J . Phys. Chem. 1986, 90, 5 131-5 134
5131
Low-Temperature Luminescence of Chromium( I I I ) Complexes Coordinated with Macrocyclic Tetraamine Ligands Leslie S. Forster*+and Ole Manstedt Department of Chemistry, University of Arizona, Tucson, Arizona 85721, and Department of Inorganic Chemistry, H . C. 0rsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen 0, Denmark (Received: March 24, 1986)
The 77 K emission spectra and lifetimes of a series of Cr(II1) complexes with macrocyclic ligands have been determined and compared with the results for tetraamine complexes obtained previously. The transition energy is a major determinant of the nonradiative decay rate but the number of N-H bonds and geometric factors are also important. No evidence for an increase in the number of accepting modes with lowered symmetry was found, indicating the applicability of a local mode description.
Introduction Cr(II1) complexes constitute a useful group of molecules in which to study the relationship between molecular structure and nonradiative relaxation rates. A number of efforts have been made to treat the nonradiative decay rates in Cr(II1) complexes theor e t i ~ a l l y l -and ~ a considerable body of relaxation rate data has recently been c o l l e ~ t e d ,but ~ . ~several significant issues remain to be clarified. In particular, the relative importance of changes in the electronic and vibrational factors that accompany ligand substitution is not yet clear. The electronic factors are mediated by vibronic coupling that involves promoting modes. In addition, spin-forbidden transitions require spin-orbit coupling. The vibrational contribution depends on the Franck-Condon factors for the accepting modes, and symmetry arguments are often invoked to infer the number of vibrations that are important. Kuhn et al. determined the lifetimes of a group of hexaaminechromium(II1) complexes coordinated with a variety of primary and secondary amine ligands, in which the number of N-H bonds, n, varied from 8 to 18.3 Since the coordinating atoms were the same, the electronic factor was assumed to be constant for all of the complexes, and the effect of changes in the transition energy, Le., the energy gap, was ignored. The two major conclusions of that study wgre (i) the nonradiative rate, k,,, increases with n and (ii) the lowered symmetry in complexes with strained five-membered rings leads to an enhanced k,, compared to complexes with unidentate ligands or unstrained six-membered rings. Configurational isomers exemplify another type of geometrical change. There are no large isomer effects when Cr(NH3)4X2and Cr(en),X, (en indicates 1,2-ethanediamine) complexes are dissolved in glassy media, but a marked difference in k,, is observed for the isomers of Cr(cy~ l a m ) ( H ~ O ) , ~(cyclam + indicates 1,4,8,1 l-tetraazacyclotetradecane). Attention has also been focused on the relationship between the relaxation rates and the number of intraligand bonds with high-frequency stretching vibrations, e.g., C-H, N-H, and 0-H. The relaxation rate in CloHmD~_, is nearly proportional to the number of C-H bonds,6 and the decay rate in aqualanthanide complexes is extremely well correlated with the number of coordinated water m o l e c ~ l e s . ~A direct proportionality between k,, and the number of high-frequency N-H and 0-H bonds was 4A2(0,) postulated by Robbins and Thomson for the ,E(O,,) transition in Cr(II1) complexes.' They suggested that this simple proportionality indicated that changes in the electronic rather than the vibrational factor are responsible for the k,, variation. However, this interpretation was based on a limited data set, and more recent work reveals marked deviations from direct prop o r t i ~ n a l i t y . ~ ~In~ fact, . ~ . ~the low-temperature lifetimes of Crn = 12, and C r ( d i a r n ~ a r ) ~(diamsar + indicates 1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane)n = 6, are
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'University of Arizona. 'University of Copenhagen.
the same in alcohol-water g l a s ~ e s . ~ Most of the published theoretical work has been based on a displaced coordinate model in which the accepting mode frequencies are assumed to be unaltered upon excitation. Kupka has included the effect of changing the accepting mode frequency and found that the frequency change becomes a more significant factor as the displacement is reduced.I0 The profusion of parameters that enter into any theoretical analysis makes unambiguous assessment of the individual quantities difficult. At this juncture it is desirable to study groups of closely related species in order to detail the connection between k,, and n, as well as to assess the role of symmetry and changing the energy gap. A start in this direction has been made by recording the emission spectra and lifetimes of tetraaminechromium(II1) complexes with ammonia and 1,Zethanediamine as the N-coordinating ligand^.^ We have now extended the previous work to encompass some Cr(II1) complexes with macrocyclic amine ligands complexes in order to examine these questions.
