Interaction of Halide and Pseudohalide Ions with Triplet

J. Phys. Chem. 1988, 92, 5151-5159

5151

Interaction of Halide and Pseudohalide Ions with Triplet Benzophenone-4-carboxylate: Kinetics and Radical Yields John K. Hurley>,f Henry Linschitz,*s+and Avner Treininl Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254, and Department of Physical Chemistry, The Hebrew University, Jerusalem 91 904, Israel (Received: November 3, 1987; In Final Form: March 22, 1988)

The interaction between triplet benzophenone-4-carboxylate(3BC) and some inorganic anions, X-, has been studied by laser flash photolysis over a wide range of anion concentration (up to 6 M), at pH 11 and constant ionic strength. Spectra and extinction coefficients of 3BC and BC’- radical were determined by using the benzophenone actinometer. Triplet lifetime measurements gave primary interaction rate constants, kqT(M-l s-’ ), close to diffusion controlled for iodide, thiocyanate, and azide ( k 2 X lo9), somewhat less for bromide (2 X lo8) and very low for chloride ( 0.1 M, ‘ p increases ~ with [X-] and equivalent amounts of BC’- and X2- radicals are formed, including N;. At high anion concentrations, plots of (OR-’ versus [X-]-’ are linear. Limiting values, extrapolated to [X-]-l = 0, range from 0.60 for SCN- to 0.06 for N3-. These results are interpreted, as in our earlier work on anthraquinonesulfonate, in terms of trapping a primary triplet exciplex by a second anion at high concentration to form a termolecular exciplex with low orbital momentum, which may dissociate to the observed radical products (intraradical spin-orbit coupling model). Competitive deactivation processes also occur from these exciplexes and possibly from other high-concentration interactions as well. The overall picture is supported by experiments with mixed anions (SCN- and Cl-) that permit separation of the primary and secondary processes and establish that the initial reacting species is the lowest ketone triplet.

-

Introduction Charge-transfer interactions of excited molecules with inorganic anions in aqueous solution are often associated with low quantum , free radicals. This was shown to be the case even yields, ‘ p ~ of for triplet excited states,I4 which usually give higher radical yields than singlets following interaction with organic reagents. Evidently, the charge-transfer intermediate in anion interactions is subject to an efficient dissipative process, competing with dissociation to radical products. For triplet excited states, this process must at some point involve intersystem crossing (ISC). Various mechanisms for inducing ISC were examined to account for the ~ the nature and concentration of the anions3 dependence of ‘ p on For a number of cases, “heavy-atom” effects and electron-nuclear hyperfine interactions could be ruled out, but spin-orbit coupling within the inorganic radical component of the exciplex was found to correlate well with the observation^.^ This led to the IRSOC (intraradical spin-orbit coupling) model, which emphasizes the function of such coupling in an incipient radical, in controlling the rate of the dissipative process. The model readily explains the striking increase in radical yield in the interaction of triplet anthraquinone-2-sulfonate (AQS) with CI-, Br-, I-, and SCN-, when the anion concentration is raised much above that required for complete quenching of the t r i ~ l e t . ~ Similar effects were observed in our early work on benzophenone-4-carboxylate3 and in recent studies on l ,4-naphthoquinone5 and iridium(II1) bipyridine complexes.6 In these cases, it is postulated that the primary exciplex is intercepted by a second anion at high concentration to produce a termolecular complex in which the spin-orbit coupling of the initial radical X is removed by its conversion to X< and consequent loss of orbital angular momentum. Spin relaxation to the ground state is thereby slowed, permitting radical separation. Flash photolysis experiments identified both organic and X, radicals as the reaction product^.^ The “high-concentration” phenomena noted above are quite specific and should be clearly distinguished from environmental or ion-pair exchange effects observed with some photochemically produced ion pairs in organic Indeed, high concentrations of “inert” salts (e.g., NaCIO,) have little effect on the radical yield in the reactions discussed here. The occurrence of Brandeis University. *Present address: Department of Dermatology, Harvard Medical School, Massachusetts General Hospital, Boston, MA 021 14. $The Hebrew University.

