J. Phys. Chem. 1995,99, 9403-9407
9403
Kinetic Analysis by Transient Raman Spectroscopy of the Photoreduction of 2,2'-Bipyrimidine by Amines in Water 0. Poizat*J and G. B u n t i d Laboratoire de spectrochimie Infrarouge et Raman (LASIR), CNRS, 2 rue Henri-Dunant, 94320 Thiais, France, and Laboratoire de spectrochimie Infrarouge et Raman (LASIR), CNRS, USTLFA, 59655 Villeneuve d'Ascq, France Received: December 5, 1994; In Final Form: March 7, 1995@
Photoproduct formation and decay kinetics in the photochemical reduction of 2,2'-bipyrimidine (22BPMY) by aliphatic amines in H20 and D20 are determined by nanosecond, time-resolved resonance Raman spectroscopy. This analysis shows that the reaction scheme in aqueous solution differs from that observed in acetonitrile solution. The difference is accounted for by the existence of hydrogen bonding with the solvent in the excited state, which inhibits the formation of contact ion pair upon electron transfer.
1. Introduction In a recent investigation by nanosecond time-resolved Wvisible absorption and resonance Raman spectroscopies of the photochemical reactivity of 2,2'-bipyrimidine (22BPMY) in solution,' we have shown in particular that this aza-aromatic molecule is reduced via electron transfer in the lowest triplet state ( T I )by the amines 1,4-diazabicyclo[2.2.2]octane(DABCO) and triethylamine (TEA). In acetonitrile the main products of reduction by DABCO and TEA have been identified as the anion radical 22BPMY'- and the N-hydro radical 22BPMYH', respectively. From the analysis of the relative time dependences of the intensities of the photoproducts Raman spectra, we have concluded' that the reaction involves the formation of a very short-lived ion pair [22BPMY'-, amine '+I (reaction 1) although we have not detected it. In the reduction by DABCO this pair dissociates into the free, solvated ions (reaction 2). In the case of TEA rapid intrapair proton transfer from the amine cation TEA'+ (reaction 3) yields the radical 22BPMYH' and quenches almost entirely the dissociation process (reaction 4). The free ions which are nevertheless produced from this minor process also lead to proton transfer after diffusion (reaction 5). This reaction scheme can account for the double kinetics (a dominant, fast component and a weak, slow component) which has been observed experimentally for the appearance of the 22BPMYH' Raman spectrum on reduction by TEA. T I 22BPMY
+ DABCO/TEA -
[22BPMY'-, DABCO*+/TEA'+] (1) [22BPMY'-, DABCO'+]
-
22BPMY'-,,,,
+ DABCO'+,,I, (2)
[22BPMY'-, TEA"]
22BPMY*-,,,,
-
+ TEA",,,,
+ TEA*(-H+) 22BPMY'-,,,, + TEA*+solv -,22BPMYH' + TEA'(-H') 22BPMYH'
(3) (4)
(5)
Assignments of the T I state, anion radical, N-hydro radical, and N-deutero radical Raman spectra have been established' on the LASIR, CNRS, Thiais.
* LASIR, CNRS, Villeneuve d'Ascq. +
@Abstractpublished in Advance ACS Abstracts, May 15, 1995.
basis of a complete normal mode analysis of the ground state molecule by Net0 et In water, it has been found' that the N-hydro radical 22BPMYH' is the only product of reduction by DABCO as by TEA. In D20 the N-deutero analog, 22BPMYD', is produced. However, Raman measurements made in H20 at a very short time (coincident pump and probe pulses), in the presence of an excess of TEA, have revealed the existence of a precursor transient species close to the anion which has been tentatively ascribed to a short-lived ion pair similar to that produced in acetonitrile. In this regard, reactions 1-4 have been assumed to take place in water as in acetonitrile. However, in water the free 22BPMY'- species produced from dissociation of the ion pair (reactions 2 and 4) have been supposedly quenched by protonation by water as soon as produced, in such a way that the apparent decay of the ion pair could be summarized by reaction 6. al.233
[22BPMY'-, DABCO'+/TEA'+] 22BPMYH'
+ DABCO*+/TEA'+ + OH-
(6)
In this report we present a more detailed kinetic analysis of the reduction of 22BPMY by TEA and DABCO in H20 and D20 by transient Raman spectroscopy. Better results were obtained by this technique than by transient W-visible absorption for two reasons. Firstly, the minimum pump/probe time delay of our Raman experiment (10-20 ns) is notably shorter than that available with our transient absorption system (2150 ns). On the other hand, the different transient species which are involved in the reduction processes, that is, the triplet state T I , the anion 22BPMY'-, and the radicals 22BPMYH' and 22BPMYD', can be identified unambiguously from each other by their Raman spectra which are very specific but are hardly distinguishable by their W-visible absorption spectra, apart from the anion which presents a characteristic absorption in the 505-530-nm region. The results presented here suggest that, in water, electron'abstraction by the triplet bipyrimidine does not lead to formation of a reactive ion pair as in acetonitrile but rather to a less reactive solvent-separated ion pair.
