Journal ofPhotochemistry and Photobiology, A: Chemistry, 50 (1989) 1 - 9 1

QUENCHING OF KETONE TRIPLET STATES BY SILANES

U. MiiLLER

Technical University “Carl Schorlemmer”, Leuna-Merseburg, Chemical Department, Otto-Nuschke-Strasse, 4200 Merseburg (G.D.R.)

W. HELMSTREIT

Central Institute of isotope and Radiation Research, Academy of Sciences of G.D.R., Permoserstrusse 15, 7050 Leipzig (G. D. R.)

H.-J. TIMPE

Technical University “Carl Schorlemmer”, Leuna-Merseburg, Chemical Department, Otfo-Nuschke-Strasse, 4200 Merseburg (G.D.R.)

(Received February 6, 1989)

Summary

Triplet state quenching of the ketones benzophenone, p-methylbenzo- phenone, p-phenylbenzophenone and benzil by silanes containing an Si-H bond was investigated in a 9:l (v/v) mixture of acetonitrile and acetone. It was found that both the n,r* and n,r*- r,r* triplet states are quenched if their energies are above 280 kJ mol-‘. Quenching rate constants I+ were determined using steady state phosphorescence quenching and the time- resolved measurement of the triplet-triplet absorption. From an analysis of the behaviour of the triplet-triplet absorption of benzophenone as a func- tion of time at variable silane concentrations, a reaction scheme was derived which includes the combination reactions of diphenyl ketyl, solvent and silyl radicals and the addition of silyl radical to the ground state of benzo- phenone. leH values obtained in this way agree with the phosphorescence quenching data and are of the order of 3 X lo6 to 3.4 X lo* dm3 mol-’ s-l. Furthermore, it was found that the log -It, values depend on the magnitude of the wavenumber of the Si-H vibration of the silanes.

1. Introduction

It is known that the triplet state of ketones reacts with the Si-H bond of silanes to form silyl and ketyl radicals by hydrogen atom abstraction [ 1,2]. The rate constants for triplet quenching of benzophenone by various organosilanes in benzene have already been investigated. Values from 5 X lo6 to 9 X lo6 dm3 mole1 s- ’ have been determined using the silanes (CH3CH2)3SiH, n-C5HIlSiHs, C6H5SiH3 and Cl,SiH [2]. In this work triplet

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quenching of benzophenone by silanes was also studied using silanes with different substituents (R, , R2, R3)

kn “Ph,CO* + R,R2R3SiH - Ph&OH. + R1R2R3Si- (I)

If any other mode of quenching is involved in addition to hydrogen abstraction (eqn. (l)), then this other process will be included in the measured rate constant kn. With this assumption a change in the kH value represents a change in the rate constant of the hydrogen atom abstraction reaction, which can be described 13, 41 by

log Izn a M = D(Si-H) - D(O-H) - ET -E, (2a)

where AEl is the enthalpy of the reaction, D is the energy of dissociation of the Si-H and O-H bonds (ketyl radical), ET is the energy of the triplet state and E, is the bond energy of the carbonyl7~ bond.

In the quenching of 3Ph&O* by various silanes, D(O-H), ET and E, are kept constant and eqn. (2a) is reduced to

log 72n a D(Si-H) (2b)

Unfortunately, most of the Si-H bond energies of the silanes used here are not available in the literature. The values given for (CH3)&M-I, 339 kJ [5], 355 kJ [6], 376 kJ 173, d o not allow us to make a satisfactory correlation between the kn value and the Si-H bond energy, It is known that the Taft constants e* can be related to the wavenumber of the Si-H vibration [8,9]. If a linear free-energy relation between these quantities is assumed, lzw should depend on the wavenumber of the Si-H vibration.

We also investigated the influence of different triplet energies and states on the rate constant of the triplet quenching by (CH$SiO)$H$iH (5) (cf. Table 1). Because of the good solubility of the silanes, all experiments were carried out in a 9: 1 (v/v) mixture of acetonitrile and acetone at room tem- perature .

