7714
J . Phys. Chem. 1991, 95, 7714-7717
Microwave Spectrum and Structure of 3,3,4,4-Tetrafluorocyciobutene Anne M. Andrews, Susan L. Maruca, Kurt W. Hillig 11, Robert L. Kuczkowski,* Department of Chemistry, University of Michigan. Ann Arbor, Michigan 481 09- 1055
and Norman C. Craig Department of Chemistry, Oberlin College, Oberlin, Ohio 44074 (Received: March 1 , 1991;
In Final Form: May 16, 1991)
The microwave spectra of the normal, deuterated (d2),and "C isotopic species of 3,3,4,4tetrafluorocyclobutenewere assigned. The rotational constants and selection rules are consistent with C, symmetry. The structural parameters are d(C=C) = 1.349 (6) A, d(HC-CF2) = 1S O 1 (6) A, d(FZC-CF2) = 1.539 (6) A, d(C-H) = I .080 (1) A, d(C-F) = 1.358 (2) A, LC=C-C = 93.6 (2)O, LC-C-C = 86.3 (2)O, and LFCF = 106.2 (2)O. The dipole moment is 3.43 (2) D. The F2C-CF2 bond is significantly shorter than the analogous bond in cyclobutene, a change that is consistent with the effect of fluorine substitution in C3 ring systems.
Introduction
TABLE 1: Observed Rotational Transitions (MHz) for 3,3,4,4-Tetrafluorocyelobutene
The effects of fluorination on the structural parameters of cyclopropane and oxirane rings have been extensively studied both experimentally' and theoretically.2 While the structural changes are somewhat complex and dependent on the pattern of substitution, it is generally true that the carbon-carbon bonds contiguous to a fluorine-substituted carbon atom are shortened. The carbon-fluorine bonds also shorten as more fluorine substitution occurs at a carbon. This parallels the results for fluoromethanes and fluoroethenes, as recently ~ummarized.~In fluoroethanes, however, a lengthening is reported in the C-C distance in the series H2FC-CFH2,4 HF2C-CF2H,5 and F3C-CFt as well as in perfluorocy~lobutane.~It has recently been found that d(C-C=) shortens while d(C-C) lengthens in perfluorocy~lobutene~ and 1,2-dichlorotetrafluorocyclobutene* In 1,4,4-trifluorocyclobutene, it appears that the carbon-carbon bonds at which fluorination occurs have become shorter? although the results are not definitive due to an incomplete isotopic data set. Explanations for the observed trends have been disc~ssed.~J They are the result of an interplay of rehybridization and Coulombic effects at the carbon atoms as well as r back-bonding from fluorine at a double bond. As part of an ongoing study of the effects of halogenation on small ring compounds, 3,3,4,4-tetrafluorocyclobutene(hereafter TFCB) and its perdeuterated modification were synthesized by us. We have assigned the microwave spectra of these species as well as two I3C species in natural abundance and have determined the structural parameters for TFCB. In contrast to the perhalogenated cyclobutene results mentioned above, a shortening ( I ) (a) Perretta, A. T.; Laurie, V. W. J. Chem. Phys. 1975,62,2469. (b) Justnes, H.; Zozom, J.; Gillies, C. W.;Sengupta, S. K.; Craig, N. C. J. Am. Chem. Soc. 1986,108,881. (c) Beauchamp, R. N.; Agopovich, J. W.; Gillies, C. W.J . Am. Chem. Soc. 1986,108,2552. (d) Chiang, J. F.; Bcrnet, W. A. Tetrahedron 1971, 27, 975. (e) Beauchamp, R.N.; Gillies, C. W.; Gillies, J. Z . J. Mol. Specrrosc. 