J . Phys. Chem. 1988, 92, 2219-2223
that other medium effects would result in dispersion of the sharp phonon peaks, resulting in a more continuous line shape (e.g., each phonon line could certainly have a phonon sideband). Still, it is not at all clear that this would be sufficient to completely eliminate the sharp behavior predicted from a single-mode D H O model. The present model suffers from none of the above objections. The direct coupling to the intermolecular mode that is obtained is an intermediate value, which yields an anomalously large hole-burning line width that does not eliminate the asymmetry from the long-wavelength band. The resonant coupling does introduce chaotic spacing of levels, so that the resultant hole is broad and featureless. Finally, the electronic interactions are treated correctly, via a model which reproduces other spectroscopic data.' These arguments are by no means definitive; the D H O model could probably be modified to yield better agreement with experiment than the crude version in ref 6. Alternatively, some combinations of the two picture may be correct; e.g., the large spectral widths required to "dress" the stick spectra in the DHO model could arise from interactions with a C T state. However, the proponents of D H O model do need to show that their model can in fact be made consistent with all available experimental data. We next turn to the question of further experimental tests of hole-burning models. Recent Stark effect measurements*' support the contention that a C T state is mixed into the Qy SP manifold. A simulation of the Stark line shape with our vibronic coupling model will be undertaken in the near future. Note, however, that the mere presence of C T character in the excited state does not guarantee that the chaotic restructuring of the absorption band proposed here actually takes place. A key issue which has emerged from various theoretical perspectives is the characterization of the (hypothesized) strongly (27) Lockhart, D. J.; Boxer, S. G. Biochemistry 1987, 26, 664. (28) Plato, M., personal communication.
2219
coupled intermolecular mode (P mode) of the SP dimer. The most direct way to address this experimentally would be a resonance Raman (RR) experiment in which excitation was made directly into the SP Qy band and the low-frequency region of the R R spectrum was obtained. In the most favorable circumstances, one could extract the Franck-Condon (FC) factor of the P mode from analyzing the RR intensity (relative to, e.g., high-frequency peaks); issues of vibronic borrowing versus direct FC displacement could be studied by analyzing R R excitation profiles, which have a very different shape for A (direct FC) and B term (vibronic borrowing) scattering mechanisms. Finally, we give a brief discussion concerning the relationship of the hole-burning analysis to primary charge separation dynamics. If the interacting C T state could be identical as P'B-, the model developed here would be in complete accord with that of ref I , which proposed that the state P+B- was quasi-resonantly coupled to the SP Qy manifold. However, this model also predicts that an initial transient bleaching of B should occur instantaneously. Such a bleaching is apparently not observed in the recent femtosecond experiments of Breton, Martin, and co-workers.lOJ1 If, on the other hand, the CT state is closer in form to PL+PM-, no conflict with the dynamical results is produced. One could argue that admixture of this state would facilitate overlap with the P'B- (or P+H-) configuration, thus accounting in part for the speed and asymmetry of electron transfer. These ideas are not new, and we have not provided any sort of proof of their validity; rather, our present results are consistent with such a picture.
Acknowledgment. This work was supported by a grant from the National Science Foundation. R.A.F. is an Alfred P. Sloan Foundation Fellow and a Camille and Henry Dreyfus TeacherScholar. We thank J. Deisenhofer for sending us the most recent set of refined X-ray crystallographic coordinates for the chromophores of the Rsp. viridis reaction center. We thank the University of Texas Center for High Performance Computing for providing Cray X-MP computation time.
Time-Resotved Studies on the Dynamics of Photoinduced Formation of M(CO),(polypyridyi) (M = Mo, Cr, and W) Complexes K. Kalyanasundaram Institut de Chimie Physique, Ecole Polytechnique FgdPrale, CH- 1015 Lausanne, Switzerland (Received: July 30, 1987; In Final Form: November 3, 1987)
The dynamics of photoinduced formation of M(CO),(LL) (M = Mo(O), Cr(O), and W(0) and LL = various polypyridyl ligands such as 5-chloro-l,lO-phen, 1,lO-phen, 2,2'-bpy, and 4,4'-Mez-2,2'-bpy) from M(C0)6 and LL in benzene has been examined via pulsed laser photolysis techniques. The formation of the polypyridyl tetracarbonyl complex occurs in several steps, extending over a wide range of time scales (from a few nanoseconds to several milliseconds). Spectral evidence is presented for the formation of a pentacarbonyl monodentate polypyridine intermediate. The rate of formation of the complexes follows the order Mo(0) > Cr(0) > W(0). For a given metal carbonyl, the reactivity of various ligands follows the order 5-Clphen 3 phen > bpy 3 Me2bpy.
