5905
J. Phys. Chem. 1993,97, 5905-5910
Photochemistry of 4,4'-Bipyridine. Nanosecond Absorption and Raman Study of the Hydrogen Atom Abstraction from Methanol and 2-Propanol 0. Poizat,'J G. Buntinx,* P. Valat,o V. Wmtgens,o and M. Bridouxs Luboratoire de Spectrochimie Infrarouge et Raman (LASIR), CNRS, 2 rue Henri Dunant. 94320 Thiais, France, Luboratoire de Spectrochimie Infrarouge et Raman (LASIR), CNRS, USTLFA, Bat. C5, 59655 Villeneuve d'Ascq, France, and Laboratoire des Matgriaux Molgculaires (LMM), CNRS, 2 rue Henri Dunant, 94320, Thiais, France Received: December 23, 1992; In Final Form: March 1 , 1993
The transients that are produced in the photolysis of 4,4'-bipyridine (44BPY) in methanol and in 2-propanol have been examined by use of transient absorption and time-resolved resonance Raman spectroscopies in the nanosecond time range. 4,4'-Bipyridine undergoes photoreduction by these alcohols, leading to the hydrogen adduct radical 44BPYH'. The reaction arises via quenching of the T I state of 44BPY by the abstraction by a nitrogen atom of a hydrogen atom from methanol ( k =~ 1.05 X lo5 M-* s-l) or 2-propanol ( k =~2.4 X los M-I s-l). The 44BPYH' radical decays via disproportionation in competition with protonation by the solvent. In methanol, an additional very fast reaction of formation of the 44BPYH' radical is observed besides the above hydrogen transfer to the TI state. It is suggested that this fast reaction results from the double transfer of an electron and a proton in the excited SIstate of H-bonded bipyridine or bipyridine aggregates.
Introduction The photochemical reactivity of aza-aromatic molecules is mainly governed by the presence of the nitrogen lone pair of electrons and presents analogies with the reactivity of carbonyl compound~.I-~In this regard, we recently observed that 4,4'bipyridine4and 2,2'-bipyridine5are photoreduced by amines such as triethylamine or 1,4-diazabicyclo(2.2.2)octane and we found from combined transient absorption and time-resolved resonance Raman analyses that the reaction arises, as for the reduction of ketones by amines, via the formation of a triplet ion pair followed by ionic dissociation in competition with intrapair proton transfer. On the other hand, photoreduction of 4,4'-bipyridine (44BPY) by hydrogen-donating solvents has also been reported. Elisei et a1.6 observed, from flash photolysis experiments, the quenching of T I 44BPY by hydrogen-atom transfer from 2-propanol, rertbutyl alcohol, and cyclohexane and ascribed the transient reaction product to the N-monohydro radical 44BPYH' (A,, = 370 and 560 nm). From time-resolved resonance Raman measurement^,^ we confirmed that the photoreduction of 44BPY by alcohols yields the 44BPYH' radical and found that hydrogen-atom abstraction arises also from diethyl ether and tetrahydrofuran, whereas in acidified aqueous and methanolic solutions the photoreduction leads to the formation of the N,N'-dihydro radical cation 44BPY&'+ (A,, = 375 and 580 nm). In neutral alcoholic or aqueous solutions, the monohydro radical is present as the hydrogen-bonded form (RO-H.. .NCSH~-CSH~N'-H) and the dihydro radical cation 44BPYH2*+is also formed to some extent. The structure and electronic configuration of the 44BPYH' and 44BPYH2*+ species have been discussed in detail from the vibrational analysis of their transient resonance Raman ~ p e c t r a . ~ However, no kinetic measurements could be carried out in this work as the delay between the pump and probe excitations was set to a fixed value of about 30 ns by the use of an optical delay line. In the present paper, we investigate in detail the photoreduction of 44BPY by methanol and 2-propanol and consider more particularly the analysis of the nanosecond reaction dynamics as followed by combined transient absorption and transient resonance +
LASIR, Thiais. Villeneuve d'Ascq. LMM, Thiais.
