2358
J . Phys. Chem. 1990, 94, 2358-2361
Time-Resolved Pump-Probe Photoionization Study of Excited-State Dynamics of Phenol-( H,O), and Phenol-(H,O), Robert J. Lipert and Steven D. Colson*tt Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 0651 1 !Received: September 26. 1989)
Time-resolved pump-probe photoionization mass spectrometry has been used to study the excited-state dynamics of phenol-( H,O), and phenol-(H,O), formed in a supersonic expansion. Singlet-state lifetimes were measured by using both soft (355 nm) and hard (193 nm) ionization. Phen~l-(H,O)~was found to have an anomalously short singlet-state lifetime ( 6 i 1 ns) compared to those of other phenol-water complexes. We interpret this as evidence that phenol-(H,O), lacks the type of hydrogen bonding found in phenol-(H20) and phenol-(H20),. In addition, we have used 193-nm ionization with a variable delay to study the fragmentation of these complexes in the triplet manifold. It was found that, after undergoing intersystem crossing from the S I origin level, both complexes rapidly lose two water molecules. This number is apparently limited by the amount of vibrational excitation acquired in the triplet manifold which, in turn, is fixed by the singlet-triplet energy gap. We also investigated the fragmentation that occurs following 193-nm ionization of the SI origin level of the complexes. Here, phen~l-(H,O)~rapidly loses only one water molecule while phenol-(H,O), loses two. Interestingly, 193-nm ionization could potentially deposit more than twice as much vibrational energy in the ion compared to the vibrational excitation in the triplet manifold. These results suggest that Franck-Condon factors are important in limiting the extent to which these complexes dissociate following ionization,
Introduction An interesting application of molecular beam technology is in the investigation of solute-solvent interactions.] Using a supersonic expansion, it is possible to form small, cold, isolated complexes between solute and solvent molecules. This allows one to systematically investigate the changes that occur in the properties of these microsolutions as solvent molecules are added to an initially isolated solute molecule. An especially useful spectroscopic probe of the properties of these complexes is resonance-enhanced multiphoton ionization mass spectrometry (MPI-MS) which allows one to probe both neutral and ionic, mass-selected species2 One variety of this technique that we have employed involves the use of two lasers; one laser is used to promote the complex to an excited electronic state, and the other to ionize the excited complex. By varying the timing between the two lasers, one can obtain information on the excited-state dynamics of the c ~ m p l e x e s . ~ We have used this approach to study hydrogen-bonded complexes formed between phenol and water. Our earlier studies revealed that p h e n ~ l - ( H ~ Ois) ~qualitatively different from phenol-water complexes containing one and three water molecules. )~ The spectrum of the S , origin region of the ~ h e n o l - ( H ~ Ocomplex is highly ~ o n g e s t e d . ~Its low-frequency vibrational progressions are reminiscent of the vibronic structure of van der Waals clusters. In contrast, the SI origin of phenol-(H20) and of phenol-( H20), are intense, single peaks.s*6 Also, the triplet-state dissociation dynamics of the complexes show interesting variations with complex size. In a previous study: we probed for triplet-state dissociation products at a fixed, 300-ns delay after pumping the SI state. In that experiment, the distribution of triplet fragments was monitored as the pump laser wavelength was varied. The results showed that, within 300 ns of intersystem crossing, both phenol-(H20) and phenol-(H,O), dissociate to bare phenol. However. phenol-(H20), loses only two water molecules. Moreover, we found that substantially more energy is required to remove two water molecules from p h e n ~ l - ( H ~ Othan ) ~ from phen~i-(H,O)~.These findings suggest that the bonding occurring in the complex containing two water molecules is significantly different from that in the one- and three-water complexes. Previous work also showed that the singlet-state lifetime of a phenol-water complex could provide information on its structure. This is because the formation of a hydrogen bond between one
' Present address: Battelle Pacific Northwest Laboratory-MSRC, P.O. Box 999, Richland. WA 99352. 0022-3654/90/2094-2358$02.50/0
