under these conditions. The spectral changes of the [Yb(tfd)2L] complex in acetonitrile solution upon irradiation with 365 nm UV light and its recoverable spectrum after irradiation with 450 nm visible light are similar to those in ethanol solution, as shown in Figures S7 and S8. The reversible isomerization reaction for more than 10 cycles shows the excellent fatigue resistance and reversibility of the photoisomerization process of the [Yb(tfd)2L] complex.

Figure 5

UV–vis spectral change of the [Yb(tfd)2L] complex in ethanol solution (2.0 × 10–5 mol/L) upon irradiation at 365 nm and recoverable irradiation at 450 nm as a function of time.

Figure 6

UV–vis spectral changes of the [Yb(tfd)2L] complex in ethanol solution (2.0 × 10–5 mol/L) upon irradiation with UV light at 365 nm and visible light at 450 nm. The inset shows the maximum absorption intensity for 10 cycles of alternating irradiation with UV light at 365 nm and visible light at 450 nm.

The first-order kinetics rate constants of the photoisomerization reaction of the [Yb(tfd)2L] complex in ethanol and acetonitrile solutions were calculated to be 3.8 × 10–3 and 4.5 × 10–3 s–1, and the quantum yields were calculated to be 0.046 and 0.035 (Table ), respectively, which are similar to those of the pure HL ligand and our previous reported azobenzene-containing lanthanide complexes.

Compared with the observed photoisomerization properties of the [Yb(tfd)2L] complex, the quantum yield and photoisomerization rate constants of [Yb(tfd)2NL] in solution are slightly smaller owing to the para electron-donating dimethylamino group substituent in the NL ligand. As shown in Figure S9, the UV–vis spectra of complex [Yb(tfd)2NL] in different solvents exhibit trends similar to those of other HNL-containing lanthanide complexes. The π–π* absorption bands of the [Yb(tfd)2NL] complex are at 326 and 421 nm in ethanol solution and at 324 and 428 nm in acetonitrile solution. In addition, the forbidden symmetry of the n−π* transition band of the azo group is not present in either spectrum. Upon 365 nm UV light irradiation, both of the π–π* maximum absorption intensities decrease. After the photostable state is reached and the sample is irradiated with 450 nm light, the intensities of the both the π–π* bands increase. However, the quantum yield of photoisomerization of the [Yb(tfd)2NL] complex is much smaller than those of HL and the [Yb(tfd)2L] complex, owing to the para electron-donating dimethylamino group substituent.

The UV–vis spectra of the [Yb2(tfd)4DL] complex are the sum of the absorption spectra of the H2DL and 4,4,4-trifluoro-1-phenylbutane-1,3-dione (Htfd) ligands, similar to the spectral form of H2DL (Figure S3). In ethanol solution, there is a strong π–π* transition absorption peak at 332 nm and a weak n−π* transition absorption peak at 451 nm. The molar absorption coefficient of the complex (ε332 = 1.04 × 105 L mol–1 cm–1) is 3 times that of the ligand, mainly because of the coordination effect of the metal ion. The photoisomerization properties of the complex are typical but slightly different from those of the mononuclear [Yb(tfd)2L] complex (see Figures S10–S13). The quantum yield of photoisomerization is much smaller than that of the mononuclear [Yb(tfd)2L] complex, and it is also smaller than that of the pure H2DL ligand. The rate constants of photoisomerization are at the same level as the [Yb(tfd)2NL] complex in ethanol and acetonitrile solutions (Table ). This may be because of the large spatial steric hindrance at both ends of the azo group by coordination to the ytterbium ion.

Photoisomerization Behavior of H2DL and the [Yb(tfd)2L] and [Yb2(tfd)4(DL)] Complexes in Polymethyl Methacrylate (PMMA) Films

The switching speed of trans-to-cis or cis-to-trans isomerization is the most rapid in solution and the slowest in a rigid environment, such as a polymer.[37] Similar to our previously studied lanthanide complexes doped in PMMA,[19,20] the photoisomerization properties of the H2DL ligand and [Yb(tfd)2L] and [Yb2(tfd)4(DL)] complexes doped in PMMA films were investigated (Figure S14). The change of the UV–vis spectrum of H2DL in PMMA upon irradiation with UV or visible light shows a trend similar to that in solution, although the extent of trans-to-cis photoisomerization is low. Upon 365 nm UV light irradiation, the intensity of the π–π* transition absorption peak at 332 nm gradually decreases. The photostable state is reached after about 20 min. With subsequent irradiation with 450 nm visible light, the intensity of the π–π* transition absorption band rapidly increases in the first 5 s and then slowly increases to a constant value with prolonged irradiation time. The first-order rate constant of the photoisomerization reaction of H2DL in PMMA is 1.2 × 10–2 s–1, which is higher than that in solution.

