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Photophysical and photochemical studies on the lanthanide complexes of metallocene-containing cryptands
J. Photochem.
Photobiol. A:
Chern., 56 (1991)
255-265
255
Photophysical and photochemical studies on the lanthanide complexes of metallocene-containing cryptands C. Dennis Hall+ and Nelson W. Sharpe Department (U.K) (Received
of Chemistry, King’s CoUege, University of London, Strand, London WCZR 2LS June
7, 1990)
Abstract Laser flash photolysis has been used to study the photophysics and photochemistry of metallocene-containing cryptands and their complexes with rare earth cations in solution. The metallocene has been shown to act as an efficient centre for the radiationless deactivation of the Lanthanide excited state. The requirement for guest metal cation coordination by the host macrocycle for intimate metallocene-cation interaction has been demonstrated by comparison with a range of model ferrocene compounds. Detailed time-resolved studies have permitted the characterization of the coordination chemistry about w+ within the cryptate and the functions within the host cryptand primarily responsible for coordination of the metal cation guest are shown to be the amide carbonyl groups. A transient absorption species associated with the excited metallocene has been observed and ascribed to a species arising from the transfer of an electron to a molecule of the solvent.
1. Introduction A number of recent reports [l-5] have appeared on studies of the photophysics of solutions of rare earth cation complexes with macrocyclic structures. These have concerned the photophysical interaction of the aromatic functions contained in the structure of the macrocycle with lanthanide excited states, or simply the photophysics of the guest lanthanide within the “cage” structure of the host molecule. In this laboratory we have synthesized a range of metallocene-containing cryptands [6], which are able to complex metal cations. These permit the extension of photophysical and photochemical studies to molecular architectures in which the lanthanide is brought into close proximity with other metal centres in solution, thereby affording the possibility for metal-metal interaction. This paper reports these flash photolysis studies on metallocene-containing cryptands and their complexes. The metallocene-containing cryptands 1 and 2 (Fig. 1) are known to complex the divalent cations of the beryllium group [7]. The solution state chemistry of the trivalent rare earth cations displays distinct similarities to that of the group II cations [8] and both electrochemical and UV-visible absorption studies [9, lo] confirm that the complexation behaviour of metallocene-containing cryptands towards alkaline earth cations and lanthanide cations is comparable. ‘Author to whom correspondence should be addressed.
Elsevier Sequoia/Printed in The Netherlands
256
Fig. 1. Schematic structure of the metallocene-containing cryptand hosts, I (Z=Fe) (Z=Ru).
and 2
The photochemistry of simple ferrocene compounds has been reviewed [ll] whilst lanthanide cations possess attractive photophysical properties, eg. relatively long-lived excited states with well-defined energies in the visible spectrum [12]. Thus the lanthanide complexes of 1 and 2 constitute macrocyclic molecular systems within which the photochemistry and photophysics of metal-metallocene donor-acceptor interactions may be investigated and perhaps exploited.
2. Experimental
details
Laser excitation was obtained from the ruby second harmonic (Spectron Laser Systems Ltd., Q switched with frequency doubling using a temperature-tuned 85 “C RDA crystal). This delivers a 347 nm pulse of about 100 mJ of 10 ns half-height pulse width with circular beam cross-sectional area of about 0.5 cm*. This light was incident upon a 1 cm’ quartz cell containing the solution under study at room temperature. All solutions were air equilibrated unless otherwise stated. Emissions perpendicular to the laser pulse were focused into a monochromator (Applied Photophysics; f/3.4; 220 mm) fitted with a Hamamatsu R928 photomultiplier shielded from electromagnetic fields and radiation as efficiently as possible. The output signal was collected on a Phillips PM3315 125 MHz digital storage oscilloscope, and the data transferred to a BBC microcomputer for processing using non-linear least-squares simplex optimalization programs. For transient absorption measurements, a monitoring beam (about 0.5 cm2 circular cross-sectional area) from a 250 W xenon arc was focused into the sample cuvette perpendicular to the laser pulse and the transmitted light analysed as before. The concentrations of the solutions under study were chosen so as to give optical in the range 0.1-0.4 in order to produce densities (ODs) at 347 nm in 1 cm cells, &,, an optimal concentration of excited species in the path of the monitoring beam for transient absorption experiments.
257 A Unicam SP1700 UV spectrophotometer was used to record the UV-visible spectra_ Low resolution excitation-emission spectra were recorded on a Perkin-Elmer MPF3 spectrofluorimeter. The cryptand ~,l’-(1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-7,l6-diyldicarbonyl)-ferrocene (1) was synthesized and purified in the manner reported earlier [13], as was the ruthenium analogue (2) by an identical method from l,l’-bis(chlorocarbonyl)ruthenocene as starting material. Both were isolated as dihydrates and gave satisfactory elemental analyses and relative molecular masses by mass spectroscopy. l,l’-Bis(diethylamido)-ferrocene was prepared as previously reported [14]. Lanthanide perchlorate hexahydrates (Ln(C104)3. 6Hz0 (Ln= europium, terbium, dysprosium)) were used directly as purchased (Ventron-Alfa Products), since time-resolved emission spectroscopy could be used to verify their authenticity in solution_ Terbium tris(acetoacetonate) (purity 99.9%) was purchased from Johnson Matthey Rare Earth Products. Acetonitrile (MeCN) was used throughout as solvent (Rathburns Chemicals Ltd., HPLC grade S); H,O was of “AnalaR” grade (BDH) and DzO was 99.8% pure (Fhrorochem Ltd.). 3. Results
and
Discussion
An equimolar solution of 1 with dysprosium perchlorate hexahydrate in acetonitrile results in the formation of the complex {Dy+:l}. The coordination of the Dy3+ cation by the host molecule 1 is evidenced by a bathochromic shift of the broad, lower energy ferrocene-centred d-d absorption band [15]. For the uncomplexed cryptand 1 this has a maximum centred at 452 nm, and for the complex with Dy3’ the band shows a maximum centred at 478 nm, with an increase in extinction coefficient at A,, from 230 to 380 mol-’ dm’ cm-‘. For the higher energy d-d absorption band a hypsochromic shift is observed upon complexation, from A,,= 347 nm (~=311) to 324 nm (~=1909 dm3 cm-‘), For the complexes formed by Eu3+ and Tb3+ with 1 these absorption mol-l data are identical. Similar shifts are noted for the complexes of 1 with Be2+, Mg”, Detailed analysis of these absorption data [lo] show that the Ca’+, Sr2+ and Ba”. stoichiometry of the complex {Df+:l} is 1:l (at room temperature in the concentration constant K of 32000 mol-’ dm3. The study also region 10d3 M) with an equilibrium includes equilibrium data for a family of ferrocene-containing cryptands related to 1 by selected chemical modifications which reveal that the amide carbonyl oxygen atoms are a prerequisite for complex formation. This implies that for the complex {Dy’:l} the internuclear Fe-Dy distance is of the order of 350 pm. Recent Mijssbauer studies [16] confirm internuclear distances of this magnitude, at least for the solid state. Fluorimetry showed that for all three complexes of 1 with Dg’, Tb3+ and Eu3+, there were no excitations into ferrocene-centred bands which resulted in detectable lanthanide-centred emissions. Ruby laser excitation (about 100 mJ at 347 nm in 10 ns) of acetonitrile solutions of the complex {Dy’:l} at concentrations in the region from 10m4 to fOW3 M gives rise to excited states of both the ferrocene and the dysprosium centres. The only emissions observed are those of Dg+, which are narrow and characteristic, the two most intense being centred at 480 nm (4F9,2-6H1,/2) and 573 nm (4Fsn-(iH13,2). Either of these may be used to monitor the time-resolved decay of the 4F, state, since both emissions display identical kinetics. No emissions between 410 and 720 nm other than those characteristic of the 4F9,2 state of the Dy+ species were detected. The D$+ emissions are considerably reduced in intensity. Figure 2(a) shows the emission monitored at 480 nm of an equimolar solution of dysprosium perchlorate hexahydrate and 1 in
(a) :
:
’ I
:
: :
:
*
:
I
i 1 e R r
i
2JJ.s
,:
I
:
:
Y w
:
:
-
il :
t Y
at 3.2X low4 Fig. 2. (a) Emission monitored at 480 nm (4F9rr6H I& from the complex {w+:l) M in MeCN subsequent to laser excitation (L-347 nm). (b) As for (a) but with 5% Hz0 component in MeCN as solvent (intensity scale less sensitive than in (a)). (c) As for (a) but with 5% DtO component in MeCN as solvent (intensity scale same as in (b)). (Scales: (a) 5 w per division; (b), (c) 2 ps per division).
