Picosecond Dynamics of Avobenzone in Solution - The Journal of

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Picosecond Dynamics of Avobenzone in Solution Adam D. Dunkelberger,*,† Ryan D. Kieda, Brett M. Marsh, and F. Fleming Crim Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States ABSTRACT: Avobenzone, a dibenzoylmethane compound commonly found in sunscreens, can photoisomerize after exposure to near-ultraviolet light. At equilibrium, avobenzone exists as a chelated enol characterized by a strong intramolecular hydrogen bond. Many nanosecond- to microsecond-scale experiments have shown that the photoisomerization involves several nonchelated enol (NCE) isomers and reaction paths, including some that reduce avobenzone’s efficacy as a sunscreen. Because some of the NCE isomers are unstable, these experiments do not directly measure their spectroscopic signatures. Here, we report the dynamics of avobenzone on the picosecond time scale. We excite avobenzone at 350 nm and observe the formation and relaxation of new isomers and vibrationally excited species with broadband visible probe pulses and 266 nm probe pulses. Our results show the first direct evidence of two unstable NCE isomers and establish the lifetimes of and the branching ratio between these isomers.

1. INTRODUCTION Dibenzoylmethane compounds, widely used as components in sunscreens, have rich photochemistry involving isomerization through multiple pathways after ultraviolet excitation.1−10 The photoreactions of dibenzoylmethanes include cis−trans isomerization, isomerization about single bonds to form rotamers, and enol−keto tautomerization.1 The variety of photoisomerization pathways present in dibenzoylmethanes makes them an attractive target for fundamental studies of isomerization dynamics. This rich photochemistry is often detrimental to sunscreen efficacy because the photochemical products do not absorb near-ultraviolet light as efficiently as the reactants from which they are formed.2,11−14 A detailed understanding of the photochemistry of the dibenzoylmethane family of compounds could aid in the design of more stable absorbers for sunscreens. Ultrafast spectroscopic techniques are a powerful tool for detailed studies of photochemical pathways. Ultrafast experiments can both answer questions about the mechanistic details of specific reactions15−19 and test the accuracy of more general theories of reaction dynamics.20−24 Advances in theoretical methods also allow for sophisticated, accurate calculations of the dynamics in photoisomerizing systems.25−29 Here, we present the results of ultrafast transient electronic absorption experiments on avobenzone, a particularly common dibenzoylmethane.30 Avobenzone has received considerable attention as the target of nanosecond to millisecond time scale flash photolysis experiments. Figure 1 shows a simple, schematic representation of the results of this body of work. In discussing the results of these experiments, we adopt Cantrell and McGarvey’s language for discussing the isomers of avobenzone.1 Avobenzone predominantly exists in a chelated enol (CE) form, characterized by an intramolecular hydrogen bond, in © XXXX American Chemical Society

most solvents. Flash photolysis experiments show that a nonchelated enol, designated NCE1, is formed through cis− trans isomerization following excitation near the absorption maximum of CE at 350 nm.1,8 Analysis of the kinetics of the reverse reaction, the isomerization of NCE1 to CE, suggests an equilibrium between NCE1 and another isomer.8 Cantrell and McGarvey identify this other isomer as NCE2, a rotational isomer formed through rotation about a single bond. Because the barrier to rotation about a single bond is much lower than to cis−trans isomerization, leading to much faster relaxation, they do not directly observe NCE2. Tobita et al. and Yamaji and Kida suggest a third nonchelated enol, NCE3 formed through rotation about the carbon−oxygen bond of the hydroxyl group.7,8 NCE3 would likely relax to CE even more quickly than does NCE2 since the moment of inertia of this rotation is quite small. Neither NCE2 nor NCE3 has been directly observed via flash-photolysis experiments. We expect NCE2 and NCE3 to relax to CE in femtoseconds or picoseconds because of the relative ease of relaxation through single-bond rotation. As Cantrell and McGarvey note, because each benzene ring of avobenzone possesses different substituents, each isomer identified above also has an equivalent isomer resulting from intramolecular proton transfer between the two oxygen atoms.1 NMR studies on dibenzoylmethane, where R1 and R2 = H, show that this proton transfer is rapid enough in solution that the proton is delocalized between the two oxygen atoms.31 We show only one set of proton-transfer isomers in our schematic diagrams for clarity. Received: February 17, 2015 Revised: May 10, 2015

