J . Phys. Chem. 1991, 95, 9071-9075 the stabilization energy exactly, the amount of the spectral shift seems to be in a good agreement with the theoretical prediction. We can safely conclude that the solvation of electrons is initiated by the reorientation of the solvent molecules in the first solvation shell and the polarization of the outer shell is established later and that the polarization of the outer shell may not be an ad-
9071
ditional small adjustment but may contribute almost 50% of the stabilization energy. Acknowledgment. The present work was partially supported by Grant-in-Aid for Special Promoted Research 6265006 to N.M. from the Ministry of Education, Science and Culture of Japan.
Reinvestigation of the Absorbing and Emitting Charge-Transfer Excited States of [Cu(NN),]+ Systems R. Michael Everly and David R. McMillin* Department of Chemistry, 1393 Brown Building, Purdue University, West hfayette, Indiana 47907- 1393 (Received: April 2, 1991)
The absorption and emission spectra of a series of [Cu(NN),]+ systems, where NN denotes a heteroaromatic, chelating ligand derived from I , IO-phenanthroline or 2,2'-bipyridine, have been investigated. Detailed group theoretical assignments of the relevant low-lying charge-transfer states are presented. For the first time, a charge-transfer absorption band has been identified with a polarization perpendicular to the axis joining the metal and the ligand centers (the z axis). One of the most novel properties of the complexes is that they exhibit thermally activated emission. Data presented include absorption and emission polarization measurements from glycerin solutions over the temperature range 90-175 K as well as data from a 4:l (v/v) ethanol-methanol solution at 90 K. The emisssion can be attributed to two different excited states in thermal communication, and both components of the emission are z polarized. The results are inconsistent with a model that assigns the emissions to spin-orbit components derived from a triplet excited state (Parker, W. L.; Crosby, G. A. J . Phys. Chem. 1989,93,5692-5696), but they are in accord with a previous proposal which assigns the thermally activated emission to a singlet state (Kirchhoff, J. R., et al. Inorg. Chem. 1983, 22, 2380-2384).
Introduction Investigation of the photochemical and photophysical properties of transition-metal complexes, fueled in part by the energy crisis of the 197Os, continues to flourish as an area of research.'-3 Aside from possible applications in solar energy conversion, there is great interest in the fundamental properties of electronic excited states. Although tris(bipyridine)ruthenium(II) has by far been the most extensively studied transition-metal system, several other complex ions have been investigated. The [Cu(NN),]+ systems (where NN denotes a derivative of 1 ,IO-phenanthroline or 2,2'-bipyridine) 5
" 2\ "
6
' /6
1 ,lOphenanthroiine
intense MLCT transitions apparent in the near-UV-visible spectrum are discussed below. Luminescence from the [Cu(NN),]+ systems in a rigid glass at 77 K was first observed by Buckner and M~Millin;~ later Blaskie and McMillin detected emission in fluid solution.I0 Emission arises from at least two different excited states in thermal communication because the emission intensity and energy increase with increasing temperature. Kirchhoff et al. originally proposed that the two states, separated by about 1800 cm-I, represent the singlet and triplet spin states derived from the lowest energy MLCT state." However, Parker and Crosby have recently proposed an alternative interpretation of the emission data.* From the emission lifetime and the shift in energy between the absorption and the emission maxima they concluded that the emission is wholly derived from a triplet state. They suggested that two components are resolved because of a substantial zero-field splitting. To account for the magnitude of the spin-orbit interaction, they assigned the emission to a 3E term derived from e(xz,yr) b,(x) excitation where the b,(x) wave function can be approximately described as a linear combination of the second
5ms 6
h;
2,2'-bpyridine
exhibit a strong absorbance in the visible region and have been of particular interest to us. The assignments of the low-lying states involved in the absorption and emission spectra have, however, been somewhat controversial. Regardless of the exact nature of the ligand, the visible absorption spectra of these [Cu(NN),]+ complexes are dominated by intense, broad absorption bands maximizing at a wavelength in the range 440-470 nm. This absorption was originally assigned by Irving and Williams4 as a metal-to-ligand charge-transfer (MLCT) transition wherein an electron is promoted from a 3d orbital of copper to a low-lying A* orbital of the ligands, and the orbital parentage of the dominant charge-transfer absorption has since been establi~hed.~-*In DZdsymmetry the excited state e($) corresponds to a IB, term associated with an e(xz,yz) excitation where in the first approximation the e($) wave functions can be regarded as linear combinations of the lowest unoccupied molecular orbitals (LUMOs) of the N N ligands. Other less
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Author to whom correspondence should be addressed.