(at 3.2~ 10e4 M,A341 = 0.37). The intensity of the emission is very small, and the emission is only collected with difficulty. In consequence, the signal-to-noise ratio is poor; however, the trace analyses reasonably well as a single exponential, with a decay rate constant k1 of 264000 (*9000) s-l. It is probable that most of the emission intensity contributing to this decay trace is from the very minor component in solution MeCN
2.59
of the uncomplexed w+ species, and that emissions from w+ complexed with 1 are even more intensity quenched. For acetonitrile solutions of dysprosium perchlorate hexahydrate at similar concentrations in the absence of 1, kl has the value 196000 (*3000) s-l, and the emission is considerably more intense. A novel observation for ferrocene systems is seen by employing time-resolved transient absorption techniques upon laser excitation of solutions of the complex {w+:I). The trace (Fig. 3(a)) consists of a 2~s delay subsequent to laser excitation, whereupon a transient develops pseudoexponentially over a period of 2 11s and then decays on a slightly longer time scale. The trace from onset at 2 ~.ls may be modelled by the difference of two rise-decay single exponentials, with rate constants k(rise) = 500000 s-l and k(decay) - 200000 s-l (Fig. 3(b)). The 2 ps time delay implies the existence of an intermediate species prior to the growth in population of the spectroscopically detectable excited species, giving rise to the transient absorption. The absorption does not display a maximum in the region 670-700 nm and therefore the transient is not the ferricenium-centred moiety (1’). However, the extinction coefficient of 1 + at these wavelengths is low and probably precludes detection by transient absorption. There is little variation in the intensity of the novel feature over the wavelength range from 395 to 630 nm, i.e. its difference optical density AOD spectrum over this wavelength range is constant, within experimental error, with (AOD),, at about 4% of transmission prior to laser excitation. Previous pulse radiolysis studies on liquid MeCN [17] have identified a room temperature equilibrium between the monomeric and dimeric anion radicals of the solvent, i.e. CH,CN’-
+ CH,CN=
(CH,CN);-
with the monomeric form being the more thermodynamically stable. The transient AOD spectra for these radicals over the wavelength range 400-650 nm are without significant maxima, with the OD of the dimeric form quoted as slightly greater. Therefore we propose one possible explanation for the transient absorption feature observed in our study is the photo-excited transfer of an electron from the metallocene centre (i.e. 1 + h v+ 1+ + e- ) to a solvent molecule (in bulk or coordinated) to yield CH3CN’-. The equilibrium then shifts to yield the dimeric anion radical (CH,CN);-, and hence the net changes in the solution’s total OD effectively monitor the rise and decay kinetics in the concentration of the dimeric anion radical, or some species which is an intermediate. Repeated flashing of the solution does not result in detectable photodecomposition, and therefore the ultimate fate of the photo-ejected electron is probably recombination with the ferrocenium centre. The intensity of the transient absorption increases with increasing power of the laser pulse (at least for laser pulse energies up to 200 mJ). Photo-excitation of ferrocene and its derivatives is known to result in electron transfer to NzO for A,,< 313 nm, by a mechanism which does not include the solvated electron [18, 191. In our study, saturation of the solution with N20 did not cause the transient absorption feature to disappear, nor its time-resolved profile to change. This may be because the concentrations of N20 and excited transient species were too low for quenching to occur over the time period during which the transient was observed, or that the reaction pathway to the anion radical dimer is unaffected by N20 saturation. The measurements were repeated on the system @y'+:l} in 95%MeCN-5% water solutions. Our previous electrochemical and absorption studies indicated that, upon addition of water to the MeCN solutions, the guest metal cation was observed to decomplex, as witnessed by the regeneration of the redox waves and the W-visible spectrum of the free host 1. Parallel nuclear magnetic resonance (NMR) studies confirm this behaviour, i.e. the 13C and ‘H spectra of the free hosts 1 and 2 (in MeCN-D20
260
;
4,
I laser flash /
(cl Fig. 3. (a) Transient absorption trace monitored at 520 nm for the complex {w*:l} M in MeCN. The intensity of the absorbance increases as shown to a maximum ps subsequent to the laser flash where (AOD),, is 4% of the transmission prior (b) Computer model of the trace in (a). The dotted line is the difference of two single with t=O set at abut 2 PS subsequent to the laser flash. (c) As for (a), but with D,O component in MeCN as solvent. (Scales: (a), (b) and (c) 2 w per division.)
