ultrafast photoprotective properties of the sunscreening ......cinnamate and benzophenone...

Ultrafast photoprotective properties ofthe sunscreening agent octocrylene

Lewis A. Baker, Michael D. Horbury, and Vasilios G. Stavros∗Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK

∗[email protected]

Abstract: Today octocrylene is one of the most common moleculesincluded in commercially available sunscreens. It provides broadbandphotoprotection for the skin from incident UV-A and UV-B radiation ofthe solar spectrum. In order to understand how octocrylene fulfils its roleas a sunscreening agent, femtosecond pump-probe transient electronicUV-visible absorption spectroscopy is utilised to investigate the ultrafast-nonradiative relaxation mechanism of octocrylene in cyclohexane ormethanol after UV-B photoexcitation. The data presented clearly shows thatUV-B photoexcited octocrylene exhibits ultrafast-nonradiative relaxationmechanisms to repopulate its initial ground state within a few picoseconds,which, at the very least, photophysically justifies its wide spread inclusionin commercial sunscreens.

© 2016 Optical Society of America

OCIS codes: (260.5130) Photochemistry; (300.6530) Spectroscopy, ultrafast; (320.7150) Ul-trafast spectroscopy; (350.5130) Photochemistry.

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1. Introduction

Ultraviolet-B radiation (UV-B; 280-315 nm) is a minor contribution (∼1%) to the daily solarspectrum, being almost completely reflected by the stratospheric ozone layer [1]. The actual %of UV-B reaching the surface of the Earth depends on many environmental variables such as;ozone depletion, cloud cover and solar incidence angle [2, 3]. What little of it that does reachthe surface of the Earth can impact the biosphere extensively [4,5]. In humans, UV-B mediatesthe photolytic reaction of 7-dehydrocholesterol to previtamin D3, an early step in the synthesisof Vitamin D [6]. Deleterious effects of UV-B exposure are widely known with photodamage toDNA leading to malignant melanoma being a prominent example. As such, humans have devel-oped extensive photoprotective mechanisms in response to UV-B overexposure. Predominately,UV-absorbing eumelanin is distributed in the skin, the concentration of which can be regulatedthrough melanogenesis in response to varied exposure e.g. tanning of the skin through overex-posure [7, 8]. However, tanning is a delayed response, after DNA damage could have alreadyoccurred. Furthermore, subsequent overexposure to UV-B, even after tanning has taken place,is not always preventative of additional photodamage to the skin [7]. This has led to the widespread use of sunscreens, which on application to the skin, behaves as a barrier reducing theprobability that high energy UV radiation reaches the underlying tissues. Sunscreens thereforework to complement the natural photoprotective mechanisms of the skin.

With respect to this, an effective sunscreen must absorb (or scatter) radiation over a broadspectral range in order to provide protection across all damaging wavelengths of radiation,typically over the ranges of UV-B and the high energy components of UV-A (<340 nm) [9].As such, sunscreens generally consist of mixtures of organic UV-A and UV-B filters such ascinnamate and benzophenone derivatives, as well as inorganic scattering molecules such asTiO2 and ZnO2 [10, 11]. The former are molecules which are photoexcited by radiation andsubsequently dissipate their excited state energy via ultrafast (femtosecond (fs) to picosecond(ps) timescales) relaxation pathways, with benzophenones [12–14] and diketones [15] beingrepresentative examples.

Octocrylene (OC, see Fig. 1) is one such organic filter whose prevalence is high amongstcommercial sunscreens [16]. It displays broadband absorption of UV-B and high energy com-ponents of UV-A radiation, as shown from its UV-visible absorption spectrum in Fig. 1, but hasalso been shown to act as a stabiliser to other sunscreen constituents, such as avobenzone [17].In general there remains much ongoing research to understand the potential adverse derma-tological (e.g. photoallergies) and physiological effects resulting from the use of sunscreens,including OC [18–20]. OC shows no significant fluorescence or phosphorescence, hinting thatultrafast processes maybe occurring that outcompete spontaneous emission [21]. Closely re-lated molecules, ethylhexylmethoxycrylene and octylmethoxycinnamate have been shown toundergo isomerisation as the dominate relaxation pathway after UV photoexcitation with highefficiency leading to the idea that OC will relax via similar processes [21, 22].

