Preparation, photophysics, and electrochemistry of segmented comonomers consisting of thiophene and pyrimidine units: new monomers for hybrid copolymers

An efficient coupling route to novel pi-conjugated comonomers consisting of pyrimidine, thiophene, and bithiophene units was developed. The novel pi-donor-acceptor-donor and pi-donor-acceptor-acceptor-donor conjugated compounds were prepared by Suzuki heterocoupling and Ni(0)-mediated Ullman homocoupling reactions. Photophysical investigation of these alternating pi-donor and acceptor compounds indicated that the deactivation of their singlet excited state proceeds predominately by fluorescence and results in high fluorescence quantum yields. Intersystem crossing to the triplet state was also present in ca. 10%. Quantification of the triplet manifold by laser flash photolysis further revealed that bithiophene produced its triplet state in only 31%. Cyclic voltammetry studies showed that the comonomers undergo both oxidation and reduction leading to their radical cations and radical anions, respectively. The radical cations are highly reactive and undergo anodic polymerization resulting in mutual p- and n-type dopable polymers. The extended conjugation resulting from polymer formation was confirmed by both absorbance and fluorescence spectroscopy and by GPC. Ruthenium binding with the conjugated homocoupled ligand was also found resulting in a hybrid alternating copolymer with significantly different spectroscopic and electrochemical properties relative to its metal-free counterpart.     

Photophysics and electrochemistry of conjugated oligothiophenes prepared by using azomethine connections

Novel conjugated azomethines consisting of 1 to 5 thiophenes and up to 4 azomethine bonds prepared from a stable diaminothiophene are presented. The effect of the number of thiophene and azomethines bonds on the photophysics and electrochemistry was examined. A high degree of conjugation was confirmed by bathochromic shifts upward of 120 and 210 nm for the absorbance and fluorescence, respectively, relative to the diaminothiophene precursor. Acid doping with methanesulfonic acid resulted in further bathochromic shifts along with lowering of the HOMO-LUMO energy gaps to 1.3 eV. Moreover, the compounds are extremely stable as evidenced by the absence of decomposition products under acid conditions. The resulting heteroatomic covalent bonds are furthermore reductively and hydrolytically resistant. Increasing the degree of conjugation shifts the nonradiative mode of singlet excited state energy dissipation from internal conversion (IC) to intersystem crossing (ISC). The resulting triplet manifold produced by ISC was efficiently deactivated by intramolecular self-quenching from the azomethine bond leading to a nonemissive triplet. Cyclic voltammetry revealed unprecedented reversible radical cation formation of the azomethines. Both one-electron oxidations and reductions were found by electrochemical measurements demonstrating the azomethines' capacity to be mutually p- and n-doped. One of the azomethines exhibited reversible electrochromic behavior with the electrochemically generated radical cation absorbing in the NIR at 1630 and 792 nm. X-ray crystallography confirmed the thermodynamically stable E isomer was formed uniquely and that the thiophenes are coplanar adopting an antiparallel arrangement.     

Étude de réactions photochimiques—XXVI: Réactions photochimiques de diènes transoïdes en série stéroïdes

A mechanism accounting for the products from excited transoid dienes is proposed. In the singlet excited state, these dienes are deactivated by an intramolecular path which competes with a bimolecular process. In the triplet excited state, dienes undergo a unique bimolecular process. The multiplicity of the excited state controls the “product” formation through the conformation of the excited dienes.     

Photochemistry of naphthalimide photoacid generators

The photophysical properties of a series of 1,8-naphthalimide photoacid generators were studied by steady state fluorescence and phosphorescence spectroscopy. Emission and excitation anisotropies, triplet quantum yields in polar and nonpolar solvent and photoacid generation were evaluated. The singlet excited state exhibits a low polarity and is strongly deactivated by an efficient intersystem crossing process. In protic solvent, a homolytic singlet cleavage of the N-O bond occurs and leads to the acid production. The existence of a triplet state close to the singlet state was clearly evidenced. The presence of close singlet excited states is supported by fluorescence anisotropy and picosecond laser spectroscopy experiments. Results of DFT calculations well confirm the experimental contentions and yield important information about the cleavage process involved in such compounds.     

