Subpicosecond Transient Absorption Analysis of the Photophysics of 2

have been correlated with those obtained in the microsecond time domain. ..... Science China Physics, Mechanics and Astronomy 2010 53 (6), 1024-10...
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J. Phys. Chem. 1996, 100, 19380-19388

Subpicosecond Transient Absorption Analysis of the Photophysics of 2,2′-Bipyridine and 4,4′-Bipyridine in Solution G. Buntinx* and R. Naskrecki† Laboratoire de Spectrochimie Infrarouge et Raman (LASIR) du CNRS, Centre d’Etudes et de Recherches Lasers et Applications, UniVersite´ de Lille I, 59655 VilleneuVe d’Ascq, France

O. Poizat Laboratoire de Spectrochimie Infrarouge et Raman (LASIR), CNRS, 2 rue Henri-Dunant, 94320 Thiais, France ReceiVed: March 25, 1996; In Final Form: September 18, 1996X

The transient absorption spectra of 2,2′-bipyridine (22BPY) and 4,4′-bipyridine (44BPY) following femtosecond excitation at 266 nm in a series of organic solvents have been measured in the 300-670-nm wavelength range with subpicosecond time resolution. The results have been correlated with those obtained in the microsecond time domain. The S1 states are characterized and attributed to nπ* levels in both molecules. The S1 state of 22BPY decays essentially via intersystem crossing to the T1 ππ* state in all solvents (τS1 ) 50-80 ps). The 44BPY S1 lifetime varies from 10 to 70 ps depending on the solvent. The shortest lifetimes are found in alcohols (10-20 ps) and alkanes (≈11 ps) where the fast formation of the N-hydro radical 44BPYH•, already evidenced in previous nanosecond experiments, is shown to occur with high yields from the S1 state in parallel to ISC. In alcohols this process is found to be activated by the protic character of the solvent and to take place within H-bonded solute/solvent complexes pre-existing in the ground state. A mechanism involving a surprisingly fast H-atom abstraction by the 44BPY S1 state via homolytic breaking of the alcohol OH bond is suggested.

1. Introduction Bipyridines are widely utilized as bases in quaternary salts1-3 and as chelating agents in charge-transfer transition-metal complexes4-6 and bimetallic mixed-valence complexes7-9 which are extensively studied with regard to their considerable photoinduced redox properties. Despite this large interest, few works are devoted to the photophysical and photochemical properties of the bipyridine ligands, and particularly little information is found in the literature concerning the reactivity of the lowest excited singlet state S1. 2,2′-bipyridine (22BPY) is reported to be very weakly fluorescent.10-13 From picosecond emission experiments and semiempirical calculations, Castellucci et al.14 have found very short fluorescence lifetimes for 22BPY in cyclohexane (160 ps) and methanol (=50 ps) and have shown that the low fluorescence quantum yield in inert solvents results from a very effective intersystem crossing to a local triplet state. A consequence of this low yield is the difficulty of avoiding alteration of the measurements due to the ability of 22BPY to form strongly emitting complexes with Zn2+ ions present as traces in solvents stored in glassware12,14 or water adducts in undried apolar solvents10 or to aggregate and form dimeric species at concentrations above 10-4 M.13,14 On the other hand, 4,4′-bipyridine (44BPY) is known to be nonfluorescent in solution,15 and there have been no reports in the literature concerning spectroscopic analyses of the S1 state of this molecule. Here we present measurements of transient absorption with subpicosecond time resolution for investigating the excited S1 state dynamics and elementary photophysical properties of 22BPY and 44BPY in various organic solvents. An important question concerning the reactivity of bipyridines is the nature

of the S1 state. In fact solvent-dependent photophysical properties can be expected from the presence of close-lying, vibronically coupled nπ* and ππ* excited states with relative energies varying with the protic character of the solvent. Whereas from fluorescence10,14 and two-photon absorption16 data the S1 state of 22BPY was assigned as nπ* in apolar solvents in agreement with semiempirical calculations,14 inversion between nπ* and ππ* states has been suggested to arise in water solutions.10 Another question of interest is the possibility, in the case of 22BPY, of cis-trans photoisomerization which has been considered as a potential reactive channel of relaxation of S1 in competition with nonreactive channels (internal conversion, radiative decay).17 However, a most important subject of interest in the present study is the comprehension of a fast reaction of photoreduction by alcohols that has been observed specifically for 44BPY from a recent nano/microsecond analysis by time-resolved resonance Raman and electronic absorption measurements.18 This analysis revealed the formation of the N-hydro radical NC5H4-C5H4NH• (44BPYH•) with doublesslow and fast (unresolved)skinetics. The slow component was shown to result from a reduction of the lowest triplet state T1 44BPY via hydrogen-atom abstraction from the alcohol. The fast component was assumed to arise from reduction of the excited S1 state but remained unexplained. In methanol comparable radical yields were measured for the two processes although the S1 lifetime was estimated to be at least 4 orders of magnitude shorter than the T1 lifetime. In regard to this surprisingly high efficiency of the fast process, a mechanism of double electron and proton transfer from the solvent to the S1 state, favored by the existence of hydrogen bonds with the solvent, was tentatively suggested.18 2. Experimental Section

†

Permanent address: Department of Physics, Poznan University, Grunwaldzka 6, 60780 Poznan, Poland. X Abstract published in AdVance ACS Abstracts, November 1, 1996.