0022-3654/88/2092-5151$01.50/0

such specific interactions emphasizes the need for better understanding of the chemical reactivity of exciplexes and, in particular, the role of termolecular exciple~es.~ We present here a detailed report on the interactions of triplet benzophenone-4-carboxylate with several inorganic anions. Because of its lower reduction potential, this triplet discriminates better than AQS among various anions, with respect to the quenching reaction, and thus provides an opportunity to study sequential processes involving mixed anion systems. A further new result of this work is the observation of diazide radical (N6-) formation at high azide concentrations. The effect of its low stability constant is discussed. In the present work, we attempt to minimize any complications caused by environmental or medium effects on transient spectra by using NaC104 to maintain constant ionic strength over very wide variations in concentrations of quenching anions.

Experimental Section Sodium halides, hydroxide, thiocyanate, and perchlorate were Fisher (certified) reagents; azide was from Aldrich (99%). Methylhydrazine (Aldrich) was distilled (bp 87.1-87.2 OC, uncorrected). Water was redistilled from alkaline permanganate and acid dichromate. 4-Carboxybenzophenone (Aldrich) was thrice recrystallized from ethanol. Titration in 25 vol 5% ethanol/water gave its pK, = 4.5. Studies described here were all done in alkaline media and involve only the benzophenone-4-carboxylate anion. We denote this anion itself as BC, and the dinegative anion radical as BC’-. The luminescence spectrum of BC at 77 K in ether/ethanol (3:l) glass, made alkaline with sodium ethoxide, was taken on a Perkin-Elmer MPF-4 spectrofluorimeter. Emission peaks were found at 419, 451, 486, and 530 nm, giving ET = 2.96 eV for the carboxylate triplet, close to that of benzophenone. The redox potential of BC at pH 11 was estimated from an irreversible cyclic voltammogram and was taken to be 30 mV less negative than the (1) Watkins, A. R. J . Phys. Chem. 1974, 78, 1885, 2555. (2) Treinin, A.; Hayon, E. J . Am. Chem. Soc. 1976, 98, 3884. (3) Treinin, A,; Loeff, I.; Hurley, J. K.; Linschitz, H. Chem. Phys. Lett. 1983, 95, 333, and references cited therein. (4) Loeff, I.; Treinin, A,; Linschitz, H. J . Phys. Chem. 1984, 88, 4931. (5) Loeff, I.; Treinin, A,; Linschitz, H., unpublished results. (6) Slama-Schwok, A,; Gershuni, S.; Rabani, J.; Cohen, H.; Meyerstein, D. J . Phys. Chem. 1985, 89, 2460. (7) Simon, J. D.; Peters, K. S . Acc. Chem. Res. 1984, 17, 277. (8) Goodman, J. L.; Peters, K. S. J . Am. Chem. Soc. 1985, 107, 6459. ( 9 ) Mattes, S. L.; Farid, S . Acc. Chem. Res. 1982, 15, 80.

0 1988 American Chemical Society

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Hurley et al.

