Excited-state Surfaces Research Articles

Ultraviolet Photodissociation Dynamics of the 1-Methylallyl Radical.

The ultraviolet (UV) photodissociation dynamics of the 1-methylallyl (1-MA) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the wavelength region of 226-244 nm. The 1-MA radicals were produced by 193 nm photodissociation of the 3-chloro-1-butene and 1-chloro-2-butene precursor. The 1 + 1 REMPI spectrum of 1-MA agrees with the previous UV absorption spectrum in this wavelength region. Quantum chemistry calculations show that the UV absorption is mainly attributed to the 3pz Rydberg state (perpendicular to the allyl plane). The H atom photofragment yield (PFY) spectrum of 1-MA from 3-chloro-1-butene displays a broad peak around 230 nm, while that from 1-chloro-2-butene peaks at ∼236 nm. The translational energy distributions of the H atom loss product channel, P (ET)'s, show a bimodal distribution indicating two dissociation pathways in 1-MA. The major pathway is isotropic in product angular distribution with β ∼ 0 and has a low fraction of average translational energy in the total excess energy, ⟨fT⟩, in the range of 0.13-0.17; this pathway corresponds to unimolecular dissociation of 1-MA after internal conversion to form 1,3-butadiene + H. The minor pathway is anisotropic with β ∼ -0.23 and has a large ⟨fT⟩ of ∼0.62-0.72. This fast pathway suggests a direct dissociation of the methyl H atom on a repulsive excited state surface or the repulsive part of the ground state surface to form 1,3-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Vibrational coherences in half-broadband 2D electronic spectroscopy: Spectral filtering to identify excited state displacements.

Vibrational coherences in ultrafast pump-probe (PP) and 2D electronic spectroscopy (2DES) provide insights into the excited state dynamics of molecules. Femtosecond coherence spectra and 2D beat maps yield information about displacements of excited state surfaces for key vibrational modes. Half-broadband 2DES uses a PP configuration with a white light continuum probe to extend the detection range and resolve vibrational coherences in the excited state absorption (ESA). However, the interpretation of these spectra is difficult as they are strongly dependent on the spectrum of the pump laser and the relative displacement of the excited states along the vibrational coordinates. We demonstrate the impact of these convoluting factors for a model based upon cresyl violet. A careful consideration of the position of the pump spectrum can be a powerful tool in resolving the ESA coherences to gain insights into excited state displacements. This paper also highlights the need for caution in considering the spectral window of the pulse when interpreting these spectra.

Non-adiabatic excited-state time-dependent GW molecular dynamics (TDGW) satisfying extended Koopmans' theorem: An accurate description of methane photolysis.

There is a longstanding difficulty that time-dependent density functional theory relying on adiabatic local density approximation is not applicable to the electron dynamics, for example, for an initially excited state, such as in photochemical reactions. To overcome this, we develop non-adiabatic excited-state time-dependent GW molecular dynamics (TDGW) on the basis of the extended quasiparticle theory. Replacing Kohn-Sham orbitals/energies with correlated, interacting quasiparticle orbitals/energies allows the full correspondence to the excited-state surfaces and corresponding total energies, with satisfying extended Koopmans' theorem. We demonstrate the power of TDGW using methane photolysis, CH4→CH3•+H, an important initiation reaction for combustion/pyrolysis and hydrogen production of methane. We successfully explore several possible pathways and show how this reaction dynamics is captured accurately through simultaneously time-tracing all quasiparticle levels. TDGW scales as O(NB3-4), where NB is the number of basis functions, which is distinctly advantageous to performing dynamics using configuration interaction and coupled cluster methods.

Non-adiabatic direct quantum dynamics using force fields: Toward solvation.

Quantum dynamics simulations are becoming a powerful tool for understanding photo-excited molecules. Their poor scaling, however, means that it is hard to study molecules with more than a few atoms accurately, and a major challenge at the moment is the inclusion of the molecular environment. Here, we present a proof of principle for a way to break the two bottlenecks preventing large but accurate simulations. First, the problem of providing the potential energy surfaces for a general system is addressed by parameterizing a standard force field to reproduce the potential surfaces of the molecule's excited-states, including the all-important vibronic coupling. While not shown here, this would trivially enable the use of an explicit solvent. Second, to help the scaling of the nuclear dynamics propagation, a hierarchy of approximations is introduced to the variational multi-configurational Gaussian method that retains the variational quantum wavepacket description of the key quantum degrees of freedom and uses classical trajectories for the remaining in a quantum mechanics/molecular mechanics like approach. The method is referred to as force field quantum dynamics (FF-QD), and a two-state ππ*/nπ* model of uracil, excited to its lowest bright ππ* state, is used as a test case.

