Inorganic Photochemistry:Past,Present, and Future

Transition metal complex, in its electronic excited state, has intriguing photophysical and photochemical properties that are substantially different from its ground state, Indeed, electronically excited metal complex can be viewed as hot chemical species that is readily synthesized by photo-excitation with UV-visible light. If the energy of excited metal complex can be properly manipulated, it may be possible to devise new catalytic system for converting light to chemical energy. In the context of energy conversion reactions and chemical sensing, it is important for biomolecular reactions at room temperature. Among the photochemical bimolecular reactions, the following three have the widest applications in photocatalysis, and these are (1) bimolecular outer-sphere electron transfer reactions, (2) bimoleculat inner-sphere atom transfer/abstract reactions, and (3) exciplex formation involving electronic excited state. The past of inorganic photochemistry has demonstrated the success of[Ru(bpy)₃]²⁺ as a powerful reagent for light-induced electron transfer reactions. Much of the current photochemistry research focus on coordinative unsaturated metal complexes, that are strongly photoluminescent and readily undergo substrate binding reactions in their excited states. In this lecture, I will review some of the past successful stories of[Ru(bpy)₃]²⁺ and discuss our current research on the luminescent metal-complexes prepared in my laboratory. I will end my lecture by proposing a clue for achieving light-induced multi-electron transfer reactions, which remains a challenge in photochemistry research.     

On the Apparent Lack of Oxygen Quenching of Triplet Excited States

The early observation that excited triplet states obtained chemically from trimethyldioxetane (and subsequent energy transfers) appeared not to be quenched by oxygen is now shown to be an artifact; acetaldehyde is a product of the reaction and via chemical reactions it scavenges oxygen present in the medium; the attendant extended lifetime of the triplet states then permits their employment in photochemical-type reactions. The effect of the aldehyde can be over-ridden through an efficient introduction of an excess of oxygen into the liquid reaction phase.     

Properties of excited singlet states of N-arylurethanes : Photo-fries reactions,fluorescence, quenching and sensitization

The deactivation of the first excited S(ππ^*) states of N-arylurethanes (produced upon irradiation with UV light) by emission (fluorescence), chemical reaction (photo-Fries rearrangement and fragmentation), energy transfer to quenchers, and radiationless transitions to ground and triplet states is investigated. Arylurethanes exhibit fluorescence (λ_f ≈ 295–350 nm, φ_f ≈ 10^?2, τ_f ≈ 1–6 ns) and phosphorescencs (λ_p ≈ 370–410 nm). The variations of the quantum yields of the fluorescence and of the photo-Fries rearrangement of N-arylurethanes by substituents and solvents are essentially due to variations of the rate constants for the radiationless processes. Fluorescence and photo-Fries reactions can be quenched by diffusion-controlled energy transfer to aliphatic ketones. Quenching is accompanied by sensitization of the ketone fluorescence. The urethane fluorescence and photo reactions may be sensitized by aromatic hydrocarbons. The results of all the quenching and sensitization experiments demonstrate that the photo-Fries reactions of N-arylurethanes proceed via the first excited singlet states of the urethanes.     

Excited state calculations on fluorene-based polymer blends: effect of stacking orientation and solvation

Polyfluorene-based polymer blends have been utilized in the development of optoelectronic devices. The constituent copolymers are chemically designed to facilitate more efficient electron/hole mobility, thereby enhancing control over exciton formation and dissociation. When appropriate pairs of these are blended together, intermolecular charged-particle localizations are induced, leading to significant intermolecular charge-transfer character and luminescence that exhibit some sensitivity to their interfacial orientation. The authors report on a time-dependent density functional theory quantum chemical investigation of the relevant excited states of the polymer blend poly[9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine]/poly(9,9-dioctylfluorene-co-benzothiadiazole. They show that the calculated excited states generally agree with experimental observations although there is a consistent underestimation of the charge-transfer states. Further, they show sensitivity to lateral shifts in interfacial stacking. Finally, solvation with a low dielectric solvent greatly stabilizes the charge-transfer states.     

On the role of van der Waals interaction in chemical reactions at low temperatures

It is shown that van der Waals interaction potential plays a crucial role in chemical reactions at low temperatures. By taking the Cl+HD reaction as an illustrative example, we demonstrate that quasibound states of the van der Waals potential preferentially undergo chemical reaction rather than vibrational predissociation. Prereaction occurs even when the wave functions of the quasibound states peak far out into the entrance channel, outside the region of the van der Waals well. It is found that chemical reaction dominates over nonreactive vibrational quenching in collisions of vibrationally excited HD molecules with ground state chlorine atoms at ultracold temperatures.     