S0022-3654(96)00900-8 CCC: $12.00

4,4′-bipyridine and 2,2′-bipyridine (Aldrich) were carefully dried in Vacuo to remove hydration water and sublimated at 80 © 1996 American Chemical Society

Photophysics of 2,2′- and 4,4′-Bipyridine °C in Vacuo prior to each measurement. Perfluorohexane (Acros), 2,2,2-trifluoroethanol (Lancaster), and nonfluorinated solvents (Prolabo) were used as received. Two types of water were used as solvent: distilled water on the one hand and doubly distilled, deionized water (Milli-Q plus ultrapure water system from Millipore) on the other hand. Similar results were obtained in both cases. No further precaution was taken in manipulating the solvents. As discussed in the Introduction, the difficulty encountered14 in measuring the fluorescence of 22BPY in the presence of trace amount of water adducts in undried apolar solvents or of strongly emitting Zn2+ complexes is due to the extremely weak fluorescence quantum yield of this bipyridine. The presence of such impurities is expected to have much less influence on absorption measurements. As a confirmation, the 308-nm absorption of the 22BPY-water adduct,11 easily distinguishable from the 281-nm band of the isolated species, was not detected in the absorption spectra of the apolar solutions studied in this work. It will be confirmed in the discussion that these impurities, if present, do not influence the observed results. All absorption measurements in the microsecond time domain were performed on deaerated, Ar-purged sample solutions at concentrations varying from 10-4 to 10-5 M. Subpicosecond measurements were carried out on 5 × 10-3 to 10-4 M solutions. These solutions were not deaerated. The subpicosecond experiments were performed using a Tisapphire based femtosecond laser system comprising a Tisapphire oscillator (MIRA 900D, Coherent) pumped by a 10-W cw argon laser (INNOVA 310, Coherent) and a regenerative amplifier (ALPHA 1000, BM Industries). The oscillator delivers 100-fs pulses (1 W) at a repetition rate of 76 MHz. The regenerative amplifier is used to increase the energy of some of the pulses emitted by the oscillator and to lower the repetition rate. It is pumped by a 10-W, 1-kHz intracavity frequencydoubled Nd:YLF laser (621D, BM Industries). This system provides 1-mJ, 100-fs pulses at 800 nm at a repetition rate of 1 kHz. Frequency doubling and tripling were achieved using 0.5mm thick BBO crystals. In order to obtain the shortest possible pulses in the third harmonic we used the following setup. The IR fundamental beam provided by the amplifier was divided into two beams with a 2-mm thick 50/50 dielectric beam splitter. One beam was sent without focusing into the first BBO crystal to produce the second harmonic with an efficiency of 20-25%. The residual fundamental light was separated from the harmonic using two dichroic mirrors. The second IR beam, whose polarization was rotated by 90° through an half-wave plate, was then temporally and spatially combined to the second harmonic beam in the second BBO crystal to yield the third harmonic with an efficiency of about 8%. This method led to shorter UV pulses than the usual method by which tripling is achieved by mixing the second harmonic to the residual IR fundamental beam emerging from the first BBO crystal, which is temporally broadened (the peak wavelengths have been much more absorbed in the crystal than the wings). The probe beam (white light continuum) was generated in a 1-mm thick calcium fluoride plate using part (a few microjoules) of the 800-nm amplified pulses. To extend the continuum spectrum as far as possible in the UV, we tested various transparent materials (such as SiO2, LiF, BaF2, MgF2, and sapphire) and solvents. The best results were obtained with CaF2 whose continuum spectrum extended down to 300 nm. Below this wavelength, its energy was too low and unstable to allow reliable measurements. Its polarization was set at the magic angle relative to the polarization of the pump light. After spatial filtering through an iris, it was separated into two parallel beams by a metal-coated beam

J. Phys. Chem., Vol. 100, No. 50, 1996 19381 splitter. Both spots (≈1-mm diameter) were sent into the sample cell without focusing, one being collinear to the pump beam. In order to avoid as much as possible group velocity dispersion, only reflective optics were used. The two spots were then sent onto the entrance slit of a 230-mm focal length stigmatic spectrograph equipped with a 150 grooves/mm grating. The two signals were detected simultaneously using an intensified double photodiode array (DILOR) with response linear on a 0-2 range of ∆OD. The time dispersion of the continuum light over the 300-670-nm region, estimated from the dependence upon the probe wavelength of the onset time of appearance of the broad-band two-photon absorption in pure n-hexane (see the first paragraph of the Results Section) was about 0.8 ps. The spectra shown in this report are not corrected for this dispersion effect. The time origin is arbitrarily chosen as the maximum overlap of the pump and probe pulses at 400 nm, which means that the value of the delay time t given for each spectrum is exact only at this wavelength. It is concluded from the dispersion characteristics that signals observed in the 540580-nm region are delayed (real time ≈ t - 400 fs) and those in the 320-340-nm region are in advance (real time ≈ t + 200 fs) with respect to this reference time. Photolysis was achieved at 266 nm (third harmonic of the laser) within strong S0 f Sn ππ* absorptions of 44BPY and 22BPY (266 ) 7500 and 10 000 M-1 cm-1, respectively). The energy of the pump pulse at the sample was 35-40 µJ for a diameter matched to that of the probe spot, i.e. 1 mm. Accordingly the energy density was about 4.5-5 mJ/cm2 per pulse. The continuum probe pulse was delayed in time relative to the pump pulse using an optical delay line (Microcontrol Model MT160-250PP driven by an ITL09 controller, precision (1 µm). Besides the group velocity dispersion of the continuum, the time resolution (fwhm of the pump/probe intensity cross-correlation) of the system was limited by the increase of the pump pulse temporal width in the two BBO crystals and by pump/probe group-velocity mismatch effects in the sample cell. The overall time resolution at a given wavelength was estimated to be about 400 fs from the two-photon absorption rise time in pure hexane. The sample solution was circulating in a flow cell with 2.5-mm optical path length. All spectra were accumulated over 3 min (≈180 000 pump-probe sequences). The transient absorption experiment in the microsecond time domain has already been described.19 The pump beam at 248 nm (1.5 mJ, 20 ns) was provided by an excimer laser (Questeck 2040) and focused on a 1 mm × 10 mm slice of the sample, yielding a 15-mJ/cm2 energy density per pulse. The probe light (1-mm diameter) was provided by a pulsed xenon lamp (Applied Photophysics). The detector, a photomultiplier (Hamamatsu 1P28) coupled to a digital oscilloscope (Tektronix TDS540), allowed a 0.1-µs time resolution. 3. Results It is important to note that, in the subpicosecond excitation configuration utilized in this work, efficient multiphoton absorption of the pump beam was observed on excitation of most of the pure solvents which were investigated. In the case of alkanes and alcohols, this effect leads to the appearance of an intense transient absorption signal, peaking around 300 nm and tailing up to 650 nm, which was used to monitor the experimental time resolution and the continuum probe dispersion. Kinetic analyses at fixed wavelengths, which are not affected by the time dispersion of the probe continuum, indicate that this signal rises within 400 fs then decays in about 1.5 ps to yield a weak, long-lived (>500 ps) absorption spectrum covering uniformly the 300-670-nm range. In cyclohexane this residual

19382 J. Phys. Chem., Vol. 100, No. 50, 1996

Buntinx et al.