In this letter we utilise femtosecond pump-probe transient electronic (UV-visible) absorption spectroscopy (TEAS) to probe the excited state dynamics of OC following photoexcitation at its UV-B absorption maximum ∼300 nm (4.13 eV). We determine that OC displays ultrafast relaxation back to its initial ground state after UV-B irradiation, which photophysically, justifies its wide spread inclusion in commercial sunscreens.


2 4 0 2 7 0 3 0 0 3 3 0 3 6 0 3 9 0

0

1

Abso

rbanc

e/arb

.unit

sW a v e l e n g t h / n m

C y c l o h e x a n e M e t h a n o l

Fig. 1. UV-visible spectra of octocrylene (structure shown in inset) in cyclohexane (blackline) and in methanol (blue line) displaying a broad absorption peak in the UV-B regioncentred at ca. 300 nm.

2. Methodology

A stock sample of 97% OC was purchased from Sigma-Aldrich and used without further pu-rification. For all reported TEAS measurements, 10 mM solutions of OC in either cyclohexane(>99%, VWR) or methanol (≥99.6%, Sigma-Aldrich) were recirculated between two CaF2windows with 100 µm PTFE spacers via a flow-through cell (Harrick Scientific). The sampleswere photoexcited by 300 nm pump pulses with fluences of ∼1–2 mJ cm−2 produced by acommercially available optical parametric amplifier (TOPAS-C, Light Conversion) seeded bya 1 kHz pulse train (1 W, 800 nm) from a Ti:sapphire chirp regenerative amplifier (Spitfire ProXP, Spectra Physics). A small portion of the 800 nm fundamental (∼5 mW) is focussed into a 1mm thick CaF2 window producing a broadband white light continuum (∼335–675 nm) used asthe probe pulses. A half-wave plate is used to hold probe polarisation at the magic angle (54.7◦)relative to the pump polarisation. All transient absorption spectra (TAS) are chirp corrected us-ing the KOALA package [23]. Further experimental details may be found in References [24]and [25].

Both the TAS of OC-cyclohexane and OC-methanol were analysed using a global fittingprocedure [26, 27]. The experimental TAS are modelled by the sum of n exponential functionsconvoluted with a Gaussian instrument response function, G(∆t):

F(λ ,∆t) =n

∑i

G(∆t)⊗Ai(λ )e

−(∆t− t0)τi , (1)

where Ai(λ ) is the decay associated spectrum (DAS) for the corresponding exponential decayfunction with lifetime τi, and t0 denotes the temporal position of pump-probe pulse overlap. Thesum of squares, F(λ ,∆t), of the modelled TAS are minimised with respect to the experimentallymeasured TAS. For both OC-cyclohexane and OC-methanol, four exponential functions (i.e. n =4) are required to fully describe the experimental TAS and the G(∆t) is taken to be∼100 fs [13].All confidence intervals assigned to lifetimes are reported to the 95% level using asymptoticstandard errors, further details of this assignment methodology can be found in Reference [27].

All ‘static’ UV-visible spectroscopic measurements were taken using a Cary 50 UV-visiblespectrophotometer with a 1 cm path length quartz cuvette, and ∼µM OC-cyclohexane andOC-methanol solutions. To investigate evidence of long-lived photoproducts, continuous wave


UV irradiation studies were performed on OC using the following procedure. First a staticUV-visible spectrum of each sample was taken (Cary 300 spectrometer), to obtain a ‘before’spectrum. Samples were then irradiated with continuous wave radiation, ∼3 W, from an arclamp (OBB, Tunable KiloArc) for 10 minutes using the a central wavelength of 300 nm andbandwidth of ∼20 nm. A second UV-visible spectrum was taken (Cary 300 spectrometer) fol-lowing irradiation, referred to as the ‘after’ spectrum. A subtraction of the before spectrumfrom the after spectrum results in the reported ‘difference spectrum’.

