J. Phys. Chem. A 2010, 114, 3045–3049
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Side-Chain Effects on the Electronic Relaxation of Radicals followed by Time-Resolved Pump-Probe Spectroscopy: 2,3-Dimethylbut-2-yl vs tert-Butyl† Bastian Noller,‡,§ Lionel Poisson,‡ Ingo Fischer,*,§ and Jean-Michel Mestdagh*,‡ Laboratoire Francis Perrin, CNRS URA 2453, CEA IRAMIS/SerVice des Photos, Atoms et Mole´cules, F-91191 Gif-sur-YVette Cedex, France, and Institute of Physical Chemistry, UniVersity of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: September 21, 2009
The excited-state lifetime of the 2,3-dimethylbut-2-yl (DMB) radical, a hexyl isomer, upon electronic excitation into the 3p Rydberg state at 265 nm, is measured by femtosecond time-resolved photoionization. It is shown that the 3p state deactivates in a two-step process, which is well described by two time constants of 25 and 400 fs. The results are compared to tert-butyl (t-C4H9), another tertiary radical investigated before. Timedependent DFT calculations confirm the earlier suggestion that curve crossings along the C-C coordinate play an important role in the excited-state deactivation. Introduction Considerable interest in the excited-state dynamics arises for radical species because small hydrocarbon radicals play key roles in combustion processes,1,2 interstellar space,3,4 polymerization,5,6 and hydrocarbon cracking.7 Despite the relevance of hydrocarbon intermediates in high-energy environments, only little spectroscopic information on such systems is found in the literature due to the experimental challenge of producing such species under isolated conditions. In particular the initial photophysical processes after excitation have been explored for only a few species. Ultrafast investigations on the excited states have been performed for ethyl, propargyl,8 and tert-butyl9 radicals, as well as a few carbenes.10,11 Exploring the dependence of the molecular dynamics on side chains or functional groups12 is a systematic way of elucidating the femtochemistry13 of polyatomics. In recent publications the excited-state dynamics of low-lying Rydberg states in ethyl and tert-butyl have been discussed.8,9 The 3s state of ethyl deactivated within 20 fs, while the 3s Rydberg state of tert-butyl deactivated with a time constant of around 125 fs. In contrast the 3p Rydberg state behaved differently, and a time constant of 2 ps was found. This observation motivated the investigation of the closely related 2,3-dimethylbut-2-yl (DMB) radical, one of the isomers of hexyl, C6H13. Both tert-butyl and DMB are tertiary radicals that only differ by two methyl groups at the side chain (Figure 1). The absorption spectrum of 2,3dimethylbut-2-yl has its maximum at around 266 nm,14 enabling excitation with the third harmonic of a Ti:Sa laser. A 3p Rydberg state is excited at this wavelength,14 with a large oscillator strength. On the other hand, the C3V-symmetrical tert-butyl radical shows only a relatively weak absorption band for the 2 A1/2E (3p) r 2A1 excitation.14 Here we want to compare their excited-state deactivation and see how it is affected by the aliphatic side chains. † Part of the “Benoıˆt Soep Festschrift”. * Corresponding authors,
[email protected] and ingo@ phys-chemie.uniwuerzburg.de. ‡ Laboratoire Francis Perrin, CNRS URA 2453, CEA IRAMIS/Service des Photos, Atoms et Mole´cules. § Institute of Physical Chemistry, University of Wu¨rzburg.
Figure 1. The tert-butyl (left) and the 2,3-dimethylbut-2-yl radical (right) are closely related and only differ in the presence of two methyl groups.
SCHEME 1: Pyrolytic Generation of 2-Iodo-2,3dimethylbutane
Experimental Section The experiments were performed in a differentially pumped standard molecular beam apparatus equipped with a time-offlight mass spectrometer (TOF-MS) and a velocity map imaging (VMI) detector used for mapping ion or electron kinetic energy distributions. The radicals were produced by jet flash pyrolysis15 from 2-iodo-2,3-dimethylbutane according to Scheme 1. The precursor was synthesized according to ref 16. It was seeded in 3 bar of argon and expanded through a short, weakly heated silicon carbide tube attached to a water-cooled solenoid pulsed valve operating at 20 Hz. A 20 Hz femtosecond Ti:Sa oscillator/amplifier system was used for the pump-probe experiments. It consists of a Verdi/ Mira (pump laser/oscillator) followed by an Offner type stretcher. The amplification is achieved by a Thales/BMI regenerative amplifier followed by a four-pass amplifier and a spatial filter. A two-pass grating compressor is used to obtain quasi Fourier transform limited 50 fs pulses. The third harmonic of the Ti:Sa laser (265 nm, 2 µJ) was used as pump pulse. Following the initial excitation, the fundamental of the Ti:Sa (795 nm, 480 µJ) was applied as probe pulse in a multiphoton ionization process. Both laser beams were horizontally polarized. The pump-probe time delay was controlled by a delay line set on the probe beam. The time intervals between two data points
10.1021/jp9062059 2010 American Chemical Society Published on Web 10/09/2009
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Figure 3. Velocity map images taken with a mass gate around m/z ) 85, using 265 nm one-color ionization.
