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.     

Mapping the Excited‐State Potential Energy Surface of a Photomolecular Motor

A detailed understanding of the operation and efficiency of unidirectional photomolecular rotary motors is essential for their effective exploitation in molecular nanomachines. Unidirectional motion relies on light‐driven conversion from a stable ( 1 a ) to a metastable ( 1 b ) conformation, which then relaxes through a thermally driven helix inversion in the ground state. The excited‐state surface has thus far only been experimentally characterised for 1 a . Here we probe the metastable, 1 b , excited state, utilising ultrafast transient absorption and femtosecond stimulated Raman spectroscopy. These reveal that the “dark” excited‐state intermediate between 1 a and 1 b has a different lifetime and structure depending on the initial ground‐state conformation excited. This suggests that the reaction coordinate connecting 1 a to 1 b differs to that for the reverse photochemical process. The result is contrasted with earlier calculations.     

Firefly Chemiluminescence and Bioluminescence: Efficient Generation of Excited States

Firefly luciferase catalyzes a light‐emitting reaction in which an excited‐state product is formed. Both experimental and theoretical methodologies are used to study this system, and the reactions catalyzed by luciferase are relatively well characterized. However, the mechanism by which an excited‐state product is formed is still unknown. This Minireview deals with the current understanding of firefly bioluminescence and chemiluminescence. Thermal decomposition of simple 1,2‐dioxetanes is also discussed, due to their role in formation of the excited‐state bioluminophore.     

A Photophysical Study of Sensitization-Initiated Electron Transfer: Insights into the Mechanism of Photoredox Activity

The development of photocatalytic reactions has provided many novel opportunities to expand the scope of synthetic organic chemistry. In parallel with progress towards uncovering new reactivity, there is consensus that efforts focused on providing detailed mechanistic insight in order to uncover underlying excited-state reactions are essential to maximise formation of desired products. With this in mind, we have investigated the recently reported sensitization-initiated electron transfer (SenI-ET) reaction for the C−H arylation of activated aryl halides. Using a variety of techniques, and in particular nanosecond transient absorption spectroscopy, we are able to distinguish several characteristic signals from the excited-state species involved in the reaction, and subsequent kinetic analysis under various conditions has facilitated a detailed insight into the likely reaction mechanism.     

从Hückel分子轨道理论导出激发态周环反应的奇偶定则

本文从Huckel分子轨道理论,导出激发态周环反应的奇偶定则。在推导中Huckel久期行列式的非零元均视为反应坐标的函数,对其数值不作特殊假定。本文的推导也有助于对周环反应的Mobius-Huckel结构理论获得进一步的了解。     

Rich Athermal Ground‐State Chemistry Triggered by Dynamics through a Conical Intersection

A fundamental tenet of statistical rate theories (such as transition state theory and RRKM) is the rapidity of vibrational relaxation. Excited‐state reactions happen quite quickly (sub‐picosecond) and thus can exhibit nonstatistical behavior. However, it is often thought that any diversity of photoproducts results from different conical intersections connecting the excited and ground electronic states. It is also conceivable that the large energy of the photon, which is converted to vibrational energy after electronic transitions could lead to athermal hot ground state reactions and that these might be responsible for the diversity of photoproducts. Here we show that this is the case for sulfines, where a single conical intersection is implicated in the electronic transition but the excited state reaction leads to nine different products within less than a picosecond.     

Preparation and Photophysical and Photoelectrochemical Properties of a Covalently Fixed Porphyrin–Chemically Converted Graphene Composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl‐functionalized CCG and porphyrin boronic ester. The covalently linked CCG–porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady‐state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin‐linked CCG revealed a short‐lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO₂ electrode coated with the porphyrin‐linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π‐conjugated systems in the excited and ground states.     

