Tuning a Lanthanide Complex To Be Responsive to the Environment

Nov 16, 2015 - An enhanced rate of intersystem crossing results in a lutetium complex with a relatively small fluorescence quantum yield (0.15%) and a...
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Tuning a Lanthanide Complex To Be Responsive to the Environment in Solution Ryan T. Golkowski, Nicholas S. Settineri, Xikang Zhao, and David R. McMillin* Department of Chemistry, 560 Oval Drive, Purdue University, West Lafayette, Indiana 47907-2084, United States ABSTRACT: The f−f emissions of lanthanide-ion complexes have predictable emission energies and many practical applications, but the emitting states are generally impervious to the surroundings. This investigation explores ligandand metal-centered emission processes for a series of mixed-ligand complexes of composition M(X-T)(NO3)3, where the metal ion is europium, gadolinium, terbium, or lutetium, and X-T denotes the tridentate ligand 2,2′:6′,2″terpyridine (H-T), 4′-phenyl-2,2′:6′,2″-terpyridine (Ph-T), or 4′-pyrrolidin-Nyl-2,2′:6′,2″-terpyridine (pyrr-T). The presence of the pyrrolidinyl substituent imparts intraligand charge-transfer (ILCT) character to the ligand-based excited states and reduces the energy gap between the singlet and the triplet excited states. An enhanced rate of intersystem crossing results in a lutetium complex with a relatively small fluorescence quantum yield (0.15%) and a gadolinium complex with an impressive phosphorescence yield of 9.6% in deaerated solution. The Tb(pyrr-T)(NO3)3 system is unique because the relatively lowenergy triplet ILCT state equilibrates with the emissive f−f state. The result is a truly remarkable f−f emission signal that is sensitive to the polarity of the local environment as well as the presence of dioxygen.

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INTRODUCTION This article reports photophysical data for a series of complexes most succinctly formulated as M(X-T)(NO3)3 systems, where M denotes a lanthanide cation and X-T denotes a 4′-substituted 2,2′:6′,2″-terpyridine ligand (Chart 1). Photoluminescent

Solid-state structures generally reveal a coordination number of 10, built up by three bidentate nitrate ions and a coordinated water molecule. The symmetry about the metal center is inevitably low, but the donor arrangement resembles a tricapped pentagonal bipyramid, shaped in part by one essentially equatorial and two out-of-plane bidentate nitrate ligands. In other complexes previous authors have also examined ligand substituent effects,16,17 including aminebearing groups.18 However, in the case of the 4′-pyrrolidin-Nyl-2,2′:6′,2″-terpyridine ligand (pyrr-T), the electron-rich nitrogen center connects directly to the terpyridine framework as opposed to being separated by a phenyl or phenylethynyl spacer. The results reveal that the N-pyrrolidinyl group successfully imparts intraligand-charge-transfer (ILCT) character to the low-lying ligand-based absorptions, the consequences of which are far reaching. Thus, ligand-based fluorescence is comparatively weak in pyrr-T complexes, while the gadolinium complex has a relatively high phosphorescence yield. The f−f emission yields also turn out to be low for the corresponding Eu(III) and Tb(III) complexes, but the emission from the Tb(pyrr-T)(NO3)3 is notable for being exquisitely sensitive to the solution environment.

Chart 1

lanthanide complexes receive increasing attention because they find application in many technological areas including lasing materials, ion sensing, and bioimaging.1−6 In terms of the choice of ligand, X-T systems have already proven to be valuable in luminescence studies of platinum and ruthenium complexes7−10 and in the current study X = H, phenyl (Ph), or N-pyrrolidinyl (pyrr). Lanthanide ions are of interest because they provide an excellent size match for the binding pocket of the terpyridine framework.11,12 The ions explored include Eu(III), Gd(III), Tb(III), and Lu(III). Europium and terbium are convenient choices because they allow studies of f−f emissions, while the lutetium and gadolinium complexes provide useful information about ligand-based feeder states. Chelating nitrate ions are convenient coligands because many structures of the mixed-ligand complexes are available.13−15 © 2015 American Chemical Society

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EXPERIMENTAL SECTION

Materials. Sigma-Aldrich was the vendor for Gd[NO3]3·4H2O, Tb[NO3]3·5H2O, Lu[NO3]3·xH2O, pyrrolidine, and 4′-chloro2,2′:6′,2″-terpyridine (Cl-T), while Eu[NO3]3·5H2O came from Received: August 26, 2015 Revised: November 9, 2015 Published: November 16, 2015 11650