All ab initio electronic structure calculations of OC were performed with the Gaussian 09computer package [28]. The ground state geometry energy minimum is determined at theDFT//B3LYP/6-311+g** level of theory [29–32]. This minimum was confirmed by normalmode analysis through searches for imaginary frequencies . The presence of imaginary frequen-cies would otherwise describe a point on the potential energy surface with negative curvature,thus a transition state. The likely excited states were characterised at the TD-DFT//B3LYP/6-311+g** level of theory. Calculations were also performed with the M052X functional forcomparison [33].

3. Results

The TAS recorded for OC-cyclohexane and OC-methanol for a range of pump-probe time de-lays, ∆t, are shown in Figs. 2(a) and 2(b) respectively. We start by considering the early time (∆t < 2 ps) OC-cyclohexane. The TAS are dominated by two positive absorption features: (i) a broad, intense absorption across probe wavelengths ∼335–475 nm, and (ii) a weaker absorp-tion signal which extends out to, and decays towards the baseline by ∼675 nm (the limit of our probe spectral window). Both of these positive signals are attributed to the excited state absorption of OC, based on subsequent analysis (vide infra). In particular, absorption feature (i) changes significantly for increasing ∆t, up to 2 ps, see Fig. 2(e). The absorption signal dis-plays an intense peak at ∆t ∼150 fs with a short-lived negative signal also observed beyond ∼550 nm for ∼250 < ∆t < 500 fs which we attribute to stimulated emission. As this stimu-lated emission signal decays, another absorption peak grows in by ∆t ∼500 fs, which is most clearly seen at probe wavelengths of λ ∼500 nm, see Fig. 2(a). By 2 ps, the ESA has almost completely returned to the baseline, with no further spectral features observed up to the maxi-mum available pump-probe time delay of ∆t = 600 ps. Similar features are observed in the TAS of OC-methanol shown in Figs. 2(b) and 2(f).

Table 1. Summary of the Lifetimes of Dynamical Processes of Octocrylene.

Lifetime / fs Cyclohexane Methanol

τ1 80 ± 10 90 ± 10τ2 120 ± 10 130 ± 10τ3 180 ± 10 200 ± 10τ4 810 ± 140 1520 ± 360a

aLower limit of unbounded interval used.

Quantitative insight into the dynamical processes observed in the TAS is gleaned from globalfitting, where four exponential functions convoluted with a Gaussian instrument response isrequired to fully describe the TAS. For OC-cyclohexane, global fitting reveals four lifetimes(τ1, τ2, τ3 and τ4) as summarised in Table 1, and are characterised by the corresponding DASgiven in Fig. 2(g). The shapes of the DAS are valuable in aiding in the interpretation of the


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Norm.Amp.

τ1 τ2 τ3 τ4G

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H

Fig. 2. (A) Raw TAS of OC-cyclohexane following 300 nm photoexcitation, the colormapindicates the change in optical density (OD), a linear timescale is used up to 2 ps afterwhich a logarithmic scale is used up to 16 ps. (B) Similar observations are seen for 300nm photoexcited OC-methanol. (C-D) The residuals between the raw TAS and the globallyfitted TAS. (E-F) Selected transients at specified wavelengths (see D) highlighting the earlydelay time changes in the absorption profile for OC-cyclohexane and OC-methanol respec-tively. Transients have been integrated over a∼5 nm spectral range. (G) The correspondingdecay associated spectrum (DAS) for OC-cyclohexane as determined by the global fittingprocedure for the four lifetimes described in the main text. (H) DAS for OC-methanol.The amplitude of the DAS for both τ4 lifetimes has been increased by a factor of five forresolution. Amplitudes are normalised by the amplitude of τ2.

