Excited-state Reaction Research Articles

Reaction blockading in a reaction between an excited atom and a charged molecule at low collision energy.

Recent advances have enabled studies of atom-ion chemistry at unprecedentedly low temperatures, allowing precision observation of chemical reactions and novel chemical dynamics. So far, these studies have primarily involved reactions between atoms and atomic ions or non-polar molecular ions, often in their electronic ground state. Here, we extend this work by studying an excited atom-polar-molecular-ion chemical reaction (Ca* + BaCl+) at low temperature in a hybrid atom-ion trapping system. The reaction rate and product branching fractions are measured and compared to model calculations as a function of both atomic quantum state and collision energy. At the lowest collision energy we find that the chemical dynamics differ dramatically from capture theory predictions and are primarily dictated by the radiative lifetime of the atomic quantum state instead of the underlying excited-state interaction potential. This reaction blockading effect, which greatly suppresses the reactivity of short-lived excited states, provides a means for directly probing the reaction range and also naturally suppresses unwanted chemical reactions in hybrid trapping experiments.

Excited-state intramolecular proton transfer reactions of 2,5-bis(2′-benzoxazolyl)hydroquinone and its water cluster exhibiting single and double proton transfer: A TD-DFT dynamics simulation

Detailed pictures of the excited-state intramolecular proton transfer (ESIPT) of 2,5-bis(2′-benzoxazolyl)hydroquinone (BHQ) and its water cluster have been investigated by dynamics simulations on the first lowest-excited energy using time-dependent density functional theory (TD-DFT). We focused on the structural, photophysical and dynamic properties of BHQ in the absence and presence of water molecules through intermolecular hydrogen bonds (interHBs). Our dynamics simulations reveal three possible mechanisms of the ESIPT processes: i) no proton transfer (No PT); ii) single PT (SPT); and iii) double PT (DPT), that could take place within the PT time of 160 fs via intrinsic intramolecular hydrogen bonds (intraHBs). The ESIPT mechanism of isolated BHQ elucidates that back PT is likely to be found at 64% rather than the SPT (32%) and DPT (4%), which is in good agreement with the experiments of dual fluorescence from di-enol and mono-keto emissions. Notably, the results from BHQ with water (BHQ-(H2O)2) reveal that the participation of water might produce a remarkable effect on promoting the SPT process up to 60% and DPT up to 7 times when compared to conditions of no water. The simulated probability of PT is well related to possible PT mechanisms regarding different tautomers in the fluorescence spectra found in previous experiments. The existence of di-keto tautomer arose from the DPT of BHQ and its water cluster and was not observed in the UV/Vis spectrum.

Divergent Hammett Plots of the Ground- and Excited-State Proton Transfer Reactions of 7-Substituted-2-Naphthol Compounds.

The rational design of photoacids requires accessible predictive models of the electronic effect of functional groups on chemical templates of interest. Here, the effect of substituents on the photoacidity and excited-state proton transfer (PT) pathways of prototype 2-naphthol (2OH) at the symmetric C7 position was investigated through photochemical and computational studies of 7-amino-2-naphthol (7N2OH) and 7-methoxy-2-naphthol (7OMe2OH). Time-resolved emission experiments of 7N2OH revealed that the presence of an electron-withdrawing versus electron-donating group (EWG vs EDG, NH3+ vs NH2) led to a drastic decline in photoacidity: p Ka* = 1.1 ± 0.2 vs 9.6 ± 0.2. Time-dependent density functional theory calculations with explicit water molecules confirmed that the excited neutral state (x = NH2) is greatly stabilized by water, with equation-of-motion coupled cluster singles and doubles calculations supporting potential mixing between the La and Lb states. Similar suppression of photoacidity, however, was not observed for 7OMe2OH with EDG OCH3, p Ka* = 2.7 ± 0.1. Hammett plots of the ground- and excited-state PT reactions of substituted 7-x-2OH compounds (x = CN, NH3+, H, CH3, OCH3, OH, and NH2) vs Hammett parameters σp showed breaks in the linearity between the EDG and EWG regions: ρ ∼ 0 vs 1.14 and ρ* ∼ 0 vs 3.86. The divergent acidic behavior most likely arises from different mixing mechanisms of the lowest Lb state with the La and possible Bb states upon substitution of naphthalene in water.