The Journal of Physical Chemistry, Vol. 92, No. 18, 1988

observed peak at -1.40 V versus saturated calomel electrode (SCE), using a carbon paste electrode.1° This value is somewhat less negative than the half-wave potential of benzophenone itself at pH 11, -1.53 V versus SCE." For Ered(BC)versus normal hydrogen electrode (NHE), we obtain -1 . I 3 V . All experiments incorporating anions were run at pH 11.2 f 0.2, adjusted with NaOH. At this concentration, OH- has a negligible quenching effect on 3BC.'2 Ionic strengths were maintained constant at 4.8 or 6.0 M, depending on the anion being studied, with NaC10, as inert component. BC concentrations were in the range 0.25-1.3 mM, giving absorbances of 0.04-0.2 at the laser excitation wavelength (extinction coefficient of BC = 160 M-' cm-' at 347 nm, in 6 M NaClO,, and at pH 11). Generally, BC absorbances were about 0.1, and initial flash transient densities (UTa at 535 nm) up to about 0.2 (M triplet) could easily be reached. The flash photolysis apparatus, utilizing a Q-switched, frequency-doubled ruby laser, pulsed xenon measuring source, and associated equipment, has been described earlier.I3 The laser was operated at constant primary output, well above threshold. Flash excitation intensities were varied, with no change in beam profile, by interposing a series of Corning glass filters (5-57 or 7-60) in the frequency-doubled beam, and were measured by deflecting part of the beam to a fast photodiode and linear pulse-integrating circuit.I4 Solutions were flashed in rectangular cells, 25 mm (along the measuring axis) X 8 mm deep, backed by aluminum foil to further improve constancy of excitation flux across the measuring beam. Samples were bubbled with nitrogen or saturated with oxygen, as desired. Solutions were replaced after every few flashes, and unwanted UV light was filtered from the measuring beam. Quenching constants (k:) were determined from the variation of observed first-order triplet decay rate constants with anion concentration: kobsd= ko + k:[X-]. Very good straight lines were obtained for k , > lo8 M-' s-l The extinction coefficients of 3BC (535 and 545 nm) and BC'(660 nm) were determined actinometrically in 6 M NaC10, solution, pH 1 1 , by the "limiting slope" rnethod,l5 referred to benzophenone triplet in benzene (eT = 7220 M-' cm-' at 530 nrn),I5 by utilizing the relation

2 [ =

dL&/dE

€ref

(1

- z/zdref

d A D r e r / d T l _ [ - fx (1 - I/Zo)x

1

Here the 0values are initial flash transient absorbance changes at appropriate wavelengths of BC and benzophenone solutions, respectively, E is flash energy, and the slopes are corrected for any small difference in exciting-light absorption at 347 nm. The quantum yields of both ketone triplets were taken to be ~ n i t y . ' ~ - ' ~ The radical anion BC'- was obtained by trapping the triplet with M methylhydrazine, which reacts by charge transfer (rate 3X constant = 2 X IO9 M-' cm-l ) to give BC'- directly, without the intermediate step of H-atom transfer followed by proton loss. Comparison with reduction by 4-carboxybenzhydrol in alkaline medium has shown that methylhydrazine reduction of 3BC gives M BC, in 0.1 M KCI (10) Cyclic voltammograms were taken at 6 X at pH 11 and 100 mV/s scan rate. The 30-mV correction assumes that the irreversibility is due to instability of the ketyl anion radical on the polarographic time scale. We thank Prof. Alan Stolzenberg for this measurement. (11) Elving, P. J.; Leone, J. T. J . Am. Chem. SOC.1958, 80, 1021. (1 2) The quenching of "BC by OH-is measurable by the present method, and the rate constant is given in Table I. However, the reaction at high [OH-] is complicated by growing-in phenomena not seen for the other anions. Our studies at high [OH-] are being published separately. (13) Andrews, L. J.; Deroulede, A.; Linschitz, H. J . Phys. Chem. 1978, 82, 2304. (14) Davis, C.; Hodginson, K. A. J . Phys. E : Sei. Instrum. 1972, 5, 544. ( 1 5 ) Hurley, J. K.; Sinai, N.; Linschitz, H. Photochem. Photobiol. 1983, 38, 9. (16) Inbar, S.;Linschitz, H.; Cohen, S. G . J . Am. Chem. SOC.1980, 102, 1419. ( I ? ) Inbar, S.; Linschitz, H.; Cohen, S. G . J . Am. Chem. SOC.1981, 103, 7323. (18) Moore, W . M.; Hammond, G. S.; Foss, R. P. J . Am. Chem. Sor. 1961, 83, 2789.