water molecule and the O H hydrogen of phenol lengthens the singlet-state lifetime over 40-f0ld.'-~ The reason for the lifetime lengthening is that the hydrogen bond reduces the ability of the O H stretching vibration to act as an accepting mode for internal conversion to the ground state. Since internal conversion is by far the fastest process depopulating the SI state of phenol, the result is a longer singlet-state lifetime.9 We therefore expect to find longer singlet-state lifetimes in phenol complexes in which this bonding arrangement occurs than in complexes in which it does not. We report here the singlet-state lifetimes of phenol(H20), and phen~l-(H,O)~, measured using time-resolved pump-probe photoionization. The structural implications of these lifetimes are then discussed. As mentioned above, we previously probed for triplet-state fragmentation products at a fixed delay after pumping the singlet state of the complexes. We have extended this work by continuously monitoring the population of the various fragments of an individual complex as the pump-probe delay was increased from 0 to 160 ns. In this experiment, the pump laser wavelength was tuned to the SI origin of the complex. Thus, the experimental procedure was the same as in the lifetime measurements except that, here, the detection of triplet species requires high-energy ionization. As has been discussed by Dietz et a1.,I0 this is because the ion must be formed in approximately the same vibrational state as the vibrationally excited triplet species. This additional energy is equal to the singlet-triplet energy gap. In phenol, this gap is approximately]' 7700 cm-I. However, energies high enough to ionize triplet species can produce highly vibrationally excited parent ions when singlet-state complexes are ionized. These ions can then dissociate. As a result, the time evolution of the fragment ion signals can be a composite of signals produced by dissociation before and after ionization. Still, the time dependence of the See,for example: Even, U.; Jortner, J. J . Chem. Phys. 1983, 78, 3445. (2) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981.85, (1)
3739. (3) Duncan, M. A.; Dietz, T. G.; Liverman, M. G.; Smalley, R. E. J . Phys. Chem. 1981, 85, 7. (4) Lipert, R. J.; Colson, S. D. Chem. Phys. Lett. 1989, 161, 303. ( 5 ) Fuke, K.; Kaya, K. Chem. Phys. Lett. 1983, 94, 97. (6) Lipert, R. J.; Colson, S. D. J. Chem. Phys. 1988, 89, 4579. (7) Sur, A,; Johnson, P. M. J . Chem. Phys. 1986, 84, 1206. (8) Lipert, R. J.; Bermudez, G.; Colson, S. D. J . Phys. Chem. 1988, 92, 3801.
(9) Lipert, R. J.; Colson, S. D. J . Phys. Chem. 1989, Y3, 135. ( I O ) Dietz, T. G.; Duncan, M . A.; Smalley, R. E. J . Chem. Phys. 1982, 76, 1227. ( 1 I ) McClure, D. S. J . Chem. Phys. 1949, 17, 905.
0 1990 American Chemical Society
Excited-State Dynamics of Phenol-Water Complexes
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2359
signals provides information on their origin. Therefore, pumpprobe photoionization can be used to reveal the dissociation channels that are open in both the triplet state and the ion. Thus, we also present here the results of time-resolved pump-probe experiments on ~ h e n o l - ( H ~ Oand ) ~ phenol-(H20), using 193-nm ionization. These, together with previous results on phenol and phenol-(HzO), provide an overview of the photofragmentation of all phenol-water complexes for which absorption spectra have been obtained.
Experimental Section The experimental arrangements were similar to those previously described.* Briefly, helium carrier gas at a pressure of 15 atm was passed through a heated (approximately 50 "C) sample holder containing phenol (B&A) dissolved in an excess of water. The phenol-water complexes were formed in a supersonic expansion as the seeded gas was continuously expanded through a 12.5-wm nozzle into a vacuum chamber maintained at lo4 Torr. Approximately 2 mm from the nozzle the free jet was crossed with the unfocused, frequency-doubled output of a Nd:YAG pumped dye laser system. The electronically excited molecules were then ionized with either 193-nm radiation from an ArF excimer laser or the third or fourth harmonics of a Nd:YAG laser. The probe beam entered the interaction region counterpropagating to the pump beam and focused with a 1-m lens. A repelling field of 500 V/cm accelerated the resulting ions into a time-of-flight mass spectrometer. The entire mass spectrum was captured after every laser shot with a Tektronix 7612D digitizer which also performed the signal averaging. The firing of both lasers was triggered by a Stanford Research Systems DG535 digital delay/pulse generator interfaced to an LSI-I 1/23 computer. The pump-probe delays were increased in steps of 1 or 2 ns. The estimated time jitter in each laser is 3 ns.