Analytical applications of nanomaterials in electrogenerated chemiluminescence

This critical review covers the use of carbon nanomaterials (single-wall carbon nanotubes, multi-wall carbon nanotubes, graphene, and carbon quantum dots), semiconductor quantum dots, and composite materials based on the combination of the aforementioned materials, for analytical applications using electrogenerated chemiluminescence. The recent discovery of graphene and related materials, with their optical and electrochemical properties, has made possible new uses of such materials in electrogenerated chemiluminescence for biomedical diagnostic applications. In electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), electrochemically generated intermediates undergo highly exergonic reactions, producing electronically excited states that emit light. These electron-transfer reactions are sufficiently exergonic to enable the excited states of luminophores, including metal complexes, quantum dots and carbon nanocrystals, to be generated without photoexcitation. In particular, this review focuses on some of the most advanced and recent developments (especially during the last five years, 2010–2014) related to the use of these novel materials and their composites, with particular emphasis on their use in medical diagnostics as ECL immunosensors.     

Potassium Poly(Heptazine Imide): Transition Metal‐Free Solid‐State Triplet Sensitizer in Cascade Energy Transfer and [3+2]‐cycloadditions

Polymeric carbon nitride materials have been used in numerous light‐to‐energy conversion applications ranging from photocatalysis to optoelectronics. For a new application and modelling, we first refined the crystal structure of potassium poly(heptazine imide) (K‐PHI)—a benchmark carbon nitride material in photocatalysis—by means of X‐ray powder diffraction and transmission electron microscopy. Using the crystal structure of K‐PHI, periodic DFT calculations were performed to calculate the density‐of‐states (DOS) and localize intra band states (IBS). IBS were found to be responsible for the enhanced K‐PHI absorption in the near IR region, to serve as electron traps, and to be useful in energy transfer reactions. Once excited with visible light, carbon nitrides, in addition to the direct recombination, can also undergo singlet–triplet intersystem crossing. We utilized the K‐PHI centered triplet excited states to trigger a cascade of energy transfer reactions and, in turn, to sensitize, for example, singlet oxygen (¹O₂) as a starting point to synthesis up to 25 different N‐rich heterocycles.     

Binding and reactivity under restricted geometry conditions: Applicability of the Pseudophase Model to thermal and photochemical processes

Reactivity under restricted geometry conditions can be quantitatively rationalized by using the Pseudophase Model. This model considers two states in equilibrium, free and bound, which are not perturbed by the reaction. That is, the reaction in which the two states participate must be slow in comparison to the exchange process between the free and bound states. This condition is fulfilled in the case of chemical reactions, but it does not hold for fast photochemical reactions. In spite of this, the Pseudo-phase Model was found to be, at least apparently, applicable for excited state reactions. However, a thorough analysis of the kinetic data brings to light important differences between the two types of reactions, particularly in the meaning of the parameters present in the equations of the Pseudophase Model.     

Novel oxa-di-pi-methane and Norrish type I reactions in the S2 (pi,pi*) excited state of a series of beta,gamma-unsaturated ketones

[reaction: see text] Beta,gamma-unsaturated methyl ketones with electron-withdrawing groups at the gamma-position of the ene moiety undergo ODPM rearrangements and Norrish type I reactions on direct irradiation at 254 nm. The results are consistent with the involvement of alkene S(2) (pi,pi*) as reactive excited states in these processes.     

UV photostability of metal phthalocyanines in organic solvents

Kinetic studies of photochemical reactions induced by UV radiation in solutions of metal phthalocyanines have been carried out to determine the factors which might have influenced the stability of photosensitized phthalocyanines. Complexes of the molecular type Mpc, M'(2)pc, and Lnpc(2) (where M = Li, Mg, Fe, Co, Zn, Pb; M'= Tl; Ln = rare earth; pc = phthalocyanine ligand, C(32)H(16)N(8)(2-)) were investigated in DMF, DMSO, and pyridine. Progressive decay of the phthalocyanine macrocycle due to absorption of UV light was observed. Phthalimide found in the final photolysis product may indicate some chemically bonded oxygen involved in the solid phthalocyanine material. Fluorescence lifetimes determined for the studied compounds (2.91-5.98 ns) have shown no particular relation to the stability of the excited macrocyclic system. The bonding strength of the photosensitized phthalocyanine moiety appears to rely on typical chemical factors, rather than on the properties of the excited states. Kinetics of the degradation process has proved to depend on the molecular structure of the complex and seems to be controlled by interactions of the macrocycle bridging nitrogen atoms with the solvent molecules. The use of electron acceptor solvents such as DMSO may enhance the molecular stability of phthalocyanines excited by UV radiation. Sandwich-type rare earth diphthalocyanines dissolved in DMSO displayed the highest photostability.     

