J. Phys. Chem. 1995,99, 17566-17572
17566
Exciplex Binding Energy and Kinetic Rate Constants of the Interaction between Singlet Excited State (Dibenzoy1methanato)boron Difluoride and Substituted Benzenes Yuan L. Chow* and Carl J. Johansson Department of Chemistry, Simon Fraser University, Bumaby British Columbia, Canada VSA 1S6 Received: July 11, 1995@
For exciplex formations of singlet excited (dibenzoy1methanato)borondifluoride (*(DBM)BF*) with a series of substituted benzenes (SB), their heat of formation (binding energy, -AHex) and kinetic rate constants were measured. In the vicinity of room temperature, the exciplexes from less substituted benzenes including xylenes were formed reversibly, as shown by oxygen perturbations of fluorescence intensity and time-resolved fluorescence analysis. For these exciplex formations, temperature dependent intensity changes were utilized to determine -AHex, which permits the calculation of the repulsive energy of the Franck-Condon ground state complex Re from exciplex energy. For other related exciplexes from irreversible formation at room temperature, these values were evaluated by extrapolations of the correlation with steric factors. The heat of formation becomes larger, the lifetime of the exciplexes becomes longer, and Lex becomes slower, as Eox(SB) becomes lower, i.e., more electron donating. The heat of formation is correlated with the stabilization energy of exciplexes depending on the extent of CT (charge transfer) and LE (locally excited) contributions; that is, either a >> c or c >> a in the exciplex wave function. In the high-CT region, A H e x Re originates primarily from the redox potential gap. An additional stabilization energy is developed for the exciplexes from trimethylbenzenes to toluene as the HOMO-HOMO gap becomes closer. In the weak CT (or high LE) contribution region, the (AHex Re) term becomes nearly a constant, and the exciplex fluorescence peaks all appears red-shifted about 0.25 eV from the *(DBM)BF* peak.
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Introduction
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In the first paper concerning the exciplex chemistry of singlet excited (dibenzoy1methanato)boron difluoride' (*(DBM)BF2), we investigated the solvatochromic shifts on a series of exciplex fluorescence peaks derived from substituted benzenes (SB) to afford the dipole moments, upon which the exciplexes are shown to possess primarily the charge transfer (CT) character (showing contact radical ion pair characteristics) and mainly locally excited (LE) contributions (showing similar fluorescence properties of *(DBM)BF2) on the basis of a two-state exciplex wave function.2-'0 In between the two extremes, exciplex formations are also driven by extra CT-LE state resonance interactions, (A-D+IH(*AD), from intermediate contributions; such stabilization energies (Us) are small in both extreme situations. The phenomena and driving forces could also be rationalized by the frontier orbital scheme4. We have used valence bond theory to illustrate the source of Usin high-CT and also LE contribution areas previously,',' which clarifies that the formation of these exciplexes was driven largely by the electrostatic attraction and some energy stabilization (Us)arising from resonance interactions IA-D+) (*ADI. This set forth the background to investigate other properties of these *(DBM)BF2 exciplexes in an endeavor to unravel the driving force and mechanism for the formation. To this end, we determined the binding energy and kinetic decay patterns for these *(DBM)BF* exciplexes. This paper is the continuation of the study that precedes this.' Figure 1 schematically describes the energetic relation of *(DBM)BF2 (excited state acceptor) interacting with a SB (donor) to form a stabilized exciple^^.^-^^^*^'^ by releasing -AHex; upon radiative or radiationless decay (hvmax).(DBM)BFz/SB reaches the Franck-Condon ground state; the complex (AD)Fc is shown to have repulsive energy Re and should relax along the ground state energy ~urface.~ In the ground state, DBMBF2
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Abstract published in Advance ACS Abstracts, November 15, 1995.
0022-365419512099- 17566$09.00/0
'(AD)
-
D
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'A
i r r DA
Figure 1. Potential energy scheme of (DBM)BFz/benzenesexciplexes.