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Results Singlet-State Lifetimes. The present lifetime measurements were made using the shortest light pulses we had available. The pump pulse consisted of the output of a dye laser pumped by a Nd:YAG laser running in fast pulse mode (fwhm = 2-3 ns). Similarly, ionization was effected by the third harmonic of a second Nd:YAG laser, also operated in fast pulse mode. For an accurate comparison of the singlet-state lifetimes of the various complexes, we have remeasured the singlet-state lifetime of phenol-(H20) using the same laser configuration. With soft (near threshold, low power) ionization, all complexes produced only a parent ion signal. The time dependence of this signal reflects the decay of the initially excited singlet state which, in all cases, was the SI origin band. The singlet-state decays are compared in Figure 1. The solid lines are nonlinear least-squares fits of the data to a convolution of a Gaussian excitation pulse with a single-exponential decay, superimposed on a sloping background. In addition, we measured all singlet-state lifetimes using hard (high energy, low power) ionization. For these measurements, either an ArF excimer laser (193 nm) or the fourth harmonic of a Nd:YAG laser (266 nm) was used. The lifetimes obtained by the two methods were effectively the same. The result for phenol-(H,O), T~ = 15 f 1 ns, is in excellent agreement with our previous hard ionization result of 15 f 1 ns.* The singlet-state lifetime of phenol-(H20), was found to be 6 f 1 ns. This was quite surprising, especially considering the 18 f 1 ns lifetime found for phenol-(HzO)3. Photofragmentation. The results obtained for the photofragmentation of phenol-water complexes are summarized in Figure 2. Each row of this figure shows the time dependence of all the ion signals produced by a particular complex. Each complex was initially excited to its S, origin level. After a variable delay, 193-nm ionization was used to detect surviving, electronically excited species. To provide an overview of the exicted-state dynamics of phenol-water complexes, we will first review the results previously obtained for phenol and phenol-(H20). Phenol. With hard ionization, the ion signal reflects the total excited-state population (singlet plus triplet states). The time
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Time (ns) Figure 1. S, origin level decays of phenol-water complexes as determined by pump-probe photoionization using 355-nm radiation for ionization. The solid lines are nonlinear least-squares fits of the data to a convolution of a Gaussian excitation pulse with a single-exponential decay. (a) Phenol-(H20), T, = 15 f 1 ns. (b) Phenol-(H,O),, T, = 6 i 1 ns. (c) Phenol-(H,O),, T, = 18 f 1 ns.
dependence of this signal is biexponential. The singlet-state lifetime (2 ns) is given by the decay of the fast component. The slower decay (T = 300 ns) is that of the hot triplet state.' Phenol-(H,O). The phenol-(H20)+ ion displays a single-exponential decay. The time dependence of phenol+, following excitation of phenol-(H20), is biexponential. Phenol-(H20),. Phen~l-(H,O)~+shows a single-exponential decay. The same behavior is seen in the phenol-(H,O)+ mass channel. In contrast, the phenol+ signal shows no obvious decay at all. Within the time window during which we were able to monitor it, the signal simply rose to a plateau. PhenoZ-(HzO),. The time dependence of the various ion signals originating from the phen~l-(H,O)~complex shows all the patterns
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2360 The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
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2' Figure 2. An overview of the excited-state dynamics of phenol-water complexes as revealed by pump-probe photoionization using hard ionization. In each row are shown the time evolutions of all the ion signals generated by 193-nm ionization of a particular complex. For phenol-(H,O)+ and phenol-(H,0)2+ produced from phenol-(H,0)2, 266-nm light was used for ionization.
mentioned above. Phenol-(H,O),+ and p h e n ~ l - ( H ~ O )both ~+ display single-exponential decay. The decay of the phenol-(H20)+ signal is biexponential while phenol' shows no decay on the time scale of our experiment.