Figure 1. Transient absorption spectra of 10-3 M 2,2′-bipyridine in cyclohexane measured at different delay times after 266-nm excitation.

Figure 2. Transient absorption spectra of 10-3 M 4,4′-bipyridine in acetonitrile measured at different delay times after 266-nm excitation.

TABLE 1: Singlet Excited State (S1) Absorption Wavelength and Lifetime and Relative Values for the Lowest Triplet State (T1) Quantum Yield and for the Intersystem-Crossing Rate Constant of 22BPY in Various Solvents

Gaussian profiles indicate clearly that intensity in the 420470-nm region of the 1-ps spectrum cannot be accounted for by the superposition of the wings of the 365- and 525-nm bands only. An additional broad absorption signal is necessarily superposed to the S1 spectrum at short time. This background absorption decays in about 20 ps and then remains almost constant. The decay of the spectral intensity at 530 nm from 4 to 500 ps has been fit to monoexponential kinetics to estimate the S1 lifetime. The corresponding τS1 values are reported in Table 1 for all solvents. In order to evaluate the solvent effect on the rate of intersystem crossing, kISC (S1 f T1), we also give in Table 1 relative values for the T1 quantum yield (ΦT1(rel)). These values are calculated as the ratios of the T1 band intensity at the end of the kinetics (500-ps spectrum) and of the initial S1 band intensity estimated by extrapolating the exponential decay fit of the S1 absorption to time zero. They are given in Table 3 normalized relative to the value found for methanol which is arbitrarily taken as unity. This procedure assumes that the T1 and S1 absorption strengths are independent of the solvent. The S1 lifetime being the result of several simultaneous deexcitation pathways, the quantum yield Φi for each process is determined by its rate constant ki

solvent

max a λS1fSn (nm)

τS1b (ps)

ΦT1(rel)c

kISC(rel)d

C6H12 CH3CN H2O CH3OH 2-PrOH

365, 525 363, 530 367, 527 365, 528 367, 527

70 64 54 77 53

0.9 0.8 0.8 1.0 0.8

0.8 0.8 1.0 0.9 1.0

a

Measured in the 4-ps spectrum. b Value established from monoT1 exponential decay fit of the S1 absorption (accuracy (5%). c I355nm (t S1 (t ) 0) ratio normalized with respect to the value in ) 500 ps)/I525nm CH3OH (see the text). d ΦT1/τS1 ratio normalized with respect to the value in water.

absorption is similar to the spectrum reported for the excited S1 state.20,21 Multiphoton absorption by the solvent was notably reduced in solutions containing the strongly absorbing bipyridine species. In no case a signal imputable to solvated electrons could be detected in the 650-nm region. In fact, the presence of solvated electrons should be characterized by a broad and structureless absorption maximizing around 700 nm in water and alcohols and 1000 nm in alkanes.22,23 Results for 2,2′-Bipyridine. Figure 1 gives a series of transient absorption spectra obtained at different times between 1 and 500 ps after 266-nm excitation of a solution of 22BPY (10-3 M) in cyclohexane. The 500-ps spectrum shows a single band at 355 nm which corresponds to the spectrum of the lowest triplet state, T1 (ππ*), as observed on the microsecond time scale.24 The 4-ps spectrum presents two bands at 365 and 525 nm which decay within 500 ps, whereas the T1 signal in the 300-340-nm region gains intensity. An isosbestic point is guessable around 340 nm. We ascribe thus the 4-ps spectrum to the precursor of the T1 state, i.e. the S1 state. A progressive narrowing of the red band edge of the S1 bands is clearly observed from 4 to 20 ps. Similar experiments on solutions of 22BPY in water, 2-propanol, methanol, and acetonitrile reveal comparable time evolution of the spectra as in cyclohexane. The peak positions (see Table 1) and relative intensities of the S1 bands measured from the 4-ps spectrum are not significantly dependent on the solvent nature. Similar profile narrowings are also observed in the 4-20-ps delay range. The decay of the S1 absorption spectrum is preceded by a spectral evolution below 4 ps which is roughly independent of the solvent. On going from the 1-ps to the 4-ps spectrum in Figure 1, the intensity rises at the visible band maximum but not in the wings. It decreases notably in the region 420-470 nm. Although the observed bands are not perfectly symmetrical, rough fits to

τS1 ) 1/∑ki

(1)

Φi ) τS1ki

(2)

i

Relative values for the ISC rate constants (kISC(rel)) in the different solvents can thus be obtained by weighting the ΦT1(rel) values by the corresponding S1 lifetimes. These values, normalized relative to that found in water, are given in Table 1. Results for 4,4′-Bipyridine. Transient absorption spectra of 44BPY in acetonitrile, n-hexane, cyclohexane, perfluorohexane, methylene chloride, water, and various alcohols (methanol, ethanol, 2-propanol, 1-butanol, tert-butyl alcohol, and 2,2,2trifluoroethanol (TFE)) were recorded at different times in the 0-500-ps range following subpicosecond excitation at 266 nm. A selection of typical spectra obtained for solutions in acetonitrile and methanol are shown in Figures 2 and 3, respectively. As for 22BPY, the latest spectrum (100 ps) in acetonitrile presents one UV band (330-340-nm range) characteristic of the T1 state.26,27 At shorter times the spectra display in addition two bands with maxima in the regions 375-380 and 580-590 nm. Their decay and the rise of the T1 absorption can be fit with the same first-order kinetics, and a clear isosbestic point

Photophysics of 2,2′- and 4,4′-Bipyridine

Figure 3. Transient absorption spectra of 10-3 M 4,4′-bipyridine in methanol measured at different delay times after 266-nm excitation.

