2548
J . Phys. Chem. 1992, 96, 2548-2553
in Table 11. All the rate constant values were based on or inferred from literature values with the exception of k , and k2, which were varied in an attempt to predict the observed induction time and disappearance rate of CF31over a variety of conditions. In general, it was found that k , was determined by the slope of the In [CF31] vs time curves and k , determined the induction times. Unfortunately, no set of values for k, and k , satisfactorily fits the data over a wide range of pressures, although all the fits point to k , N (2-5) X and k , N (1-3) X lo-" cm3/(molecule s). The most striking shortcoming of the model is that it predicts a higher CFJ disappearance rate for higher initial CFJ partial pressures, in contrast to the data of Figure 6. It would seem that at least one key step is missing from the rate package. For that matter, direct experimental evidence for the postulated initiation step is still lacking. Conclusions The reaction stoichiometry and kinetics of F2 CF31have been studied through an examination of their end products and by collecting data on the disappearance rate of CF31. The observed rate is a function of initial species concentration but over a wide range of conditions is on the order of cm3/(molecule s)
+
(45) Ogawa, T.; Carlson, G. A.; Pimentel, G. C. J . Phys. Chem. 1970, 74, 2090. (46) Whittle, E. MTP Int. Rev. Sci. Phys. Chem., Ser. I 1972, 9, 75. (47) Appelman, E. H.; Clyne, M. A. A. J . Chem. Soc., Faraday Trans. I 1975, 71, 2072. (48) Whitefield, P. D.; Davis, S. J. Chem. Phys. Lett. 1981, 83, 44. (49) Lilenfeld. H. V.; Bradburn, G. R. Chem. Phys. Lett. 1986,131,276.
following an induction period. This behavior can be explained only by a chain reaction, and the existence of IFS,C2F6,and CF, as end products strongly suggests a significant role for both I F and CF3 as part of this chain. The disappearance rate displays no strong dependence on F, concentration, but 0, can clearly play an important role. Important details of the kinetic sequence remain unknown, such as precise knowledge of the intermediate species. The probable existence of IF as an intermediate lends support to the hypothesis that the IF(B) emission observed during 248-nm photolysis of F2/He/CF31 m i x t ~ r e s l arises ~ ~ ' ~ from sequential pumping of I* onto IF(X), in analogy with O,(lA) pumping of IF(X),6 rather than from recombination processes. This has important implications for the design of any chemically pumped IF(B) laser, since I* pumping of IF(X) represents another channel for producing IF(B). At this time it is not known whether this channel is more or less efficient than 0 2 ( ' A ) pumping of IF(X), but given the near resonance of I* and 02('A), these two channels could be expected to compete with each other in any pumping scheme involving 0 2 ( ' A ) .
Acknowledgment. We thank Jan Marien and Dave Campbell for fine glass-blowing work, Marty Valles, Chris Berst, Jim Clay, Jeff Saland, McSean McGee, Wayne Wasson, and Charles Miglionico for other technical assistance, and Gerry Streit and Steve Davis for helpful discussions. This work was supported by the Air Force Office of Scientific Research under Contract 2303Y205. Registry No. F,, 7782-41-4; CFJ, 2314-97-8; IFS, 7783-66-6; CF4, 75-73-0.