Aminonaphthalene and aminoquinoline photocages: meta-effect and photo-release of carboxylic acids and alcohols

Three series of compounds were tested as photocleavable protective groups (photocages), which are derivatives of 1,2-substituted aminonaphthalene 4, 1,3-substituted aminonaphthalene 5, or 5,6-substituted aminoquinolines 6. Their photophysical properties and photochemical reactivity in releasing alcohols and carboxylic acids were investigated. Moreover, mechanism of photochemical reaction was studied computationally and by transient absorption spectroscopy with time-resolution from femtoseconds to milliseconds. Photocages 4 and 5 are generally fluorescent molecules with high ΦF, whereas quinoline 6 is weekly fluorescent due to photoinduced charge transfer. Derivatives 4 release alcohol with low quantum yield (release of methanol from 4d is ΦR = 0.00010 ± 0.00005), but upon prolonged irradiation can be applied in organic synthesis to deprotect both aliphatic and aromatic alcohols. On the contrary, photocages 5 undergo relatively efficient release of carboxylic acids, (release of acetic acid from 5b is ΦR = 0.22 ± 0.01), in accord with the meta-effect in photochemistry, which is our main discovery that is based on the rational design of the chromophore. The photochemical reaction mechanism involves homolytic cleavage on the singlet excited state surface and formation of radicals which were detected by transient spectroscopy. The meta-effect did not promote the heterolytic cleavage as it was the case with aniline chromophores. The subsequent electron transfer delivers naphthalenmethyl carbocation and released carboxylates. The applicability of photocages 5 in organic synthesis was demonstrated on aliphatic and aromatic carboxylic acids. The potential use in biology was suggested due to their very weak photoinduced antiproliferative effect, which was essayed on three human cancer cell lines (MCF-7 breast carcinoma, HCT 116 colon carcinoma, and H460 lung carcinoma).

How a Single 5 eV Electron Can Induce Double-Strand Breaks in DNA: A Time-Dependent Density Functional Theory Study.

Low-energy (<20 eV) electrons (LEEs) can resonantly interact with DNA to form transient anions (TAs) of fundamental units, inducing single-strand breaks (SSBs), and cluster damage, such as double-strand breaks (DSBs). Shape resonances, which arise from electron capture in a previously unfilled orbital, can induce only a SSB, whereas a single core-excited resonance (i.e., two electrons in excited orbitals of the field of a hole) has been shown experimentally to cause cluster lesions. Herein, we show from time-dependent density functional theory (TDDFT) that a core-excited resonance can produce a DSB, i.e., a single 5 eV electron can induce two close lesions in DNA. We considered the nucleotide with the G-C base pair (ds[5'-G-3']) as a model for electron localization in the DNA double helix and calculated the potential energy surfaces (PESs) of excited states of the ground-state TA of ds[5'-G-3'], which correspond to shape and core-excited resonances. The calculations show that shape TAs start at ca. 1 eV, while core-excited TAs occur only above 4 eV. The energy profile of each excited state and the corresponding PES are obtained by simultaneously stretching both C5'-O5' bonds of ds[5'-G-3']. From the nature of the PES, we find two dissociative (σ*) states localized on the PO4 groups at the C5' sites of ds[5'-G-3']. The first σ* state at 1 eV is due to a shape resonance, while the second σ* state is induced by a core-excited resonance at 5.4 eV. As the bond of the latter state stretches and arrives close to the dissociation limit, the added electron on C transfers to C5' phosphate, thus demonstrating the possibility of producing a DSB with only one electron of ca. 5 eV.

Cavity Quantum Electrodynamics Complete Active Space Configuration Interaction Theory.

