DOI: 10.1002/chem.201402268
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& Chirality
Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand Junpei Yuasa,* Hiroshi Ueno, and Tsuyoshi Kawai*[a]
Abstract: This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge-like twisting motion of a bidentate ligand, 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO), in a cis–trans isomerization of chiral europium(III) complexes. X-ray diffraction analysis revealed that twisting motion of BIPYPO provides scis and s-trans geometries of a chiral EuIII complex containing either tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (d-1) or tris[3-(heptafluoropropylhydroxymethylene)(+)-camphorate] (d-2). The s-cis EuIII complexes show eightcoordinate geometry around the EuIII ion, in which the chelate between the phosphoryl oxygen and the EuIII ion forces the s-cis geometry of BIPYPO. In contrast, the phosphorus– nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand, inducing a quasi-
Introduction Manipulation of optical handedness is a potentially very useful phenomenon for biological probing, display technologies, and molecular optical devices. For further development of these technologies, bistable molecular systems without change in chemical composition are of increasing interest. These systems often exhibit structural bistability based on the twisting motion of the chiral structural unit, leading to a clear reversal of the circular dichroism (CD) spectrum.[1–9] Circularly polarized luminescence (CPL) is often regarded as the luminescence analogue of CD, but CPL systems have an enormous amount of potential for highly sophisticated optical devices, such as quantum information technology[10–12] and fluorescent molecular probes.[13–22] Thus, CPL sign reversal based on bistable molecular materials that show a marked difference in the intensity of left- and right-circularly polarized light will be especially useful.[23, 24] However, switching of CPL properties is generally achieved only with a change in the composition of materials [a] Dr. J. Yuasa, H. Ueno, Prof. Dr. T. Kawai Graduate School of Materials Science Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan) Fax: (+ 81) 743-72-6179 E-mail: [email protected] [email protected]
seven-coordinate EuIII complex. The difference in coordination geometry causes the sign change of the CPL signals between the s-cis and s-trans isomers, whereby the s-cis and strans isomers of EuIII complexes exhibit the positive and negative CPL signals, respectively, for the 5D0 !7F1 transition. The proportion of the s-trans-d-1 against s-cis-d-1 increases upon changing the solvent from [D3]acetonitrile to [D6]acetone, inducing a sign change of the CPL signals. The complexes d-1 and d-2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s-cis and s-trans isomers of EuIII complexes. In both cases, the proportions of the longer lifetime components (t1) decrease and instead the shorter lifetime components (t2) increase upon changing the solvent from [D3]acetonitrile to [D6]acetone.
by chemical stimuli, ligand exchange, or their inclusion in host molecules.[25–27] Among a number of CPL active compounds including various coordination compounds, p-conjugated molecules, and polymers, europium(III) complexes have been the subject of great interest because of the significantly large circularly polarized dissymmetry in their emission bands based on magnetic-dipole (MD) and electric-dipole (ED) transitions.[28–40] However, systematic control of CPL activity of EuIII complexes still remains challenging because of flexibility on their coordination chemistry.[41] We report herein the first example of sign reversal of large CPL signal based on the hinge-like twisting motion of a bidentate ligand (BIPYPO). The twisting motion[42] of BIPYPO is capable of providing structural bistability for EuIII complexes containing chiral camphor ligands, inducing opposite signs of CPL signals (Scheme 1). In this work, both crystal structures that exist in two stable states were successfully determined by Xray analysis. The chelate between the phosphoryl oxygen and the EuIII ion forces the s-cis geometry of the bidentate ligand (BIPYPO),[43] but in contrast the phosphorus–nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand (Scheme 1). The molecular conformation changes provided for our system then become of interest in finding a way to control the handedness of the large circular polarization by inducing a conformation change in the ligand.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402268. Chem. Eur. J. 2014, 20, 8621 – 8627
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Figures 1 and 2 (top panels) show the circularly polarized luminescence (CPL) spectra of a 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO) EuIII complex containing tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (d-1) and tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate] (d-2), respectively, in the spectral range of the 5D0 !7F1 (MD) and 5D0 !7F2 (ED) transitions (CPL spectra of d-1 and l-1, and CD spectra,
see Figures S1 and S2 in the Supporting Information).[44–46] In both of these complexes, the sign or magnitude of the CPL signals are significantly dependent on the solvent used; that is, the CPL of d-1 is remarkably enhanced when [D6]acetone is used as the solvent instead of [D3]acetonitrile (Figure 1 top, solid line to dashed line) and on the other hand, a sign reversal of CPL for d-2 is observed upon changing the solvent from [D3]acetonitrile to [D6]acetone (Figure 2 top, solid line to dashed line). The CPL sign reversal was further investigated with various solvent ratios of [D6]acetone/[D3]acetonitrile by measuring the dissymmetry factor (glum) for the 5D0 !7F1 transition. The dissymmetry factor is defined as glum = 2(ILIR)/(IL + IR), in which IL and IR refer to the intensity of left- and right-circularly polarized light, respectively. The glum value of d-2 for the 5D0 !7F1 (gMD) transition continuously changes from the positive value of gMD = 0.66 to the negative value of gMD = 0.17 with increasing [D6]acetone content (Figure 3 circles and also see Figure S3 in the Supporting Information). Similarly, the emission intensity ratio (Irel) of 5D0 !7F2 to 5D0 !7F1 sequentially changes from Irel = 13.3 to 22.1 (Figure 3 triangles). The trends of both changes (gMD and Irel) agree well with each other (Figure 3 and also see Figure S4 in the Supporting Information). In general, the emission intensity of the 5D0 !7F2 electricdipole (ED) transition band is sensitive to the local symmetry of EuIII surroundings, while the 5D0 !7F1 magnetic-dipole (MD) transition band is less sensitive to the ligand field around the EuIII ion, and hence their emission intensity ratio (Irel) often reflects the coordination environment of the EuIII ion.[47] The significant change in emission intensity ratio Irel = 13.3 to 22.1 (Figure 3 triangles) suggests that the coordination structure changes depending on the solvent composition. Hence, the good agreement between gMD and Irel profiles (Figure 3) indicates that changes in the coordination surroundings of the EuIII
Figure 1. CPL (top) and emission (bottom) spectra of d-1 (1.0 103 m) in [D3]acetonitrile (solid line) and [D6]acetone (dashed line) at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
Figure 2. CPL (top) and emission (bottom) spectra of d-2 (1.0 103 m) in [D3]acetonitrile (solid line) and [D6]acetone (dashed line) at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
Scheme 1. Equilibrium between s-cis and s-trans geometries of the chiral EuIII complexes and their sign of dissymmetry factors in CPL at the 5D0 !7F1 (gMD) and 5D0 !7F2 (gED) transitions. Top panels show schematic representation for the s-cis and s-trans geometries, in which the s-cis and s-trans geometries are locked into the metal–ligand interaction and phosphorus–nitrogen interaction, respectively.