is seen around 350 nm. This short-time spectrum is thus unambiguously attributed to the S1 state. A similar S1 spectrum is observed in all solvents in the first picoseconds following the pump excitation. In methanol the residual spectrum observed after complete decay of the S1 bands (50-ps spectrum in Figure 3) is quite different from that in acetonitrile or water. It shows three absorptions at 335, 361, and 540 nm. This spectrum is comparable (band shapes, relative intensities, and peak positions) to that previously reported for a time delay of 0.4 µs (spectrum A in Figure 4 of ref 18). In this respect, the 335-nm band is assigned to the T1 state, whereas the 361- and 540-nm absorptions characterize the N-hydro radical, 44BPYH•. An isosbestic point is still clearly apparent around 340 nm. The visible absorption profiles can be fit by a superposition of the 590-nm S1 band observed in water and the 540-nm radical band with contributions respectively rising and decreasing from 5 to 50 ps. The radical species is thus produced from the S1 state. It is also present at the end of the S1 de-excitation in most of the alcohols examined in this study and, to a lower extent, in cyclohexane and n-hexane, but not in water, TFE, or acetonitrile. In alkanes the radical absorption maxima are shifted to 350 and 500 nm, respectively. Similar shifts are observed on the S1 band positions (Table 2). No measurements could be made in perfluorohexane where the solubility of 44BPY is very low and instantaneous precipitation is induced on femtosecond excitation. In all solvents the T1 and radical band intensities remain constant after complete decay of the S1 spectrum, in the 50-500-ps time range. As for 22BPY, the decay of the S1 spectrum is preceded by a fast spectral evolution in the 0-3-ps time range. There is first a solvent-dependent change in shape of the S1 bands, more perceptible on the visible one: the band profile narrows and rises in intensity, whereas its maximum is red-shifted by 5-20 nm. Figure 4 displays, for a few solvents, the plot of the 590nm intensity measured from spectra recorded at different times in the 4-300-ps range. All plots are normalized to the same maximum to facilitate comparisons of their decay. In the inset is plotted the short-time dependence of the 590-nm intensity measured in methanol and trifluoroethanol. It shows that the rise in intensity of the S1 absorption is slightly longer than the estimated 400-fs pump/probe correlation time. Moreover, a background signal in the 420-470-nm domain shows complex intensity and time evolution dependence on the solvent and the sample concentration. In all cases, the rise time of this background corresponds to the pump/probe correlation time, and a decay component of =20 ps contributes to its kinetics. The origin of these short time effects will be briefly discussed below

J. Phys. Chem., Vol. 100, No. 50, 1996 19383 but will not be analyzed in more detail in this report as much more quantitative kinetics measurements are needed. The least-squares fits of the decay part of these plots to single exponentials, also shown in Figure 4, have been calculated to evaluate the S1 lifetimes. The corresponding τS1 values and the S1 peak position (4-ps spectrum) are given in Table 2. Relative values for the triplet quantum yields (ΦT1(rel)) and intersystemcrossing rate constants (kISC(rel)), derived as described above for the 22BPY molecule from the ratio of the triplet absorption, OD335nm, at 100 ps and of the S1 absorption, OD580nm extrapolated at t ) 0, are presented in Table 2. In solvents where the radical is formed in parallel to the T1 state, the radical contribution to the total absorption at 335 nm, estimated as equal to the radical band intensity at 540 nm from reference spectra previously obtained for 44BPYH•,18,27,28 was subtracted to obtain the pure triplet contribution. Data recorded for the fully deuterated isotopomer, 44BPY-d8, in TFE reveal that whereas the T1 quantum yield remains unchanged, the S1 lifetime increases from 67 to 104 ps and thus the relative kISC value (kISC(rel)) decreases from 0.4 to 0.26 by deuteration. Relative values for the radical quantum yield (ΦRH•(rel)) and formation rate constant (kr(rel)) in the different solvents were established similarly from the ratios of the 540-nm band intensity at 100 ps and of the S1 absorption at t ) 0, assuming constant absorption strengths for 44BPYH• and S1 in all solvents. They are also listed in Table 2. Finally, from a series of measurements performed on 10-4 to 5 × 10-3 M solutions of 44BPY in methanol, the S1 lifetime and relative ΦT1 and ΦRH• quantum yields appeared constant on this concentration range. In order to get further experimental data for understanding the fast mechanism of formation of the 44BPYH• radical in alcohols, subpicosecond measurements of 44BPY (10-3 M) in binary solutions of ethanol with water, n-hexane, or acetonitrile were carried out. The dependence upon the ethanol concentration of the radical yield as measured by the intensity of the 540-nm band at the end of the S1 state decay are plotted in Figure 5 (symbol +). No measurements could be realized in perfluorohexane/n-hexane, perfluorohexane/ethanol, and methylene chloride/n-hexane mixed solutions because of problems of low solubility and of precipitation upon excitation. In parallel we have evaluated the concentration ratio of the “free” and H-bonded 44BPY populations as a function of the ethanol mole fraction in n-hexane and in acetonitrile by analyzing the ground state Raman spectrum of 44BPY in these solutions. This analysis is based on the fact that, as reported by Cabac¸ o et al. for pyridine,28 the “free” and H-bonded forms of 44BPY are characterized by two components at 995 ( 1 and 1004 ( 2 cm-1, respectively, of the Raman active ring-breathing mode. The relative Raman intensity of the H-bonded component, Ibound/ Ifree + Ibound, as a function of the ethanol mole fraction is plotted in Figure 5 (symbol O) for the ethanol/n-hexane and ethanol/ acetonitrile solutions. These plots represent the population of the H-bonded 44BPY species relative to the total 44BPY population. In order to compare the reactivities induced upon femtosecond and nanosecond excitations and to estimate the influence of multiphoton absorption in the former case (the instantaneous photon density at the sample in the case of the femtosecond pump is about 25 000 times that of the nanosecond pump), we have recorded the transient spectra and kinetics after nanosecond excitation of 44BPY in the solvents listed in Table 2. All measurements were made with solutions of constant optical density (OD ) 0.8, [44BPY] ) 1.5 × 10-4 M) at the excitation wavelength. For all solvents we have compared the relative amounts of the 44BPYH• species produced from the S1 state,

19384 J. Phys. Chem., Vol. 100, No. 50, 1996

Buntinx et al.