2. Experimental Section 2,2'-Bipyrimidine-hs (Lancaster) and 2,2'-bipyrimidine-d6 (synthesis given in ref 1, deuterium content 97%) were sublimed in vacuo. 1,4-Diazabicycl0[2.2.2]octaneand triethylamine were from Prolabo. Triethylamine was purified by distillation. Water
0022-3654/95/2099-9403$09.00/0 0 1995 American Chemical Society
9404 J. Phys. Chem., Vol. 99, No. 23, 1995
Poizat and Buntinx
was bidistilled and deionized. D20 (CEA, '99.8%) was used as received. All solutions were deoxygenated by purging with argon. The nanosecond flash photolysis and transient Raman systems-both based on classical pump and probe optical arrangements-have been described in detail p r e v i ~ u s l y . ' ~ ~ ~ ~ Pump excitation at 248 nm (excimer laser Questeck 2040, 15 ns, 1.5 d) was in resonance with the strong SO S,, (.nn*) absorption of 22BPMY peaking at 240 nm in aqueous solution2 (E248nm 7500 M-I cm-I). The 355-nm probe excitation for Raman measurements was provided by a 10-Hz Nd:Yag laser (Quantel 581C, 8 ns, -1 d).The spectral resolution and the analyzed spectral field at 355 nm were about 9 and 1750 cm-I, respectively. The total jitter between the pump and probe pulses and the detector gate (20 ns) was better than 4 ns.
-
-
3. Results The main chemical intermediates involved in the photochemical reduction of 22BPMY at the nanosecond time level have been characterized by transient W-visible absorption and timeresolved resonance Raman scattering.' These are the lowest triplet TI state, the anion radical and its protonated form, the N-hydro radical (or the N-deutero radical in the case of deuterated water or deuterated alcohols as solvent). The 355nm Raman probe for investigating the kinetics of the reduction process was in resonance with a strong W absorption of the different transient species (Amax = 335,368, and 345 nm for TI 22BPMY, 22BPMY'-, and 22BPMYH', respectively'). The Raman spectra of these transients were thus all enhanced by resonance at this wavelength, and their kinetic evolutions could be studied simultaneously from the same measurements by plotting the intensity of Raman lines specific to each one of these species. Such specific Raman features thus needed to be chosen. In this regard Figure 1 shows reference Raman spectra for these transient species recorded with probe excitations in close resonance with the maximum of their W absorption bands (340 nm for T I , 370 nm for 22BPMY'-, and 355 nm for 22BPMYH' and 22BPMYD'). With comparison of these spectra, several Raman bands can be selected to identify each transient unambiguously. These are lines at 1485 cm-I for T I , at 1789, 1730, and 1572 cm-' for 22BPMY'-, at 1769,1560,1439,1286, 1237, and 646 cm-I for 22BPMYH', and at 1557, 1439, 1380, 1300, and 645 cm-' for 22BPMYD'. To investigate the mechanism of reduction of TI 22BPMY by amines in aqueous solution, a series of Raman spectra were recorded after photolysis of solutions of 22BPMY ( lop3M) in H20 and D2O in the presence of different amounts of TEA or DABCO and for various pump/probe time delays. A quantity of 5 % (v/v) acetonitrile (ca. 1 M) was added to the solutions to provide an internal reference of intensity. For each solution, the spectra recorded at different times were normalized with respect to the 920-cm-' line of acetonitrile. Then the probeonly spectrum was subtracted to