Results and Discussion Opposite sign in CPL spectra of two EuIII complex isomers
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Figure 3. Plots of glum at 595 nm (gMD, circles) and relative emission intensity (Irel = I613/I595, triangles) versus [D6]acetone content (vol %) for CPL and emission spectra of d-2 (1.0 103 m) in [D3]acetonitrile containing [D6]acetone [0, 25, 50, 75, and 100 vol %] at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
ion are responsible for the sign reversal of the CPL signals of d-2. In light of these results, we postulate an equilibrium between two EuIII complex isomers (Scheme 1), which have inherent coordination geometries and hence opposite CPL signs. Their proportion ratio changes according to the degree of [D6]acetone content in [D3]acetonitrile. This causes the change in magnitude of the CPL signal for d-1 (Figure 1, top) and sign reversal of CPL for d-2 (Figure 2, top) (a more detailed discussion is given below), being associated with changes in the emission spectra (Figures 1 and 2, bottom). X-ray crystal structures of two EuIII complex isomers In this work, we successfully determined the crystal structures of both isomers of EuIII complexes (s-cis- and s-trans-d-1) in the equilibrium (vide infra). Suitable crystals of s-cis- and s-trans-d1 were obtained by recrystallization from acetonitrile and acetone, respectively (see Table 1 for crystallographic data and refinement details). The X-ray crystal structure of s-cis-d-1 obtained from acetonitrile shows eight-coordinate geometry around the EuIII ion (Figure 4 b); the chelate between the phos-
Table 1. Crystallographic parameters and refinement details for s-cis-d1 and s-trans-d-1.
formula Mr crystal system space group a [] b [] c [] V [3] T [K] Z F000 1 calcd [g cm3] R1 [I > 2s(I)] wR2 [I > 2s(I)] goodness of fit
s-cis-d-1
s-trans-d-1
C146H136Eu2F18N7O16P4 3014.50 orthorhombic P212121 (#19) 13.3700(3) 19.2887(4) 28.7667(6) 7418.6(3) 123.0 2 3074.00 1.349 0.0569 0.1663 1.195
C140H122Eu2F18N4O16P4 2886.31 triclinic P1 (#1) 13.5751(3) 14.0088(3) 20.4218(4) 3293.5(1) 123.0 1 1466.00 1.455 0.0402 0.1042 1.075
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Figure 4. ORTEP view of a) s-cis-d-1 and c) s-trans-d-1. Skeleton structures of b) s-cis-d-1 and d) s-trans-d-1. Hydrogen atoms and solvent molecules are omitted for clarity. Ellipsoids are drawn at the 50 % probability level.
phoryl oxygen and the EuIII ion serves to lock the s-cis geometry at the BIPYPO ligand (Figure 4 a). On the other hand, recrystallization of d-1 from acetone generated the s-trans-d-1 structure (Figure 4 c), in which one pyridine nitrogen (N2) from BIPYPO binds to the EuIII ion instead of phosphoryl oxygen (O2). In the crystal structures of s-trans-d-1, there are two similar complexes in the unit cell, one of which has a disordered 3(trifluoromethylhydroxymethylene)-(+ )-camphorate (d-facam) ligand (see Figure S5 in the Supporting Information). For the purpose of clarity, Figure 4 c shows the other crystal structure of s-trans-d-1. The distance between P2 and N1 (2.978 ) in strans-d-1 BIPYPO is significantly shorter than the sum of van der Waals radii of phosphorus and nitrogen (3.35 ), a clear indication of the phosphorus–nitrogen interaction between the two atoms,[43, 48] which provides a conformational lock for the trans conformation in the BIPYPO ligand (Figure 4 c).[43] The metal–ligand atom distance for s-cis- and s-trans-d-1 are summarized in Table 2. In the case of s-cis-d-1, all eight positions of the coordination sphere are occupied by oxygen atoms (O1– O8, Figure 4 b), and the metal–oxygen distance is in the range of 2.344–2.460 (Table 2). The metal–oxygen distance is conTable 2. Metal–ligand atom distance for s-cis- and s-trans-d-1. Ligand atom
s-cis-d-1
Distance [] s-trans-d-1
O1 O2 O3 O4 O5 O6 O7 O8 N2
2.434 2.364 2.344 2.460 2.356 2.421 2.383 2.403 5.497[a]
2.372 6.784[a] 2.329 2.424 2.376 2.384 2.360 2.393 2.742
[a] Not bound to EuIII.
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Full Paper siderably shorter than the sum of van der Waals radius of oxygen and ionic radius of EuIII (2.74 ) owing to the strong coordination interactions between EuIII and oxygen atoms. The coordination sphere of s-trans-d-1 is occupied by seven oxygen atoms (O1 and O3–8) and one nitrogen atom (N2) from the BIPYPO ligand (Figure 4 d). The metal–oxygen distance of s-trans-d-1 is in the range of 2.329–2.424 , and the metal–nitrogen distance (EuN2) is 2.742 (Table 2). The metal–nitrogen (EuN2) distance (2.742 ) is close to the sum of van der Waals radius of nitrogen and ionic radius of EuIII (2.77 ), suggesting the weak coordination interaction of the EuIII ion with the nitrogen atom (N2). Thus, there are two types of coordination geometry possible for the EuIII ion depending on the twisting motion of the BIPYPO ligand: eight-coordinate for s-cis-d-1 and quasi-seven-coordinate for s-trans-d-1.[49] The difference in coordination geometry results in the sign change of the CPL signals between s-cis- and s-trans-d1 (Scheme 1).[50–52] On the other hand, recrystallization of d1 gave the s-trans-d-1 crystal instead of s-cis-d-1 when acetone was used as the recrystallization solvent instead of acetonitrile (vide supra). These findings indicate that the proportion of the s-trans-d-1 against s-cis-d-1 increases upon changing the solvent from acetonitrile to acetone.[53–55] The ratio between the s-cis and s-trans isomers of EuIII complexes may depend on the solvent polarity. In acetonitrile (dielectric constant: 37.5), the polar solvent may weaken the phosphorus–nitrogen interaction of the s-trans EuIII complex, resulting in the shift of the equilibrium toward the s-cis EuIII complex (Scheme 1 left). By changing the solvent from acetonitrile to acetone (dielectric constant: 21), the ratio of the strans EuIII complex increases. In such a case, the change in magnitude of the CPL of d-1 (Figure 1 top) can be explained by the equilibrium shifts (Scheme 1), assuming that s-cis- and s-trans-d-1 exhibit positive and negative CPL signals, respectively, for the 5D0 !7F1 transition. Coexistence of s-cis- and strans-d-1 with opposite CPL signs provides the rather weak CD spectrum in [D3]acetonitrile (Figure 1 top, solid line); however, upon changing the solvent to [D6]acetone, the relative ratio of s-trans-d-1 showing the negative signal at the 5D0 !7F1 transition increases. This causes the development of the negative CPL band at 5D0 !7F1 and the positive CPL band at 5D0 !7F2 (Figure 1 top, dashed line). Although s-cis- and s-trans-d-2 are difficult to crystallize relative to those of d-1, owing to conformational flexibility of alkyl chains (-CF2CF2CF3), the equilibrium shown in Scheme 1 can give an adequate explanation of sign reversal of the CPL signal for d-2 (Figure 2 top), assuming that s-cis- and s-trans-d-2 also show the positive and negative CPL signs, respectively, for the 5 D0 !7F1 transition. The complex s-cis-d-2, exhibiting the positive CPL signal for the 5D0 !7F1 transition, is the major component in [D3]acetonitrile, leading to a positive CPL band (+) for 5 D0 !7F1 and the negative band () for the 5D0 !7F2 transition (Figure 2 top, solid line). This situation is reversed to an , + sign sequence for the 5D0 !7F1 and 5D0 !7F2 transitions, respectively, in [D6]acetone (Figure 2 top, dashed line); hence, s-transd-2, showing the negative CPL signal for 5D0 !7F1, becomes the major component in [D6]acetone. Chem. Eur. J. 2014, 20, 8621 – 8627
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Emission lifetimes of two EuIII complex isomers In order to verify these hypotheses, we performed emission lifetime measurements of d-1 and d-2 (Figure 5 a and 5b, respectively) in [D3]acetonitrile and [D6]acetone. Each sample shows a biexponential decay [I(t) = A1exp(t/t1) + A2exp(t/t2)] with two different lifetimes (t1 and t2), which is a clear indication of two emitting species, that is, the s-cis and s-trans isomers of EuIII complexes. Emission lifetimes of d-1 and d-2 are summarized in Table 3. The longer (t1) and shorter (t2) lifetime
Figure 5. Emission decay profiles of a) d-1 (1.0 103 m) and b) d-2 (1.0 103 m) in [D3]acetonitrile and [D6]acetone at 298 K. Excitation wavelength l = 371 nm.