1 LASIR,
0022-3654/93/2097-5905%04.00/0
Raman scattering. For this study, a two-laser Raman excitation system equipped with a tunable electronic pumpprobe delay device was employed. The results confirm that T I 44BPY is quenched by alcohols to yield the hydrogen-adduct radical 44BPYH'. However, they indicate that the SIstate is, in some cases, also a precursor for the formation of this radical and suggest that a very fast reduction process involving the transfers of an electron and of a proton takes place in the picosecond time domain. Finally, it is found that the 44BPYH' species decay mainly via disproportionation in competition with N-protonation.
Experimental Section 4,4'-Bipyridine (Aldrich) was sublimed in vacuo prior to each spectroscopic measurement. Methanol, 2-propanol, and tertbutyl alcohol (Prolabo) were purified by distillation over calcium. Aqueous solutions were prepared with doubly distilled water. Solutions were deoxygenated with an Ar purge directly in the spectroscopic cells. Excitation of 44BPY was carried out at 266 or 248 nm in the strong SO S, absorption lying in the 220270-nm region (€266 = 7500 M-I cm-I, e248 = 12 500 M-I cm-I). The time-resolved Raman system was composed of an excimer laser (Questek 2040) as pump source (248 nm, 15 ns, 1.5 mJ), a 10-Hz Q-switched Nd:YAG laser coupled to a Dye laser (Quantel YG581C + TDLSO) as probe source (370 nm, 8 ns, 1.5 mJ), and a homebuilt multichannel spectrometer (detailed description in ref 8) coupled to a gated intensified diode array. A pulse and delay generator (description in ref 9) triggers the two lasers and the detector gate (20 ns) with a total jitter better than 4 ns. The spectral resolution and analyzed field in the 370nm region were about 8 cm-' and 1600 cm-I, respectively. The nanosecond flash photolysis apparatus for transient absorption measurements has been described elsewhere.10 It included a frequency-quadrupled Nd:YAG laser (BMI) as pump source (266 nm, 7 ns, 0.2 mJ), a pulsed xenon lamp (Applied Photophysics) as probe source (crossed-beam arrangement), and a spectrometer (resolution better than 4.6 nm) equipped with a digitalized oscilloscope (Tektronix 7A13 + 7912AD).
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Results Before discussing the results concerning the photolysis of 44BPY in methanol and in 2-propanol, a few general considerations on the flash photolysis and transient resonance Raman 0 1993 American Chemical Society
Poizat et al.
5906 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 15871
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I
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1250
1040Ns I
750
2000 (Cfl-l)
Figure 1. Time-resolved resonance Raman spectra of TI44BPY at different times following photolysis at 248 nm of a deaerated solution of 44BPY (lo-' M) in acetonitrile. Probe excitation: 340 nm. The solvent peaks have been subtracted. The dashed lines indicate Raman lines not due to the TI state (see the text). The insert shows a plot of the intensity of the 1377-cm-I TIband, measured relatively to the solvent spectrum, as a function of the pump/probe time delay.
