J. Phys. Chem. 1996, 100, 19417-19424
19417
Competition between Geminate Recombination and Solvation of Polar Radicals following Ultrafast Photodissociation of Bis(p-aminophenyl) Disulfide T. Bultmann and N. P. Ernsting* Institut fu¨ r physikalische und theoretische Chemie der Humboldt-UniVersita¨ t, Bunsenstrasse 1, D-10117 Berlin, Germany ReceiVed: July 17, 1996; In Final Form: September 26, 1996X
Supercontinuum probe pulses are used for studying the ultrafast photodissociation of bis(p-aminophenyl) disulfide into two p-aminophenylthiyl radicals and their subsequent geminate recombination in polar solvents. The kinetic investigations are complicated by a spectral absorption shift of the photolytically generated radicals due to solvation. The dissociation and recombination dynamics are separated from the solvation dynamics by a moment analysis of the transient spectra. Geminate recombination in polar solvents is observed only if the time scale for dielectric relaxation of the solvent is comparable to or slower than the time scale for recombination. The latter depends on the initial distance distribution of the radical pairs.
(p-aminophenyl) disulfide (BPADS) into two p-aminophenylthiyl (PAPT) radicals:
I. Introduction The photodissociation of a molecule into neutral species and its reversalsthe recombination of two geminate atoms or radicalssis still a subject of intense research. However, only for the diatomic molecules I2 and Br2 have the two aspects been investigated for different environments ranging from gases at low and high pressures to apolar as well as polar solvents (for reviews see refs 1 and 2). Troe and co-workers3 assessed the kinetics and quantum yield for the geminate recombination of two iodine atoms in the liquid phase with a diffusive model.4 Recently, the photochemistry of I2 has been investigated by Lienau and Zewail5 in different rare gases over a wide pressure range with femtosecond time resolution. That work focuses on the excited-state dynamics of I2 and on the influence of ultrafast solvation on dissociation and recombination. It is the last topic that the present paper is devoted to primarily, but we concentrate instead on large organic radicals in polar solvents. In contrast to the halogen measurements, where the dissociation products are not observed, we monitor the rise and decay of the product absorption due to the photodissociation and recombination reactions. Compared to the knowledge about the dissociation of diatomic molecules and the geminate recombination of atoms, little is known about the corresponding processes of larger organic compounds.6-12 A molecule that has been investigated with some continuity in this respect is tetraphenylhydrazine.7-12 The photodissociation of this compound into diphenylaminyl radicals was reported to occur along two different channels, with respective time constants of 80 fs and about 20 ps in aliphatic solvents.10 The recombination of the radicals could be described adequately with a diffusive model for a reactive pair in solution8 as developed by Shin and Kapral.13 The same description was applied to the recombination of two phenylthiyl radicals formed by the photodissociation of diphenyl disulfide in alkane solvents.6 This reaction has also been studied in refs 14-16. In the present paper we investigate the photodissociation of bis * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02151-X CCC: $12.00
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Using femtosecond transient absorption spectroscopy, we seek a qualitative picture of the competition between geminate recombination and solvation of the photolytically generated radicals. The solvation is evidenced by a bathochromic shift of the radical absorption with time, which correlates with the dielectric relaxation of the solvent. Partial results have already been published by one of us for solutions in diethyl ether,17 in methanol and ethanol,18 and in propylene carbonate.19 This reaction has also recently been studied in hexane solution on a picosecond time scale, but with an emphasis on dimer formation11,20 which appears to be a minor reaction channel in the polar solvents treated here. II. Experimental Section The Laser System. Femtosecond pulses at 620 nm are generated with a self-built colliding-pulse-mode dye laser. They are amplified up to 240 µJ, with less than 1% of energy background, in a dye amplification scheme which is driven by an excimer laser (Lambda-Physik LPX 103) at 30 Hz.21 The amplified pulses are recompressed to the original pulse width of 90 fs (assuming a Gaussian pulse shape); the pulse energy at this point is 120 µJ. The corresponding basic beam (diameter ≈ 5 mm) is split in two equal parts by a wedged dielectric beam splitter. One part is used to generate UV pump pulses by frequency doubling. It is focused with a f ) 1330 mm lens onto a KDP crystal cut for type I phase matching. The crystal thickness of 300 µm limits the UV pulse width to 105 fs. Pulse energies between 12 and 14 µJ at 310 nm are obtained for 60 µJ input pulses. The other part of the basic beam is used to generate supercontinuum probe pulses. It is focussed with a f ) 400 mm lens into a water flow cell (300 µm quartz windows separated by 1.7 mm). The Broad-Band Spectrometer. The femtosecond pump/ supercontinuum probe (PSCP) spectrometer is shown in Figure © 1996 American Chemical Society
19418 J. Phys. Chem., Vol. 100, No. 50, 1996
Figure 1. Optical setup for the pump/supercontinuum probe measurements with anastigmatic, dispersion-free imaging.