0022-365419 112095-907 1$02.50/0
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( 1 ) Hoffman, M. 2.;Bolletta, F.; Moggi, L.; Hug, G. L. J . Phys. Chem. Ref Dafa 1989, 18, 219-543. (2) Ferraudi, G. J. Elemenfs of Inorganic Phofochemisfry;Wiley-Interscience: New York, 1988. (3) Sfkora, J.; Sima, J. Coord. Chem. Reo. 1990, J07, 1-225. (4) Irving, H.; Williams, R. J. P. J . Chem. SOC.1953, 3192-3210. (5) Day, P.; Sanders, N. J . Chem. Soc. A 1967, 1536-1541. (6) Ichinaga, A. K.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker, C. 0.;Marnot, P. A.; Sauvage, J. P. Inorg. Chem. 1987, 26, 4290-4292. (7) Dad, C.; Schllpfer, C. W.; Goursot, A,; Penigault, E.; Weber, J. Chem. P h p . Leff. 1981, 78, 304-310. (8) Parker, W. L.;Crosby, G. A. J . Phys. Chem. 1989, 93, 5692-5696. (9) Buckner, M. T.; McMillin, D. R. J . Chem. Soc., Chem. Commun.
1978, 759-761.
(IO) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519-3522. (11) Kirchhoff, J. R.; Gamache, R. E.,Jr.; Blaskie, M. W.; Dei Paggio, A. A,; Lengel, R. K.; McMillin, D. R. Inorg. Chem. 1983, 22, 2380-2384.
0 1991 American Chemical Society
Everly and McMillin
9012 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
650
740
I
,
850
Wovelength (nm) 350
410
Wovelength
550
(nm)
Figure 2. Emission spectrum of [Cu(tmbp),]BF, in glycerin as a function of temperature. From top to bottom T = 175, 150, 125,90, and 77 K. Excitation wavelength was 450 nm.
Figure 1. Absorption spectrum of [Cu(dmp),]N03 in glycerin at different temperatures. From top to bottom T = 90, 135, 175, and 293 K.
lowest unoccupied molecular orbitals (SLUMOs) of the N N ligands. However, Kirchhoff et al. have shown that [C~(tmbp)~]+, where tmbp denotes 4,4’,6,6’-tetramethyI-2,2’-bipyridine,exhibits the same type of temperature-dependent emission.” In the case of the bipyridine complex the emission cannot possibly arise from the ’E state proposed by Parker and Crosby because the ligand x orbital occurs at much too high an energy.12 Furthermore, the same type of temperature-dependent emission is exhibited by mixed-ligand complexes of copper( I), where the symmetry is too low to allow for orbitally degenerate terms.”J4 In an attempt to refine the state assignments further, we have gathered emission and excitation polarization data for a series of [ C U ( N N ) ~ ] + systems over a range of temperatures. The results of these studies are presented below.