at 3.2~ lop4 at about 4.5 to the flash. exponentials 5% Hz0 or
as solvent) are regenerated upon addition of water. However, laser studies reveal conclusively that complexation phenomena persist upon addition of water. Ako, addition of D20 and E&O components to the MeCN solution of 1 with dysprosium permits utilization of the deuterium isotope effect to gain information concerning the coordination
261
sphere about the D$’ centre. This methodology is well established in Eu3+ and Tb3+ solution chemistries [8]. We have shown 1201 that for the dysprosium aquo ion species in 95% MeCN-5% water the rate constant for the decay of the “I& excited state is linearly related to the ratio of O-H to O-D oscillators contained in the aquo ligands at dysprosium. Experimental determination of the rate constant krr when the aqueous component is Hz0 and the rate constant kD when the aqueous component is DzO, allows calculation of the number 4 of coordinated aqueous moieties at the dysprosium centre by the relationship 4 =2&l X 10e5(kH-k,). For the fully aquated w+ ion in MeCN solution, measured values of kH and kD yield the value q = 9.3, the “spectroscopic” coordination number. A commonly encountered coordination number for dysprosium in the solid state is nine. The spectroscopic value of 9.3 reflects the dynamic exchange of coordinated aquo species through a transition state of slightly expanded coordination number, where this exchange is significantly faster than the rate of decay of the 4F,, z excited state of the w+ ion. Figure 2(b) shows the decay of the emission monitored at 480 nm for an equimolar solution of 1 with dysprosium perchlorate hexahydrate at 3.2~ 10m4 M in 95% MeCN and 5% Hz0 as solvent @I347 = 0.10). The emission decay trace was collected within about 5 min of the addition of the aqueous component to the MeCN solution; the trace changes negligibly over the succeeding few hours, and repeated flashing causes no discernible decomposition. It is immediately apparent that the emission, in the presence of HzO, is far more intense than in the absence of water (by a factor of 20 at time zero and therefore shows a much improved signalto-noise ratio), and that the lifetime is shorter than for the nominal [Dy(H20)9]3+ ion, for which the 4F,, state decays exponentially with kl-395000 s-l (&6900) in MeCN-water as solvent. The trace in Fig. 2(b) analyses exactly as a single-exponential decay, with kH=532000 (*8000) s-'.Figure 2(c) shows the decay of the emission monitored at 480 nm for an identical solution except that the aqueous component is D20 rather than H,O. The trace was collected about 5 min subsequent to addition of the DzO to the MeCN solution. The decay analyses exactly as a single-exponential decay with kD = 235 000 ( k 2000) s- ‘. This decay rate is considerably greater than for the fully aquated aquo ion, the nominal [Dy(DzO)9]3+ under identical conditions, for which the 480 nm emission decays exponentially with rate kl=39000 (k400) s-l. Again the emission is much more intense than in the absence of DzO, by a factor of 15 at time zero. The observation that the 4F,, state of the w+ ion has a shorter lifetime indicates that coordination of the cation by the ferrocene-containing macrocycle 1 persists despite the presence of the aqueous component in the solvent which might be expected to cause decomplexation. The resultant decay constant for the 4Fg/2 excited state of the Dy+ ion in the complex {[Dy(X,0)J3’:1), where X=H or D, is given by the linear summation of the quenching rate kFc owing to the ferrocene centre, and the contribution by the total number q of coordinated aqueous moieties with individual quenching rates k,,of 44000 s-’ and kd =4340 s-l,which affords the simultaneous equations: k-r = kc + qk, and kD = kc + qkd Inserting the observed experimental rate constants yields the values kFc= 202500 ( f 3500) S -l, the contribution of the complexed ferrocene centre to the quenching of the ion, and also the number of coordinated aqueous 4F~,z excited state of the w’ moieties q = 7.5 ( st 0.25) in the complex {[Dy(X,0),J3+:1}. The expected “spectroscopic” saturated coordination number of 9.3 at dysprosium implies that, in addition to the
262
7.5 coordinated aqueous moieties, there are two additional donor functions. The host macrocycle 1 therefore acts as a bidentate ligand in wet MeCN solutions. In the presence of water the complex is unlikely to be static since in the NMR experiments the signals show that the carbonyls of 1 are trans. In the absence of water, NMR indicates that within the complex (for MeCN as solvent) the carbonyls become cis in order to accommodate metal cation complexation. However, the NMR experiment operates in the millisecond domain, and for dynamic equilibria the collected data are time averaged. Hence, NMR indicates that in the presence of water the equilibrium lies in favour of uncomplexed Df+_ This flash photolysis study is on a microsecond time scale. The decay of the Dy’ excited-state emission is observed as a single exponential because the rate of equilibrium in the presence of water between complexed and uncomplexed Dy’ is faster than the rate of decay of the 4F9n excited state for both the complexed and the uncomplexed D$+ species. The data indicate the macrocycle to be bidentate, and it seems likely that the two carbonyl oxygen atoms are involved in complex formation, and that the carbonyls must be c&. An equally unexpected observation for the {[Dy(X,0),13’:1} complex in wet MeCN solutions is the fate of the transient absorption feature which is observed in dry MeCN solution (see Fig. 3(a)). Figure 3(a) and 3(c) show the transient absorption trace before and after addition of the aqueous component and it is clear that the addition of water causes the transient absorption feature to disappear. Since the addition of the aqueous component causes the OD, A 347, at 347 nm of the solutions under study to decrease from 0.37 to 0.10, the number of photons absorbed in the path of the monitoring beam decreases by about 50%. Therefore the disappearance of the transient absorption feature is not an experimental artefact due to the geometry of the apparatus. The transient absorption feature is observed for solutions of 1 alone in MeCN, either with or in the absence of added water. The suppression of CH&N’and (CH,CN);formation upon pulse radiolysis of MeCN-water mixtures has been observed previously
P71.