Polariton chemistry has attracted great attention as a potential route to modify chemical structure, properties, and reactivity through strong interactions among molecular electronic, vibrational, or rovibrational degrees of freedom. A rigorous theoretical treatment of molecular polaritons requires the treatment of matter and photon degrees of freedom on equal quantum mechanical footing. In the limit of molecular electronic strong or ultrastrong coupling to one or a few molecules, it is desirable to treat the molecular electronic degrees of freedom using the tools of ab initio quantum chemistry, yielding an approach we refer to as ab initio cavity quantum electrodynamics, where the photon degrees of freedom are treated at the level of cavity quantum electrodynamics. Here, we present an approach called Cavity Quantum Electrodynamics Complete Active Space Configuration Interaction theory to provide ground- and excited-state polaritonic surfaces with a balanced description of strong correlation effects among electronic and photonic degrees of freedom. This method provides a platform for ab initio cavity quantum electrodynamics when both strong electron correlation and strong light-matter coupling are important and is an important step toward computational approaches that yield multiple polaritonic potential energy surfaces and couplings that can be leveraged for ab initio molecular dynamics simulations of polariton chemistry.

Spin-Vibronic Intersystem Crossing and Molecular Packing Effects in Heavy Atom Free Organic Phosphor.

We present a detailed investigation into the excited state properties of a planar D3h symmetric azatriangulenetrione, HTANGO, which has received significant interest due to its high solid-state phosphorescence quantum yield and therefore potential as an organic room temperature phosphorescent (ORTP) dye. Using a model linear vibronic coupling Hamiltonian in combination with quantum dynamics simulations, we observe that intersystem crossing (ISC) in HTANGO occurs with a rate of ∼1010 s-1, comparable to benzophenone, an archetypal molecule for fast ISC in heavy metal free molecules. Our simulations demonstrate that the mechanism for fast ISC is associated with the high density of excited triplet states which lie in close proximity to the lowest singlet states, offering multiple channels into the triplet manifold facilitating rapid population transfer. Finally, to rationalize the solid-state emission properties, we use quantum chemistry to investigate the excited state surfaces of the HTANGO dimer, highlighting the influence and importance of the rotational alignment between the two HTANGO molecules in the solid state and how this contributes to high phosphorescence quantum yield.

Acid Violet 3: A Base-Activated Water-Soluble Photoswitch.

Acidic azo dyes are widely used for their vibrant colors. However, if their photophysics were better understood and controllable, they could be integrated into many more applications such as photosensing, photomedicine, and nonlinear optics. Here, the proton-controlled photophysics of a widely used acid, hydrazo dye, acid violet 3 (AV3) is explored. Density functional theory is used to predict the ground- and excited-state potential energy surfaces, and the proposed photoisomerization mechanism is confirmed with spectroscopic experiments. The ground-state and first two excited-state surfaces of the three readily accessible protonation states, AV3-H, AV3, and AV3+H, are investigated along both the dihedral rotation and inversion coordinates. The deprotonated AV3-H undergoes photoisomerization with blue light (λex = 453 nm) through a dihedral rotation mechanism. Upon the formation of the cis-isomer, the reversion of AV3-H is predicted to occur through a mixed rotational and inversion mechanism. In contrast, AV3 and its protonated form, AV3+H, do not undergo photoisomerization because there is no driving force for either the rotation or inversion of the azo bond in the excited state. In addition, when the azo bond is acidic, the ground-state dihedral rotation reversion mechanism barrier is lower. The mechanistic insights gained here through the combination of theory and experiment provide a roadmap to control the reactivity of AV3 across 11 orders of magnitude of proton concentration, making them interesting candidates for a range of pharmaceuticals.

Azobenzene-Imidazolium ionic liquid crystals: Phase properties and photoisomerization in solution state

This is the first report of phase properties and photoisomerization behaviour of a series of amphiphilic azobenzene-imidazolium derivatives. The cationic imidazolium moiety was functionalised with varying alkyl chain length (C10-C18) and connected to an azobenzene via a flexible ether spacer. Their phase transitional properties and photoisomerization in solution were modulated by the alkyl chains in the imidazolium moiety. C14 was the shortest chain length to induce the formation of a liquid crystal phase, wherein compounds having C14-C18 exhibited stable smectic A phase. All compounds photo switched from trans-to-cis isomers in solution when illuminated with UV radiation. DFT calculations revealed that the trans isomers could adopt two geometries, however, folded geometry (global minima) was thermodynamically more stable than the open chain structure. The long conjugate (C16) had a higher trans-cis isomerization barrier due to the multilayer folding geometry that reduced its structural flexibility as compared to the C10 homologue. The photo conversion efficiency (CE) was alkyl chain length dependent. The C10 salt had a larger CE than C16 homologue due to the transition gap between S1 excited state and ground surface is smaller in the former (0.211 eV) than that in the latter (0.278 eV), thus resulting in efficient photoisomerization in the former.

