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J. Phys. Chem. 1980, 84. 2688-2690
thank R. P. Van Duyne for helpful discussions regarding electrode properties and dynamics. References and Notes
photochemical perturbation of the electrode double layer. Double layers are a general consequence of the meeting of two phases at a boundary. The simple existence of a boundary for an electrolyte implies an anisotropy on the forces operating on the particles in the interphase region.1° There are actually several double layers or electrified interfaces in our experiment: the solution-quartz interface; the quartz-semiconductor interface; and the semiconductor-silver bus bar interface. At the onset of the experiment, the double-layer potentials all cancel. We speculate the laser pulse alters the chemical composition of the electrolyte at the boundary of the cell. Now all the electrified interfaces are still symmetric except the solution interphasial region. Immediately after the light pulse the two electrodes (front and back) find themselves in different chemical environments as a result of the differential absorption of light at the two electrodes.ll Since the rest of the system remains symmetric, the potential difference A4 we observe reflects the unrelaxed difference in chemical environment between the two electrodes. Although this study establishes for the first time a clear relation between photoreduction quantum yield, laser intensity, and observed photopotential, the detailed mechanism how a photochemical perturbation alters the double layer remains unknown, and without such a model we are presently unable to offer an interpretation for the magnitude, sign, and temporal decay of this phenomenon.
D. D. MacDonald, “Translent Techniques In Electrochemistry", Plenum, New York, 1977. (a) H. W. Russel, Phys. Rev., 32,667 (1928); (b) S. Paszyc and R. G. W. Norrish, Rozn. Chem., 37, 305 (1963). A. Bergman, C. R. Dicksan, S. D. Udofsky, and R. N. Zare, J. Chem. Phys., 65, 1188 (1978). S. P. Perone, J. H. Richardson, B. S. Shepard, J. Rosenthal, J. E. Hatter, and S. M. George, ACS Symp. Ser., No. 85 (1978). V. Balzani and V. Carasslti, “Photochemistry of Coordination Compounds”, Academic Press, New York, 1970. A. W. Adamson, W. L. Watts, E. Zinayo, D. W. Watts, P. P. Fleischauer, and R. D. Lindholm, Chem. Rev., 68, 541 (1908). G. Zimmerman, J. Chem. Phys., 23, 825 (1955). R. D. W. Kemmkt and R. D. Peacock, “The Chemistry of Manganese Technology and Rhenium”, Vd. 22, Pergamn Press, New Yak, 1973. We used a fink difference model to simulate the evolution of charge density p = Ct - C- and total ion density u = Ct f C-in the cell. We approximated the tlme derlvative with the forward difference @,v(x,t) = [v(x,t+h) - v(x,t)]/hand the second-order space derivative with the centered difference @P,,v(x,t) = [ v(x+i,t) - 2v(x,t) v(x-i,t)]/l*. The electroneutialit); of the solution i O c p dxchecked the stability of the calculatlon. We solved for the electric potentlal by numerical lntegratlon of LaplacB’s equatlon d24/dx2 = P. The equations for diffusion In the absence of recomblnation are analytically tractable. No reasonable photochemlcal perturbatlon produces a concentration gradient large enough to separate charge due to the unequal diffusion constants of the positlve and negative specles and then relax that charge separation on the time scale of the experlment. The superpostlon of recombination and diffusion yields an analytically intractable model for the phenomenon. We solved these equations by numerical simulation and found no translent potential. This has a simple explanation. The time scale for recombination is -5 orders of magnltude smaller than that for diffusion. Hence, the system recombinatively relaxes the concentration gradient produced by photochemlcal perturbation well before diffuslon can create a charge separation. On the time scale of recombination, there is no bulk dlfjusion. J. O’M. Bockrls and A. K. N. Reddy, “Modern Electrochemlstty”, Vol. 2, Plenum Press, New York, 1970, pp 630-632. In other experiments we used a cell geometry in which the back electrode is not exposed to light. We observed comparable or larger photopotentials.