The UV–vis spectral changes of the [Yb(tfd)2L] complex in a PMMA film upon photoirradiation are shown in Figure S14b–d. The absorption spectrum of [Yb(tfd)2L] in the PMMA film shows the characteristic π–π* absorption band at 331 nm and a weak n−π* absorption band at 447 nm. With irradiation at 365 nm, the photostable state is reached in only 15 min. The trans-to-cis first-order reaction rate constant for the [Yb(tfd)2L] complex in the PMMA film was determined to be 7.4 × 10–3 s–1, which is higher than that in solution and for the ligand HL itself. When 365 nm UV light irradiation is stopped, 95% of the cis form returns to the trans form with 450 nm light irradiation for 5 s. The recovery extent of the trans form by visible light indicates that the photochemical pathway of the [Yb(tfd)2L] complex is similar to that in solution and for the HL ligand. In addition, for 10 cycles of alternating irradiation with UV light at 365 nm and visible light at 450 nm, the [Yb(tfd)2L] complex shows no photodegradation in PMMA (Figure S15).

The [Yb2(tfd)4(DL)] complex in PMMA shows the same UV–vis absorption variation as in solution. Upon continuous 365 nm UV light irradiation, the characteristic π–π* transition absorption band gradually decreases, whereas the n−π* transition absorption band variation is not obvious because of the low intensity. When 365 nm UV light is changed to 450 nm visible light, the π–π* transition intensity gradually increases. This shows that the [Yb2(tfd)4(DL)] complex is capable of reversible trans–cis–trans isomerization in the PMMA film. In addition, because of the poor solubility of the complex, reversible photoisomerization in the PMMA film decreases with increasing cycling number by alternating light irradiation (365 nm UV and 450 nm visible light) (Figure S16). The linearity of the first-order kinetic photoisomerization behavior is not ideal, so the photoisomerization rate constant in PMMA was not determined.

Thermal Cis-to-Trans Isomerization of the H2DL Ligand and [Yb(tfd)2L] and [Yb2(tfd)4(DL)] Complexes

Azobenzene and its derivatives are sensitive to light-triggered trans-to-cis and cis-to-trans isomerization. Cis-to-trans isomerization can also spontaneously occur in the dark.[38,39] Normally, after azobenzene derivatives are irradiated with UV light for a certain time, they begin to revert to the transisomer after standing in the dark. Cis-to-trans thermal isomerization occurs on the order of hours to days for azobenzenes and seconds to milliseconds for pseudo-stilbenes.[38,40] Cis-to-trans thermal isomerization of the [Yb(tfd)2NL] complex is not sensitive to the dark even at high temperature, so only the first-order rate constants and half-lives of H2DL and the [Yb(tfd)2L] and [Yb2(tfd)4(DL)] complexes were determined. Thermal cis-to-trans isomerization was also investigated at 50 °C. The calculated first-order rate constants (k) and half-lives (τ1/2) at 25 and 50 °C are given in Table . A representative example of thermal cis-to-trans isomerization of the [Yb2(tfd)4(L)] complex in ethanol solution is shown in Figure . At 25 °C, the rates of cis-to-trans thermal isomerization of H2DL and the [Yb(tfd)2L] and [Yb2(tfd)4(DL)] complexes in solution are very slow. The half-lives of the cis isomers of H2DL, [Yb(tfd)2L], and [Yb2(tfd)4(DL)] are higher than at about 85 days in solution, indicating that the cis isomers are stable at 25 °C in the dark. Thermal isomerization of azobenzene and azobenzene derivatives becomes faster by increasing the temperature, and higher cis-to-trans isomerization rate constants are obtained at a higher temperature. At 50 °C, the first-order rate constants for cis-to-trans thermal isomerization of H2DL, [Yb(tfd)2L], and [Yb2(tfd)4(DL)] in solution are similar to those of similar azobenzene-containing lanthanide complexes and higher than those of other azobenzene-containing metal complexes.[41,42]