Theoretical insights into the excited‐state process of 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol

In this paper, we theoretically explore the motivation and behaviors of the excited‐state intramolecular proton transfer (ESIPT) reaction for a novel white organic light‐emitting diode (WOLED) material 4‐tert‐butyl‐2‐(5‐(5‐tert‐butyl‐2‐methoxyphenyl)thiazolo[5,4‐d]thiazol‐2‐yl)‐phenol (t‐MTTH). The “atoms in molecules” (AIM) method is adopted to verify the formation and existence of the hydrogen bond O? H···N. By analyzing the excited‐state hydrogen bonding behaviors via changes in the chemical bonding and infrared (IR) vibrational spectra, we confirm that the intramolecular hydrogen bond O? H···N should be getting strengthened in the first excited state in four kinds of solvents, thus revealing the tendency of ESIPT reaction. Further, the role of charge‐transfer interaction is addressed under the frontier molecular orbitals (MOs), which depicts the nature of the electronic excited state and supports the ESIPT reaction. Also, the electron distribution confirms the ESIPT tendency once again. The scanned and optimized potential energy curves according to variational O? H coordinate in the solvents demonstrate that the proton transfer reaction should occur in the S₁ state, and the potential energy barriers along with ESIPT direction support this reaction. Based on the excited‐state behaviors reported in this work, the experimental spectral phenomenon has been reasonably explained.     

Mechanistic Insights into RNA Transphosphorylation from Kinetic Isotope Effects and Linear Free Energy Relationships of Model Reactions

Phosphoryl transfer reactions are ubiquitous in biology and the understanding of the mechanisms whereby these reactions are catalyzed by protein and RNA enzymes is central to reveal design principles for new therapeutics. Two of the most powerful experimental probes of chemical mechanism involve the analysis of linear free energy relations (LFERs) and the measurement of kinetic isotope effects (KIEs). These experimental data report directly on differences in bonding between the ground state and the rate‐controlling transition state, which is the most critical point along the reaction free energy pathway. However, interpretation of LFER and KIE data in terms of transition‐state structure and bonding optimally requires the use of theoretical models. In this work, we apply density‐functional calculations to determine KIEs for a series of phosphoryl transfer reactions of direct relevance to the 2′‐O‐transphosphorylation that leads to cleavage of the phosphodiester backbone of RNA. We first examine a well‐studied series of phosphate and phosphorothioate mono‐, di‐ and triesters that are useful as mechanistic probes and for which KIEs have been measured. Close agreement is demonstrated between the calculated and measured KIEs, establishing the reliability of our quantum model calculations. Next, we examine a series of RNA transesterification model reactions with a wide range of leaving groups in order to provide a direct connection between observed Brønsted coefficients and KIEs with the structure and bonding in the transition state. These relations can be used for prediction or to aid in the interpretation of experimental data for similar non‐enzymatic and enzymatic reactions. Finally, we apply these relations to RNA phosphoryl transfer catalyzed by ribonuclease A, and demonstrate the reaction coordinate–KIE correlation is reasonably preserved. A prediction of the secondary deuterium KIE in this reaction is also provided. These results demonstrate the utility of building up knowledge of mechanism through the systematic study of model systems to provide insight into more complex biological systems such as phosphoryl transfer enzymes and ribozymes.     

Rapid freeze-quench 57Fe Mössbauer spectroscopy: monitoring changes of an iron-containing active site during a biochemical reaction

Nuclear gamma resonance spectroscopy, also known as M?ssbauer spectroscopy, is a technique that probes transitions between the nuclear ground state and a low-lying nuclear excited state. The nucleus most amenable to M?ssbauer spectroscopy is 57Fe, and 57Fe M?ssbauer spectroscopy provides detailed information about the chemical environment and electronic structure of iron. Iron is by far the most structurally and functionally diverse metal ion in biology, and 57Fe M?ssbauer spectroscopy has played an important role in the elucidation of its biochemistry. In this article, we give a brief introduction to the technique and then focus on two recent exciting developments pertaining to the application of 57Fe M?ssbauer spectroscopy in biochemistry. The first is the use of the rapid freeze-quench method in conjunction with M?ssbauer spectroscopy to monitor changes at the Fe site during a biochemical reaction. This method has allowed for trapping and subsequent detailed spectroscopic characterization of reactive intermediates and thus has provided unique insight into the reaction mechanisms of Fe-containing enzymes. We outline the methodology using two examples: (1) oxygen activation by the non-heme diiron enzymes and (2) oxygen activation by taurine:alpha-ketoglutarate dioxygenase (TauD). The second development concerns the calculation of M?ssbauer parameters using density functional theory (DFT) methods. By using the example of TauD, we show that comparison of experimental M?ssbauer parameters with those obtained from calculations on model systems can be used to provide insight into the structure of a reaction intermediate.     