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DOI: 10.1021/acs.jpca.5b01641 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

diverts half of the amplifier output to a single-wavelength probing instrument, and we use the other half in a broadband probing arrangement. In both cases, a home-built β-barium borate (BBO) optical parametric amplifier (OPA) converts the 800 nm pulses to the near-infrared. Subsequent BBO crystals successively double the frequency of the higher energy signal pulses from the OPA to generate 350 nm pulses that we use to excite avobenzone electronically. The OPA and frequencyquadrupling processes are inefficient, resulting in 350 nm pulse energies of 0.5 μJ in the broadband experiment and 1 μJ in the single-wavelength experiment. We generate 266 nm pulses via two-stage third-harmonic generation for the single-wavelength measurements, in which we reduce the intensity of these probe pulses to avoid photoproduct buildup on the windows of the sample cell. For the broadband measurements, we focus a small portion of the 800 nm amplifier output into a 3 mm CaF2 substrate to generate a white-light supercontinuum between 350 and 600 nm that we use to interrogate the dynamics of avobenzone. Lenses focus and collimate the infrared, visible, and ultraviolet beams that we generate in the frequency quadrupling process. We use a parabolic mirror to collimate the continuum probe beam achromatically. A neutral density filter splits the continuum pulses into probe and reference pulses and an ultraviolet beamsplitter (ThorLabs) does the same for the 266 nm probe pulses. In both cases, retroreflectors on computercontrolled delay stages allow us to set the delay between the excitation and probe pulses. A peristaltic pump circulates the sample solution between two CaF2 windows surrounding a 1 mm spacer in both the broadband and single-wavelength experiments.33 The 350 nm excitation pulses intersect either the single-wavelength, 266 nm probe pulses or broadband visible probe pulses at a small angle within the sample cell. In either case, a parabolic mirror with a focal length of 100 mm focuses the probe beam into the sample and a lens with a focal length of 200 mm focuses the excitation beam. Matched integrating photodiodes detect each 266 nm probe and reference pulse. A slotted-wheel chopper blocks every other excitation pulse and allows us to calculate the transient absorption of the 266 nm probe induced by the excitation pulse. We average the transient absorption signal obtained from 1500 pairs of laser shots to obtain noise levels on the order of tens of μOD. The broadband detection scheme32 uses a home-built spectrometer to disperse the probe and reference pulses and a pair of matched, 1024-channel photodiode arrays to detect these pulses. For the avobenzone experiments described below, we collected five exposure pairs comprising 200 shots at each time delay. We use avobenzone and all solvents as purchased (SigmaAldrich). For each solvent, we prepared 0.1 mM solutions of avobenzone in that solvent. This concentration and the 1 mm path length of our sample cell yields absorption near 1 OD at 350 nm in each solvent. Figure 2 shows steady-state absorption spectra of avobenzone in cyclohexane, methanol, and acetonitrile. Because of the well-studied photodegradation of avobenzone, we compared the results of our transient absorption experiments with fresh samples to the results from samples irradiated by our excitation and probe pulses for several hours. Long irradiation can cause photoproducts to accumulate on the surfaces of the cell, and the attendant increased absorption of the probe light degrades the signal-to-noise ratio. We move the sample cell between scans to avoid this degradation.

Figure 1. Schematic representation of the isomerization pathways of avobenzone. Three distinct rotation pathways lead from CE− avobenzone to NCE−avobenzone isomers. Other authors have identified NCE1 in transient absorption measurements on slower time scales than those reported here. Other studies show strong evidence for NCE2 and NCE3 but do not directly report their formation and relaxation kinetics.