Table 3

Thermal Isomerization Rate Constants (kiso) and Half-Lives (τ1/2) of H2DL, [Yb(tfd)2L], and [Yb2(tfd)4(DL)] in Different Solvents at 25a and 50 °C

	ethanol (25 °C)a 103		acetonitrile (25 °C)a 103		ethanol (50 °C)		acetonitrile (50 °C) 104
compounds	kiso (d–1)	τ1/2 (d)	kiso (d–1)	τ1/2 (d)	104kiso (s–1)	τ1/2 (min)	kiso (s–1)	τ1/2 (min)
H2DL	4.9	140	3.2	213	2.2 ± 0.10	53 ± 3.0	2.1 ± 0.11	53 ± 2.8
[Yb(tfd)2L]	3.8	178	4.6	149	1.7 ± 0.07	63 ± 3.1	1.2 ± 0.06	97 ± 4.5
[Yb2(tfd)4(DL)]	4.2	164	8.1	85	2.1 ± 0.11	53 ± 2.9	3.1 ± 0.14	38 ± 2.1

At 25 °C, data obtained from one measurement.

Figure 7

Thermal cis-to-trans isomerization of the [Yb(tfd)2L] complex in ethanol solution (2.0 × 10–5 mol/L) at 50 °C.

Thermal cis-to-trans isomerization of both the ligands and their complexes in ethanol solution was investigated as a function of temperature (Figure S17). The results show that the rates of thermal isomerization of the Yb(III) complexes are almost the same as those of the pure ligands. For example, at 70 °C for 15 min, the pure HL and H2DL ligands achieve 71 and 87% cis-to-trans reversal, whereas their Yb(III) complexes [Yb(tfd)2L] and [Yb2(tfd)4(DL)] achieve 68 and 86% reversal, respectively. This indicates that the thermal isomerization pathway in these Yb(III) complexes is simple and similar to that of the free azobenzene ligands.

NIR Photoluminescence Properties of the [Yb(tfd)2L], [Yb(tfd)2NL], and [Yb2(tfd)4(DL)] Complexes

Recently, NIR-based emission ytterbium(III) complexes have attracted interest because of their potential applications in laser systems, light-emitting diodes, and medical imaging for bioscience.[11,24,26,30,31,43−47] NIR emission from Yb3+ (980 nm) has efficient transmission in biological tissues owing to NIR light ranging from 0.9 to 1.5 μm having little interference with biomaterials. Our previous report shows that such azo-containing lanthanide complexes can emit luminescence because the excitation energy of tfd can be transferred to the rare earth ion, owing to the triplet state of the ligand being higher than the resonance level of the rare earth ion, although the azo-containing ligand does not participate in energy transfer to the lanthanide ion because of its lower triplet-state energy levels. The three [Yb(tfd)2L], [Yb(tfd)2NL], and [Yb2(tfd)4(DL)] complexes have relatively strong NIR luminescence properties in aerated solutions at room temperature. The energy of the excited triplet state of the H2DL ligand was calculated by time-dependent density functional theory (TD-DFT) with the Becke–Lee–Yang–Parr (B3LYP) hybrid exchange–correlation functional and 6-31+G(d) basis set using the Gaussian 09 program. The lowest excited triplet state of the H2DL ligand is 1.7498 eV, which is the lowest energy among the ligands but still higher than the resonance level of Yb3+ (2F5/2, 1.2722 eV). It is well-known that there is no report on the optimum energy gap for an efficient energy transfer from organic ligands to the Yb(III) ion.[48−50] Because the absorption spectra of tfd and azo-containing ligands seriously overlap around 310 nm, the tfd ligand is well-known for being an “antenna ligand” in lanthanide complexes.[20] Thus, these azo-containing ligands may have the chance to participate in energy transfer to the lanthanide ion, as shown in Figure . Furthermore, the NIR luminescence intensity of the three Yb3+ complexes does not change upon 365 nm UV light irradiation as a function of time (Figure S19). These Yb3+ complexes are the first examples of azo-group-containing complexes showing NIR emission.