+
Acknowledgment. This work was supported by the National Science Foundation under NSF CHE 78-10019. The use of the YAG laser was made possible through the support of the Stanford Center for Material Research. W.F.C. thanks the Board of Regents of the University of New Mexico for sabbatical leave. M.G.P. thanks NRCC for providing both grant money and guest privileges to pursue computer studies of the phenomena. The authors
Magnetic Circular Dichroism and Cryogenic Optical Study of Photochemically Important, Pnictide-Containing Transition Metal Carbonyls A. F. Schrelner,” S. Amer,t W. M. Duncan,’ and R. M. Dahlgrens Lkpartments of Chemistry, North Carolina State University, Raleigh, North Carolina 27650, and University of Toronto, Toronto, Canada; Texas Instruments, Dallas, Texas 75221; and the Proctor and Gamble Company, Cincinnati, Ohio 45247 (Received: June 24, 1980; I n Final Form: August 20, 1980)
Magnetic circular dichroism (MCD) analysis of pnictide-containing complexes, [M(CO),E], where M = Cr or W and E = PPh3, AsPh3,or SbPh3,gives direct evidence through the presence of a positive A term that the lowest energy, spin-allowedoptical band is lE lA1, of excited configuration [6e(-dxz, d,z)]3[4al(-dz2)]1. The lower ratio of IA/DI for [Cr(CO)SE](E = PPh3, AsPh3,SbPh3) compared to [Cr(CO)~,(amine)] indicates the presence of less orbital angular momentum in the excited state of the former.
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For reasons of their diverse photochemical and thermal reaction pathways, photocatalytic activities, photophysics, *Author to whom correspondence should be addressed at the North Carolina State University. ‘University of Toronto. t Texas Instruments. *North Carolina State University. 0022-3654/80/2084-2688$01.00/0
and frequently encountered high quantum yields of their photoreactions, the 3d 4d, and 5d transition metal carbonyls are a most interesting and important class of molecu1es.l Much of such research either benefits from or demands detailed howledge of the involved excited and excited MO, so that we are prompted to present such direct-assignment information from new MCD experimental data and its analysis, Our present results are es0 1980 American Chemical Society
The Journal of Physlcal Chem;stty, Vol. 84, No. 21, 1980 2889
Letters
h
(nd
A
Figure 1. MCD and optical spectra of [Cr(CO),AsPh,]: (a) MCD and optical spectra in CH2Ci, at 300 K (b) and (c) at 300 and 77 K in EPA solvent. ([e], is the molar ellipticity per gauss, and E is in units of L mot-' cm- .)
(nrrS
Flgure 3. MCD and optical spectra of [Cr(CO)5PPh3]: for a-c, see caption of Figure 1.
-
*o.m.
X%PRs
(nm) Figure 2. MCD and optical spectra of [Cr(CO),SbPh,]: caption of Figure 1.
(nm) for a-c, see
pecially pertinent for assigning the state symmetry and MO fate for t,he photochemically much used, and spectroscopically red-most, spin-allowed photoexcitation band I1 a t -350 nm (2.86 pm-l) of the molecules [M(CO)&I, where E is the Lewis base with donor atom P, As, or Sb. The MCD approach employed here consists of (i) observing s-shape MCD dispersions, or A terms,2 whose phenomenological origin is the differential absorption of left- and right-circularly polarized radiation for excitation to components of degenerate, orbital angular momentumcontaining, excited states split by the longitudinal Zeeman effect, and (ii) theoretically analyzing the sign of the MCD A term which is determined by the MO fate and MO origin of the photoexcitation. In this way we first differentiate an excitation to a degenerate, orbital momentum containing tetragonal excited state, lE, from transitions to nondegenerate tetragonal states, lA1, lA2,lB1, or IB2,and second we determine the very important excited molecular orbital fate of the electron. The MCD analysis will follow the presentation of experimental data. We first discuss the data of the arsine, [Cr(C0)&Ph3], because it is clear from Figure 1that the photochemically important band I1 at -360 nm (2.77 pm-l, optical dispersion a2)has a positive MCD A term fully exposed. At cryogenic temperature (77 K) the optical band (dispersion c, Figure 1)is clearly resolved; while its maximum is expectedly blue shifted at low temperature, no fine-structure emerges (Figure 1). The presence of the A term identifies band I1 as having excited-state symmetry lE. Figure 2 shows our room temperature optical and MCD spectra and cryogenic optical data for the stibine, [Cr(CO)6SbPh3]. Here band I1 (-355 nm or 2.82 pm-l) is optically less clearly isolated, but both lobes of a positive MCD A term are still visible. However, for the phosphine, [Cr(C0)&'h3], only the negative lobe of the A term is visible for optical band I1 (-355 nm, dispersion a2, Figure 3); the positive lobe of the A term is cancelled by the much larger, adjacent blue band at 330 nrn (300 pm-l) of negative ellipticity ([d]~(max) -0.27, where [e], is the molecular ellipticity per gauss). Finally, Figure 4 shows the MCD and optical
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Figure 4. MCD and optical spectra of [W(CO),PPh,]: caption of Figure 1.