DOI: 10.1021/acs.jpca.5b08310 J. Phys. Chem. A 2015, 119, 11650−11658

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The Journal of Physical Chemistry A Acros. The supplier-quoted metal purities were 99.9% for the terbium and europium salts and 99.99% for the gadolinium and lutetium analogues. Solvents used in this study including dichloromethane (DCM), chloroform (CHCl3), and ethanol were commercial products of Macron as was concentrated nitric acid. The acetonitrile (MeCN) came from Sigma-Aldrich, and the silanizing solution (5% dimethyldichlorosilane in heptane) came from Fluka. The quantum yield standard [Ru(bpy)3](PF6)2 was available from a previous study.10 Methods. For f−f emission measurements the slit settings for excitation and emission were 2.5 nm, except as noted, and appropriate filters were in place to shield the detector from the excitation beam. Correction factors for backing out variations in detector sensitivity as a function of wavelength came from the manufacturer. The flash lamp in the Cary Eclipse delivers most of the excitation pulse within 2 μs, but it takes 20 μs for the output to extinguish entirely. An app, supplied by Cary, makes it possible to set the delay time prior to initiating emission detection, the gate time for accumulating the signal, and the total elapsed time between pulses. The function is necessary because the standard mode of operation uses a fixed gate time of 40 μs, which differentially suppresses long-lived emission signals. Typical app settings for recording the total emission signal from a rare-earth complex were 0 μs delay, 5 ms gate, and 7 ms total decay time. The complexes are labile, and the trivalent metal ions are capable of binding to the walls of cuvettes. As a result metal ion impurities deposited during a previous study can sometimes appear in a nominally pure sample. One way to help minimize retention of metal ions involves periodic exposure of the cuvette to a 6 M nitric acid followed by silanization of the inner walls.19 An effective extra measure involves saturating the walls of the cuvette with the metal ion of interest by prior exposure to a solution of its nitrate salt dissolved in acetonitrile. Special precautions were necessary to keep dioxygen out of solution for certain lifetime measurements. The model number GL14-S quartz cuvettes came from Starna, but the cap was an in-house design machined from Teflon. At one end the cap had threads that matched the cell top and held in place a standard flat septum available from Starna. A standard laboratory septum sealed the other end of the cap, resulting in a small inner volume. Insertion of an argon-line inlet needle and an exhaust needle permitted a series of two purges. The first purge involved the inside of the cap, and the second the inside volume of the cuvette. After the removal of both needles the last step involved covering the needle holes with black tape. Presaturation of the argon with solvent vapor minimized any volume change during the procedure. The solvent study involved using mixtures of a stock solution containing of 0.020 g of Gd(pyrr-T)(NO3)3 (30 μmol) in 50 mL of MeCN and chloroform. The method described by Parker and Rees20 was convenient for the estimation of quantum yields where the standard was [Ru(bpy)3]2+ dissolved in aerated MeCN (φs = 0.01821). Equation 1 yields the estimated quantum yields (ϕ)