Analogies between photochemical reactions and ground-state post-transition-state bifurcations shed light on dynamical origins of selectivity.

Revealing the origins of kinetic selectivity is one of the premier tasks of applied theoretical organic chemistry, and for many reactions, doing so involves comparing competing transition states. For some reactions, however, a single transition state leads directly to multiple products, in which case non-statistical dynamic effects influence selectivity control. The selectivity of photochemical reactions-where crossing between excited-state and ground-state surfaces occurs near ground-state transition structures that interconvert competing products-also should be controlled by the momentum of the reacting molecules as they return to the ground state in addition to the shape of the potential energy surfaces involved. Now, using machine-learning-assisted non-adiabatic molecular dynamics and multiconfiguration pair-density functional theory, these factors are examined for a classic photochemical reaction-the deazetization of 2,3-diazabicyclo[2.2.2]oct-2-ene-for which we demonstrate that momentum dominates the selectivity for hexadiene versus [2.2.2] bicyclohexane products.

Tracking the conical intersection dynamics for the photoinduced Jahn-Teller switch of a Mn(iii) complex.

Octahedral Mn(iii) ions predominantly exhibit an axially elongated Jahn-Teller (JT) distortion, which is responsible for their large uniaxial magnetic anisotropy. As a result, they are often used in the synthesis of single-molecule magnets (SMMs). Modulation of the JT distortion using femtosecond laser pulses could offer a route to controlling the magnetisation direction, and therefore is promising for the development of data storage devices that work on ultrafast timescales. Photoinduced switching of the distortion from an axially elongated to an axially compressed structure has been demonstrated for various Mn(iii) complexes. However, the dynamics around the region of the conical intersection for the photoinduced JT switch remains unclear. Here, ultrafast transient absorption spectra were recorded for solutions of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)manganese(iii) (Mn(dpm)3) in ethanol to further explore the dynamics of the photoinduced JT switch. We observe the generation of a vibrational wavepacket on the excited state surface, which has a frequency of approximately 155 cm-1 and encompasses a JT-active vibrational mode. This coherent motion is maintained after passage through the conical intersection back to the ground state, which launches wavepackets along the ground state potential energy surface (PES) with frequencies of approximately 180 and 110 cm-1 that we assign to the elongated and compressed state, respectively. Inspection of the relative phases of the frequencies reveals phase shifts that are consistent with a one-mode reaction coordinate, and passes through the conical intersection at 1/4 and 3/4 of the excited state vibrational period. Our results provide direct insights into the non-adiabatic dynamics of Mn(iii) complexes, which can be used to guide the synthesis of optically controlled SMMs.

Fast and accurate excited states predictions: machine learning and diabatization.

The efficiency of machine learning algorithms for electronically excited states is far behind ground-state applications. One of the underlying problems is the insufficient smoothness of the fitted potential energy surfaces and other properties in the vicinity of state crossings and conical intersections, which is a prerequisite for an efficient regression. Smooth surfaces can be obtained by switching to the diabatic basis. However, diabatization itself is still an outstanding problem. We overcome these limitations by solving both problems at once. We use a machine learning approach combining clustering and regression techniques to correct for the deficiencies of property-based diabatization which, in return, provides us with smooth surfaces that can be easily fitted. Our approach extends the applicability of property-based diabatization to multidimensional systems. We utilize the proposed diabatization scheme to achieve higher prediction accuracy for adiabatic states and we show its performance by reconstructing global potential energy surfaces of excited states of nitrosyl fluoride and formaldehyde. While the proposed methodology is independent of the specific property-based diabatization and regression algorithm, we show its performance for kernel ridge regression and a very simple diabatization based on transition multipoles. Compared to most other algorithms based on machine learning, our approach needs only a small amount of training data.

Ultraviolet photochemistry of the 2-buten-2-yl radical.