Figure 8

Energy level of the first excited triplet states of the ligands and the related levels of the ytterbium ion.

The characteristic luminescence emission spectra of the Yb3 + complexes were recorded in the NIR region under excitation by 310 nm UV light in different solvents (Figures and S18). The NIR luminescence of the three Yb3+ complexes has a sharp line emission at about 980 nm, corresponding to the 2F5/2 → 2F7/2 transition. It is noted that the appearances of another two broad bands centered at 1012 (or 1018) and 1048 (or 1053) nm attributed to the Stark splitting of the crystal field.[51] For the [Yb(tfd)2L] and [Yb(tfd)2NL] complexes, the emission bands are centered at 976, 1012, and 1048 nm, and the slight red shifts of the bands at 981, 1018, and 1053 nm for the [Yb2(tfd)4(DL)] complex are because of the two metal centers. The NIR emission for the binuclear [Yb2(tfd)4(DL)] complex is more intense than for the mononuclear complex with the same number of ligands because there are two emissive metal centers in the binuclear complex compared with only one in the mononuclear complex. The solvent does not affect the luminescence emission spectral shapes of the Yb3+ complexes, but it does affect the emission peak intensities. The intensity of luminescence is slightly higher in acetonitrile solution than in ethanol solution. The trans-to-cis isomerization behavior of the azobenzene group does not affect the luminescence intensities of the complexes.

Figure 9

NIR luminescence spectra of the [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] complexes (λex = 310 nm) in acetonitrile solution (2.0 × 10–5 mol/L).

To better understand the NIR luminescence of the three complexes, the time-resolved luminescence decay responses in the NIR region were measured by the time-correlated single photon counting technique. The NIR luminescence decay curves of the complexes in ethanol and acetonitrile solutions were measured at room temperature (λex = 310 nm). The emission decay curves were well-fitted by a single-exponential function to obtain the luminescence lifetimes (Table and Figure S20). The intrinsic luminescence quantum yield of the Yb3+ ions can be estimated by Φ = τobs/τ0, where τobs is the observed emission lifetime and τ0 is the natural lifetime (2.0 ms for the Yb3+ ion).[52,53] The maximum estimated NIR quantum yields of the three complexes are in the range of 0.4–0.6% (Table ). NIR luminescence emission from these complexes is similar to that for other reported Yb3+ complexes.[53,54] These complexes will be good dual function materials in biological system applications, such as controlling the DNA morphology to regulate the biological processes.

Table 4

NIR Luminescence Lifetime and Intrinsic Luminescence Quantum Yield of [Yb(tfd)2L], [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] in Different Solvents

		acetonitrile		ethanol
complexes	λem (nm)	τ (μσ)	ϕyb (%)	τ (μσ)	ϕyb (%)
[Yb(tfd)2L]	976	10.77	0.54	7.52	0.37
[Yb(tfd)2(NL)]	976	11.40	0.57	8.06	0.40
[Yb2(tfd)4(DL)]	981	11.26	0.56	9.00	0.45

Conclusions

We have synthesized novel ternary β-diketonate mononuclear ytterbium complexes of HL and HNL and binuclear ytterbium complex of H2DL and structurally characterized. The X-ray crystal structures show that the Yb(III) ions in all three complexes are eight-coordinated to four oxygen atoms of the two β-diketonate ligands and one oxygen atom and three nitrogen atoms of the deprotonated ligand from HL, HNL, or H2DL, forming a trigonal dodecahedron geometry with similar ranges of −N=N– distances. Among the complexes, only the mononuclear [Yb(tfd)2L] complex can undergo reversible and easily observable trans-to-cis photoisomerization in different organic solvents and in a solid PMMA film. The mononuclear [Yb(tfd)2NL] complex-attached azobenzene groups with a donor substituent and the binuclear [Yb2(tfd)4(DL)] complex both show small photoisomerization quantum yields. A thermal isomerization study of the complexes shows that the thermal isomerization pathway in these Yb(III) complexes is simple and similar to that of the free azobenzene ligands. All three Yb3+ complexes show NIR emission in aerated solutions at room temperature. They are the first examples of azo-group-containing complexes showing NIR emission. The [Yb(tfd)2L] complex with dual photoisomerization and NIR luminescence functions has potential applications as a smart material in biological systems.