Introduction In recent years there has been an enormous growth in studies of photochemistry of organometallic compounds, in particular those of metal carbonyls.' Metal carbonyl derivatives containing pyridyl (L) or polypyridyl (LL) ligands exhibit low-lying metal-to-ligand charge-transfer (MLCT) states. Photophysical and (1) (a) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic: New York, 1979. (b) Meyer, T. J.; Caspar, J. V. Chem. Rev. 1985, 85, 187. (c) Steigman, A. E.; Tylor, D. S. Coord. Chem. Rev. 1985, 63, 217.
0022-3654/88/2092-2219$01.50/0
photochemical investigations of these complexes have revealed room temperature emission and also rich and diverse forms of photoreactivity. References 2-5, for example, list some recent photochemical studies on group VIB metal carbonyl polypyridyl derivatives: M ( CO)5L,2 M ( C0)4L2,3 M ( CO),( LL) ,, and (2) (a) Wrighton, M. S.; Abrahamson, H. B.; Morse, D. L. J. Am. Chem. SOC.1976, 98, 4105. (b) Boxhoorn, G.; Oskam, A.; Gibson, E. P.; Narayanaswamy, R.; Rest, A. J. Inorg. Chem. 1981, 20, 783. (c) Lees, A. J.; Adamson, A. W. J. Am. Chem. SOC.1980, 102, 6874; J . Am. Chem. SOC. 1982, 104, 3804. (d) Lees, A. J.; Adamson, A. W. Inorg. Chem. 1981, 20, 4381.
0 1988 American Chemical Society
2220 The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
Kalyanasundaram
(CO),M-L-M(C0)5;5 M = Cr(O), Mo(O), and W(0). Abundant literature on the substitutional photochemistry of metal carbonyls exists. Despite these, relatively few time-resolved studies of mechanistic aspects of their photochemistry are a~ailable.~.’ In this report we focus our attention on the dynamics of photoinduced formation of M(CO),(polypyridyl) complexes ( M = Mo(O), Cr(O), and W(0)). In a widely used, high-yield synthetic procedure, these polypyridyl derivatives are prepared by UV irradiation of the corresponding M(CO)6 in the presence of an excess of polypyridyl (LL) ligands: M(CO),
+ LL -% M(CO),(LL) + 2CO
+ co
(k,)
(2)
+ solv + M(CO)5(solv) ( k , ) (3) + LL M(CO),(LL) + solv (k3) (4) M(CO),(LL) + co (k4) (5) M(CO),(LL)
M(CO), M(CO)S(solv)
-
I
390
2, 2 ’ - b p y
= o
(1)
Scheme I outlines a sequence of steps that is widely accepted as a possible mechanism for the final formation of these highly colored diimine complexes. Two noteworthy features of Scheme I are the involvement of M(CO), fragments associated with the solvent M(CO),(solv) and the diimine ligand LL as a monodentate M(CO),(LL) as distinct intermediates. SCHEME I M ( C O ) ~-L M ( C O ) ~
Mo (CO)6/LL,’benzene
+
From a mechanistic point of view, the process of sequential chelation of a bidentate ligand is worthwhile to investigate. Herein we present a detailed laser flash photolysis investigation (extending over a wide time scale from a few nanoseconds to several milliseconds) that demonstrates the occurrence of the above scheme for Cr, Mo, and W analogues. Through an elegant use of a rapid scan diode array spectrophotometer, Lees et a1.’ recently have provided some direct evidence for the occurrence of Scheme I in the formation of various W(CO),(diimine) complexes.