Benzophenone -4-carboxylate a

Radical anion

0

Triplet

01

I

500

400

600

700

X , nm

Figure 1. Absorption spectra: (0)triplet benzophenone-4-carboxylate, jBC, in 6 M NaC104, pH 11.3, [BC] = 0.80 mM, normalized at 535 nm; (0)radical anion, BC'-in 6 M NaClO,, pH 11.6, [BC] = 0.61 mM with 5.5 m M methylhydrazine, normalized at 660 nm. I

AD

I

I

I

,

I

Determination of 6(3CB) ond E(CB,-) > ,/,

0 201

1 I

0 3BC, 6 M NaC104

X = 535 nm benzene, X : 530nm (347)Abs E 0 121 B C - in 6 M NaC104 (347)Abs = 0 I62 X = 660nm

0 BP in

0 05

1

,

I

2

I

I

I

3

4

5

I

Laser energy, arbitrary units

Figure 2. Determination of extinction coefficients of 'BC and BC'- by limiting slope method, referred to benzophenone in benzene; flash transient absorbance, AD, versus flash energy (arbitrary units): I ( 0 ) benzophenone in benzene, A (347) = 0.121, X = 530 nm;I1 (0) BC in 6 M NaC104, p H 11.5, A (347) = 0.162, X = 535 nm; 111 (A)as in 11, with 3 mM methylhydrazine, X = 660 nm.

BC'- with primary quantum yield = 1.0 f 0.05.'9 In previous flash measurements of radical yields from triplets, we could directly compare the initial triplet and final radical absorbances resulting from the same f l a ~ h . ' ~ However, ?'~ the present study is mainly concerned with experiments at such high quencher (reductant) concentrations that no initial triplet can be seen within our time resolution (-30 ns). In this region, radical yields (a) were also determined by the limiting slope method, using BC itself in 6 M NaC104 as actinometer solution, and the triplet and radical extinction coefficients at 535 and 660 nm, respectively, as determined above. In cases where very low yields rendered extrapolation of ALI versus E plots impractical, a was obtained from measurements of ORo at a high excitation energy in solutions containing reductant, combined with values of ADTO at the same laser energy (read from a calibration curve) for matching solutions without reductant. Identification of transient species was facilitated by comparing solutions bubbled with N, and 0,. With N,, using enough quencher to trap all the triplet, we observe initially the combined (19) Kodwin, P. Ph.D. Thesis, Brandeis University, 1984.

The Journal of Physical Chemistry, Vol. 92, No. 18. 1988 5153

Triplet Interactions with Anions absorbance of any BC'- and inorganic radicals. With 02,BC'disappears rapidly, permitting observation of the relatively long-lived inorganic radical spectrum alone.4

TABLE I: Rate Constants for Quenching of Triplet BC by Anions and Their Redox Properties (versus NHE) ~