Experimental Section Materials. cis-[Cr(cyclam)F2]C104,trans-[Cr(cyca)F2]C104 1,4,8,1 l-tetraazacyclo(cyca = mes0-5,5,7,12,12,14-hexamethyltetradecane), trans- [Cr(cyclam) F2]C104, trans- [Cr(cyclam)(NH3)2]C13.HCI.4H20, cis-[Cr-cis-(cyclam)(NH3)2]13~H20, cis-Cr[Cr(cycb)F2]C104 (cycb = rac-5,5,7,12,12,14-hexamethyl- 1,4,8,1l-tetraazacyclotetradecane),trans- [Cr(cyclam)C12]C1, trans- [Cr(~yca)(OH)~]C10~.3H,O, trans- [Cr(cyclam)(OH)(H20)](C104),, cis- [Cr(cyclam)Cl,]Cl, cis-[Cr(cyclam)(en)] Br3.2H20, and cis- [Cr(cycb)(OH),]C104.2H20 were prepared by literature methods or by modification of published procedures."-'3 cis-Cr(cyclam)(H20)2+was prepared by making an aqueous solution of cis-Cr(cyclam)C12+ basic and acidifying after the solution stood for a few minutes. The diaqua complexes were prepared by acidifying solutions of the corresponding hydroxo complexes. Spectral and Lifetime Measurements. Samples were dissolved (1) Robbins, D. J.; Thomson, A. J. Mol. Phys. 1973, 25, 1103. (2) Strek, W.; Ballhausen, C. J. Mol. Phvs. 1978. 36. 1321. (3) Kiihn, H.; Wasgestian, F.; Kupka, H.*J. Phys.'Chem. 1981, 85, 665. (4) Fucaloro, A. F.; Forster, L.S.;Rund, J. V.; Lin, S.H.J. Phys. Chem. 1983.87. 1796. (5) Forster, L. S.; Rund, J. V.; Fucaloro, A. F.; Lin, S.H.J . Phys. Chem. 1984,88, 5020. (6) Lin, S.H.; Bersohn. R. J. Chem. Phys. 1968, 48, 2732. (7) Sudnick, D.; Dew. Horrocks, W. J. Am. Chem. SOC.1979,101,335. (8) Forster, L. S.; Rund, J. V.; Castelli, F.; Adams, P. J . Phys. Chem. 1982, 86, 2395. (9) Comba, P.; Mau, A. W. H.; Sargeson, A. M. J. Phys. Chem. 1985,89, 394. (10) Kupka, H.Mol. Phys. 1979, 37, 1673, 1682. (1 1) Ferguson, I.; Tobe, M. Inorg. Chim. Acta 1970, 4, 109. (1 2) Eriksen, J.; Molnsted, 0.Acta Chem. Scand., Ser. A 1983, A37, 579. (13) Kane-Maguire, N. A. P.; Wallace, K. C.; Miller, D. B. Inorg. Chem. 1985, 24, 597.
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0022-3654/86/2090-5131.$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986
5132
Forster and Mansted
, trans
-
trans
-
On-18 0-0
MOL
. k
Y
5.10’
I
\
\ \
CrlNH31,1H201~ trans c,5
-
_------
1.L5
1.50
-
vE/prn-l
= k,, on the 2E(Oh) 4A2(Oh)energy for Cr(am),’+ complexes (except as noted in Table I, data are from ref 3). Figure 2. Dependence of
790
690 7LO 690 6LO Alnm hlnm Figure 1. 77 K emission spectra in ethylene glycol-water (2:l (v/v)). 7LO
in ethylene glycol-water (2:1 (v/v)) and immersed directly into liquid Nz.Excitation was at 337 nm with an N2 laser. The emission was passed through a 0.25-m monochromator (4.4-nm bandwidth) and onto a RCA C-31034 photomultiplier. Care was exercised to ensure reliability in the spectral positions and the 4Az(oh)emission energies, vE, were reproducible to %(oh) 0.0040 pm-I. In those cases where comparison is possible, our results are ca. 0.0050 pm-’ smaller than those of Kuhn et aL3 For lifetime measurements the output was digitized with a Biomation Model 805 transient recorder and averaged over 1024 sweeps. Lifetime reproducibility was 2-3%. The spectral measurements were made with boxcar detection.