Experimental Section Materials. The 4,4’,6,6’-tetramethyl-2,2’-bipyridinel5(tmbp) and 2,9-di-n-butyl- 1,l O-phenanthrolineI6 (dbp) ligands were synthesized as previously reported. The 2,9-dimethyl- 1,lophenanthroline (dmp) and 2,9-dimethyl-4,7-diphenyl-l,10phenanthroline (bcp) ligands were purchased from Aldrich and were used without further purification. The [Cu(NN),]+ complexes were usually isolated as the BF, salts by a literature procedure.” All complexes gave satisfactory spectroscopic and elemental analyses. High-purity distilled in glass methanol and anhydrous ethanol were purchased from American Scientific (Burdick & Jackson) and Midwest Solvent Co., respectively. Double-distilled high-purity glycerin was purchased from Aldrich. Equipment. All spectroscopic measurements were made in an Oxford Instruments Model DN 1704 cryostat. Absorption data were obtained by using a Cary 17D or a Perkin-Elmer Lambda 4C spectrophotometer. Corrected emission spectra were measured on a SLM-Aminco S P F 500C spectrofluorometer. Emission polarization measurements were determined by using a PerkinElmer MPF 448 fluorometer with a standard polarization attachment. Methods. To dissolve the compounds in glycerin, the solutions were heated to -80 OC in a water bath. While hot, the solution was then introduced into a 1 cm X 1 cm polystyrene fluorescence cell from Precision Cell. Samples dissolved in a 4:1 (v/v) alcohol (12)Ceulemans, A.; Vanguickenborne, L. G. J. Am. Chem. SOC.1981, 103,2238-2241, (13)Breddels, P. A.; Berdowski, P. A. M.; Blasse, G.; McMillin, D. R. J . G e m . Soc., Faraday Trans. 2 1982,78, 595-601. (14) Palmer, C. E. A,; McMillin, D. R. Inorg. Chem. 1987.26.3837-3840. (15) Linnell, R. H.J . Org. Chem. 1957,22, 1691-1692. (16) Dietrich-Buchecker. C.0.: Marnot. P. A.; Sauvage, J. P. Terrahedron k t i . 1982, 23,5291-5294. (171 McMillin, D. R.; Buckner. M. T.: Ahn. B. T. Inore. Chem. 1977,16, 943-945.
Wovelength ( n m )
Figure 3. Emission data from [Cu(dmp),]NO3.2H,O in glycerin. From the top, emission spectrum at 175, 150, 125, and 90 K. The polarization spectrum (horizontal trace) is at 175 K (scale at right). Excitation wavelength was 450 nm. (Inset) Difference spectra obtained when the 90 K spectrum is subtracted from the others.
Wovelength (nml Figure 4. Emission data from [Cu(dbp)],BF4 in glycerin. From the top, emission spectrum at 90, 125, 150, and 175 K. The polarization spectrum (horizontal trace) is at 175 K (scale at right). Excitation wavelength was 455 nm; isoemissive point was at 605 nm.
mixture were examined in a 1-cm cylindrical quartz cell. Prior t o spectral measurements, the sample was allowed t o equilibrate at the desired temperature (fO.l K) for at least 1 h. Polarization ratios were calculated according to the method outlined by Lakowicz.’* As required, spectra were smoothed by a simple moving window average technique.
Results The absorption spectra of the [Cu(NN),]+ systems are temperature dependent as illustrated in Figure 1. In particular, the most intense absorption band sharpens somewhat and exhibits a small red shift at lower temperatures, However, in the range between 90 and 175 K when the main band is excited in the trough ( I 8) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983;pp 112-153.
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The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9073
Excited States of [Cu(NN),]+
az,b,(X)
I\ I
JI
e(+)
=_ _ _ _ _ _ _ _ _ - --- bl,a(X) -
--________
-- - bz,b3(S)
~
a,(zZ)
-___________
a(& 02
D2d
Figure 6. Schematic orbital splitting diagrams for [Cu(NN),]+. Left,
Du symmetry; right, D2symmetry. The flattening distortion is assumed to occur along the x axis.
4
,
I
390
1
I
I
I
.
470
,
, \ - t o 550
Wavelength (nm)
expected for a molecule with parallel (perpendicular) absorption and emission dipoles. Consistent with the notion of temperature-dependent solubility problems, a much increased emission polarization of 0.38 was measured from [C~(dmp)~lBF, in glycerin at 215 K, which appears to be just above the glass transition temperature.
