New insights into the dissociation dynamics of methylated anilines

Aniline, an important model system for biological chromophores, undergoes ultrafast H-atom loss upon absorption of an ultraviolet photon. By varying the number and position of methyl substituents on both the aromatic ring and amine functional group, we explore the ultrafast production of photofragments as a function of molecular structure. Both N-methyl aniline and 3,5-dimethyl aniline show altered H-atom loss behaviour compared to aniline, while no evidence for CH3 loss was found from either N-methyl aniline or N,N-dimethyl aniline. With the addition of time-resolved photoelectron spectroscopy, the photofragment appearance times are matched to excited state relaxation pathways. Evidence for a sequential excited state relaxation mechanism, potentially involving a valence-to-Rydberg decay mechanism, will be presented. Such a global, bottom-up approach to molecular photochemistry is crucial to understanding the dissociative pathways and excited state decay mechanisms of biomolecule photoprotection in nature

Wavepacket insights into the photoprotection mechanism of the UV filter methyl anthranilate

Meradimate is a broad-spectrum ultraviolet absorber used as a chemical filter in commercial sunscreens. Herein, we explore the ultrafast photodynamics occurring in methyl anthranilate (precursor to Meradimate) immediately after photoexcitation with ultraviolet radiation to understand the mechanisms underpinning Meradimate photoprotection. Using time-resolved photoelectron spectroscopy, signal from the first singlet excited state of methyl anthranilate shows an oscillatory behavior, i.e. quantum beats. Our studies reveal a dependence of the observed beating frequencies on photoexcitation wavelength and photoelectron kinetic energy, unveiling the different Franck-Condon overlaps between the vibrational levels of the ground electronic, first electronic excited, and ground cationic states of methyl anthranilate. By evaluating the behavior of these beats with increasing photon energy, we find evidence for intramolecular vibrational energy redistribution on the first electronic excited state. Such energy redistribution hinders efficient relaxation of the electronic excited state, making methyl anthranilate a poor choice for an efficient, efficacious sunscreen chemical filter

Ultrafast dissociation dynamics of 2-Ethylpyrrole

To explore the effects of ring substitution on dissociation dynamics, the primary photochemistry of 2-ethylpyrrole has been explored using ultrafast ion imaging techniques. Photoexcitation to the S1 state, a πσ* state, in the range of 238 to 265 nm results in cleavage of the N–H bond with an H-atom appearance lifetime of ca. 70 fs. The insensitivity of this lifetime to photon energy, combined with a small kinetic isotope effect, suggests that tunneling does not play a major role in N–H bond cleavage. Total kinetic energy release spectra reveal modest vibrational excitation in the radical counter-fragment, increasing with photon energy. At wavelengths ≤ 248 nm a second, low kinetic energy H-atom loss mechanism becomes available with an appearance lifetime of approximately 1.5 ps and possibly due to the population of higher lying 1ππ* states

Photodissociation dynamics of the methyl perthiyl radical at 248 nm via photofragment translational spectroscopy

Photofragment translational spectroscopy was used to study the photodissociation of the methyl perthiyl radical CH 3 SS at 248 nm. The radical was produced by flash pyrolysis of dimethyl disulfide (CH 3 SSCH 3 ). Two channels were observed: CH 3 + S 2 and CH 2 S + SH. Photofragment translational energy distributions indicate that CH 3 + S 2 results from C-S bond fission on the ground state surface. The CH 2 S + SH channel can proceed through isomerization to CH 2 SSH on the ground state surface but also may involve production of electronically excited CH 2 S

Substituent position effects on sunscreen photodynamics : a closer look at methyl anthranilate

Towards the development of a bottom-up rationale for sunscreen design, the effects of substituent position on the ultrafast photodynamics of the sunscreen precursor methyl anthranilate (MA, an ortho compound) were evaluated by studying para- and meta-MA in vacuum. Time-resolved ion yield (TR-IY) measurements reveal a long-lived S1 excited state (≫ 1.2 ns) for para-MA, proposed to be the result of a weakly fluorescent, bound excited state. In the case of meta-MA, TR-IY transients reveal a much faster (∼2 ns) excited state relaxation, possibly due to multiple low-lying S1/S0 conical intersections of prefulvenic character. While meta-MA may not be an ideal sunscreen ingredient due to a low ultraviolet absorbance, its comparatively efficient relaxation mechanism may constitute an alternative to common sunscreen relaxation pathways. Thus, our results should prompt further studies of prefulvenic relaxation pathways in potential sunscreen agents