Table 3. Emission lifetimes of d-1 and d-2 in [D3]acetonitrile and [D6]acetone at 298 K.
t1 [ms]
d-1[a] t2 [ms]
[D3]acetonitrile 0.15 (56 %)[b] 0.019 (44 %)[c] [D6]acetone 0.25 (31 %)[b] 0.030 (69 %)[c]
t1 [ms]
d-2[a] t2 [ms]
0.36 (66 %)[b] 0.036 (34 %)[c] 0.39 (36 %)[b] 0.070 (64 %)[c]
[a] Emission decay curves were analyzed by biexponential curve fittings [I(t) = A1exp(t/t1) + A2exp(t/t2)]. [b] The value in parentheses denotes A1/(A1+A2). [c] The value in parentheses denotes A2/(A1+A2).
components should correspond to the s-cis and s-trans isomers of EuIII complexes, respectively. On one hand, eight-coordinate geometry for the s-cis EuIII complex defined by eight oxygen atoms (Figure 4 b) suppresses the nonradiative processes and on the other hand, the weak EuIIIN coordination bond for the s-trans EuIII complex (Figure 4 d) causes an increase in the rate of the excited state deactivation. In both d-1 and d-2, the proportions of the longer lifetime components (t1) decrease and instead the shorter lifetime components (t2) increase upon changing the solvent from [D3]acetonitrile to [D6]acetone (Table 3). This observation is consistent with the above assumption: the relative ratio of the s-trans EuIII complex against the s-cis EuIII complex increases with increasing acetone content. The observed CPL spectra (Figures 1 and 2, top) are a result of a combination of CPL from s-cis and s-trans EuIII complexes with opposite CPL signs, and hence the resulting CPL sign depends on emission quantum yields and glum values of s-cis and s-trans EuIII complex isomers found in
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Full Paper [D3]acetonitrile and [D6]acetone. Although each of these parameters is difficult to evaluate individually, a contribution of the s-trans EuIII complex to the total emission intensity is not so large, judging from the short emission lifetime of the s-trans EuIII complex (t2) as compared to that of the s-cis EuIII complex (t1). However, the s-trans EuIII complex significantly influences the CPL, for which an increase ratio of the s-trans EuIII complex results in the increase of the negative CPL band for the 5D0 ! 7 F1 transition and the positive CPL band for the 5D0 !7F2 transition (Figure 1 and 2 top, dashed lines) upon changing the solvent from [D3]acetonitrile to [D6]acetone. This indicates that the s-trans EuIII complex with quasi-seven-coordinate geometry exhibits large circular polarization in emission, and hence a highly dissymmetric seven-coordinate environment should be a unique approach for the design of the lanthanide complex that shows a marked difference in the intensity of leftand right-circularly polarized light.[56]
The relative emission intensity (I617/612) of the second band (l = 617 nm) with respect to the first band (l = 612 nm) for the 5 D0 !7F2 transition decreases with increasing the temperature (see Figure S6 in the Supporting Information), suggesting the equilibrium shift toward the s-trans EuIII complex (Scheme 1 right) with increasing temperature. The dissymmetry factors for the 5D0 !7F1 (gMD) and 5D0 !7F2 (gED) transitions are plotted against temperature (Figure 7 circles and squares, respectively).
Figure 7. Temperature dependence of glum at 594 nm (gMD, circles) and at 612 nm (gED, squares) for CPL spectra of d-1 (1.0 103 m) in [D6]acetone. Excitation wavelength l = 375 nm.
Temperature dependence of emission and CPL spectra The possibility of the equilibrium between the s-cis and s-trans isomers of EuIII complexes (Scheme 1) is also confirmed by the temperature dependence of CPL spectrum for d-1 (vide infra). Figure 6 shows emission and CPL spectra of d-1 at low temperature (183 K) in [D6]acetone (bottom and top panels, respectively). The emission band for the 5D0 !7F2 transition is formally split into two levels at l = 612 and 617 nm (Figure 6, bottom); the CPL spectrum of d-1 shows positive and negative signs at first (l = 612 nm) and second bands (l = 617 nm), respectively, of the 5D0 !7F2 transition (Figure 6, top). The negative CPL sign for 5D0 !7F2 (l = 617 nm) may result from s-cis-d-1 exhibiting the negative CPL signals for the 5D0 !7F2 transition [gED ()].
The gMD value (circles) begins to decrease (that is, larger negative values), and conversely, gED value (squares) begins to increase (that is, larger positive values) above about 253 K. This indicates that the proportion of the negative CPL component for 5D0 !7F1 [gMD ()] and the positive CPL component for 5 D0 !7F2 [gED (+)] increase with increasing temperature.[57–60] The existence of temperature dependence of the dissymmetry factor is also indicative of the equilibrium between the s-cis and s-trans isomers of EuIII complexes (Scheme 1). The equilibrium shifts to the s-trans EuIII complex (Scheme 1 right) exhibiting the negative CPL signal for the 5D0 !7F1 transition [gMD ()] and the positive CPL signal for the 5D0 !7F2 transition [gED (+)] with increasing temperature.