⎛ I ⎞⎛ 1 − 10−A s ⎞⎛ nx ⎞2 ϕx = ϕs⎜ x ⎟⎜ ⎟⎜ ⎟ ⎝ Is ⎠⎝ 1 − 10−Ax ⎠⎝ ns ⎠

was intense enough to measure by a deconvolution method using data obtained by excitation with a strobed 280 nm LED. In terms of compounds, the method of Clark et al. yielded 4′pyrrolidin-N-yl-2,2′:6′,2″-terpyridine.23 The route to 4′-phenyl2,2′:6′,2″-terpyridine was a modification of the Kröhnke synthesis.24,25 Anal. Calcd for C21H15N3: C, 81.53; H, 4.89; N, 13.58. Found: C, 81.41; H, 4.95; N, 13.64. Midwest Microlab, LLC (Indianapolis, IN) carried out all elemental analyses. The same synthetic method yielded all of the rare-earth complexes; the procedure used for Tb(pyrrT)(NO3)3 is representative. The complex deposited after combining 0.050 g (0.165 mmol) of pyrr-T ligand dissolved in 5 mL of DCM with 0.108 g (0.248 mmol) of Tb[NO3]3 dissolved in 5 mL of MeCN. All reagents were in plastic tubes. Upon redissolving in acetonitrile, the ESI mass spectrum confirmed that the [Tb(pyrr-T)(NO3)2]+ ion (m/z = 585.4) was present in solution as a dissociation product. Recrystallization from hot methanol served as a purification step. ESI measurements and the match between absorbance and emission excitation spectra signaled sample purity in all cases, and microanalytical data provided confirmation in the case of the pyrr-T complexes, which consistently showed the most surprising and informative photophysical properties. Anal. Calcd for C19H22N7O11Tb or Tb(pyrr-T)(NO3)3·2H2O: C, 33.40; H, 3.25; N, 14.35. Found: C, 33.60; H, 3.01; N, 14.21. Anal. Calcd for C19H20N7O10Eu or Eu(pyrr-T)(NO3)3·H2O: C, 34.66; H, 3.06; N, 14.89. Found: C, 34.65; H, 2.85; N, 14.42. Anal. Calcd for C19H19N7O9.5Gd or Gd(pyrr-T)(NO3)3·0.5H2O: C, 34.86; H, 2.93; N, 14.98. Found: C, 35.09; H, 3.03; N, 13.99. Instrumentation. All absorbance spectra came from a Varian Cary 100 spectrometer. The emission spectrophotometer was a Varian Cary Eclipse unit equipped with a R3896 phototube. The VSL-337-NDS nitrogen laser was a product of Laser Science. A description of the associated detection system is in the literature.26 An Optical Building Blocks EasyLife V instrument operating with a 280 nm LED was useful for measuring the emission lifetime of Lu(Ph-T)(NO3)3. The mass spectrometer employed was a single-quadrupole Waters Micromass ZQ ESI unit operating at a cone voltage of 40 V and a desolvation temperature of 200 °C.

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RESULTS Absorbance Studies. Figure 1 depicts the absorbance spectra obtained in acetonitrile for the series of Tb(XT)(NO3)3 complexes which are representative of other systems studied. The Lu(III) systems differ slightly in that the maxima systematically shift to shorter wavelengths by 1−3 nm, possibly because the absence of a solvent ligand reduces the coordination number to nine.14 With a couple of qualifications, the absorption maxima centering around 280 and 330 nm, respectively, are attributable to π−π* transitions of the terpyridine ligand framework.27 One qualifier is that the absorption of pyrr-T complexes extends to longer wavelengths due to the possibility of intraligand charge transfer (ILCT).7 Charge-transfer absorption is accessible in the pyrr-T complexes because of the presence of (1) the electron-rich pyrrolidinyl substituent and (2) the coordinated lanthanide ion which stabilizes charge buildup in the π system of the terpyridine moiety. Another qualifier is that the nitrate ligand also exhibits absorption in the vicinity of 280 nm (Figure 2). The nitrate absorption is, however, less than 1% of that for a complex bearing an X-T ligand. Emission Studies with the H-T and Ph-T Ligands. When the tridentate ligand is Ph-T, the energy, intensity, and orbital parentage of the observed emission vary dramatically, depending on the central metal ion (Figure 3). Thus, Lu(PhT)(NO3)3 exhibits a Ph-T-based fluorescence signal shifted to slightly longer wavelengths than the corresponding absorption. One cannot directly compare this signal with the fluorescence

(1)

where subscript s (x) denotes the standard (unknown), I is the integrated area of a corrected emission spectrum, A denotes absorbance at the exciting wavelength, and n is always the refractive index of the solvent (MeCN). When the emission lifetime was longer than 25 μs it was possible to estimate the lifetime from data obtained with the Cary Eclipse spectrophotometer by means of a semilog plot of integrated spectral intensity vs delay time. For aerated solutions of Tb(pyrr-T)(NO3)3 the lifetime is relatively short, so the measurement involved exciting the sample with a nanosecond-time-scale N2 laser at 337 nm and monitoring the emission decay with a sampling oscilloscope. It was possible to extract the decay constant from a three-parameter fit of the exponential decay by implementing the Solver function of Excel.22 A residual plot justified an analysis based on single-exponential decay even though a second very weak component of the decay was present, vide infra. The subnanosecond emission signal from Lu(Ph-T)(NO3)3 11651

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Figure 1. Room-temperature absorbance spectra of Tb(X-T)(NO3)3 complexes in acetonitrile, where X is H (), phenyl (···), or Npyrrolidinyl (---).