2. Results and discussion

To demonstrate the formation of ketyl radical as a result of reaction (l), the absorption spectra of the ketyl radical in the systems benzophenone-benzhydrol and benzophenone-silane 5 were recorded in the acetonitrile-acetone mixture. Figure 1 shows the absorption spectra obtained 30 ps after a 15 ~1s flash. Under these experimental conditions, 3Ph2CO* is completely converted into the diphenyl ketyl radical, Ph&OH.. In comparison with the system benzophenone-silane 5, it can be seen that in benzophenone-benzhydrol twice the quantity of ketyl radicals is generated; this is in agreement with the fact that quenching of the benzophenone triplet by benzhydrol yields only the ketyl radical [IO]. The absorption maxima at 530 nm and 525 nm agree with the value of 530 nm observed in

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TABLE 1

Rate constants 12~ of benzophenone triplet quenching by organosilanes and comparison with values given in ref. 2

Silane hH x lo+ (dm3 rnol-l s-l)

Method/solventb

1 (CHs)sSiOSi(CH& <O.l 2 (CzH5)4Si co.1 3 (GHsO)&i 1 4 (CzHsO)$iH 1.7 5 (CH3SiO)&HsSiH 3.4; 4; 2.5 6 (CH&C&SiH 8.8 7 C&sSiHs 5a 8 n-C5Hr1SiHs 8.8a 9 (GH&SiH 24; 9.6a

10 (( CH&SiCH2)2CHsSiH 27 11 (n-C&)&!HsSiH 30 12 ((CHs)sSiCH,)(CHs),SiH 49 13 ((CHs)sSi)&H 340 14 (CHs)sSiO(SiHCHsO),Si(CH& 0.55

(n = 50)

P/SH PfSH P/SH P/SH P/SH; LP/SH; P/B P/SH LP/Ba LP/Ba P/SH; LP/Ba P/SH P/SH P/SH P/SH

P/B

aValues obtained from ref. 2. bP, phosphorescence; LP, laser photolysis; B, benzene; SH, 9:l (v/v) solvent mixture of acetonitrile and acetone.

450 500 550 A,nm Cs ’ mot dm -3

Fig. 1. Transient absorption spectra of 5 X 10h3 mol dm‘-3 benzophenone solutions con- taining 0.1 mol dmA3 benzhydrol (a) and 0.1 mol dmB3 triethylsilane (b) in 9 :l (v/v) mixture of acetonitrile-acetone 30 I.CS after a 15 ps flash.

Fig. 2. Transient phosphorescence intensity I of benzophenone (a), p-methylbenzo- phenone (b), p-phenylbenzophenone (c) and benzil (d) us. (CH#iO)$H&GiH (5) concen- tration in a 9:l (v/v) mixture of acetonitrile-acetone (IO is the intensity without silane).

acetonitrile [ 11). Rate constants of phosphorescence quenching of benzo- phenone can be deduced from the slope of the Stern-Volmer plot (as shown in Fig. 2) using the measured 7p value of 14 ps. The rate constants obtained in this way and the kH values already published are summarized in Table 1.

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It is obvious that silyl-substituted silanes have the highest reactivity towards 3Ph,CO* followed by methylene-, phenyl-, siloxyl- and ethoxy- substituted derivatives. An exception to this relation is found for the silane 14. The lower reactivity of 14 may be caused by steric hindrance of the hydrogen atom abstraction from the Si-H bond. The reactivity of the silanes 1, 2 and 3, which do not have an Si-H bond, may be due to hydrogen atom abstraction from C--H bonds. The increased reactivity of the silane 3 in comparison with the silanes 1 and 2 is probable due to the slightly weakened C-H bond in the &ether position (the C-H bond energy in 3 is 383 kJ malli [12], whereas the C-H bond energy in 1 and 2 is 414 kJ mol-’ [ 131). Furthermore, using the measured kn value of silane 4, which is correct for the overall reaction, we can calculate 12n = 1 X lo6 dm3 mol-’ s-l for the Si-H abstraction in 4.

As already pointed out above, a correlation should also exist between the Jtu values and the wavenumber of the Si-H vibration. The plot of log kH us. the wavenumber of the Si-H vibration yields a straight line as shown in Fig. 3.

To investigate the influence of different triplet states on the kH value, quenching of the triplet state of benzophenone, p-methylbenzophenone, p-phenylbenzophenone and benzil by 5 was measured. The phosphorescence lifetimes rp of the compounds in the absence of 5 were determined to be 14 +- 0.5 I_CS, 12 * 0.5 ps, 19 +- 0.5 ps and 11 f 0.5 ps respectively. Employing these lifetimes, rate constants kH of 3.4 X 10” dm3 mall’ s-i for benzo- phenone and 2.7 X 106 dm3 mall* s-i for p-methylbenzophenone were deduced from the slope of the plot as shown in Fig, 2. It can be inferred from this that the mixed n,r*-rr,r* triplet state of p-methylbenzophenone has a lower reactivity towards this silane than that of the n,r* state of benzophenone, and both the 7~,7r* triplet state of p-phenylbenzophenone and the n,r* state of benzil do not react with silane 5 under these conditions.