eliminate the solvent bands. Figures 2 and 3 show two characteristic series of spectra obtained with D20/TEA solutions at several typical delays for M (Figure 2) and 3 x two concentrations of TEA: M (Figure 3). We examine now precisely these two examples of spectral evolution. In both cases, the final spectrum of the kinetics corresponds to the radical 22BPMYD'. The 10-ns spectra are the superposition of two groups of bands with relative intensities dependent on the TEA concentration. The first one (essentially the line at 1485 cm-') can be ascribed to the triplet state and the second one (in particular the 1577-, 1746-, and 1809-cm-I lines) resembles the anion spectrum. The spectra at intermediate delays in Figures 2 and 3 seem to result from
h Ln
m
4
2000
1500
1000
(CM-1)
500
Figure 1. Reference time-resolved resonance Raman spectra for the lowest triplet state (solvent H20, pump 248 nm, probe 340 nm, delay 20 ns), the anion radical (solvent CH,CN, electron donor DABCO (5 x IO-, M), pump 248 nm, probe 370 nm, delay 100 ns), and the N-hydro and N-deutero radicals (solvents H20 and D20, respectively, electron donor TEA (5 x M), pump 248 nm, probe 355 nm, delay 500 ns) of 22BPMY (lo-, M in all cases). The solvent lines (probeonly spectrum) are subtracted.
combinations of these three basis spectra. In order to verify this point and to accurately analyze the kinetics of the transients, all spectra were decomposed, as shown for example in Figure 4 for the 30-ns spectrum of Figure 2, by using the following processing. Subtraction of the 250-ns spectrum after normalization with respect to the 1300- and 1439-cm-' bands leads to a processed spectrum (trace B in Figure 4) where the contribution of 22BPMYD' is taken away. Subtracting this 30-11s processed spectrum from the 10-ns spectrum yields spectrum D in Figure 4 which corresponds unambiguously to the TI spectrum observed in pure water (see Figure 1). Finally, subtracting this TI spectrum from the 30-ns processed spectrum (trace B) after normalization relative to the 1485-cm-' band leads to trace C in Figure 4 which is close to the 22BPMY'- reference spectrum of Figure 1 but differs from it by several weak frequency shifts. This spectrum corresponds to that observed in our previous
Kinetics of Photoreduction of 2,2'-Bipyrimidine
J. Phys. Chem., Vol. 99, No. 23, 1995 9405
h
I I
I I
I
2000
I
I
1500
1000
(CM-1) 500
Figure 3. Time-resolved resonance Raman spectra (probe 355 nm) of a deaerated solution of 22BPMY M) and TEA ( 3 x lo-* M) in DzO at different times after 248-nm laser photolysis. All spectra are normalized with respect to the solvent lines, which have then been subtracted. Several typical band wavenumbers are indicated and vertical lines characterize typical bands of the TI state (solid lines), anion radical (dashed lines), and N-deutero radical (dotted lines).
h
In In d
2000
1500
1000
(CM-11
500
Figure 2. Time-resolved resonance Raman spectra (probe 355 nm) of M) and TEA a deaerated solution of 22BPMY M) in D20 at different times after 248-nm laser photolysis. All spectra are normalized with respect to the solvent lines, which have then been subtracted. Several typical band wavenumbers are indicated, and vertical lines characterize typical bands of the TI state (solid lines), anion radical (dashed lines), and N-deutero radical (dotted lines).