Conclusion
Figure 6. CPL (top) and emission (bottom) spectra of d-1 (1.0 103 m) in [D6]acetone at 183 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra. Chem. Eur. J. 2014, 20, 8621 – 8627
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In conclusion, we have demonstrated the sign reversal of large CPL signal by twisting motion of the bidentate ligand (BIPYPO). As determined by an X-ray diffraction analysis, the twisting motion of BIPYPO provides structural bistability, leading to s-cis and s-trans geometries for the chiral EuIII complexes. The phosphorus–nitrogen interaction assists the twisting of the BIPYPO ligand from the s-cis to the s-trans geometry. The cis/trans ratio is strongly dependent on solvent, for which the proportion of the s-trans EuIII complex increases with increasing acetone content in acetonitrile. Emission lifetime measurements in comparison with CPL analysis demonstrated that the s-cis and the s-trans isomers exhibit opposite CPL signs: the positive sign for the s-cis isomer and negative sign for the s-trans isomer for the 5D0 !7F1 transition of EuIII.[61] This control of the handedness of the large circular polarization by inducing the twisting motion of the ligand is unique and may open up new opportunity to create intelligent molecular systems. 8625
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Keywords: chirality · europium luminescence · twisting motion
Materials Europium(III) tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] [EuIII(d-facam)3(H2O)2], europium(III) tris[3-(trifluoromethylhyand droxymethylene)-()-camphorate] [EuIII(l-facam)3(H2O)2], europium(III) tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate]europium [EuIII(d-hfbc)3(H2O)2] were purchased from Aldrich. 3,3-Bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO) was synthesized with the procedure described in the previous report.[43]
Synthesis For [EuIII(BIPYPO)(d-facam)3] (d-1), [EuIII(d-facam)3(H2O)2] (690 mg, 0.75 mmol) and BIPYPO (420 mg, 0.75 mmol) were dissolved in methanol (75 mL) and heated to reflux under stirring for 12 h. The reaction solution was evaporated using a rotary evaporator. The obtained powder was dissolved in hexane, and then filtrated to remove insoluble solids. The hexane solution was evaporated by a rotary evaporator. The obtained powder was recrystallized using an acetonitrile (yield: 60 %). HRMS [ESI-MS (positive)]: m/z calcd for C58H54EuF6N2O6P2 [Md-facam] + : 1201.25597; found: 1201.25605. [EuIII(BIPYPO)(l-facam)3] (l-1) and [EuIII(BIPYPO)(d-hfbc)3] (d-2) were synthesized by using the similar procedures to d-1. l-1: HRMS [ESIMS (positive)]: m/z calcd for C58H54EuF6N2O6P2 [Ml-facam] + : 1201.25597; found: 1201.25604. d-2: HRMS [ESI-MS (positive)]: m/z calcd for C62H54EuF14N2O6P2 [Md-hfbc] + : 1401.24320; found: 1401.24355.
Spectral measurements Emission lifetimes of the EuIII complexes (in [D3]acetonitrile and [D6]acetone) were recorded using a streak camera (Hamamatsu, picosecond fluorescence measurement system, C4780). A schematic diagram of the CPL measurement system is given in Figure S8 in the Supporting Information.
Crystallography Suitable crystals of s-cis- and s-trans-d-1 were obtained by recrystallization from acetonitrile and acetone, respectively. Since d1 shows much higher solubility in acetone than in acetonitrile, a small amount of hexane was used as a poor solvent to grow the single crystal in acetone. Single crystals of the EuIII complexes were mounted with epoxy resin on a glass fiber. X-ray diffraction intensity was collected with a Rigaku RAXIS RAPID (3 kW) imaging plate area detector with graphite monochromated MoKa radiation at 123 K. All calculations were performed with the Rigaku CrystalStructure 3.8.1 software. CCDC-916002 (s-cis-d-1) and CCDC-916001 (s-trans-d-1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan, through Grantin-Aid for Young Scientists (A) and Scientific Research (C) (No. 21750147), Scientific Research on Innovative Areas “Science of Super-molecular Structure and Creation of Chemical Elements”, and Scientific Research (A) (No. 21107520). Chem. Eur. J. 2014, 20, 8621 – 8627
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ligand
design
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[46] We have also tried to measure CPL spectra of the EuIII complexes (d1 and d-2) in other solvents ([D6]ethanol and [D]chloroform). Unfortunately, the EuIII complexes show no emission to reasonable detection levels for CPL measurement in these solvents. [47] Y. Hasegawa, M. Yamamuro, Y. Wada, N. Kanehisa, Y. Kai, S. Yanagida, J. Phys. Chem. A 2003, 107, 1697. [48] D. W. Allen, D. E. Hibbs, M. B. Hursthouse, K. M. A. Malik, J. Organomet. Chem. 1999, 572, 259. [49] S.-H. Lin, Z.-C. Dong, J.-S. Huang, Q.-E. Zhang, X.-L. Lu, Acta Crystallogr. Sect. C 1991, 47, 426. [50] P. D. Knight, P. Scott, Coord. Chem. Rev. 2003, 242, 125. [51] J. Crassous, Chem. Commun. 2012, 48, 9684. [52] K. Dehnicke, A. Greiner, Angew. Chem. 2003, 115, 1378; Angew. Chem. Int. Ed. 2003, 42, 1340. [53] In contrast to the intramolecular interactions in the EuIII complexes, there is no appreciable intermolecular interaction between the complexes in the molecular packing diagrams. Thus, crystal structures of the s-cis- and s-trans-d-1 (Figure 4 a and 4c) should reflect the average arrangement of d-1 in acetonitrile and acetone solutions, respectively, albeit the crystallization process may somewhat change the distribution of species. [54] Piguet et al. pointed out the effects of crystallization process on the distribution of species in the extensive study of luminescent polynuclear single-stranded oligomers, see: A. Zaı¨m, N. D. Favera, L. Gune, H. Nozary, T. N. Y. Hoang, S. V. Eliseeva, S. Petoud, C. Piguet, Chem. Sci. 2013, 4, 1125. [55] 1H NMR spectra of d-1 and d-2 exhibit broad signals ranged from 15 to 0 ppm, indicating rapid exchange between the s-cis and s-trans isomers of EuIII complexes on 1H NMR timescale. Thus, the proportion ratio between the s-cis and s-trans isomers of EuIII complexes could not be determined accurately from 1H NMR spectra of the EuIII complexes. [56] The solid-state emission of the EuIII complexes studied here is too small to be determined the emission lifetimes of the solid samples. [57] Emission intensity of d-1 for the 5D0 !7F1 transition at high temperatures (293–313 K) is too small to determine the gMD values accurately by our CPL measurement system. [58] The gMD value (Figure 7 circles) barely decreases and gED value (Figure 7 squares) barely increases with decreasing temperature at 183–253 K. This may result from the different temperature dependence of the emission quantum yields of s-cis- and s-trans-d-1. The obtained dissymmetry factors (gMD and gED) are combinations of glum values of s-cis and s-trans EuIII complexes with opposite CPL signs. In such a case, the temperature dependence of gMD and gED (Figure 7) reflects the change in the proportion ratio between s-cis- and s-trans-d-1 as well as the emission quantum yields of s-cis- and s-trans-d-1 at each temperature. [59] The emission intensity of d-1 shows significant temperature dependence; d-1 exhibits intense emission at low temperatures in [D6]acetone. Although the anomalous temperature dependence of emission efficiency has not been clarified fully, these insights would provide important implications for rational design of EuIII complexes exhibiting large CPL with high emission quantum yields, since EuIII complexes of camphor derivatives show particularly large circular polarization.[28, 29] The detailed study of temperature dependence of emission efficiency for EuIII complexes containing camphor derivatives will be reported elsewhere. [60] d-1 also shows a temperature-dependent change in the UV/Vis absorption spectra (see Figure S7 in the Supporting Information), in which the absorbance decreases with increasing the temperature. There is no isosbestic point observed because the absorbance decreases in the whole spectral region. However, the reverse process is observed when temperature is increased and the absorbance change depending on temperature is reversible. These results are consistent with the two-state equilibrium (Scheme 1). [61] Such sign reversal of CPL signal based on the cis–trans isomerisation could not be limited to the lanthanide coordination chemistry, because the twisting motion process has also been found in transition metal complexes.[42] Received: February 20, 2014 Published online on June 24, 2014
