Photochemical Pathways John F. Endicott Wayne State University, Detroit, MI 48202
Much of the lure of transition metal photochemistry has derived from the idea that excitations of visible absorption hands might be used to generate intermediate species whose unusual electronic configurations would have some special catalytic significance. There are, indeed, many unusual chemical reactions which are promoted by light absorption in transition metal systems. Several other contributions to this symposium feature detailed discussions of the various types of photoreactions found among transition metal systems. This paper will only briefly survey the simple transition metal photoreactions observed in homogeneous systems; the major purpose of this paper is to outline some of the general principles which are likely to govern various photoreaction pathways. These principles will be illustrated with examples chosen from three classes of simple photoreactions. Emphasis will he placed on reaction pathways which are unique to excited states, and have no direct ground state parallels. Most transition metal excited states can he conveniently categorized as "ligand field" or "charge transfer" electronic states depending on whether the ground state electronic orhit,als -~~~~ have oredominantlv metal d-orbital character (i.e., LF-states) or differ significantly in the amount of metal and lieand contribution (CT-states) in the electronic wave funct i k The net chemistry observed is a function of the excited state electronic configuration and the possibilities are (the asterisk indicates the electronically excited species) as follows. ~
originally populated by light absorption: the dominant photoreaction need not proceed from a state with the electronic confieuration ~ e n e r a t e dby the initial light absorption. The ch&cterization of these various states and the determination of the factors which govern the probability of the excited system populating states of different electronic configurations in the course of thermal equilibration are the basic problems of nhotoohvsics. The starting- noint for a discussion of these processes is the Born-Oppenheimer approximation that electronic motion is rapid compared to inter-nuclear motion. This permits us to treat each different electronic configuration as a separate, more or less paraholic, potential energy surface. The vihrational motions of nuclei can then he represented by vihrational levels within each surface. In such a model. each electronic surface can he regarded as a sort of howl, and the escape from a eiven surface can he attributed to classical kinetic pathways (i.e., involving activation energies, transition states, etc., as in the strong coupling limit discussed below) or by quantum mechanical pathways with no classical analogs (i.e., involving electronic and nuclear "tunneling" as in the weak coupling limit helow). Those pathways which involve net chemical changes tend to require relatively strong reactant-product surface coupling (especially when hond breaking occurs). In view of this, there is a strong tendency, especially for reactions which occur from vibrationally equilibrated excited states, to discuss these chemical reactions in classical -
-
L
~
-
~
~d ~
~
.
Unimolecular processes originating in LF-states
-
1) Substitutional processes (for S = solvent species,L = ligand),
*ML6+ S
ML5S + L
2) Isomerization processes (AA = bidentate or two monodentate
hgands)
-
'(cis or trans, d or 1); M(AA12XY
3) Deactivation processes
(trans or as; d
or l)M(AA)zXY
-
*ML6 MLB+ heat 4)
Radiative processes *MLs
-
ML6 + hu
Unimolecular processes originating from C-T states 1) Oxidation-reduction processes (.L = free radical)
21-5) Substitutional, isomerization, deactivation, and radiative processes as in 1)-4) above.
These processes are rarelv com~letelvindependent and a ~ r a n c k h m d o nexcited states) from ele&onic excited states whose excess vihrational energy has heen transferred to the sdvent medium 1vil~r311u1ii1ll;r~1lilihratedexcited s t ; ~ t t + ~ . 011cmust a.w take account t iimroctiOnabctwrn stiltes with diiierent ex,lrt.d cltrtr that study of thesr rearticms will rive cunaidrrablr insiaht - lntu I he elertn~nicselection ruleh for reaction pathways.