corresponding TAS. Positive components of the DAS indicate the decay of population from aparticular state whereas negative going components indicate a rise in the population of a state.In particular, a negative going component (an exponential rise) concomitant with a positivecomponent (an exponential decay) can be interpreted as a flow of population from the positiveregion into the negative region, which can be induced by a change in electronic state or by vibra-tional energy transfer within a single electronic state [34]. The DAS of τ1, τ3 and τ4 are positiveindicating a decaying absorption signal whilst τ2 is negative indicating a growing absorption orstimulated emission feature which agrees qualitatively with the observed absorption features (i)and (ii) of the TAS (Figs. 2(a) and 2(e)). Following an identical procedure, global fitting of theOC-methanol TAS (Fig. 2(b)) reveals four dynamical processes with lifetimes summarised inTable 1 and are characterised by the corresponding DAS given in Fig. 2(h) and display similarfeatures to those discussed for OC-cyclohexane. The confidence interval of the lifetime τ4 forOC-methanol is unbounded on the postive limit. We suggest this is because of the convolutionwith an increased photoproduct absorption signal compared to OC-cyclohexane (see Fig. 3), as


such, the lower limit is used (Table 1) [27].The presence of a ground state bleach (GSB) is not observed which is likely due to the con-

volution with the strong positive ESA as well as the limit of the probe window being at the tailend of OC’s absorption profile (Fig. 1). Continuous wave irradiation studies were used to inves-tigate any long-lived photoproducts which would typically be signalled by an incomplete GSBrecovery. Following the procedure described (vide supra), the resulting difference spectra forOC-cyclohexane and OC-methanol are shown in Fig. 3. There are small discrepancies betweenthe OC-cyclohexane difference spectrum with the corresponding TAS (Fig. 3(a)) whereby thedifference spectrum displays a more prominent negative absorption. A similar observation ismade for OC-methanol, where a pronounced offset between the difference spectrum and the∆t = 600 ps spectrum around ∼350 nm is seen. These features may indicate the presence ofa long-lived photoproduct which contributes a minor channel to the relaxation mechanism ofOC. Additionally, a broad absorption spans the probe window in both OC-cyclohexane andOC-methanol TAS at ∆t = 600 ps. For OC-methanol, there is an emergence of a defined peakcentred at ∼375 nm. There is no evidence for either of these features in the corresponding dif-ference spectra which suggests that these features may originate from long lived triplet stateabsorption [17, 21].

2 8 0 3 0 0 3 2 00

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rption

D i f f e r e n c e s p e c t r u m ∆t = 6 0 0 p sA

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B 3 L Y PM 0 5 2 X C

Norm.Counts


D

Fig. 3. (A) For OC-cyclohexane, continuous wave irradiation results in the difference spec-trum (black line) using the procedure described in the main text. Overlaid is a ∆t = 600ps spectrum (blue line). (B) Similarly for OC-methanol. Absorption signals have been nor-malised with respect to the most negative going signals. (C) Calculated transition energiesbetween triplet states, Tn ← T1. The accessible wavelengths by the probe is highlighted ingray. (D) The output of the KiloArc lamp used for irradiation experiments.