An enol to keto tautomerization also plays a role in the photochemistry of avobenzone. The keto product is a key intermediate in photodegradation of avobenzone. Yamaji and Kida recently reported the results of flash photolysis experiments in which they observed the formation of keto avobenzone in acetonitrile but not in other solvents. Their results suggest that the enol to keto reaction is more favorable in the NCE1 geometry than in CE but that protic solvents stabilize NCE1, preventing the reaction to the keto isomer.8 The work we describe here uses ultrafast transient absorption spectroscopy and electronic structure calculations to study the isomerization reactions of avobenzone. We excite avobenzone in the CE form at 350 nm and monitor the differential absorption of the sample at 266 nm, where the nonchelated enol and keto isomers absorb more strongly than CE− avobenzone, and between 350 and 600 nm, where we observe signatures of electronically and vibrationally excited avobenzone molecules. The signals we obtain with visible probing identify relaxation from the electronically excited state in hundreds of femtoseconds followed by vibrational relaxation in several picoseconds. Analysis of the dynamics observed with 266 nm light in different solvents, in combination with our calculations, shows that we observe the prompt formation and subsequent decay of NCE2. Long-lived offsets from zero in our transient absorption measurements arise from the comparatively slow dynamics of NCE1 and the keto isomer. We find no evidence of reaction from NCE2 to the keto isomer, but we do observe NCE2 directly.

2. EXPERIMENTAL METHODS Our instrument, described previously,32,33 relies on a Ti:sapphire regenerative amplifier (Coherent Legend Elite) that generates 3.5 mJ pulses centered at 800 nm. A beamsplitter B


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time, but one decays in hundreds of femtoseconds while the other persists for picoseonds. The spectrum of the fast component contains the stimulated emission between 440 and 500 nm and a positive feature between 375 and 425 nm that we assign to excited-state absorption from the first excited singlet state (S1) to higher energy states. The slow component comprises the ground-state bleach and another feature that overlaps the positive feature in the fast component. We assign this more slowly decaying positive feature to vibrationally excited reactant molecules with their characteristic shift of the electronic absorption to lower energy. Our weak excitation pulses lead to small signals compared to the noise of our instrument. This low signal-to-noise ratio precludes our using SVD analysis quantitatively. We instead use our SVD results to inform our data analysis. We expect that the stimulated emission should decay with the faster time constant from the SVD results and that the ground-state bleach should recover with the slow time constant. We fit the dynamics globally, constraining the prominent visible feature to decay biexponentially with a fast component equal to the rate of stimulated emission decay and a slow component equal to the ground-state bleach recovery. Figure 4 shows representative

Figure 2. Steady-state absorption spectra of avobenzone in cyclohexane, methanol, and acetonitrile. All three spectra are from 10−4 M solutions of avobenzone in the solvent indicated in a cell with 1 mm path length.

3. RESULTS 3.1. Broadband Visible Detection. Figure 3 shows a contour plot of the transient absorption and emission of

Figure 3. Contour plot generated from one scan of transient absorption of 10−4 M avobenzone in cyclohexane after excitation at 350 nm. Labels indicate stimulated emission from the electronically excited state, transient absorption from electronically and vibrationally excited avobenzone, and ground-state bleaching.

Figure 4. Cuts through the transient spectrum of avobenzone in cyclohexane at representative wavelengths after excitation at 350 nm. Open circles are transient absorption values obtained by averaging across 20 detector pixels centered at the indicated wavelength. Solid lines are fits to the time evolution as described in the text.

avobenzone after excitation of the CE isomer at 350 nm. The spectrum of the continuum probe pulse overlaps with the absorption of reactant CE molecules, leading to a strong ground-state bleach signal between 340 and 375 nm. As the reaction regenerates CE molecules, this feature recovers to a negative baseline that indicates the formation of long-lived photoproducts. Below a wavelength of 340 nm, we have too little probe intensity to detect the absorption. We assign the short-lived feature between 440 and 500 nm to stimulated emission from the electronically excited state populated by the 350 nm excitation pulse. The decay of the stimulated emission reflects depopulation of the electronically excited state. The wavelength and time evolution of this feature are consistent with stimulated emission in other photoisomerization systems.33−35 The strong, positive transient feature between 375 and 425 nm is asymmetric and appears to have a biexponential decay. Because of these observations, we turned to singular value decomposition (SVD) to examine whether multiple species contribute to this feature. We used the SVD routine in the Transient Absorption software package written by Sfeir to decompose our transient spectra into their component species.36 The SVD analysis clearly identifies spectral signatures of two species. Both appear with an instrument limited rise