shows weak tendency to form EDA complexes, showing Ka < 0.15 M-I. The small dip in the ground state surface represents this association, which is small and not significant for less substituted SB than for mesitylene. The minimum representation of the reaction scheme on the basis of the work from this lab~ratory'~ is given in Scheme 1. In polar solvent, it is assumed that the exciplex is partitioned into two types of reactions, i.e., electron transfer (radical ion) and cycloaddition reactions. Our aims are to clarify the size and source of binding energy -AHex and kinetic parameters shown in Scheme 1. Experiments Materials and Instruments. The same descriptions given in the previous paper' also apply here. Time-resolved fluorescence decays were performed using PTI LS-1 using singlephoton counting, and the details of the measurements are described in the previous publication.I 0 1995 American Chemical Society
Exciplex Binding Energy and Kinetic Rate Constants
J. Phys. Chem., Vol. 99, No. 49, 1995 17567
SCHEME 1
I
hv
Results Thermodynamics of Exciplex Formation. Under steady state illumination, *(DBM)BF;! in the presence of a SB in cyclohexane gave the parent, (cu. 400 nm) as well as exciplex (wavelengths above 500 nm) fluorescence. The ratio of fluorescence quantum yields of the two species could be related by that of the intensities, l a and lex. The reversibility of the exciplex formation can be tested using oxygen (air) as a perturbing quencher of the two states according toI5.I6 eq 1; biff[O2]was 5.9 x lo7 s-' where the oxygen concentration in cyclohexane was takenI7J8as M. The reversibility of the reactions are expressed by eqs l a and l b with required conditions that primarily depend on the sizei6of k-ex. It is clear that the relative size of k-ex with respect to k? k: determines the second term in equation 1, which provides the indicator for the reversibility as in eqs l a and lb. The ratio of fluorescence intensity quenching at room temperature by air for both species was easily determined at 21 and 41 f 3 "C. In Table 1, the ratio c1.05 was noted as fully reversible cases at room temperature; for mono- and disubstituted benzene cases, the ratio became almost 1.OO at 41 "C and was unambiguously reversible.
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370
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470
570
370
470
570
Wavelength (nm)
Figure 2. Variation of fluorescence profiles of (DBM)BFz (4.3 x M) in the presence of SB at various temperatures in cyclohexane under argon: (left) SB = 4-rert-butyl-o-xylene (0.18 M) at 14.5, 2 1.6, 29.1, 35.6, 40.1, 46.3, and 54.9 C from a to g; (right) SB = 1,2,4trimethylbenzene (0.18 M) at 13.3, 17.7, 22.0, 29.0, 35.8, 41.5, 46.4, 53.5, and 59.8 C from a to i.
temperature between 12 and 60 "C. Of necessity, some tertbutyl-substituted benzenes were also used for determinations to apply well-understood steric effects. The fluorescence intensities at 390 and 550 nm were taken as la and lex, respectively, and plots according to eq 2b were constructed for various SB. Examples are shown in Figure 3. The benzene derivatives with less than two methyl groups or with a tertbutyl substitution gave straight lines with positive slopes (=-AHex/R) in the temperature range required for reversible exciplex formations,I2.l9 as in Figure 3a. For electron rich benzenes, such as PMB (pentamethylbenzene) and HMB (hexamethylbenzene), the exciplex formation would be strongly irreversible (i.e., k-ex 2 M. These parameters are listed in Table 3. As expected, kexquickly reached kd,ff even in interactions with xylenes and more substituted SB. Beyond
lo9 s-l confirms the full reversibility of the exciplex formation and provides justification for the calculation of the binding energy by eq 2b. The kinetic results are in agreement with the oxygen quenching results and directly define the range of reversible exciplex formation up to xylenes in the ambient temperature range. Therefore, up to xylenes experimentally determined equilibrium constants and binding eneriges of the exciplex formation (Tables 1 and 3) provide reliable data.
Exciplex Binding Energy and Kinetic Rate Constants CAlP
3.4
1
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High lo Medium CT-
LOW
J. Phys. Chem., Vol. 99, No. 49, 1995 17571 Cl
E, (DBMBF,)
TABLE 4: Thermodynamic Properties of Some (DMB)BFd Benzene Exciplexes in Cyclohexane under Argon benzene benzene to1uen e o-xylene m-xylene p-xylene mesitylene tert-butylbenzene p-tert-butyltoluene 5-tert-butyl-m-xylene
‘red
(”)
Figure 6. Plot of the exciplex fluorescence maxima as a function of redox energy differences (cited from ref 1).