TABLE 2: Singlet Excited State (S1) Absorption Wavelength and Lifetime and Relative Values for the Lowest Triplet State (T1) Quantum Yield, for the Intersystem-Crossing Rate Constant, and for the N-Hydro Radical Quantum Yield and Formation Rate Constant of 44BPY in Various Solvents (The Solvent Viscosity (η), Dielectric Constant (E), Dipolar Moment (µ), and Electron-Pair Acceptance Ability (ET) Are Also Given) solvent

η (10-3 Pa s)



µ (D)

ETa (kcal mol-1)

b (nm) λSmax 1fSn

τS1c (ps)

ΦT1(rel)d

kISC(rel)e

CH2Cl2 C6H14 C6H12 CH3CN CF3CH2OH H2O MeOH EtOH 1-PrOH 1-BuOH 2-PrOH 2-BuOH t-BuOH

0.40 0.30 0.90 0.34 1.65 0.89 0.54 1.08 1.96 2.59 2.07 3.66 5.12

8.9 2.0 1.9 37.5 26.7 78.4 32.7 24.6 20.3 17.5 19.9 16.6 12.5

1.14 0.09 0.00 3.44 2.52 1.83 1.87 1.66 3.09 1.75 1.66 1.55 1.66

41.1 30.9 31.2 46.0 59.5 63.1 55.4 51.9 50.7 50.2 48.6 47.1 43.9

381, 590 365, 562 366, 562 374, 580 378, 590 378, 590 378, 590 378, 590 378, 588 378, 589 378, 586 378, 586 377, 585

45.0 11.0 10.0 41.0 67.0 27.0 11.5 11.5 12.0 13.0 18.0 18.0 20.0

0.95 0.16 0.16 1.0 0.76 0.70 0.24 0.26 0.24 0.23 0.35 0.35 0.44

0.8 0.6 0.6 0.9 0.4 1.0 0.8 0.9 0.8 0.7 0.7 0.7 0.8

ΦRH•(rel)f

kr(rel)g

0.35 0.25

0.36 0.28

1.0 0.80 0.65 0.62 0.48 0.45 (0.48)

1.0 0.80 0.60 0.55 0.30 0.29 (0.27)

From ref 25. b Measured on the 4-ps spectrum. c Value established from monoexponential decay fit of the S1 absorption (accuracy (5%). S1 (t ) 100 ps)/I580nm (t ) 0) ratio normalized with respect to the value in acetonitrile (see the text). e ΦT1/τS1 ratio normalized with respect to RH• S1 (t ) 100 ps)/I580nm (t ) 0) ratio normalized with respect to the value in methanol (see the text). g ΦRH•/τS1 ratio the value in water. f I530nm normalized with respect to the value in methanol. a

d I T1 330nm

Figure 4. Time-dependent absorption change at 590 nm after 266-nm excitation of 44BPY in methanol (4), ethanol (1), acetonitrile (O), and trifluoroethanol (0). Each point is averaged over several measurements. The symbol size represents the experimental error. The solid lines are least-square fits to single-exponential kinetics. Inset: dilated view of the 0-5-ps time range for the methanol and trifluoroethanol solutions. All plots are normalized to the same maximum.

measured as the intensity of the fast, unresolved rise at 540 nm, and of the T1 species determined by extrapolating the decay trace at 340 nm to t ) 0 and subtracting if necessary the radical contribution as described above. The important results of this investigation are the following: (i) The formation of the N-hydro radical 44BPYH• from the S1 state is observed exclusively in the alcohols and in n-hexane and cyclohexane. (ii) The corresponding radical yield appears proportional to the pump power. (iii) The relative ΦT1 and ΦRH• values found in this way for the different solvents agree within 10% with those listed in Table 2. Measurements could be achieved in perfluorohexane, and a relative ΦT1 value of 1.0 is found whereas no radical is detected. (iv) In all solvents, the spectrum recorded at the early times (0.2-0.5 µs) of the kinetics is almost similar to that found in the picosecond range at the end of the S1 state decay; i.e., the ratios of the T1 and radical amounts produced in both experiments are comparable. As an example Figure 6 shows that the 0.3-µs spectrum recorded in these conditions for 44BPY in n-hexane is comparable to that obtained at 100 ps by using the subpicosecond experiment. Finally, additional measurements were performed after nanosecond excitation of 10-5 M solutions of 44BPY in methanol and n-hexane. We

Figure 5. Relative yield (symbol +) of the N-hydro radical 44BPYH• produced on reaction of ethanol with the photoexcited S1 state of 4,4′bipyridine as a function of the alcohol concentration in binary solutions ethanol/water (top), ethanol/hexane (middle), and ethanol/acetonitrile (bottom). The fraction of H-bonded molecules in these solutions measured as the Raman intensity ratio Ibound/(Ibound + Ifree) for the 4,4′bipyridine ring-breathing mode (see the text) is also displayed (symbol O).

observe that the relative T1 and radical amounts are approximately the same as for the 1.5 × 10-4 M solutions. Discussion Although the sample solutions used for the subpicosecond measurements have not be deaerated, we consider that the results are not influenced by the presence of oxygen. In fact, according

Photophysics of 2,2′- and 4,4′-Bipyridine

Figure 6. Comparison of the 100-ps transient absorption spectrum recorded for a 10-3 M solution of 4,4′-bipyridine in n-hexane by using the subpicosecond experiment and of the 0.3-µs spectrum obtained for a 10-4 M solution of 4,4′-bipyridine in n-hexane with the submicrosecond experiment.