Experimental Section

Materials and Methods

YbCl3·6H2O was purchased from Jinan Camolai Trading Company (Jinan, China). Analytical grade Htfd and PMMA were purchased from Alfa Aesar and used as received. The analytical grade solvents were redistilled before use. The melting points were determined with an X-4 micromelting point apparatus without correction. The electrospray ionization mass spectrometry data were acquired with a Bruker ESQUIRE-3000 Plus LC-MS/MS spectrometer. The elemental analyses were carried out with a Vario EL III elemental analyzer. The infrared spectra (KBr pellets) were recorded with a Nicolet AVATAR FT-IR 330 spectrometer. The solution proton nuclear magnetic resonance (1H NMR) spectra were recorded at 400 MHz with a Bruker ADVANCE 400 spectrometer at 25 °C. The chemical shifts (ppm) were measured using tetramethylsilane as the internal reference. TGA was performed with a SDTQ 600 thermogravimeter under a nitrogen atmosphere with a heating rate of 10 °C/min. The UV–vis absorption spectra were recorded with a Shimadzu UV224012PC absorption spectrometer. The NIR photoluminescence spectra and lifetimes were determined with an Edinburgh instruments FLS980 fluorescence spectrometer equipped with a Hamamatsu R5509 NIR photomultiplier tube, and the emission and excitation slits were set to 2 and 5 nm, respectively. PMMA thin film preparation and the photoisomerization quantum yield (ϕ) measurements were the same, as previously reported.[19] The 365 nm UV light was produced with a Shanghai Yihui ZF-2 UV analyzer. The 450 nm visible light was produced with a 300 W PLS-SXE300CUV Xe lamp and isolated by a sharp cut filter. The thermal recoverable experiments were performed by placing a 1 cm cuvette in the thermostated cell of a spectrophotometer, and the spectrum was measured as a function of temperature. The temperature error was controlled to ±1 °C. The DFT calculations were executed with the three-parameterized B3LYP hybrid exchange–correlation functional and 6-31+G(d) basis set using the Gaussian 09 program. At the optimized geometry, the low-lying singlet as well as the triplet excitation energies were calculated by TD-DFT using the same basis set without considering the solvent effect.

Ligand Synthesis

(E)-4-(Pphenyldiazenyl)-N,N-bis (pyridin-2-ylmethyl)benzohydrazide (HL) and (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis (pyridin-2-ylmethyl)benzohydrazide (HNL) were synthesized and characterized, according to our previous report.[20]

Synthesis of (E)-4,4′-N′,N′-Bis(pyridin-2-ylmethyl)benzohydrazide Azobenzene (H2DL)

H2DL was synthesized from (E)-4,4′-benzohydrazide azobenzene and 2-(chloromethyl)pyridine hydrochloride. (E)-4,4′-Benzohydrazide azobenzene (0.06 g, 0.2 mmol) and 2-(chloromethyl)pyridine hydrochloride (0.20 g, 1.2 mmol) were dissolved in 10.0 mL of absolute ethanol and heated to 80 °C. To this solution, 12.0 mL of 0.2 mol/L NaOH aqueous solution was added dropwise, and the resulting mixture was refluxed for 12 h. During the reaction, a yellow precipitate was formed, and the solution was then cooled to room temperature. The yellow solid product was filtered and washed with water, followed by ethanol/water (1:1). H2DL was obtained as a pure pale yellow powder (yield 89%). Mp 198–200 °C; ESI(+)-MS (m/z, methanol): 685.3 [M + Na+]; 1H NMR (400 MHz, DMSO-d6): δ ppm = 9.84 (s, 2 H), 8.47 (d, J = 4.5, 4 H), 7.89 (d, J = 8.6, 4 H), 7.81 (d, J = 8.6, 4 H), 7.75 (dt, J = 10.1, 4.5, 8 H), 7.24 (ddd, J = 6.7, 4.9, 1.6, 4 H), 4.31 (s, 8 H). FT-IR (KBr, cm–1): 3311 (s, O–H), 1654 (s, C=O), 1588 (m, C=C), 1475 (s, N=N), 1309 (s, C–N), 864, 761, 694 (m, Ph–H); elemental analysis: calculated for C38H38N10O4 (%): C, 65.31; H, 5.48; N, 20.04. Found (%): C, 65.25; H, 5.32; N, 20.40.