The determination of binding energy allows us to calculate the Franck-Condon ground state energy Re from the relation in Figure 1 and further, by the correlation with Charton’s steric parameters,21 provides access to Re of exciplexes with high-CT characters up to contact radical ion pairs. Higher Re values, by 1.3 kcal/mol, for those carrying a corresponding rert-butyl group serve to demonstrate steric effects imposed by the bulky group with respect to a methyl group. Steric effects of tert-butyl groups on the exciplexes are more complex to determine and are being investigated now. The qualitative relation of electron-donating properties, E,,(D), with respect to Ered(A)is readily perceived from binding energies and rate constants of exciplex formation. The thermochemical energies of exciplex formations are intimately related to the electronic structure; in Figure 6, the plot of hv,,, vs E,,(D) (i.e. the HOMO-LUMO gap) is reproduced from the previous report’ to visualize the driving force for the exciplex formation over the wide variation of contributions in Ye, = alA-D+) cl*AD). A summary of published rep ~ r t ~shows ~ ~that . hvmax(eV) ~ * ~ ~ = E,, - Ered - 0.15 - Us = ECRIP- Us. The size of AH.., Re(=Es - hv,,,) is represented by “x” in Figure 6, which shows that for the high-CT exciplexes (those from tetramethyl-substituted or higher) this quantity coincides with the CRIP (contact radical ion pair) correlation line; that is, the electromotive force provides the entire driving force for the formation, as demonstrated by the correlation with E,, - Ere,+ This also means no extra stabilization, Us = 0, in the range from trisubstituted benzenes to chlorobenzene, whereas the size “x” decreases proportionally as the CT contribution tapers to 30% (and the LE contribution increases to 70%).An additional stabilization energy US arising from state resonance interaction, IA-D+) c-, I*AD), appears as “y” in Figure 4. We have discussed that Us is proportional to the overlap integral S and inversely proportional to the HOMO-HOMO gaps’v5 AE, e.g., Us = p/AE;as S is relatively small, it must be a small AE that is enhancing U S . Beyond chlorobenzene is the region of low-CT contributions; the Ye, is primarly contributed by I *AD), and hvmaxis relatively independent of redox potential differences and show a horizontal plot sightly below E, of *(DBM)BF2 by US Re, which is about 0.25 eV from that in Figure 4; the former is insignificant, and the latter is nearly constant. In this region exciplex hvmaxand shape are essentially those of *(DBM)BF2 from (*ADIHI*AD) E (*AIHI *A) = Es. Qualitatively, these exciplex fluorescences borrow the pattem and nature from those of *A. Similar cases have been published.28 This is indeed observed clearly from
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-AGex (kcal/mol)
0.06 f 0.11 0.89 f 0.08 2.07 f 0.08 2.01 f 0.08 2.69 f 0.07 3.22 f 0.07 0.31 f 0.07 1.99 f 0.07 3.01 f 0.07
- M e x
(kcaVmo1) 2.5 f 0.2 3.2 f 0.2 4.4 i 0.3 4.5 i 0.3 5.1 & 0.5 5.6 i 0.5 2.4 f 0.2 3.7 i0.3 4.3 i .3
-Asex (eu) 8.2 i 0.6 7.8 i 0.6 7.9 0.8 8.4 i 0.8 8.1 i 1.2 8.0 & 1.2 7.1 i 0.6 5.8 f 0.8 4.4 i 0.8
*
the fluorescence spectra of the cyanobenzene exciplex, which shows vibrational structures and near overlap similar to those of *(DBM)BF2. These interactions under various CT and LE contributions have been discussed in terms of valence bond and frontier orbital theories in the previous paper. Returning to Table 3, it is possible to calculate the free energy of exciplex formations from AGex = -RT In Kex; in tum the corresponding entropy can be ~ b t a i n e d ’ . ~ , ’ ~from , ’ ~ ,-Ace, ~~,~~ =-AHex TASex. These thermodynamic parameters are given in Table 4. Weller’s g r ~ u p ~ has , ~ , demonstrated ’~ that for exciplexes with large dipole moment (pex > 10 D) AGex and AHe, are linear functions of E,, - Ered with unit slope with constant -ASex = 18 eu. Calculated -AS in Table 4 are much smaller, but nearly constant at about 8 eu for benzene to mesitylene in spite of variations in their AHe, and AG,,. As the plot in Figure 6 starts to deviate from the CRIP correlation from trisubstituted benzene exciplexes and onward, these (most likely the whole series *(DBM)BFz/SB exciplexes obviously do not obey the Weller’s relation.