to typical concentrations of O2 in air-saturated organic solvents at 25 °C of 10-3-10-4 M and assuming a pseudo-unimolecular quenching constant with a maximum value of 1011 M-1 s-1, the lifetime limitation for a reactive species falls in a range 10-8-10-7 s. The probability for diffusional quenching processes by oxygen is thus negligible in the picosecond domain. Nature of the Excited States. Semiempirical14 and ab initio29 calculations predict that the lowest excited singlet (S1) and triplet (T1) states of 22BPY correspond to planar 1Au (nπ*) and 3Bu (ππ*) configurations, respectively. The ππ* character and planar conformation of the T1 state are confirmed experimentally from transient absorption10,30 and luminescence, EPR, and ODMR 31-34 measurements. The much lower fluorescence quantum yield (ΦF ∼5 × 10-4) and lifetime (τF ) 160 and 50 ps in cyclohexane and methanol, respectively) reported for 22BPY14 compared to biphenyl (ΦF ) 0.15, τS1 ) 16 ns in n-hexane at 300 K35) are in agreement with the El-Sayed’s prediction that fast ISC must occur between nπ* singlet and ππ* triplet states of N-heterocyclics due to allowed spin-orbit coupling36 and favorable Franck-Condon overlaps.37 The kinetics observed in the present work for the decay of the S1 f Sn absorption and the rise of the T1 f Tn absorption confirm that ISC takes place in a time of the order of 100 ps although there is slight disagreement between the measured S1 lifetimes (Table 1) and the fluorescence lifetimes given above. Possible aggregation effects (formation of dimeric species), expected at concentrations above 10-4 M,13,14 may arise in the solutions utilized for our absorption measurements (10-3 M) and contribute to these discrepancies, although the proportion of such aggregates is presumably minor in a concentration range of 10-4 -10-3 M.13 On the other hand, the 54-ps S1 lifetime observed in water (Table 1) does not correspond at all to the 1640-ps fluorescence lifetime reported in this solvent and ascribed to the Zn(22BPY)2+ impurity.14 This result indicates clearly that this impurity does not contribute to the observed absorption spectra which represent thus the first characterization of S1 22BPY in water. By scaling the relative triplet quantum yields in Table 1 according to the ΦT1 ) 0.83 value found from flash photolysis in cyclohexane,30 one obtains kISC rate constants varying from 1.2 × 1010 s-1 in cyclohexane or acetonitrile to 1.4 × 1010 s-1 in water. These values are thus nearly independent upon the solvent, which indicates that ISC is the dominant decay route of the S1 state in all solvents and that the nπ* nature of this state is not markedly affected by the polarity or the protic character of the solvent. This conclusion is confirmed by the similarity of the S1 absorption spectra (band

J. Phys. Chem., Vol. 100, No. 50, 1996 19385 wavelengths and relative intensities) in all solvents. The suggestion10 of an inversion of the nπ* and ππ* excited singlet states of lowest energy in going from cyclohexane to water solution thus seems unlikely. According to the T1 f Tn molecular extinction coefficient of 5.36 × 104 M-1 cm-1 and the triplet quantum yield of 0.83 reported for 22BPY in cyclohexane,30 absorption intensities of max ≈ 5.5 × 104 and 1.5 × 104 M-1 cm-1 can be evaluated for the UV (λmax ) 365 nm) and visible (λmax ) 525 nm) S1 f Sn transitions, respectively. In the case of 44BPY, the general analogy with 22BPY and biphenyl concerning the triplet state lifetime (100 ( 30 µs) and transient absorption spectrum is strongly in accordance with a ππ* T1 configuration. As a confirmation, the time-resolved resonance Raman spectrum of T1 44BPY18 is close to that found for T1 biphenyl38,39 regarding the peak frequencies as well as the relative resonance Raman intensity enhancements. The ππ* nature of T1 44BPY is also in agreement with electron paramagnetic resonance data.31 By contrast the S1 state of 44BPY differs from the S1 (ππ*) state of biphenyl for the much shorter lifetime (τS1 < 100 ps; see Table 2). In this regard it can be rather related to S1 22BPY. In addition the S1 f Sn absorption spectrum (band positions and relative intensities) of 44BPY (Figures 2 and 3) resembles closely the 22BPY spectrum (Figure 1) but is notably different from that of biphenyl (not shown) which is characterized by a strong visible band at 655 nm and a weaker UV band at 390 nm. Accordingly, we assume S1 44BPY to be essentially of nπ* nature. As for 22BPY, the S1 spectrum (peak position and relative intensities) remains nearly unchanged in the various solvents used in this investigation and the kISC value (Table 2) is weakly solvent dependent. We conclude therefore that the nπ* character of S1 44BPY is preserved in all these solvents; i.e., there is no inversion of the nπ* and ππ* states in going from aprotic to protic solvents. Finally the reduction of the ISC rate constant observed for 44BPY in TFE upon deuteration is usual in aromatic hydrocarbons and is due to a decrease of the Franck-Condon overlaps between the singlet and triplet wave functions.40 Short-Time Spectral Evolution. The narrowing of the red band edge observed in a 4-20-ps delay for the S1 bands of 22BPY and, less apparently, of 44BPY, is likely due to vibrational relaxation. In fact, since an upper state Sn (ππ*) is initially populated on excitation of both molecules, hot S1 (nπ*) species are produced by internal conversion and vibrational cooling is expected. The excess of vibrational energy deposited in the S1 molecule must correspond to the S1-Sn energy gap, which is about 3800 cm-1 in the case of 22BPY.14 A sound analysis of the complex and solvent-dependent spectral changes observed at shorter delays (0-4 ps) would require much more experimental data, in particular detailed kinetic analyses, than reported here. Several tentative assignments may be proposed for the short-lived background signal which appears distinctly in the 420-470-nm region and decays notably within the first 20 ps. As mentioned above, there is obviously a contribution of the wings of the unrelaxed S1 bands (hot molecules) which narrow in the same 0-20-ps range. A second possible contribution is the absorption of the precursor Sn state itself, for which a broad and diffuse spectrum is expected in regard to its very short-lived nature. Accordingly the opposite spectral changes observed in the 1-3-ps delay following photolysis for the background and the S1 bands (the former decreases in intensity whereas the latter rise) could correspond to the Sn f S1 electronic relaxation. A third possible contribution to the short-lived background absorption may arise from an excited solvent species produced via multiphoton absorption.

