Bridging the Gap between the Gas Phase and Solution Phase: Solvent Specific Photochemistry in 4-tert-Butylcatechol

Solvent effects on catechol's binding affinity: investigating the role of the intra-molecular hydrogen bond through a multi-level computational approach

The subtle interplay between the inter-molecular interactions established by catechol with the surrounding solvent and the intra-molecular hydrogen bond (HB) characterizing its conformational dynamics is investigated through a multi-level computational approach. First, quantum mechanical (QM) calculations are employed to accurately characterize both large portions of the catechol's potential energy surface and the interaction energy with neighboring solvent molecules. The acquired information is thereafter exploited to develop a QM derived force-field (QMD-FF), in turn employed in molecular dynamics (MD) simulations based on classical mechanics. The reliability of the QMD-FF is further validated through a comparison with the outcomes of ab initio molecular dynamics, also purposely carried out in this work. In agreement with recent experimental findings, the MD results reveal remarkable differences in the conformational behavior of isolated and solvated catechol, as well as among the investigated solvents, namely water, acetonitrile or cyclohexane. The rather strong intramolecular HB, settled between the vicinal phenolic groups and maintained in the gas phase, loses stability when catechol is solvated in polar solvents, and is definitively lost in protic solvents such as water. In fact, the internal energy increase associated with the rotation of one hydroxyl group and the breaking of the internal HB is well compensated by the intermolecular HB network available when both phenolic hydrogens point toward the surrounding solvent. In such a case, catechol is stabilized in a chelating conformation, which in turn could be very effective in water removal and surface anchoring. Besides unraveling the role of the different contributors that govern catechol's conformational dynamics, the QMD-FF developed in this work could be in future employed to model larger catechol containing molecules, due to its accuracy to reliably model both internal flexibility and solvent effects, while exploiting MD computational benefits to include more complex players as for instance surfaces, ions or biomolecules.

Excited-state hydrogen detachment from a tris-(o-phenylenediamine) iron(ii) complex in THF at room temperature

We previously reported that a tris-(o-phenylenediamine) iron(ii) complex promotes photochemical H₂ generation and C-H carboxylation of o-phenylenediamine without any additives under N₂ and CO₂ atmospheres, respectively, in tetrahydrofuran at room temperature. Herein, the key mechanistic process, namely, excited-state hydrogen detachment from the o-phenylendiamine moiety, is demonstrated under an N₂ atmosphere.

Conservation of ultrafast photoprotective mechanisms with increasing molecular complexity in sinapoyl malate derivatives

Sinapoyl malate is a natural plant sunscreen molecule which protects leaves from harmful ultraviolet radiation. Here, the ultrafast dynamics of three sinapoyl malate derivatives, sinapoyl L-dimethyl malate, sinapoyl L-diethyl malate and sinapoyl L-di-t-butyl malate, have been studied using transient electronic absorption spectroscopy, in a dioxane and methanol solvent environment to investigate how well preserved these dynamics remain with increasing molecular complexity. In all cases it was found that, upon photoexcitation, deactivation occurs via a trans-cis isomerisation pathway within ∼20-30 ps. This cis-photoproduct, formed during photodeactivation, is stable and longed-lived for all molecules in both solvents. The incredible levels of conservation of the isomerisation pathway with increased molecular complexity demonstrate the efficacy of these molecules as ultraviolet photoprotectors, even in strongly perturbing solvents. As such, we suggest these molecules might be well-suited for augmentations to further improve their photoprotective efficacy or chemical compatibility with other components of sunscreen mixtures, whilst conserving their underlying photodynamic properties.