Figure 3. Room-temperature f−f emission spectra of the Ph-T complexes of Tb(III) (blue) and Eu(III) (red). (Inset) Corresponding ligand-based fluorescence spectra of the same Ph-T complexes in blue and red, respectively, as well as the fluorescence spectrum of the Lu(III) analogue (dashed line) and the ligand-based fluorescence and phosphorescence spectra of the Gd(III) analogue (solid black line, x 5.9).

respectively. See Table 1 for a compilation of lifetimes and quantum yields. The antenna effect of the X-T ligand is easy to appreciate. Figure 2 depicts the contrast between the absorption spectrum of a 2.0 mM solution of Tb(NO3)3 and the excitation spectrum of a 10-times more concentrated solution. Note that the f−f transitions, which appear in the excitation spectrum, are so weak they do not even register in the absorption spectrum. At the same time, the nitrate band is missing in the excitation spectrum of Tb(NO3)3 even though it defines the absorption spectrum. Finally, the only bands evident in the excitation spectrum of a 0.20 mM solution of Tb(H-T)(NO3)3 are the π−π* transitions of the coordinated H-T ligand. Emission Studies of pyrr-T Complexes. Ligand-based absorption extends to appreciably longer wavelengths in pyrr-T complexes. Even more impressively, the fluorescence of Lu(pyrr-T)(NO3)3 shifts to dramatically longer wavelengths, centering about 465 nm, compared with 360 nm for the corresponding signal from Lu(Ph-T)(NO3)3. The magnitude of the emission shift reflects the stabilization the polar solvent affords the dipolar excited state, vide infra. However, the fluorescence yield decreases by a couple orders of magnitude vis à vis the Ph-T analogue (Table 1). Again, in acetonitrile, Gd(pyrr-T)(NO3)3 exhibits a ligand-based fluorescence signal with nearly the same quantum yield as the lutetium analogue. Under argon, the Gd(III) complex also exhibits a comparatively strong phosphorescence signal, centering around 500 nm and exhibiting a lifetime of 120 μs (Table 1). The f−f emission from Tb(pyrr-T)(NO3)3 is unique in that the signal is highly dioxygen dependent. Under argon at room temperature, the lifetime of the emission signal is 730 μs, whereas the lifetime of the principal component of the emission

Figure 2. Room-temperature absorption (solid red) and excitation spectra (dashed red) of Tb(H-T)(NO3)3 in acetonitrile. Corresponding absorption (solid black) and excitation spectra (dashed black) of Tb(NO3)3 in acetonitrile.

of the free Ph-T ligand because of the conformation change in the ligand required for complexation to a metal center.28 The Gd(III) analogue exhibits a similar fluorescence signal but with about a 6-fold decrease in intensity as well as a broad ligandbased phosphorescence signal at longer wavelengths with a lifetime of 565 μs under an argon atmosphere. Dissolved oxygen quenches the 3π−π* emission signal in an air-saturated, room-temperature solution. In contrast, the Tb(III) and Eu(III) complexes almost exclusively exhibit f−f emission signals which originate from 5D4 and 5D0 excited states, 11652

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The Journal of Physical Chemistry A Table 1. Photophysical Data from MeCN Solutions at Room Temperaturea M(III) Lu(III) Gd(III) Tb(III)

Eu(III)

X-T Ph-T pyrr-T Ph-T pyrr-T H-T Ph-T pyrr-T H-T Ph-T pyrr-T

ϕ(1f l)b 0.14 0.0015 0.023 0.0012

ϕ(3p)b

ϕ(5l)b

τ (μs) −4

0.024d 0.096d 0.72 0.71 0.27 0.46 0.44 (0.0012)g

krad,c (s−1)

7 × 10

2.0 × 108

565e 120e 1600 1500 730e (28)f 1350 1450 1200

4.2 8.0 4.5 4.7 3.7 3.4 3.0

× × × × × × ×

101 102 102 102 102 102 102

a Entries independent of O2 except as noted. bSymbol 1f l (3p) designates ligand-based fluorescence (phosphorescence) yield; 5l designates f−f luminescence yield. ckfl, kp, or kl in the Jablonski diagram; calculated as ϕ/τ. dMeasured under Ar. eτ under Ar; used for krad calculation. fValue for aerated solution. gOn the basis of the total absorbance, but the signal actually derives from a minor component.