Unexpected Role of Ru(II) Orbital and Spin Contribution on Photoinduced Ligand Exchange: New Mechanism To Access the Photodynamic Therapy Window

A series of 12 Ru(II) complexes of the type [Ru(tpy)(L)(CH3CN)]n+, 1–12, containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands, L, were synthesized and evaluated for their ability to photodissociate CH3CN, a model for nitrile-containing drugs. Although the bidentate ligands chosen display a similar degree of steric bulk around the metal center in the ground state, the photosubstitution efficiencies of 1–12 vary by approximately an order of magnitude. The complexes containing the most electron-donating bidentate ligands, 8–11, exhibit the larger quantum yield values for ligand exchange in water. Complexes 8–11 also possess the smallest energy gap between the ground state and the lowest energy triplet metal-to-ligand charge transfer (3MLCT) excited state. In addition, density functional theory calculations indicate that a large degree of ligand character is present in their highest occupied molecular orbitals (HOMOs) and in their 3MLCT excited states. These results show that ligand mixing affects the π-backbonding ability of the metal center in the excited state, increasing the quantum yields of nitrile ligand dissociation. Linear relationships were observed between the quantum yield of ligand exchange and both the Mulliken spin density on the metal center in the excited state and the percent of Ru(d) character in the ground state HOMO. These findings, driven by the presence of π-donating ancillary ligands that enhance excited state reactivity, can be used to design new complexes with higher quantum yields for the release of nitrile-containing drugs and biological probes without affecting their stability in the dark.

Aqueous reactions of organic triplet excited states with atmospheric alkenes

Abstract. Triplet excited states of organic matter are formed when colored organic matter (i.e., brown carbon) absorbs light. While these “triplets” can be important photooxidants in atmospheric drops and particles (e.g., they rapidly oxidize phenols), very little is known about their reactivity toward many classes of organic compounds in the atmosphere. Here we measure the bimolecular rate constants of the triplet excited state of benzophenone (3BP∗), a model species, with 17 water-soluble C3–C6 alkenes that have either been found in the atmosphere or are reasonable surrogates for identified species. Measured rate constants (kALK+3BP∗) vary by a factor of 30 and are in the range of (0.24–7.5) ×109 M−1 s−1. Biogenic alkenes found in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and methyl jasmonate – react rapidly, with rate constants above 1×109 M−1 s−1. Rate constants depend on alkene characteristics such as the location of the double bond, stereochemistry, and alkyl substitution on the double bond. There is a reasonable correlation between kALK+3BP∗ and the calculated one-electron oxidation potential (OP) of the alkenes (R2=0.58); in contrast, rate constants are not correlated with bond dissociation enthalpies, bond dissociation free energies, or computed energy barriers for hydrogen abstraction. Using the OP relationship, we estimate aqueous rate constants for a number of unsaturated isoprene and limonene oxidation products with 3BP∗: values are in the range of (0.080–1.7) ×109 M−1 s−1, with generally faster values for limonene products. Rate constants with less reactive triplets, which are probably more environmentally relevant, are likely roughly 25 times slower. Using our predicted rate constants, along with values for other reactions from the literature, we conclude that triplets are probably minor oxidants for isoprene- and limonene-related compounds in cloudy or foggy atmospheres, except in cases in which the triplets are very reactive.