to For the complex of 1 with Tb34 in MeCN solutions at similar concentrations the dysprosium study, again considerable intensity quenching of the Tb3+-centred D ‘ 4 excited-state emission monitored at 545 nm (jD4-‘F 5) is observed. The emission is seen to decay with a pseudo-first-order rate coefficient k=2900 s-l.For the Tb3’ species using solutions of terbium perchlorate hexahydrate alone in MeCN, this decay is single exponential with kl= 1540 s-l. Therefore in the complex (Tb3’:l) it is again probable that the complexed Tb3+ species is very efficiently quenched by the metallocene centre, and that the bulk of the emission monitored is from terbium uncomplexed by I, which is a minor component of the solution. Therefore, in the absence of an aqueous component within the solvent, the equilibrium between complexed and uncomplexed cation is not dynamic (or very slow). The addition of a 5% aqueous component to the solution indicates that the metal-metallocene interaction persists. With 5% HzO, the 545 nm emission is seen to decay quasi-exponentially with pseudo-first-order rate s-’ and, for 5% DzO, similarly with a rate kD -29000 s-l.However, in kH ~42000 this case, addition of an aqueous component to the solution causes a 50% reduction in emission intensity at time zero. For the species [T~J(H~O)~]~+ in MeCN, the D ‘ 4 state is observed to decay exponentially with kl= 2500 s-i, whilst m(DzO),]3+ decays exponentially with kl=290 s-l.Clearly the terbium excited state has a considerably shorter lifetime for solutions containing 1 even in the presence of an aqueous component, indicating persistence of interaction with the metallocene centre. The non-singleexponential character of the emissions precludes calculation of the number of coordinated aquo species at Tb3 + . The emission is quasi-exponential since in the presence of water
263
the equilibrium between complexed and uncomplexed Tb3’ is dynamic and on a time scale comparable with the lifetime of the 5D4 state of ([Tb(X,0),]3’:l}. This dissimilarity of complexation behaviour towards Df+ and Tb 3+ by 1 is consistent with observations noted in our previous electrochemical and UV-visible absorption studies 19, lo]. The characteristics of the transient absorption feature with Th3+ as complexed cation are identical with those reported for Dyj+ above. Identical behaviour of this transient is also found for Eu3+ as guest cation. No emissions from the complex formed by Ed+ with 1 were observed, since ruby laser excitation is inefficient for ‘Do state population, and at these concentrations even the relatively intense 616 nm emission (5D0-7F2) was not detected. MeCN solutions of the complex {Dff:2} formed by Dg+ with the ruthenocenecontaining cryptand 2 were also investigated. At the optimum concentrations the emissions from the laser-excited dysprosium 4F 9R state in the complex {Dy+:2) are less intense than the corresponding complex with 1 and were collected with an inferior signal-to-noise ratio. However, it is clear that the ruthenocene centre causes 4Fgn lifetimes to be quenched even more than the corresponding ferrocene-centred complex. For the complex {Dg+:2} in MeCN, the 480 nm emission from the 4Fgn excited state of D?’ subsequent to laser excitation is seen to decay as a single exponential with rate k,= 260000 (f 25000)s-l.Upon addition of 5% Hz0 to the solvent, this becomes kH = 690000 (~-40000)s-l,and, for 5% D20, it becomes kD -320000 (&20000) s-i. Therefore it is clear that complexation persists in the presence of water. Solving the simultaneous equations (as above) yields the value q, the number of coordinated aqueous moieties at dysprosium, of 9.2 ( f 1.5). However, the accuracy of the data is such that bidentate coordination by 2, as observed for 1, is still possible. Also, the contribution of 2 to the total quenching of the dysprosium excited state is evaluated at kRc= 275000 ( f 25000) s-i. The ratio kRc/kFc = 1.36indicates that the metallocenecentred state responsible for the quenching is closer in energy to the 4F9r2 state of Df+ for 2 than for 1. This is consistent with the interpretation that the quenching mechanism by the metallocene centres involves the forbidden ground-state singlet to excited-state triplet transition, and is electric dipole-dipole in character [21] with spin conservation for the donor-acceptor interaction. The transient absorption species observed for the corresponding complex with 1 were observed for (Dy+:2}. The characteristics were identical in every respect except that at 347 nm the quantum yield of the transient species is, qualitatively, less by about 20%. That the quenching phenomena are directly related to metal-metal interaction brought into play by coordination may be verified by studies on model complexes. Figure 4 shows the luminescent emission from excited terbium (5Dq-7Fg) monitored at 545 nm for an equimolar solution of l,l’-bis(diethylamido)ferrocene and terbium tris(acetoacetonate) at 5.2 x 10m4 M in MeCN as solvent subsequent to ruby laser excitation. The trace indicates an equilibrium between complexed and uncomplexed with the bis(diamide) competing for ligation at the sterically crowded Tb(acac),, lanthanide centre. The major component of the equilibrium is uncomplexed Tb(acac), which is responsible for the longer-lived decay which analyses as a near single exponential with rate constant 1550 (f 50) s-l. In the absence of the ferrocene derivative under similar conditions the laser-excited 5D4 state of Tb(acac), is observed to decay with rate constant 1580 ( f 70) s-r and exhibits no fast depopulating process of the excited state. (The rate constant for the decay of the ‘Da state of Tb(acac)3 is concentration dependent between 10m5 M and 1O-4 M, within the range between 1200 s-l and 1600 S -’ respectively). The observed decay rate of 1550 s-l in the presence of the ferrocene bis(diamide) indicates that the complexation equilibrium is effectively static compared
264
Fig. 4. Emission monitored at 545 nm (jD,,-‘F5) for an equimolar solution of Fe(C&eC0.NEt2)2 and l%(acac), at 5.2X 10m4M (~l~~,=O.15), in MeCN as solvent. (Scale, 50 w per division.)
with the millisecond time scale for the lifetime for the uncomplexed lanthanide species. In Fig. 4 there is also evident an additional fast decay process, which results from the superposition of emissions from the minor component of the equilibrium, which is Tb(acac), complexed by the ferrocene bis(diamide). The rate constant for the fast decay process may be estimated at about 30000 (+SOOO) s-l which is of the same order as the decay rate for the species {[Tb(X,0)J3+:1} noted earlier. This demonstrates clearly that the ferrocene centre acts as a quencher of the lifetime of the lanthanide excited state. The characteristic ferrocene-centred transient absorption feature is corresponding observed throughout. The co-solution of Tb(acac), with Fe(C,Ha. CO - Me), in MeCN under similar conditions results in no significant change in the rate constant for decay of the 545 nm emission, and no fast component is observed. Clearly the Tb(acac), species is not coordinated by the acyl carbonyl functions of Fe(CsH,. CO * Me)2. The transient absorption feature characteristic of the ferrocene centre is evident throughout. The corresponding co-solutions of Tb(acac), with ferrocene result in no quenching of the excited lanthanide state. Underivatized ferrocene possesses no complexing capabilities and therefore these photophysical metal-metallocene interactive phenomena cannot result from simple collisional encounter complexes. Ruby laser excitation of ferrocene resulted in the observation of the transient absorption feature but with low quantum yield and is independent of the solution’s water content.
4. Conclusions From this study on the photophysics and photochemistry of metallocene-containing cryptates and their complexes with lanthanide cations the following points emerge. (i) The metallocene centres act as efficient quenchers of the lifetimes of the excited states of complexed lanthanide cations. (ii) 347 nm pulsed laser excitation gives rise to a transient species associated, in origin, with the metallocene centre.
(iii) Coordination of lanthanide cations by the metallocene-containing cryptands persists in wet solutions with the host cryptand acting as a bidentate ligand. (iv) The fate of the transient species is influenced by coordination numbers of aquo ligands at the complexed lanthanide cation and therefore the mode of coordination by the host cryptand. (v) It is the amide carbonyls within the host molecular framework which are primarily responsible for the complexation of the cation. (vi) Coordination of the cation is a prerequisite for these metal-metallocene interactions to occur.
Acknowledgments We D. Flint
thank RTZ Chemicals Ltd. (Birkbeck College, London)
for financial support for the use of laser
(N.W.S.) facilities.
and Professor
C.
