Deactivation Path Research Articles

Femtosecond Infrared Spectroscopy Resolving the Multiplicity of High-Spin Crossover States in Transition Metal Iron Complexes.

Tuning the photophysical properties of iron-based transition-metal complexes is crucial for their employment as photosensitizers in solar energy conversion. For the optimization of these new complexes, a detailed understanding of the excited-state deactivation paths is necessary. Here, we report femtosecond transient mid-IR spectroscopy data on a recently developed octahedral ligand-field enhancing [Fe(dqp)2]2+ (C1) complex with dqp = 2,6-diquinolylpyridine and prototypical [Fe(bpy)3]2+ (C0). By combining mid-IR spectroscopy with quantum chemical DFT calculations, we propose a method for disentangling the 5Q1 and 3T1 multiplicities of the long-lived metal-centered (MC) states, applicable to a variety of metal-organic iron complexes. Our results for C0 align well with the established assignment toward the 5Q1, validating our approach. For C1, we find that deactivation of the initially excited metal-to-ligand charge-transfer state leads to a population of a long-lived MC 5Q1 state. Analysis of transient changes in the mid-IR shows an ultrafast sub 200 fs rearrangement of ligand geometry for both complexes, accompanying the MLCT → MC deactivation. This confirms that the flexibility in the ligand sphere supports the stabilization of high spin states and plays a crucial role in the MLCT lifetime of metal-organic iron complexes.

Quantum-Chemistry Study of the Photophysical Properties of 4-Thiouracil and Comparisons with 2-Thiouracil.

DNA in living beings is constantly damaged by exogenous and endogenous agents. However, in some cases, DNA photodamage can have interesting applications, as it happens in photodynamic therapy. In this work, the current knowledge on the photophysics of 4-thiouracil has been extended by further quantum-chemistry studies to improve the agreement between theory and experiments, to better understand the differences with 2-thiouracil, and, last but not least, to verify its usefulness as a photosensitizer for photodynamic therapy. This study has been carried out by determining the most favorable deactivation paths of UV-vis photoexcited 4-thiouracil by means of the photochemical reaction path approach and an efficient combination of the complete-active-space second-order perturbation theory//complete-active-space self-consistent field (CASPT2//CASSCF), (CASPT2//CASPT2), time-dependent density functional theory (TDDFT), and spin-flip TDDFT (SF-TDDFT) methodologies. By comparing the data computed herein for both 4-thiouracil and 2-thiouracil, a rationale is provided on the relatively higher yields of intersystem crossing, triplet lifetime and singlet oxygen production of 4-thiouracil, and the relatively higher yield of phosphorescence of 2-thiouracil.

One-pot synthesis of alginate-antimicrobial peptide nanogel.

Nanosized alginate-based particles (NAPs) were obtained in a one-pot solvent-free synthesis procedure, achieving the design of a biocompatible nanocarrier for the encapsulation of IbM6 antimicrobial peptide (IbM6). IbM6 is integrated in the nascent nanosized hydrogel self-assembly guided by electrostatic interactions and by weak interactions, typical of soft matter. The formation of the nanogel is a dynamic and complex process, which presents an interesting temporal evolution. In this work, we optimized the synthesis conditions of IbM6-NAPs based on small-angle X-ray scattering (SAXS) measurements and evaluated its time evolution over several weeks by sensing the IbM6 environment in IbM6-NAPs from photochemical experiments. Fluorescence deactivation experiments revealed that the accessibility of different quenchers to the IbM6 peptide embedded in NAPs is dependent on the aging time of the alginate network. Lifetimes measurements indicate that the deactivation paths of the excited state of the IbM6 in the nanoaggregates are reduced when compared with those exhibited by the peptide in aqueous solution, and are also dependent on the aging time of the nanosized alginate network. Finally, the entrapment of IbM6 in NAPs hinders the degradation of the peptide by trypsin, increasing its antimicrobial activity against Escherichia coli K-12 in simulated operation conditions.

One Dianionic Luminophore with Three Coordination Modes Binding Four Different Metals: Toward Unexpectedly Phosphorescent Transition Metal Complexes.