decay drops to only 28 μs in aerated acetonitrile. The emission quantum yield also drops by a factor of about 20. A weaker component of the decay is also evident with a lifetime on the order of hundreds of microseconds. Figure 4 shows that the

The lifetime is also unaffected by the presence of dioxygen in solution. However, the excitation spectrum does not match the absorption spectrum in that it evinces a hypsochromic shift, much like that of the weak component in the emission of the Tb(pyrr-T)(NO3)3 system. Varying the solvent is useful in probing polar excited states,10,29 and this is particularly true in the case of the Tb(pyrr-T)(NO3)3 system. The experiment takes two practical considerations into account: (1) Use of a noncoordinating solvent minimizes any changes that occur in the coordination sphere of the central lanthanide ion, and (2) a nonpolar solvent provides an important contrast to acetonitrile. Solubility considerations provide yet another constraint. The best compromise proves to be using a mixture of acetonitrile and chloroform. The results in Figure 5 reveal how the energy of the emission from Gd(pyrr-T)(NO3)3 shifts to higher energy as the solvent changes from pure acetonitrile to 75% CHCl3.

Figure 4. Room-temperature absorbance spectra of Tb(pyrr-T)(NO3)3 in acetonitrile (black) and the corresponding excitation spectrum measured with a 40 μs gate and zero time delay (red). The excitation spectrum measured with a time delay of 1 ms (blue) represents the longer lived component of the emission.

excitation spectrum of the major component tracks the absorption spectrum of the Tb(pyrr-T)(NO3)3 complex, exhibiting two principal UV bands that straddle the minimum at 322 nm. However, the long-lived component of the emission shows a hypsochromically shifted excitation spectrum in which the bands merge together with no minimum separating them. The presence of a large excess of Tb(NO3)3 in solution has no effect on the biphasic decay signal or the excitation spectrum of the minor component. In the case of the europium analogue, the f−f emission exhibits strictly monophasic decay, but the apparent quantum yield is quite low at 0.12%. Nevertheless, the emission lifetime is 1.2 ms, comparable to those observed for the analogous europium complexes involving H-T or Ph-T.

Figure 5. Room-temperature emission spectrum of Gd(pyrr-T)(NO3)3 in acetonitrile under air (dashed blue line) and Ar (dashed black line). Corresponding spectra in 75% chloroform under air (solid blue line) and under Ar (solid black line). 11653

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The Journal of Physical Chemistry A Similarly, results depicted in Figure 6 reveal how the f−f emission signal from Tb(pyrr-T)(NO3)3 increases in intensity

Figure 7. Jablonski diagram with subscripts fl, p, and l for radiative ligand-based fluorescence, ligand-based phosphorescence, and f−f emission processes, respectively. Subscript n designates a nonradiative decay process, and kisc is the rate constant for intersystem crossing between ligand-based levels. ΔEa is the activation energy for thermal repopulation of the intraligand excited state.

series, it is convenient to divide the complexes into three tranches39,40 in order of increasingly complex electronic structure. Class A: Lu(X-T)(NO3)3 Systems. The lutetium complexes are in a class of their own due to the f14 ground state and the absence of f−f excited states. The heavy atom effect of the lutetium ion is modest at best because none of the complexes exhibits a detectable room-temperature phosphorescence signal. Indeed, the ligand-based fluorescence signal of the PhT complex has a very respectable quantum yield of 14%. The pyrr-T complex is different, however, as the fluorescence quantum yield drops by about 2 orders of magnitude. The rate of intersystem crossing from the singlet to the triplet intraligand excited state is apparently very rapid in the pyrr-T complex. Incorporating a nitrogen atom with a lone pair into a chromophore can, in fact, speed up intersystem crossing by orders of magnitude in accordance with El-Sayed’s rules.41 However, the pyrrolidinyl moiety is ordinarily nearly coplanar with the core terpyridine framework due to a significant mesomeric interaction that occurs.10,23 Indeed, coordination to a metal ion generally ensures that the lowest energy intraligand excitation entails charge transfer from the electron-rich substituent into the π system of the trpy moiety.7 Electronic excitation consequently involves charge displacement across the Npyrr−Ctrpy bond axis, as opposed to a rotation about the axis which would be more conducive to intersystem crossing.41 Chart 2 crudely depicts the type of charge distribution anticipated for the intraligand charge-transfer (ILCT) state. Note that the drawing omits the other two bidentate nitrate

Figure 6. Room-temperature f−f emission spectrum of Tb(pyrrT)(NO3)3 in acetonitrile (solid line) and 75% chloroform (dashed line).

as the average solvent environment becomes less polar. In contrast, virtually no shifts occur in the corresponding excitation spectra. Consistent with the increased emission yield, the emission lifetime of Tb(pyrr-T)(NO3)3 increases to 930 μs in the aerated solution containing 75% CHCl3.