cuts through the ground-state bleach (375 nm), transient absorption (395 nm), and stimulated emission (475 nm) features we observe after exciting avobenzone in cyclohexane at 350 nm. Each trace is a cut through the same single scan and is averaged over 20 pixels, corresponding to about 6 nm. To obtain time constants, we fit the evolution of these traces to a convolution of our approximately 200 fs Gaussian instrument response function with the appropriate exponential or biexponential decay. We studied the dynamics of avobenzone in methanol, cyclohexane, and acetonitrile to explore the effects of solvent polarity and hydrogen-bonding character. The transient spectra we observe are qualitatively the same. Table 1 shows the time constants we obtain from our global fits in each solvent. We estimate the uncertainty by averaging the fitting results across multiple scans and reporting the standard deviation in the time constants. The fast component of the dynamics that corresponds to stimulated emission and excited-state absorption, τES, decays significantly faster in cyclohexane, but the slow component linked to vibrational relaxation, τvib, is insensitive to the identity of the solvent. 3.2. Single-Wavelength Ultraviolet Detection. We assign the positive transient absorption at 266 nm that we C


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The Journal of Physical Chemistry A Table 1. Time Constants from Visible Transient Absorption Analysis Time Constants from Fits to Time Evolution of Stimulated Emission, Transient Absorption, and GroundState Bleach Features of Avobenzone after Excitation at 350 nma τES τvib

methanol

cyclohexane

acetonitrile

1.4 ± 0.2 ps 6.3 ± 1.0 ps

420 ± 40 fs 5.2 ± 0.6 ps

1.1 ± 0.2 ps 5.9 ± 1.2 ps

Table 2. Time Constants from Ultraviolet Transient Absorption Analysis Time Constants and Amplitude Factors Obtained from Fits to Time Evolution of Avobenzone Probed at 266 nm after Excitation at 350 nma methanol time constants 1.3 τUV, fast τUV, slow 30 relative amplitudes fast 0.36 slow 0.22 offset 0.42

a Uncertainties are standard deviations of results from at least five scans.

observe after 350 nm excitation to newly formed nonchelated enol isomers of avobenzone. Figure 5 shows the time evolution

± 0.3 ps ± 10 ps ± 0.04 ± 0.04 ± 0.02

cyclohexane

acetonitrile

1.1 ± 0.2 ps 59 ± 8 ps

1.4 ± 0.2 ps 80 ± 20 ps

0.54 ± 0.04 0.22 ± 0.02 0.24 ± 0.04

0.44 ± 0.04 0.13 ± 0.02 0.44 ± 0.03

a

Uncertainties are standard deviations of results from at least seven scans.

collects the results from the analysis of the time evolution of the transient absorption of avobenzone at 266 nm following excitation at 350 nm. We report the amplitudes of the fast component that decays in about 1 ps, the slow component that decays in tens of ps, and the long-time offset as relative amplitudes. The relative amplitude is the amplitude of the component that we obtain from the fit divided by the sum of the amplitudes of all three components.

4. DISCUSSION We observe prompt excited-state absorption and stimulated emission from the S1 state of CE−avobenzone after excitation with 350 nm light. As Table 1 shows, the lifetime of this electronically excited state is very sensitive to solvent identity. In cyclohexane this excited state lifetime is one-third the lifetime in methanol. We infer from this strong sensitivity that the excited-state is significantly less stable in nonpolar solvents than in polar solvents, suggesting a structure with localized charge. After the excited-state absorption and stimulated emission decay with time constant τES, ground-state bleach and transient absorption features remain whose time evolution likely reflects the relaxation of vibrationally excited CE− avobenzone. In other photoisomerizing systems, we and others have observed similar transient absorption signals that rise exponentially with a time constant in the hundreds of fs before decaying.33−35 Here we do not resolve a rising exponential component to the transient absorption of vibrationally energized molecules. We suspect the spectral overlap between the excited-state absorption and vibrationally excited CE absorption features obscures the growth of this feature. The transient absorption of vibrationally excited CE decays with the same time constant that the ground-state bleach recovers, τvib. The majority of the ground-state bleach recovers on this time scale, but a small negative offset persists that is consistent with formation of long-lived nonchelated isomer, either NCE1 or keto. Either of these isomers is effectively permanent on the picosecond time scale. The relaxation rate of vibrationally excited CE is insensitive to solvent identity, suggesting that intramolecular relaxation to low-energy modes, which can then transfer energy to various solvents with equal efficiency, dominates the energy-transfer process. The transient absorption signals at 266 nm clearly reflect multiexponential kinetics in avobenzone. After accounting for a strong coherent response at early times, we find evidence for three distinct relaxation processes, likely arising from the formation and subsequent relaxation of three distinct isomers of avobenzone. We assign each component to an isomer on the