Figure 2. TOF mass spectra of DMB at different conditions. With pyrolysis off, the precursor is visible at m/z ) 212 (upper trace). With pyrolysis on, the precursor is fully converted.
were not constant in a given time scan and adjusted to the slope of the decay signal. Around zero time delay, data points were taken typically every 8 fs, whereas at early and late delay times, longer intervals were chosen. The beams were overlapped in a small angle and focused into the interaction region by a 70 cm lens for the 265 nm and a 50 cm lens for the 795 nm. The 795 nm beam was focused 5 cm away from the interaction region; the focus for the 265 nm beam was 14 cm away. For pump-probe contrast optimization the probe and the pump beam were attenuated until the one-color background signal was minimized. The full width at half-maximum of the laser crosscorrelation was determined to be 119 fs using nonresonant ionization of 2,3-dimethyl-2-butene (see below). In the timedelay scans, 256 shots were averaged per data point. Results In a first step the radical generation was optimized by timeof-flight mass spectrometry, with pump and probe laser set around the zero in time. As the ionization energy of DMB was estimated to be around 6.9 eV,12 at least two 795 nm probe photons are required in addition to the 265 nm pump photon for the radical ionization. The upper trace of Figure 2 shows the time-of-flight spectrum obtained when employing the precursor with the pyrolysis turned off. Mass peaks are visible at m/z ) 43, m/z ) 84, m/z ) 85, m/z ) 128, and m/z ) 212 corresponding to propyl, 2,3-dimethyl-2-butene, DMB, HI, and 2-iodo-2,3-dimethylbutane. The precursor thus shows partial photodissociation17 or dissociative photoionization18 (DPI). The intensity of the mass peak at m/z ) 84 (2,3-dimethyl2-butene) increased over the day and could be strongly reduced by pulling vacuum on the seeding line for ≈1 min. In addition the precursor changed its color from colorless to red after around 1 h at room temperature. Hence 2,3-dimethyl-2-butene is partially produced from decomposition of the precursor forming HI not linked to the pyrolysis or to dissociative photoionization. When the pyrolysis is turned on, the precursor is fully converted (Figure 2, bottom trace). The radical signal decreases as well, indicating that the cross section for dissociative
photoionization of the precursor is higher than the cross section for photoexcitation of the radical at this wavelength. To distinguish pyrolytically generated intermediates from those produced by dissociative ionization, we used VMI to optimize the pyrolysis conditions. Figure 3 presents velocity map images when using 2-iodo-2,3-dimethylbutane as precursor and gating the detector to m/z ) 85. They were recorded with different pyrolysis conditions and at an increased laser intensity (150 µJ) with horizontally polarized 265 nm light alone. At this wavelength the DMB radical is excited into the 3p Rydberg state, while the precursor is excited into the dissociative A-band, which is typical for many iodides. The images include masses around m/z ) 85 ( 2, because the mass selectivity of the photoion images is limited by the duration of the high-voltage pulse applied to the front MCP of the imaging detector. Hence in contrast to the TOF spectra in Figure 2, the mass resolution is not sufficient to separate the two mass peaks at m/z ) 85 and m/z ) 84. However, since m/z ) 84 corresponds to 2,3dimethyl-2-butene and is not formed by dissociative photoionization, it will appear as a sharp central spot. Thus it does not disturb the interpretation. With pyrolysis off, the image shows a sharp central spot and two polarized hourglass-shaped extensions (Figure 3, left-hand side). An hourglass shape has been observed for several other halides in VMI experiments.19 After the pyrolysis is turned on, the polarized part disappears and the signal intensifies to a sharp spot. In addition, the central peak shifts, indicating a slightly different velocity of the molecular beam. Even though the difference is not as pronounced as was demonstrated for the diazirines in ref 11, the images enable a differentiation between bond fissions induced by laser radiation and those induced pyrolytically. With the pyrolysis turned off, the 2-iodo-2,3-dimethylbutane dissociates into I and DMB. Due to momentum conservation the DMB radical fragment picks up a considerable amount of kinetic energy. The direct dissociation of the precursor leads to the polarized part of the image. After the pyrolysis is turned on, no precursor is present and the polarized part of the image disappears. We conclude that at these conditions DPI and/or photodissociation of the precursor are reduced and the mass signal at m/z ) 85 originates from pyrolytically produced DMB radicals. The mass spectrum recorded with pyrolysis on shows that several different species are present. Thus the examination was restricted to time-resolved mass spectrometry and no photoelectron spectra were recorded. The instrument response function (IRF) as well as the zero in time were deduced by analyzing the time dependence of the 2,3-dimethyl-2-butene mass peak, ionized nonresonantly by the 265 nm pump and 795 nm probe pulses. As visible in Figure 4, its time dependence was fitted by a Gaussian function, with a full width at half-
Side-Chain Effects on Electronic Relaxation
Figure 4. Signal intensity of mass m/z ) 84 as a function of the time delay between excitation and ionization (pump 265 nm, probe 795 nm).