This work reports on a battery of coordination compounds featuring a versatile dianionic luminophore adopting three different coordination modes (mono, bi, and tridentate) while chelating Pd(II), Pt(II), Au(III), and Hg(II) centers. An in-depth structural characterization of the ligand precursor (H2 L) and six transition metal complexes ([HLPdCNtBu], [LPtCl], [LPtCNtBu], [LPtCNPhen], [HLHgCl], and [LAuCl]) is presented. The influence of the cations and coordination modes of the luminophore and co-ligands on the photophysical properties (including photoluminescence quantum yields (ΦL ), excited state lifetimes (τ), and average (non-)radiative rate constants) are evaluated at various temperatures in different phases. Five complexes show interesting photophysical properties at room temperature (RT) in solution. Embedment in frozen glassy matrices at 77K significantly boosts their luminescence by suppressing radiationless deactivation paths. Thus, the Pt(II)-based compounds provide the highest efficiencies, with slight variations upon exchange of the ancillary ligand. In the case of [HLPdCNtBu], both ΦL and τ increase over 30-fold as compared to RT. Furthermore, the Hg(II) complex achieves, for the first time in its class, a ΦL exceeding 60% and millisecond-range lifetimes. This demonstrates that a judicious ligand design can pave the way toward versatile coordination compounds with tunable excited state properties.

ReNeGate: A Reaction Network Graph-Theoretical Tool for Automated Mechanistic Studies in Computational Homogeneous Catalysis.

Exploration of the chemical reaction space of chemical transformations in multicomponent mixtures is one of the main challenges in contemporary computational chemistry. To remove expert bias from mechanistic studies and to discover new chemistries, an automated graph-theoretical methodology is proposed, which puts forward a network formalism of homogeneous catalysis reactions and utilizes a network analysis tool for mechanistic studies. The method can be used for analyzing trajectories with single and multiple catalytic species and can provide unique conformers of catalysts including multinuclear catalyst clusters along with other catalytic mixture components. The presented three-step approach has the integrated ability to handle multicomponent catalytic systems of arbitrary complexity (mixtures of reactants, catalyst precursors, ligands, additives, and solvents). It is not limited to predefined chemical rules, does not require prealignment of reaction mixture components consistent with a reaction coordinate, and is not agnostic to the chemical nature of transformations. Conformer exploration, reactive event identification, and reaction network analysis are the main steps taken for identifying the pathways in catalytic systems given the starting precatalytic reaction mixture as the input. Such a methodology allows us to efficiently explore catalytic systems in realistic conditions for either previously observed or completely unknown reactive events in the context of a network representing different intermediates. Our workflow for the catalytic reaction space exploration exclusively focuses on the identification of thermodynamically feasible conversion channels, representative of the (secondary) catalyst deactivation or inhibition paths, which are usually most difficult to anticipate based solely on expert chemical knowledge. Thus, the expert bias is sought to be removed at all steps, and the chemical intuition is limited to the choice of the thermodynamic constraint imposed by the applicable experimental conditions in terms of threshold energy values for allowed transformations. The capabilities of the proposed methodology have been tested by exploring the reactivity of Mn complexes relevant for catalytic hydrogenation chemistry to verify previously postulated activation mechanisms and unravel unexpected reaction channels relevant to rare deactivation events.

On the Notation of Catastrophes in the Framework of Bonding Evolution Theory: Case of Normal and Inverse Electron Demand Diels-Alder Reactions.

This paper generalizes very recent and unexpected findings [J. Phys. Chem. A, 2021, 125, 5152-5165] regarding the known "direct- and inverse-electron demand" Diels-Alder mechanisms. Application of bonding evolution theory indicates that the key electron rearrangement associated with significant chemical events (e. g., the breaking/forming processes of bonds) can be characterized via the simplest fold polynomial. For the CC bond formation, neither substituent position nor the type of electronic demand induces a measurable cusp-type signature. As opposed to the case of [4+2] cycloaddition between 1,3-butadiene and ethylene, where the two new CC single bonds occur beyond the transition state (TS) in the activated cases, the first CC bond occurs in the domain of structural stability featuring the TS, whereas the second one remains located in the deactivation path connecting the TS with the cycloadduct.

Nucleic Acids as a Playground for the Computational Study of the Photophysics and Photochemistry of Multichromophore Assemblies.