Photodimerization of Phenanthrene-9-carbonitrileand Methyl Phenanthrene-9-carboxylate Frederick D. Lewis,* Steven V. Barancyk, and Eric L. Burch Department of Chemistry, Northwestern University, Evanston, lllinois 60208-31 13 (Received: August 30, 1991) The fluorescence self-quenching and photodimerization of three phenanthrene derivatives have been investigated. Methyl phenanthrene-9-carboxylate undergoes self-quenching with a rate constant of I .2 X lo9 M-' s-' in dichloromethane solution to form a weakly bound, nonfluorescent excimer from which the syn head-to-tail dimer is formed efficiently. The well-known photodimerization of phenanthrene-9-carbonitrileis found to occur predominantly via the excitation of a ground-state n-complex or aggregate. The structure and stability of the n-complex have been investigated using both IH NMR spectroscopy and molecular mechanics. The failure to observe self-quenching for either the nitrile or N,N-dimethylphenanthrene-9-carboxamide is attributed to an enthalpy of excimer formation for these and other phenanthrenes which is too small to overcome the entropy of excimer formation at room temperature in fluid solution. Introduction Self-quenching of the lowest singlet state of aromatic hydrocarbons in fluid solution at room temperature results in the formation of singlet excimers when the enthalpy of formation is sufficiently large (ca. 4-5 kcal/mol) to overcome the entropy of association.'s2 This is the case for most simple aromatic hydrocarbons, with the exception of phenanthrene and ~ h r y s e n e . ~ Aromatic hydrocarbons can also form n-complexes in the ground state;4 however, the enthalpies of association are generally much smaller in the ground state than in the singlet state. Thus, studies of the absorption and emission of 9-complexes have generally been limited to the vapor phase,5 low-temperature matrices,6 or arenes ( I ) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; Chapter 7. (2) Stevens, B. Ado. Photochem. 1971, 8, 161, (3) Birks, J . B. In The Exciplex; Gordon, M . , Ware, W. R..Eds.; Academic Press: New York, 1975; p 39. (4) Hunter, C. A.; Sanders, J. K. M. J . Am. Chem. Soc. 1990,112,5525. ( 5 ) Saigusa, H.; Lim. E. C. J . Phys. Chem. 1991, 95, 1194.
SCHEME I
connected by covalent bonds in solution.' Saigusa and LimS recently reported that excitation of the van der Waals or n-complex of fluorene in a supersonic jet yields an excited n-complex in which the excitation is initially localized on one molecule. Geometric (6) Chandross, E. A.; Ferguson, J.; McRae, E. G. J . Chem. Phys. 1966, 45, 3546. (7) Reynders, P.; Kuhnle, W.; Zachariasse, K. A . J . Am. Chem. SOC.1990, 112, 3929.
0022-3654/92/2096-2548$03.00/00 1992 American Chemical Society
Photodimerization of P N and PE
The Journal of Physical Chemistry, Vol. 96,No. 6,1992 2549
relaxation of the excited fluorene *-complex yields the singlet excimer. Thus electronic excitation of a *-complex might provide a route for excimer formation in solution in cases where the enthalpy of formation is too small to overcome the entropy of association (Scheme I). Strongly-polarizing, electron-withdrawing groups such as nitriles and esters are known to favor ground-state *-complex formation! We report here the results of our investigation of the photodimerization and self-quenching of phenanthrenes with electron-withdrawing ester, amide, and nitrile substituents a t the 9-position (eq 1). We find that (a) methyl phenanthrene-9X
Y
PN: X = C N PE: X = C02Me PA: X=CONMe2
DN: X = CN DE: X = CO,Me
carboxylate (PE) undergoes self-quenching and dimerization via a normal excimer mechanism, (b) N,N-dimethylphenanthrene9-carboxamide (PA) fails to undergo either self-quenching or photodimerization, and (c) phenanthrene-9-carbonitrile (9cyanophenanthrene, PN) fails to undergo self-quenching and undergoes dimerization predominantly via excitation of a ground-state r-complex. The final observation resolves a longstanding puzzle concerning the mechanism of P N photo-
'
Experimental Section General Methods. Melting points were taken on a Fisher-Johns apparatus and are uncorrected. ' H N M R spectra were recorded on an EM390 spectrometer with T M S as an internal standard or on a Varian XLA 400 spectrometer with TMS and cyclohexane (0.1% v/v) as internal standards. The change in the cyclohexane resonance (0.006 ppm) relative to TMS was insignificant when campared to the dilution shifts for PN over the concentration range examined (0.0008-0.2 M). UV-visible absorption spectra were measured with a Hewlett-Packard 8452 A diode array spectrometer. Fluorescence spectra of degassed solutions were recorded with either a Perkin Elmer MPF-44A spectrophotometer or a PTI LS-1 single photon counting instrument with a high-pressure Xe lamp and steady-state software. Fluorescence lifetimes were measured with a PTI LS-1 single photon counting instrument with a gated hydrogen arc lamp (time resolution ca. 0.5 ns). Front face geometries were used for concentrated solutions (>0.005M). All lifetimes reported for solutions were obtained from a singleexponential fit to the fluorescence decay ( x 2 < 1.2). Low-temperature (77 K) studies were performed using methylcyclohexane glasses (0.05 M) using the mathematical model derived by Dimicoli and Hblhe2*for dimer formation provides (27) Kloepfer, R.; Morrison, H. J . A m . Chem. SOC.1972, 94, 2 5 5 . (28) Dimicoli, J.-L.; Helene, C. J . Am. Chem. SOC.1973, 95, 1036.