&loL -
-
-
+
(14) Forster, L S.; Rund, J. V.; Fucaloro, A F J Phys Chem 1984,88, 5012
nOH-
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Results Spectra. The emission spectra of the tetraamine complexes coordinated with macrocyclic ligands resemble those of the 1,2ethanediamine and ammonia analogues in a general way. Trans complexes containing fluoride and hydroxide ligands show the 4Az(Oh) broad emission spectra characteristic of 2T,(Oh) transition^,'^ but the remaining spectra are of the sharp 2E(Oh) 4A2(Oh)type. In the latter group, cis complex emissions are dominated by the 0-0 transition with weak vibronic sidebands (Figure 1). Trans complexes, however, exhibit a relatively weak 0-0 band. Assignment of the 0-0 band in the trans-dichlorotetraaminechromium(111) complex is based on a comparison with the spectra of a group of hexaamine, pentamine, and trans-tetraaminechromium(II1) species, which indicates that the weak shoulder on the short wavelength edge of the designated 0band is not the spectral origin. In no case is the spectral origin as weak as the shoulder in the trans-dichlorotetraamine and trans-dichloro-“cyclam” spectra. For both cis and trans complexes, the transition energies increase with the number of N-H bonds, Le., in the order Cr(cyclam)X2 < Cr(en),X2 < Cr(NH3),X2. Lifetimes. The low-temperature excited-state relaxation rates for rigid glass solutions are collected in Table I. The Cr(NH3)4X, and Cr(en)2Xzresultss are included for comparison. Except as noted by a range of rate constants, the decays were exponential. In all cases the nonradiative rate is much larger than the radiative k, = knr. rate and r-’ = k,, Except for the cis-dihydroxo and trans-difluoro complexes the trend in k,, parallels that observed for vE, Le., Cr(cyclam)X2 < Cr(en2)X2< Cr(NH3)4X2. The cis-Cr(cycb) complexes relax nearly twice as fast as their cyclam counterparts, but contrary
T-’
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, 1.LO
Figure 3. Dependence of
T-’
1.L5 vE I wn-’
1.50
-
= k,, on the 2E(Oh)
cis-Cr(ax)4X2complexes.
4A2(Oh)energy for
to this there is no change in either the decay rate or spectrum when and cyclam is replaced by cyca in trans-Cr(~yclam)(H~O)~~+ trans-Cr (cyclam) F2+.
Discussion In a displaced but undistorted coordinate model with a single high-frequency accepting mode, vM, and a single promoting mode, vp, the low-temperature limiting expression for the nonradiative b in the weak coupling case isL5 rate for a
-
k,, = (r3/hvMAE?1/2vp(H’~b12 exp(-S (In (AE’/SMhvM) - I)AE’/(huM))
(1)
S is a measure of the total horizontal displacement between the
potential surface minima and SMthe corresponding quantity along the accepting mode coordinate. AE’ is the electronic energy is a matrix difference less the promoting mode energy and element that involves the spin-orbit and vibronic coupling operators. Even for the simple model embodied in eq 1, ligand changes can affect k,, by altering any of the quantities in the equation. Effect of Energy Gap on kn,. In some previous attempts to rationalize nonradiative decay rates in Cr(II1) complexes, variations in vE were ign0red.4~~~~ While this neglect is inconsequential for the analysis of the ammineaquachromium(II1) data where the range of the 0-0 energy, vE, is very small, the effect of changing the energy gap cannot always be overlooked, as demonstrated in Figure 2 by the hexaaminechromium(II1) data of Kuhn et al. k,, decreases with increasing vE for all series of complexes grouped (15) Freed, K .