1. A technical problem with the PSCP technique is the need to image the supercontinuum source onto the sample with dispersion-free optics at relatively large numerical apertures when compared to laser beams. Low dispersion over the entire visible range is readily achieved by reflective optics. However, if spherical mirrors are employed, the usual off-axis arrangement introduces astigmatism, which results in an elliptical probe spot on the sample. In this case, unnecessarily high pump pulse energies are required. The main idea behind our setup is to achieve a well-defined, 120 µm probe spot on the sample in order to reduce the energy requirements for the pump pulses and hence to scale down all amplifiers. The potential of this approach has recently been demonstrated.22 Here we describe the optical setup for the photodissociation experiments. In principle, astigmatism may be avoided with a Cassegrain arrangement, but the necessary mirrors are difficult to obtain and the combination is cumbersome to align. Another approach uses elliptical mirrors,23 which are relatively expensive and may exhibit significant deviations from the ideal surface. Here we put into practice an alternative, inexpensive, and flexible scheme for anastigmatic dispersion-free imaging of the supercontinuum which relies on spherical mirrors only.24 It consists of a concave imaging mirror which is used at the smallest possible input angle. The resulting astigmatism is compensated by a convex mirror with a large radius of curvature, set at an angle of nearly 45°. In this way, the polarized continuum is first imaged onto a 100 µm pinhole for spatial filtering, mainly to reduce lateral fluctuations and to ease the subsequent alignment. The imaging mirror, with a radius of curvature R1 ) 224 mm (diameter ) 46 mm), is almost filled by the supercontinuum. The spectral component at the generating wavelength is reduced by blocking a central area (diameter ) 7 mm) on the mirror. The compensating mirror has R2 ) -1220 mm. (The choice of radii for the spherical mirrors was determined by available stock rather than planning. Astigmatism was then eliminated by an appropriate choice for the input angles.) Then the pinhole is imaged onto the sample cell by a combination of a concave mirror and a convex quartz meniscus (R2 ) -4780 mm, thickness 2 mm). The reflection from the front surface of the meniscus serves as the probe beam, and the rear reflection constitutes the reference beam; the transmitted supercontinuum is discarded. The diameter of the probe spot on the sample cell is 120 µm, and the pulse energies are about 80 nJ. The sample cell has a thin quartz entrance window (300 µm), and the thickness of the flown solution is 300 µm.
Bultmann and Ernsting Since the reference beam passes through the curved front surface of the meniscus twice, it obtains an additional astigmatism. The latter is compensated behind the sample cell with another concave mirror (R ) 224 mm) which focuses the reference beam into a quartz fiber. The probe beam is focused into a second quartz fiber simply by an achromatic lens (f ) 100 mm). The spectra of both beams are evened out by a Schott BG 24 glass and an optimized solution of different dyes. The fibers are coupled to a polychromator (Jobin-Yvon H25), and the dispersed probe and reference continuua are registered with a double intensified diode array (Princeton Instruments DIDA 512) as in ref 18. The quartz fibers (Ceram Optec, UV 943/ 1000, diameter ) 1 mm) have a length of 20 m. They should broaden the probe input pulses; group velocity dispersion alone contributes a broadening of 20 ps.25 This results in a linear dynamic range of 5 × 103 for the detection. The UV pump beam is routed through a variable delay line and focused behind the sample cell by a spherical mirror (R ) 400 mm). A thin dielectric mirror is used to superimpose the pump beam collinearly onto the probe beam. The diameter of the pump beam on the sample cell is about 0.8 mm in these experiments. The pump polarization is set to the magic angle with respect to the probe polarization. The dispersion of time zero and the wavelength-dependent time resolution was measured by transient bleaching of malachite green and by two-photon absorption in benzene.18 The pump/probe intensity cross-correlation has a width e200 fs (fwhm) for all probe wavelengths. The dispersion of time zero is found to be equivalent to the dispersion which is obtained from a 2 mm thick quartz plate. The latter is therefore used to correct the transient spectra for the chirp of the supercontinuum probe pulses. We also measured the spectra of the PAPT radicals on a microsecond time scale with a Xe flash lamp instead of a femtosecond supercontinuum and an excimer laser for optical pumping. In this case, the sample cell was a 40 mm long prismatic capillary cell.26 Materials. Bis(p-aminophenyl) disulfide (Aldrich) was recrystallized several times from ethanol. All solvents where HPLC grade. Concentrations were 2.0-4.5 mM for the femtosecond measurements and 0.04 mM in the case of the microsecond measurements, which were performed under nitrogen. III. Results A. Analysis of the Transient Spectra. Figure 2 shows the evolution of the radical absorption in methanol (a) and