Experimental Section Mo, Cr, and W carbonyls (M(CO),) and polypyridyl ligands L L (2,2’-bipyridine, 1,lO-phenanthroline, 5-chloro-1,lOphenanthroline, etc.) were purum-grade chemicals from Fluka and were used as supplied. The polypyridyl carbonyl complexes M(CO),( LL) are conveniently prepared via UV-light photolysis of M(CO), with excess ligand LL in N,-purged isooctane solutions, according to literature p r o c e d ~ r e .The ~ products are sparingly soluble and precipitate out of solution. Purification was achieved by washing the precipitate repeatedly with isooctane. UV-visible (3) (a) Abrahamson, H. B.; Wrighton, M. S. Inorg. Chem. 1978,17,3385. (b) Chun, S.; Getty, E. E.; Lees, A. J. Inorg. Chem., 1984, 23, 2155. (4) (a) Wrighton, M. S.; Morse, D. L. J . Organomet. Chem. 1975, 97,405. (b) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1983, 22, 572, 3825. (5) Lees, A. J.; Fobare, J. M.; Mattimore, E. F. Inorg. Chem. 1984, 23, 2709. (6) (a) Kelly, J. M.; Bent, D. V.; Herrmann, H.; Schulte-Frohlinde, D.; Koerner von Gustof, E. J . Organomet. Chem. 1974,69, 259. (b) Bonneau, R.; Kelly, J. M. J . Am. Chem. SOC.1980, 102, 1220. (c) Kelly, J. M.; Long, C.; Bonneau, R. J . Phys. Chem. 1983, 87, 3344 and references cited therein. (d) Lees, A. J.; Adamson, A. W. Inorg. Chem. 1981, 20,4381. ( e ) Lees, A. J.; Adamson, A. W. J . Am. Chem. SOC.1982,104,3804. (0 Vlcek, A. Inorg. Chem. 1986, 25, 522. ( 9 ) Herrmann, H.; Grevels, F.-W.; Henne, A,; Schaffner, K. J . Phys. Chem. 1982, 86, 5151. (h) Nasielski, J.; Kirsch, P.; Wilputte-Steinert, L. J . Organomet. Chem. 1971, 29, 269. (i) Welch, J. A,; Peters, K. S.; Vaida, V. J . Phys. Chem. 1982, 86, 1941. G) Flamigni, L. Radiat. Phys. Chem. 1979, 13, 133. (7) (a) Yesaka, H.; Kobayashi, T.; Yasufuku, K.; Nagakura, S. J . Am. Chem. SOC.1983, 105, 6249 and references cited therein. (b) Lee, K.-W.; Hanckel, J. M.; Brown, T. L. J . Am. Chem. SOC.1986, 108, 2266. (c) Hughey, J. L.; Anderson, C. P.; Meyer, T. J. J . Organomet. Chem. 1977,125, C49. (d) Rothberg, L. J.; Cooper, N. J.; Peters, K. S.;Vaida, V. J . Am. Chem. SOC.1982, 104, 3536. (e) Herrick, R. S.; Herrinton, T. R.; Walker, H. W.; Brown, T. L. Organometallics 1985, 4, 42. ( f ) Wegman, R. W.; Olsen, R. J.; Gard, D. R.; Faulkner, L. R.; Brown, T. L. J . Am. Chem. Sor. 1981, 103, 6089. (8) (a) Chan, L.;Lees, A. J. J . Chem. Soc., Dalton Trans. 1987, 513. (b) Marx, D. E.; Lees,A. J. Inorg. Chem. 1987, 26,620. (c) Schadt, M. J.; Lees, A. J. Inorg. Chem. 1986, 25, 672. (d) Schadt, M. J.; Gresalfi, N. J.; Lees, A. J. Inorg. Chem. 1985,24,2942; J . Chem. SOC.,Chem. Commun. 1984, 506.
U
c
TJ
Ll L 0
,“ 0 . 0 4 8 Q
0-
0.064r
0 032
0
Wavelength
(nm)
Figure 1. Absorption spectra (difference spectra) of transient intermediates produced after 353-nm laser photolysis of Mo(CO), M)polypyridyl ligand solltions in benzene: top, 2,2’-bipyridine (lo-* M); middle, 1,lO-phenanthroline M); and bottom, 5-chloro-1,lOphenanthroline (lo-) M).
absorption spectral data of bpy and phen complexes agree well with those p ~ b l i s h e d . ~ Flash photolysis studies employed a Nd:YAG laser system (delivering 10-ns, 353-nm laser pulses) or a Ruby laser system (1 5-ns, 347-nm pulses) coupled to a fast kinetic spectroscopic detection system. Solutions for flash photolysis contained invariably M of M(CO), and 10-1-10-3 M of the polypyridyl ligand in benzene. The solutions were purged with ultrapure N 2 for at least 15 min prior to use.