Figure 5. Time dependence of mass m/z ) 85, fit by a monoexponential model.
maximum (fwhm) of 119 fs. We assume this value to be close to the cross correlation of the two laser pulses. For extracting the decay time constants of the DMB radical, the IRF was initially convoluted with a monoexponential molecular response function20 and the convolution compared to the experimental results by a least-squares fit. Time constant and amplitude of the function were optimized, whereas the fwhm and the zero in time were held fixed and taken from the timedependent ion signal of 2,3-dimethyl-2-butene. As visible in Figure 5, a monoexponential model describes either the early or the late part of the time dependence, but the overall description of the experimental data is poor. Thus an alternative fit was performed assuming a two-step deactivation mechanism20 as described in detail in ref 11. Here the two-step molecular response function is convoluted by the IRF. This mechanism reflects a decay of the initially prepared state, with a time constant τ1, down to a lower-lying transient and its consecutive deactivation with a second time constant τ2. Hence in this model the detection efficiency of the initial and intermediate transient differs and the final level is no longer ionizable. This model provides a good overall fit to the data and yields time constants of τ1 e 25 fs and τ2 ) 400 ( 50 fs, as shown in Figure 6. Note that the fast initial time constant τ1 can only be determined because an accurate zero in time is available from the m/z ) 84 peak.
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Figure 6. Signal intensity of mass m/z ) 85 as a function of the time delay between excitation and ionization (pump 265 nm, probe 795 nm). The measured signal is fit to a two-step decay model.
the C-C coordinate between the radical center and the isopropyl side chain of DMB (C2-C3 distance, Figure 1). In contrast, all C-C bonds are identical in tert-butyl. The stretching mode breaks the C3V symmetry of tert-butyl, whereas the Cs symmetry of DMB is maintained along the coordinate. TDDFT calculations were performed at the UB3P86/6-311+G** level of theory. TDDFT has recently been reported to be more precise for predicting potential energy surfaces than CIS21 and comes close to the results of CASPT2 in some cases at lower computational expense.21 For acquiring the geometries along the C-C coordinate a relaxed potential energy surface scan, using the UBMK/6-311+G** method, was performed on the ground state surface first. These structures were employed for the excited state calculations. The structure of the cation was also calculated via this method of theory. The results for tert-butyl and the DMB radical are depicted in Figure 7. The figure includes the first four roots of the excited state calculation. The calculations show that the 3p and the 3s state approach each other closely at 1.9-2.0 Å. They also show a crossing of the ground state and the 3s Rydberg state at an elongated C-C bond length (approximately 2.6 Å). This coordinate thus provides an efficient deactivation pathway. The potential energy surfaces of tert-butyl could be reproduced at the CASSCF level of theory (not depicted). The calculated vertical excitation energies slightly overestimate the experimental values; however, the qualitative trends are fully reproduced as shown in Table 1. The excited state energies for tert-butyl are very close to those published in ref 22. The 3s state of DMB is blue-shifted compared to the 3s state of tert-butyl. On the other hand, the 3p state is red-shifted. This observation is confirmed by the experiments of Wendt et al.12 A geometry optimization of the DMB+ cation is likely to give a similar molecular structure as the equilibrium geometry of the Rydberg states. Further calculations on the cation conclude that the radical center is no longer pyramidal, but planar. Also the methyl rotors slightly turn. A similar observation was also found for tert-butyl.9 However, the calculations presented in ref 9 show no indication of strong coupling to the ground state along the umbrella mode. Discussion
Computational Results Extensive calculations on the potential energy surfaces of tertbutyl have already been performed by our group at the CASSCF and CAS-PT2 level of theory.9 The calculations gave indications that a very important coordinate for the deactivation could be the C-C distance. Hence the following studies are focused on
The 3p state of the DMB radical deactivates in a two-step process most likely via the 3s state to the electronic ground state. This scenario differs from the deactivation of the 3p state of the closely related tert-butyl radical, which has also been studied in a [1 + 2′] process.9 In contrast to tert-butyl, in DMB the excitation at 265 nm is stronger due to relaxed symmetry
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Figure 7. Potential energy surfaces along the C-C stretching mode of tert-butyl and DMB. The energy surfaces are rather similar in appearance and show a crossing between the 3s state and the ground state at around 2.6 Å.