The ultraviolet (UV) photodissociation dynamics of the 2-buten-2-yl (C4H7) radical were studied using the high-n Rydberg atom time-of-flight (HRTOF) technique in the photolysis region of 226-246 nm. 2-Buten-2-yl radicals were generated by 193 nm photodissociation of the precursor 2-chloro-2-butene. The H-atom photofragment yield (PFY) spectrum of 2-buten-2-yl is broad, peaking at 234 nm. Quantum chemistry calculations show that the UV absorption is due to the 3py and 3px Rydberg states (parallel to the plane of CC double bond). The translational energy distributions of the H-atom loss product channel, P(ET)'s, of 2-buten-2-yl show a bimodal distribution indicating two dissociation pathways. The major pathway peaks at ET ∼ 7 kcal mol-1 with a nearly constant fraction of average ET in the total excess energy, 〈fT〉, at ∼0.11-0.12. This main pathway has an isotropic product angular distribution with β ∼ 0, consistent with the unimolecular dissociation of a hot 2-buten-2-yl radical following internal conversion from the electronically excited state, resulting in the formation of 2-butyne + H (∼84%) and 1,2-butadiene + H (∼16%). Additionally, there is a minor non-statistical pathway with an isotropic angular distribution. The minor pathway peaks at ET ∼ 35 kcal mol-1 in the P(ET) distributions and exhibits a large 〈fT〉 of ∼0.40-0.46. This fast pathway suggests a direct dissociation of the methyl H-atom on a repulsive excited state surface or on the repulsive part of the ground state surface, forming 1,2-butadiene + H. The fast/slow pathway branching ratio is in the range of 0.03-0.08.

Machine learning accelerated photodynamics simulations

Machine learning (ML) continues to revolutionize computational chemistry for accelerating predictions and simulations by training on experimental or accurate but expensive quantum mechanical (QM) calculations. Photodynamics simulations require hundreds of trajectories coupled with multiconfigurational QM calculations of excited-state potential energies surfaces that contribute to the prohibitive computational cost at long timescales and complex organic molecules. ML accelerates photodynamics simulations by combining nonadiabatic photodynamics simulations with an ML model trained with high-fidelity QM calculations of energies, forces, and non-adiabatic couplings. This approach has provided time-dependent molecular structural information for understanding photochemical reaction mechanisms of organic reactions in vacuum and complex environments (i.e., explicit solvation). This review focuses on the fundamentals of QM calculations and ML techniques. We, then, discuss the strategies to balance adequate training data and the computational cost of generating these training data. Finally, we demonstrate the power of applying these ML-photodynamics simulations to understand the origin of reactivities and selectivities of organic photochemical reactions, such as cis–trans isomerization, [2 + 2]-cycloaddition, 4π-electrostatic ring-closing, and hydrogen roaming mechanism.

Hot-Exciton Mechanism and AIE Effect Boost the Performance of Deep-Red Emitters in Non-Doped OLEDs.

Luminescent materials possessing a "hot-exciton" mechanism and aggregation-induced emission (AIE) qualities are well-suited for use as emitting materials in nondoped organic light-emitting diodes (OLEDs), particularly in deep-red regions where their ground state and singlet excited state surfaces are in proximity, leading to the formation of multiple nonradiative channels. However, designing molecules that artificially combine the hot-exciton mechanism and AIE attributes remains a formidable task. In this study, a versatile strategy is presented to achieve hot-exciton fluorescence with AIE property by increasing the first singlet excited (S1 ) state through modulation of the conjugation length of the newly created acceptor unit, matching the energy level of high-lying triplet (Tn ) states, and enhancing exciton utilization efficiency by employing suitable donor moieties. This approach reduces the aggregation-caused quenching (ACQ) in the aggregate state, resulting in the proof-of-concept emitter DT-IPD, which produces an unprecedented external quantum efficiency (EQE) of 12.2% and Commission Internationale de I'Eclairage(CIE)coordinates of (0.69, 0.30) in a deep-red non-doped OLED at 685nm, representing the highest performance among all deep-red OLEDs based on materials with hot-exciton mechanisms. This work provides novel insights into the design of more efficient hot-exciton emitters with AIE properties.

Exploring the Effects of Mutagenesis on FusionRed by Using Excited-State QM/MM Dynamics and Classical Force Field Simulations.

Fluorescent proteins (FPs) are a powerful tool for examining tissues, cells, and subcellular components in vivo and in vitro. FusionRed is a particular FP variant mutated from mKate2 that, in addition to lower cytotoxicity and aggregation rates, has shown potential for acting as a tunable photoswitch. This was posited to stem partially from the presence of a bulky side chain at position 158 and a further stabilizing residue at position 157. In this work, we apply computational techniques including classical molecular dynamics (MD) and combined quantum mechanics/molecular mechanics simulations (QM/MM) to explore the effect of mutagenesis at these locations in FusionRed on the chromophore structure, the excited-state surface, and relative positional stability of the chromophore in the protein pocket. We find specific connections between the statistical sampling of the underlying protein structure and the nonradiative decay mechanisms from excited-state dynamics. A single mutation (C158I) that restricts the motion of the chromophore through a favorable hydrophobic interaction corresponds to an increase in fluorescence quantum yield (FQY), while a second rescue mutation (C158I-A157N) partially restores the flexibility of the chromophore and photoswitchability with favorable water interactions on the surface of the protein that counteracts the original interaction. We suggest that applying this understanding of structural features that inhibit or favor rotation on the excited state can be applied for rational design of new, tunable and red photoswitches.