Discussion Singlet-State Lifetimes. The singlet-state lifetime of phenol-(H,O),, 6 f 1 m,is shorter than expected, based on the results obtained for phenol-( H 2 0 ) and phenol-( H20),. In phenol-( H,O), the lengthening of the singlet-state lifetime, compared with that of phenol, has been interpreted as being due to the lower phenol O H stretching vibrational frequency in the complex. The lower vibrational frequency is a result of the formation of a hydrogen bond between the oxygen of water and the phenol O H hydrogen. This lower frequency leads to smaller Franck-Condon factors for internal conversion and hence to a lower rate of internal conversion.I2 Since the singlet-state lifetime of phenol is primarily determined by this rate, hydrogen bonding lengthens the singlet-state lifetime. If hydrogen bonding to the phenol OH hydrogen also occurs in phenol-(H20),, one expects that it would result in a singlet-state lifetime that is similar to that of phenol-(H20). However, we find that phenol-(H20), has a singlet-state lifetime that is about one-half that of phenol-(H,O). This brings into question either the above interpretation of the effects of hydrogen bonding or the existence of a hydrogen bond (12) See ref 8 and references therein.
in p h e n ~ l - ( H ~ O )The ~ . result for phenol-(H,O), throws some light on this issue. We obtained a singlet-state lifetime of phenol-(H,O), of 18 f 1 ns. This is 3 ns longer than the lifetime of phenol-(H,O) and 2-3 times longer than the lifetime of pher~ol-(H,O)~.This lifetime is consistent with the above description of the effects of hydrogen bonding on singlet-state lifetimes. Thus, phenol-(H20), is seen to have an anomalously short singlet-state lifetime. This finding is also consistent with the SIstate spectra of phenol-water complexes. The highly congested SI origin region of phenol(H20)2is quite unlike those of phenol-(H,O) and phenol-(H20),. Thus, the spectra and lifetimes of the Complexes containing one and three water molecules suggest these complexes contain a similar hydrogen bond between a water oxygen and the O H hydrogen of phenol. This type of bonding rearrangement apparently does not exist in phen~l-(H,O)~. Our earlier indicated that the singlet-state lifetimes of phenol complexes are correlated with the phenol OH vibrational frequency, at least until this frequency becomes comparable to the C H stretching frequency. (At that point C H vibrations will dominate radiationless transitions in phenol, just as they do in benzene.],) Purcell and Drago14 found that, in solutions containing a hydrogen-bonding partner, the shift in OH vibrational frequency in phenol is roughly correlated with hydrogen-bond (13) See, for example: Knee, J. L.; Otis, C. E.; Johnson, P. M. J . Chem. Phys. 1984, 81, 4455, and references therein. (14) Purcell, K. F.; Drago, R. S . J . Am. Chem. SOC.1967, 89, 2874.
Excited-State Dynamics of Phenol-Water Complexes strength. The conclusion is that phenol-(H,O), contains either a particularly weak hydrogen bond or none at all. This is in line with our earlier finding6 that appreciably less energy is consumed when two water molecules are lost from triplet phenol-(H,O), than when the same number is lost from triplet phenol-(H20),. Photofragmentation. Phenol-(H,O). The lack of a triplet component in the parent ion signal is taken as evidence that no measurable amount of triplet complex exists.’ This is reasonable because the triplet formed through intersystem crossing contains approximately 7700 cm-I of vibrational energy while a typical hydrogen-bond dissociation energy is approximately 2000 cm-l. One then expects to detect a long-lived triplet fragment (phenol, cooled by the evaporation of one water molecule) in the phenol+ mass channel. This expectation is confirmed. The phenol+ signal also contains a short-lifetime component, Le., a decay that follows the singlet decay observed in the parent ion. Since we are pumping the vibrationless level of SI,the excited singlet-state monomer cannot be formed through dissociation. Therefore, dissociation of phenol-(H20) must be occurring following ionization. Apparently, Franck-Condon factors for ionization from the SI0-0 result in the population of vibrational levels both above and below the threshold for observable dissociation of the ionized complex. This results in decays in both phenol+ and phenol-(H20)+ that follow the singlet-state lifetime.8 One could also argue that the same dissociation is occurring following ionization of a triplet complex, except that here the fragmentation is complete. This could then account for there being signal in only the