spectroscopies must be given. Very complementary results can be obtained from these two techniques and their confrontation is very instructive. On the one hand, Raman spectra provide detailed structural information, which is generally not available from electronic absorption. Moreover, they allow unquestionably a better discremination between species that have comparable absorption spectra. On the other hand, we have remarked in previous combined investigationsof reaction kinetics by transient absorption and transient Raman spectro~copies~*~ that very different quantitative kinetic data are reached by the two techniques. As a general rule, the decay kinetics measured from Raman experiments appear much shorter than from absorption experiments. In fact, because of the low sensitivity of the Raman technique compared to absorption, the time-resolved Raman detection of a photogenerated transient species requires much higher sample concentration and pump excitation energy. Under these conditions,the concentrations of the photoproduced transient species are themselves much higher and bimolecular (or multimolecular) processes appear notably promoted to the detriment of unimolecular processes. A clear illustration of this effect is given by the time decay of the lowest triplet state TI of 44BPY in inert solvents (CH3CN, for example). T I 44BPY is characterized in the 25G800-nm region by a strong absorption band peaking at 340 nm, which decaysvia mixed first-order and secondorder kinetic^.^^^ A natural lifetime of 70 ps corresponds to the first-order kinetics as measured from transient absorption.4 In contrast, the T I Raman spectrum probed at 340 nm decays via pure second-order kinetics (Figure 1) with a lifetime of approximately 200 ns, indicating that triplet-triplet annihilation is the dominant deactivation process of T I under these conditions. Note that a very weak residual Raman spectrum is observed in addition to the T I bands at a delay of 1 ps (trace d in Figure 1). Its frequencies correspond to those found for the 44BPYH' radical spectrum probed at 370 nm in apratic solvent^.^,^ Its
1500
1000
(CM-1)
Figure 2. Time-resolved resonance Raman spectra of a deaerated solution of 44BPY (10-3M) in methanol at different times following laser photolysis at 248 nm. Probe excitation: 370 nm. The intensities of the different spectra are normalized with respect to the solvent bands, which have then been subtracted.
intensity appears nearly constant from 40 ns to 1 ps, indicating that the formation of this radical is probably independent of the decay of the T I species but results rather from a minor process at the SI level. This process will not be studied here. Photolysis of 44BPY in Methanol and 2-Proponol. TimeResolved Resonance Raman Analysis. The main absorption band of the different reduction products 44BPY'-, 44BPYH', and 44BPYHz'+ lies at 385, 370, and 375 nm, respectively. Accordingly, most of the Raman measurements destined for the investigation of the process of photoreduction of 44BPY by alcohols were carried out with a mean probe excitation at 370 nm, for which the spectra of the three reduced speciesare strongly enhanced by resonance. However, at this wavelength, the tripletstate spectrum of 44BPY is very weakly enhanced and appears completely masked by the intense spectra of the reduced species when present. Some measurements were thus also performed with 340-nm probe excitation. Figure 2 shows the time-resolved Raman spectra obtained on 370-nm probe excitation for a deaerated solution of 44BPY ( M) in methanol, at different times after laser photolysis at 248 nm. As observed previously? the short-time spectrum (40 ns) is essentially characteristic of the hydrogen-adduct radical 44BPYH (Raman lines at 590, 740, 967, 997, 1198, 1228, 1337, 1506, 1587, 1650, 1736, 1968, and 1990 cm-I). The intensities of the four lines at 1587, 1506, 1337, and 1228 cm-I, which are specifically representative of this increase from 20 to 150 ns then decrease continuously from 150 ns to 1 ms with a common kinetics, which is mainly of second-order type. The lifetime of the 44BPYH' radical, measured from this kinetics, is approximately 4 ps in the present conditions but depends strongly on the pump intensity, as expected for a second-order decay process. Raman measurements made with 340-nm probe excitation (notshown) indicate that theTl spectrumdecays completely from 20 to 150 ns, i.e., within the same time domain as the 44BPYH' spectrum grows. The radical species is thus produced, at least in part, from the T I state.