Dispersion-induced structural preference in the ultrafast dynamics of diphenyl ether

Dispersion interactions are omnipresent in large aromatic systems and influence the dynamics as intermolecular forces. The structural preference induced by dispersion interactions is demonstrated to influence the excited state dynamics of diphenyl ether (DPE) using femtosecond time-resolved transient absorption (TA) associated with quantum chemical calculations. The experimental results in aprotic solvents show that the S₁ state is populated upon irradiation at 267 nm with excess vibrational energy dissipating to solvent molecules in several picoseconds, and then decays via internal conversion (IC) within 50 ps as well as intersystem crossing (ISC) and fluorescence with a lifetime of nanoseconds. The polarity of the solvent disturbs the excited state energies and enhances the energy barriers of the ISC channel. Furthermore, the intermolecular dispersion interactions with protic solvents result in the OH–π isomer dominating in methanol and the OH–O isomer is slightly preferred in t-butanol in the ground state. The hydrogen bonded isomer measurements show an additional change from OH–O to OH–π geometry in the first 1 ps besides the relaxation processes in aprotic solvents. The time constants measured in the TA spectra suggest that the OH–O isomer facilitates IC. The results show that the OH–π isomer has a more rigid structure and a higher barrier for IC, making it harder to reach the geometric conical intersection through conformer rearrangement. This work enables us to have a good knowledge of how the structural preference induced by dispersion interactions affects excited state dynamics of the heteroaromatic compounds.

Dispersion interactions are omnipresent in large aromatic systems and influence the dynamics as intermolecular forces.

Examining solvent effects on the ultrafast dynamics of catechol

We consider the effect of a polar, hydrogen bond accepting, solvent environment on the excited state decay of catechol following excitation to its first excited singlet state (S₁). A comparison of Fourier transform infrared spectroscopy and explicit-solvent ab initio frequency prediction suggests that 5 mM catechol in acetonitrile is both nonaggregated and in its "closed" conformation, contrary to what has been previously proposed. Using ultrafast transient absorption spectroscopy, we then demonstrate the effects of aggregation on the photoexcited S₁ lifetime: at 5 mM catechol (nonaggregated) in acetonitrile, the S₁ lifetime is 713 ps. In contrast at 75 mM catechol in acetonitrile, the S₁ lifetime increases to 1700 ps. We attribute this difference to aggregation effects on the excited-state landscape. This work has shown that explicit-solvent methodology is key when calculating the vibrational frequencies of molecules in a strongly interacting solvent. Combining this with highly complementary steady-state and transient absorption spectroscopy enables us to gain key dynamical insights into how a prominent eumelanin building block behaves when in polar, hydrogen bond accepting solvents both as a monomer and as an aggregated species.

Effects of Intra- and Intermolecular Hydrogen Bonding on O-H Bond Photodissociation Pathways of a Catechol Derivative

The catechol functional motif is thought to play both a structural and photochemical role in the ubiquitous natural pigment, eumelanin. Intramolecular and intermolecular hydrogen bonding interactions lead to a variety of geometries involving the two O-H groups in catechol, but its photophysical behavior in these situations has not been comprehensively characterized. Toward this end, we monitor the UV-induced O-H bond photodissociation reaction in an exemplar catechol derivative, 4- tert-butylcatechol, possessing different intramolecular and intermolecular hydrogen bonding geometries using femtosecond transient absorption spectroscopy measurements in the UV-visible and mid-infrared regions following 265 nm photoexcitation. Three different hydrogen bonding arrangements are obtained by tuning solution complexation equilibria of the catechol with the hydrogen bond acceptor, diethyl ether (Et₂O), and are verified computationally. We find that intermolecular hydrogen bonding to the free O-H group in catechol increases its first excited singlet state (S₁) lifetime by 2 orders of magnitude (i.e., ∼ 16 to 1410 ps), and that O-H bond dissociation is prevented because Et₂O is a poor hydrogen atom acceptor. Complexation of both O-H groups with multiple Et₂O molecules further elongates the S₁ lifetime to 1670 ps due to shifting of the solution equilibria that describe complex formation. Weakening of the characteristic, intramolecular hydrogen bond of the catechol derivative by intermolecular hydrogen bonding to one or more Et₂O molecules does not enhance the rate of O-H bond dissociation.