Results I . Triplet and Radical Spectra. The absorption spectra of triplet benzophenone-4-carboxylate (3BC) and its radical anion (BC'-) in 6 M NaC104 are shown in Figure 1. Spectral shapes and peak positions in the visible region agree well with those found p r e v i ~ u s l y 'in~ dilute phosphate buffer, with new maxima now observed at 350 nm for the triplet and 360 nm for the radical. The triplet peak is slightly shifted from 535 to 545 nm in passing to 6 M salt. Figure 2 gives representative results illustrating the limiting slope method. From such plots we obtain, for the molar decadic extinction coefficients (M-l cm-I), c(~BC)= 5200 (545 nm), 5000 (535 nm), 8800 (350 nm); t(BC'-) = 7100 (660 nm), 14300 (360 nm); all 4~5%.For comparison, c(~BC)given previ~usly'~ in dilute buffer (peak at 535 nm, and now corrected for a change from 7630 to 7220 in extinction coefficient of the reference benzophenone triplet)I5 was 6250, while e(BC'-) at 660 nm, similarly corrected, was 7660. II. Flash Photolysis of BC in Aqueous Media. The limiting first-order decay constant for 3BC in aqueous media, with or without added inert salt, is (6 f 1) X lo4 s-l (measured at pH 10). At concentrations and flash intensities used in this work, T-T interactions reduce the initial lifetime to 8-10 1 s . Decay of the triplet with no added quenchers leaves a long-lived residual species, unambiguously identified by its spectrum as BC'- (pK = 8.2;17 pH >9), with quantum yield 0.08 f 0.02. Studies over a range of BC concentration show a slight quenching of the triplet by the ground state ( k 5 X lo6 M-' s-l). However, it does not appear that radical formation can be assigned to this process, since the quantum yield remains constant (f25%) over an %fold variation in BC concentration (0.22-1.8 mM). Similarly, no marked effect was found when initial triplet concentrations were varied IO-fold by attenuating filters in the exciting laser beam. While the reaction certainly does not involve biphotonic excitation, the possibility of a T-T reaction leading to some radical formation cannot yet be entirely excluded, since at very low flash energies (where transient decays can be followed only over limited times) it becomes difficult to distinguish firstand second-order kinetic components, and errors in measuring low radical yields become even larger. Within the stated uncertainty, the radical yield was also independent of the presence or absence of inert salt (6 M NaCI04) and was the same in H 2 0 and D20. Despite these latter results, we tentatively attribute this radical formation, in absence of other possibilities, to direct reaction of 3BC with water. Related observations of even higher ketyl radical yields from triplet benzophenone in water have been interpreted in this way."J1 In any case, this relatively slow reaction introduces no complication in discussing processes occurring at high reductant concentrations, where triplet decay is entirely dominated by fast reactions with added quenchers. This is our main concern in this paper. III. Reactions of 3BC with Anions. A . Low Anion Concentration. At low concentrations, various halide and pseudohalide anions (X-) quench 'BC with little or no radical formation. Production of radicals via the triplet-water reaction is correspondingly suppressed by the competing triplet-anion process (see Discussion). Table I gives bimolecular quenching constants, kqT; thiocyanate, iodide, and azide quench at close to diffusion-controlled rates (k: > lo9 M-' s-I ), bromide is somewhat less active, hydroxide still less, and chloride almost ineffective. Sulfate, nitrate, and perchlorate (as sodium salts) do not interact with 3BC, even up to molar concentrations.

-

E(X-/X),b V

AGET*'