The interaction between light and multichromophoric assemblies (MCAs) is the primary event of many fundamental processes, from photosynthesis to organic photovoltaics, and it triggers dynamical processes that share remarkable similarities at the molecular scale: light absorption, energy and charge transfer, internal conversions, emission, and so on. Those events often involve many chromophores and different excited electronic states that are coupled on an ultrafast time scale. This Account aims to discuss some of the chemical physical effects ruling these processes, a fundamental step toward their control, based on our experience on nucleic acids.In the last 15 years, we have, indeed, studied the photophysics and photochemistry of DNA and its components. By combining different quantum mechanical methods, we investigated the molecular processes responsible for the damage of the genetic code or, on the contrary, those preventing it by dissipating the excess energy deposited in the system by UV absorption. Independently of its fundamental biological role, DNA, with its fluctuating closely stacked bases stabilized by weak nonbonding interactions, can be considered a prototypical MCA. Therefore, it allows one to tackle within a single system many of the conceptual and methodological challenges involved in the study of photoinduced processes in MCA.In this Account, by using the outcome of our studies on oligonucleotides as a guideline, we thus highlight the most critical modellistic issues to be faced when studying, either experimentally or computationally, the interaction between UV light and DNA and, at the same time, bring out their general relevance for the study of MCAs.We first discuss the rich photoactivated dynamics of nucleobases (the chromophores), highlighting the main effects modulating the interplay between their excited states and how the latter can affect the photoactivated dynamics of the polynucleotides, either providing effective monomer-like nonradiative decay routes or triggering reactive processes (e.g., triplet generation).We then tackle the reaction paths involving multiple bases, showing that in the DNA duplex the most important ones involve two stacked bases, forming a neutral excimer or a charge transfer (CT) state, which exhibit different spectral signatures and photochemical reactivity. In particular, we analyze the factors affecting the dynamic equilibrium between the excimer and CT, such as the fluctuations of the backbone or the rearrangement of the solvent.Next, we highlight the importance of the effects not directly connected to the chromophores, such as the flexibility of the backbone or the solvent effect. The former, affecting the stacking geometry of the bases, can determine the preference between different deactivation paths. The latter is particularly influential for CT states, making very important an accurate treatment of dynamical solvation effects, involving both the solvent bulk and specific solute-solvent interactions.In the last section, we describe the main methodological challenges related to the study of polynucleotide excited states and stress the benefits derived by the integration of complementary approaches, both computational and experimental. Only exploiting different point of views, in our opinion, it is possible to shed light on the complex phenomena triggered by light absorption in DNA, as in every MCA.

Photophysical characteristics of Phenylimidazo(4,5-f)1,10-phenanthroline Rhenium(I) complexes - A theoretical approach to their potential applications in lighting devices

In the search for new luminescent semiconductors to be applied in energy conversion devices, we report via DFT/TD-DFT calculations the structure, UV–Vis absorption, and phosphorescent properties of Re(I) complexes denoted as [ReCl(CO)3(Ln)] and [ReTMP(CO)3(Ln)]+, where TMP is 2,4,6-trimethylpyridine and Ln is a bidentate ligand that corresponds to phenylimidazo(4,5-f)1,10-phenanthroline. This last ligand was analyzed for their electron-donating and -withdrawing characters with different substituents on the peripheral phenyl ring (Ln = -H, –CH3, o –F). Regarding the absorptions, at lower energy in the [ReCl(CO)3(Ln)], it was possible to identify a mixture of metal-to-ligand and ligand-to-ligand charge-transfer excitations, while the [ReTMP(CO)3(Ln)]+ series showed ligand centered transitions as responsible for absorption maximum. The phosphorescence of [ReCl(CO)3(Ln)] complexes exhibits an emission wavelength of 609–623 nm, whose deactivation path comes from π* orbital located at Ln ligands, towards the metal and ligands orbitals. By contrast, the emission process of the [ReTMP(CO)3(Ln)]+ complexes are mainly due to ligand-centered electron promotion (70%) in line with absorption features, showing emission wavelengths in the range of 521–527 nm. The change of the substituent Cl for TMP significantly influences the absorption and emission energies, unlike substitution on the Ln ligand. However, this last substitution pattern becomes relevant when considering injection/transport of holes-electrons where the reorganization energy (λ) for [ReCl(CO)3(L3)] neutral complexes improves due to the more balance charge transfer ability required for organic light-emitting diodes (OLED), while [ReTMP(CO)3(L2)]+ is the best choice for a light-emitting electrochemical cell (LEEC). Then, rhenium complexes can be considered as promising luminophores for device applications.

Mechanistic Photophysics of Tellurium-Substituted Uracils: Insights from Multistate Complete-Active-Space Second-Order Perturbation Calculations.