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Photobiol. A:
Chern., 56 (1991)
255-265
255
Photophysical and photochemical studies on the lanthanide complexes of metallocene-containing cryptands C. Dennis Hall+ and Nelson W. Sharpe Department (U.K) (Received
of Chemistry, King’s CoUege, University of London, Strand, London WCZR 2LS June
7, 1990)
Abstract Laser flash photolysis has been used to study the photophysics and photochemistry of metallocene-containing cryptands and their complexes with rare earth cations in solution. The metallocene has been shown to act as an efficient centre for the radiationless deactivation of the Lanthanide excited state. The requirement for guest metal cation coordination by the host macrocycle for intimate metallocene-cation interaction has been demonstrated by comparison with a range of model ferrocene compounds. Detailed time-resolved studies have permitted the characterization of the coordination chemistry about w+ within the cryptate and the functions within the host cryptand primarily responsible for coordination of the metal cation guest are shown to be the amide carbonyl groups. A transient absorption species associated with the excited metallocene has been observed and ascribed to a species arising from the transfer of an electron to a molecule of the solvent.
1. Introduction A number of recent reports [l-5] have appeared on studies of the photophysics of solutions of rare earth cation complexes with macrocyclic structures. These have concerned the photophysical interaction of the aromatic functions contained in the structure of the macrocycle with lanthanide excited states, or simply the photophysics of the guest lanthanide within the “cage” structure of the host molecule. In this laboratory we have synthesized a range of metallocene-containing cryptands [6], which are able to complex metal cations. These permit the extension of photophysical and photochemical studies to molecular architectures in which the lanthanide is brought into close proximity with other metal centres in solution, thereby affording the possibility for metal-metal interaction. This paper reports these flash photolysis studies on metallocene-containing cryptands and their complexes. The metallocene-containing cryptands 1 and 2 (Fig. 1) are known to complex the divalent cations of the beryllium group [7]. The solution state chemistry of the trivalent rare earth cations displays distinct similarities to that of the group II cations [8] and both electrochemical and UV-visible absorption studies [9, lo] confirm that the complexation behaviour of metallocene-containing cryptands towards alkaline earth cations and lanthanide cations is comparable. ‘Author to whom correspondence should be addressed.
Elsevier Sequoia/Printed in The Netherlands
256
Fig. 1. Schematic structure of the metallocene-containing cryptand hosts, I (Z=Fe) (Z=Ru).
and 2
The photochemistry of simple ferrocene compounds has been reviewed [ll] whilst lanthanide cations possess attractive photophysical properties, eg. relatively long-lived excited states with well-defined energies in the visible spectrum [12]. Thus the lanthanide complexes of 1 and 2 constitute macrocyclic molecular systems within which the photochemistry and photophysics of metal-metallocene donor-acceptor interactions may be investigated and perhaps exploited.
2. Experimental
details
Laser excitation was obtained from the ruby second harmonic (Spectron Laser Systems Ltd., Q switched with frequency doubling using a temperature-tuned 85 “C RDA crystal). This delivers a 347 nm pulse of about 100 mJ of 10 ns half-height pulse width with circular beam cross-sectional area of about 0.5 cm*. This light was incident upon a 1 cm’ quartz cell containing the solution under study at room temperature. All solutions were air equilibrated unless otherwise stated. Emissions perpendicular to the laser pulse were focused into a monochromator (Applied Photophysics; f/3.4; 220 mm) fitted with a Hamamatsu R928 photomultiplier shielded from electromagnetic fields and radiation as efficiently as possible. The output signal was collected on a Phillips PM3315 125 MHz digital storage oscilloscope, and the data transferred to a BBC microcomputer for processing using non-linear least-squares simplex optimalization programs. For transient absorption measurements, a monitoring beam (about 0.5 cm2 circular cross-sectional area) from a 250 W xenon arc was focused into the sample cuvette perpendicular to the laser pulse and the transmitted light analysed as before. The concentrations of the solutions under study were chosen so as to give optical in the range 0.1-0.4 in order to produce densities (ODs) at 347 nm in 1 cm cells, &,, an optimal concentration of excited species in the path of the monitoring beam for transient absorption experiments.
257 A Unicam SP1700 UV spectrophotometer was used to record the UV-visible spectra_ Low resolution excitation-emission spectra were recorded on a Perkin-Elmer MPF3 spectrofluorimeter. The cryptand ~,l’-(1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-7,l6-diyldicarbonyl)-ferrocene (1) was synthesized and purified in the manner reported earlier [13], as was the ruthenium analogue (2) by an identical method from l,l’-bis(chlorocarbonyl)ruthenocene as starting material. Both were isolated as dihydrates and gave satisfactory elemental analyses and relative molecular masses by mass spectroscopy. l,l’-Bis(diethylamido)-ferrocene was prepared as previously reported [14]. Lanthanide perchlorate hexahydrates (Ln(C104)3. 6Hz0 (Ln= europium, terbium, dysprosium)) were used directly as purchased (Ventron-Alfa Products), since time-resolved emission spectroscopy could be used to verify their authenticity in solution_ Terbium tris(acetoacetonate) (purity 99.9%) was purchased from Johnson Matthey Rare Earth Products. Acetonitrile (MeCN) was used throughout as solvent (Rathburns Chemicals Ltd., HPLC grade S); H,O was of “AnalaR” grade (BDH) and DzO was 99.8% pure (Fhrorochem Ltd.). 3. Results
and
Discussion
An equimolar solution of 1 with dysprosium perchlorate hexahydrate in acetonitrile results in the formation of the complex {Dy+:l}. The coordination of the Dy3+ cation by the host molecule 1 is evidenced by a bathochromic shift of the broad, lower energy ferrocene-centred d-d absorption band [15]. For the uncomplexed cryptand 1 this has a maximum centred at 452 nm, and for the complex with Dy3’ the band shows a maximum centred at 478 nm, with an increase in extinction coefficient at A,, from 230 to 380 mol-’ dm’ cm-‘. For the higher energy d-d absorption band a hypsochromic shift is observed upon complexation, from A,,= 347 nm (~=311) to 324 nm (~=1909 dm3 cm-‘), For the complexes formed by Eu3+ and Tb3+ with 1 these absorption mol-l data are identical. Similar shifts are noted for the complexes of 1 with Be2+, Mg”, Detailed analysis of these absorption data [lo] show that the Ca’+, Sr2+ and Ba”. stoichiometry of the complex {Df+:l} is 1:l (at room temperature in the concentration constant K of 32000 mol-’ dm3. The study also region 10d3 M) with an equilibrium includes equilibrium data for a family of ferrocene-containing cryptands related to 1 by selected chemical modifications which reveal that the amide carbonyl oxygen atoms are a prerequisite for complex formation. This implies that for the complex {Dy’:l} the internuclear Fe-Dy distance is of the order of 350 pm. Recent Mijssbauer studies [16] confirm internuclear distances of this magnitude, at least for the solid state. Fluorimetry showed that for all three complexes of 1 with Dg’, Tb3+ and Eu3+, there were no excitations into ferrocene-centred bands which resulted in detectable lanthanide-centred emissions. Ruby laser excitation (about 100 mJ at 347 nm in 10 ns) of acetonitrile solutions of the complex {Dy’:l} at concentrations in the region from 10m4 to fOW3 M gives rise to excited states of both the ferrocene and the dysprosium centres. The only emissions observed are those of Dg+, which are narrow and characteristic, the two most intense being centred at 480 nm (4F9,2-6H1,/2) and 573 nm (4Fsn-(iH13,2). Either of these may be used to monitor the time-resolved decay of the 4F, state, since both emissions display identical kinetics. No emissions between 410 and 720 nm other than those characteristic of the 4F9,2 state of the Dy+ species were detected. The D$+ emissions are considerably reduced in intensity. Figure 2(a) shows the emission monitored at 480 nm of an equimolar solution of dysprosium perchlorate hexahydrate and 1 in
(a) :
:
’ I
:
: :
:
*
:
I
i 1 e R r
i
2JJ.s
,:
I
:
:
Y w
:
:
-
il :
t Y
at 3.2X low4 Fig. 2. (a) Emission monitored at 480 nm (4F9rr6H I& from the complex {w+:l) M in MeCN subsequent to laser excitation (L-347 nm). (b) As for (a) but with 5% Hz0 component in MeCN as solvent (intensity scale less sensitive than in (a)). (c) As for (a) but with 5% DtO component in MeCN as solvent (intensity scale same as in (b)). (Scales: (a) 5 w per division; (b), (c) 2 ps per division).