Role of the Pyridinyl Radical in the Light-Driven Reduction of Carbon Dioxide: A First-Principles Study

The reduction of carbon dioxide to fuels or chemical feedstocks with solar energy is one of the grand goals of current chemistry. In recent years, electrochemical, photoelectrochemical, and photochemical experiments provided hints of an unexpected catalytic role of the pyridine molecule in the reduction of carbon dioxide to formic acid or methanol. In particular, it has been suggested that the 1-pyridinyl radical (PyH) may be able to reduce carbon dioxide to the hydroxy-formyl radical. However, extensive theoretical studies of the thermodynamics and kinetics of the reaction called this interpretation of the experimental observations into question. Using ab initio computational methods, we investigated the photochemistry of the hydrogen-bonded PyH···CO2 complex. Our results reveal that carbon dioxide can be reduced to the hydroxy-formyl radical by a proton-coupled electron-transfer (PCET) reaction in excited states of the PyH···CO2 complex. In contrast to the ground-state PCET reaction, which exhibits a substantial barrier, the excited-state PCET reaction is barrierless but requires the passage through two conical intersections. Our results provide a tentative explanation of the catalytic role of the PyH radical in the reduction of CO2 with the qualification that the absorption of a photon by PyH is necessary.

Coupled Excited-State Dynamics in N-Substituted 2-Methoxy-9-Acridones.

Fluorophores of the acridone family have been widely employed in many applications, such as DNA sequencing, the detection of biomolecules, and the monitoring of enzymatic systems, as well as being the bases of intracellular sensors and even antitumoral agents. They have been widely used in fluorescence imaging due to their excellent photophysical properties, in terms of quantum yield and stability. However, frequently, the fluorescence emission data from acridones are not easily interpretable due to complex excited-state dynamics. The formation of π-stacking aggregates and excimers and excited-state proton transfer (ESPT) reactions usually result in emission features that are dependent on the experimental conditions. Therefore, an in-depth understanding of the dynamics involved in the excited-state transients of these dyes is mandatory for their appropriate application. Herein, we synthesized and fully characterized different 2-methoxy-9-acridone dyes. Their transient fluorescence emission spectra exhibited a complex dynamic behavior that can be linked to several excited-state reactions. We performed a thorough study of the excited-state dynamics of these dyes by means of time-resolved fluorimetry supported by computational calculations. All this allowed us to establish a multistate kinetic scheme, involving an ESPT reaction coupled to an excimer formation process. We have unraveled the rich dynamics behind this complex behavior, which provides a better understanding of the excited states of these dyes.

Near-IR Photochemistry for Biology: Exploiting the Optical Window of Tissue.

Photoactive molecules enable much of modern biology and biochemistry-a vast library of fluorescent chromophores is used to track and label cellular structures and macromolecules. However, photochemistry is better known to the synthetic or physical organic chemist as a "light switch" that turns on unusual excited-state reactivity, isomerization, or dynamic adjustment of structure. This review details a rapidly growing approach to biophotochemistry that uses low-energy near-IR wavelengths not only for imaging, but also for close spatial control over chemical switching events in biosystems. Emphasis is placed on topics of biomedical interest: release of gaseous biological messengers, uncaging of drugs, nano-therapeutics, and modification of biomaterials.

Carbon monoxide activation by atomic thorium: ground and excited state reaction pathways.

Multi-reference configuration interaction (MRCI) and single reference coupled cluster calculations are performed for the ThCO and OThC isomers. Scalar and spin-orbit relativistic effects are considered through a relativistic pseudopotential and the coupling of MRCI wavefunctions via the Breit-Pauli spin-orbit Hamiltonian. Optimized geometries, excitation energies, and vibrational frequencies are reported for both isomers. Full potential energy profiles are constructed for the Th+CO reaction and the conversion of the produced ThCO to OThC. Linear ThCO was found to be more stable than the highly ionic bent OThC system by about 4 kcal mol-1. The interconversion barrier is estimated to be around 30 kcal mol-1. Our results are in agreement with earlier experimental data for the two isomers. The lowest lying states of Th do not populate f-orbitals and resemble the electronic structure of Ti. Therefore, the ability of the two atoms to activate the C[triple bond, length as m-dash]O bond is compared. OTiC is found to be about 40 kcal mol-1 less stable than TiCO revealing the efficiency of Th and possibly other f-block elements to activate multiple chemical bonds as opposed to d-block metals.