Photochemistry of 4,4'-Bipyridine Onecan see in Figure 2 that, besides the decay of the 44BPYH' spectrum, new Raman peaks appear at long time delays. First, two shoulders become visible at 1525 and 1357 cm-I in the spectrum probed at 1.5 ps, These signals correspond to two dominant lines of the N,N'-dihydro radical cation, 44BPYH2'+,7J1J2which is likely produced from protonation of the 44BPYH' radical by methanol. As a confirmation, slight frequency upshifts of the 44BPYH' lines at 1650 and 1736 cm-I on going from the 150-11s spectrum to the 1.5-ps one indicate the presence of growing peaks around 1660 and 1745 cm-I which correspond also to strong features of the 44BPYH2'+ Raman spectrum. The other main bands of 44BPYH2'+ (1003 and 740 cm-I) are superposed to 44BPYH lines and cannot be distinguished from them. The radical cation spectrum is much weaker, as the radical spectrum and the time-evolution of its intensity cannot be evaluated accurately. However, a careful examination of the 44BPYH2*+bands shows that the intensity of the 1660-cm-I line increases more rapidly than the intensities of the 1525- and 1357-cm-I lines in the 1.5 ps-1 ms timedomain. Twocomponents are thus superposed at 1660 cm-I: one characterizes the 44BPYH2*+ cation with an intensity of about two-third of the 1525- and 1357-cm-1band intensities and the second corresponds to a third transient species, the concentration of which increases continuously. Two other Raman features at 1539 and 1992 cm-I can be clearly ascribed to this third compound, as they show similar intensity behaviors. Finally, the 997-cm-I line in the 50ps and 1-ms spectra appears too intense to result from the sum of the 44BPYH' and 44BPYH2*+components (997 and 1003 cm-1, respectively) and contains probably also a third contribution. These four unassigned bands at 997, 1539, 1660, and 1992 cm-I correspond nicely to the spectrum of the neutral N,N'-dihydro species 44BPYH2, which has been first characterized by Lu et al." from two-electron reductionof theN,N'-diprotonated species 44BPYHz2+(main lines at 996,1536, and 1652 cm-I). We have also observed4the formation of this species from disproportionation of the 44BPYH' radical produced from photoreduction of 44BPY by triethylamine (four lines were found at 996, 1539, 1658, and 1992 cm-I). In the present case, 44BPYH' is likewise the precursor of 44BPYH2, since the increase of the 1539-cm-I Raman peak, ascribed to this last species, and the decay of the 44BPYH' spectrum have nearly the same kinetics. Disproportionation of 44BPYH' is thus likely to occur. Figure 3 shows the time-resolved resonance Raman spectra recorded at 370 nm at different times after 248-nm photolysis of a deaerated solution of 44BPY ( l w M) in 2-propanol. As for the solutions of methanol, the 44BPYH' spectrum is observed at short times and decays with second-order kinetics in the millisecond time scale whereas the lines ascribed to the neutral N,N'-dihydro species 44BPYH2 rise. However, in this case the growth of the 44BPYH2*+spectrum a t long time delays is notably weaker than in methanol, indicating that the protonation of 44BPYH' by 2-propanol is less efficient than by methanol. In contrast, the 44BPYH' radical produced on photolysis in aqueous solutions containing small amounts of methanol or 2-propanol (1:lO mol) decays via first-order kinetics in the microsecond time scale and leads exclusively to the formation of the 44BPYH2*+cation. Protonation of 44BPYH' is thus much faster and more efficient than in pure alcoholic solutions. Transient Absorption Analysis. A series of transient absorption spectra have been recorded at different times following 266-nm excitation of deaerated solutions of 44BPY ( lo4 M) in methanol and 2-propanol and in aqueous solutions of these alcohols. Figure 4 shows typical spectra probed at delays of 400 ns, 2 ps, and 14 ps in pure methanol solutions. Three signals with different kinetics can be discerned on examination of the time evolution of these spectra. First, a band peaking at 345 nm decreases continuously with time and corresponds to the triplettriplet absorption of the TI state of 44BPY. Then, a line a t 370
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 5907
Figure 3. Time-resolved resonance Raman spectra of a deaerated solution M) in 2-propanol at different times following laser of 44BPY photolysis at 248 nm. Probe excitation: 370 nm. The intensities of the different spectra are normalized with respect to the solvent bands, which have then been subtracted.