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& Chirality
Sign Reversal of a Large Circularly Polarized Luminescence Signal by the Twisting Motion of a Bidentate Ligand Junpei Yuasa,* Hiroshi Ueno, and Tsuyoshi Kawai*[a]
Abstract: This work demonstrates sign reversal of large circularly polarized luminescence (CPL) signal based on the hinge-like twisting motion of a bidentate ligand, 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO), in a cis–trans isomerization of chiral europium(III) complexes. X-ray diffraction analysis revealed that twisting motion of BIPYPO provides scis and s-trans geometries of a chiral EuIII complex containing either tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (d-1) or tris[3-(heptafluoropropylhydroxymethylene)(+)-camphorate] (d-2). The s-cis EuIII complexes show eightcoordinate geometry around the EuIII ion, in which the chelate between the phosphoryl oxygen and the EuIII ion forces the s-cis geometry of BIPYPO. In contrast, the phosphorus– nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand, inducing a quasi-
Introduction Manipulation of optical handedness is a potentially very useful phenomenon for biological probing, display technologies, and molecular optical devices. For further development of these technologies, bistable molecular systems without change in chemical composition are of increasing interest. These systems often exhibit structural bistability based on the twisting motion of the chiral structural unit, leading to a clear reversal of the circular dichroism (CD) spectrum.[1–9] Circularly polarized luminescence (CPL) is often regarded as the luminescence analogue of CD, but CPL systems have an enormous amount of potential for highly sophisticated optical devices, such as quantum information technology[10–12] and fluorescent molecular probes.[13–22] Thus, CPL sign reversal based on bistable molecular materials that show a marked difference in the intensity of left- and right-circularly polarized light will be especially useful.[23, 24] However, switching of CPL properties is generally achieved only with a change in the composition of materials [a] Dr. J. Yuasa, H. Ueno, Prof. Dr. T. Kawai Graduate School of Materials Science Nara Institute of Science and Technology 8916-5 Takayama, Ikoma, Nara 630-0192 (Japan) Fax: (+ 81) 743-72-6179 E-mail: [email protected] [email protected]
seven-coordinate EuIII complex. The difference in coordination geometry causes the sign change of the CPL signals between the s-cis and s-trans isomers, whereby the s-cis and strans isomers of EuIII complexes exhibit the positive and negative CPL signals, respectively, for the 5D0 !7F1 transition. The proportion of the s-trans-d-1 against s-cis-d-1 increases upon changing the solvent from [D3]acetonitrile to [D6]acetone, inducing a sign change of the CPL signals. The complexes d-1 and d-2 show a biexponential decay with two different lifetimes, suggesting two emitting species, that is, the s-cis and s-trans isomers of EuIII complexes. In both cases, the proportions of the longer lifetime components (t1) decrease and instead the shorter lifetime components (t2) increase upon changing the solvent from [D3]acetonitrile to [D6]acetone.
by chemical stimuli, ligand exchange, or their inclusion in host molecules.[25–27] Among a number of CPL active compounds including various coordination compounds, p-conjugated molecules, and polymers, europium(III) complexes have been the subject of great interest because of the significantly large circularly polarized dissymmetry in their emission bands based on magnetic-dipole (MD) and electric-dipole (ED) transitions.[28–40] However, systematic control of CPL activity of EuIII complexes still remains challenging because of flexibility on their coordination chemistry.[41] We report herein the first example of sign reversal of large CPL signal based on the hinge-like twisting motion of a bidentate ligand (BIPYPO). The twisting motion[42] of BIPYPO is capable of providing structural bistability for EuIII complexes containing chiral camphor ligands, inducing opposite signs of CPL signals (Scheme 1). In this work, both crystal structures that exist in two stable states were successfully determined by Xray analysis. The chelate between the phosphoryl oxygen and the EuIII ion forces the s-cis geometry of the bidentate ligand (BIPYPO),[43] but in contrast the phosphorus–nitrogen interaction provides a conformational lock for the s-trans geometry of the BIPYPO ligand (Scheme 1). The molecular conformation changes provided for our system then become of interest in finding a way to control the handedness of the large circular polarization by inducing a conformation change in the ligand.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402268. Chem. Eur. J. 2014, 20, 8621 – 8627
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Figures 1 and 2 (top panels) show the circularly polarized luminescence (CPL) spectra of a 3,3-bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO) EuIII complex containing tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] (d-1) and tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate] (d-2), respectively, in the spectral range of the 5D0 !7F1 (MD) and 5D0 !7F2 (ED) transitions (CPL spectra of d-1 and l-1, and CD spectra,
see Figures S1 and S2 in the Supporting Information).[44–46] In both of these complexes, the sign or magnitude of the CPL signals are significantly dependent on the solvent used; that is, the CPL of d-1 is remarkably enhanced when [D6]acetone is used as the solvent instead of [D3]acetonitrile (Figure 1 top, solid line to dashed line) and on the other hand, a sign reversal of CPL for d-2 is observed upon changing the solvent from [D3]acetonitrile to [D6]acetone (Figure 2 top, solid line to dashed line). The CPL sign reversal was further investigated with various solvent ratios of [D6]acetone/[D3]acetonitrile by measuring the dissymmetry factor (glum) for the 5D0 !7F1 transition. The dissymmetry factor is defined as glum = 2(ILIR)/(IL + IR), in which IL and IR refer to the intensity of left- and right-circularly polarized light, respectively. The glum value of d-2 for the 5D0 !7F1 (gMD) transition continuously changes from the positive value of gMD = 0.66 to the negative value of gMD = 0.17 with increasing [D6]acetone content (Figure 3 circles and also see Figure S3 in the Supporting Information). Similarly, the emission intensity ratio (Irel) of 5D0 !7F2 to 5D0 !7F1 sequentially changes from Irel = 13.3 to 22.1 (Figure 3 triangles). The trends of both changes (gMD and Irel) agree well with each other (Figure 3 and also see Figure S4 in the Supporting Information). In general, the emission intensity of the 5D0 !7F2 electricdipole (ED) transition band is sensitive to the local symmetry of EuIII surroundings, while the 5D0 !7F1 magnetic-dipole (MD) transition band is less sensitive to the ligand field around the EuIII ion, and hence their emission intensity ratio (Irel) often reflects the coordination environment of the EuIII ion.[47] The significant change in emission intensity ratio Irel = 13.3 to 22.1 (Figure 3 triangles) suggests that the coordination structure changes depending on the solvent composition. Hence, the good agreement between gMD and Irel profiles (Figure 3) indicates that changes in the coordination surroundings of the EuIII
Figure 1. CPL (top) and emission (bottom) spectra of d-1 (1.0 103 m) in [D3]acetonitrile (solid line) and [D6]acetone (dashed line) at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
Figure 2. CPL (top) and emission (bottom) spectra of d-2 (1.0 103 m) in [D3]acetonitrile (solid line) and [D6]acetone (dashed line) at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
Scheme 1. Equilibrium between s-cis and s-trans geometries of the chiral EuIII complexes and their sign of dissymmetry factors in CPL at the 5D0 !7F1 (gMD) and 5D0 !7F2 (gED) transitions. Top panels show schematic representation for the s-cis and s-trans geometries, in which the s-cis and s-trans geometries are locked into the metal–ligand interaction and phosphorus–nitrogen interaction, respectively.