(at 3.2~ 10e4 M,A341 = 0.37). The intensity of the emission is very small, and the emission is only collected with difficulty. In consequence, the signal-to-noise ratio is poor; however, the trace analyses reasonably well as a single exponential, with a decay rate constant k1 of 264000 (*9000) s-l. It is probable that most of the emission intensity contributing to this decay trace is from the very minor component in solution MeCN
2.59
of the uncomplexed w+ species, and that emissions from w+ complexed with 1 are even more intensity quenched. For acetonitrile solutions of dysprosium perchlorate hexahydrate at similar concentrations in the absence of 1, kl has the value 196000 (*3000) s-l, and the emission is considerably more intense. A novel observation for ferrocene systems is seen by employing time-resolved transient absorption techniques upon laser excitation of solutions of the complex {w+:I). The trace (Fig. 3(a)) consists of a 2~s delay subsequent to laser excitation, whereupon a transient develops pseudoexponentially over a period of 2 11s and then decays on a slightly longer time scale. The trace from onset at 2 ~.ls may be modelled by the difference of two rise-decay single exponentials, with rate constants k(rise) = 500000 s-l and k(decay) - 200000 s-l (Fig. 3(b)). The 2 ps time delay implies the existence of an intermediate species prior to the growth in population of the spectroscopically detectable excited species, giving rise to the transient absorption. The absorption does not display a maximum in the region 670-700 nm and therefore the transient is not the ferricenium-centred moiety (1’). However, the extinction coefficient of 1 + at these wavelengths is low and probably precludes detection by transient absorption. There is little variation in the intensity of the novel feature over the wavelength range from 395 to 630 nm, i.e. its difference optical density AOD spectrum over this wavelength range is constant, within experimental error, with (AOD),, at about 4% of transmission prior to laser excitation. Previous pulse radiolysis studies on liquid MeCN [17] have identified a room temperature equilibrium between the monomeric and dimeric anion radicals of the solvent, i.e. CH,CN’-
+ CH,CN=
(CH,CN);-
with the monomeric form being the more thermodynamically stable. The transient AOD spectra for these radicals over the wavelength range 400-650 nm are without significant maxima, with the OD of the dimeric form quoted as slightly greater. Therefore we propose one possible explanation for the transient absorption feature observed in our study is the photo-excited transfer of an electron from the metallocene centre (i.e. 1 + h v+ 1+ + e- ) to a solvent molecule (in bulk or coordinated) to yield CH3CN’-. The equilibrium then shifts to yield the dimeric anion radical (CH,CN);-, and hence the net changes in the solution’s total OD effectively monitor the rise and decay kinetics in the concentration of the dimeric anion radical, or some species which is an intermediate. Repeated flashing of the solution does not result in detectable photodecomposition, and therefore the ultimate fate of the photo-ejected electron is probably recombination with the ferrocenium centre. The intensity of the transient absorption increases with increasing power of the laser pulse (at least for laser pulse energies up to 200 mJ). Photo-excitation of ferrocene and its derivatives is known to result in electron transfer to NzO for A,,< 313 nm, by a mechanism which does not include the solvated electron [18, 191. In our study, saturation of the solution with N20 did not cause the transient absorption feature to disappear, nor its time-resolved profile to change. This may be because the concentrations of N20 and excited transient species were too low for quenching to occur over the time period during which the transient was observed, or that the reaction pathway to the anion radical dimer is unaffected by N20 saturation. The measurements were repeated on the system @y'+:l} in 95%MeCN-5% water solutions. Our previous electrochemical and absorption studies indicated that, upon addition of water to the MeCN solutions, the guest metal cation was observed to decomplex, as witnessed by the regeneration of the redox waves and the W-visible spectrum of the free host 1. Parallel nuclear magnetic resonance (NMR) studies confirm this behaviour, i.e. the 13C and ‘H spectra of the free hosts 1 and 2 (in MeCN-D20
260
;
4,
I laser flash /
(cl Fig. 3. (a) Transient absorption trace monitored at 520 nm for the complex {w*:l} M in MeCN. The intensity of the absorbance increases as shown to a maximum ps subsequent to the laser flash where (AOD),, is 4% of the transmission prior (b) Computer model of the trace in (a). The dotted line is the difference of two single with t=O set at abut 2 PS subsequent to the laser flash. (c) As for (a), but with D,O component in MeCN as solvent. (Scales: (a), (b) and (c) 2 w per division.)