Figure 5. Excitation data from [Cu(bcp)2]BF4at 90 K monitored at 680 nm. Top, excitation spectrum measured from glycerin; bottom, excitation spectrum measured from 4: 1 (v/v) ethanol-methanol. The excitation
Discussion Absorption and Excitation. The absorption spectra of the [Cu(NN),]+ complexes typically reveal three distinct band systems polarization (scale at right) is the noisy trace. Spectra were smoothed by the moving window average technique. at lower temperatures as shown in Figure 1. Detailed assignments are hampered by uncertainties about the ground-state symmetry. In crystalline materials the angle between the planes of the two TABLE I: Emission Polarization Data ligands departs from the ideal 90° expected for Du symmetry. exc emission polarization However, the angle deviates as substituents are introduced on the wavelength,” 4: 1 ligands or as the anion is varied;I9 hence lattice-packing intercomplex nm glycerin ethanol-methanol actions may determine the dihedral angle in the solid state. Since [Cu(dmp)21N03 450 0.32 0.44 the structure in solution is unknown, the results will be first [CU(&P)~IBF, 470 0.10 0.40 interpreted in terms of D M symmetry, and then the effect of a [Cu(db~)2lBFd 455 0.32 0.44 flattening distortion will be considered. The pertinent one-electron [Cu(tmbp)21N03 450 0.23 orbitals of [Cu(NN),]+ in D M symmetry are depicted on the left-hand side of Figure 6 . In the figure the z axis is defined to In glycerin. be collinear with the C2axis passing through the ligands of the between the vibronic maxima, e.g., 450 nm in Figure 1, the ab[Cu(NN),]+ system, and the x and y axes are defined to be sorption is essentially constant. collinear with the other two C2symmetry elements. The e(xzyz) Figures 2-4 show the emission spectra of [ C ~ ( t m b p ) ~ ] B F ~ , orbitals of the copper have been placed above b2(xy) in accordance [Cu(dmp),]NO3.2H2O, and [Cu(dbp),]BF, as a function of with MS SCF-Xa calculations pertaining to a related system.’ temperature in glycerin. For convenience the emission polarization The accessible ?r* orbitals of the ligands form bases for e(+), a2(x), has only been plotted for the highest temperature spectrum, but and bl(x) levels of the D M complex, where denotes the LUMO it is essentially temperature independent over the entire range. and x denotes the S L U M 0 of the free N N ligand. (By conWith the exception of [ C ~ ( d b p ) ~the ] + emission intensity and the ventionI2 the T orbital is labeled (x) if it is antisymmetric emission energy increase with increasing temperature. Figure 5 (symmetric) with respect to a twofold rotation of the ligand.) provides a representative excitation polarization spectrum, namely There are several possible symmetry-allowed MLCT transitions that of [Cu(bcp),] BF4 dissolved in 4: 1 (v/v) ethanol-methanol within the scheme of orbitals presented in Figure 6. However, at 90 K. For comparison, Figure 5 also includes an excitation as first pointed out by Day and SandersSand later confirmed by spectrum measured in glycerin at the same temperature. The the analysis of Phifer and McMillin,2° the absorption intensity spectra reveal that the polarization of the transition centered is largely confined to the transitions polarized along the axis that around 410 nm is orthogonal to that of the other visible bands. joins the metal and the ligand centers. As noted above, virtually In general, the degree of emission polarization varied with all authors agree that the intense visible absorption maximum of conditions. Although stronger emission signals were typically the [Cu(NN),]+ systems can be ascribed to a ‘B2 term associated observed from samples in a glycerin matrix, at 90 K the polariwith an e(xz,yz) e(+) This transition will be zation usually increased by about 0.1 when 4:l ethanol-methanol referred to as band I1 in accordance with the numbering scheme was used as the solvent. The most dramatic enhancement was of Ichinaga et ale6 observed with [Cu(bcp),]BF,, in which case the polarization In the same numbering scheme a low-energy shoulder, which increased from 0.1 to 0.4 (Table I). The choice of anion also appears at around 530 nm in Figure 1, represents band 1. Ichinaga influenced the results in glycerin. For example, in the case of et al. have suggested that this band could correspond to one of [Cu(tmbp),]+ the emission polarization increased from 0.18 to the allowed xy-polarized MLCT transitions,6 but one of the im0.23 when the nitrate salt was used in place of the tetrafluoroborate portant points established by Parker and Crosby is that band I and band 11 have the same polarization.8 However, there are only salt, while for [Cu(dmp),]+ the corresponding increase was from 0.23 to 0.32. Complexes that dissolved readily typically exhibited two z-polarized transitions allowed in Du symmetry, and the the strongest emission polarization. One possible explanation for the variation between samples is microcrystallite formation, be(19)Klemens, F. K.;Fanwick, P. E.; Bibler, J. K.;McMillin, D. R. fnorg. cause scattering by particulates would tend to depolarize the signal Chem. 1989,28, 3076-3079. and to reduce the polarization from the ideal value of 0.5 (-0.33) (20) Phifer, C. C.; McMillin, D. R. fnorg. Chem. 1986, 25, 1329-1333.
