Femtosecond ground state recovery: measuring the intersystem crossing yield of polyspirobifluorene

Measurement of the quantum yield of triplet formation has been made for the prototypical conjugated polymer polyspirobifluorene in solution and solid state. An updated method has been described based on femtosecond time resolved ground state recovery following photoexcitation of the polymer. The two components to the recovery of the ground state due to the decay of the singlet and triplet excited states are clearly visible and from these it is possible to calculate Phi(T)=0.05+/-0.01 in solution, this gives k(isc)=5.4 x 10(7) s(-1) which compares favorably with other conjugated polymers. In polymer films an increased triplet yield of Phi(T)=0.12+/-0.02 is found to be independent of temperature, the increased yield is attributed to triplet recombination from charged states.     

Enhancement of the large stokes-shifted fluorescence emission from the 2-(2'-hydroxyphenyl)benzoxazole core in a dendrimer

The photochemical properties of a series of newly synthesized dendrimers, 4-6, having a 2-(2'-hydroxyphenyl)benzoxazole (HBO) core, were studied in benzene. The fluorescence quantum yields (Phi(f)) were determined to be 0.022, 0.030, and 0.038 for 4, 5, and 6, respectively, increasing in higher generation dendrimers. With transient absorption spectroscopy, the quantum yields of the isomerization from the (E)-keto form ((1)K(E)*) to the (Z)-keto form ((1)K(Z)) (Phi(E)(-->)(Z)) and those of intersystem crossing (Phi(isc)) can be estimated. Whereas Phi(E)(-->)(Z) values decreased in higher generation dendrimers, Phi(isc) values were almost the same among 4-6. The quantum yields of nonradiative decay (Phi(nr)) increased in higher generation dendrimers. The dendrimer structure also affected the reverse tautomerization process.     

Gemini型表面活性剂疏水链中肉桂酸基团的光致二聚反应

采用紫外光谱、 红外光谱和质谱研究了Gemini型表面活性剂N,N'-二对丁氧基肉桂酰胺基胱氨酸钠(SDBCC)溶液的光化学反应过程和产物, 分析了SDBCC浓度对分子疏水链中肉桂酸基团光化学反应产物分布和光致二聚产率的影响. 结果表明, 在不同浓度下, SDBCC分子疏水链中的肉桂酸基团的光化学反应过程均以光致二聚反应为主, 且为分子内光致二聚产物. 但聚集体形成前后在光照过程中肉桂酸基团的光致二聚反应产率随SDBCC浓度的变化趋势不同, 聚集体形成前光致二聚反应产率随浓度的增加而增大, 聚集体形成后光致二聚反应产率随浓度的增大而减小. 综合光化学反应过程、 拓扑条件和聚集体结构进一步分析了胶束和囊泡的存在对SDBCC分子疏水链中肉桂酸基团光致二聚反应的影响.     

Oxygen uptake and involvement of superoxide radicals upon photolysis of ketones in air-saturated aqueous alcohol, formate, amine or ascorbic acid solutions

The photolysis of acetophenone, benzophenone, 4-carboxybenzophenone and benzil was studied in air-saturated aqueous solution in the presence of alcohols. The overall reaction is an oxidation of 2-propanol to acetone. The quantum yield of oxygen uptake (Phi(-O(2))) increases with increasing 2-propanol concentration up to 0.9. The photoreaction can also be initiated by quenching of the ketone triplet state by ascorbic acid, formate or an amine e.g. triethylamine. Subsequent reactions of the involved radicals with oxygen yield the superoxide radical and eventually hydrogen peroxide. For the ketones in the presence of 3-30 mM ascorbic acid or triethylamine Phi(-O(2)) = 0.3-0.9. The specific properties of ketones, including 4-methoxyacetophenone and 2-acetonaphthone, the radicals involved and the pH and concentration dependences of Phi(-O(2)) are discussed.     

反-4-(4-R-苯乙烯基)吡啶光二聚反应的研究

通过反-4-(4-R-苯乙烯基)吡啶[R=H(Ⅰa),Me(Ⅰb),OMe(Ⅰc),OH(Ⅰd),Me2N(Ⅰe)］在稀盐酸中的光二聚反应合成了4个r-1,c-2,t-3,t-4-1,3-双(4-R-苯基)-2,4-二(4-吡啶基)环丁烷[R=H(Ⅱa),Me(Ⅱb),OMe(Ⅱc),OH(Ⅱd)］,除Ⅱa外,其余为新化合物.光二聚反应具有高度立体选择性且几乎是定量完成的.研究发现,随着取代基供电子能力的提高,光二聚反应速率下降.反应的高度立体选择性以及该反应不受空气中氧气影响的事实表明光二聚反应是按激发单重态历程进行的.研究还发现Ⅰa～Ⅰd在有机溶剂中主要发生反-顺异构化反应,随着溶剂极性增加,反-顺异构化速度加快,表明其反-顺异构化反应亦经激发单重态历程.本文检测到了Ⅰa在稀盐酸溶液中的激基缔合物荧光.     