1
0,15
M i --i--1 ---
B
I
O
'
0 300
5
\
Y I
I
400
I
I
500
I
I
600
WAVELENGTH / NM
Figure 4. Transient absorption spectra of a deaerated solution of 44BPY (lo4 M) in methanol at different times following 266-nm laser photolysis: (A) 400 ns; (B) 2 ps; (C) 14 ps. Inserts I and I1 show the kinetic traces taken at 340 and 530 nm, respectively, and insert I11 shows a kinetic trace at 530 nm after addition of 4 X M 1,3-cyclohexadiene to the above solution.
nm and a broad band around 540 nm increase slightly from 400 ns to 4 MS, then decrease a t longer time delays. They characterize the N-monohydro radical 44BPYH'.6 Finally, a thin line at 385 nm and a broad band maximizing in the 560-600-nm region appear as the 44BPYH' spectrum decreases and may be ascribed either to the anion 44BPY'- I 3 . l 4 or to the N,N'-dihydro cation 44BPYH2*+,I5 which is the protonated form of radical 44BPYH'. In basicified solutions of methanol, the TI and 44BPYH' absorptions are still present but the third spectrum is no longer observed. We thus assign it to the radical cation. The decay of the TI state, illustrated by the kinetic trace at 340 nm (insert I in Figure 4), can be correctly fitted with a firstorder kinetics plus a background. The latter corresponds to the residual 44BPYH' species, which absorbs slightly a t this wavelength. The triplet-state lifetime calculated from this pseudo-
5908
The Journal of Physical Chemistry, Vol. 97, No. 22, 1993
Poizat et al.
I
4-
A
. 500NS/DIV.
I
I
0.5
1.0
1.5
IALCOHOLI /
2.0
N
Fipre5. Plotoftherateoftripletdecayfor44BPY(104M)indeaerated acetonitrile as a function of the concentration of (A) 2-propanol and (B) methanol.
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first-orderkinetics(T1/2 1 . 4 ~ismuchshorterthanthenatural ~) lifetime measured in acetonitrile (q L 70 ps). On the other hand, the increase of the kinetic trace a t 530 nm (insert I1 in Figure 4), which represents the growth of the 44BPYH' radical, comprises a very fast, initial rise followed by a slow component. The latter has the same kinetics as the decay trace of the triplet state. Therefore, the triplet state TI of 44BPY is quenched by methanol and yields the radical 44BPYH'. However, no isobestic point is observed between the triplet and radical absorption features. In fact the vertical rise that characterizes the increase of the 44BPYH' species at short time suggests that part of the radical is produced via a parallel, very fast process involving probably the SI state. In order to confirm this double origin of the 44BPYH' radical, we have analyzed the kinetic data obtained in the presence of a powerful triplet quencher, 1,3-cyclohexadecadiene. Upon addition of up to 4 X 1t5 M, 1,3-~yclohexadiene to a solution of 44BPY (10-4 M) in methanol, we observe the efficient quenching of the triplet state ( k t 8 X 1O1OM-I s-I) and the simultaneousdisappearance of the slow-rising component of the kinetic trace taken at 530 nm, whereas the initial fast rise remains unchanged (insert I11 in Figure 4). This result is consistent with the fact that the fast and slow rises of the 44BPYH' radical result from processes involving the SIand TI states of 44BPY, respectively. The rate constant for quenching of the TI state by methanol has been obtained precisely from a plot of the pseudo-first-order decay rate constant of the triplet transient (probed at 340 nm) as a function of methanol concentration in acetonitrile (trace B in Figure 5). The contribution of the 44BPYH' absorption a t 340 nm, estimated by transposing the kinetic trace at 530 nm after normalization a t long delay time, was taken into account in the fit of the kinetic trace at 340 nm. Both the starting acetonitrile solution and the quenching methanol solution were deaerated and contained equal amounts of 44BPY (2 X M). A quenching rate constant of kC(Me0H) = (1.05 f 0.10) X los M-I s-I is found from these data. Note finally that, in these solutions of acetonitrile containing methanol, the kinetic trace at 530 nm is similar to that in pure methanol, whereas in aqueous solutions of methanol it displays only the slow component correlated to the triplet-state decay. The fast process originating from the SIstate is thus inhibited in the presence of water. Consider now the passage from the 44BPYH' spectrum to the 44BPYH2*+ one. In the 2-20-ps time range, this passage is characterized by three isobestic points at 377,395, and 565 nm. This observation is consistent with our assumption that the N,N'dihydro radical cation of 44BPY arises via protonation of the N-monohydro radical by methanol. As a matter of fact, the increase of the 44BPYH2'+ absorption at 385 nm or at 590 nm and the decay of the 44BPYH' absorption at 345 nm or at 530 nm (half-life 8 ps) can be fitted by the same pseudo-first-order
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325
350
375
400
WAVELENGTH / INN
Figure 6. Transient absorptionspectra of a deaerated solution of 44BPY (10-4 M) in 2-propanol at different times following 266-nm laser photolysis: (A) 250 ns; (B) SO0 ns; (C) 1000 ns; (D)2000 ns; (E)3500 ns. The insert shows the kinetic traces at (I) 340 nm and (11) 540 nm.