19386 J. Phys. Chem., Vol. 100, No. 50, 1996 However, the characteristic broad absorption of the solvated electron (at =700 nm in water and alcohols and 1000 nm in alkanes22,23) is not detected. Whatever the relative importance of these different contributions in the short-time spectral relaxation, the important point is that, as it will be shown further in the discussion, they do not interfere significantly with the reactivity observed at longer time. Photophysics of the S1 State of 44BPY. As can be seen in Table 2, the S1 lifetime varies in the 10-70-ps range depending on the solvent, but the kISC rate is only weakly solvent dependent. In this regard intersystem crossing from S1 to T1 is not responsible for the fluctuations of τS1 with the solvent. Since 44BPY is nonfluorescent,15,41 the rate kIC of internal conversion from S1 to S0 is probably a factor affecting significantly the S1 lifetime. For example, a change in kIC is likely responsible for the shortening of τS1 observed on going from acetonitrile (41 ps) to water (27 ps), as comparable ISC rate constants are found in both cases. However the shortest S1 lifetimes and lowest T1 yields are measured in solvents where the 44BPYH• radical is produced simultaneously to the T1 state. A crucial factor governing the S1 lifetime is thus certainly the rate kr of radical formation. Before discussing the nature of this reaction, let us analyze its kinetic aspect in more detail. Consider the dependence of the radical yield on the ethanol concentration measured from the binary solvent experiments (symbol + in Figure 5). In ethanol/water mixtures (upper graph), the radical yield varies linearly with the ethanol mole fraction. In ethanol/hexane mixtures (middle graph), the radical yield increases rapidly to its maximum value as the mole fraction of ethanol varies from 0 to 0.1 and then remains nearly constant for higher ethanol concentrations. A similar evolution is observed in ethanol/acetonitrile solutions (lower graph), but the maximum radical yield is reached for an ethanol mole fraction of about 0.35. These dependences of the 44BPYH• yield in aprotic solvents/ethanol mixtures match closely the evolution of the relative population of the H-bonded 44BPY species in the initial solution (symbol O in Figure 5). This correlation indicates that the yield of formation of the radical from ethanol is not proportional to the alcohol concentration but to the concentration of the hydrogen-bonded 44BPY‚‚‚HO-R complex in the ground state. This dependence means that the radical formation in the presence of alcohols does not result from a diffusional, pseudo-first-order process but rather from a direct intracomplex reaction. In water/ethanol mixtures all the 44BPY molecules are H-bonded either to water or to methanol molecules. The concentration ratio of these two types of complexes is thus expected to be proportional to the mole fraction ratio of these two solvents, which is in agreement with the linear dependence of the radical yield upon the ethanol mole fraction. In pure alcoholic solutions where almost all the 44BPY molecules are H-bonded, the decay kinetics of S1 is determined essentially by the relative rates of the IC, ISC, and radical formation processes: τS1[ROH] ) (kIC + kISC + kr)-1. In pure water, the 44BPY molecules are present in a H-bonded form comparable to that in alcohols but no radical is produced from the S1 state: τS1[H2O] ) (kIC + kISC)-1. Considering the IC and ISC rate constants as approximately similar in water and in alcohols, i.e. imputing the shorter S1 lifetime in alcohols to be due exclusively to the additional radical formation process, allows the expression of the kr constant as kr ) (τS1[ROH])-1 - (τS1[H2O])-1. The absolute kr rates, krcalc, calculated in this approximation in the different solvents, and their relative values, krcalc(rel), are given in Table 3. There is undoubtedly a certain analogy between these relative values and those measured from the radical quantum yields (kr(rel) values in Table 2), which

Buntinx et al. TABLE 3: Rate Constants for the Formation of the 44BPYH• Radical from the S1 State in Alcohols Estimated from the Shortening of the S1 Lifetime Relative to Water (See the Text)

a

solvent

τS1 (ps)

krcalc (s-1)

krcalc(rel)a

H2O MeOH EtOH 1-PrOH 1-BuOH 2-PrOH 2-BuOH t-BuOH

27.0 11.5 11.5 12.0 13.0 18.0 18.0 20.0

0 5.0 × 1010 5.0 × 1010 4.6 × 1010 4.0 × 1010 1.8 × 1010 1.8 × 1010 1.3 × 1010

0 1.0 1.0 0.9 0.8 0.36 0.36 0.26

Normalized with respect to the value in methanol.

confirms that the notable shortening of the S1 lifetime in alcohols is to a large part due to quenching by the radical formation process. In n-hexane and cyclohexane the very short 44BPY S1 lifetime (10-11 ps) is not attributable to a fast ISC process since the rate constant kISC is notably lower than that in acetonitrile, water, or alcohols. The quite low triplet quantum yield compared to nonpolar, aprotic solvents where no radical is produced (perfluorohexane, methylene chloride) suggests a quenching of the ISC process by the reaction leading to the formation of the 44BPYH• radical, as in alcohols. However the relatively low rate constants found in n-hexane and in cyclohexane assuming equal maximum absorption strengths for the radical visible band in alkanes and alcohols (kr(rel) values in Table 2) are in contradiction with this hypothesis. Unfortunately the S1 lifetime and relative kISC rate in perfluorohexane could not be measured. Considering the 45-ps τS1 value in methylene chloride and assuming the radical formation process to be responsible for the shortening of the lifetime in n-hexane and cyclohexane leads to hypothetical relative kr values of 1.37 and 1.55, respectively, which are too far from the measured 0.36 and 0.28 kr(rel) values (Table 2). Faster internal conversion from S0 to S1 is thus likely contributing to the short S1 lifetime of 44BPY in alkanes. Fast Mechanism of Formation of the 44BPYH• Radical in Alcohols. The reaction leading to the formation of the H-adduct radical from the S1 state of 44BPY in alcohols is puzzling in several aspects. It is worthwhile to mention that this reaction is observed also in the case of 4-phenylpyridine,42 indicating that the second pyridyl ring in 44BPY is not involved in the mechanism. The fact that, in methanol, comparable relative T1 and radical yields are obtained on nanosecond excitation of 10-5 to 10-4 M solutions and on femtosecond excitation of 10-4 to 10-3 M solutions indicates clearly that there is no influence of aggregate formation in the reaction yielding the radical. Intradimer processes can be thus ruled out. Considering its product, this reaction is equivalent to a transfer of a hydrogen atom H• from alcohol to excited bipyridine. However it differs drastically from the H-atom transfer reaction arising at the T1 level as to its rate constant. Whereas quenching rate constants of the order of 105 M-1 s-1 were found for the T1 state,18 those estimated above for the S1 state (krcalc values in Table 3) are higher by more than 5 orders of magnitude. In this respect, they are more characteristic of electron transfer than of H-atom transfer. A mechanism based on the reduction of 44BPY by electrons produced from multiphoton ionization of the solvent, followed by protonation by the solvent, could be imagined. However several experimental observations are not consistent with this reaction scheme. If multiphoton ionization were effective in alcohols it should be effective also in water, and a comparable reactivity should be observed in both cases. This is in contradiction with the fact that no radical