The photophysical mechanisms of tellurium-substituted uracils were studied at the multistate complete-active-space second-order perturbation level with a particular focus on how the position and number of tellurium substitutions affect their nonadiabatic relaxation processes. Electronic structure analysis reveals that the lowest several excited states are closely concerned with the n and π orbitals at the Te7-C2 [Te8-C4] moiety of 2-tellurouracil (2TeU) [4TeU and 24TeU]. Both planar and twisted minima were optimized for 2TeU, whereas only planar ones were obtained for 4TeU and 24TeU, except for a twisted T1 minimum of 4TeU. Based on intersection structures and linearly interpolated internal coordinate paths, we proposed several feasible excited-state deactivation paths. It is found that the relaxation channels for 2TeU are more complicated than those of 4TeU and 24TeU. The electronic population transfer to the T1 state for 2TeU is easier than that for 4TeU and 24TeU in consideration of the barrier heights from the S2 Franck-Condon point to the S2/S1 or S2/T2 intersections. In addition, the recovery of the ground state from the T1 state for 2TeU will be more efficient than that for the other two systems as well.

The case of [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (bpm: 2,2′-bipyrimidine) towards multi-metallic ReI species and their properties

The reaction of [Re(CO)3(THF)Br]2 and 2,2′-bipyrimidine (bpm) in dichloromethane at room temperature employing a stoichiometric 2:1 ratio leads to a mixture of [(bpm)Re(CO)3Br] (21%) and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (79%). When a 1:1 ratio is used, only anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] is obtained in a 89% yield. The reaction of [Re(CO)5Br] and (bpm) leads to [(bpm)Re(CO)3Br] as a single product. [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] display one and two rhenium(I) centers of distorted octahedral coordination geometry respectively, each with a facial ligand correlation. The intramolecular metal to metal distance is 5.8508(16) Å in anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. The HOMO-LUMO gap for [(bpm)Re(CO)3Br] results 0.74 eV larger (3.03 vs 2.29 eV) than the computed for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. In the same sense, the anti-isomer is computed to be 0.14 eV (13.4 kJ mol−1) more stable compared to the hypothetical syn-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Cyclic voltammetry shows that the difference between the ReI/Re0 reduction and its ReI/ReII oxidation in terms of potential is about 3.46 V for [(bpm)Re(CO)3Br], while this value diminishes to 2.62 V for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Following this tendency, the MLCT band for [(bpm)Re(CO)3Br] is centered at 385 nm (DMF), while it is red-shifted almost 100 nm for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. While [(bpm)Re(CO)3Br] emits at 680 nm (excited at 380 nm), anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] remains non-emissive, which is attributed to a favored vibrational deactivation path.

Pore Blocking by Phenolates as Deactivation Path during the Cracking of 4-Propylphenol over ZSM-5

Cracking of propyl side chains from 4-propylphenol, a model compound for lignin monomers, is studied for a commercial ZSM-5 zeolite catalyst. The decline of 4-propylphenol conversion with time on stream can be delayed by co-feeding water. FTIR spectroscopy shows the formation of chemisorbed phenolates during reactions and significant amounts of phenolics are detected by GC-MS of the extract from the spent catalysts. Thus, chemisorbed phenolates are identified as the main reason for deactivation in the absence of water. Regardless of the amount of co-fed water, substituted monoaromatics and polyaromatic species are formed. Comprehensive characterization of the spent catalysts including Raman and solid-state 27Al NMR spectroscopy, and thermogravimetric analysis points to a combination of deactivation processes. First, phenolates bind to Lewis acid sites within the zeolite framework and hinder diffusion unless they are hydrolyzed by water. In addition, light olefins created during the cracking process react to form a polyaromatic coke that deactivates the catalyst more permanently.

Impact of Heteroatom Speciation on the Activity and Stability of Carbon‐Based Catalysts for Propane Dehydrogenation

AbstractMetal‐free carbon catalysts represent attractive alternatives to platinum and chromium‐containing systems for the on‐purpose production of propylene, a crucial chemical building block, via non‐oxidative propane dehydrogenation. However, the role of heteroatoms in determining reactivity and stability is poorly understood. Here, the study of a platform of polyaniline‐derived and activated carbons, with distinct porous properties, and heteroatom type, content, and speciation, enables establishing improved structure‐performance relationships. In‐depth characterization reveals that, while several oxygen species potentially catalyze the reaction, carbonyl groups are the main functionality governing the activity. Oxygen loss via propane‐driven water evolution is the main deactivation path. Six‐fold enhanced stability evidenced over carbon catalysts derived from KOH‐post‐treated polyanilines demonstrates the importance of controlling the nanostructure for enhancing longevity.

Implementation of the interacting quantum atom energy decomposition using the CASPT2 method.