--
Acknowledgments
The contributions of several coworkers have stimulated many of the ideas contained in this and are noted among the references. The National Science Foundation, The National Institutes of Health and the Department of Energy have provided some of the financial support for the various contributions from our laboratory. Literature Cited (11 Turro, N. J., "Modern Moiecular Photochernistiy..' Benjamin-Curnrnings. Meno Park, CA, 1978. (21 Hullebono, B. R.. Lanplord. C. H., and Serpnne, N.. Coord. Chem. Reos.. 39, 181 11981). 191 Sutin,N., and Cmutr, C., J. CHEM EDUC.,60,809 (19831. (41 Scando1a.F.. andBalrani.V.. J. CHEM. FDUC.. 60, 814 (19831. 161 Kirk.A.D..Caord. Chrm Riu,39,225(198ll. (6) Kirk. A. D., J. CHEM. EouC.,60,843 11983). (71 Kirk.A. D. and Porter. G. B.. J. P h y s Chem.,84,887 (1980). (8) Walterr. R.T. and Adarns0n.A. W.,Aclo Chem. Srond. Part A,33,53 (19791. (9) la) Flint, C. D. and Greenough,P.. Trans. Forodoy Suc.. 69,897 11971);(hi Flint. C. D. and Matthews. A. P.. T r a m Foroday Soc, 70,419 (1973): Ic) Flint. C. D. and Matthew8.A. P.. Trans. FoiodoySoc., 71.1807 (1974). (101 Endieoft. J. F. Brubaker, G. R., Lessard, R., and Tamilsmsn, R., unpublished work (11) (a) Zink, J. I.. J. Amer. Chem. Soi., 94,8039 (19721; (bl Zink. J. 1,lnoig. Chrm., 12. 1957 (19731: lc) Zink. J. I., J . Amei. Chem Soe.. 96,4464 11974). (12) Vanquickenborne.L. G. and Cuelernans, A.. J . Amai Chrm Soc.99.2208 (19771.
.
(13) wi1son.R.B. and So1omon.E. 1,Inorp.Chem., 17,1729 (1977). (14) Endieoft. J. F., Rarnssarni,T.. Tarnilaman. R.. and Brubaker, G. R.. in "Structuie ,~ . in i I ~D ~ t I chemistry..' ~i ~ ~~ ~ ~C~ o d ~ g s o nD. . (liditor) Academic Press, F U D C ~ ~ O "~ Lac. mew r "I&. m press. (15) P. K.. Cnppen, WS.. and Kane-Maguire, N. A. P., Inoig Chem., 22, 696 (1983). (16) A11mp.S. R . . C O X , A . . K ~ ~ J..andReed, ~.T. W. J . J . Chem S o c I h r o d w Trans. I , 76.162 (1980). (17) K,,, pi^^,,, in~eoctio.~ineiic., lo. 301 (19801. (18) Forster, L. S., Rund. J, V.,Castelli, F..Adams, P., J . Phys. Chpm ,86,2395 119821. (19) Englrnan, R. and Jortner, J.,Mol. Phyr , 16,145 (19701. (20) Raso1o.F. and Pearson, R. G.."Mschanisrns of lnorganicResciions..iJohn W h y and Sons, Inc, New York. 1968. (21) Strek, W.,andBallhausen,C. 3.. Mof. Phyi.,36,1321 (19781. 1221 P1ib"~h.R. A . poon. C. K..Br"ce.C. M.,andAdamson,A. W.. J. Amrr Chrm S " i , 96, 3027 (1fi91. (231 (a) Langford. C. H.. and Vuik, C. P. J., J. Amor. Chem. Soc.. 98, 5409 (1976): (hi Lsngford, C. H..and Mdkhssiah, A. Y. S., J Chem. Soi. Chem Cammun, 1210 11982). (24) Wilson.R.B..snd Solornon.E.1.. J. Amor. Chsm. Sac.. 102.4085 (1980). (25) Endicott. J. F., sndFerrsudi, G. J.. J. Phys Chom., 80,950 (1976). (26) la) Mok,C. Y.. andEndicott, J.F., JAmei. Cham. Sor., 160,123 (1978):(b)Endicott. J.F., Wong. C.~L..Ciskowski,J. M.,and Balak~ishnsn,K.P.,J Amer Chem.Sac.. lOZ.21W (1980): lc! Endicotf, J.F.,and Cmkowski, J. M.. unpublished work. (27) Flint. C. D., Greonough,P.,Matthews, A. P . , J Chrm. Suc Doilan.368 11972). (28) Tarnilarasan, R., and Endicotf. J. F.. unpublished work. lb)Chm,S.N..and (291 la) Chen.S. N..sndPorter,G.B.,Cham.Phys.Lttt..6.41(19701; Porter. G. B . , d Amer Chom. Sac., 92,3196 (1970). (30) Sriram, R., Ho1frnsn.M. 2 , Jsmiesan,M. A.,and Serpone.N.. J. Amer Chem. S a c . 102,1754 119801. (31) Jsrnieson, M. A , Serpone, N., and Hoffman, M. Z., Courd. Chrm R e u s , 39, 121 (19811. (32) Dexter,D. L.. J. Chom Phys.. 21,836 (19531.
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Volume 60 Number 10
October 1983
829