Photophysics of 2,2′- and 4,4′-Bipyridine is produced in water. Moreover, the typical absorption of the solvated electron (λmax ) 650 nm in methanol, 700 nm in water22) should be superposed to the S1 spectrum at the earliest times, before the appearance of the radical spectrum. There is no evidence of such a signal in the observed spectra in any solvent. Finally, the fact that comparable relative T1 and radical yields are obtained with both the femtosecond and nanosecond excitations although the photon density in the former is considerably higher than in the latter shows unambiguously that there is not more contribution from multiphoton processes in the formation of the radical than in that of the T1 state. We conclude that the strong multiphoton absorption that is observed in pure solvents, and which contributes probably to the shortlived background superposed to the 44BPY S1 spectrum in the earliest times, has apparently no influence on the 44BPY photoreactivity. Another essential difference between the S1 and T1 processes concerns the relative reactivities of the alcohols. As expected, the rate constants for the formation of 44BPYH• from the T1 state were found18 to be proportional to the hydrogen-atom donor character of the alcohols with a maximum value for 2-propanol (the reaction results from the homolytic break of a C-H bond of the alcohol in position R to the hydroxyl group). In contrast the relative rate for the S1 state (kr (rel) values in Table 2) is maximum in the case of methanol and nearly minimum for 2-propanol. As can be seen by comparing the solvent parameters in Table 2, there is no relationship between this rate and the alcohol viscosity and polarity. On the other hand, if we consider only the seven alkyl alcohols in which the radical is produced, the kr(rel) value appears to increase with the electronpair acceptance, i.e. the protic character of the alcohol. This observation is in agreement with the above conclusion that the radical formation at the S1 state occurs within a H-bonded complex, 44BPY‚‚‚H-OR. This indicates moreover that the reaction rate increases with the strength of the hydrogen bond, which suggests a two-step mechanism: a photoinduced proton transfer yielding an excited ion pair followed by an intrapair electron transfer from the alcoholate to the protonated bipyridine.

Scheme 1 hν

44BPY‚‚‚HO-R 98 44BPY*‚‚‚HO-R f [44BPYH+‚‚‚-OR]* f 44BPYH• + •OR However several points are in contradiction with this reaction scheme. First, no reaction is observed in water and in TFE although these solvents are characterized by a notably stronger electron-pair acceptance or protic character than methanol. Therefore the dependence of the reaction efficiency upon the strength of the alcohol-bipyridine H-bond, well established for the alkyl alcohols, breaks down for water and TFE. Moreover the above reaction scheme assumes a stronger basicity of 44BPY in the excited singlet state than in the ground state, which is unexpected with regard to the nπ* nature presumed for the S1 state since, in this case, a nitrogen lone pair orbital is depleted of an electron upon excitation to S1. Finally such a double mechanism based on proton and electron transfer is excluded in alkanes. According to these remarks, a H-atom transfer seems more probable. In order to account for the absence of reactivity in the most acidic solvents, water, and TFE, we propose tentatively an alternative mechanism involving the contribution of both the initially pumped Sn (ππ*) state and the relaxed S1 (nπ*) state. ππ* states being polarizable and zwitterionic in nature, a reinforcement of the 44BPY‚‚‚HO-R hydrogen bond with a possible proton jump from the alcohol to the bipyridine must

J. Phys. Chem., Vol. 100, No. 50, 1996 19387 arise on going from S0 to Sn. On the contrary, nπ* states being inherently diradical in nature, the fast relaxation from Sn to S1 must be accompanied by a decrease of the hydrogen bond strength and favor radical processes. We suggest thus that the radical formation in alcohols results from H-atom abstraction by the 44BPY S1 (nπ*) state via the homolytic break of the OH bond of a H-bonded alcohol molecule (Scheme 2). In this assumption the fast rate of this transfer which takes place during the short S1 lifetime is guaranteed by the favorable orientation and proximity of the donor and acceptor species achieved in the ground state through the hydrogen bonding.

Scheme 2 hν

44BPY‚‚‚HO-R 98 44BPY*(Sn)‚‚‚H‚‚‚OR V IC kr

44BPYH• + •OR 79 44BPY*(S1)‚‚‚HO-R The absence of reactivity in solvents of stronger acidity as water and TFE could be explained by assuming that protonation of 44BPY arises in the Sn (ππ*) state and modifies the spatial orientation of the reactants in such a way that when the proton is released upon relaxation to S1, the configuration of the system is no more favorable to a fast H-atom transfer (Scheme 3).

Scheme 3 hν

44BPY‚‚‚HO-R 98 [44BPYH+ (Sn), -OR]* V IC 44BPY*(S1) + H+ + -OR This mechanism proposed tentatively for the fast radical formation at the S1 level in alcohols differs thus from that at the T1 level in the fact that it assumes an homolytic breaking of the OH bond instead of a CH bond (the OH bond is too energetic to be cleaved at the T1 level). It is based on the existence of preassociated acceptor and donor entities in the ground state in a configuration favorable to fast H-atom transfer on excitation. Several points remain nevertheless obscure. In particular the values of the rate constant kr are much higher than those usually encountered for H-atom abstraction processes. On the other hand, since the proposed mechanism is dependent on an excited state acid-base equilibrium, a decrease of its efficiency rather than its complete inhibition would be expected on going from methanol to water or TFE. Finally a different mechanism must be considered for the radical formation in alkanes where H-bonding is excluded. However the high rate observed for the H-atom abstraction by the S1 state suggests also in this case that the reaction arises within predisposed solute/solvent entities such as exciplexes. A series of photochemical measurements in binary solvents are planned in order to obtain better insight into this process. On the other hand, picosecond time-resolved resonance Raman experiments and quantum chemical calculations are currently in progress to investigate the structure and relaxation dynamics of the shorttime intermediates as a function of the solvent nature. Acknowledgment. The Centre d’Etudes et de Recherches Lasers et Applications (CERLA) is supported by the Ministe`re

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