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kinetics and a rate constant of kTPYH'(MeOH) 5 X lo3 M-I s-I can be estimated for the protonation of 44BPYH' by methanol. The transient absorption spectra obtained for a deaerated solution of 44BPY (10-4 M ) in 2-propanol in the 0-10 ps-time region show essentially the decay of the TI band at 345 nm (71/2 0.7 ps) and the simultaneousgrowth of the 44BPYH' spectrum (370- and 540-mm absorptions). Figure 6 shows typical spectra recorded from 200 ns to 3.5 ps in the 310-385-nm region. Contrary to the observation made in the case of methanol solutions, two isobestic points at 360 and 380 nm indicate clearly that, in 2-propanol, the TI state is the only precursor of the 44BPYH' radical, as previously reported by Elisei et aZ.6 The increase of the 44BPYH' absorption at 540 nm corresponds satisfactorily to the decay of the triplet trace at 340 nm (inserts I and I1 in Figure 6). The kinetic trace at 540 nm does not comprise any fast rise a t short time, confirming that the SIstate of 44BPY does not react with 2-propanol as with methanol. The Stern-Volmer analysis of the plot of the pseudo-first-order decay rate constant of TI as a function of 2-propanol concentration (trace A in Figure 5), obtained as described above, leads to a quenching rate constant of k;l(2PrOH) = (2.4 f 0.1) X lo5 M-1 SKI. The decay of the44BPYH species takes place in the millisecond time scale. The corresponding kinetic trace at 540 nm can be fitted by mixed first-order and second-order kinetics. A parallel increase of the absorption strength at 385 and 590 nm is observed simultaneously to the decrease of the 540-nm absorption. By analogy with the results found with methanol solutions, we ascribe the growing spectrum to the 44BPYH2'+ radical cation resulting from protonation of 44BPYH' by 2-propanol. If we correlate this process to the first-order component of the radical decay kinetics, an approximate rate constant of ktBPYH'(2PrOH) 15 M-I s-I is found. Thisvalue is much weaker than that observed in methanol, which confirms the previous observation from Raman data that the protonation of 44BPYH' by 2-propanol is less efficient than by methanol. We propose to ascribe the secondorder component of the 44BPYH' decay to the process of disproportionation, which has been clearly characterized in the above Raman analysis. Note finally for comparison that the rate constant for quenching of the TI state of 44BPY by tert-butyl alcohol (k:' (tBuOH) IO4 M-I s-l) is notably weaker than the constants of quenching by methanol or 2-propanol.