DEACTIVATION OF EXCITED AZO DYE MOLECULES BY ENERGY TRANSFER

Abstract— Förster energy transfer from an excited azo dye monolayer to a luminescent cyanine dye monolayer was investigated. It is concluded that higher vibronic levels of the excited singlet state are efficiently deactivated by processes other than thermal equilibration.     

THE ENERGETICS OF ELECTRON-TRANSFER REACTIONS OF CHLOROPHYLL AND OTHER COMPOUNDS*

Abstract— Quite often the primary photochemical reaction of an excited state molecule is transfer of an electron to or from another molecule in its ground state. Rates of such reactions are closely dependent on differences between ground and excited state redox potentials of the reagents. The solvent also plays an important role in stabilizing ion pairs formed by the electron transfer. This Review discusses experimental data relating rates to electrochemical energy parameters in the context of a scheme which portrays the energy and electron transactions in a unified manner. Three consequences of reaction of a singlet excited state are distinguished: (S1) quenching without detectable products, (S2) exciplex fluorescence, (S3) transient radical ion production, and energetically necessary conditions are derived for each. Similarly, four kinds of reactions involving the triplet state are distinguished, which depend on the relation between the energy of the triplet state and that of the ion pair states: (TI) rapid quenching, (T2) slow quenching, (T3) accelerated intersystem crossing and (T4) generation by reaction between radical ions of like spin. The last may be followed by electrochemiluminescence. Classes of compounds for which data are available include chlorophylls, porphyrins and a few other molecules of biological interest, aromatic hydrocarbons and their derivatives, heterocyclic systems, carbonyl compounds, dyes, and complexes of Ru and U. A Table compiling median or selected values of ground and excited state electrochemical potentials of chlorophylls, some porphyrins, and a few other compounds is presented.     

抗氧化剂防止品红甲亚胺染料光褪色的作用机制

本文利用激光光解动态吸收光谱研究了氧与品红甲亚胺染料间的作用,发现染料的光褪色不仅与单重态氧有关,氧的自由基负离子也可能是引起褪色的一个重要因素.还研究了抗氧化剂对品红甲亚胺染料激发态的影响,证实前者对染料激发态有猝灭作用,据此对抗氧化剂防止品红甲亚胺染料光褪色机理作了进一步探讨.     

Zur elektrochemischen lumineszenz an halbreitern

The electrochemical luminescence of aromatic compounds in aprotic solvents was investigated using the following semiconductor electrodes: n-GaP, n-ZnO and n-SnO₂. Electrochemical luminescence by direct heterogeneous electron transfer to the excited singlet or triplet state and subsequent luminescence could in no case be detected. The discussion of the mechanism of the redox processes involved is based on experimental observations and energetic correlations which were partly obtained by measuring the capacity of the semiconductor—solvent systems used. We presume that the directly excited states are deactivated by radiationless transitions due to a heavy atom effect. This conclusion was confirmed to the extent that the naphthalin luminescence was quenched by these semiconductors at 77 K.     

Singlet energy migration along an alternating block copolymer of oligothiophene and oligosilylene in solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the F?rster theory, the energy migration process was attributed to an incoherent hopping mechanism.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

EXCITED SINGLET AND TRIPLET PRODUCTS GENERATED FROM OXIDATION OF THE -CO-CH- CONTAINING MOLECULES

Abstract— In a previous study. we found indirect chemiluminescence from air oxidation of various simple ketones in alkaline aprotic solvents containing 9,10-diphenylanthracene and 9,10-dibromoanthracene. By summarizing the experimental results. it was concluded that excited products. mainly in the triplet state. were generated from air oxidation of those simple compounds having a -CO-CH-group via dioxetane intermediates.
Recently. we have succeeded in observing an intense direct chemiluminescence due to the generation of the excited singlet product by the air oxidation of such anthracene derivatives having a -CO-CH-group as 9,10-diisobutyrylanthracene and related compounds. The finding lends strong support to the hypothesis that dioxetane or dioxetanone may be the critical intermediate to give fluorescent singlet products.     