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DISCUSSION Lanthanide Complexes. Complexes of a lanthanide ion with a partially filled 4f shell exhibit color-pure2 emission due to the shielding effect the filled 5s and 5p shells have on the 4f electrons.1,30,31 A complication with Gd(III) complexes is that the first metal-centered excited state does not occur until well into the UV region, due to the f7 configuration, which results in an exceptionally stable S = 7/2 ground state.32 With Eu(III) or Tb(III), however, the ground states are not quite as stable (S = 6/2) and the f−f states are comparatively easy to access. Their emissions occur in the visible region with lifetimes as long as a millisecond.33 In spite of the long lives, the excited states are usually impervious to quenching by dioxygen due to the aforementioned shielding of the 4f electrons. Population of Laporte-forbidden f−f states generally occurs by energy transfer from a ligand-based triplet state (Figure 7),34−36 but direct energy transfer from the absorbing singlet state to an f−f state can occur in parallel with intersystem crossing.37 Although one traditionally regards the feeder states as ligand based, the presence of a paramagnetic rare-earth ion definitely influences the rate constants for intersystem crossing (kisc) and radiative decay (kp).38 To see all this play out in the M(X-T)(NO3)3

Chart 2a

a

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excited states that bridges the gap between the ligand-based excited states and the ground state. As a consequence, energy transfer to f−f excited states outcompetes fluorescence and phosphorescence, enabling both ligand-based states to be conduits for funneling energy into spectroscopically forbidden f−f states. As before, electron-exchange-mediated interactions play a key role, but here the mixing coefficients that connect the ligand-based triplet state to f−f excited states (as opposed to the ground state) are dominant. The reason is the magnitude of the coefficient varies inversely with the energy gap between interacting states.45 Energy transfer is clearly efficient because the f−f emission yields for the Ph-T and H-T complexes range from 45% to 70%, comparing favorably with those reported for other systems. For example, a europium complex of the substituted acetylacetonato ligand TTA has an emission yield of 56%,48 while its adduct with a terpyridine ligand bearing an anilinoethynyl substituent has a reported emission yield of 27%.18 The excited state lifetimes of the Ph-T and H-T complexes range as high as 1500 μs, in most cases regardless of the presence or absence of molecular oxygen in solution. The f−f emission of Tb(pyrr-T)(NO3)3 is exceptional in that it depends on the concentration of dioxygen in solution. In addition, even in the absence of dioxygen, the f−f emission yield and lifetime of Tb(pyrr-T)(NO3)3 come in at relatively low values of 27% and 730 μs, respectively. In an aerated solution of MeCN, the emission yield is much lower still at 0.13%. Under the same conditions the principal component of the emission decay has a lifetime of just 28 μs along with a time-resolved excitation spectrum that comports with the absorption spectrum of the complex. A second, much weaker component of the decay has a longer lifetime and a different excitation spectrum, vide infra. Sensitivity of f−f emission to the presence of dioxygen is classic evidence of back population of the lowest energy intraligand excited state (Figure 7),49 because direct dioxygen-induced quenching is implausible for an f−f excited state. In the absence of dioxygen, thermal repopulation of the ligand-based excited state and decay via the kn′ process (Figure 7) similarly accounts for the lifetime being approximately one-half that of the Ph-T and H-T analogues. Back population of the intraligand state depends, of course, upon there being a relatively small energy gap between states. One can often gauge the energy of the ligand-based state from the phosphorescence spectrum of the Gd(III) analogue, but here it is hard to discern the exact energy due to the absence of resolved vibrational structure. By tuning the solvent polarity it is, nevertheless, possible to confirm the basic model. While the energy of the f−f emission is not dependent on the solvent, the energy of the ILCT excited state is because it has a dipole moment that is larger than that of the ground state (Chart 2). Accordingly, the midpoint of the phosphorescence signal of Gd(pyrr-T)(NO3)3 shifts from 501 nm in MeCN to 482 nm in 75% chloroform. Also, consistent with an increase in the barrier to repopulation of the ILCT state, the lifetime of the f−f emission signal increases to 930 μs in an aerated solution that contains 75% chloroform. Using the emission spectrophotometer, delay times of 20 μs and longer, as well as a 5 ms gate time setting, one can measure the decay of the emission of Tb(pyrr-T)(NO3)3 in aerated solution. In this way a weak, long-lived component of the decay becomes evident with a lifetime in the hundreds of microseconds range. However, the associated excitation spectrum does not match the absorption of Tb(pyrr-T)(NO3)3 (Figure 4). Because the terbium ion is labile, the long-lived component