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Discussion At first sight, the results obtained on the photoreactivity of 44BPY in alcohols from resonance Raman scattering on the one
Photochemistry of 4,4'-Bipyridine hand and from electronic absorption on the other hand seem to be not in complete agreement. However, it will be shown in the following discussion that these two types of results are complementary and that the discrepancies can be accounted for by differences in the relative yields of the monomolecular and bimolecular decay processes within the overall reaction scheme, as discussed in the preamble of the section Results. The analysis of the Raman spectra has established that, in methanol and 2-propanol, the lowest triplet state of 44BPY leads to the formation of the H-atom adduct radical 44BPYH'. This radical is shown to decay via disproportionation in competition with protonation by the solvent. Protonation appears to be more efficient in methanol than in 2-propanol, as expected from the fact that the former solvent is more acidic and more polar than the latter. However, in both cases, the second-order kinetics that characterizes the decay of the 44BPYH' spectrum indicates that, in the experimental conditions adopted for the Raman measurements, disproportionationis the dominant process of deactivation of 44BPYH'. In contrast, in aqueous solutions, containing small amounts of alcohol, this radical yields essentially the 44BPYHz*+ cation via a first-order kinetics, Le., protonation is the dominant way of deactivation of 44BPYH'. Two explanations may be mentioned to account for this effect. First, the acidic character of water, related to its high dielectric constant, is expected to stimulate notably the reaction of protonation. Second, also in consequence of the acidity of water, important solvation of 44BPY may be assumed in the ground state due to hydrogen bonding between water molecules and the nitrogen atoms of the heterocyclic species. Such association may hinder the approach of the alcohol molecules and thus partly inhibit the quenching of the TI state of 44BPY and reduce the concentration of the 44BPYH' radical formed in this reaction. In this case the bimolecular decay process of this radical by disproportionation must be restrained. The results obtained from transient absorption confirm that the TI state of 44BPY is quenched by alcohols and yields the 44BPYH' radical. Thevalue of the quenching constant increases in going from tert-butyl alcohol ( lo4 M-I SKI)to methanol (1.05 X M-I SKI) and then to 2-propanol (2.4 X lo5 M-1 s-I); Le., it varies as the hydrogen-atom donor ability of the alcohol. This observation confirms the previous statement6,' that the production of 44BPYH' from the TI state arises via hydrogen-atom transfer from the alcohol. However, a second, very fast process takes place in methanol, probably from the SIstate. Its mechanism cannot be studied with an experiment with nanosecond time resolution and remains unexplained. Several remarks concerning this process are nevertheless worth making from the present analysis. To our knowledge, the lifetime of the SI state of 44BPY has not been reported. The SI state of the neighboring molecule 2,2'-bipyridine has been found tovary from 50 to 160ps, depending on the solvent.5-I6 In any way, it is certain from the present Raman and absorption measurements that the SIstate of 44BPY is shorter than 10 ns and thus the TI/Sl lifetime ratio is at least 104. According to the relative heightsof the fast and slow increases of the kinetic trace at 530 nm (insert I1 in Figure 4), we conclude that approximately equal amounts of radical 44BPYH' are produced from the SIand TI states. Therefore, the rate constant for quenching the SIstate by methanol is a t least lo4times higher than that measured for the TI state Le., k t ( M e 0 H ) Z109 M-I s-1. Such a value is close to the diffusion-controlled rate constant and is plausible for an electron-transfer reaction, but it cannot characterize a hydrogen-atom transfer process. However, methanol is not a donor of electron. On the other hand, we have observed that the formation of 44BPYH' from the SIstate of 44BPY occurs in pure methanol but not in 2-propanol nor in aqueous solutions of methanol. Since these solvents differ mainly in polarity and thus in H-bonding ability, it is probable that the interaction of the solvent with the 44BPY species plays an