1990, 144, 269. (2) (a) Deakyne, C. A.; Allen, L. C.; Craig, N. C. J. Am. Chem. Soc. 1977, 99,3895. (b) Deakyne, C. A.; Cravero, J. P.;Hobson, W. S.J. Phys. Chem. 1984,88,5975. (c) Shancke, A.; Boggs, J. E.J. Am. Chem. SOC.1979,101, 4063. (d) Cremer. D.; Kraka, E.J. Am. Chem. Soc. 1985, 107, 3800. (e) Durmaz, S.;Kollmar, H. J. Am. Chem. Soc. 1980,102,6942. (0 Boggs, J. E.; Fan, K. Acra Chem. Scand., Ser. A 1988, A42, 595. (3) CsBzSr, A. C.; Hedberg, K. J. Phys. Chem. 1990, 94, 3525. (4) Freisen, D.; Hcdberg, K. J . Am. Chem. Soc. 1980, 102, 3987. (5) Brown, D. E.;Beagley, B. J. Mol. Srrucr. 1977, 38, 167. (6) Gallaher, K. W.; Yokozeki, A.; Bauer, S. H.J. Phys. Chem. 1974,78, 2389. (7) (a) Chiang, C. H.; Porter, R. F.; Bauer, S.H. J . Mol. Srrucr. 1971, 7,89. (b) Alekseev, N. V.; Barzdain, P. P. Zh. Strukr. Khim. (Engl. Tram/.) 1974, 15, 171. (8) Thomassen, H.; Hedberg, K. J . Phys. Chem. 1990, 94, 4847. (9) Craig, N. C.; Kim, H.; Lorencak, P.; Sibley, S. S.; Kuczkowski, R. L. J . Mol. Srrucr. 1990, 223, 45. 0022-3654/9 1 /2095-77 14$02.50/0
,
t
CF,CH=CHCF2 transition u(obs) 615-505 624-514 625-515 633-523 634-524 64,15336 65,-54, 66,-55, 716-606 726-616 734-624 735-625 7,-73, 75a-74, 76,-75, 77,-66,
24 460. I3 26072.78 26 196.91 27 895.67 27 897.73 29660.98 31 425.29 33 189.18 28 405.75 30 137.16 31 808.78 31 813.14 33 575.48 35 339.60 37 103.63 38 867.63
Au' 0.13 -0.03 -0.18 0.01 -0.1 1 -0.09 0.1 I -0.04 -0.02 -0.04 0.13 0.15 0.06 0.02 -0.01 -0.03
CF2CMDCFz transition u(obs) 616-505 22 865.99 625-514 24566.67 642-533 28 232.16 63,-54, 29816.42 66,-55, 31 401.47 717-606 26499.65 28 155.91 726-615 735-624 30 254.68 734-625 30 548.80 744633 32000.37 743-634 32013.99 75,-64, 33596.48 76,-65, 35 182.18 77,-66, 36767.07
AU 0.04 -0.08 0.01 0.03 0.06 -0.06 0.05 0.05 -0.06 0.04 -0.06 0.07 0.03 -0.01
Au = u(obs) - u(calc). bAn x for the KO subscript denotes an unresolved K-doublet transition. (I
TABLE 11: Observed Rotational Transitions (MHz) for '% Species of 3,3,4,4-Tetrafluorocyclobutene b
1
C F2"C H=C HC F2 transition v(obs) AP 212-10, 313-202 414-303 221-110 220-1II 322-21, 321-212 303-212 4a-313 414-321 413-322 423-330 422-331
8626.059 12503.529 16376.817 10351.217 10359.838 14232.944 14258.932 10825.075 14727.577 12942.958 13028.833 I 1269.540 I 1270.503
0.002 0.000 0.001 -0,002 0.000 -0.001
0.001 0.000 0.000 -0.001 -0.000 -0.002 0.002
I3CF2CH=CHCF2 transition v(obs) Au 211-101 8710.580 0.002 312-202 12628.969 0.000 413-303 16551.711 0.001 221-111 10475.283 -0.001 220-110 10466.811 0.000 322-212 14389.387 0.000 321-211 14364.089 -0.001 16 149.860 0.000 331-221 330-220 16 149.800 0.001 303-211 10816.986 0.000 4a-312 14709.153 0.000 414-322 12919.568 0.001 413-321 13004.605 -0.001 423-331 1 1 189.083 -0.005 422-330 1 1 190.016 0.005
' A u = u(obs) - u(calc).
of all the bonds around the fluorinated carbon centers has been observed.
