FREE ENERGY DEPENDENCE OF THE QUENCHING OF CHLOROPHYLL a FLUORESCENCE BY SUBSTITUTED QUINONES*

The quenching of chlorophyll a (Chl a) fluorescence hy a series of substituted benzoquinones. naphthoquinones and anthraquinones has been examined employing ethanol and acetonitrile as solvents. All quinones are good quenchers of fluorescence. There is an excellent linear relation between the Stern-Volmer quenching constants, K, and the polarographic half wave potentials (E₁₂) of the quinones, with more oxidizing quinones being better quenchers. The quenching data are consistent with the excited state half wave potential of ?1.31 eV predicted theoretically, demonstrating that the kinetically estimated value of the Chl a excited state reduction potential agrees with that expected on spectroscopic grounds. The results of quenching are not in agreement with the conventional Marcus theory of electron-transfer reactions, as there is no evidence of quenching constant. K_q. decrease vsΔG⁰ even for free energy changes nearly twice that expected for the onset of the Marcus inverted region. However, the kinetically estimated K_q values are in good agreement with the ones calculated by using the Rehm and Weller equation for fluorescence quenching by electron transfer. Our experimental results support the electron transfer mechanism of quenching proposed by Seely.     

Synthesis, characterization and theoretical analysis on a oxygen-sensing phosphorescent copper(I) complex

The fluorescence quenching of 9-aminoacridine by certain substituted uracils in water was studied using absorption, steady state and time resolved measurements. The bimolecular quenching rate constants (k_q), binding constant K and number of binding sites (n) were calculated based on the fluorescence quenching data. The free energy change (ΔG_et) for electron transfer process was calculated by Rehm–Weller equation. From lifetime measurement we observed that the quenching was mainly due to static mechanism involving ground state complex formation.     

Dipole moments of aryl cis - and trans -styryl sulphides and sulphones

The dipole moments of thirteen aryl styryl sulphides and eleven aryl styryl sulphones have been determined. Among them are eightcis-trans isomeric pairs, four sulphides and four sulphones. The styryl group is found to conjugate with the sulphide function as an electron-withdrawing group. The angles which the styrylthio and styrylsulphonyl groups make with their axes of rotation are calculated. The observed dipole moments of the sulphides and sulphones are compared with the moments calculated by the vector addition of group moments. The dipole moments ofcis aryl styryl sulphides and sulphones are found to be generally lower than those of theirtrans isomers. An explanation is offered attributing the cause to sterically enhanced styryl-sulphur conjugation in thecis isomers.     

Effects of electron tunneling and nonresonance quenching of photoluminescence in semiconducting CdSe/ZnS AND CdSe nanocrystals by porphyrin molecules in joint complexes

The quenching of photoluminescence (PL) in semiconducting CdSe/ZnS and CdSe nanocrystals (NC) of various sizes during surface passivation by molecules of tetrapyridylporphyrins (P) in toluene at 295 K was investigated. It was shown that resonance transfer of energy NC → P plays a minor role in PL quenching (<10%), while photoinduced electron transfer NC → P is absent. On the basis of experimental data and quantum-mechanical calculations it was established that with identical molar ratio x = C_P/C_NC the probability of quenching k _q decreases with increase in the size of the NC while the PL quenching process itself under conditions of quantum confinement is due to electron tunneling of the excited electron–hole pair on the surface of the NC followed by localization of the organic ligand (P) on anchor groups. The obtained results are of interest for investigating the mechanisms of the blinking of PL in single semiconductor nanocrystals. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 1, pp. 17–26, January–February, 2009.     

Electron Transfer Fluorescence Quenching of Blepharisma japonicum Photoreceptor Pigments

Abstract— The hypericin analogs blepharismin (BP), oxyblepharismin (OxyBP) and stentorin (ST), the photosensing chromophores responsible for photomotile reactions in the ciliates Blepharisma japonicum (red and blue cells) and Stentor coeruleus, represent a new class of photoreceptor pigments whose chemical structures have recently been determined. In the case of ST it has been shown that the first excited singlet state can be deactivated by donation of an electron to an appropriate acceptor molecule (e.g. a quinone molecule). This charge transfer can be considered a possible mechanism for the primary photoprocess for the photomotile responses in S. coeruleus. To determine whether an electron transfer process also occurs in the deactivation of excited blepharismin, we studied the fluorescence quenching of OxyBP in dimethyl-sulfoxide (DMSO) and in ethanol using electron acceptors with different reduction potentials. Under our experimental conditions ground state and excited state complexes (like fluorescent exciplexes) are not formed between the fluorophore and the quenchers. In DMSO the bimolecular quenching constant values (k_q) calculated on the basis of the best fitting procedures clearly show that the quenching efficiency decreases with the quencher negative reduction potential, E⁰. The k_q (M^-1 s^-1) and E⁰ (V) values are, respectively, 7.8 times 10⁹ and -0.134 for 1,4-benzoquinone, 8.9 times 10⁹ and -0.309 for 1,4-naphthoquinone, 2.4 times 10⁹ and -0.8 for nitrobenzene, 0.009 times 10⁹ and -1.022 for azobenzene and 0 and -1.448 for benzophenone. These findings point to the conclusion that upon formation of the encounter complex between OxyBP and the quencher, an electron is released from excited OxyBP to the quencher, similar to what happens in ST. It is suggested that in the pigment granules such a light-induced charge transfer from excited blepharismin to a suitable electron acceptor triggers sensory transduction processes in B. japonicum.     