The Journal of Physical Chemistry, Vol. 97,No. 22, 1993 5909 important role in the reactivity of the SI state. These remarks suggest that, in methanol, hydrogen bonding in the ground state predisposes the system to a fast double transfer, concerted or not, of a proton and an electron when 44BPY is excited to the SIstate. Wemayimagineforinstancethat,iftheexcitedS~stateisassumed to be more polar and more basic than the ground state, rapid protonation of this SIstate, favored in methanol, which is more polar and more acidic than 2-propanol, yields an excited pair '(44BPYH+,-OCH3)* and is followed immediately by internal electron transfer. Another possible scheme is based on the hypothesis that electron transfer could arise initially within bipyridine aggregates stabilized in methanol. The strongly basic radical anion produced is this process would then be instantaneously protonated by methanol to yield the 44BPYH' radical. However, these tentative interpretations are not entirely convincing. In particular they do not explain the absence of reaction from the SIstate in water and water/methanol solutions. The comprehension of this fast process requires an analysis with a picosecond resolution experiment. One observes in Figure 5 that, unexpectedly, the extrapolations to infinite dilution of the plots of the rate of triplet decay as a function of the concentration of alcohol in acetonitrile do not lead to the same rate values in the cases of methanol and of 2-propanol. Whereas the plot obtained with 2-propanol as quencher leads to an approximate value in agreement with that measured in pure acetonitrile (- lo4s-l), a notably higher value (-3 X 104 s-I) is found in the case of methanol. This effect has been confirmed by repetitive measurements. A possible explanation would be that, besides the quenching reaction by H-atom transfer, the presence of methanol lowers the natural triplet lifetime by 44BPY due to the presence of a particular interaction with the heterocyclic molecule and/or the presence of aggregates. This assumption is in agreement with the above discussion of the reactivity observed for the S I state of 44BPY in methanol. Whereas the decay of the 44BPYH' radical was found to be essentially bimolecular when probed by transient Raman scattering, it appears rather of first-order nature when observed by transient absorption. This effect results from the fact that, as discussed above, the concentration of the 44BPYH' species produced in the flash photolysis experiment is much weaker than that generated in the Raman experiment in such a way that the bimolecular process of deactivation by disproportionation becomes negligible compared to the monomolecular process of protonation by the solvent. The bimolecular kinetics is nevertheless observed in 2-propanol solutions for which the rate of protonation of 44BPYH' is very slow. A comparable observation can be made concerning the tripletstate deactivation in alcohols. As measured from absorption, TI 44BPY decayswith pseudo-first-order kinetics by hydrogen-atomabstraction from the solvent. However, thevery short TIlifetime measured from Raman data (-80 ns) compared to those found from absorption (1.4 and 0.7 ps in methanol and 2-propanol, respectively) indicates that efficient triplet-triplet annihilation takes place in the experimental conditions adopted for the Raman measurements in addition to quenching by alcohol. As a conclusion, the different results obtained from transient Raman and absorption experiments on the photoreactivity of 44BPY in alcohols can be comprehensively summarized by the following reaction scheme: 440PY
- $1 hv
'440PY*
tr!!et-tfplet annihilation
exclusively in CH30H (undetermined
'12(44BPYH2 + 44BPy)
m i )
5910 The Journal of Physical Chemistry, Vol. 97, No. 22, 1993 The rates of triplet reduction by different alcohols follow the H-atom donor ability of the Alcohol, as it is observed, for example, in the case of aromatic ketones. In contrast, the fast process involving the SIstate does not conform to the H-atom donor aptitude of the alcohol but rather to its ability to form H-bonded associations with the bipyridine molecule in the ground state. Picosecond absorption and Raman experiments are planned to investigate this later reaction, the mechanism of which remains undetermined.
References md Notes (1) Beak, P.; Mesrcr, W. R.In Organic Phorochemistry; Chapman, 0. L., Ed.; Marcel Dekker: New Yotk, 1969; Vol 2, p 117. (2) Cowan, D. 0.; Drisko, R. L. Elements of Organic Chemistry; Plenum: New York, 1976; p 75. ( 3 ) T u r r o , N . J. M o d e r n M o l e c u l a r P h o t o c h e m i s t r y ; Benjamin/Cummings: London, Amsterdam, 1978; p 362. (4) Buntinx, 0.;Valat, P.; Wintgens, V.; Poizat, 0.1. Phys. Chem. 1991, 95, 9347.
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