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The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
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bl(xly2) a2(x) transition is expected to occur at much higher energy.20 To account for the polarization, Parker and Crosby have assigned band I to the 'A2 term derived from e(xz,yz) e($) excitation, which becomes z allowed under the influence of a D2 flattening distortiom8 As emphasized earlier, this type of distortion has been observed in the solid state, and there is strong evidence that a significant flattening distortion occurs in the emissive CT excited states2' The qualitative effect of a D2 distortion on orbital splitting is depicted in Figure 6, where, with no loss of generality, the flattening is assumed to occur along the x axis. The prediction is that the lowest energy, z-polarized transition in D2 symmetry should be b3(yz) b2($). The third band system, band Ill, is a weak, vibronically structured transition that is centered at about 410 nm in Figures 1 and 5 . Within the framework of Du symmetry Ichinaga et al. have assigned band 111 as the other z-polarized transition6 (b,(x2-y2) a2(x)) since it could also be expected to carry significant oscillator strength. However, the excitation polarization spectrum in Figure 5 clearly reveals that the polarizations of bands 11 and I11 are mutually orthogonal. Therefore, the bulk of the absorption in the region of band 111 must be xy polarized and, in Dzd symmetry, can be assigned as a transition to a 'E excited state. Because there is no analogous transition in the spectrum of [C~(tmbp)~]+,(' the transition can be assumed to terminate in either the al(x) or the b,(x) orbital and hence to originate from the e(xzyz) level. In Figure 5 the polarization ratio never quite reaches the theoretical value of -0.33, and in the corresponding region of the spectrum of [Cu(dbp)JBF, the polarization ratio never dips below about -0.1. These results suggest that there could be some contribution from an underlying z-polarized absorption, possibly the leading edge of band 11, which extends to around 400 nm in the spectrum of [ C ~ ( t m b p ) ~ If ] . the actual symmetry is D2,the physics are basically unchanged. In the lower symmetry the transition would be expected to originate from the HOMO b&yz). The transition to b,(x) would b e y polarized, while the transition to a(x) would be x polarized. In principle, MCD spectroscopy could be used to establish the molecular symmetry. Thus, in Du symmetry a transition to a bona fide 'E state would show up as an A term because the excited state is orbitally degenerate. In their studies of [ C ~ ( p h e n ) ~ ] + Hollebone et al. have, in fact, assigned an A term to a transition that falls on the high-energy side of the main MLCT absorption band.22 On the other hand, Parker and Crosby have measured the MCD spectrum of [Cu(bcp),]+ and have assigned the signal to two B terms with opposite signs: consistent with their suggestion that the ground state suffers a D, flattening distortion. However, even if the ground state is not significantly flattened, the excited state is clearly distorted,2' and this, too, complicates the interpretation of the MCD spectrum. On balance, the MCD data do not shed much light on the spectral analysis. As a matter of fact, more bands have been resolved in the absorption spectrum than in the MCD spectrum. Emission Spectra. The emission spectra of the [ C U ( N N ) ~ ] + complexes are quite unusual in that the energy and intensity of emission increase with increasing temperature (Figures 2-4). Since the lifetime decreases at the same time, the only plausible interpretation is that there are at least two emissive states in thermal communication. Indeed, Kirchhoff et al." have proposed that the emission spectrum reflects two components separated by about 1800 cm-I. The fact that the difference spectra in Figure 3 all have the same maximum and the same band shape provides additional support for the two-component model. The difference spectrum defines the emission from the higher energy state, whereas the 90 K spectrum represents the best approximation of the other component. When the two components are corrected for instrumental distortions and then converted to a wavenumber scale,24the difference in the emission maxima provides an estimate
