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Direct Spectroscopic Evidence for a Hitherto Elusive УZwitterionicФ Excited State.

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HIGHLIGHTS
Direct Spectroscopic Evidence for a Hitherto Elusive
“2 witterionic” Excited State
Lutz H. Gade*
Direct experimental support for a theoretical concept in science is sometimes obtained in an area for which it was not
originally proposed. The case we describe here is the simplest
example of chemical bonding, the interaction of two electrons in
weakly coupled orbitals on two atomic centers. As early as 1932
Mulliken analyzed the situation in the x-orbital domain of twisted ethylene[’] and a few years later the (related) electronic structure of the 0 domain in “stretched” dihydrogen (i.e. an H,
molecule in which the interatomic distance is increased well
beyond its equilibrium value) .[’I In both cases four electronic
states are specified (Fig. 1): two low-energy “diradical” states
b)
--f
zwitterionic
I -7760t
t[“I
0-0
ionic states do not have a net dipole moment in homosymmetric
molecules. The fact that one of the states would represent the
ground state in a real zwitterion is the reason for this potentially
confusing terminology. However, in a polar environment (solution) these states may be subject to “sudden polarization” and
display zwitterionic r e a ~ t i v i t y . ~ ~ ]
Whereas the diradical states have been comprehensively studied over the past decades and provide the foundation for the
description of the extensive organic chemistry of biradicals, the
zwitterionic states have proved to be far more elusive (though
invoked, for example, to explain the photoisomerization and
cyclization of 1,3-dienes).[41 Recent studies of the solvent and
substituent effects on the lifetime of the transient species generated by n K* excitation of alkenes have provided some indirect evidence for the existence of the postulated manifold of
zwitterionic excited
However, its direct detection has to
date not been achieved with these systems. The short lifetime
and thus the transitory nature of the weak coupling, which is
characteristic of the zwitterionic states during the stretching of
bonds preceding dissociation or the twisting of bonds in isomerizations, is the main reason for this situation.
D. G. Nocera and co-workers have now reported the direct
spectroscopic detection of the zwitterionic states in a weakly
coupled two-electron two-orbital system.f61As their object of
study they chose the dinuclear complex 2 with a quadruple
-
0-0
Fig. 1. The four electronic states of the o(1s)- and rr(2p) manifolds in H, and
ethylene. respectively. a) “Stretching” of H,. VB description of the four
states (atomic orbitals centerd on atoms A and B): ‘ Z l = ‘[l~(A)ls(B)]~
ax+ = 3[Is(A)ls(B)J. ‘Z: = ‘[ls(A)ls(A) - Is(B)ls(B)], ‘Z: = ‘[ls(A)ls(A) +
Is(B)ls(B)J. b) “Twisting” of C,HI. In weakly coupled systems (H, at R > 4u,;
twist angle T z 9 0 ) the four states may be clearly distinguished as either of diradical
or zwitterionic nature (from ref. [3aJ). E in Hartree, R in a,.
(singlet and triplet) with the electrons in separate orbitals, and
two higher energy “zwitterionic” singlet
The latter are
derived from the symmetric and antisymmetric linear combination of wave functions representing the pairing of the two electrons in the two orbitals. It should be noted that these zwitter[*] Dr. L. H. Gade
Institut f i r Anorganische Chemie der Universitit
Am Hubland, D-97074 Wiirzburg (Germany)
Telefax.: Int. code
+ (931)888-4605
Angetv. Cltem. I n r . Ed. Engl. 1995, 34,
No. 5
0 VCH
1
2
metal-metal bond, which was first synthesized and characterized by F. A. Cotton et al.[’l What is it that almost predestines
this particular compound for an investigation into the nature of
the zwitterionic states? First of all, systems with a quadruple
bond have a nontransitory weakly coupled set of orbitals virtually built into them: the 6 and 6* molecular orbitals derived
from the d,, metal orbitals. Their spatial arrangement is structurally reinforced by the G and x bond framework which maintains their weak coupling. Secondly, the relatively bulky phosphane ligands sterically lock the system in its ground state
conformation, and therefore the orientation of the d,, orbital is
Verlagsgesellschaft mbH, 0.69451 Weinheim. 1995
o57o-ax3319510505-0547 $ l0.00+ .25/0
547
HIGHLIGHTS
fixed. The choice of the PMe,-substituted complex thus avoids
complications arising from internal rotation about the metal metal axis in an excited state of the molecule, as observed for
instance for [Re,Cl,]'- (1) in solution. Finally, previous work
performed in the groups of H. B. Gray,[*' F. A. Cotton,['] and
others["%
has provided a reliable assignment of relevant
bands in the UV/Vis absorption and emission spectra of several
quadruply bonded compounds, which developed simultaneously with the theoretical understanding of the electronic structure[*] of these species.'"]