ligands which extend along axes running out of the page in either direction.13,14,42 Consistent with charge-transfer orbital parentage,43 results obtained with Gd(III) analogues suggest the splitting between the singlet and the triplet intraligand excited states is two-thirds smaller when pyrr-T is the ligand in comparison with Ph-T. Thus, another possibility is that the relatively small energy gap promotes more effective singlet− triplet mixing and faster intersystem crossing. Class B: Gd(X-T)(NO3)3 Systems. The Gd(III) complexes behave very differently because they have seven unpaired electrons (S = 7/2) and an associated f−f excited state, albeit at relatively high energy (ca. 313 nm).32 To begin with, the ligand fluorescence yield of the Gd(pyrr-T)(NO3)3 complex is down by a factor of about 6, relative to the Lu(III) analogue (Table 1). The explanation is that intersystem crossing rates are typically much faster in paramagnetic Gd(III) complexes.38 In the absence of dioxygen the Gd(III) complexes also exhibit ligand-based phosphorescence signals in room-temperature fluid solution. Interestingly, Gd(pyrr-T)(NO3)3 exhibits a higher phosphorescence yield even though it has a shorterlived signal. The contrasting results arise because the pyrr-T complex has a 20-times higher phosphorescence rate constant kp (Table 1), yet another indication of the ease with which nominally spin-forbidden processes become allowed in complexes involving the pyrr-T ligand. Previous workers have also reported observing ligand phosphorescence from gadolinium complexes in solution, and there is wide agreement that the paramagnetism is central to the phenomenon.38,44 Robinson provided a theoretical framework for understanding mechanisms for relaxing the spin selection rule, and paramagnetic systems represent a special case.45 Even when heavy atoms are present, theory suggests that electron-exchange terms, which arise because of the antisymmetric nature of the electronic wave function, can be responsible for the requisite state mixing. Here, the participating charge distributions involve electrons in π orbitals of the ligand as well as electrons in f orbitals of the metal center. Of course, similar interactions occur in lutetium complexes; it is the paramagnetic character of gadolinium that relaxes the spin restriction that otherwise inhibits the rate of intersystem crossing.45 For a simplified view of the mechanism, consider the triplet (S = 1) state of the X-T ligand. It can combine with the S = 7/2 ground state of Gd(III) to generate 3 combination states. More specifically, vector addition of the two spins results in combination states with multiplicities 2SNet + 1 = 10, 8, or 6, where SNet is the total spin quantum number of the combined state.40,46 Now the fluorescent ligand-based state has S = 0 and so does the ground electronic state. Each therefore generates a combination state that has SNet = 7/2 and a multiplicity of 2SNet + 1 = 8. It follows that, even ignoring spin−orbit coupling interactions, the ligand-based processes of intersystem crossing and phosphorescence both represent at least partially spin-allowed processes in gadolinium complexes. No change in the total spin state is necessary because each process can operate between term states that have the same (octet) multiplicity. The phosphorescence yields in Table 1 are nonetheless modest, but that is probably because the parallel kn′ decay process (Figure 7) also becomes partially spin allowed.47 Class C Ions Eu(III) and Tb(III). The Eu(III) and Tb(III) complexes yield very different results. Even though both ions have a paramagnetic S = 6/2 ground state, none of their complexes exhibits an appreciable ligand-based phosphorescence signal. The difference is both ions have a ladder of f−f 11655

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complete. When weakly donating nitrate ions act as coligands, the LMCT state may just be that much more accessible.

is likely to be the product of a ligand rearrangement reaction. Equations 2−4 are therefore plausible candidates for the equilibrium reaction involved Tb(pyrr‐T)(NO3)3 ⇄ Tb(NO3)3 + pyrr‐T