for a-c, see
data for a tungsten analogue, [W(CO),PPh,]; the large negative MCD lobe at -355 nm is assigned to contain part of the A term which, similar to the 3d analogue, [Cr(CO)$Ph3], has its positive lobe cancelled by the adjacent high-intensity, negative MCD band at 300 nm. The new, low-intensity band I at -375 nm is present in the W molecule (Figure 4) but absent in the analogous Cr molecule (Figure 3), for which reason it is assigned to the metal-localized, spin-orbit coupling allowed transition, 3T1(2e12ti) lA1(2t:). To conclude, molecule [M(CO)$] has a MCD A term of positive sign for ita photochemically important band I1 near 350 nm (2.86 pm-I). The MCD band analysis is as follows. The first preliminary point of the analysis is that band I1 is considered to be a -d--d, metal-localized band (of configuration change 2t&?e: 2t$ in Oh)because (i) band I1 of [M(CO)$] is of lower energy than the lowest energy A$ = 0 d-d band3 of the Oh parent, [M(C0),J3, (ii) the magnitudes of the ligand-induced electric field perturbations are Dq(E) < Dq(CO), and (iii) intensities ( 1000 L mol-l cm-') of optical band I1 we not unreasonable in such noncentric, covalent molecules, [M(CO),E]. Second, in regard to the potentially involved high-energy occupied MOs, it is known from photoelectron spectroscopy4 for several molecules like [M(CO),E] that the HOMO of molecules studied here is 6e4(-dZz,dyz,M;2t2J > 2bi(-d,,,M;2tzp). The third preliminary point in regard to the MO fate for band I1 is that the present MCD analysis will be used to decide between MOs 4al(-dd,aM) and 3bl( -d,a-,,zM) as derived from 2e M of Oh. The differentiation between choices of the M 6 fate requires the computational evaluation of the ratio, AID, of A-(MCD) and Dj,,(optical), the electric dipole strength. There are two MCD excitation intensity (e) mechanism~ operative ~ ~ ~ for these closed-shell molecules, i.e., those due to quantized molecular A and B parameters. Here, it suffices to evaluate A parameters in order to interpret excitation, lj) la) (1) Aj, = 1/2C(jJF.)j).Im(alfi)j)X (jlfila) +
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The Journal of Physical ChemMty, Vol. 84, No. 21, 1980
2690
Letters
where riz and p are the real electric dipole (fix+ fiy)and magnetic dipole @), operator la) and are in the complex basis ('Al and 'E, respectively), and spans the componenta of I'E). Converting to the basis-free reduced matrix element form,6 excitation to 'E becomes
I
u)
I
AE+A~= -(2)-3/2i(EllPllE)I(Aill~llE)12 (2) Only the first state integral, (p),will need to be numerically evaulated, viz., for the same transition, 'E 'A1,the electric dipole strength is 1 (3) Dj-a = --Gl(almlj)12
-
da J which equals I( 1Alllml11E12in basis-free form, so that the experimentally and theoretically accessible AID ratio for 'E 'Al is
-
A/D = --(2y3l2i('EJJpll'E) (4) We evaluated6 the letter magnetic dipole integral, (lEllpll'E), ~s_~~/~/?('E,IZ,I'E ), since 1.1, = /?I,. In order to evaluate ( 1,) ,we now need to consider the molecular orbital alternatives within the determinants of 'E, and 'Ey.. The 0ne;electron MO equivalent' of excited-state integral, ('EllllllE), in eq 4 is the following (lEaIIllIIEa) = ( e l h ) (5) ( 'Eblllll'Eb) =
-(ellille) (6) where E, and Eb are, respectively, E[e3(-dlE,d M)a:(-d,zM)] and ~[e3(-d,,,dy,M)b~(-d,%aM)]. UY2e elect to evaluate (ellllle) as -21/2(e,lZ,le ), where MOs e, and ey are approximateda as d and d,,, is metal based, so that (e,llJe,) equals i, a n f e q 5 and 6 equal t i and -i, respectively. Substitutions into eq 4 yield