TABLE 1: Vertical Excited-State Energies of the DMB and tert-Butyl Radical Predicted by TD-B3P86/6-311+G** Calculationsa 3s 3p a
DMB (eV)
tert-butyl (eV)
4.18 (3.82) 4.77 (4.64)
4.05 (3.75) 4.92 (4.86)
Experimental values (vertical) are given in brackets.14
restrictions for the DMB radical as discussed in ref 14. Hence the excitation has a considerably higher oscillator strength, and the former by C3V symmetry forbidden excitation and deactivation transitions might become accessible for DMB. For the tertbutyl radical the deactivation of the 3p Rydberg state was surprisingly slow (τ ) 2 ps), especially when compared to the lower-lying 3s state, which deactivated on a time scale of around 100 fs.9 In the DMB radical a methyl group is replaced by a more “floppy” isopropyl group. This increases the vibrational state density of the molecule and breaks the C3V symmetry. These aspects can explain the accelerated deactivation of the DMB radical as compared to the tert-butyl. A similar effect of functional groups on the deactivation times of excited states has recently been reported for several benzenes.12 When the potential energy surfaces of tert-butyl and DMB in Figure 7 were analyzed, a crossing between the ground and the first excited state is apparent along the C-C stretching coordinate, which indicates the possible presence of a conical intersection and provides a possible deactivation channel.9 In DMB the 3s and 3p Rydberg states are energetically closer than those in tertbutyl. This might induce stronger coupling and accelerate the deactivation of the 3p state in DMB. However, since the predicted energy surfaces of both radicals have a similar appearance, the difference in deactivation time cannot be explained by the surfaces alone. The higher state density of the DMB is likely to influence the coupling between the 3s and 3p state allowing a faster deactivation as compared to tert-butyl. A second interesting point to discuss is the two-step deactivation of the 3p state of DMB when compared to tert-butyl. Since the reported time-dependent ion signal of tert-butyl in ref 9 has a lower signal-to-noise ratio and the zero in time was not determined as accurately, it is possible that the first deactivation step is simply not resolved in those measurements. The rapid decay of 25 fs in DMB is likely assigned to an irreversible movement of the initial wavepacket out of the Franck-Condon region of excitation, whereas the longer time constant of 400 fs would correspond to a deactivation initiated by the 3p to 3s energy transfer. The time constant of 400 fs is five times shorter than the time constant of the corresponding energy transfer in tert-butyl, in line with the discussion above on the higher state density in DMB compared to tert-butyl.