Lewis et al. SCHEME 111 PN + PN
e (PN PN) Ka
('PN' PN)
I 'PN'
kn
+
PN
'(PN PN)'
I
ON
a self-association constant of ca. 0.4 for PN in chloroform solution. The association constant would be expected to be higher in a hydroxylic solvent such as ethanol and lower in an aromatic solvent such as benzene, in agreement with the observed effect of solvent on OD (Table I). The ' H N M R data can equally well be fit to models for higher aggregate formation. Higher aggregates, including microcrystalline PN, are presumably responsible for the observation of light scattering and long-wavelength emission with multiexponential decay in concentrated (>0.05 M) dichloromethane solutions. The greater concentration dependence of the lH NMR chemical shifts for H(4), H(5), H(8), and H(10) compared to the other hydrogens indicates that there may be a preferred *-complex geometry or geometries in which these hydrogens are in the deshielding region of the nitrile or aromatic *-electrons. The MM2 calculations (Table V) indicate that the potential energy surface for sandwich pairs of P N molecules is very shallow, but that head-to-tail geometries (Figure 2) are more stable than are head-to-head geometries and slipped geometries are more stable than fully eclipsed. These geometrical preferences are consistent with the Hunter-Sanders4 model for ?T-T interactions which is based upon optimization of the attractive interactions between *-electrons and the a-framework. We have attempted to fit the concentration dependence of quantum yield data for conversion of P N to D N (Table I) to a simple model in which the PN monomer and a dimer r-complex are the only light-absorbing species and it is assumed that DN is formed only via excitation of the *-complex (Scheme 111). According to this model and the additional assumption that the absorbance of the *-complex is twice that of the monomer, the quantum yield for dimer formation can be described by eq 4. The
best fit to the high-concentration data can be obtained for a value of K, = 0.75, somewhat higher than the value estimated from the ' H N M R data. The failure of the model to fit the low-concentration quantum yield data presumably is due to the inadequacy of the simplifying assumptions. Absorption of light by a *-complex either in a 77 K matrix or in solution is expected to yield an excited complex in which the excitation is localized on one of the two PN molecules (Scheme I).5 The similar lifetime for the excited complex and isolated monomer a t 77 K (Table IV) could provide an explanation for the observation of oxygen quenching of both the monomer fluorescence (Table 111) and photodimerization (Table I) to a similar extent. Dissociation of the weakly bound excited *-complex to yield 'PN* and PN would be expected to compete with relaxation to the excimer, in accord with the low value of aD = 0.094 obtained from the intercept of Figure 3. This would require that the excimer, once formed, undergo very efficient dimerization, as in the case of PE. High dimerization and cross addition efficiencies for the excimers and exciplexes of phenanthrene derivatives are consistent with the very high frontier orbital coefficients of the C9-Clo bond.29 Concluding Remarks. The differences in the photochemical behavior of the three phenanthrene derivatives PE, PN, and PA can be explained on the basis of their ability to form ground-state and excited-state *-complexes. Of the three, only PE undergoes self-quenching. The self-quenching rate constant, k , = 1.2 X lo9 M-l s-l, is slower than the rate of diffusion (kdiM. = 3 X 1O1O M-' SKI in dichloromethane solution), indicative of an enthalpy of (29) Caldwell,
R. A. J . Am. Chem. SOC.1980, 102, 4004.
2553
J. Phys. Chem. 1992, 96,2553-2561 excimer formation (ca. 4-5 kcal/mol) marginally larger than the entropy of a~sociation.'-~Since the singlet lifetimes of P N and PA are similar to that of PE, the failure to observe self-quenching for PN and PA must result from smaller enthalpies of formation (