M

I-

1.33

-0.50

SCN-

1.62 1.32' 1.93 2.3d 2.6d

-0.21 -0.51

6.0 6.0 4.8

+0.10 +0.40

6.0

+0.7

5.0

X-' N< Br-

OH-

c1-

~~~~

ionic strength,'

5.0

10-9kqT, M-1 s-I 2.3 2.2 2.1

0.22 0.01 1 -8 x 10-5

'As sodium salt. *Values for I-, Br-, CI-, N3-, and SCN- from: Alfassi, Z. B.; Harriman, A,; Huie, R. E.; Mosseri, S . ; Neta, P. J. Phys. Chem. 1987, 91, 2120, and references therein. CPreviousvalues are 1.9 V (see, e.g., ref 27), which seems to accord better with other quenching studies2and with spectroscopic properties of N3-. dShizuka, H.; Obuchi, H. J . Phys. Chem. 1982,86, 1297. 'AGET* = E ( X - / X ) E,,,(BC) - ET(BC); E,,,(BC) = -1.13 V, ET = 2.96 eV, both from the present work. (NaCIO, used to maintain ionic strength as [X-] varied.

-

AD BC- and (SCN),-

I I

05!

Flash Transients

Bh

I

-

n- &??

O*OO

450

500

550

660

650

A , nrn

-A-*-A

700

750

'

0

Figure 3. Flash transient spectra in BC (1.3 mM) + NaSCN (6 M) solution: (0)initial spectrum in deoxygenated solution, (BC'- + (SCN),-); (A) residual spectrum after 40 ps in oxygenated solution

((SCN),-), pH 11.3. B. High Anion Concentration: Identification of Radical Products and Stoichiometry. With increasing anion concentration, the yield of BC'- radical passes through a broad minimum ( ' p ~ 0 around 10-3-10-2 M) for the strongly quenching ions treated here. At anion concentrations above about 0.1 M, BC'- radical formation then rises sharply, accompanied by an equivalent appearance of X, radicals, as shown below. The radical yield tends towards a saturation limit that varies with the ion. I . Thiocyanate. Figure 3 shows the transient spectra obtained by flashing BC solutions with high concentrations of sodium thiocyanate, bubbled with nitrogen and oxygen. The spectrum of the deoxygenated solution is given directly after the flash, while that containing oxygen is shown after a time sufficient to allow the BC'- radical to disappear. The initial spectrum displays the characteristic BC'- bands at 660 and 360 nm and a broad band around 500 nm. In the presence of oxygen, the latter absorption is long-lived and corresponds closely to the well-established spectrum of (SCN) klsc(I). In the analogous situation, group I anions at high concentration form X; radicals within the exciplex, again with zero orbital momentum, and we expect similarly that kfJ2) > krsc(’) and that Bfr(’) will be high. To account for these low yields, we therefore find it necessary to include reaction 6 in the mechanism given above. Further direct evidence for such a process is found in the observation that the radical yield in the 3AQS-azide reaction decreases strongly at N3- concentrations above 0.5 M.4 While k q ( ’ )and kIsc(’), Le., reactions 6 and 8, cannot be separated kinetically, the concept of two such independent quenching pathways is also helpful, in that it suggests detailed consideration of possible new reaction intermediates or complexes in reaction 6 that differ structurally and dynamically from 3(M-.-+X;). This must evidently be the case for anion systems that cannot form dianion radicals, either X; or XY-. But even when such radicals are stable, we may write termolecular exciplex structures that might be formally represented as resonance hybrids of 3(X-. M-. .X). Thus, in accord with the IRSOC model, the structure of 3(X. .M2-. .X) may lead to enhanced intersystem crossing and thereby quenching because of additional spin-orbit coupling in the second X radical. Moreover, the distinction between reactions 6 and 8 may become blurred if different structures of the termolecular exciplex can interconvert. The rate of such interconversion may then determine to what extent these structures act separately. An additional effect appearing at high concentration may be termed “nearest-neighbor quenching” or “static” exciplex formation. This results from interaction of the initially excited singlet with an immediately adjacent molecule. Under these conditions, second-order kinetics become first order, and transitions directly from the singlet exciplex to the ground state will cause a drop in the radical yield. Indeed, such a process may explain the slight but definite decrease in ‘ p that ~ is observed at high chloride or thiocyanate concentration in the AQS ~ y s t e m .Any ~ corresponding effect in the BC case is small and does not influence the extrapolation of p-’versus [X-]-’, in Figure 9. The effect of such “static” exciplex formation on the absorption spectrum will evidently depend on the strength and range of the interaction. Qualitatively, one would expect a broadening on the long-wave side of the absorption that would be difficult to detect. Table 11 also gives data on strengths of spin-orbit coupling, tx,within the several inorganic radicals X. Since theory3spredicts direct proportionality between kIsc and lx2,Table I1 includes values of the ratio ~ l o p e / < For ~ ~ .the ions I-, Br-, and SCN-, which have comparable X2- formation constants (Table 11), the slope increases with lX, but lacking more information on Of,(’) and particularly on kx, one cannot fully establish the real dependence from these results. We note that kx should depend of klsc(I)on lX2 not only on the rate of X- X X2- (as considered earlier)4 but also on the binding between X or X2- and M-, in the exciplexes. 2. Azide Ion. In the high-concentration region, the behavior of azide as a group I anion toward 3BC is expressed as a rise in & i with increasing [N3-j, with concomitant formation of N6radical, in contrast to the decrease in a found with 3AQS. Since k-x for azide must be comparable to kx (Kq is only 0.33 M-I). it is useful to consider the full expression for ‘ p (eq ~ 14) and discuss two extreme cases: (a) kfr(’)>> kxeff[N3-]. In this situation, eq 14 reduces to

-

.

+

= 1

-

+ klsc(l)/kfr(’)+ (k,(’)/kfr(’))[N3-]

(16)

which was found to apply to the reaction between azide and 3AQS.4 (b) kfr(’)