at 3.2~ lop4 at about 4.5 to the flash. exponentials 5% Hz0 or
as solvent) are regenerated upon addition of water. However, laser studies reveal conclusively that complexation phenomena persist upon addition of water. Ako, addition of D20 and E&O components to the MeCN solution of 1 with dysprosium permits utilization of the deuterium isotope effect to gain information concerning the coordination
261
sphere about the D$’ centre. This methodology is well established in Eu3+ and Tb3+ solution chemistries [8]. We have shown 1201 that for the dysprosium aquo ion species in 95% MeCN-5% water the rate constant for the decay of the “I& excited state is linearly related to the ratio of O-H to O-D oscillators contained in the aquo ligands at dysprosium. Experimental determination of the rate constant krr when the aqueous component is Hz0 and the rate constant kD when the aqueous component is DzO, allows calculation of the number 4 of coordinated aqueous moieties at the dysprosium centre by the relationship 4 =2&l X 10e5(kH-k,). For the fully aquated w+ ion in MeCN solution, measured values of kH and kD yield the value q = 9.3, the “spectroscopic” coordination number. A commonly encountered coordination number for dysprosium in the solid state is nine. The spectroscopic value of 9.3 reflects the dynamic exchange of coordinated aquo species through a transition state of slightly expanded coordination number, where this exchange is significantly faster than the rate of decay of the 4F,, z excited state of the w+ ion. Figure 2(b) shows the decay of the emission monitored at 480 nm for an equimolar solution of 1 with dysprosium perchlorate hexahydrate at 3.2~ 10m4 M in 95% MeCN and 5% Hz0 as solvent @I347 = 0.10). The emission decay trace was collected within about 5 min of the addition of the aqueous component to the MeCN solution; the trace changes negligibly over the succeeding few hours, and repeated flashing causes no discernible decomposition. It is immediately apparent that the emission, in the presence of HzO, is far more intense than in the absence of water (by a factor of 20 at time zero and therefore shows a much improved signalto-noise ratio), and that the lifetime is shorter than for the nominal [Dy(H20)9]3+ ion, for which the 4F,, state decays exponentially with kl-395000 s-l (&6900) in MeCN-water as solvent. The trace in Fig. 2(b) analyses exactly as a single-exponential decay, with kH=532000 (*8000) s-'.Figure 2(c) shows the decay of the emission monitored at 480 nm for an identical solution except that the aqueous component is D20 rather than H,O. The trace was collected about 5 min subsequent to addition of the DzO to the MeCN solution. The decay analyses exactly as a single-exponential decay with kD = 235 000 ( k 2000) s- ‘. This decay rate is considerably greater than for the fully aquated aquo ion, the nominal [Dy(DzO)9]3+ under identical conditions, for which the 480 nm emission decays exponentially with rate kl=39000 (k400) s-l. Again the emission is much more intense than in the absence of DzO, by a factor of 15 at time zero. The observation that the 4F,, state of the w+ ion has a shorter lifetime indicates that coordination of the cation by the ferrocene-containing macrocycle 1 persists despite the presence of the aqueous component in the solvent which might be expected to cause decomplexation. The resultant decay constant for the 4Fg/2 excited state of the Dy+ ion in the complex {[Dy(X,0)J3’:1), where X=H or D, is given by the linear summation of the quenching rate kFc owing to the ferrocene centre, and the contribution by the total number q of coordinated aqueous moieties with individual quenching rates k,,of 44000 s-’ and kd =4340 s-l,which affords the simultaneous equations: k-r = kc + qk, and kD = kc + qkd Inserting the observed experimental rate constants yields the values kFc= 202500 ( f 3500) S -l, the contribution of the complexed ferrocene centre to the quenching of the ion, and also the number of coordinated aqueous 4F~,z excited state of the w’ moieties q = 7.5 ( st 0.25) in the complex {[Dy(X,0),J3+:1}. The expected “spectroscopic” saturated coordination number of 9.3 at dysprosium implies that, in addition to the
262
7.5 coordinated aqueous moieties, there are two additional donor functions. The host macrocycle 1 therefore acts as a bidentate ligand in wet MeCN solutions. In the presence of water the complex is unlikely to be static since in the NMR experiments the signals show that the carbonyls of 1 are trans. In the absence of water, NMR indicates that within the complex (for MeCN as solvent) the carbonyls become cis in order to accommodate metal cation complexation. However, the NMR experiment operates in the millisecond domain, and for dynamic equilibria the collected data are time averaged. Hence, NMR indicates that in the presence of water the equilibrium lies in favour of uncomplexed Df+_ This flash photolysis study is on a microsecond time scale. The decay of the Dy’ excited-state emission is observed as a single exponential because the rate of equilibrium in the presence of water between complexed and uncomplexed Dy’ is faster than the rate of decay of the 4F9n excited state for both the complexed and the uncomplexed D$+ species. The data indicate the macrocycle to be bidentate, and it seems likely that the two carbonyl oxygen atoms are involved in complex formation, and that the carbonyls must be c&. An equally unexpected observation for the {[Dy(X,0),13’:1} complex in wet MeCN solutions is the fate of the transient absorption feature which is observed in dry MeCN solution (see Fig. 3(a)). Figure 3(a) and 3(c) show the transient absorption trace before and after addition of the aqueous component and it is clear that the addition of water causes the transient absorption feature to disappear. Since the addition of the aqueous component causes the OD, A 347, at 347 nm of the solutions under study to decrease from 0.37 to 0.10, the number of photons absorbed in the path of the monitoring beam decreases by about 50%. Therefore the disappearance of the transient absorption feature is not an experimental artefact due to the geometry of the apparatus. The transient absorption feature is observed for solutions of 1 alone in MeCN, either with or in the absence of added water. The suppression of CH&N’and (CH,CN);formation upon pulse radiolysis of MeCN-water mixtures has been observed previously
P71.