Results and Discussion Opposite sign in CPL spectra of two EuIII complex isomers
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Figure 3. Plots of glum at 595 nm (gMD, circles) and relative emission intensity (Irel = I613/I595, triangles) versus [D6]acetone content (vol %) for CPL and emission spectra of d-2 (1.0 103 m) in [D3]acetonitrile containing [D6]acetone [0, 25, 50, 75, and 100 vol %] at 298 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra.
ion are responsible for the sign reversal of the CPL signals of d-2. In light of these results, we postulate an equilibrium between two EuIII complex isomers (Scheme 1), which have inherent coordination geometries and hence opposite CPL signs. Their proportion ratio changes according to the degree of [D6]acetone content in [D3]acetonitrile. This causes the change in magnitude of the CPL signal for d-1 (Figure 1, top) and sign reversal of CPL for d-2 (Figure 2, top) (a more detailed discussion is given below), being associated with changes in the emission spectra (Figures 1 and 2, bottom). X-ray crystal structures of two EuIII complex isomers In this work, we successfully determined the crystal structures of both isomers of EuIII complexes (s-cis- and s-trans-d-1) in the equilibrium (vide infra). Suitable crystals of s-cis- and s-trans-d1 were obtained by recrystallization from acetonitrile and acetone, respectively (see Table 1 for crystallographic data and refinement details). The X-ray crystal structure of s-cis-d-1 obtained from acetonitrile shows eight-coordinate geometry around the EuIII ion (Figure 4 b); the chelate between the phos-
Table 1. Crystallographic parameters and refinement details for s-cis-d1 and s-trans-d-1.
formula Mr crystal system space group a [] b [] c [] V [3] T [K] Z F000 1 calcd [g cm3] R1 [I > 2s(I)] wR2 [I > 2s(I)] goodness of fit
s-cis-d-1
s-trans-d-1
C146H136Eu2F18N7O16P4 3014.50 orthorhombic P212121 (#19) 13.3700(3) 19.2887(4) 28.7667(6) 7418.6(3) 123.0 2 3074.00 1.349 0.0569 0.1663 1.195
C140H122Eu2F18N4O16P4 2886.31 triclinic P1 (#1) 13.5751(3) 14.0088(3) 20.4218(4) 3293.5(1) 123.0 1 1466.00 1.455 0.0402 0.1042 1.075
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Figure 4. ORTEP view of a) s-cis-d-1 and c) s-trans-d-1. Skeleton structures of b) s-cis-d-1 and d) s-trans-d-1. Hydrogen atoms and solvent molecules are omitted for clarity. Ellipsoids are drawn at the 50 % probability level.
phoryl oxygen and the EuIII ion serves to lock the s-cis geometry at the BIPYPO ligand (Figure 4 a). On the other hand, recrystallization of d-1 from acetone generated the s-trans-d-1 structure (Figure 4 c), in which one pyridine nitrogen (N2) from BIPYPO binds to the EuIII ion instead of phosphoryl oxygen (O2). In the crystal structures of s-trans-d-1, there are two similar complexes in the unit cell, one of which has a disordered 3(trifluoromethylhydroxymethylene)-(+ )-camphorate (d-facam) ligand (see Figure S5 in the Supporting Information). For the purpose of clarity, Figure 4 c shows the other crystal structure of s-trans-d-1. The distance between P2 and N1 (2.978 ) in strans-d-1 BIPYPO is significantly shorter than the sum of van der Waals radii of phosphorus and nitrogen (3.35 ), a clear indication of the phosphorus–nitrogen interaction between the two atoms,[43, 48] which provides a conformational lock for the trans conformation in the BIPYPO ligand (Figure 4 c).[43] The metal–ligand atom distance for s-cis- and s-trans-d-1 are summarized in Table 2. In the case of s-cis-d-1, all eight positions of the coordination sphere are occupied by oxygen atoms (O1– O8, Figure 4 b), and the metal–oxygen distance is in the range of 2.344–2.460 (Table 2). The metal–oxygen distance is conTable 2. Metal–ligand atom distance for s-cis- and s-trans-d-1. Ligand atom
s-cis-d-1
Distance [] s-trans-d-1
O1 O2 O3 O4 O5 O6 O7 O8 N2
2.434 2.364 2.344 2.460 2.356 2.421 2.383 2.403 5.497[a]
2.372 6.784[a] 2.329 2.424 2.376 2.384 2.360 2.393 2.742
[a] Not bound to EuIII.
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Full Paper siderably shorter than the sum of van der Waals radius of oxygen and ionic radius of EuIII (2.74 ) owing to the strong coordination interactions between EuIII and oxygen atoms. The coordination sphere of s-trans-d-1 is occupied by seven oxygen atoms (O1 and O3–8) and one nitrogen atom (N2) from the BIPYPO ligand (Figure 4 d). The metal–oxygen distance of s-trans-d-1 is in the range of 2.329–2.424 , and the metal–nitrogen distance (EuN2) is 2.742 (Table 2). The metal–nitrogen (EuN2) distance (2.742 ) is close to the sum of van der Waals radius of nitrogen and ionic radius of EuIII (2.77 ), suggesting the weak coordination interaction of the EuIII ion with the nitrogen atom (N2). Thus, there are two types of coordination geometry possible for the EuIII ion depending on the twisting motion of the BIPYPO ligand: eight-coordinate for s-cis-d-1 and quasi-seven-coordinate for s-trans-d-1.[49] The difference in coordination geometry results in the sign change of the CPL signals between s-cis- and s-trans-d1 (Scheme 1).[50–52] On the other hand, recrystallization of d1 gave the s-trans-d-1 crystal instead of s-cis-d-1 when acetone was used as the recrystallization solvent instead of acetonitrile (vide supra). These findings indicate that the proportion of the s-trans-d-1 against s-cis-d-1 increases upon changing the solvent from acetonitrile to acetone.[53–55] The ratio between the s-cis and s-trans isomers of EuIII complexes may depend on the solvent polarity. In acetonitrile (dielectric constant: 37.5), the polar solvent may weaken the phosphorus–nitrogen interaction of the s-trans EuIII complex, resulting in the shift of the equilibrium toward the s-cis EuIII complex (Scheme 1 left). By changing the solvent from acetonitrile to acetone (dielectric constant: 21), the ratio of the strans EuIII complex increases. In such a case, the change in magnitude of the CPL of d-1 (Figure 1 top) can be explained by the equilibrium shifts (Scheme 1), assuming that s-cis- and s-trans-d-1 exhibit positive and negative CPL signals, respectively, for the 5D0 !7F1 transition. Coexistence of s-cis- and strans-d-1 with opposite CPL signs provides the rather weak CD spectrum in [D3]acetonitrile (Figure 1 top, solid line); however, upon changing the solvent to [D6]acetone, the relative ratio of s-trans-d-1 showing the negative signal at the 5D0 !7F1 transition increases. This causes the development of the negative CPL band at 5D0 !7F1 and the positive CPL band at 5D0 !7F2 (Figure 1 top, dashed line). Although s-cis- and s-trans-d-2 are difficult to crystallize relative to those of d-1, owing to conformational flexibility of alkyl chains (-CF2CF2CF3), the equilibrium shown in Scheme 1 can give an adequate explanation of sign reversal of the CPL signal for d-2 (Figure 2 top), assuming that s-cis- and s-trans-d-2 also show the positive and negative CPL signs, respectively, for the 5 D0 !7F1 transition. The complex s-cis-d-2, exhibiting the positive CPL signal for the 5D0 !7F1 transition, is the major component in [D3]acetonitrile, leading to a positive CPL band (+) for 5 D0 !7F1 and the negative band () for the 5D0 !7F2 transition (Figure 2 top, solid line). This situation is reversed to an , + sign sequence for the 5D0 !7F1 and 5D0 !7F2 transitions, respectively, in [D6]acetone (Figure 2 top, dashed line); hence, s-transd-2, showing the negative CPL signal for 5D0 !7F1, becomes the major component in [D6]acetone. Chem. Eur. J. 2014, 20, 8621 – 8627
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Emission lifetimes of two EuIII complex isomers In order to verify these hypotheses, we performed emission lifetime measurements of d-1 and d-2 (Figure 5 a and 5b, respectively) in [D3]acetonitrile and [D6]acetone. Each sample shows a biexponential decay [I(t) = A1exp(t/t1) + A2exp(t/t2)] with two different lifetimes (t1 and t2), which is a clear indication of two emitting species, that is, the s-cis and s-trans isomers of EuIII complexes. Emission lifetimes of d-1 and d-2 are summarized in Table 3. The longer (t1) and shorter (t2) lifetime
Figure 5. Emission decay profiles of a) d-1 (1.0 103 m) and b) d-2 (1.0 103 m) in [D3]acetonitrile and [D6]acetone at 298 K. Excitation wavelength l = 371 nm.