Photophysics of the red-form Kaede chromophore.

The green fluorescent protein (GFP) drove revolutionary progress in bioimaging. Photoconvertible fluorescent proteins (PCFPs) are an important branch of the FP family, of which Kaede is the prototype. Uniquely, PCFPs can be permanently switched from green to red emitting forms on UV irradiation, facilitating applications in site-specific photolabelling and protein tracking. Optimisation and exploitation of FPs requires understanding of the photophysical and photochemical behaviour of the chromophore. Accordingly, the principal GFP chromophore has been the subject of intense experimental and theoretical investigation. In contrast, the photophysics of the red emitting PCFP chromophore are largely unstudied. Here we present a detailed investigation of the excited-state properties of the Kaede chromophore in solution, utilising steady state measurements, ultrafast time-resolved electronic and vibrational spectroscopies, and electronic structure theory. Its excited state dynamics are very different to those of the parent GFP. Most remarkably, the PCFP chromophore has highly complex wavelength-dependent fluorescence decays and a mean lifetime an order of magnitude longer than the GFP chromophore. Transient electronic and vibrational spectroscopies suggest that these dynamics arise from a range of excited-state conformers that are spectrally and kinetically distinct but chemically similar. These conformers are populated directly by excitation of a broad thermal distribution of ground state structures about a single conformer, suggesting an excited-state potential surface with several minima. Temperature-dependence confirms the existence of barriers on the excited-state surface and reveals the radiationless decay mechanism to be internal conversion. These experimental observations are consistent with a model assuming a simple ground state potential energy surface accessing a complex excited state possessing multiple minima.

Excited state potential energy surfaces of N-phenylpyrrole upon twisting: reference values and comparison between BSE/GW and TD-DFT.

The puzzling case of the mixing between the charge transfer (CT) and local excited (LE) characters upon twisting of the geometry of N-phenylpyrrole (N-PP) is investigated considering the six low-lying singlet excited states (ES). The theoretical calculations of the potential energy surfaces (PES) have been performed for these states using a Coupled Cluster method accounting for the impact of the contributions from the triples, many-body Green's function GW and Bethe-Salpeter equation (BSE) formalisms, as well as Time-Dependent Density Functional Theory (TD-DFT) using various exchange-correlation functionals. Our findings confirm that the BSE formalism is more reliable than TD-DFT for close-lying ES with mixed CT/LE nature. More specifically, BSE/GW yields a more accurate evolution of the excited state PES than TD-DFT when compared to the reference coupled cluster values. BSE/GW PES curves also show negligible exchange-correlation functional starting point dependency in sharp contrast with their TD-DFT counterparts.

Photochemical Degradation of the UV Filter Octyl Methoxy Cinnamate Probed via Laser-Interfaced Mass Spectrometry.

Octyl methoxycinnamate (OMC) is a common UVA and UVB filter molecule that is widely used in commercial sunscreens. Here, we used gas-phase laser photodissociation spectroscopy to characterise the intrinsic photostability and photodegradation products of OMC by studying the system in its protonated form, i.e., [OMC·H]+. The major photofragments observed were m/z 179, 161, and 133, corresponding to fragmentation on either side of the ether oxygen of the ester group (m/z 179 and 161) or the C-C bond adjacent to the ester carbonyl group. Additional measurements were obtained using higher-energy collisional dissociation mass spectrometry (HCD-MS) to identify fragments that resulted from the breakdown of the vibrationally hot electronic ground state. We found that the m/z 179 and 161 ions were the main fragments produced by this route. Notably, the m/z 133 ion was not observed through HCD-MS, revealing that this product ion is only produced through a photochemical route. Our results demonstrate that UV photoexcitation of OMC is able to access a dissociative excited-state surface that uniquely leads to the rupture of the C-C bond adjacent to the key ester carbonyl group.