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

Methyl-E-4-methoxycinnamate (E-MMC) is a model chromophore of the commonly used commercial sunscreen agent, 2- ethylhexyl-E-4-methoxycinnamate (E-EHMC). In an effort to garner a molecular-level understanding of the photoprotection mechanisms in operation with E-EHMC, we have used time-resolved pump-probe spectroscopy to explore E-MMC’s and E-EHMC’s excited state dynamics upon UV-B photoexcitation to the S1 (11ππ*) state in both the gas- and solution-phase. In the gas-phase, our studies suggest that the excited state dynamics are driven by non-radiative decay from the 11ππ* to the S3 (11nπ*) state, followed by de-excitation from the 11nπ* to the ground electronic state (S0). Using both a non-polar-aprotic solvent, cyclohexane, and a polar-protic solvent, methanol, we investigated E-MMC and EEHMC’s photochemistry in a more realistic, ‘closer-to-shelf’ environment. A stark change to the excited state dynamics in the gas-phase is observed in the solution-phase suggesting that the dynamics are now driven by efficient E/Z isomerisation from the initially photoexcited 11ππ*state to S0

Kinetics of the hydrolysis of atmospherically relevant isoprene-derived hydroxy epoxides

Isoprene (the most abundant nonmethane hydrocarbon emitted into the atmosphere) is known to undergo oxidation to 2-methyl-1,2,3,4-butanetetraol, a hydrophilic compound present in secondary organic aerosol (SOA) in the atmosphere. Recent laboratory work has shown that gas phase hydroxy epoxides are produced in the low NOx photooxidation of isoprene and that these epoxides are likely to undergo efficient acid-catalyzed hydrolysis on SOA to 2-methyl-1,2,3,4-butanetetraol at typical SOA acidities. In order to confirm this hypothesis, the specific hydroxy epoxides observed in the isoprene photooxidation experiment (as well as several other related species) were synthesized, and the hydrolysis kinetics of all species were studied via nuclear magnetic resonance (NMR) techniques. It was determined that the isoprene-derived hydroxy epoxides should undergo efficient hydrolysis under atmospheric conditions, particular on lower pH SOA. An empirical structure−reactivity model was constructed that parametrized the hydrolysis rate constants according to the carbon substitution pattern on the epoxide ring and number of neighboring hydroxy functional groups. Compared to the previously studied similar nonfunctionalized epoxides, the presence of a hydroxy group at the α position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 20, and the presence of a hydroxy group at the beta position to the epoxy group was found to reduce the hydrolysis rate constant by a factor of 6

Formation And Stability Of Atmospherically Relevant Isoprene-derived Organosulfates And Organonitrates

Isoprene is the precursor for number of alcohol, organosulfate, and organonitrate species observed in ambient secondary organic aerosol (SOA). Recent laboratory and field work has suggested that isoprene-derived epoxides may be crucial intermediates that can explain the existence of these compounds in SOA. To confirm this hypothesis, the specific hydroxy epoxides observed in gas phase isoprene photooxidation experiments (as well as several other related species) were synthesized and the bulk phase aqueous reactions of these species in the presence of sulfate and nitrate were studied via nuclear magnetic resonance (NMR) techniques. The results indicate that both primary and tertiary organosulfates and organonitrates are efficiently formed from the potential SOA reactions of isoprene-derived epoxides. However, the tertiary organonitrates are shown to undergo rapid nucleophilic substitution reactions (in which nitrate is substituted for by water or sulfate) over the whole range of SOA pH, while the tertiary organosulfates are found to undergo a much slower acid-dependent hydrolysis reaction. The primary organonitrates and organosulfates under study were found to be stable against nudeophilic substitution reactions, even at low pH. This finding provides a potential explanation for the fact that organosulfates are more commonly detected in ambient SOA than are organonitrates