Photophysical studies of dipolar organic dyes that feature a 1,3-cyclohexadiene conjugated linkage: the implication of a twisted intramolecular charge-transfer state on the efficiency of dye-sensitized solar cells

A detailed study of the synthesis and photophysical properties of a new series of dipolar organic photosensitizers that feature a 1,3‐cyclohexadiene moiety integrated into the π‐conjugated structural backbone has been carried out. Dye‐sensitized solar cells (DSSCs) based on these structurally simple dyes have shown appreciable photo‐to‐electrical energy conversion efficiency, with the highest one up to 4.03 %. Solvent‐dependent fluorescence studies along with the observation of dual emission on dye 4 b and single emission on dyes 4 a and 32 suggest that dye 4 b possesses a highly polar emissive excited state located at a lower‐energy position than at the normal emissive excited state. A detailed photophysical investigation in conjunction with computational studies confirmed the twisted intramolecular charge‐transfer (TICT) state to be the lowest emissive excited state for dye 4 b in polar solvents. The relaxation from higher‐charge‐injection excited states to the lowest TICT state renders the back‐electron transfer process a forbidden one and significantly retards the charge recombination to boost the photocurrent. The electrochemical impedance under illumination and transient photovoltage decay studies showed smaller charge resistance and longer electron lifetime in 4 b ‐based DSSC compared to the DSSCs with reference dyes 4 a and 32 , which further illustrates the positive influence of the TICT state on the performance of DSSCs.     

Proton transfer in phenol–amine complexes: Phenol electronic effects on free energy profile in solution

Free energy profiles for the proton transfer reactions in hydrogen‐bonded complex of phenol with trimethylamine in methyl chloride solvent are studied with the reference interaction site model self‐consistent field method. The reactions in both the electronic ground and excited states are considered. The second‐order Møller‐Plesset perturbation (MP) theory or the second‐order multireference MP theory is used to evaluate the effect of the dynamical electron correlation on the free energy profiles. The free energy surface in the ground state shows a discrepancy with the experimental results for the related hydrogen‐bonded complexes. To resolve this discrepancy, the effects of chloro‐substitutions in phenol are examined, and its importance in stabilizing the ionic form is discussed. The temperature effect is also studied. In contrast to the ground state, the ππ^* excited state of phenol–trimethylamine complex exhibits the proton transfer reaction with a low barrier. The reaction is almost thermoneutral. This is attributed to the reduction of proton affinity of phenol by the ππ^* electronic excitation. We further examine the possibility of the electron–proton–coupled transfer in the ππ^* state through the surface crossing with the charge transfer type πσ^* state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Primary processes in the photosensitized polymerization of cationic monomers

A detailed time-resolved laser spectroscopy investigation has been carried out on the electron transfer reactions of substituted thioxanthone derivatives with diphenyliodonium (Ph-I⁺) salts having different metal halide counterions (MX^?_n). Quenching of thioxanthones' triplet state has been followed under various conditions, by changing the number and nature of substituents on the thioxanthone skeletone, using anion with different nucleophilicity and employing different solvents, namely methanol and acetonitrile. A Photosensitization mechanism is proposed involving an electron transfer from thioxanthone to diphenyliodonium salt. The absorption spectra of the thioxanthone's excited state and the formed new transient are recorded and the rate constants of the excited state processes are measured. The triplet state of thioxanthone derivatives has been quenched by cationically polymerizable monomers and the quantum yield of the major processes has been evaluated. Photolytic experiments have been performed to measure the extent of acid formation. Form photopolymerization experiments using different photoinitiating systems, the rate of polymerization and percentage of monomer conversion have been determined. Both the reactivity in the excited states and the nucleophilicity of the anions affect the efficiency of the photopolymerization reaction.     