4. Discussion

We now begin to discuss the implications to the photoprotective properties of OC drawing onthe different experimental results presented here. We have observed the generation of a pho-toproduct(s) using continuous wave irradiation, suggesting that there exists a minor relaxationpathway which generates photoproduct(s), suggesting a small probability that OC undergoesphotodegradation after UV irradiation, a scenario that has been previously reported [35, 36].When compared to the TAS of OC for ∆t = 600 ps, we observe a weak absorption (∼100 µOD)across the probe window, which is clearly not present in the difference spectrum obtained fromcontinuous wave irradiation measurements. Ab initio calculations of triplet-triplet transitionenergies, as described in the methodology, are shown in Fig. 3(c). These suggest there are anumber of triplet states accessible within the probe window (335–675 nm). We therefore sug-gest this feature can be attributed to triplet state absorption, which is known to be accessibleunder these conditions, and a property that has been exploited in mixtures with avobenzoneto improve its photostability [17, 21, 37]. The spectrum at ∆t = 600 ps shows a clear positivepeak centred around λ ∼375 nm for OC-methanol compared to OC-cyclohexane. This mightindicate that the rate of intersystem crossing to the triplet state, and/or the absorption crosssection between this triplet state and higher lying states increases in the more polar solventmethanol, which may be explained by an increase in solvent perturbations on the excited stateenergy levels. These observations have an important consequence of OC’s use in sunscreens;the relaxation mechanism of OC only has an apparently minor contribution that involves tripletstate absorption or photodegradation, which means there may be a lower probability of OC-containing sunscreens causing adverse dermatological effects, a finding which complementsthe current literature [16, 35, 36]. Another possibility remains that given the presence of a CNsubstituent in OC, internal conversion (IC) to a long-lived charge transfer state is another plau-sible assignment of the photoproduct signal.

Considering next the dynamical processes extracted from the global fitting of the TAS wecan attempt to rationalise the dynamics operating in OC after UV-B photoexcitation, although,as we reiterate below, we acknowledge that further work is essential; this discussion howeverserves as an important starting point. Precedence in the mechanism comes from simple excitedstate calculations of octocrylene. We found that the likely first excited state, initially populatedthrough excitation by the pump pulse is a ππ∗ ← S0 transition, see Table 2 (for molecularorbitals (TD-DFT//B3LYP/6-311+g** shown), transition wavelengths and oscillator strengths)and the static absorption spectra shown in Fig. 1. We also note that both OC-methanol andOC-cyclohexane display similar lifetimes and DAS, suggesting that the proposed dynamics arevery similar in both, hence the dynamics discussed are applicable to both systems.

We propose that an initial photoexcitation likely populates an ensemble of close in energyn1ππ∗ states (n ≥ 1). The subsequent decay of this population to a lower lying excited stateis assigned the lifetime τ1. We suggest this in turn populates another excited 1ππ∗ state withlifetime τ2. We draw confidence with the assignment of τ1 and τ2 by considering the DASassociated with these similar lifetimes (cf. Fig. 2 and Table 1). The positive-going DAS asso-ciated with τ1 closely mirrors the negative-going DAS associated with τ2 which might imply apopulation flow between two states. Furthermore, the negative feature in the TAS assigned tostimulated emission onsets from ∼250 fs (see Figs. 2(c) and 2(d)) suggest that it may originatefrom a state other than the initially populated one(s). Thus τ2 predominately captures the pop-ulation flowing between these two states by IC, likely via a n1ππ∗/m1ππ∗ conical intersection(CI, n 6= m) given the efficiency this occurs with. We suggest the population on this secondstate subsequently relaxes with the lifetime of τ3. Since photoexcited OC appears to almostcompletely recover its ground state, we suggest this state will couple back to the ground statevia a m1ππ∗/S0 CI and subsequently will relax to the ground vibrational state by vibrational


energy transfer, likely mediated by a combination of intramolecular vibrational energy redistri-bution and vibrational energy transfer to the surrounding solvent molecules [38, 39]. This laststep is captured by the lifetime τ4, especially the internal conversion to the ground state. An in-teresting possibility remains in that this relaxation may occur via the isomerisation around thealiphatic C=C, a process suggested to occur in the closely related molecule ethylhexylmethoxy-crylene [21], however, confirmation of this would require further theoretical and experimentalstudies; for example, one could envisage aromatic ring substitution as one possible technique,but is beyond the scope of this work, and in keeping with the thesis of this work, may triggerfurther investigations (see below).