ethylene glycol (b) over a time range of several picoseconds. The spectra have been corrected for the dispersion of time zero as mentioned above. At early time a transient spectrum consists of several components: excited-state absorption by the electronically excited parent disulfide, the developing radical absorption, and an unidentified background that extends over the entire spectral range. The excited-state absorption is seen only while the pump and probe pulses overlap in time. The unidentified spectral background appears to be broad and unstructured; it is therefore approximated by a linear interpolation between the optical densities at 450 and 700 nm on either side of the radical absorption band. This background contributes less than 5% to the optical density at the maximum of the radical absorption and lives for a few picoseconds depending on the solvent. On the microsecond time scale, the transient spectrum shows no background to the radical absorption although it was measured with a much higher signal-to-noise ratio. Also, the pure solvents gave no transient signal at all. For the following analysis we therefore subtract the linear background from the observed
Photodissociation of Bis(p-aminophenyl) Disulfide
J. Phys. Chem., Vol. 100, No. 50, 1996 19419 B. Determination of Relative Radical Concentrations. The Lambert-Beer law is written as
OD(ν˜ , t) ) σ(ν˜ ,t) N(t)x
(2)
Here OD denotes the optical density, ν˜ the wavenumber, σ the absorption cross section, N the particle density, and x the optical path length. In the present investigation, the radical absorption spectrum shifts significantly to the red and changes its shape within the first 10 ps. It is reasonable to assume that these changes reflect the vibrational and solvent relaxation of the radicals only. In this case the dipole strength ∫bandσ(ν˜ ,t)/ν˜ dν˜ for the corresponding electronic transition should be conserved.29,30 We therefore use the extended relation
OD(ν˜ ,t) dν˜ ν˜ N(t) ) σ(ν˜ ,t) x∫band dν˜ ν˜
∫band
(3)
which for a conserved transition dipole strength leads to Figure 2. Evolution of the p-aminophenylthiyl spectra following ultrafast UV photodissociation of the parent disulfide in methanol (a) and ethylene glycol (b). The spectra have been corrected for the chirp of the supercontinuum probe pulse.
c(t) ) I′∫band
spectra, so that at all times (and at wavelengths longer than 450 nm) there remains only the developing radical absorption band. The radical absorption spectrum at a given time delay between the pump pulse at 310 nm and the probe continuum is fitted to a log-normal distribution over wavenumbers.27,28 This function is given by
{
{
}
ln 2 g0 exp [ln(1 + R)]2 for R > -1 g(ν˜ ) ) b2 0 for R e -1; R ) 2b[(ν˜ - ν˜ p)/∆]
(1)
Here ∆ gauges the bandwidth, b is the asymmetry and ν˜ p gives the band position. Figure 3 shows a transient PAPT absorption spectrum from the series of Figure 2a, together with the best log-normal fit. At the present state of data evaluation, the transient spectra are not deconvoluted with the pump-probe cross-correlation. The width of the latter depends on wavelength. But this dependence is small over the probed spectral range, and its neglect does not influence the results significantly. The relative concentrations as a function of time, which are derived below, can be deconvoluted with the (mean) cross-correlation directly.
(4)
Here I′ is a constant which also contains the optical path length. The integral
I ) ∫band
Figure 3. Transient radical absorption spectrum in methanol at 1.9 ps, together with the best log-normal fit. The spectrum is taken from the series shown in Figure 2a. Fit parameters are g0 ) 0.293, b ) 0.0536, ∆ ) 2657.2 cm-1, and ν˜ p ) 17 537.6 cm-1.
OD(ν˜ ,t) dν˜ ν˜
OD(ν˜ ) +∞ dν˜ ) ∫-∞ g(ν˜ )ν˜ -1 dν˜ ν˜
(5)
involves the log-normal function g(ν˜ ) which was fitted to the transient spectra OD(ν˜ ). Figure 4 shows the relative radical concentration as a function of time for the first few picoseconds. In ethanol and 2-propanol (Figure 4a,b)sas well as in methanol, 1-propanol, and 2-butanol for which curves are not shownsthe radical concentration rises within ≈300 fs (inset in Figure 4a) to a level which remains constant up to the investigated time limit of 150 ps (inset in Figure 4b). Fits to the experimental data give a deconvoluted rise time between 40 and 100 fs, and no significant solvent influence on the rise time can be observed. For the solvents 1-butanol (Figure 4c), ethylene glycol (Figure 4d), and propylene carbonate (Figure 4e) the ultrafast initial rise (with rate constant kA) is followed by a significant decrease of the radical concentration during the first few picoseconds. This is described with a biexponential response function
Φ ∝ [1 - A exp{-kAt}] + B exp{-kBt}
(6)
Solid lines represent the best fit of this response function to the data, after convolution with a Gaussian function (200 fs fwhm) representing the time resolution of the pump/probe spectrometer. The corresponding parameters are collected in Table 1. Here a comment is in order about the unidentified broad background, which is by necessity ignored in the present treatment (cf. the previous section). The well-defined radical absorption band appears with a deconvoluted rise time