We present an implementation of the interacting quantum atom (IQA) energy decomposition scheme using the complete active space second-order perturbation theory (CASPT2). This combination yields a real-space interpretation tool with a proper account of the static and dynamic correlation that is particularly relevant for the description of processes in electronic excited states. The IQA/CASPT2 approach allows determination of the energy redistribution that takes place along a photophysical/photochemical deactivation path in terms of self- and interatomic contributions. The applicability of the method is illustrated by the description of representative processes spanning different bonding regimes: noble gas excimer and exciplex formation, the reaction of ozone with a chlorine atom, and the photodissociations of formaldehyde and cyclobutane. These examples show the versatility of using CASPT2 with the significant information provided by the IQA partition to describe chemical processes with a large multiconfigurational character.

A theoretical study on the excited-state deactivation paths for the A-5FU dimer.

The photostability of DNA plays a key role in the normal function of organisms. A-5FU is a base pair derivative of the A-T dimer where the methyl group is replaced by a F atom. Here, accurate static TDDFT calculations and non-adiabatic dynamic simulations are used to systematically investigate the excited-state decay paths of the A-5FU dimer related to the proton transfer and the out-of-plane twisting deformation motion of A and 5FU in the 1ππ* and 1nπ* states. CC2 is used to check the accuracy of the current TDDFT calculations. Our results show that the deformation of the C[double bond, length as m-dash]C or C[double bond, length as m-dash]N double bond in A and 5FU provides an efficient pathway for the depopulation of the lowest excited states, which can compete with the excited-state proton transfer paths in the dimer. This finding indicates that monomer-like decay paths could be important for the photostability of weakly hydrogen-bonded DNA base pairs and provide a new insight into the excited-state decay paths in base pairs and their analogues.

(Invited) Activation and Deactivation in Ultra-Highly Doped n-Type Epitaxy for nMOS Applications

Since ultra-highly doped, highly tensile strained Si:P epitaxy was reported about 10 years ago, it quickly replaced Si:CP and become key processes for advanced nodes. With greater than 3E21 doping level, these films exhibit significant tensile stress and low resistivity, both enhanced the nMOS performance. As the technology scales, contact resistance is becoming a major factor for device performance, even higher activation level is desired. In this paper, the mechanisms for high doping level and tensile strain are briefly reviewed and discussed. Despite epitaxially grown, these films showed similar thermal equilibrium behavior as implant-annealed samples and could be predicted with well established model. Diffusion and strain analysis shed some lights on the processes involved in the carrier activation and deactivation. Formation of group V - vacancy complex, V4V, or Si3P4-like, structure is key to understand the behavior, especially dopant diffusion and interaction with vacancy. Unlike Si:As, Si:P showed different behavior with different anneal and deposition approaches that might indicating a much more complicate deactivation path. And strong surface influence of P diffusion might present a challenge for contact engineering.

Numerical simulation of cyclic behavior of a ductile metal with a coupled damage-plasticity model with several damage deactivation paths

In this article, the damage analysis of a thin walled rod has been investigated by employing the continuum damage mechanics (CDM). The rod is under cyclic tension and compression loadings and the elstoplastic damage analysis has been carried out by using the developed finite element code. Moreover, unilateral effect of damage has been investigated and the obtained results are compared to the experimental data, available in the literature. The results are obtained for two kinds of damage deactivation regimes, discontinuous and continuous damage deactivation with different damage deactivation paths. The obtained results show that, using continuous damage deactivation concept yields to more aligned results with experimental data, both qualitatively and quantitatively. Moreover, it is shown that using second order function of stress as a damage deactivation path leads to better numerical convergence in solution algorithm and also it makes more accurate prediction of material behavior under cyclic loading.

Decoupling structure-sensitive deactivation mechanisms of Ir/IrOx electrocatalysts toward oxygen evolution reaction

Deterioration of intrinsic activity of Ir-based electrocatalysts during the oxygen evolution reaction (OER) has received much less attention compared to the metal dissolution. Combining chronoamperometry with operando electrochemical impedance spectroscopy and cyclic voltammetry, we show that the deactivation via active site phase transformation from hydrous Ir oxide/hydroxide into anhydrous Ir oxide, is concomitant with the dissolution-induced loss of electrochemical surface area. The relative contributions from these deactivation paths were found to be structure sensitive. Systematic evaluation of different Ir-based catalysts at identical electro-oxidative conditions showed that hydrous IrOx with structural short-range order exhibited an initial minor degradation of intrinsic activity but the most significant dissolution after the extended stability test. In contrast, newly-reported Ir superstructures with higher crystallinity and larger proportion of low-index crystal terminations exhibited enhanced resistance to dissolution but a major degradation of intrinsic activity, as the performance-relevant hydrous oxide/hydroxide species developed only on the surface of metallic Ir. The Ir/IrOx catalyst regeneration was demonstrated.