F. Top. Appl. Phys. 1976, 15.
The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 5133
Luminescence of Chromium( 111) Complexes
I
I
TABLE I
2
no. N-H
bonds 18 12 10 trans-Cr(cyclam)(NH,),”+ 10 ~is-Cr(cyclarn)(NH,),~~ 10“ Cr(ditn),,+g 1 0“ Cr(dien),,+ 8 cis-Cr(cyctam)(en)’+ 6b Cr(diamsar)” 6c Cr(tacn),,+ 12 cis-Cr(NH,),F2+ 8 cis-Cr(en),F2+ cis-Cr(cyclam)F,+ 4 4 cis-Cr(cycb)F,+ 1 2d trans-Cr(NH3),F2+ 86 trans-Cr(en),F2+ trans-Cr(cyclam)F,+ 46 4d trans-Cr(cyca)F2+ rrans-Cr(NH,),C12t 12 8 trans-Cr(en),CI2+ trans-Cr(cyclam)CI,+ 4 cis-Cr(NH,),CI,+ 12 ~is-Cr(en)~Cl,+ 8 4 cis-Cr(cyclam)C12+ r r ~ n s - C r ( N H ~ ) ~ ( H ~ 0 ) , ~ +12 tran~-Cr(en),(H,O),~+ 8 tr~ns-Cr(cyclam)(H,O)~~~ 4 trans-Cr(~yca)(H~O)~~~ 4 c~~-C~(NH,)~(H,O),”+ 12 8 ~is-Cr(en),(H,O)~~’ cis-Cr(cyclam)(H20)23t 4 cis-Cr(cycb)(H20),”+ 4 C~S-C~(NH,)~(OH),+ 12 8 ~is-Cr(en),(OH)~+ 4 cis-Cr(cyclam)(OH),+
vn/w& 1.517 1.490 1.488 1.481 1.498 1.465 1.470 1.457 1.471 1.508 1.484 1.462 1.458 1.414 1.294 1.265 1.268 1.453 1.431 1.427 1.477 1.439 1.418 1.502 1.493 1.479 1.474 1.497 1.486 1.443 1.432 1.462 1.437 1.383
lO-,/s-’ 1.47 0.90 O.5Oe 0.89 0.50 1.03 0.74 0.90 0.29 2.86 1.92 1.11 1.85-2.08 2.04 2.86 1.52-1.92 1.47-2.04 3.0-3.6 2.00 1.15 2.94 2.50 2.10 2.50 1.37 0.79 0.80 2.44 1.61 1.35 2.38 3.27 2.94 3.7-4.5
1
I
\
!-I
1-l X
-
“Reference 3. *Reference 9. CReference21. d2T,(Oh) ,A2@,) emission. Energy corresponds to band maximum. e0.56in Me2S0 (ref 13) and 0.59 in N,N-dimethylformamide. /The same value in Me2S0 (ref 13). gditn = 1,5,9-triazanonane. hdien = 1,4,7-triazaheptane. according to the number of N-H bonds. The same trend prevails for the three series of cis-tetraaminechromium(II1) complexes in Figure 3. This behavior is in accord with predictions based on eq 1 if all the parameters other than AE’ = vE - vP are assumed to be constant. In this case eq 1 reduces to where a and /3 are constants. @ depends upon the accepting modes characteristics while a is a measure of promoting mode and spin-orbit coupling effectiveness. Coordination of thiocyanate, acetylacetonate, and cyanide leads to especially large vE variations which are associated with a nearly tenfold increase in k,, induced when the methyl groups in tris(acetylacetonate)chromium(III) are replaced by hydrogen atoms or phenyl moieties.I6 The vE variation in the diketonate complexes was attributed to delocalization of metal electrons onto the T acceptor ligands where the C-H vibrations are localized. Delocalization onto cyanide or thiocyanate should not enhance the relaxation, however, since these ligands have no effective accepting vibrations, but the vE reduction would increase k,,. Effect of Number of N-H and 0 - H Bonds on knr. A definite correlation between the decay rates and the number of N-H bonds prevails in all groups of complexes with a common skeleton (Figures 2 and 3). The very large deuterium isotope effects4 indicate that the N-H modes with vM = 0.32 Fm-’ are the principal accepting vibrations in hexaaminechromium(II1) complexes. In tetraaminechromium( 111) complexes with monatomic ligands N-H stretching vibrations are still the dominant accepting modes, but in complexes containing hydroxo or water ligands 0-H vibrations will also contribute. In the harmonic approximation a mode can
1.5
\
\
I
I
I
I
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0
+0.02
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Figure 4. Effect of transition energy difference on the k,, ratio in cis and
trans isomers: (0)dichloro complexes; ( 0 )diaqua complexes. be accepting only if it is totally symmetrical or if its frequency changes during the nonradiative transition. Equation 1 applies only in the former case, but in either event k,, will increase with the number of accepting modes, which, if a local mode representation is valid, is equal to the number of N-H and 0-H bonds. The theoretical interpretation of the k,, dependence on the number of such bonds in chromium(II1) complexes has long been the subject of some contention. Robbins and Thomson’ recognized that eq 1 predicts a much larger dependence of k,, upon n than observed and minimized the role of accepting modes