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References and Notes (1) Chow, Y. L.; Johansson, C. J. J . Phys. Chem. 1995, 99, 17558. (2) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, CA, 1978. (3) (a) Beens, A.; Weller, A. Acta. Phys. Pol. 1968,4,593. (b) Knibbe, H. Ph.D. Thesis, Free University of Amsterdam, Sept 1969. (c) Beens, H. Ph.D. Thesis, Free University of Amsterdam, Jan 1970. (d) Beens, H.; Weller, A. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: New York, 1975; Vol. 2, p 159. (4) (a) Weller, A. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975. (b) Leonhardt, H.; Weller, A. Ber. BunsenGes. Phys. Chem. 1963, 67, 791. ( 5 ) (a) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Phys. Chem. 1991, 95, 2068. Chung, W.-S.; Turro, N. J.; Gould, I. R.; Farid, S. J . Phys. Chem. 1991, 95,7752. (b) Gould, I. R.; Farid, S. J . Phys. Chem. 1992, 96, 7635. (c) Gould, I. R.; Gomez-Jahn, L.; Goodman, J. L.; Farid, S. J . Am. Chem. SOC.1993, 115, 4405. (d) Gould. I. R.: Farid. S. J . Am. Chem. SOC.1993, 115, 4814. (e) Gould, I. R.; Farid, S. J . Phys. Chem. 1993, 97, 13067. (6) Weller, A. Z . Phys. Chem. N . F. 1982, 133, 93. (7) (a) Rehm, D.; Weller, A. Z . Phys. Chem. N . F. 1970,69, 183. (b) Beens, H.; Knibbe, H.; Weller, A. J . Chem. Phys. 1967, 47, 1183. (8) (a) Mataga, N.; Okada, T.; Yamamoto, N. Chem. Phys. Left. 1967, 1, 119. (b) Okada, T.; Matsui, H.; Oohari, H.; Matsumoto, H.; Mataga, N. J . Chem. Phys. 1968, 49, 4717. (c) Taniguchi, Y.; Nishima, Y.; Mataga, N. Bull. Chem. SOC.Jpn. 1972, 45, 764. (9) (a) Ware, W. R.; Richter, H. P. J . Chem. Phys. 1968, 48, 1595. (b) Kuzmin, M. G.; Guseva, L. N. Chem. Phys. Lett. 1969, 3, 71. (c) McDonald, R. J.; Selinger, B. K. Aust. J . Chem. 1971,24, 1797. (d) Talyor, G. N. Chem. Phys. Lett. 1971, 10, 355. (e) Ottolenghi, M. Acc. Chem. Res. 1973, 6, 153. (0 Tavares, M. A. F. J . Chem. Phys. 1980, 72, 43. (10) Michl, J.; Bonacic-Koutecky, V. Ektronic Aspects of Organic Photochemistry; Wiley: New York, 1990. (11) Johansson, C. J. Ph.D. Thesis Dissertation, Simon Fraser University, 1994. (12) Stevens, B. Adv. Photochem. 1971, 8, 161. (13) Mataga, N.; Ottolenghi, M. In Molecular Association; Foster, R., Ed.; Academic: New York, 1979. (14) (a) Chow, Y. L.; Cheng, X. Can. J. Chem. 1991, 69, 1575. (b) Chow, Y. L.; Ouyang, X. Can. J . Chem. 1991,69, 423. (c) Chow, Y. L.; Wu, S. P.; Ouyang, X. J . Org. Chem. 1993, 59, 421. (d) Chow, Y. L.; Wang, S. S.; Cheng, X. Can J. Chem. 1993,71, 846. (e) Liu, Z.-L.; Zhang,
17572 J. Phys. Chem., Vol. 99, No. 49, I995 M.-X.; Yang, L.; Liu, Y.-C.; Chow, Y. L.; Johansson, C. I. J . Chem. SOC., Perkin Trans. 2 1994, 585. (15) (a) Cohen, M.; Selinger, B. Mol. Phorochem. 1969, 1, 371. (b) McDonald, R. J.; Selinger, B. K. Mol. Photochem. 1971, 3, 99. (16) Caldwell, R. A,; Creed, D.; DeMarco, D. C.; Melton, L. A,; Ohta, H.; Wine, P.H. J . Am. Chem. SOC.1980, 102, 2369. (17) (a) Chow, Y. L.; Johansson, C. I. Res. Chem. Intermed. 1993, 19, 191; J. Photochem. Phofobiol. A: Chem. 1993, 74, 171. (b) Chow, Y. L.; Johansson, C. I. J . Chin. Chem. SOC.1993, 40, 531. (18) Murov, S. L. Handbook of Photochemistry; Marcel Dekker: New York, 1973. (19) Knibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 839. (20) (a) O’Connor, D. V.; Ware, W. R. J. Am. Chem. SOC.1976, 98, 4708. (b) O’Connor, D. V.; Ware, W. R. In 12th Informal Conference on Photochemistry; NBS Special Publication 526; United States Department of Commerce, NBS: Washington, DC, October, 1978. (c) O’Connor, D. V.; Ware, W. R. J. Am. Chem. SOC.1979, 101, 121. (d) Cheung, S . T.; Ware, W. R. J. Phys. Chem. 1983, 87, 466.
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