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Everly and McMillin of the energy gap between states. Unfortunately, the emission occurs at relatively long wavelengths where the instrumental distortion is severe and difficult to correct accurately. On the basis of these data the energy gap can be estimated to be 2000 f 500 cm-' in all cases; the large error margin is a consequence of the experimental problem associated with spectral corrections. Estimated values of the respective radiative rate constants led Kirchhoff et al. to assign the two states as the singlet and triplet states associated with an excited electronic configuration having MLCT character." In contrast to the above, Parker and Crosby have offered a different interpretation of the temperature-dependent emission behavior; in particular, they propose that the emission arises from a 3E state which is split by spin-orbit coupling into three distinct components.8 They attribute the emission to the lower two energy levels which are separated from each other by 1000 cm-I. To account for the magnitude of the spin-orbit interaction, they have further proposed that the 'E state is connected with a e(xz,yz) b,(x) transition. One of the spin-orbit states is a B, state from which z-polarized emission is expected. Accordingly, this component has been assigned to be the low-energy component, and the higher energy emitting state has been assigned to the E component which should give xy-polarized emission. However, this model has several shortcomings. In the first place, the 3E excited state in question is clearly thermally inaccessible in [ C ~ ( t m b p ) ~due ] + to the intrinsically high energy of the ligand SLUM0.6s'2 Second, the model requires a large spin-orbit splitting in spite of the fact that the complex involves a first-row transition ion and the fact that a Jahn-Teller distortion removes the orbital degeneracy in the excited state.,' Finally, the data in Figures 2-4 clearly show that both emitting states have the same polarization as opposed to the orthogonal relationship predicted by Parker and Crosby. New State Assignments. The most logical approach is to assign the emitting states under the assumption of D2 symmetry in view of the flattening distortion which is proposed to occur in the C T excited state.2' Under a flattening distortion, the C2 axes along x, y , and z are preserved; hence, the emission polarization can still be used to guide the assignments. The difference spectra in Figure 3 reveal the contribution from a higher energy emitting state. As discussed in detail previously,'' the radiative rate constant of the upper emitting state is sufficiently large (ca. lo7 s-I) that there can be little doubt but that it is a singlet state. Since the emission from this state is z polarized, the state can be assigned as a 'BI state in D2 symmetry. On the basis of Figure 6, this state would logically arise from a b3(yz) b,($) transition, Le., the state associated with band I in the absorption spectrum. The polarization results suggest that the lower energy emitting state, which according to the lifetime data has triplet multiplicity,'-8.'' is also a B, state. From Figure 6 it follows that there are two low-lying triplet states in the D2 structure: a 3Bl and a 3A associated with b3(yz) b2($) and b3(yz) b3($) transitions, respectively. According to the direct product rules for multiplying the spin and orbital components of the wave function,23only the term gives rise to a spin-orbit state with B I symmetry. Therefore, the surprising prediction is that the z-polarized emitting states of the D, structure are associated with different electronic configurations. Of course, in addition to the two emitting states, there are several other states, including the other spin-orbit states associated with the 3A term as well as those associated with the 3B,term, that may populated. Preferential occupation of less emissive states may explain why the emission intensity decreases further at very low temperature^.^,'^ In principle, the unusual temperature dependence that is ob4) could be served in the emission from [ C ~ ( d b p ) ~(Figure ]+ explained by the thermal population of a third, nonemissive energy level. In this view the net decrease in the emission intensity at
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(21) Everly, R. M.; Ziessel, R.; Suffert, J.; McMillin, D. R. Inorg. Chem. 1991, 30, 559-561.
(22) Hollebone, B. R.: Mason, S.F.; Thomson, A . J. Symp. Faraday Soc. 1969, 3, 146-1 60.