Dynamics of electronically excited states in the eumelanin building block 5,6-dihydroxyindole

We report the first excited state dynamics study of gas-phase 5,6-dihydroxyindole (5,6-DHI), a key building block of eumelanin pigments that are found throughout nature and serve as important photo-protective compounds. Time-resolved ion-yield measurements over the 241-296 nm ultraviolet photoexcitation region revealed non-adiabatic processes occurring on up to three distinct timescales. These reflect ultrafast (i.e. sub-picosecond) internal conversion within the excited state singlet manifold, and much longer-lived processes ranging from 10 ps to in excess of 1 ns. Our investigation paves the way for precisely targeted future studies of 5,6-DHI that exploit more differential measurement techniques. The work was facilitated by the use of soft laser-based thermal desorption to introduce 5,6-DHI samples into the gas phase. This approach, based on low-cost, readily available diode lasers, is straightforward, easily controllable and potentially applicable to a wide range of non-volatile molecular species.

Intermolecular Hydrogen Bonding Modulates O-H Photodissociation in Molecular Aggregates of a Catechol Derivative

The catechol functional group plays a major role in the chemistry of a wide variety of molecules important in biology and technology. In eumelanin, intermolecular hydrogen bonding between these functional groups is thought to contribute to UV photoprotective and radical buffering properties, but the mechanisms are poorly understood. Here, aggregates of 4-t-butylcatechol are used as model systems to study how intermolecular hydrogen bonding influences photochemical pathways that may occur in eumelanin. Ultrafast UV-visible and mid-IR transient absorption measurements are used to identify the photochemical processes of 4-t-butylcatechol monomers and their hydrogen-bonded aggregates in cyclohexane solution. Monomer photoexcitation results in hydrogen atom ejection to the solvent via homolytic O-H bond dissociation with a time constant of 12 ps, producing a neutral semiquinone radical with a lifetime greater than 1 ns. In contrast, intermolecular hydrogen bonding interactions within aggregates retard O-H bond photodissociation by over an order of magnitude in time. Excited state structural relaxation is proposed to slow O-H dissociation, allowing internal conversion to the ground state to occur in hundreds of picoseconds in competition with this channel. The semiquinone radicals formed in the aggregates exhibit spectral broadening of both their electronic and vibrational transitions.

Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents

Evolution has ensured that plants and animals have developed effective protection mechanisms against the potentially harmful effects of incident ultraviolet radiation (UVR). Tanning is one such mechanism in humans, but tanning only occurs post-exposure to UVR. Hence, there is ever growing use of commercial sunscreens to pre-empt overexposure to UVR. Key requirements for any chemical filter molecule used in such a photoprotective capacity include a large absorption cross-section in the UV-A and UV-B spectral regions and the availability of one or more mechanisms whereby the absorbed photon energy can be dissipated without loss of the molecular integrity of the chemical filter. Here we summarise recent experimental (mostly ultrafast pump-probe spectroscopy studies) and computational progress towards unravelling various excited state decay mechanisms that afford the necessary photostability in chemical filters found in nature and those used in commercial sunscreens. We also outline ways in which a better understanding of the photophysics and photochemistry of sunscreen molecules selected by nature could aid the design of new and improved commercial sunscreen formulations.

A Perspective on the Ultrafast Photochemistry of Solution-Phase Sunscreen Molecules

Sunscreens are one of the most common ways of providing on-demand additional photoprotection to the skin. Ultrafast transient absorption spectroscopy has recently proven to be an invaluable tool in understanding how the components of commercial sunscreen products display efficient photoprotection. Important examples of how this technique has unravelled the photodynamics of common components are given in this Perspective, and some of the remaining unanswered questions are discussed.

Observing and understanding the ultrafast photochemistry in small molecules: applications to sunscreens

In this review, we discuss the importance of biological and artificial photoprotection against overexposure to harmful ultraviolet radiation. Transient electronic and transient vibrational absorption spectroscopies are highlighted as important tools in understanding the energy transfer in small molecules, with a focus on the application to commercial sunscreens with representative examples given. Oxybenzone, a common ingredient in commercial sunscreens and sinapoyl malate, a biological sunscreen in plant leaves are presented as case studies.