Phototautomerism of isocytosine in a water medium: Theoretical and experimental study

We have carried out a combined theoretical (at the TDDFT BLYP/6-311++G** level of theory) and experimental study of the mechanism of phototautmerization of the N(3)H oxo tautomer of isocytosine dissolved in a water medium. The study reveals that this photoreaction occurs along the 1πσ* excited-state reaction paths of the oxo and hydroxy tautomers of the compound through conical intersections S0/S1. The last strucutres are located at the CASSCF level for both tautomers.

Experimental Evidence for a Triplet Biradical Excited-State Mechanism in the Photoreactivity of N,C-Chelate Organoboron Compounds.

N,C-chelate organoborates represent an emerging class of photoresponsive materials due to their photochromic switching at a boron center. Despite the promising applicability of such systems, little is known about the excited-state processes that lead to their unique photoreactivity, which is detrimental to the design of next-generation smart materials based on boron. As part of our ongoing effort to understand and improve the utility of these organoboron compounds, we report some of the first experimental evidence to support an excited-state mechanism for N,C-chelate organoborates. Femtosecond transient absorption spectroscopy combined with steady-state UV/vis and fluorescence measurements gives direct insight into their underlying photochemical processes, such as the formation of a common triplet charge-transfer state which either relaxes radiatively or undergoes the desired photoisomerization through a biradical intermediate. With this information, a complete mechanistic picture of the excited-state reactivity of N,C-chelate organoborates has been established, which is anticipated to lead to new smart materials with improved performance.

Tuneable solid-state emitters based on benzimidazole derivatives: Aggregation induced red emission and mechanochromism of D-π-A fluorophores

Organic fluorophores with photoluminescent properties in the solid state are widely applied in modern technology. Of special interest are those molecular materials capable of luminescent “switching” when exposed to external stimuli. However, the development of solid-state molecular systems capable of luminescence tuning over a broad spectral range, especially in the red region, remains a major challenge. In this work an aggregation induced emission phenomenon (AIE) is shown as a favourable strategy for design and development of novel solid-state emitters, especially in combination with excited-state proton transfer reactions (ESIPT). In general, the demanding and complex synthesis of AIE molecular systems (commonly large rotor molecules with luminescence in the blue and green spectral region) is known to be a major drawback of this approach.We present a systematic study of the spectral properties of five AIE- and ESIPT-based solid-state emitters comprising facile synthesis of benzimidazole (BI) derivatives, a new class of candidates for solid–state fluorophores. The five BI-based Schiff bases (compounds 1–5) were characterised in solution and solid state using spectroscopic and diffraction methods. All compounds are D-π-A structures with bulky aromatic groups that can undergo twisted molecular conformational change. The presence of the OH group was found to cause excited state intramolecular proton transfer (ESIPT) while different substituents on the π-conjugated backbone allow tuning of solid-state emission from blue to red. The compounds tend to aggregate in water, exhibiting new red-shifted emission bands. Compounds 1–5 display very weak emission in solutions, while in solid-state the emission is enhanced to various levels as a result of aggregation induced emission phenomena (AIE). Since AIE can easily be disrupted, the spectral properties of solid-state fluorophores are sensitive to external influence, such as mechanical or chemical stimuli. Reversible mechanochromic luminescence induced by mechanical grinding and subsequent chemical fuming and recrystallisation, is observed for fluorophores 2–5 with wavelength shifts of 16 nm, 19 nm, 26 nm and 48 nm, respectively. Fluorophore 5 shows red fluorescence with mechanochromic hysteresis. These results are discussed from a structural point of view and confirm the applicability of the dyes as mechanochromic materials.