Experimental Section Synthesis. TFCB was synthesized in modest yield by reaction of sodium borohydride (Aldrich) and perfluorocyclobutene 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7715
Structure of 3,3,4,4-Tetrafluorocyclobutene
-
TABLE III: Rotational a d Centrifugal Distortion Constants for 3,3,4,4-Tetnfluorocyclobutene
,
A," MHz
B, MHz
MHz Dj, kHz DJK,kHz DK, kHz d , , kHz dz, kHZ
nc
MHz P,, amu A2 Au,,d
amu
CFZCD-CDCFZ 2682.810 (5) 1922.440 (4)
1961.521 (7) 1952.947 (1 1) 0.265 (55) -0.37 (20) 0.42 (16) 0.054 (29) 0.008 (7) 50 0.09 169.2147 89.5630 88.4318
C,
Pbb,
I
CFzCHzCHCF2 2839.291 (8)b
A2
1857.119 (4) 0.128 (24) -0.19 (9) -0.05 (9) 38 0.15 173.3190 98.8116 89.5651
CF2"CH.-CHCF2
"CF2CHeHCFz 2839.41 1 (I)
2803.454 ( I ) 1949.426( I ) 1940.870 (1) 0.179 (13) 0.399 (39)
1957.058 ( I ) 1948.523 ( I ) 0.225 (21) -0.213 (52) 0.242 (54)
13 0.001 169.6814 90.7065 89.5637
15 0.002 169.8060 89.5592 88.4281
pee, amu A2 a Watson S reduction ( I r ) .*One standard deviation in parentheses. CNumberof transitions in the fit. " A u = u(obs) - u(ca1c). TABLE IV: Principd Axis Coordinates of 3,3,4,CTetrafluorocyclobu tene b
a
IOa
KP
10
C
Kr
Kr
10
0 -1.5170 -1.5165 Ci,? h0.6742 h0.6750 0 0 -0.0183 -0.0197 Cl,, h0.7682 h0.7708 0 0 -2.2895 -2.2913 Hj,6 f1.4280 h1.4281 0 F,,8 +1.3575 +1.3569 h1.0856 Al.0856 0.5456 0.5460 Fp,io -1.3575 -1.3569 h1.0856 h1.0856 0.5456 0.5460 'Coordinates from a least-squares fit of 12 moments of inertia. * Kraitchman substitution coordinates. Fluorine coordinates and c for C3,, from the first and second moment equations of normal species. CSeeFigure 1 for atom numbers.
(Peninsular Chemical Research).gJo The solvent was diglyme (Aldrich) that had been dried by refluxing over sodium metal. The oven-dried apparatus was purged with dry nitrogen prior to use. Forty millimoles of sodium borohydride dissolved in 60 mL of diglyme was added dropwise over 2.5 h to 22 mmol of perfluorocyclobutene dissolved in 20 mL of diglyme. The contents of the reaction flask were well stirred while being maintained at about -15 O C with an icesalt mixture. During the reaction, the flask was connected to a dry ice-acetone-cooled trap open to the atmosphere through a drying tube. After all of the sodium borohydride solution had been added and stirring had been continued for 2 h, the dry ice around the trap was replaced with liquid nitrogen, and readily volatile material was vacuum distilled into the trap. TFCB was separated from the crude products by preparative gas chromatography at room temperature on tricresyl phosphate-on-Fluoropak 80 (80/100-mesh Teflon from Analabs) columns in I-m and 5-m lengths. Finally, TFCB was dried by passage over phosphorus pentoxide. Mass spectrometry [parent peak at 126 (intensity = 66), C2F4(71), C3F2H(76), C3FH2(loo), C F (44)]" and N M R spectroscopy confirmed TFCB.9 The deuterated modification of TFCB was prepared by the same reaction on a smaller scale with sodium borodeuteride (Aldrich) in place of the hydride. Spectrometers. The microwave spectra of the normal and d2 isotopic species were recorded from 18.0 to 40.0 GHz with a conventional Hewlett-Packard 8460A Stark modulation spectrometer. The waveguide cells were cooled with dry ice, and pressures of 10-20 mTorr were employed. After assignments were made, frequency data were collected with a PDP-I 1/23 computer interfaced to the spectrometer. Frequency measurements were accurate to 0.10 MHz. The dipole moment was measured on this spectrometer by calibrating the electric field using OCS.lz The spectra of the two singly substituted 13C species were measured with a Fourier transform microwave (FTMW) spectrometer employing a pulsed nozzle molecular beam ~0urce.l~The sample entering the nozzle was < I % TFCB and 99+% Ar at a
Figure 1. Atom numbering and structure for 3,3,4,4-tetrafluorocyclo-
butene.