Photodynamics of potent antioxidants: ferulic and caffeic acids

The dynamics of ferulic acid (3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) and caffeic acid (3-(3,4-dihydroxyphenyl)-2-propenoic acid) in acetonitrile, dioxane and water at pH 2.2 following photoexcitation to the first excited singlet state are reported. These hydroxycinnamic acids display both strong ultraviolet absorption and potent antioxidant activity, making them promising sunscreen components. Ferulic and caffeic acids have previously been shown to undergo trans-cis photoisomerization via irradiation studies, yet time-resolved measurements were unable to observe formation of the cis-isomer. In the present study, we are able to observe the formation of the cis-isomer as well as provide timescales of relaxation following initial photoexcitation.

Ultrafast photoprotective properties of the sunscreening agent octocrylene

Today octocrylene is one of the most common molecules included in commercially available sunscreens. It provides broadband photoprotection for the skin from incident UV-A and UV-B radiation of the solar spectrum. In order to understand how octocrylene fulfils its role as a sunscreening agent, femtosecond pump-probe transient electronic UV-visible absorption spectroscopy is utilised to investigate the ultrafastnonradiative relaxation mechanism of octocrylene in cyclohexane or methanol after UV-B photoexcitation. The data presented clearly shows that UV-B photoexcited octocrylene exhibits ultrafast-nonradiative relaxation mechanisms to repopulate its initial ground state within a few picoseconds, which, at the very least, photophysically justifies its wide spread inclusion in commercial sunscreens.

Gas-Phase Femtosecond Particle Spectroscopy: A Bottom-Up Approach to Nucleotide Dynamics

We summarize how gas-phase ultrafast charged-particle spectroscopy has been used to provide an understanding of the photophysics of DNA building blocks. We focus on adenine and discuss how, following UV excitation, specific interactions determine the fates of its excited states. The dynamics can be probed using a systematic bottom-up approach that provides control over these interactions and that allows ever-larger complexes to be studied. Starting from a chromophore in adenine, the excited state decay mechanisms of adenine and chemically substituted or clustered adenine are considered and then extended to adenosine mono-, di-, and trinucleotides. We show that the gas-phase approach can offer exquisite insight into the dynamics observed in aqueous solution, but we also highlight stark differences. An outlook is provided that discusses some of the most promising developments in this bottom-up approach.

Ultrafast Photoprotecting Sunscreens in Natural Plants

We explore the ultrafast photoprotective properties of a series of sinapic acid derivatives in a range of solvents, utilizing femtosecond transient electronic absorption spectroscopy. We find that a primary relaxation mechanism displayed by the plant sunscreen sinapoyl malate and other related molecular species may be understood as a multistep process involving internal conversion of the initially photoexcited 1(1)ππ* state along a trans-cis photoisomerization coordinate, leading to the repopulation of the original trans ground-state isomer or the formation of a stable cis isomer.

Bottom-up excited state dynamics of two cinnamate-based sunscreen filter molecules

We have used time-resolved pump–probe spectroscopy to explore E-MMC's and E-EHMC's excited state dynamics upon UV-B photoexcitation.

Probing Rotational Motion in 4-tert-Butylcatechol through H Atom Photofragmentation: Deviations from Axial Recoil

The time-resolved photofragmentation dynamics of 4-tert-butylcatechol were studied following one photon excitation to the S1 (1(1)ππ*) state with ultraviolet radiation in the range 260 ≤ λ ≤ 286 nm. The preparation of an aligned molecular ensemble via photoexcitation leads to anisotropy in the H atom photofragments. These H atoms originate from the decay of the S1 state through coupling onto the S2 ((1)πσ*) state, which is dissociative along the nonintramolecular hydrogen bonded "free" O-H bond. The degree of anisotropy of these photogenerated H atoms decreases with increasing pump-probe time delay. This is attributed to rotational dephasing of the initially aligned molecular ensemble. The measured dephasing occurs on a time scale akin to the appearance time of these H atoms, which likely places an intrinsic lower bound on the dephasing lifetime. The present work demonstrates how a careful balance between the appearance time of the H atoms, determined by the S1 lifetime, and the rotational dephasing in 4-tert-butylcatechol provides an opportune window to probe rotational motion in real time.

Broadband ultrafast photoprotection by oxybenzone across the UVB and UVC spectral regions

Recent studies have shed light on the energy dissipation mechanism of oxybenzone, a common ingredient in commercial sunscreens.