Photodissociation Dynamics of Cyclopropenylidene,c‐C3H2

In this joint experimental and theoretical study we characterize the complete dynamical “life cycle” associated with the photoexcitation of the singlet carbene cyclopropenylidene to the lowest lying optically bright excited electronic state: from the initial creation of an excited‐state wavepacket to the ultimate fragmentation of the molecule on the vibrationally hot ground electronic state. Cyclopropenylidene is prepared in this work using an improved synthetic pathway for the preparation of the precursor quadricyclane, thereby greatly simplifying the assignment of the molecular origin of the measured photofragments. The excitation process and subsequent non‐adiabatic dynamics have been previously investigated employing time‐resolved photoelectron spectroscopy and are now complemented with high‐level ab initio trajectory simulations that elucidate the specific vibronic relaxation pathways. Lastly, the fragmentation channels accessed by the molecule following internal conversion are probed using velocity map imaging (VMI) so that the identity of the fragmentation products and their corresponding energy distributions can be definitively assigned.     

有机分子三重激发态的调控与应用

对近期有机分子三重激发态调控的研究进展进行了总结评述。控制分子的三重激发态性质,可以制备多种具有新颖性质的分子,如用于可激活光动力治疗(PDT)的光敏剂、磷光分子探针与生物标识试剂,以及可控的三重态湮灭上转换等。但目前对三重态控制方面的研究相对较少,其中的规律也很不明确。近期有文献陆续报道了使用超分子方法和共价修饰法进行的三重态调控,利用的光物理过程有单重态能量转移、三重态能量转移、电子转移等等。现有研究结果表明,三重态的调控规律与单重态的调控规律有所不同,例如:发色团的单重激发态(荧光)往往可以被光诱导电子转移(PET)所猝灭,但是在多个例子中已发现,相同发色团的三重态并不能被PET所猝灭。本文总结的研究结果及所作的分析,将对该领域的分子结构设计及后续研究起到一定的促进作用。     

Norrish Type I Cleavage of Aliphatic Carboxylic Acids and Esters in Solution. A CIDNP.-study

UV.-irradiation of pivalic and isobutyric acid and their methyl esters as well as of α-hydroxy isobutyric and malonic acid in solution lead to chemically induced dynamic nuclear polarization (CIDNP.) of parent compounds and of various reaction products. CIDNP.-effects and product distributions confirm α-cleavage of the C(α), CO bond to be the major mode of photodecomposition. Scavenger experiments indicate that decomposition of the molecules from a triplet excited state is roughly equally or more probable than from a singlet excited state. Quantum yields of educt disappearance are also given for the methyl esters of pivalic and isobutyric acid.     

Electronic absorption and fluorescence of cinchophen, cinchoninic acid and their methyl esters; biprotonic phototautomerism of the singly-protonated species

The pH and Hammett acidity dependences of the absorption and fluorescence spectra of cinchoninic acid (quinoline-4-carboxylic acid), cinchophen (2-phenylquinoline-4-carboxylic acid) and their methyl esters, were studied. The predominant uncharged ground-state species derived from the free acids are zwitterions. Prototropic equilibria are too slow to compete with fluorescence for deactivation of the excited state at hydrogen ion concentrations represented by the pH scale. However, fluorescence shifts accompanying protonation indicate that the carboxyl group is more basic than the ring nitrogen atom in the excited state. In the Hammett acidity range the singly-charged cations of all the compounds studied undergo phototautomerism in the lowest excited singlet state. The rate of this process is acidity dependent. In very concentrated sulphuric acid solutions doubly-charged cations are formed in the excited state but not in the ground state. The intense emissions of these compounds in moderately concentrated sulphuric acid may be suitable for quantitative analysis if great care is taken to control solution acidity.     