Figure 5. (a) Expanded view of the time evolution before 15 ps. Open circles are data from one scan through the transient absorption in the solvent indicated. The gray feature is the result from a solvent-only scan of the transient absorption in cyclohexane. Other solvents show similar, albeit smaller, responses. Solid lines are fits to the time evolution as described in the text. (b) The evolution of the transient absorption of avobenzone probed at 266 nm in cyclohexane, methanol, and acetonitrile after excitation at 350 nm.

of the transient absorption of avobenzone at 266 nm after excitation at 350 nm in cyclohexane, methanol, and acetonitrile. In each solvent, the decay kinetics are biexponential and lead to an offset that does not decay on the time scale of our experiment. Previous flash photolysis experiments show that the NCE isomers have broad electronic absorption features in this region.1,8 We surmise that the two components of the relaxation and the long-time offset arise from the relaxation of separate NCE isomers. Coherent interaction between the excitation and probe pulses in pure solvent creates a strong, positive signal when the pulses are coincident, complicating our data analysis. In methanol and acetonitrile, this response is small enough that we simply fit the data after 0.5 ps to a biexponential decay with an offset. In cyclohexane, the coherent response is so intense that it interferes with our measurement of the faster time constant in the biexponential decay. We fit these data to the sum of an offset, biexponential decay and a Gaussian coherent response to extract time constants and amplitudes. Table 2 D


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relaxation is confined to a small portion of the molecule between the large ring structures, suggesting that the solvent does not influence the relaxation rate. The second component of the time evolution of the transient absorption, τUV,slow, at 266 nm decays with a time constant of 30 ± 10 ps in methanol, 59 ± 8 ps in cyclohexane, and 80 ± 20 ps in acetonitrile. Because these lifetimes are similar to those for reactions across torsional barriers, such as the trans to cis isomerization in stilbene,37−39 it is likely that we are observing isomerization of NCE2 to CE. To test this idea, we performed DFT (B3LYP, 6-311+G(d,p)) calculations with the Gaussian 09 software package to determine the energetics of the NCE2 to CE reaction.40 After optimizing the structures of CE and NCE2 in vacuo, we used the QST3 method to determine the structure of the transition state between the two isomers. We then used the IEFPCM solvation model to calculate the energy of the isomers and transition state in methanol, cyclohexane, and acetonitrile. Figure 6 collects the results of our calculations. We find that the transition state energies ΔE⧧ range from 500 to 1000 cm−1 among the three solvents. The transition-state energies in acetonitrile and cyclohexane are different by less than 100 cm−1, while the barrier in methanol is 400 cm−1 higher. These results support our hypothesis in that the reaction rate is statistically identical in cyclohexane and acetonitrile, but the calculated barrier height in methanol suggests that NCE2 to CE reaction in methanol should be much slower than we measure. Flash photolysis experiments show that hydrogen bonding is a key factor in enhancing the relaxation rate of nonchelated isomers of avobenzone.1,7 Our calculations do not take hydrogen bonding into account. It is likely that hydrogen bonding stabilizes the hydroxyl group in the transition state of the NCE2 to CE reaction and, thus, lowers the barrier. The third component of the time evolution is the long-time offset. Numerous previous experiments have established that

Figure 6. Calculated energies of NCE2, CE, and the transition state between NCE2 and CE in methanol, acetonitrile, and cyclochexane. Each energy is referenced to the energy of CE−avobenzone in methanol.