Photoinduced intramolecular electron transfer of fluorescein and violgen, carbazole

A series of polyads consisting of covalently-(CH2)4-linked fluorescein with carbazole and violger.Live been synthesized and characterized The studies of absorption,emission spectra and fluorescence lifetime quenching indicated that the intramolecular fluorescence quenching of fluorescein by violgen is mainly a static process through the formation of non emission complex (fluorescence quenching efficiency φQ=0.97,lifetime quenching efficiency φH 0,quenching efficiency of formation of non-emission complex φC=0.97); while the quenching by carbazole is mainly a dynamic electron transfer process (φQ=0.63,φET=0.63,φC=0).In the violgen-fluorescein-carbazole triads,φQ=0.97,Q ET=0.65,φC=0.32,which suggests that the photoinduced interaction of fluorescein-carbazole pair and that of violgen-fluorescein pair are in a competitive process,the dynamic electron transfer from carbazole to fluorecein is dominant in the process The free energy change of the photoinduced electron transfer and the back reac-tiorns i     

Fluorescence Quenching of Tris(2,2′-bipyridine)Ruthenium(II) Dichloride by Certain Organic Dyes

Fluorescence quenching of [Ru(bpy)₃]²⁺ by a series of organic dyes has been investigated by using the steady state fluorescence technique in aqueous medium. The dyes used are anthraquinone dyes: uniblue, acid blue 129, alizarin, alizarin red S and the azo dyes: congo red, sunset yellow, methyl orange, tartrazine, acid orange 63, methyl red and erichrome black T. The quenching of [Ru(bpy)₃]²⁺ was found to obey the Stern-Volmer equation and the corresponding Stern-Volmer plots were linear indicating dynamic quenching. The quenching rate constants (k _q) were calculated from the fluorescence data. The mechanism of quenching was discussed on the basis of the quenching rate constants as well as the reduction potential of dyes. The electron transfer mechanism has been proved by the calculation of Gibbs energy changes (ΔG _et) by applying the Rehm-Weller equation.     

光致噻吨酮与胺类、酚类、醇类的电子转移和氢转移反应

利用激光闪光光解方法研究了一系列胺类、酚类、醇类在脱氧乙腈中猝灭噻吨酮(TX)三重态的反应,得到了相应的瞬态吸收光谱和猝灭速率常数(kq).通过对光谱演变特性的分析,推断出三重态噻吨酮与不含有活泼氢的胺发生了电子转移反应,与含有活泼氢的胺发生了电子-质子转移反应.三重态噻吨酮与酚类、醇类反应中观察到噻吨酮加氢自由基的生成,据此推断出三重态噻吨酮与酚类、醇类发生了氢转移反应.胺类的猝灭速率常数随着反应自由能变(ΔG)的增大而减小,说明电子转移影响了噻吨酮三重态的猝灭.酚类的猝灭速率常数先随ΔG增大而减小,后随酚阳离子的酸性增强逐渐增大,可能是猝灭过程中电子转移影响减弱的同时氢转移影响逐渐增强.醇类的猝灭速率常数随着醇的α-C—H键能的增大而减小,说明α-C—H键能是影响噻吨酮三重态猝灭的关键因素.比较以前研究的胺类、酚类、醇类与三重态呫吨酮(XT)、芴酮(FL)反应的结果可知,由于分子结构差异性的影响,相关的猝灭速率常数按照呫吨酮、噻吨酮、芴酮的顺序逐渐减小.     

Systematic Modulation of the Fluorescence Brightness in Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC)–Gold–Thiolates

Five different highly fluorescent boron‐dipyrromethene (BODIPY)‐tagged N‐heterocyclic carbene NHC–gold halide complexes were synthesized. The substitution of the halogeno ligand by 4‐substituted aryl thiolates leads to a decrease in the brightness of the complexes. This decrease depends on the electronic nature of the thiols, being most pronounced with highly electron‐rich thiols (4‐R=NMe₂). The brightness of the gold thiolates also depends on the distance between the sulfur atom and the BODIPY moiety. The systematic variation of the electron density of [(NHC–bodipy)Au(SC₆H₄R)] (via different R groups) enables the systematic variation of the fluorescence brightness of an appended BODIPY fluorophore. Based on this and supported by DFT calculations, a photoinduced electron‐transfer quenching appears to be the dominant mechanism controlling the brightness of the appended BODIPY dye.     

Accessing the Triplet State in Heavy‐Atom‐Free Perylene Diimides

Previous studies of perylenediimides (PDIs) mostly utilized the lowest singlet excited state S₁. Generation of a triplet excited state (T₁) in PDIs is important for applications ranging from photodynamic therapy to photovoltaics; however, it remains a formidable task. Herein, we developed a heavy‐atom‐free strategy to prompt the T₁←S₁ intersystem crossing (ISC) by introducing electron‐donating aryl (Ar) groups at the head positions of an electron‐deficient perylenediimide (PDI) core. We found that the ISC efficiency increases from 8 to 54 % and then to 86 % by increasing the electron‐donating ability of head‐substituted aryl groups from phenyl (p‐PDI) to methoxyphenyl (MeO‐PDI) and then to methylthioxyphenyl (MeS‐PDI). By enhancing the intramolecular charge‐transfer (ICT) interaction from p‐PDI to MeO‐PDI, and then to MeS‐PDI, singlet oxygen generation via energy‐transfer reactions from T₁ of PDIs to ³O₂ was demonstrated with the highest yield of up to 80 %. These results provide guidelines for developing new triplet‐generating PDIs and related rylene diimides for optoelectronic applications.