fz
(A/D), = +f/,P (A/D)b = -'/z/3
(-dzzM) * (WdxzdyzM) (-dx2-yZM)
+
(7)
( ~ d z z , d y z M (8) )
We can now draw our major conclusions that (i) the presence of the MCD A term indicates that excitation for band I1 is to a degenerate excited state, either 'E, or 'Eb, and (ii) the agreement of the positive sign of the experimentally observed MCD A term with the computed positive sign of (AID), directly establishes the excited state as being 'E, and that the orbital fate is 4al(-d,zM), ruling out 3bl(-d,Z-,zM). These are direct assignments for the photochemically important band of a class of fascinating molecules, and they substantiate those inferred indirectly from other data. It is also interesting to point out that AID ratios of Cr amines: e.g., [Cr(C0)5(C6HloNH)](Figure 5 ) , are approximately twice the magnitude of AID for the analogous P, As, and Sb pnictides, [Cr(C0)5E] (Figures 1-3). From this and eq 4 it is concluded that the pnictide-containing molecules have less orbital angular momentum, ('EIJLII'E), in their excited states. This momentum reduction may
Figure 5. MCD and optical spectra of [Cr(CO),(C5H,,NH)]: see caption of Figure 1.
for a-c,
be a direct result of either one or both these cause^:^ (i) greater excited state covalency in Cr-P, -As, -Sb bonds than in Cr-N, and (ii) significant excited state d"d'(Cr)-d"d'(E) bonding between pyramidally symmetric (C3J MOs of PPh3,AsPh,, and SbPh3to tetragonal (C4J ?r MOs of fragment Cr(C0I5, an interaction expectedly absent in the Cr-N bond. Each effect in [Cr(CO)J3]would reduce the d character in MO type e (eq 5 and 6), subsequently reducing IA/DI. Finally, there is present the less important optical band at -300 nm for each molecule. It is assigned to MTLCT, of 'Tlu lAlg Oh parentage (ti,2tig or -II*(CO) -3d(Cr)), because (i) the M(CO)6parent molecules have such a band at -290 nm, (ii) the large optical intensity (emm > 5000 L mol-' cm-l), and (iii) the band has large negative B term activity analogous to that foundlo in the parents, M(CO)@
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-
Acknowledgment. We thank D. J. Darensbourg and M. Y. Darensbourg for the P-, As-,and Sb-containing complexes. References and Notes (1) D. J. Darensbourgand M. A. Murphy, J. Am. Chem. Soc., 100,463 (1978);R. M. Dahlgren and J. I. Zink, Inorg. Chem., 16,3154(1977); 0. Boxhoorn, D. J. StuRtens, and A. Oskam, Inorg. Chim. Acta, 33, 215 (1979),and references therein; T. M. Hugh, A. J. Rest, and J. R. Sodeau, J. Chem. Soc., Dalton, 184 (1979). (2) D. A. Bucklngham and P. J. Stephens, Annu. Rev. Phy5. Chem., 17,399 (ISM),and references therein. (3) N. Beach and H. B. Qray, J . Am. Chem. Soc., 90, 5713 (1968). (4) H. Daamen and A. Oskam, Inorg. CMm. Acta, 26, 81 (1979);8. R. Hlggins, D. R. Lloyd, J. A. Connor, and J. H. Hiiller, J. Chem. W., Faraday Trans. 2, 70, 1418 (1974). (5) A. F. Schrelner,S. Amer, W. M. Duncan, R. M. Dahlgren, and J. Zlnk, Presentedat the 178th National Meeting of the American Chemical Society, Washington, D.C., Sept 1979, Abstract 219, Inorganic Chemlstry. (6) J. S. Griffith, "The Theory of Transition Metal Ions", Cambridge Universlty Press, London, 1961. (7) R. M. E. Vllck and P. J. Zandstra, Chem. Phy5. Lett.,31,487 (1975). (8) R. S.Evans, A. F. Schreiner, P. J. Hauser, and T. C. Caves, Inorg. Chem., 14, 163 (1975). (9) Excited state Jahn-Teller effects are not considered here. (10) Unpublished observatlons of thls laboratory (NCSU); manuscript submitted.