Excited state transformations of heterostilbenes: Pathways to polycyclic skeleta

For the past 40 years M. Šindler-Kulyk's group has been studying, among other things, excited state reactivity of various heteroaromatic analogues of o -vinylstilbenes, o -vinylstyryl- and o -phenylenedivinylene-aromatic heterocycles, simply called o -vinyl-heterostilbenes and di-heterostilbenes. A concise overview of this vast research is presented. To date furan, pyrrole, sydnone, thiophene, oxazole, and most lately pyridine nuclei were incorporated into the basic stilbene-type skeleton. The photochemistry of these compounds is of great importance as it gives a pathway to new polycyclic products that are taxing or even impossible to obtain via ground state synthetic organic chemistry approaches. It is also an interesting subject because of illustration of diverse photoproducts formed by introducing different substituents into the system, thereby changing the conformation and electronic distribution in the molecule. Star representatives of the group of polycycles that were obtained are the benzobicyclo[3.2.1]octadiene photoproducts, which might show promising biological activity.

Chiral Inductions in Excited State Reactions: Photodimerization of Alkyl 2-Naphthoates as a Model.

Enantioselectivity in organic transformations continues to be a topic major interest in organic photochemistry. In the last decade, synergistic combination of photocatalysis and organocatalysis has emerged as a powerful strategy to gain enantioselectivity in photochemical reactions, and remarkable achievements have been obtained. In this strategy, the asymmetric induction is provided in ground state. In contrast, in the conventional enantioselective photochemistry, the chiral induction is controlled in electronic excited state, and to achieve high stereoselectivity is still a formidable challenge. Because the reactions of excited states often yield strained products with unique structures in single step that are difficult to form by thermal reactions, the development of new strategies attempted to achieve enantioselectivity in excited state reactions is still highly desired. Since the short excited state lifetime and low activation energy for reaction in excited state leave little room for manipulating the chiral induction, in order to gain enantioselectivity the substrate molecule has to already reside in a chiral environment during the excitation step. Chiral auxiliaries and chiral supramolecular hosts can provide such environments. In this presentation, we summarize the studies employing chiral auxiliary and chiral microreactor approaches to achieve high asymmetric inductions in excited state reactions performed in our laboratory. We chose the photodimerization of alkyl 2-naphthoates as a reaction model to give deeper insights into the basic factors controlling chiral induction in excited state.

Photochemistry of SO2 at the Air-Water Interface: A Source of OH and HOSO Radicals.

The photochemistry of sulfur dioxide in the near UV-vis energy range has been studied in aqueous environments. The combination of previously reported experimental measurements with accurate quantum chemical calculations achieved in this work reveals that the process represents an important source of OH radicals in the troposphere. It implicates the reaction of the lowest triplet excited state of SO2 with a water molecule. When the process occurs in the gas-phase, photochemical OH production is only significant under high humidity conditions and high SO2 concentrations as those measured in polluted urban areas. However, the OH production rate increases by several orders of magnitude when the process takes place at the surface of water droplets. The present study indicates therefore that the atmospheric importance of sulfur dioxide goes beyond its well-known role as acid rain and aerosol formation precursor.

Ultrafast Pump-Repump-Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching.

We demonstrate pump-repump-probe (PRP) transient hole burning as a spectroscopic tool for differentiating reactive from nonreactive deactivation of excited photochemical reactants observed by transient absorption spectroscopy (TAS). This method utilizes a time-delayed, wavelength-tunable ultrafast pulse to alter the excited reactant population, with the impact of "repumping" quantified through depletions in photoproduct absorption. We apply this approach to characterize dynamics affecting the nonadiabatic photocyclization efficiency to form S0 dihydrotriphenylene (DHT) following 266 nm excitation of ortho-terphenyl (OTP). TAS studies revealed bimodal deactivation of OTP*, but neither relaxation time scale (700 fs and 3.0 ps) could be assigned unambiguously to DHT formation due to overlap of excited-state and product spectra. PRP studies reveal that S1 OTP only cyclizes on the slower of these time scales, with the faster process attributable to nonreactive deactivation. We demonstrate that this method offers greater photochemical insights without assuming models to globally fit spectral transients collected by TAS.