Synthesis of Yb(III) Complexes

General Procedures

All three complexes were synthesized by the same method. HL (HNL or H2DL) (0.25 mmol) and Htfd (0.162 g, 0.75 mmol) were added to 15.0 mL of absolute ethanol. The reaction mixture was stirred and heated to 80 °C. To this solution, 0.75 mL of 1.0 mmol/L NaOH aqueous solution was added dropwise in 10 min. YbCl3·6H2O (0.25 mmol) in deionized water (5 mL) was added dropwise, and the resulting mixture was stirred for an additional 10 h at 80 °C. The precipitate appeared after cooling. The crude product was filtered and washed with 30.0 mL, 1:1 water/ethanol several times and dried under reduced pressure to give a pure powder. Crystals of the complexes suitable for X-ray analysis were obtained by slow evaporation from a mixed solution of DMF and acetonitrile for about 1 month.

[Yb(tfd)2(L)]

Orange solid (0.27 g, yield 89%); mp 234–235 °C, ESI(+)-MS (m/z, methanol): 1026.2 [M + H+]; FT-IR (KBr, cm–1): 3393 (s, O–H), 1637 (s, C=O), 1615 (s, C=C), 1578 (m, N–H), 1544 (s, N=N), 1368, 1140 (s, C–N), 1289, 1192 (s, C–F), 864, 810, 792, 768 (m, Ph–H); elemental analysis: calculated for YbC45H33F6N6O5 (%): C, 52.74; H, 3.25; N, 8.20. Found (%): C, 51.36; H, 3.16; N, 8.04.

[Yb(tfd)2(NL)]

Red solid (0.25 g, yield 78%); mp 260–262 °C, ESI(+)-MS (m/z, methanol): 1069.3 [M + H+]; FT-IR (KBr, cm–1): 3434 (s, O–H), 2917 (m, C–H), 1643 (m, C=O), 1603 (m, C=C), 1577 (m, N–H), 1539 (s, N=N), 1364, 1140 (s, C–N), 1290, 1188 (s, C–F), 858, 825, 765 (m, Ph–H); elemental analysis: calculated for YbC47H38F6N7O5 (%): C, 52.86; H, 3.59; N, 9.18. Found (%): C, 52.35; H, 3.63; N, 8.96.

[Yb2(tfd)4(DL)]

Orange solid (0.30 g, yield 66%); ESI(+)-MS (m/z, methanol): 1869.0 [M + H+]; FT-IR ( KBr, cm–1) 3435 (s, O–H), 1636 (s, C=O), 1614 (m, C=C), 1578 (m, N–H), 1541 (s, N=N), 1319, 1141 (s, C–N), 1290, 1191 (s, C–F), 816, 764, 715, 702 (m, Ph–H); elemental analysis: calculated for Yb2C78H56N10O10F12 (%): C, 50.17; H, 3.02; N, 7.50. Found (%): C, 50.11; H, 2.96; N, 7.76.

Structure Determination of H2DL, [Yb(tfd)2L],[Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] by X-ray Diffraction

The single-crystal data of H2DL, [Yb(tfd)2(NL)], and [Yb2(tfd)4(DL)] were collected on a Bruker SMART Apex CCD diffractometer using graphite-monochromatic Mo Kα (λ = 0.71073 Å) radiation at 173(2) K. Data integration and reduction were performed with SAINT software, and the absorption correction was applied to the data by the SADABS program.[55] The data for [Yb(tfd)2L] were collected on an Oxford Gemini S Ultra CCD area detector at 173(2) K. Absorption correction was applied using the multiscan CrysAlis Red program.[56] All of the structures were solved by direct methods and refined by full-matrix least-squares calculations based on F2 using the SHELXTL-2014 software package.[57] All of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in their calculated positions and included as riding atoms with isotropic displacement parameters of 1.5 times Ueq (the equivalent isotropic displacement parameter) of the attached C atoms.