pressure of about 1.5 atm. The gas expansion cools the vibrational and rotational levels enormously, making it quite easy to detect low-J transitions of low-abundance isotopes guided by predictions from spectroscopic data already in hand, as recently demonstrated with pr0pana1.I~ Many factors, including rotational constants, vibrational rigidity, large dipole moments, and a natural abundance of -2% for each I C species, made TFCB an especially propitious case. Along with the planar moment inertial data mentioned below, several other checks verified that the observed transitions arose from the I3Cspecies. The reduced intensities of the transitions were appropriate, although quantitative measurements relative to the normal species are very difficult on a FTMW spectrometer and were not seriously undertaken. For each species, there is a K-doublet pair split by 1 MHz for which it could be ascertained that an alternation of intensity was absent. This unaltered intensity indicates the loss of the C2symmetry axis, as expected for the "C species. A careful check was also made to confirm that there were no other transitions in the regions expected for the 13Cspecies and that the lowest vibrational satellite was depopulated and absent in the FTMW spectrum. The high resolution and reduction in Doppler and pressure broadening resulted in frequency measurements accurate to 0.002 MHz for these species.
-
Results and Discussion Preliminary structural models indicated that TFCB would be a near prolate top with either b-dipole or c-dipole selection rules. Broad spectral traces displayed several overlapping series of lines spaced about 1764 MHz apart. Each series consisted of R-branch transitions having the same values of J arising from the AKp = 1 "perpendicular" transitions of a near prolate top. This spacing
~~
(!O) Burton, D,,J.;Johnson, R. L. J . Am. Chem. Soc. 1%4,86,5361 and
a private communication. ( I I ) Kim, H. Unpublished results. (12) Muenter, J. S.J. Chem. Phys. 1968, 48, 4544.
(13) Hillig, K. W., 11; Matos, J.; Scioly, A.; Kuczkowski, R. L. G e m . Phys. Left. 1987, 133, 359. (14) Rande!l, J.; Cox, A. P.;Hillig, K. W., 11; Imachi, M.; LaBarge, M. S.; Kuczkowski, R. L. 2.Nafurforsch. 1988, 13a, 271.
Andrews et al.
7716 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 TABLE V Experimental Structural Parameters lor 3,3,4,4-Tetnfluorocyclobuteneand Related Molecules TFCB MW, this work d(C=C) d(=C-C) d(C-C) d(C-CI) d(C-F) d(C-H) L(C=C-C) L(C-c-C) L(C-c-x y L( F-C-F) L(C-C-H)
MW. ref 15
cyclobutene
hexafluorocyclobutene ED. ref 3
1,2-dichlorotetrafluorocyclobutene ED, ref 8
1.324(23) 1.502(5) 1.584(1 1) 1.347 (4)
1.359 (9) 1.500 (6) 1.599 (IO) 1.687 (3) 1.340 (2)
95.0(5) 85.0(2) 134.6(IO) 107.6 (5)
94.6 (2) 85.4 (2) 135.0 (IO) 108.2(4)
1"
KP
1.348 (3) 1.502 (3) 1.536 (3)
1.350 1 SO0 1.542
1.342 (4) 1.517 (3) 1.566(3)
1.358(2) 1.079 ( I ) 93.6 ( I ) 86.4 (2) 136.3 (5) 106.2 (2) 132.1 (2)
1.357 1.080 93.7 86.3 136.2 106.2 132.2(5)
1.083 (5) 94.2 (3) 85.8 (2) 135.7 (IO) 132.3 (5)
"See footnotes a and b in Table IV. b X is a point on the bisector of the FCF angle; this is angle C,-C4-X (Figure I ) . TABLE VI: Stark Coefficients and Dipole Moment of 3,3,4,4-Tetrafluorocyclobutene AvJe2"
transition
IM
615-505
0
927-817
1 1
2 3 4
obs 0.380 (1 I ) b 5.87 (12) 0.370 (24) 1.84 (4) 4.29 (9) 7.76 (11)
"Stark coefficient in units of MHz cm2 k V 2 .