basis of time scale alone because their electronic spectra overlap at this probe wavelength. The fastest component of the dynamics probed at 266 nm, τUV, fast, decays with a time constant of 1.3 ps and varies by less than 10% among solutions in cyclohexane, methanol, and acetonitrile. Our visible results show that the excited-state absorption is sensitive to the solvent identity. That this feature is insensitive to the solvent suggests that a different process governs the relaxation. We tentatively assign this decay to the relaxation of NCE3 to CE. Because this relaxation depends on hydroxyl group torsion, it should be a facile process involving only the motion of a light atom. The motion involved in the

Figure 7. Schematic representation of the results of fitting to the transient absorption of avobenzone after excitation at 350 nm. After excitation, electronically excited CE−avobenzone relaxes in less than 1 ps to form three NCE isomers and vibrationally excited CE. E



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both NCE1- and keto-avobenzone relax to CE on the microsecond to millisecond time scale and have substantial absorption cross sections at 266 nm. Because we probe the dynamics by observing the evolution of broad, electronic transitions, we are unable to differentiate between NCE1 and the keto isomer. Yamaji and Kida found enhanced formation of keto-avobenzone in acetonitrile compared to other solvents, but the long-lived offset we measure in acetonitrile is not markedly different from the others we measure.8 If we were, in fact, probing keto-avobenzone, we would observe different dynamics in acetonitrile, and thus, we conclude that the longlived offset is a signature of NCE1-avobenzone. While we lack an accurate description of the electronically excited state we populate with our excitation pulse, we conclude from our transient absorption results that the excited-state evolution is complex and leads to at least four products. We infer formation of vibrationally excited CE and three distinct nonchelated enol isomers following excitation of CE at 350 nm. From the amplitudes we obtain when fitting the time evolution probed at 266 nm, we can estimate the fractional population of each NCE isomer. We estimate that each isomer has the same electronic absorption cross-section at 266 nm, presuming that the absorption comes from a π* ← n transition in the carbonyl group. We then conclude that the amplitude factor, defined above, for each component is the fractional population of the NCE associated with that component. Importantly, this fraction is the fraction of ultraviolet-absorbing products formed, not a direct quantum yield. Using Yamaji and Kida’s result for the extinction coefficient of keto-avobenzone at 266 nm,8 we estimate that our quantum yield for ultraviolet-absorbers is less than 1%. The fractions we report for each NCE are fractions of that small quantum yield. Figure 7 summarizes the results of our analysis. After excitation at 350 nm, we quickly produce vibrationally excited CE−avobenzone and three distinct nonchelated avobenzone isomers. The vibrationally excited CE, NCE2, and NCE3 isomers relax on the ultrafast time scale of our experiment, but the NCE1 products persist far longer. In methanol and acetonitrile, we observe roughly equal production of NCE3 and NCE1 with somewhat less NCE2 formed. In cyclohexane, NCE3 production is enhanced while NCE1 production is suppressed. Polar solvents might enhance the formation of NCE1 by stabilizing transition states that contain a free hydroxyl group, but these results rest on a simplistic approximation regarding the absorption coefficients of each NCE isomer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address †

NRC RAP Fellow, U.S. Naval Research Laboratory, Washington, DC, 20375. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Air Force Office of Scientific Research for funding this work through Grant No. FA9550-10-1-0116. REFERENCES

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5. CONCLUSION We present the first measurements of the femtosecond and picosecond dynamics of avobenzone after excitation at 350 nm. Immediately after excitation, we observe excited-state absorption and stimulated emission from the electronically excited state. Subsequently, the system evolves to form vibrationally excited CE and three distinct rotational isomers. The vibrationally energized chelated enols relax via primarily intramolecular pathways in 6 ps. NCE3-avobenzone relaxes in 1.3 ps through hydroxyl group torsion, NCE2−avobenzone relaxes in tens of picoseconds through single-bond torsion, and NCE3 persists throughout the time scale of our experiment. We estimate the branching ratio between the three nonchelated isomers by analyzing the amplitude of their corresponding components in the time evolution of the ultraviolet transient absorption. F


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Picosecond Dynamics of Avobenzone in Solution - The Journal of

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