Fig. 3. Semilogarithmic plot of the rate constant kH of benzophenone phosphorescence quenching vs. wavenumber of the Si-H vibration for various silanes (numbers on the straight line represent the silanes (cf. Table 1)).

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The hydrogen atom abstracting action of the n,r* triplet state of benzil and its very low reactivity towards triethylsilane (kH < 5 X lo4 dm3 molTi s-r) are known [Z]. Therefore it seems that the triplet energy of benzil is too low to abstract a hydrogen atom from silane 5. In addition, there is no fluores- cence quenching of benzil at the concentrations used (see Fig. 2).

In addition to the steady state phosphorescence quenching experi- ments, the behaviour of the triplet-triplet absorption of Ph,CO as a function of time at various concentrations of silane 5 was studied in the time-resolved regime at 530 nm. In this way it was possible to obtain information about the various reactions which caused the time profile observed.

As is known, the transient absorption of 3Ph,CO* and Ph,COH* is superimposed on this time profile. To simulate this, the reactions of 3Ph&O* (eqns. (3) - (5)) and Ph&OH- (eqns. (6) - (9)) must be taken into consideration. At elevated silane concentrations, the reaction of silyl radicals (RSi.) with Ph,CO (eqn. (9)) must also be considered. Because of the high benzophenone concentration (2 X IO-* mol dmp3), this reaction proceeds with pseudo-first-order kinetics in competition with radical combination (eqn. (7)). Th ere f ore reaction (9) also has an effect on the behaviour of the Ph,COH- absorption as a function of time.

3Ph&O* + O2 - product (3)

3Ph,CO* + SH - Ph&OH. + S. (4)

3Ph2CO* + RSiH --+ Ph&OH- + RSi- (5)

Ph$ZOH- + S- - product 1 (6)

Ph&OH- + RSi- --+ product 2

Ph&OH- + Ph&OH- __f pinacol

RSi- + Ph,CO - Ph&OSiR*

17)

(S)

(9)

where O2 is the remaining oxygen in the solution (3 X 10d5 mol dmm3 or less; bubbled with purest-grade nitrogen), SH is the solvent mixture and RSiH is the silane 5.

The behaviour of the 3PH,CO* decay as a function of time is given by the rate equation

dT/dt = p(t) - ks[Tl LO21 - k,[T] [SH] - kn[T] [RSiH] (10)

where T represents 3Ph2CO* and p(t) is the pulse shape of the nitrogen laser (related to the concentration of 3Ph2CO* generated). It is (5.5 f 0.5) X 10V5 mol dmp3 per laser pulse.

The behaviour of Ph&OH- (KR) as a function of time is approximated by the rate equations

d[KR]/dt = k,[T][SH] - (2k6 + 2k,)[R,]* (11)

Fig. 4. Benzophenone triplet quenching by (CHaSiO)zCHaSiH (5). Comparison of model with experimental data monitored at 530 nm. Numbers on the curves refer to the rate equations given in the text (concentration of silane 5 given in parentheses). (a) Full lines: (10) 3Ph$ZO* decay; (11) PhzCOH. build up; (i) summation of (10) and (11); (0) experi- mental time profile observed in a solution without silane; broken lines: numbers have the same meaning as above; (A) experimental time profile observed in 0.1 mol dms3 silane. (b) Numbers have the same meaning as above; full and broken lines refer to (e) 0.25 mol dme3 and (A) 0.5 mol dmW3 silane respectively. The curves are normalized to their maxima. The actual absorbance maxima are: (a), (e) 0.059; (A) 0.07; (b), (e) 0.061; (A) 0.061.

d[KR]/dt = kn[T] [RSiH] - (2h, + 2k8)[RJ2 - k9[Rz] [Ph&O] (12)

where RI = Ph&OH- = S. according to eqn. (4) and Rz = Ph,COH- = RSi- according to eqn. (5).

Equations (11) and (12) assume that the absorption at 530 nm is caused mainly by Ph$ZOH* . Equations (lo), (11) and (12) can be solved numerically using the Runge-Kutta method. The various components of these solutions are summed to give the time profile measured. Figure 4 depicts the results obtained at silane 5 concentrations of 0, 0.1, 0.25 and 0.5 mol dme3. This figure shows that the observed behaviour as a function of time can be approximated adequately by these rate equations. The rate constants k3, k4, k6 and k8 were obtained in a solution without silane. The value of k3 was determined independently in Ph,CO solutions with various oxygen concentrations. The value of k8 was taken from refs. 14 and 15. Values of k4 and k6 were obtained from the fit. The rate constants kH, k7 and k, were obtained in the presence of various concentrations of silane 5.