C20in parentheses.
+
calc 0.377 5.96 0.369 1.83 4.28 7.70 lo in
parentheses.
set the value for 2A - ( E 0. Optimization of structural models to fit this value readily identified the J and K values for each series, leading to the assignment of the c-dipole transitions. At low K , where the asymmetry doublets were resolved and not widely separated, it was apparent that they exhibited an alternation of intensity due to nuclear spin statistics. This finding confirmed that the c-dipole moment coincided with a molecular C, symmetry axis, which exchanged two pairs of fluorine and one pair of hydrogen atoms (all I = The TFCB-d2spectrum was assigned similarly except that the R-branch series were more difficult to recognize initially since this species was more asymmetric ( K = -0.84 compared to -0.98 for the normal isotope) and the selection rules switched to b dipole. An alternation of intensity consistent with the change in dipole axis and deuterium ( I = 1) substitution was also observed in this case. A selected set of transitions for each isotope is given in Table I. After the spectra of the normal and the d2species were assigned, their inertial data were used to obtain preliminary structures and predict regions where the I3C species could be found. The regions to be searched by using the conventional absorption spectrometer were rich and cluttered. The spectral search regions were smaller with the FTMW spectrometer (8-18 GHz) due to the lower J values accessible at these lower frequencies. Also, the vibrational and rotational cooling greatly simplified the spectrum. With this instrument, the transitions were readily found. One type of carbon atom lies close to the a principal axis, and this I3C species had c-dipole transitions like the normal isotopic species. The other "C species had b-dipole selection rules like the d2species. The complete set of assigned transitions is listed in Table 11. The derived rotational and centrifugal distortion constants for all the isotopic species are listed in Table 111. The planar moments Pbband P, where P,, = xm,x;, are listed in Table 111 and provide some insight into the symmetry of the molecule. When isotopic substitution occurs in a symmetry plane, the planar moment is not affected by substitution in that plane. It can be seen that Pbb of the normal species is nearly identical with fbb of one of the I3C species and P,, of the other two species. This result implies that all the isotopic substitutions occur in a symmetry plane that is either the ac or ab inertial plane. The switch in the labeling of the inertial plane is consistent with the
change in selection rules upon isotopic substitution mentioned above. The near prolate top nature of TFCB is the origin of this behavior. Another symmetry condition is reflected in the spin statistics observed for the normal and d2species. This requires the presence of a C2axis. These symmetry elements establish C, symmetry for TFCB. This finding is consistent with previous work on cyclob~tene'~ and perfluorocy~lobutene.~ For C, symmetry, eight internal parameters are required to define the structure of TFCB. Since there are nine independent moments of inertia, the structure can be determined without any constraints. A Kraitchman substitution procedure and a leastsquares fitting method were used to derive structural parameters. In the Kraitchman procedure, the in-plane coordinates for deuterium and carbon were calculated from Kraitchman's equations;16 their out-of-plane coordinates were fixed to zero. The fluorine coordinates and the c coordinate for the fluorinated carbon atom (which is small and unreliable from Kraitchman's equations) were obtained from fitting the three moments of inertia and Cm,q for the normal isotopic species. The other procedure involved least-squares fitting of all 12 observed moments of inertia. The coordinates obtained