(23) McGlynn, S.P.;Azumi, T.; Kinoshita, M. Molecular Spectroscopy ofrhe Tripler Srare; Prentice-Hall: Englewood Cliffs, NJ. 1969; pp 183-208. (24) Reference 18, pp 42-43.
J. Phys. Chem. 1991, 95,9075-9080 higher temperatures would be ascribed to population of a nonemitting state close in energy to the two BIstates identified earlier. The presence of the butyl substituents, which would displace solvent molecules, could result in a less polar local environment and a unique excited-state ordering for this complex.2s On the other hand, the dbp complex is also unique in that it has floppy substituents in the 2- and 9-positions of the phenanthroline. It is therefore possible that the emission is influenced by some type of thermally activated quenching process that is peculiar to the complex with the butyl groups.
Conclusions Excitation and emission polarization data provide a means of assigning the absorbing and emitting charge-transfer excited states of C U ( N N ) ~ systems. + Although the absorption spectrum can for the most part be treated within the context of Du symmetry, a static or dynamic flattening distortion is invoked to account for a low-energy shoulder designated band I. In line with theory, the dominant absorption (band 11) has been assigned to a z-polarized 'B2 term, where z is the axis connecting the metal and the ligand centers. A high-energy shoulder (band HI),which is absent in the bipyridine complex, has been shown to be x y polarized and is attributed to a 'E state involving excitation to the ligand SLUMO. At least two emissive states in thermal communication are required to explain the temperature dependence of the emission intensity, and the emission polarization data reveal that both
9075
emissions are z polarized. Consequently, they cannot be assigned to spin-orbit components of a 'E state as proposed by Parker and Crosby.8 The polarization data can, however, be reconciled with the model of Kirchhoff et al., which assigns the emission to singlet and triplet CT states." However, the complex is believed to undergo a significant flattening distortion in the C T excited state, and in the reduced symmetry the z-polarized emissions have to be assigned to singlet and triplet terms associated with different electronic configurations. In the absence of more specific information about structure, the emission assignments have been couched in terms of D2 symmetry which is believed to be the highest possible symmetry for the C T states. However, it is possible that this assumption will be inadequate for explaining results obtained with other techniques. In fact, resonance Raman data have been interpreted to indicate that the electron localizes on a single N N ligand in the excited state,26in which case symmetry could be as low as C,. The uncertainties regarding the molecular symmetry in no way affect the conclusion that the thermally activated emission is associated with an excited state with singlet multiplicity.
Acknowledgment. This research was funded by N S F Grant CH-9024275. (25) For a related effect see: Reitz, G.A.; Demas, J. N.; DeGraff, B. A.; Stephens, E. M. J . Am. Chem. S a . 1988, 110, 5051-5059. (26) McGarvey, J. J.; Bell, S.E. J.; Gordon, K. C. fnorg. Chem. 1988.27, 4003-4006.
Conformational Changes on Electronic Excitation of the Mercury-Water van der Waals Complex Marie Christine Duval and Benoit Seep* Laboratoire de Photophysique Mol5culaire du CNRS, Bcitiment 21 3, and Institut de Physicochimie Mol5culaire de I'Universit5 Paris Sud, UniuersitC de Paris Sud, 91405 Orsay, France (Received: April 2, 1991; In Final Form: June 21, 1991)
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The van der Waals complex of mercur with water has been characterized in a supersonic expansion by optical absorption in a region close to the mercury 'PI YSOatomic transition. After excitation the complex dissociates into metastable mercury (3P0)and water, and this appears to be the only open channel. The rotational contours of the bands of a Hg-H20 state assigned to a parallel transition reveal a drastic reduction by 9 wavenumbers of the A rotational constant. This reduction has been explained as being due to a change in geometry from a floppy ground state to a near-C, excited complex.