Facial and Meridional Isomers of Tris(bidentate) Ir(III) Complexes: Unravelling Their Different Excited State Reactivity.

The use of tris(bidentate) Ir(III) complexes as light active components in phosphorescent organic light-emitting diodes (PhOLEDs) is currently the state-of-the-art technology to attain long-lasting and highly performing devices. Still, further improvements of their operational lifetimes are required for their practical use in lighting and displays. Facial/meridional stereoisomerism of the tris(bidentate) Ir(III) architectures strongly influences their emissive properties and thereto their PhOLEDs performances and operational device stabilities. This work underpins at the first-principles level the different excited state reactivities of facial and meridional stereoisomers of a series of tris(bidentate) Ir(III) complexes, which is found to originate in the presence of distinct triplet metal-centered (3MC) deactivation pathways. These deactivation pathways are herein presented for the first time for the meridional isomers. Finally, we propose some phosphor design strategies.

Excited-State Reactivity of [Mn(im)(CO)3 (phen)]+ : A Structural Exploration.

The electronic excited state reactivity of [Mn(im)(CO)3 (phen)]+ (phen = 1,10-phenanthroline; im = imidazole) ranging between 420 and 330 nm have been analyzed by means of relativistic spin-orbit time-dependent density functional theory and wavefunction approaches (state-average-complete-active-space self-consistent-field/multistate CAS second-order perturbation theory). Minimum energy conical intersection (MECI) structures and connecting pathways were explored using the artificial force induced reaction (AFIR) method. MECIs between the first and second singlet excited states (S1 /S2 -MECIs) were searched by the single-component AFIR (SC-AFIR) algorithm combined with the gradient projection type optimizer. The structural, electronic, and excited states properties of [Mn(im)(CO)3 (phen)]+ are compared to those of the Re(I) analogue [Re(im)(CO)3 (phen)]+ . The high density of excited states and the presence of low-lying metal-centered states that characterize the Mn complex add complexity to the photophysics and open various dissociative channels for both the CO and imidazole ligands. © 2018 Wiley Periodicals, Inc.

On-the-fly molecular dynamics study of the excited-state branching reaction of α-methyl-cis-stilbene

The branching reaction of α-methyl-cis-stilbene (cis-mSB) into its trans-mSB and 4a,4b-dihydrophenanthrene (DHP) forms upon ππ∗ excitation was examined theoretically by exploring the excited-state potential energy surface and using on-the-fly molecular dynamics simulations at the spin-flip time-dependent density functional theory (SF-TDDFT) level of theory. The branching ratio of trajectories was calculated as DHP:twist = 11:29, where twist denotes a mid-region between the cis-form and trans-form, indicating that the trans-mSB is a dominant product. The branching mechanism was analyzed by comparison with the corresponding theoretical studies on stilbene (SB) and 1,1′-dimethyl-stilbene (dmSB). The present computations elucidate the origin of variations in the branching ratio in the photoreactions of cis-SB, cis-mSB, and cis-dmSB. We also found that, because of loss of the slow component of the decay to the ground state, cis-mSB shows a faster decay rate to the ground state than cis-SB and cis-dmSB.

Experimental and theoretical study of the excited-state tautomerism of 6-azauracil in water surroundings

The irradiation of water solution of 6-azauracil showed that one of the photoreactions is the phototautomerization of the oxo tautomer of the compound into the hydroxy tautomer. The photoreaction was identified by a thorough analysis of experimentally obtained UV-spectra and the theoretical spectra simulation of several hydroxy tautomers of the compound. Furthermore we performed DFT level (BLYP/aug-cc-pVDZ) investigations of the mechanism of phototautomerization of the oxo tautomer of 6-azauracil in water surroundings applying PCM. The complete excited-state reaction path of the mechanism was proposed. It was found that the phototautomerization occurs along the 1πσ∗ excited-state reaction path. This reaction path was found to connect the two conical intersections S0/S1 of the H-detachment processes in the oxo and hydroxy tautomers of 6-azauracil.