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.     

Triplet Excited States and Singlet Oxygen Production by Analogs of Red Wine Pyranoanthocyanins

During the maturation of red wines, the anthocyanins of grapes are transformed into pyranoanthocyanins, which possess a pyranoflavylium cation as their basic chromophore. Photophysical properties of the singlet and triplet excited states of a series of synthetic pyranoflavylium cations were determined at room temperature in acetonitrile solution acidified with 0.10 mol dm^?3 trifluoroacetic acid (TFA, to inhibit competitive excited state proton transfer) and at 77 K in a rigid TFA‐acidified isopropanol glass. In solution, the triplet states of these pyranoflavylium cations are efficiently quenched by molecular oxygen, resulting in sensitized formation of singlet oxygen, as confirmed by direct detection of the triplet‐state decay by laser flash photolysis and of singlet oxygen monomol emission in the near infrared. The strong visible light absorption, the relatively small singlet‐triplet energy differences, the excited state redox potentials and the reasonably long lifetimes of pyranoflavylium triplet states in the absence of molecular oxygen suggest that they might be useful as triplet sensitizers and/or as cationic redox initiators in polar aprotic solvents like acetonitrile.     

Transient absorption spectra and photochromic mechanism of schiff bases

The transient absorption spectra of salicylideneaniline (SA) and salicylidenebenzylamine (SBA) have been investigated by means of the nanosecond laser flash photolysis. It has been found that the intermediates from the two compounds have different properties. According to the properties of the intermediates for compound SA, the primary photoproduct is a zwitterion produced by the excited singlet state, rather than a trans- keto isomer. The intermediate of compound SBA also was found to be a zwitterion, but produced not only by the excited singlet state but also by the excited triplet state. The photochromic mechanisms of the two compounds are proposed and discussed respectively.     

Photoinduced Electron Transfer‐based Halogen‐free Photosensitizers: Covalent meso‐Aryl (Phenyl,Naphthyl, Anthryl,and Pyrenyl) as Electron Donors to Effectively Induce the Formation of the Excited Triplet State and Singlet Oxygen for BODIPY Compounds

Pristine BODIPY compounds have negligible efficiency to generate the excited triplet state and singlet oxygen. In this report, we show that attaching a good electron donor to the BODIPY core can lead to singlet oxygen formation with up to 58 % quantum efficiency. For this purpose, BODIPYs with meso ‐aryl groups (phenyl, naphthyl, anthryl, and pyrenyl) were synthesized and characterized. The fluorescence, excited triplet state, and singlet oxygen formation properties for these compounds were measured in various solvents by UV/Vis absorption, steady‐state and time‐resolved fluorescence methods, as well as laser flash photolysis technique. In particular, the presence of anthryl and pyrenyl showed substantial enhancement on the singlet oxygen formation ability of BODIPY with up to 58 % and 34 % quantum efficiency, respectively, owing to their stronger electron‐donating ability. Upon the increase in singlet oxygen formation, the fluorescence quantum yield and lifetime values of the aryl‐BODIPY showed a concomitant decrease. The increase in solvent polarity enhances the singlet oxygen generation but decreases the fluorescence quantum yield. The results are explained by the presence of intramolecular photoinduced electron transfer from the aryl moiety to BODIPY core. This method of promoting T₁ formation is very different from the traditional heavy atom effect by I, Br, or transition metal atoms. This type of novel photosensitizers may find important applications in organic oxygenation reactions and photodynamic therapy of tumors.     

Excited singlet state kinetics on a subnanosecond time-scale

It is possible to study directly the absorption time-profiles of short-lived excited singlet states by sampling the transient excitation spectra for fluorescence from upper states. This approach has many advantages over direct absorption measurements, since the effects of triplet state absorption can be suppressed and the population time-profiles of excited singlet states can be studied in detail. The first direct measurements are reported of singlet state absorption time profiles in a number of aromatic aza- and carbonyl compounds.