Table 3. Emission lifetimes of d-1 and d-2 in [D3]acetonitrile and [D6]acetone at 298 K.
t1 [ms]
d-1[a] t2 [ms]
[D3]acetonitrile 0.15 (56 %)[b] 0.019 (44 %)[c] [D6]acetone 0.25 (31 %)[b] 0.030 (69 %)[c]
t1 [ms]
d-2[a] t2 [ms]
0.36 (66 %)[b] 0.036 (34 %)[c] 0.39 (36 %)[b] 0.070 (64 %)[c]
[a] Emission decay curves were analyzed by biexponential curve fittings [I(t) = A1exp(t/t1) + A2exp(t/t2)]. [b] The value in parentheses denotes A1/(A1+A2). [c] The value in parentheses denotes A2/(A1+A2).
components should correspond to the s-cis and s-trans isomers of EuIII complexes, respectively. On one hand, eight-coordinate geometry for the s-cis EuIII complex defined by eight oxygen atoms (Figure 4 b) suppresses the nonradiative processes and on the other hand, the weak EuIIIN coordination bond for the s-trans EuIII complex (Figure 4 d) causes an increase in the rate of the excited state deactivation. In both d-1 and d-2, the proportions of the longer lifetime components (t1) decrease and instead the shorter lifetime components (t2) increase upon changing the solvent from [D3]acetonitrile to [D6]acetone (Table 3). This observation is consistent with the above assumption: the relative ratio of the s-trans EuIII complex against the s-cis EuIII complex increases with increasing acetone content. The observed CPL spectra (Figures 1 and 2, top) are a result of a combination of CPL from s-cis and s-trans EuIII complexes with opposite CPL signs, and hence the resulting CPL sign depends on emission quantum yields and glum values of s-cis and s-trans EuIII complex isomers found in
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Full Paper [D3]acetonitrile and [D6]acetone. Although each of these parameters is difficult to evaluate individually, a contribution of the s-trans EuIII complex to the total emission intensity is not so large, judging from the short emission lifetime of the s-trans EuIII complex (t2) as compared to that of the s-cis EuIII complex (t1). However, the s-trans EuIII complex significantly influences the CPL, for which an increase ratio of the s-trans EuIII complex results in the increase of the negative CPL band for the 5D0 ! 7 F1 transition and the positive CPL band for the 5D0 !7F2 transition (Figure 1 and 2 top, dashed lines) upon changing the solvent from [D3]acetonitrile to [D6]acetone. This indicates that the s-trans EuIII complex with quasi-seven-coordinate geometry exhibits large circular polarization in emission, and hence a highly dissymmetric seven-coordinate environment should be a unique approach for the design of the lanthanide complex that shows a marked difference in the intensity of leftand right-circularly polarized light.[56]
The relative emission intensity (I617/612) of the second band (l = 617 nm) with respect to the first band (l = 612 nm) for the 5 D0 !7F2 transition decreases with increasing the temperature (see Figure S6 in the Supporting Information), suggesting the equilibrium shift toward the s-trans EuIII complex (Scheme 1 right) with increasing temperature. The dissymmetry factors for the 5D0 !7F1 (gMD) and 5D0 !7F2 (gED) transitions are plotted against temperature (Figure 7 circles and squares, respectively).
Figure 7. Temperature dependence of glum at 594 nm (gMD, circles) and at 612 nm (gED, squares) for CPL spectra of d-1 (1.0 103 m) in [D6]acetone. Excitation wavelength l = 375 nm.
Temperature dependence of emission and CPL spectra The possibility of the equilibrium between the s-cis and s-trans isomers of EuIII complexes (Scheme 1) is also confirmed by the temperature dependence of CPL spectrum for d-1 (vide infra). Figure 6 shows emission and CPL spectra of d-1 at low temperature (183 K) in [D6]acetone (bottom and top panels, respectively). The emission band for the 5D0 !7F2 transition is formally split into two levels at l = 612 and 617 nm (Figure 6, bottom); the CPL spectrum of d-1 shows positive and negative signs at first (l = 612 nm) and second bands (l = 617 nm), respectively, of the 5D0 !7F2 transition (Figure 6, top). The negative CPL sign for 5D0 !7F2 (l = 617 nm) may result from s-cis-d-1 exhibiting the negative CPL signals for the 5D0 !7F2 transition [gED ()].
The gMD value (circles) begins to decrease (that is, larger negative values), and conversely, gED value (squares) begins to increase (that is, larger positive values) above about 253 K. This indicates that the proportion of the negative CPL component for 5D0 !7F1 [gMD ()] and the positive CPL component for 5 D0 !7F2 [gED (+)] increase with increasing temperature.[57–60] The existence of temperature dependence of the dissymmetry factor is also indicative of the equilibrium between the s-cis and s-trans isomers of EuIII complexes (Scheme 1). The equilibrium shifts to the s-trans EuIII complex (Scheme 1 right) exhibiting the negative CPL signal for the 5D0 !7F1 transition [gMD ()] and the positive CPL signal for the 5D0 !7F2 transition [gED (+)] with increasing temperature.