Results and Discussion In order to obtain some direct s p t r a l evidence and kinetic data M) in on Scheme I, laser flash photolysis of M(CO), (ca. benzene was undertaken in the presence of several polypyridyl ligands such as 2,2’-bipyridine, 4,4’-dimethyl-2,2’-bipyridine, 1,IO-phenanthroline, and 5-chloro- 1,lO-phenanthroline. Benzene was chosen as a solvent for detailed examination due to the high solubility of the reactants and the product in this medium and minimal spectral overlap of absorbances of the transients. A . Laser Photolysis of Group VU3 Metal Carbonyls. Flash photolysis of group VIB metal carbonyls in various solvents has been the subject of several investigations., Excitation at 353 nm of M(C0)6 in perfluorocarbon solvents leads to very rapid (Cns?) formation of naked pentacarbonyl fragment M(CO),:
In most of the organic solvents, this is followed by insertion of
Photoinduced Formation of [M(CO),(polypyridyl)] Complexes
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2221
TABLE I: Absorption Spectral and Kinetic Data on the Photoinduced Formation of M(CO),(LL) Complexes in Benzene at 20 OC M(CO),(LL) M
LL
Mo 5-Clphen 5-Clphen/isooctane phen bPY MezbPY 1,4-(Bu)dabd dafd Cr
465 465 465 465 465
5-Clphen phen bPY Me2bPY 1,4-(Bu)dabd dafd
390 410 390 390 395 380 390
6.66
440 435 415
6.7 X los 1.3 X lo6 2.5 x 104
X
lo6
5.0 X lo6 1.3 x 105
390 390 385 390
5-Clphen phen bPY Me2bPY 1,4-(Bu)dabd dafd Reference 8b.
395 395 406 404 385 399
-
In this study using 10-ns, 353-nm, Nd laser pulses we have confirmed the above essential observations of Kelly and others., The absorption maximum of the solvent-associated pentacarbonylmetal complex is medium dependent. M ~ ( C O ) ~ ( s o l vfor ) , example, has its maximum located at 390, 410, and 415 nm in benzene, isooctane, and cyclohexane, respectively. The solventassociated species M(CO),(solv), however, do not accumulate as a stable product, decaying slowly over several hundreds of milliseconds. B. Laser Photolysis of (Mo(CO),+ LL] in Benzene. Figure 1 (bottom) presents the evolution of the transient spectra, recorded at various times after excitation of Mo(CO), solutions containing 5-chlor+l,lO-phenanthroline,with a lO-ns, 353-nm Nd laser pulse. The initial spectrum recorded at a few nanoseconds after laser pulse excitation has a maximum at 390 nm, and it can be identified as the M(CO)5(solv) species due to its similarity with the spectrum of the product obtained in M(CO), solutions devoid of the ligand. In solutions containing polypyridyl ligand, small spectral changes occur over the 20-50-ps time scale, indicating formation of another intermediate (spectrum B). This is likely to be the pentacarbonyl complex with the polypyridyl ligand associated in a monodentate fashion M(CO),(LL):
-
3.3 2.5 2.4 c2.0 0.4) lo2 (>0.4) 10-2 10-2 10-4 10-5
b C
a a, b a, b
b b b C
1,4-(Bu)dab = 1,4-di-tert-butyl-1,4-diazabutadiene; daf = 4,5-diazafluorene
solvent molecule over a period of a several nanoseconds to yield M ( C O ) ~ ( S O:~ V ) M(CO), + solv M(CO),(solv) (3 1
M(CO)5(solv) + L L
395, 502 525, 570 395, 490 390, 490 375, 532 388, 478
410 412
W
“This work. *Reference 8.
390 390 410 390 390 390
M(CO),(LL)
+ solv
(4a)
The assignment of the transient B as the M(CO),(LL) intermediate is based principally on two observations: (i) the spectral similarity of the intermediate with those of stable M(CO)5L complexes with monopyridinate type ligands (absorption maxima for M(CO),(piperidine), for example, are at 394 (Mo), 420 (Cr), and 403 nm (W), respectively1a~2a; (ii) at a fixed concentration of M(C0)6 and laser light intensity, the decay of the M(CO),(solv) species (monitored readily in the 550-600-nm region) and the final absorbance growth of M(CO),(LL) (monitored at 500 nm), both enhanced with increasing concentration of the ligand LL. After an intermediate period of C50 ps (the time range depends on the L L concentration), transient absorbance in the 500-nm region increases slowly over several hundreds of microseconds. Mo(CO),(LL) Mo(CO),(LL) + C O (5a) +
The spectrum of the final product recorded at several milliseconds (with maxima at 500 and 395 nm) agrees very well with the