Conclusion The 3p state of the dimethylbutyl radical (DMB) deactivates in a two-step process after excitation at 265 nm. The first time constant is very fast, around 25 fs or shorter, and is likely to correspond to a quick movement of the molecule out of the Franck-Condon region after optical excitation. The second time constant (τ2 ) 400 fs) is assigned to the lifetime of the 3p state. The fast deactivation time is in agreement with the broad and unstructured absorption spectra. In contrast, after excitation of the 3p state of tert-butyl, a lifetime of 2 ps was observed and no second time constant was apparent in the experiment. We believe that the first time constant was not resolved in those experiments. The possible crossing between the first excited 3s state and the ground state at elongated C-C bond lengths computed for both DMB and tert-butyl will produce radicals far from the equilibrium geometry after internal conversion. This might contribute to an explanation of the unusually slow photodissociation observed experimentally for several alkyl radicals and also found in trajectory calculations.23 In these studies, quasi-periodic trajectories, which are located away from the equilibrium geometry, were found to prohibit C-H dissociations on the ground state surface. Acknowledgment. B.N. gratefully acknowledges a scholarship by the “Fonds der Chemischen Industrie”. Financial support was provided by the Deutsche Forschungsgemeinschaft (Fi 575/ 3-4), Laserlab Europe (Sixth Framework Program of the EU, contract no. RII3-CT-2003-506350), and the DAAD, Egide (Procope). We thank the technical laser staff of the CEA for their support. References and Notes (1) Franklin, J. L. Annu. ReV. Phys. Chem. 1967, 18, 261. (2) Baulch, D. L. G., J. F.; Richter, R. Chem. Eng. Sci. 1991, 46, 2315. (3) Thaddeus, P.; Gottlieb, C. A.; Mollaaghababa, R.; Vrtilek, J. J. Chem. Soc., Faraday Trans. 1993, 89, 2125. (4) Flasar, F. M. A., R. K.; Conrath, B. J.; Gierasch, P. J.; Kunde, V. G.; Nixon, C. A.; Bjoraker, G. L.; Jennings, D. E.; Romani, P. N.; SimonMiller, A. A.; Bezard, B.; Coustenis, A.; Irwin, P. G. J.; Teanby, N. A.; Brasunas, J.; Pearl, J. C.; Segura, M. E.; Carlson, R. C.; Mamoutkine, A.; Schinder, P. J.; Barucci, A.; Courtin, R.; Fouchet, T.; Gautier, D.; Lellouch, E.; Marten, A.; Prange, R.; Vinatier, S.; Strobel, D. F.; Calcutt, S. B.; Read, P. L.; Taylor, F. W.; Bowles, N.; Samuelson, R. E.; Orton, G. S.; Spilker, L. J.; Owen, T. C.; Spencer, J. R.; Showalter, M. R.; Ferrari, C.; Abbas, M. M.; Raulin, F.; Edgington, S.; Ade, P.; Wishnow, E. H. Science 2005, 308, 975. (5) Sawamoto, M. K., M. Trends Polym. Sci. 1996, 4, 371. (6) Uemura, T. K. Kana; Horike, Satoshi; Kawamura, Takashi; Kitagawa, Susumu; Mizunob, Motohiro; Endo, Kazunaka Chem. Commun. 2005, 48, 5968. (7) G. A. Olah, A. M. Hydrocarbon Chemistry; John Wiley & Sons, Inc.: New York, 1995. (8) Zierhut, M.; Noller, B.; Schultz, T.; Fischer, I. J. Chem. Phys. 2005, 122, 094302.
Side-Chain Effects on Electronic Relaxation (9) Noller, B.; Maksimenka, R.; Fischer, I.; Armone, M.; Engels, B.; Alcaraz, C.; Poisson, L.; Mestdagh, J.-M. J. Phys. Chem. A 2007, 111, 1771. (10) Noller, B.; Poisson, L.; Maksimenka, R.; Fischer, I.; Mestdagh, J.-M. J. Am. Chem. Soc. 2008, 130, 14908. (11) Noller, B.; Poisson, L.; Maksimenka, R.; Gobert, O.; Fischer, I.; Mestdagh, J. M. J. Phys.Chem. A 2009, 113, 3041. (12) Lee, S.-H.; Tang, K.-C.; Chen, I.-C.; Schmitt, M.; Shaffer, J. P.; Schultz, T.; Underwood, J. G.; Zgierski, M. Z.; Stolow, A. J. Phys. Chem. A 2002, 106, 8979. (13) Zewail, A. H. J. Phys. Chem. 1996, 100, 12701. (14) Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1984, 81, 717. (15) Kohn, D. W.; Clauberg, H.; Chen, P. ReV. Sci. Instrum. 1992, 63, 4003. (16) Stone, H.; Shechter, H. J. Org. Chem. 1950, 15, 491.
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3049 (17) Kadi, M.; Davidsson, J.; Tarnovsky, A. N.; Rasmusson, M.; Åkesson, E. Chem. Phys. Lett. 2001, 350, 93. (18) Waits, L. D.; Horwitz, R. J.; Daniel, R. G.; Guest, J. A.; Appling, J. R. J. Chem. Phys. 1992, 97, 7263. (19) Roeterdink, W. G.; Janssen, M. H. M. Chem. Phys. Lett. 2001, 345, 72. (20) Pedersen, S.; Zewail, A. H. Mol. Phys. 1996, 89, 1455. (21) Sobolewski, A. L.; Domcke, W. Phys. Chem. Chem. Phys. 1999, 1, 3065. (22) Lengsfield, B. H. S., P. E. M., III; Liu, B. J. Chem. Phys. 1984, 81, 710. (23) Bach, A.; Hostettler, J. M.; Chen, P. J. Chem. Phys. 2005, 123, 021101.
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