Involvement of excited triplet state in the photodissociation of cyclobutane

The potential energy curves (PECs) of the ground state and the low‐lying excited states for the photodissociation of cyclobutane have been calculated at the multi‐reference configuration interaction with singlet and doublet excitation (MRCISD) and the multi‐reference second order perturbation theory (MRPT2). Firstly, the PECs are constructed following a reaction path determined by semiclassical dynamics simulation, which suggests that the lowest triplet state of tetramethylene is involved in the photodissociation of cyclobutane. Then, the adiabatic PECs are calculated for the breaking processes of C1? C3 and C2? C4 bond respectively. The singlet‐triplet PECs' intersections have been found in the two breaking C? C bond processes. During the breaking process of the second C2? C4 bond, a local minimum has been found on the PEC of the lowest triplet state, which gives us some insight to reinterpret the experimental observed diradical intermediate as being trapped in its triplet state. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S₁ lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S₁ state.     

The Host/Guest Type of Excited‐State Proton Transfer; a General Review

Contemporary progress regarding guest/host types of excited‐state double proton transfer has been reviewed, among which are the biprotonic transfer within doubly H‐bonded host/guest complexes, the transfer through a solvent bridge relay, the intramolecular double proton transfer and solvation dynamics coupled proton transfer. Of particular emphases are the photophysical and photochemical properties of excited‐state double proton transfer (ESDPT) in 7‐azaindole and its corresponding analogues. From the chemical aspect, two types of ESDPT reaction, namely the catalytic and non‐catalytic types of ESDPT, have been classified and reviewed separately. For the case of static host/guest hydrogen‐bonded complexes both hydrogen‐bonding strength and configuration (i.e. geometry) play key roles in accounting for the reaction dynamics. In addition to the dynamical concern, excited‐state thermodynamics are of importance to fine‐tune the proton transfer reaction in the non‐catalytic host/guest type of ESDPT. The mechanisms of protic solvent assisted ESDPT, depending on host molecules and proton‐transfer models, have been reviewed where the plausible resolution is deduced. Particular attention has been given to the excited‐state proton transfer dynamics in pure water, aiming at its future perspective in biological applications. Finally, the differentiation in mechanism between solvent diffusive reorganization and solvent relaxation to affect the host/guest ESPT dynamics is made and discussed in de tail.     

Complete Proton Transfer Cycle in GFP and Its T203V and S205V Mutants

Proton transfer is critical in many important biochemical reactions. The unique three‐step excited‐state proton transfer in avGFP allows observations of protein proton transport in real‐time. In this work we exploit femtosecond to microsecond transient IR spectroscopy to record, in D₂O, the complete proton transfer photocycle of avGFP, and two mutants (T203V and S205V) which modify the structure of the proton wire. Striking differences and similarities are observed among the three mutants yielding novel information on proton transfer mechanism, rates, isotope effects, H‐bond strength and proton wire stability. These data provide a detailed picture of the dynamics of long‐range proton transfer in a protein against which calculations may be compared.     

Solid‐State NMR Spectroscopy to Probe Photoactivation in Canonical Phytochromes

The photoreceptor phytochrome switches photochromically between two thermally stable states called Pr and Pfr. Here, we summarize recent solid‐state magic‐angle spinning (MAS) NMR work on this conversion process and interpret the functional mechanism in terms of a nano‐machine. The process is initiated by a double‐bond photoisomerization of the open‐chain tetrapyrrole chromophore at the methine bridge connecting pyrrole rings C and D. The Pr‐state chromophore and its surrounding pocket in canonical cyanobacterial and plant phytochromes has significantly less order, tends to form isoforms and is soft. Conversely, Pfr shows significantly harder chromophore–protein interactions, a well‐defined protonic and charge distribution with a clear classical counterion for the positively charged tetrapyrrole system. The soft‐to‐hard/disorder‐to‐order transition involves the chromophore and its protein surroundings within a sphere of at least 5.5 Å. The relevance of this collective event for signaling is discussed. Measurement of the intermediates during the Pfr → Pr back‐reaction provides insight into the well‐adjusted mechanics of a two‐step transformation. As both Pr → Pfr and Pfr → Pr reaction pathways are different in ground and excited states, a photochemically controlled hyper‐landscape is proposed allowing for ratchet‐type reaction dynamics regulating signaling activity.