A qualitative energy level diagram showing the four-state 6
manifold of ME M complexes of D,, symmetry (Did in the case
of 2) is displayed in Figure2. The energy gap between the
25
20
hv(IR)
'w em
hv (IR)
5
shows instantaneous and intense emission. The second zwitterionic excited singlet state corresponds approximately to a (6*)'
configuration (neglecting configuration interaction with the
ground state 1'A,, (1 'Al)) and is thus not allowed according to
the selection rules for one-photon, electric-dipole transitions.
Nocera and co-workers located this state in 2 by two-photon
excitation fluorescence spectroscopy. Upon excitation into the
2'A, state the system undergoes transition to 'B,; the emission
from the latter state is then detected. That the primary excited
state is indeed populated by a two quantum transition was established by the dependence of the fluorescence intensity on the
square of the incident excitation intensity.["] The polarization
ratio (0 = Icirc/Zlin
< 1 ; circ, lin = circular and linear polarized
irradiation) of the excitation fluorescence spectrum furthermore
indicates that the excited state is indeed of 'A, symmetry,['*I as
required for the second zwitterionic state of the investigated
system. The small 21A1-'B2 energy gap of 4800 cm-' relative
to the 'B2-1'A, gap of 17100cm-' indicates the very ionic
nature of the doubly excited state 2'A, (the authors give a
preliminary estimate of 68 %). With both zwitterionic states of
the 6-6* manifold now unambiguously located, the four-state
structure of a weakly coupled two-electron two-orbital sytem
has been established for the first time.
What is the significance of this study beyond the direct verification of an important model for chemical bonding dating back
to the very beginning of quantum chemistry? For the inorganic
photochemist it sheds new light on the potential photochemical
reactivity of the metal-metal quadruple bond. The pairing of
two electrons on one metal center and two "holes" on an adjacent one predisposes this type of system for multielectron reactivity. Recent investigations by the same group give first indications of such a reactive pattern, for instance, the two-electron
photoreduction of CH,I by [W,(dppm),CI,] [Eq.(a)].I1']
0
Fig. 2. A qualitative energy level diagram for the 6-6* orbital manifold based on a
valence bond model (orbitals centered on the two metal atoms A and B). The states
are designated by their symmetry labels in D,, and D,, symmetries (the latter in
brackets). The two-photon excitation (by absorption of two near-infrared photons)
into the zwitterionic 2'A,, (2'A,) state is indicated, followed by internal conversion
to the highly emissive 'A,, ( l B 2 ) state. The red fluorescence (hv.,) from this state
was detected in the fluorescence experiment with two-photon excitation (adapted
from ref. [ 6 ] ) .
ground state 1 'A,, (1 'A,) and the triplet state 3A,, (3B,) in 2
has been extrapolated recently from magnetic measurements['
as well as from an elegant application of paramagnetic 31P
N M R spectroscopy to a series of related systems.['61 The lowfrequency band in the UV/Vis spectra of the quadruply bonded
dinuclear compounds corresponds to the excitation into the
dipole-allowed zwitterionic 'A,, ('B,) state. In "sterically
locked" complexes such as 2 relaxation into a twisted excited
singlet state (as in 1) is not feasible; the 'A,, ('B2) state therefore
[*I
I t should be mentioned in this context, however, that an "isolated 6-6* manifold" model is entirely inadequate for a quantitative theoretical description of
the 6-6' photochemistry, as has been recently discussed by Bursten and Clayton
[I31 (and, previously, by others [14]). It should therefore be taken for what it is
worth: a convenient approximation to the actual Chemical bonding in dinuclear
complexes with quadruple metal- metal bonds. The accurate calculations presented in ref. [121 include configuration interaction with states outside the 6-6*
manifold.
548
C
VCH Verlugsgestrilsihufi mhH, 0-69451 Wernherm, 199s
These recent developments have shown that the metal -metal
quadruple bond continues to fascinate the chemical community even three decades after it was discovered by F. A. Cotton et al.['O]
German version: Angew. Chem. 1995, 107, 595
Keywords: fluorescence spectroscopy . metal -metal multiple
bonds . photochemistry . zwitterionic states
[l] R. S. Mulliken, Phys. R ~ P1932.