Origin of the Photoinduced Geometrical Change of Copper(I) Complexes from the Quantum Chemical Topology View.

Copper(I) complexes (CICs) are of great interest due to their applications as redox mediators and molecular switches. CICs present drastic geometrical change in their excited states, which interferes with their luminescence properties. The photophysical process has been extensively studied by several time-resolved methods to gain an understanding of the dynamics and mechanism of the torsion, which has been explained in terms of a Jahn-Teller effect. Here, we propose an alternative explanation for the photoinduced structural change of CICs, based on electron density redistribution. After photoexcitation of a CIC (S0 →S1 ), a metal-to-ligand charge transfer stabilizes the ligand and destabilizes the metal. A subsequent electron transfer, through an intersystem crossing process, followed by an internal conversion (S1 →T2 →T1 ), intensifies the energetic differences between the metal and ligand within the complex. The energy profile of each state is the result of the balance between metal and ligand energy changes. The loss of electrons originates an increase in the attractive potential energy within the copper basin, which is not compensated by the associated reduction of the repulsive atomic potential. To counterbalance the atomic destabilization, the valence shell of the copper center is polarized (defined by ∇2 ρ(r) and ∇2 Vne (r)) during the deactivation path. This polarization increases the magnitude of the intra-atomic nuclear-electron interactions within the copper atom and provokes the flattening of the structure to obtain the geometry with the maximum interaction between the charge depletions of the metal and the charge concentrations of the ligand.

Photosensitization Versus Photocyclization: Competitive Reactions of Phenylphenalenone in Its Role as Phytoanticipins in Plant Defense Strategies.

Phenalenone (PN) derivatives are involved in plant defense strategies, producing molecular singlet oxygen in a photosensitization process. Many experimental and theoretical studies determined that PN can performe this process with a quantum yield close to 1. However, it has been observed that the efficiency of some of its derivatives is much lower. This is the case of 9-phenylphenalenone (9-PhPN). To elucidate the factors that determine the different photochemistry of PN and its derivate 9-PhPN, we developed a complete active space self-consistent field/multi-configurational second-order perturbation theory study where several deactivation paths through the lowest excited states were explored. We found that the characteristics of the low-lying excited states are similar for both PN and 9-PhPN in the areas near the geometry of excitation. Consequently, the first processes that take place immediately after absorption are possible in both systems, including the population of the triplet state responsible for oxygen sensitization. However, 9-PhPN can also undergo cyclization by a bond formation between the carbonyl oxygen and a carbon atom of the phenyl substituent. This process competes favorably with population of triplet states and is responsible for the decrease of the quantum yield of oxygen sensitization in 9-PhPN relative to PN.

Topology-dependent hydrocarbon transformations in the methanol-to-hydrocarbons reaction studied by operando UV-Raman spectroscopy.

The methanol-to-hydrocarbons (MTH) reaction represents a versatile, industrially viable alternative to crude-oil based processes for the production of chemicals and fuels. In the MTH reaction, the shape selectivity of acidic zeolites is exploited to direct the synthesis towards the desired product. However, due to unavoidable side reactions occurring under processing conditions, all MTH catalysts suffer deactivation due to coke formation. Though it is likely that some common characteristics for carbon formation exist for all zeolite topologies, it has been proposed that the differences in shape selectivity among the different catalysts will also influence the individual deactivation mechanisms. As deactivating species are mostly aromatic compounds, highly methylated benzenes and/or polycyclic aromatic hydrocarbons (PAHs) have been discussed. In some cases, these can further grow to extended carbon structures. Here, we have investigated the hydrocarbon reactivities and carbon formation for five topologically different zeolite catalysts through an operando UV-Raman approach, taking advantage of the high sensitivity of this technique towards aromatic and other carbonaceous species. The combination of the spectroscopic tool with activity measurements allowed us to obtain valuable details and some general trends on the deactivation paths during MTH. This approach made accessible unique insight on the complex chemistry of MTH by allowing the real-time observation of hydrocarbon transformations typical for the peculiar topology of each catalyst, usually inaccessible by ex situ techniques.