TiO2光催化反应机理及动力学研究进展

光催化处理环境污染物是基于催化反应过程中的一些自由基对污染物的氧化或还原作用,反应途径通常是HO·攻击或穴直接攻击,对可见光敏感的化合物也可能通过激发态来分解。动力学的表述多数符合L－H模式,广泛研究了L－H模式下的吸附与催化活性的关系,对动力学的研究也是了解其反应机理的重要途径。     

Electron photodetachment dissociation of DNA anions with covalently or noncovalently bound chromophores

Double stranded DNA multiply charged anions coupled to chromophores were subjected to UV-Vis photoactivation in a quadrupole ion trap mass spectrometer. The chromophores included noncovalently bound minor groove binders (activated in the near UV), noncovalently bound intercalators (activated with visible light), and covalently linked fluorophores and quenchers (activated at their maximum absorption wavelength). We found that the activation of only chromophores having long fluorescence lifetimes did result in efficient electron photodetachment from the DNA complexes. In the case of ethidium-dsDNA complex excited at 500 nm, photodetachment is a multiphoton process. The MS³ fragmentation of radicals produced by photodetachment at λ = 260 nm (DNA excitation) and by photodetachment at λ > 300 nm (chromophore excitation) were compared. The radicals keep no memory of the way they were produced. A weakly bound noncovalent ligand (m-amsacrine) allowed probing experimentally that a fraction of the electronic internal energy was converted into vibrational internal energy. This fragmentation channel was used to demonstrate that excitation of the quencher DABSYL resulted in internal conversion, unlike the fluorophore 6-FAM. Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Förster resonance energy transfer (FRET).     

Electronically nonadiabatic thermal reactions of organic molecules

This critical review seeks to bring together organic reactions in which thermal generation of electronic excited states plays an important role. The best known such reactions are those producing chemiluminescent products. However, it appears that there may exist at least as many nonadiabatic reactions in which the excited molecules react before they luminesce. An effort is made to understand the efficiency of excited state production. The crucial roles played by reactive intermediates are highlighted.     

Multicolor directional surface plasmon-coupled chemiluminescence

In reports over the past several years, we have demonstrated the efficient collection of optically excited fluorophore emission by its coupling to surface plasmons on thin metallic films, where the coupled luminescence was highly directional and polarized. This phenomenon is referred to as surface plasmon-coupled emission (SPCE). In this current study, we have extended this technique to include chemiluminescing species and subsequentially now report the observation of surface plasmon-coupled chemiluminescence (SPCC), where the luminescence from chemically induced electronic excited states couples to surface plasmons in thin continuous metal films. The SPCC is highly directional and predominantly p-polarized, strongly suggesting that the emission is from surface plasmons instead of the luminophores themselves. This indicates that surface plasmons can be directly excited from chemically induced electronic excited states and excludes the possibility that the plasmons are created by incident excitation light. This phenomenon has been observed for a variety of chemiluminescent species in the visible spectrum, ranging from blue to red, and also on a variety of metals, namely, aluminum, silver, and gold. Our findings suggest new chemiluminescence sensing strategies on the basis of localized, directional, and polarized chemiluminescence detection, especially given the wealth of assays that currently employ chemiluminescence-based detection.     

Rearrangements of Aromatic Nitrile Oxides and Nitrile Ylides: Potential Ring Expansion to Cycloheptatetraene Derivatives Mimicking Arylcarbenes

Nitrile imines, nitrile oxides and nitrile ylides are widely used in 1,3-dipolar cycloaddition reactions. They also undergo thermal and photochemical rearrangements to carbodiimides, isocyanates, and ketenimines, respectively. Calculations at DFT and CASPT2 levels of theory reveal novel, potential rearrangements, in which the aromatic 1,3-dipoles mimic phenylcarbene and undergo ring expansion to cycloheptatetraene derivatives. These rearrangements can potentially take place in both the singlet ground states and the triplet excited states, and they are accelerated by m,m’-bis(dimethylamino) substitution on the phenyl moieties. The new rearrangement becomes the energetically preferred path for m,m’-bis(dimethylamino)benzonitrile oxide in the triplet state. In the m,m’-bis(dimethylamino)benzo nitrile ylide, the cyclization to the 2-phenyl-1-azirine is favored over the ring expansion to a cycloheptatetraene by ca. 5 kcal mol⁻¹ in the singlet state. In the bent triplet states, 1,3-hydrogen shifts interconverting nitrile ylides are potentially possible.     

微反应器中的不对称光化学反应

化学反应的手性诱导一直备受化学家的关注,虽然不对称热化学合成和手性技术已经取得了巨大的进展,但不对称光化学反应的研究远远没有取得相应的成功.激发态寿命短、活化能低是导致其对映选择性低的主要原因.最新的研究表明,采用含手性空间或经手性修饰的微环境可以使光化学反应的立体选择性大大提高.本文针对这一热点问题,综述在微反应器中进行不对称光化学反应的研究进展.