from the two procedures are given in Table IV, and the structural parameters are listed in Table V. Very small differences exist between the two derived structures, probably smaller than is typical between r, and ro structures perhaps due to the rigidity of the ring. The differences can be exaggerated somewhat if the small c coordinate of the carbon (CFJ is set to zero in the Kraitchman calculation and the fluorine coordinates are determined solely from the moments of inertia. This alternative leads to differences no greater than 0.01 A in distances and 1.0' in angles. This alternative does not affect, however, the CFz-CF,distance, which does not depend on the c coordinate. The average of the two structures is listed in the abstract as the preferred rs/roparameters. The errors are twice the statistical ones from the fitting process. The structure of TFCB is compared with cyclobutene and two other fluorinated cyclobutenes in Table V. It is apparent that the C-C bonds at the fluorinated carbon have become shorter compared to cyclobutene, similar to the fluorinated ethenes and cyclopropanes mentioned in the Introduction. A simple model can rationalize the trends in terms of changes in hybridization and size of the substituted carbon a t ~ m ; ~ . a~ more J ' sophisticated MO analysis has been applied to the cyclopropane examples.'-2 The shortening of the CF-CFz bond is in contrast to the lengthening observed in perduorocyclobutene and the 1,2-dichlorotetrafluoro analogue in Table V. It is unlikely that this difference arises from the neglect of vibrational effects in the microwave analysis. As pointed out above, this distance depends only on the a coordinate of the carbon, which is well determined by the isotopic data. Explanations for this contrast between the systems must await further studies. Theoretical work, such as (IS) Bak, 8.;Led, J.; Nygaard. L.;RastrupAnderson, J.; Sorensen, G.0.
J . Mol. Srrucr. 1969, 3, 369. (16) (a) Kraitchman, J . Am. J . Phys. 1953, 21. 17. (b) Nygaard, L. J . Mol. Spectrosc. 1976, 62, 292. (17)Bent, H.A. J . Inorg. Nucl. Chem. 1961, 19, 43.
J. Phys. Chem. 1991, 95, 7717-7721 ab initio calculations, may shed light on this matter. We also plan to explore the microwave spectra of the two perhalogenated compounds. This investigation may provide rotational constants for a combined microwave-electron diffraction analysis to further refine the parameters. TFCB, not surprisingly, is a very polar species. Measurements of Stark splittings for six M components of two transitions are given in Table V1. These data gave a dipole moment of 3.43 (2) D. The most intense vibrational satellite for TFCB was also assigned using the conventional spectrometer. It had rotational constants of A = 2838.59 (2) MHz, B = 1958.45 (1) MHz, and C = 1954.29 ( I ) MHz. Rough relative intensity measurements indicated that the vibrational frequency was 95 f 20 cm-I. While the data are not completely conclusive, it appears that these transitions had the same spin statistics as the ground state, in-
7717
dicating that this vibration state is probably symmetric, i.e., species A, or Ala Acknowledgment. This work was supported by the Summer Undergraduate Research Program of the UM Chemistry Department with a fellowship to S.L.M. A.M.A. was the recipient of a Regents fellowship from the University of Michigan (1987-1991). The microwave spectrometers were purchased and supported with funds from National Science Foundation grants awarded to R.L.K. The work at Oberlin was supported by a grant from the Petroleum Research Fund, administered by the American Chemical Society. We appreciate discussions with Professor Kenneth Hedberg. We are grateful to Hidong Kim for preliminary work on TFCB and to Elizabeth A. Dudley and Daniel s. Schullery for their assistance in preparing the two samples of TFCB used in microwave spectroscopy.