in the decay process, focusing instead on the promoting modes. Strek and Ballhausen’ denied a direct connection between SMand n and attributed the increase in decay rate with n to changes in the electronic factor. However, Strek et a1.I’ ascribed the k,, variation in Cr(NCS),(Me2S0)6-~-*complexes entirely to changes in the vibrational factor and fitted the results by assuming a proportionality between SMand n. Finally, Kuhn, et al. used Kupka’s’O treatment, in which changes in vibrational frequencies between the ’E(0,) and 4Az(0,) states are included, to interpret their data. However, they had to assume a twofold change in SMbetween strained ligands, Le., those giving five-membered chelate rings, and unstrained ligands. More recently Kupka has extended his treatment by assuming SMto be proportional to n and has found that the inclusion of the change in vibrational frequency between the ground and excited states yields a much smaller change in k, with n than predicted by eq l,’*in accord with the results. The present data for series of complexes with a common set of coordinated ligands and a fixed number of N-H bonds, rationalized in terms of eq 2, clearly show that @ increases with a decreasing number of N-H bonds (Figures 2 and 3). This is in qualitative agreement with eq 1 since SMis expected to increase with n. For the cis-tetraamine data there is also good proportionality between p and n-l but this may be accidental as a similar relationship is not observed for the hexaamines. Configurational and Conformational Effects on knr. Kiihn et al. suggested that the increased k,, associated with five-membered rings arises from geometric distortions that increase with the number of accepting modes.3 Their data for the series of hexaaminechromium(II1) complexes with 12 N-H bonds indicate that, in general, vE is smaller in complexes with five-membered rings than in complexes where ligand strain is minimal (Figure 2). In a sense it is therefore correct to assert that ring strain is the cause of the k,, increase. However, the data for complexes with strained and unstrained rings lie on the same line in Figure 2 and the relatively small increases in k,, can be ascribed to a reduction in vE without significant effect on either a or (3; Le., neither new accepting nor promoting modes are introduced. Geometrical changes pertinent to nonradiative decay in metal complexes can be classified as conformational or configurational. In assessing the changes in k,, associated with different configurations, it is necessary to exclude data for isomer pairs involving fluoride and hydroxide ligands since the electronic transition is different in the cis and trans isomers of the complexes coordinated (17) Strek, W.; Lukowiok, E.; Jezowska-Trzebiatowska, J . Lumin. 1977,
(16)DeArmond, K.; Forster, L. S . Spectrochim. Acta 1963, 19, 1403, 1687.
15, 437.
(18)Kupka, H.,private communication.
5134 The Journal of Physical Chemistry, Vol. 90, No. 21, 1986 with these two ligands. This leaves the dichloro, diammine, and diaqua isomer pairs as suitable for examination of any isomer effect. The relationship between the cis to trans k,, ratio to the vE difference in an isomer pair is shown in Figure 4 for diaqua and dichloro complexes. The v E differences within the Cr(cyclam)C12+and C r ( ~ y c l a m ) ( N H ~ pairs ) ~ ~ +are so small as to leave little doubt that in these cases vE differences are not the only source of the k,, variation. The vE difference within the diaquacyclam pair is 0.036 pm-l and some of the k,, enhancement in cis-Cr( c y ~ l a r n ) ( H ~ Oover )~~+ the trans complex is attributable to the decrease in the transition energy. It can be argued that a possible configurational effect in the Cr(NH3)4C12+isomers is obscured by the 0.024-m-I vE difference. The cis to trans rate ratio increases < Cr(cyclam)X2. in the series C T ( N H ~ ) ~-Cr(~yca)(H,O),~~, 86916-07-6; cisCr(NH3)4(H20)23+, 42402-01-7; ci~-Cr(en),(H,O)~~', 22432-36-6; cisCr(~yclam)(H~O)~'+, 99572-59-5; cis-Cr(~ycb)(H,O)~~+, 88546-58-1; cis-Cr(NHJ4(0H),+, 57349-68-5; cis-Cr(en),(OH),+, 22432-35-5; cisC r ( ~ y c l a m ) ( O H ) ~103882-36-6. +,