report for a H20/TEA solution with 0-ns pump/probe delay and was tentatively ascribed to the anion radical engaged in an ion pair. All the transient spectra recorded during the photoreduction process could be fir by the above procedure to a linear combination of the same three basis spectra (the TI state, the 22BPMYD' radical, and the "disturbed" 22BPMY'- anion spectra), with relative contributions depending on the pump/ probe delay. These contributions, obtained from the different coefficients of subtraction used in the decomposition process, are presented in Figure 5. In part A, which concerns the DzO/ M TEA solution, the three series of experimental data are arbitrarily normalized to unity at their respective maximum value. In part B, which refers to the 3 x lo-* M TEA solution, only data related to the final photoproduct, 22BPMYD', are
normalized to the maximum value. The anion and triplet state data are normalized relative to the radical ones using the same ratios as those applied in part A. Accordingly, although the relative resonance Raman cross sections for the triplet state and the anion and radical species are not known, the variations of the relative Raman contributions of the three transient species on going from one experiment to the other reflect variations in the relative concentrations of these species. For example, it can be concluded from a comparison of parts A and B in Figure 5 that the ratios of the initial concentration of TI 22BPMY (0) and of the concentration of 22BPMYD' (A)at the end of the reaction are comparable in the two experiments, whereas the maximum amount of 22BPMY'- (W) relative to the initial amount of TI in (B) is twice that in (A). The data in parts A and B of Figure 5 were fit using a pseudofirst-order consecutive two-stage reaction model (reaction 7):
T,22BPMY
k,
22BPMY'-
k2
22BPMYD'
(7)
with
k, = kq[TEA] and k2 = k',[D20]
(8)
where kg is the rate constant for quenching of T I by electron transfer from TEA and ,'k the rate constant for quenching of the 22BPMY'- anion by transfer of 2H+from DzO. The kinetics of appearance of TI from intersystem crossing from SIcan be neglected as the SIlifetime (a few tens of picoseconds6)is much shorter than the T I decay. According to this model, and assuming that Raman intensities are proportional to concentra-
Poizat and Buntinx
9406 J. Phys. Chem., Vol. 99, No. 23, 1995 I
1 .o
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0.5
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td
'1,
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x
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0
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,
50
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,
'
100
I60
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200
.
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Time / ns Figure 5. Raman intensity vs time of the triplet state (O),anion radical (W), and N-deutero radical (A)species produced by 248-nm photolysis of 22BPMY (10-3 M) in DzO solutions containing M (A) and 3 x M (B) amounts of TEA and best fit to the data by pseudofirst-order consecutive two-stage kinetics.
2000 1500 1000 (CM-1) 500 Figure 4. Typical example of decomposition of the complex transient Raman spectra constituting the kinetics of Figures 2 and 3 (see the text): trace A, parent spectrum (30 ns spectrum of Figure 2); trace B, difference spectrum obtained by subtracting the 250 ns spectrum of Figure 2 from trace A; trace C, difference spectrum resulting from subtraction of the TI spectrum of Figure 1 from trace B; trace D, difference spectrum resulting from the subtraction of trace B from the 10 ns spectrum of Figure 2.
tions, the intensities predicted for the T I , 22BPMY'-, and 22BPMYD' Raman spectra as a function of time are given by
(9)
Z(TJ = ue-klt kl 1(22BPMY'-) = b k, - k ,
- e-k2']
(10)
The coefficients a, b, and c are the products of the initial triplet concentration [Tllo and of the resonance Raman scattering cross section for the related species. The following fit method was adopted. Data related to the M TEA solution (part A in Figure 5) were first processed. The T I decay was satisfactorily least-squares fit to a single exponential with a rate constant kl of 1.6 x lo7 s-l or, according to eq 8, a quenching rate constant k, = 1.6 x lo9 M-' s-l. The anion and radical appearances were then correctly least-squares fit using eqs 10 and 11, respectively, leading to a rate constant k2 or 8 x lo7 s-l (k', = 1.4 x lo6 M-' s-l ). At this point, the experimental data
obtained for the 3 x M TEA solution (part B in Figure 5) were adequately plotted without adjustment by using the same set of parameters for a, b, c, k,, and k', and by setting the value of kl to 4.8 x lo7 SKI, Le., 3 times the preceding value, to account for the fact that the TEA concentration has been tripled (eq 8). The good agreement between the experimental data and theoretical fits for the two series of measurements supports the above reaction scheme (eqs 7 and 8). Both the increase in the reaction rates and the changes in the relative amounts of photoproducts in going from the less to the most basic solution are correctly accounted for by this kinetic model. In particular, the inversion of the triplet state and anion relative Raman intensities observed at short times between the two experiments (10-ns spectra in Figures 2 and 3) is well-reproduced. Complementary to these results, the transient Raman analysis of the 22BPMY photoreduction in a solution of M TEA in D20 shows exclusively the appearance of the radical 22BPMYD' with the same kinetics as that for the TIstate decay. The anion spectrum is not detected even at short times. This observation is also in agreement with the above kinetic model which predicts that, for low TEA concentrations (kl