Cage Escape Yields in the Quenching of "Ru(bpy)t+ by Methylviologen. Presence of Triethanolamine as a Sacrificial Electron Donor Mikhael Georgopoulos and Morton Z. Hoffman* Department of Chemistry, Boston University, Boston, Massachusetts 0221 5 (Received: May 29, 1990; In Final Form: May 20, 1991)
Continuous and pulsed laser flash photolysis techniques have been used to determine the efficiency (7,) with which the redox products are released into the bulk upon the oxidative quenching of *Ru(bpy):+ by MV2+ in the absence and presence of TEOA as a sacrificial electron donor in aqueous solution. The value of 7, is diminished as the ionic strength (p) of the solution increases, although the extent of the effect depends on whether or not TEOA is present: in the absence of TEOA, I, = (3.8 + 4.8p1l2)-l; in the presence of 0.1 M TEOA in alkaline solution, 7, = (3.8 + 9.Op1I2)-I. TEOA is an effective scavenger of R ~ ( b p y ) ~at~concentrations + 20.05 M and in the pH 8.5-1 1.5 range. In less alkaline solution, its protonation diminishes its reducing ability; in more alkaline solution, reactions between its degradation products and MV2+yield additional equivalents of MV". The value of 7, is also a function of [ R ~ ( b p y ) ~with ~ + ]a transition at 20-50 pM between upper and lower plateau values.
Introduction The factors that control the efficiency with which redox products are formed in the bulk solution upon the bimolecular electrontransfer quenching of excited states of metal complexes are becoming increasingly known and quantified.' The interpretation of the value of the efficiency of cage escape (a,) is based on the mechanistic model (reactions 1-3)2 in which the diffusional en*M + Q + [M+/-...Q- /+I (1) [M+/-...Q/+] [M+/-...Q/+I
kt4
+M
k,
+Q
M+/- + Q-/+
(2)
(3)
counter of an excited state (*M) by an electron-transfer quencher (Q) results in the formation of a geminate redox pair within the solvent cage; escape of the redox species out of the cage into bulk solution competes kinetically with back electron-transfer within the cage to reform the original ground-state materials. Inasmuch as is equal to k,/(k, + kb(), the problem of understanding the dynamics of the quenching process and the events within the solvent cage becomes a matter of knowing the parameters that govern the values of the two rate constants that ( I ) Tazuke, S.;Kitamura, N.; Kim, H.-B. In Supramolecular Photochemistry; Balzani, V., Ed.; Reidel: Dordrecht, 1987; p 87. (2) Balzani, V.; Scandola, F. In Energy Resources through Photochemistry and Catalysis; GrBtzel, M., Ed.; Academic: New York, 1983; p I .
0022-3654/9 1 /2095-77 17$02.50/0
describe the competing intramolecular processes. Unfortunately, the events within the solvent cage occur in the subnanosecond time frame, making reaction 1 the ratedetermining step for the usual concentrations of Q;a direct measure of kbt and k, in the bimolecular quenching process cannot be achieved. However, back electron transfer in photoexcited covalently bonded donor-acceptor molecules has been determined and can be used as a model for reaction 2.3 It is well-known that kbt depends on the exergonicity of reaction 2; kbt describes a "bell-shaped" curve as a function of ACbto of the back electron-transfer reaction, carrying it through the "normal" and "inverted" Marcus regions! As well, kbt depends on the distance between the donor-acceptor centers, the nature of the connecting moieties, and the specific pathway of electron t r a n ~ f e r . ~Values of k, can be calculated from theoretical diffusional equations$ k, is expected to be dependent on the charges on the species, solution ionic strength, solvent dielectric constant and viscosity, temperature, and the extent of the specific interactions among the various species within the solvent cage. For bimolecular quenching reactions under conditions where k , has a constant value, kb,/k, (= a,-1 - 1) also describes the "bell-shaped" curve as a function of AGblO.' (3) Mataga, N . In Photochemical Energy Conuersion;Norris, J. R., Jr.; Meisel, D., Eds.; Elsevier: New York, 1989; p 32. (4) Ohno, T. Prog. React. Kinet. 1986, 14, 219. (5) Closs, G. L.; Miller, J . R. Science 1988, 240, 440, and references therein. (6) Comprehensive Chemical Kinetics; Bamford, C . H., Tipper, C. F. H., Compton, R . G.,Eds.; Elsevier: Amsterdam, 1985.
0 199 1 American Chemical Society