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CONCLUSIONS Complexes with the composition M(X-T)(NO3)3 exhibit metal- or ligand-centered emission in solution, where the metal ion is europium, gadolinium, terbium, or lutetium and XT is a 4′-substituted derivative of 2,2′:6′,2″-terpyridine. Within the series, the emission lifetimes vary by 7 orders of magnitude, and the emission quantum yields vary by 3 orders or more. Results obtained with the 4′-pyrrolidin-N-yl-2,2′:6′,2″-terpyridine ligand, denoted pyrr-T, are the most striking. Due to the presence of the electron-rich substituent, the lowest energy ligand-centered excited states exhibit ILCT character and a relatively small singlet−triplet splitting. An enhanced rate of intersystem crossing partly explains why the ligand-based fluorescence is uniformly weak. In Tb(pyrr-T)(NO3)3 the relatively low energy triplet ILCT state tends to equilibrate with the emissive 5D4 state, producing an f−f emission signal that is unusually sensitive to the local environment. More specifically, dioxygen is a potent quencher in acetonitrile, whereas introduction of a less polar solvent dramatically enhances the lifetime and the emission yield. In sharp contrast, Eu(pyrrT)(NO3)3 is virtually nonluminescent, presumably due to quenching by an LMCT state which is uniquely accessible in the europium complex.

(2)

2Tb(pyrr‐T)(NO3)3 ⇄ Tb(pyrr‐T)2 (NO3)2+ + Tb(NO3)3 + NO3−

(3)

Tb(pyrr‐T)(NO3)3 ⇄ Tb(pyrr‐T)(NO3)2+ + NO3−

(4)

Equation 2 finds precedent in rare-earth acetylacetonato complexes,18 but the excitation spectrum of Tb(NO3)3 is no match for the signal in Figure 4. As a referee pointed out, however, it is possible that the free pyrr-T chromophore is capable of sensitizing terbium ion emission in solution by means of a collisional process. The formation of Tb(pyrrT)2(NO3)2+ via eq 3 is, on the other hand, an implausible explanation for the long-lived component because the signal persists after the addition of excess Tb(NO3)3. In the end a number of observations suggest that eq 4 is most likely the operative equilibrium. Number one is that the existence of the equilibrium accounts for the observation of Tb(pyrr-T)(NO3)2+ in the electrospray-based mass spectrum. Structural studies have also shown that a dinitrato cation can crystallize with nitrate as counterion.14,42 Finally, at least in the solid state, the nitrate ion that dissociates is the one that resides opposite the terpyridine ligand in Chart 2.14 If so, the loss of the anion reduces the net dipole moment of the charge-transfer state and in a polar solvent raises the energy relative to the f−f excited state. This could account for the enhanced lifetime of the second component. Dissociation of a nitrate or pyrr-T ligand probably also conditions the results obtained with the Eu(pyrr-T)(NO3)3 analogue, which has the distinction of producing the weakest emission signal of the entire series. In principle, one might expect Eu(pyrr-T)(NO3)3 to exhibit a relatively strong signal because the emitting f−f state is too low in energy to repopulate the ILCT state. In reality, however, the emission yield is much lower than 0.12% because the observed signal exhibits an incompatible excitation spectrum. (In fact, the excitation spectrum has shifted to higher energy, much as observed for the emission ascribed to Tb(pyrr-T)(NO3)2+ above.) With regard to Eu(pyrr-T)(NO3)3 itself, a dark electronic state evidently quenches the emission. Previous workers have encountered a similar dark state in europium complexes, and recognizing the stability of the f7 dipositive europium ion, they identify the dark state as having ligand-tometal charge-transfer (LMCT) character.50−52 The presence of an electron-rich N-pyrrolidinyl substituent is logically compatible with that model, but predicting the impact of LMCT states is problematic, despite decades-long recognition of their existence. In crypate complexes, on the one hand, the efficiency of energy transfer from an LMCT state to the emitting f−f state is reportedly well below unity.53 On the other hand, in halogenated solvents complexes based on dipicolinate ligands exhibit high f−f emission yields.29 In another study involving a substituted tridentate nitrogen donor, Fu et al. concluded that the CT state quenched only the ligand-based singlet excited state and had comparatively little impact on the f−f emission.54 The Eu(pyrr-T)(NO3)3 system studied here is unusual in that the quenching of the f−f emission appears to be virtually

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors are grateful to the NSF for funding during the early stages of the work through grant number CHE 0847229. REFERENCES

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