to For the complex of 1 with Tb34 in MeCN solutions at similar concentrations the dysprosium study, again considerable intensity quenching of the Tb3+-centred D ‘ 4 excited-state emission monitored at 545 nm (jD4-‘F 5) is observed. The emission is seen to decay with a pseudo-first-order rate coefficient k=2900 s-l.For the Tb3’ species using solutions of terbium perchlorate hexahydrate alone in MeCN, this decay is single exponential with kl= 1540 s-l. Therefore in the complex (Tb3’:l) it is again probable that the complexed Tb3+ species is very efficiently quenched by the metallocene centre, and that the bulk of the emission monitored is from terbium uncomplexed by I, which is a minor component of the solution. Therefore, in the absence of an aqueous component within the solvent, the equilibrium between complexed and uncomplexed cation is not dynamic (or very slow). The addition of a 5% aqueous component to the solution indicates that the metal-metallocene interaction persists. With 5% HzO, the 545 nm emission is seen to decay quasi-exponentially with pseudo-first-order rate s-’ and, for 5% DzO, similarly with a rate kD -29000 s-l.However, in kH ~42000 this case, addition of an aqueous component to the solution causes a 50% reduction in emission intensity at time zero. For the species [T~J(H~O)~]~+ in MeCN, the D ‘ 4 state is observed to decay exponentially with kl= 2500 s-i, whilst m(DzO),]3+ decays exponentially with kl=290 s-l.Clearly the terbium excited state has a considerably shorter lifetime for solutions containing 1 even in the presence of an aqueous component, indicating persistence of interaction with the metallocene centre. The non-singleexponential character of the emissions precludes calculation of the number of coordinated aquo species at Tb3 + . The emission is quasi-exponential since in the presence of water
263
the equilibrium between complexed and uncomplexed Tb3’ is dynamic and on a time scale comparable with the lifetime of the 5D4 state of ([Tb(X,0),]3’:l}. This dissimilarity of complexation behaviour towards Df+ and Tb 3+ by 1 is consistent with observations noted in our previous electrochemical and UV-visible absorption studies 19, lo]. The characteristics of the transient absorption feature with Th3+ as complexed cation are identical with those reported for Dyj+ above. Identical behaviour of this transient is also found for Eu3+ as guest cation. No emissions from the complex formed by Ed+ with 1 were observed, since ruby laser excitation is inefficient for ‘Do state population, and at these concentrations even the relatively intense 616 nm emission (5D0-7F2) was not detected. MeCN solutions of the complex {Dff:2} formed by Dg+ with the ruthenocenecontaining cryptand 2 were also investigated. At the optimum concentrations the emissions from the laser-excited dysprosium 4F 9R state in the complex {Dy+:2) are less intense than the corresponding complex with 1 and were collected with an inferior signal-to-noise ratio. However, it is clear that the ruthenocene centre causes 4Fgn lifetimes to be quenched even more than the corresponding ferrocene-centred complex. For the complex {Dg+:2} in MeCN, the 480 nm emission from the 4Fgn excited state of D?’ subsequent to laser excitation is seen to decay as a single exponential with rate k,= 260000 (f 25000)s-l.Upon addition of 5% Hz0 to the solvent, this becomes kH = 690000 (~-40000)s-l,and, for 5% D20, it becomes kD -320000 (&20000) s-i. Therefore it is clear that complexation persists in the presence of water. Solving the simultaneous equations (as above) yields the value q, the number of coordinated aqueous moieties at dysprosium, of 9.2 ( f 1.5). However, the accuracy of the data is such that bidentate coordination by 2, as observed for 1, is still possible. Also, the contribution of 2 to the total quenching of the dysprosium excited state is evaluated at kRc= 275000 ( f 25000) s-i. The ratio kRc/kFc = 1.36indicates that the metallocenecentred state responsible for the quenching is closer in energy to the 4F9r2 state of Df+ for 2 than for 1. This is consistent with the interpretation that the quenching mechanism by the metallocene centres involves the forbidden ground-state singlet to excited-state triplet transition, and is electric dipole-dipole in character [21] with spin conservation for the donor-acceptor interaction. The transient absorption species observed for the corresponding complex with 1 were observed for (Dy+:2}. The characteristics were identical in every respect except that at 347 nm the quantum yield of the transient species is, qualitatively, less by about 20%. That the quenching phenomena are directly related to metal-metal interaction brought into play by coordination may be verified by studies on model complexes. Figure 4 shows the luminescent emission from excited terbium (5Dq-7Fg) monitored at 545 nm for an equimolar solution of l,l’-bis(diethylamido)ferrocene and terbium tris(acetoacetonate) at 5.2 x 10m4 M in MeCN as solvent subsequent to ruby laser excitation. The trace indicates an equilibrium between complexed and uncomplexed with the bis(diamide) competing for ligation at the sterically crowded Tb(acac),, lanthanide centre. The major component of the equilibrium is uncomplexed Tb(acac), which is responsible for the longer-lived decay which analyses as a near single exponential with rate constant 1550 (f 50) s-l. In the absence of the ferrocene derivative under similar conditions the laser-excited 5D4 state of Tb(acac), is observed to decay with rate constant 1580 ( f 70) s-r and exhibits no fast depopulating process of the excited state. (The rate constant for the decay of the ‘Da state of Tb(acac)3 is concentration dependent between 10m5 M and 1O-4 M, within the range between 1200 s-l and 1600 S -’ respectively). The observed decay rate of 1550 s-l in the presence of the ferrocene bis(diamide) indicates that the complexation equilibrium is effectively static compared
264
Fig. 4. Emission monitored at 545 nm (jD,,-‘F5) for an equimolar solution of Fe(C&eC0.NEt2)2 and l%(acac), at 5.2X 10m4M (~l~~,=O.15), in MeCN as solvent. (Scale, 50 w per division.)
with the millisecond time scale for the lifetime for the uncomplexed lanthanide species. In Fig. 4 there is also evident an additional fast decay process, which results from the superposition of emissions from the minor component of the equilibrium, which is Tb(acac), complexed by the ferrocene bis(diamide). The rate constant for the fast decay process may be estimated at about 30000 (+SOOO) s-l which is of the same order as the decay rate for the species {[Tb(X,0)J3+:1} noted earlier. This demonstrates clearly that the ferrocene centre acts as a quencher of the lifetime of the lanthanide excited state. The characteristic ferrocene-centred transient absorption feature is corresponding observed throughout. The co-solution of Tb(acac), with Fe(C,Ha. CO - Me), in MeCN under similar conditions results in no significant change in the rate constant for decay of the 545 nm emission, and no fast component is observed. Clearly the Tb(acac), species is not coordinated by the acyl carbonyl functions of Fe(CsH,. CO * Me)2. The transient absorption feature characteristic of the ferrocene centre is evident throughout. The corresponding co-solutions of Tb(acac), with ferrocene result in no quenching of the excited lanthanide state. Underivatized ferrocene possesses no complexing capabilities and therefore these photophysical metal-metallocene interactive phenomena cannot result from simple collisional encounter complexes. Ruby laser excitation of ferrocene resulted in the observation of the transient absorption feature but with low quantum yield and is independent of the solution’s water content.
4. Conclusions From this study on the photophysics and photochemistry of metallocene-containing cryptates and their complexes with lanthanide cations the following points emerge. (i) The metallocene centres act as efficient quenchers of the lifetimes of the excited states of complexed lanthanide cations. (ii) 347 nm pulsed laser excitation gives rise to a transient species associated, in origin, with the metallocene centre.
(iii) Coordination of lanthanide cations by the metallocene-containing cryptands persists in wet solutions with the host cryptand acting as a bidentate ligand. (iv) The fate of the transient species is influenced by coordination numbers of aquo ligands at the complexed lanthanide cation and therefore the mode of coordination by the host cryptand. (v) It is the amide carbonyls within the host molecular framework which are primarily responsible for the complexation of the cation. (vi) Coordination of the cation is a prerequisite for these metal-metallocene interactions to occur.
Acknowledgments We D. Flint
thank RTZ Chemicals Ltd. (Birkbeck College, London)
for financial support for the use of laser
(N.W.S.) facilities.
and Professor
C.
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