spectrum of the product M(CO),(LL) prepared via the isooctane photolysis procedure and the absorption maxima observed in the steady-state photolysis of the solutions. Table I presents a summary of the absorption maxima of various transients and products and their assignments. The transient behavior observed with 1,lo-phenanthroline (Figure 1, middle) is very similar. With 2,2’-bipyridine (Figure 1, top), slow changes in the initial transient evolution are observed only in solutions that contain a large excess of bipyridine (ca. 0.1 M compared to M in the case of phenanthroline and its derivatives). The conversion of M~(CO)~(solv) to Mo(CO),(bpy) itself occurs over a millisecond. Though we could observe a slow growth of Mo(CO),(bpy) over several hundreds of milliseconds in our laser flash photolysis system designed for fast kinetic measurements, we could not follow fully the slow formation of this final product. This suggests that the reactivity of bipyridine is considerably slower than phenanthroline, both bidentate ligands of same family. We will return to a more specific discussion on the kinetics latter. C. Laser Photolysis of (Cr(CO), LL] in Benzene. Figure 2 presents the evolution of the absorption spectra of the transients generated upon photolysis of Cr(C0)6-5-chlorophenanthroline solutions in benzene. The end of the laser pulse transient has a broad absorption maximum at 465 nm. Laser photolysis of Cr(CO), alone in benzene yields the same transient, and this is assigned as Cr(CO)S(solv). (Earlier studies of Peters and Kelly et a1.6cJhave shown that the Cr(CO), fragment is very reactive and rapidly solvates in less than a nanosecond in neat benzene.) Subsequently over a period of 500 hs, transient absorptions evolve to spectrum B with absorption maxima located at 435 and 415 nm. On the basis of the absorption spectral similarity of Cr(CO)5(L) complexes (L = a monopyridinate ligand such as at 420 nm),laqh transient B is assigned piperidine, in benzene, A, as Cr(CO),(LL) with the 5-chlorophenanthroline ligand coordinated in a monodentate fashion.
+
Cr(CO),(solv)
+ LL
-
Cr(CO),(LL)
+ solv
(4b)
In conformity with this assignment, the growth rate and the final yield of transient B depend on the concentration of the LL ligand. We have been unable to successfully follow the conversion to the transient B to the final product Cr(CO),(LL) with extrusion of CO, due to nonreproducible results in the growth and decay of transients, especially on slow time scales. Failure to see a gradual increase in the concentration of the product Cr(CO),(LL) with repeated laser pulse excitation suggests that the product Cr-
2222
Kalyanasundaram
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 Cr
W (CO), / L L / b e n z e n e
(CO), / L L / b e n z e n e
I
I
0.048
I
2. 2 '
395
0.024
0
I
1, 10-phen
QJO. 048 -
I
440
'J
i , 10-phen
2
0.096
. .e- ;
E IV n
c m n
i
0
L
ffl
0
2
Lo
00.024Q
0
c
0.048
0
I
5-C1 phen
435
0.032-
I
I
5-C1 phen
0.064
520
t
0.016
0
& t - 9 ms
-
0 400
500
(nm) Figure 2. Absorption spectra (difference spectra) of transient intermediates produced after 353-nm laser photolysis of Cr(CO), M)polypyridyl ligand solutions in benzene: top, 2,2'-bipyridine (1 O-* M); middle, 1,lO-phenanthroline M); and bottom, 5-chloro-1,lOphenanthroline (lo-? M).
Wavelength
Wavelength
(CO),(LL) is unstable with respect to excitation with 353-nm light pulses (decomposition?). Cr(CO),(LL) complexes are also known to be thermally unstable.6 Cr(CO),(LL) Cr(CO),(LL) + C O (5b)
-
D. Laser Photolysis of (W(C0),+ LLJ in Benzene. Figure 3 presents absorption spectra of transients produced during the laser flash photolysis of W(CO)6 in the presence of various polypyridyl ligands. The early transient (end of laser pulse) is identified as W(CO),(solv). Its absorption maximum of 390 nm in benzene is in good agreement with earlier reports. Compared to the Mo carbonyls at comparable polypyridyl ligand concentrations, the various steps that lead to the formation of W(CO),(LL) all occur relatively slowly. The pentacarbonyl monodentate complexes W(CO),(LL) all have spectra very similar to M(CO),(solv) with absorption maxima located around 390 nm with slightly stronger absorbances. As shown in Figure 3 (middle and bottom), for the phenanthroline complex, we could successfully follow the conversion of W(CO)&L to W(CO),(LL). The process occurs ca. 20 times slower than for the corresponding Mo complex. The formation of W(CO),(bpy) from W(CO),(bpy) occurs too slowly to observe its formation in our flash photolysis set up. As mentioned in the Introduction, Lees et a1.8 have successfully used a diode array spectrophotometer to monitor the conversion of W(CO),(bpy) to W(CO),(bpy) in the dark, after a brief (