The van der Waals binding of rare gas atoms to metals' has been quite extensively studied; however, there are much fewer examples for molecules attached to metals.*-' (1) (a) Smalley, R. E.; Auerbach, D. A.; Fitch, P. S.;Levy, D. H.; Wharton, L. J . Chem. Phys. 1977.66, 3778. (b) Tellinghuisen, J.; Ragone, A.; Kim, M.S.;Auerbach, D. J.; Smalley, R. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1979, 71. 1283. (c) Zanger, E.; Schmatloch, V.; Zimmermann, D. J . Chem. Phys. 1988, 88, 5396. (d) Funk, D. J.; Kvaran, A.; Breckenridge, W. H.J . Chem. Phys. 1989, 90,2915. (e) Kowalski, A.; Funk, D. J.; Breckenridge, W. H. Chem. Phys. Lctr. 1986,132, 263. (f) Bennett, R. R.; Mc Caffrey, J. G.;Breckenridge, W. H. J . Chem. Phys. 1990,92,2740. (g) Tsuchizawa, T.; Yamanouchi, K.; Tsuchiya, S . J . Chem. Phys. 1988,89, 4646. (h) Callender, C. L.; Mitchell, S.A.; Hackett, P. A. J . Chem. Phys. 1989. 90, 2535. (i) Callender. C. L.; Mitchell, S. A.; Hackett, P.A. J. Chem. Phys. 1989, 90, 5252. Q) Dedonder-Lardeux, C.; Jouvet, C.; Richard-Viard, M.;Solgadi, D. J. Chem. Phys. 1990, 91, 2828. (k) Cheng, P. Y.;Willey, K. F.; Duncan, M. A. Chem. Phys. Left. 1989, 163, 469. (I) Jouvet, C.; Lardeux-Dedonder, C.; Martenchard, S.;Solgadi, D. J . Chem. Phys., in press. (2) (a) Fuke, K.; Saito, T.; Kaya, K. J . Chem. Phys. 1984,81, 2951. (b) Fuke, K.; Saito, 7.;Nonose, S.;Kaya, K.J . Chem. Phys. 1987,86,4745. (c) Duval, M. C.; Jouvet, C.; Soep, 8. Chem. Phys. Lett. 1985, 119, 317. (d) Castelman, A. W., Jr.; Keese, R. G. Science 1988, 36, 241. (e) Lessen, D. E.; Asher, R. L.; Brucat, P. J. J . Chem. Phys. 1990, 93, 6102. (3) (a) Y:manouchi, K.; Isogai, S.;Tsuchiya, S.;Duval, M.C.; Jouvet, Soep, B. J . Chem. Phys. 1988,89, 2975. (b) Duval, C.; Benoist d Azy, 0.; M. C.; Soep, B. Chem. Phys. Lett. 1987, 141, 225.
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Mercury in its gound electronic state forms van der Waals complexes which are ~ e a k l ybound ~ * ~ owing to the 6s2 closed shell configuration of the metal atom, but conversely, the excited states of 6s6p open shell character are deeply bound or reactive."Js This change of the binding energy upon excitation can be drastic-from 100 cm-l in the ground state to 6000 cm-' in the excited state, discussed in a forthcoming paper6-and may be related to chemisorption at metal surfaces. The connection, amply discussed in theoretical results from the possible similarity of the hybrid spd character of metal surface electrons to the 6p character of excited mercury. As an extension of our previous studies on the mercury-ammonia complex, we present here results on the related mercurywater complex. In the Hg-NH3 complex4 the ammonia lone pair points toward the mercury atom and there is no noticeable change in conformation between ground and excited (63PI) states. (4) Jouvet, C.; Soep, B. Chem. Phys. Lefr. 1983, 96,426. Breckenridge, W. H.; Jouvet, C.; Soep, B. J . Chem. Phys. 1986,84, 1443. ( 5 ) Callear, A. B. Chem. Rev. 1987, 87, 335. (6) Breckenridge, W. H.; Duval, M. C.; Soep, B. To be published. (7) Saillard, J. Y.;Hoffmann, R. J . Am. Chem. Soc. 1984, 106, 2006. (8) Garcia-Prieto, J.; Ruiz, M. E.;Novaro, 0. J . Am. Chem. Soc. 1985, 107. 7512.
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