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product 1 pinacul product 2 Pt&SiR

Fig. 5. Benzophenone triplet quenching by (CH$3i0)&H3SiH (5) (RSiH) in a 9:l (v/v) acetonitrile-acetone mixture (SH). Reaction pathways and their rate constants (dm3 mole1 s-l) deduced from time-resolved measurements.

The values of these rate constants are shown in the reaction scheme depicted in Fig. 5. We can see that the IzH value obtained in this way agrees with those deduced from phosphorescence measurements. From this fitting procedure it can be seen (cf. Fig. 4) that up to a silane concentration of about 0.1 mol dme3 the time profile of the tail is largely due to the radical combination reaction described by eqn, (11). Above this concentration the radical combination reaction described by eqn. (12) becomes more and more dominant. This last feature can be explained by the reaction sequence given by eqns. (5), (7) and (9). According to eqn. (5), the RSi- and Ph&OH. con- centrations are increased if the silane concentration is increased. RSi- reacts with Ph,CO (eqn, (9)) with pseudo-first-order kinetics and the cross-reaction (eqn. (7)) is suppressed with time, since [RSi. ] < [Ph&OH* J_ It follows that the Ph,COH- concentration is increased with time (cfi Fig. 4(a) and 4(b)) and therefore the combination reaction (eqn. (8)) dominates. All the above discussions are supported by the fact that 3Ph,CO* without any additive has a lifetime of 1.6 ps under the experimental conditions used. Therefore the tail shown in Fig. 4 cannot be caused by incomplete triplet quenching of benzophenone.

The reaction pathways and their rate constants are summarized in the reaction scheme given in Fig. 5. The rate constant k4 was determined to be (8 + 0.5) X lo4 dm3 mol-’ s-l, which is higher than the values given in refs. 3 and 16 (k4 = lo3 dm3 mol-’ s-l) and ref. 17 (k2 = lo2 dm3 mol-’ s-l). How- ever, the lifetime of jPh2CO* in acetonitrile has been measured as 0.71 /.JS [18], which yields a h4 value of 7.2 X lo4 dm3 mold1 s-l. These deviations indicate a reaction which has not been considered. A contribution of the 3PhzCO* triplet-triplet annihilation could be responsible for the high value of the rate constant deduced. Within the framework of these investigations, the rate constant of the addition of RSi* to Ph&O could be determined only roughly as (5 + 3) X lo6 dm3 mole1 s- I. The order of magnitude of this rate constant agrees with values found for the addition of the triethylsilyl radical to carbonyl compounds [ 191.

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3. Experimental details

The experimental set-up used for phosphorescence quenching has been described previously [20]. The phosphorescence quenching of the carbonyl triplets by silanes was measured at the following wavelengths: benzophenone and p-methylbenzophenone at 450 nm, p-phenylbenzophenone at 440 nm and benzil at 560 nm. Excitation wavelengths of 360 nm, 350 nm, 370 nm and 410 nm respectively were employed. The phosphorescence lifetimes of the ketones were detected using the flash photolysis apparatus described in ref. 21. This experimental set-up was connected in an on-line regime to a microcomputer. The lifetimes measured at 460 nm were calculated by deconvolution of the response function with correction for the scattering light. The full width at half-maximum (FWHM) of the flash was 15 ps. The concentration of the carbonyl compounds was 5 X 10e3 mol dme3.

The investigations in the nanosecond time range were carried out using a nitrogen laser (FWHM = 2 ns) for excitation and an optical detection system consisting of a pulsed xenon lamp, a monochromator, a photodiode and an automatic back-off system. The signals were fed into a 250 MHz real time oscilloscope. The optical path length in the sample cell was 2 mm and the angle between the excitation light and detection light was 45” [22). The Ph,CO concentration was chosen to be 2 X IO-* mol dmA3.

The solvents and ketones used in this work were commercial materials which were purified by standard methods before use. The silanes were synthesized. (C,H,),SiH and (CzH50)3SiH were commercial substances. All the experiments were performed under oxygen-free conditions. During the time-resolved experiments, the sample flowed in a cycle consisting of the sample cell and the degassing vessels,

Acknowledgments

We are grateful to Dr. Popowski (Wilhelm-Pieck-University, Restock), Dr. Nietzschmann (Martin-Luther-University, Halle-Wittenberg) and Dr. Rijsler (VEB Chemiewerk Niinchritz) for supplying the silanes.

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