.
41. 751.
[2] R. S. Mulliken, Plrys. Rev. 1936, 50, 1028.
[3] a) L. Salem, C. Rowland, Angeit'. Chem. 1972, H4. 8 6 ; Angew. Chem. I n r . Ed.
EngI. 1972, If. 92: b) W. G . Dauben. L. Salem, N. J. Turro, Acc. Chem Rex
1975. H. 41: C) L. Salem. Pure Appi. Chem 1973, 33, 317 d) V. BonaEiCKoutecky. J. Koutecky, J. Michl. Angriv. Chem. 1987. 99, 216; Angew. Chen?.
In/. Ed. Engl. 1987, 26. 170.
[4] W. G. Dauben, M . S. Kellogg, J. I. Seeman, N . D. Vietmeyer. P. H. Wendschuh. P w c Appl. Chern 1973, 33. 197 and references therein. See also. N. J.
Turro. Moclrnr Mole1 ulur Photochrmistr~,2nd ed.. University Science Books.
Hill Valley. CA. USA. 1991.
0S70-0833/YSl0SOS-OSHJ 1 0 . O O i ,2510
A e g e n . Chem hi.Ed. Engi. 1995,34, No. 5
HIGHLIGHTS
[S] C . L. Schilling. E. F. Hilinski, J. Am. Chem. So?. 1988, 110, 2296; J. Morais, J.
Ma. M. B Zimmt, J. Phys. Chem. 1991, 95, 3885, J. Ma. M. B. Zimmt. J. Am.
Chetn. So(. 1992. 114, 9723.
(61 D. S. Engebrelson, J. M. Zaleski, G. E. Leroi, D. G . Nocera. Science 1994.265,
759
[7] F. A. Cotton. M. W. Extine, T. R. Felthouse, B. W. S. Kolthammer, D. G. Lay.
J. A m C/lrnr. So(.. 1981. /03, 4040.
[XI W. C . Trogler. H. B. Gray. Arc. Chem. Rrs. 1978, I f , 233; See for instance:
C . D. Cowinan. H. B. Gray. J. Am. Chem. Soc. 1973, 95, 8177: M. D. Hopkins,
H. B. Gray. ihid. 1984, 106, 2468; M. D. Hopkins. H. B. Gray, V. M.
Miskowski, Pdrherlron 1987, 4, 705. V. M. Miskowski, H. B. Gray, M. D.
Hopkins, Inorg. Chem. 1992, 31. 2085.
191 F A . Cotton. P. E. Fanwick, L. D. Gage, B. Kalbacher. D. S. Martin, J. Am.
Ch~wi.Suc. 1977. YY, 5042; P. E. Fanwick. D. S.Martin, F. A. Cotton. T. R.
Webh. Inorg. Cheni. 1977. 16, 2103.
[lo] J. R Winkler. D. G. Nocera. T. L. Netzel, J Am. Chrm. Sol. 1986, 108, 4451;
H. W. Huang. D. S. Martin, Inorg. Chem. 1985, 24. 96.
[I 11 For a completc account. see: F. A. Cotton. R. A. Walton, Multiple Bundy
Bi~/iiwrrW i v d Acorns. 2nd ed.. Clarendon Press, Oxford 1993. Chapter 10.
[12] P. J. Hay, J. A m . Chem. Soc. 1982.104.7007;F. A. Cotton. X. Feng. ihrd. 1993.
f15, 1074.
[13] B. E. Bursten, T. W. Clayton. Jr., J. Clirsler Science 1994. 5 , 157.
1141 L. Noodleman. J. G. Norman.Jr..J. Chem. P h w 1979,70.4903;W. C. Trogler,
J. Chem. Ed. 1980. 57, 424; B. E. Bursten. D. L. Clark, Pulxhedron 1987. 6,
695.
[15] M. D. Hopkins, T. C . Zietlow. V. M. Miskowski. H. B. Gray. J A m . Chem.
SOC.1985, 107, 510.
[16] F. A. Cotton, J. L. Eglin, B. Hong, C. A. James. J. Am. C%rm.Soc. 1992. 114,
4915; Inorg. Chem. 1993, 32, 2104.