Conclusion
Figure 6. CPL (top) and emission (bottom) spectra of d-1 (1.0 103 m) in [D6]acetone at 183 K. Excitation wavelength l = 375 nm for CPL spectra and 365 nm for emission spectra. Chem. Eur. J. 2014, 20, 8621 – 8627
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In conclusion, we have demonstrated the sign reversal of large CPL signal by twisting motion of the bidentate ligand (BIPYPO). As determined by an X-ray diffraction analysis, the twisting motion of BIPYPO provides structural bistability, leading to s-cis and s-trans geometries for the chiral EuIII complexes. The phosphorus–nitrogen interaction assists the twisting of the BIPYPO ligand from the s-cis to the s-trans geometry. The cis/trans ratio is strongly dependent on solvent, for which the proportion of the s-trans EuIII complex increases with increasing acetone content in acetonitrile. Emission lifetime measurements in comparison with CPL analysis demonstrated that the s-cis and the s-trans isomers exhibit opposite CPL signs: the positive sign for the s-cis isomer and negative sign for the s-trans isomer for the 5D0 !7F1 transition of EuIII.[61] This control of the handedness of the large circular polarization by inducing the twisting motion of the ligand is unique and may open up new opportunity to create intelligent molecular systems. 8625
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Keywords: chirality · europium luminescence · twisting motion
Materials Europium(III) tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] [EuIII(d-facam)3(H2O)2], europium(III) tris[3-(trifluoromethylhyand droxymethylene)-()-camphorate] [EuIII(l-facam)3(H2O)2], europium(III) tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorate]europium [EuIII(d-hfbc)3(H2O)2] were purchased from Aldrich. 3,3-Bis(diphenylphosphoryl)-2,2-bipyridine (BIPYPO) was synthesized with the procedure described in the previous report.[43]
Synthesis For [EuIII(BIPYPO)(d-facam)3] (d-1), [EuIII(d-facam)3(H2O)2] (690 mg, 0.75 mmol) and BIPYPO (420 mg, 0.75 mmol) were dissolved in methanol (75 mL) and heated to reflux under stirring for 12 h. The reaction solution was evaporated using a rotary evaporator. The obtained powder was dissolved in hexane, and then filtrated to remove insoluble solids. The hexane solution was evaporated by a rotary evaporator. The obtained powder was recrystallized using an acetonitrile (yield: 60 %). HRMS [ESI-MS (positive)]: m/z calcd for C58H54EuF6N2O6P2 [Md-facam] + : 1201.25597; found: 1201.25605. [EuIII(BIPYPO)(l-facam)3] (l-1) and [EuIII(BIPYPO)(d-hfbc)3] (d-2) were synthesized by using the similar procedures to d-1. l-1: HRMS [ESIMS (positive)]: m/z calcd for C58H54EuF6N2O6P2 [Ml-facam] + : 1201.25597; found: 1201.25604. d-2: HRMS [ESI-MS (positive)]: m/z calcd for C62H54EuF14N2O6P2 [Md-hfbc] + : 1401.24320; found: 1401.24355.
Spectral measurements Emission lifetimes of the EuIII complexes (in [D3]acetonitrile and [D6]acetone) were recorded using a streak camera (Hamamatsu, picosecond fluorescence measurement system, C4780). A schematic diagram of the CPL measurement system is given in Figure S8 in the Supporting Information.
Crystallography Suitable crystals of s-cis- and s-trans-d-1 were obtained by recrystallization from acetonitrile and acetone, respectively. Since d1 shows much higher solubility in acetone than in acetonitrile, a small amount of hexane was used as a poor solvent to grow the single crystal in acetone. Single crystals of the EuIII complexes were mounted with epoxy resin on a glass fiber. X-ray diffraction intensity was collected with a Rigaku RAXIS RAPID (3 kW) imaging plate area detector with graphite monochromated MoKa radiation at 123 K. All calculations were performed with the Rigaku CrystalStructure 3.8.1 software. CCDC-916002 (s-cis-d-1) and CCDC-916001 (s-trans-d-1) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan, through Grantin-Aid for Young Scientists (A) and Scientific Research (C) (No. 21750147), Scientific Research on Innovative Areas “Science of Super-molecular Structure and Creation of Chemical Elements”, and Scientific Research (A) (No. 21107520). Chem. Eur. J. 2014, 20, 8621 – 8627
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[46] We have also tried to measure CPL spectra of the EuIII complexes (d1 and d-2) in other solvents ([D6]ethanol and [D]chloroform). Unfortunately, the EuIII complexes show no emission to reasonable detection levels for CPL measurement in these solvents. [47] Y. Hasegawa, M. Yamamuro, Y. Wada, N. Kanehisa, Y. Kai, S. Yanagida, J. Phys. Chem. A 2003, 107, 1697. [48] D. W. Allen, D. E. Hibbs, M. B. Hursthouse, K. M. A. Malik, J. Organomet. Chem. 1999, 572, 259. [49] S.-H. Lin, Z.-C. Dong, J.-S. Huang, Q.-E. Zhang, X.-L. Lu, Acta Crystallogr. Sect. C 1991, 47, 426. [50] P. D. Knight, P. Scott, Coord. Chem. Rev. 2003, 242, 125. [51] J. Crassous, Chem. Commun. 2012, 48, 9684. [52] K. Dehnicke, A. Greiner, Angew. Chem. 2003, 115, 1378; Angew. Chem. Int. Ed. 2003, 42, 1340. [53] In contrast to the intramolecular interactions in the EuIII complexes, there is no appreciable intermolecular interaction between the complexes in the molecular packing diagrams. Thus, crystal structures of the s-cis- and s-trans-d-1 (Figure 4 a and 4c) should reflect the average arrangement of d-1 in acetonitrile and acetone solutions, respectively, albeit the crystallization process may somewhat change the distribution of species. [54] Piguet et al. pointed out the effects of crystallization process on the distribution of species in the extensive study of luminescent polynuclear single-stranded oligomers, see: A. Zaı¨m, N. D. Favera, L. Gune, H. Nozary, T. N. Y. Hoang, S. V. Eliseeva, S. Petoud, C. Piguet, Chem. Sci. 2013, 4, 1125. [55] 1H NMR spectra of d-1 and d-2 exhibit broad signals ranged from 15 to 0 ppm, indicating rapid exchange between the s-cis and s-trans isomers of EuIII complexes on 1H NMR timescale. Thus, the proportion ratio between the s-cis and s-trans isomers of EuIII complexes could not be determined accurately from 1H NMR spectra of the EuIII complexes. [56] The solid-state emission of the EuIII complexes studied here is too small to be determined the emission lifetimes of the solid samples. [57] Emission intensity of d-1 for the 5D0 !7F1 transition at high temperatures (293–313 K) is too small to determine the gMD values accurately by our CPL measurement system. [58] The gMD value (Figure 7 circles) barely decreases and gED value (Figure 7 squares) barely increases with decreasing temperature at 183–253 K. This may result from the different temperature dependence of the emission quantum yields of s-cis- and s-trans-d-1. The obtained dissymmetry factors (gMD and gED) are combinations of glum values of s-cis and s-trans EuIII complexes with opposite CPL signs. In such a case, the temperature dependence of gMD and gED (Figure 7) reflects the change in the proportion ratio between s-cis- and s-trans-d-1 as well as the emission quantum yields of s-cis- and s-trans-d-1 at each temperature. [59] The emission intensity of d-1 shows significant temperature dependence; d-1 exhibits intense emission at low temperatures in [D6]acetone. Although the anomalous temperature dependence of emission efficiency has not been clarified fully, these insights would provide important implications for rational design of EuIII complexes exhibiting large CPL with high emission quantum yields, since EuIII complexes of camphor derivatives show particularly large circular polarization.[28, 29] The detailed study of temperature dependence of emission efficiency for EuIII complexes containing camphor derivatives will be reported elsewhere. [60] d-1 also shows a temperature-dependent change in the UV/Vis absorption spectra (see Figure S7 in the Supporting Information), in which the absorbance decreases with increasing the temperature. There is no isosbestic point observed because the absorbance decreases in the whole spectral region. However, the reverse process is observed when temperature is increased and the absorbance change depending on temperature is reversible. These results are consistent with the two-state equilibrium (Scheme 1). [61] Such sign reversal of CPL signal based on the cis–trans isomerisation could not be limited to the lanthanide coordination chemistry, because the twisting motion process has also been found in transition metal complexes.[42] Received: February 20, 2014 Published online on June 24, 2014
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