There are two important caveats in order here. Firstly, since the lifetimes of the dynamicalprocesses described are similar in magnitude and are very fast, the underlying assumption ofthe global fitting procedure is that the processes are not sequential begins to break down. Thishas the effect of clouding the onset of one process with that of another. Ultimately, this meanseach lifetime will also capture some of the preceding and/or proceeding dynamics, making theabsolute assignment of a lifetime with any one process unrealistic [40]. Secondly, there willlikely be a contribution to the lifetime(s) (in particular τ1, τ2 and τ3) from an evolution out ofthe Franck-Condon window, as well as any solvent rearrangement. These processes typicallypersist for comparable timescales as the extracted lifetimes, meaning the absolute assignment ofpopulation decay from one state to another is highly complex. Furthermore, any spectral shift inthe positive absorption signal would likely effect τ1 and τ2. In this case, τ1 and τ2 may describe aspectra shift (on a single potential energy surface), which is consistent with the closely mirrored

Table 2. First Five Transitions Determined at the TD-DFT//B3LYP(M052X)/6-311+g**Level of Theory.

Transition, Sn ← S0 Wavelength / nm Osc. strength

S1 ← S0 ← 336 (295) 0.649 (0.687)

S2 ← S0 ← 315 (268) 0.657 (0.437)

S3 ← S0 ← 307 (257) 0.675 (0.477)

S4 ← S0 ← 298 (251) 0.658 (0.480)

S5 ← S0 ← 279 (240) 0.643 (0.412)


DAS (Fig. 2(g) and 2(h)). This would mean that the likely relaxation mechanism would be via an n1ππ∗/S0 CI on the timescale τ3 followed by vibrational relaxation in the S0 state captured by τ4. However, the overall picture of the relaxation dynamics observed in OC is clear; UV-B photoexcited OC undergoes ultrafast non-radiative relaxation which repopulates the ground state with high efficiency. The vast majority of the dynamics are over in the first ∼5 ps after photoexcitation, with most of the processes over within the first 2 ps of photoexcitation (Fig. 2). This has major implications for OC’s role as a sunscreening agent since the ability to dissipate energy from UV-B photoexcitation through ultrafast non-radiative processes is vital for a safe and efficient organic filter [17].

Further experimental studies are required to fully understand the states likely to be involvedin OC. Specifically, we suggest sequential kinetic studies would provide valuable insight intothe observed population flow in both OC-cyclohexane and OC-methanol, a limitation of bothour instrument response (∼100 fs) and our global fitting procedure [40], which will likely proveuseful in the deconvolution of the τ1, τ2 and τ3 lifetimes. Theoretical studies will be invaluablein understanding the states involved in the relaxation mechanism observed in our TAS andindicate if such an isomerisation provides an energetically favourable route to couple back tothe ground state. We hope this work will provide a stimulus for further studies along bothexperimental and theoretical directions.

5. Conclusion

In summary we have provided initial ultrafast measurements of the commonly used sunscreen molecule octocrylene, which is shown to exhibit an ultrafast relaxation mechanism predomi-nately via nonradiative pathways, with high efficiency. Furthermore, non-polar and polar sol-vents show little effect on the predominate dynamics, but with some noticeable effects on po-tential triplet state formation. These measurements highlight the efficiency of octocrylene as used as a sunscreen molecule and suggest there is minimal triplet state absorption and photodegradation, important properties for sunscreens.

Acknowledgments

The authors are grateful to Dr Michael Staniforth (University of Warwick) for helpful discus-sions and to Prof. Martin Paterson (Heriot-Watt) for computational resources. The authors alsothank Prof. Peter J. Sadler (University of Warwick) for the use the KiloArc. L.A.B. thanks theEngineering and Physical Sciences Research Council (EPSRC) for providing a studentship un-der grant EP/F500378/1, through the Molecular Organisation and Assembly in Cells DoctoralTraining Centre. M.D.H. thanks the University of Warwick for an EPSRC studentship. V.G.S.thanks the EPSRC for an equipment grant (EP/J007153) and the Royal Society for a UniversityResearch Fellowship.


ultrafast photoprotective properties of the sunscreening ......cinnamate and benzophenone...

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