[I71 S. H. Lin, Y. Fujimura, H. J. Neusser, E. W Schlag, Multiphoron Spertrmropy
of Molecules, Academic Press, Orlando. FL. USA, 1984. Chapter 4.
[18] P. R. Monson, W. M . McClain. J. Chem. Phys. 1970, 53. 29; W, M. McClain,
hid. 1971, 55, 2789; M. A. Nasciemento. Chrm. Phy.7. 1983. 74. 51.
[I91 C. M. Partigianoni, D. G. Nocera, Inorg. Chem. 1990,2Y.2033. See also: D. G.
Nocera, J. Clusrer S ~ I1994,
.
5, 185; C. M . Partigianoni. 1:J. Chang, D. G.
Nocera, Courd. Chem. Rev. 1990, 97, 105.
[20] F. A. Cotton, N. F. Curtis, C. B. Harris, B. F. G. Johnson. S. J. Lippard. J. T.
Mague, W. R. Robmson, J. S. Wood, Scimce 1964. 145, 1305.
Conical Intersections and the Mechanism of Singlet Photoreactions
Martin Klessinger*
The reaction pathway of a nonadiabatic photochemical reaction begins on the potential energy surface of the excited state
(S,) in the Franck-Condon region or at a spectroscopic minimum of the reactant and ends at the product minimum on the
ground-state surface (S,,). The two stages on the S, and So
surface are separated by a funnel[’] at which radiationless decay
from the excited state to
the ground state becomes possible (Fig. 1 ) .
For a long time it was
believed that the funnels
mostly correspond to
surface crossings (or
touchings) that are at
least weakly avoided,
but recent work has
shown that for a great
Y
number of inorganicI2l
Fig. 1 . Schematic representation of the poand organicc3] systems
tential energy hurfiiceh of the ground state
the crossings are actual(S,,)and the excited state (S,) of a nonadialy unavoided and correbatic photoreaction of reactant R. Depending on the w a y the classical trajectories enter
the coiiical intersection region, different
ground-state vai~eys.which lead to products
P, and PL. C:m be reached.
’pond to
conical
intersections.[41 Nowadays it appears that conical intersections are a
common feature in most nonadiabatic singlet photoreactions.
A conical intersection is defined as follows: Two states, even
if they have the same symmetry, intersect along an (F-2)-dimensional hyperline as the energy is plotted against the Fnuclear coordinates ( F = 3N-6).[’’ While for any point of the (F-2)[*I
Prof. Dr. M . Klessinger
Organisch-(‘liemisches lnstitut der Universitiit
Corrensstrasse 40. D-48149 Munster (Germany)
Telefax: Int. code + (251)83-9772
Angiw . Chrm In! M. End. 1995. 34, No. 5
dimensional intersection space the energies of the two states are
the same, the degeneracy is lifted along the two remaining linearly independent coordinates x1 and x 2 .Thus, when the energy
of the two states is plotted against these two geometrical variables (they are combinations of bond distances, angles, etc.), the
corresponding energy surfaces have the shape of a double cone,
as shown in Figure 1.
The gradient difference vector x l is defined by Equation (a).
In this direction the difference in the slopes of the upper and
the lower surface is largest. The nonadiabatic coupling vector xz
is given by Equation (b). This is the direction of nuclear dis-
displacement that mixes the states two adiabatic wave functions
at the cone point the best. If Y o and Y i are of different symmetries at the apex of the cone, x2 is the symmetry-lowering
coordinate that permits them to mix. The vectors x1 and x2 are
often nearly orthogonal. The gradient of neither of the touching
surfaces is zero at the tip of the cones, as it would be for a true
stationary point. Rather, it is the projection of the gradient onto
the (F-2)-dimensional subspace orthogonal to x, and x2 that
goes to zero when the geometry of the conical intersection is
optimized. Therefore, in the F-dimensional space, gradient-based
search routines will not recognize the bottom of the funnel as a
minimum, although it represents the lowest energy point on that
surface (unless the funnel is strongly tilted).
From a mechanistic point of view a conical intersection funnel plays a similar role in a photochemical reaction as the transition state in a thermal
Both describe the geometry
of the “reactive conformation” of the system. For a thermal
reaction, the transition state corresponds to the point of the
c) VCH Verlug.~jiesi~l/srhu/r
mhH, 0-69451 Wmnheim, 1995
0570-C)833i95,’0505-054Y$ 11).00+ 2 5 4
549
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