phenol+ channel, at long times. However, the neutral triplet and the ion formed through ionization of this triplet should have roughly comparable amounts of vibrational excitation. Moreover, the dissociation energy of the ionized complex will be greater than that of the neutral complex as a result of ion-dipole interactions between the ionized chromophore (phenol) and water. Therefore, it is more likely that dissociation is occurring in the neutral species. Phenol-(H20),. The arguments used above carry over to the larger complexes. Thus, phenol-(H,O), is seen to dissociate completely following intersystem crossing. We detected no triplet parent or triplet phenol-(H20). The only signal observed at long times was due to the ionization of triplet phenol. However, ionization of the singlet complex yields only parent ion and phenol-(H20)+. No significant amount of phenol+ is formed, as evidenced by the lack of a singlet lifetime component in the phenol+ signal decay. Therefore, unlike intersystem crossing, ionization at 193 nm does not pump sufficient vibrational energy into the complex to lead to complete dissociation, even though sufficient energy is available in the 193-nm photon. Phenol-(H,O),. Since no ion signal was seen at long times in the parent or phenol-(H20),+ mass channels, phenol-(H,O),, like phenol-(H20),, loses at least two water molecules following intersystem crossing. However, signals were seen for both phenol-(H20)+ and phenol+ at long times. The most likely interpretation of this is that phenol-(H,O) is formed by fragmentation following intersystem crossing. This complex then fragments, to
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2361 an extent determined by Franck-Condon factors, following ionization. Thus, a signal is seen at long times in both the phenol-(H,O)+ and phenol+ ion channels. Hard ionization of singlet phenol-(H,O), is seen to produce phenol-(H,O),+, phenol-(H20),+, and phenol-(H,O)” but no phenol’. Therefore, unlike phenol-(H20),, phenol-(H,O), can lose up to two water molecules following ionization from the SI origin. This is interesting considering our earlier conclusion that phenol-(H,O), contains a weaker hydrogen bond than does phenol-(H20),. On this basis alone, we expect that, in the ion, phenol-(H,O), would be more likely to lose two water molecules than would phenol-(H,O),, contrary to what is observed. Apparently, Franck-Condon factors are controlling the extent of fragmentation in the ion. Thus, these fragmentation patterns are measures of the relative geometry differences that exist between the neutral and ionized forms of the complexes. Our results then show that phen~l-(H,O)~undergoes a larger geometry change following ionization than does phenol-(H20),. This allows higher vibrational levels to be populated during ionization and hence a larger degree of fragmentation to occur. However, neither complex fragments all the way down to the bare monomer ion following ionization of the S Iorigin level.
Conclusions Lifetime measurements on phenol-(H,O), and phen~l-(H,O)~ confirm the unusual nature of the phen~l-(H,O)~complex. It seems unlikely that the properties of this complex would be as different as they are from those of the other phenol-water complexes if it were formed by the simple addition of a water molecule to phenol-(H20), i.e., by hydrogen bonding either to the phenol oxygen or to the first water molecule. It might be that the original hydrogen bond between water and phenol is weakened by a large amount of strain in the phenol-(H,O), complex. Alternatively, it might be energetically more favorable for the water dimer to form multiple hydrogen bonds with the aromatic ring than to form a single hydrogen bond to the phenol O H hydrogen. A time-resolved study of the photofragmentation of these complexes reveals that both rapidly lose two water molecules after undergoing intersystem crossing. In contrast, the triplet phenol-(H20) fragment of phenol-(H,O), is knownI5 to live longer than 40 ws. It therefore might be stable with respect to the loss of the final water molecule. That phenol-(H,O), does not lose two water molecules following hard ionization, while phenol-(H,O), does, suggests the importance of Franck-Condon factors in controlling this form of photofragmentation. This in turn is an indication of the relative differences in geometry between the neutral and ionized forms of these two complexes. Acknowledgment. This research was supported by the National Institutes of Health. (15) Goto, A.; Fujii, M.; Mikami, N.; Ito, M. J . Phys. Chem. 1986, 90,
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