Photochemical elimination of leaving groups from zwitterionic intermediates generated via electrocyclic ring closure of alpha,beta-unsaturated anilides

Methacrylanilides, ArN(CH3)COC(CH2LG)=CH2, with allylic leaving groups (LG(-) = BocAla, PhCO2(-), PhCH2CO2(-), PhO(-)) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an alpha-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG(-) = PhO(-), although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450-700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and Phi(tot)(para-CO2CH3) = 0.04-0.06 at 310 nm and Phi(tot)(para-COPh) = 0.08-0.1 at 365 nm, for which Phi(isc) = 0.15.     

Self-assembled coordination cage as a reaction vessel: triplet sensitized [2+2] photodimerization of acenaphthylene, and [4+4] photodimerization of 9-anthraldehyde

Inner cavity of Pd-nanocage has been used as a reaction vessel for performing triplet sensitized [2+2] photodimerization of acenaphthylene using water soluble xanthene dyes (Eosin Y and Rose Bengal) as sensitizers, and [4+4] photodimerization of 9-anthraldehyde. Although the [4+4] photodimerization of 9-anthraldehyde gave similar results to solution reaction, the xanthene dye sensitized [2+2] triplet state photodimerization of acenaphthylene encapsulated within Pd-nanocage yielded the syn dimer in quantitative yield. The results obtained from the triplet state [2+2] photodimerization of acenaphthylene within Pd-nanocage is remarkable given the fact that the photodimerization reaction when performed in methanol in the presence of Eosin Y and Rose Bengal gave the syn and anti dimers in the ratio 0.5 and 0.6, respectively. Preaggregation of molecules encapsulated inside Pd-nanocage in a syn fashion seems to be the governing factor for such a behavior.     

Electron transfer from aromatic amino acids to triplet quinones

The photoreduction of 1,4-benzoquinone, 1,4-naphthoquinone, 9,10-anthraquinone (AQ) and several methylated or halogenated derivatives in argon-saturated acetonitrile-water mixtures by indole, N-acetyltryptophan and N-acetyltyrosine was studied by time-resolved UV-vis spectroscopy using 20 ns UV laser pulses. The quinone triplet state is quenched by the aromatic amino acids and the rate constants are (1-5)x10(9)M(-1)s(-1). The semiquinone radical anion Q.(-) is the major observable transient after electron transfer from amino acids to the quinone triplet state. Termination of Q.(-) and amino acid derived radicals takes place in the mus-ms range. The effects of structure and other specific properties of quinones and amino acids are discussed. The radicals are subjects of intercept with oxygen, whereby hydrogen peroxide is eventually formed. The quantum yield of oxygen uptake Phi(-O2) as a measure of formation of hydrogen peroxide increases with increasing amino acid concentration, approaching Phi(-O2) for AQ in air-saturated solution.     

Structural effects on the solid-state photodimerization of 5-chloro and 5-nitro-2(1<Emphasis Type="Italic">H</Emphasis>)-pyridone in molecular compounds

Abstract  

The preparation and crystal structure of six molecular compounds consist of light-stable host molecules [(1-cyclohexyl-4-hydroxybenzene, 1,1,6,6-tetraphenyl-2,4-hexadiyne-1,6-diol, 4-((10-[4-(ethoxycarbonyl)phenoxy]decyl)oxy)benzene, 1,2,4,5-benzenetetracarboxylic acid, resorcinol] and light-sensitive guest molecules [(5-chloro-2(1H)-pyridone or 5-nitro-2(1H)-pyridone] are described in light of their failure to undergo [4 + 4] photodimerization in the solid-state upon irradiation with UV light. Unlike in many cases were pyridone and its derivatives undergo such dimerization in the solid-state due to the packing arrangement in the crystalline form in which the geometric requirement for photodimerization meet, the compounds described here failed to pack in ways to enable such photodimerization. The distances between the potentially reactive atoms ranged from 4.620 to 8.408 ? which are too long to react.     

Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 6. Chemical and redox properties and highly effective photosensitizing activity for singlet oxygen production of penta- and monopalladated complexes in dimethylformamide solution

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.     

Photoprocesses of molecules with 2-nitrobenzyl protecting groups and caged organic acids

The 308 nm photoinduced formation of the nitroso product and the intermediacy of the aci-nitro form(s) were studied for a series of 2-nitrobenzyl alkyl and aryl esters (1a-4e) and bis-(nitrophenyl)methyl acetates (5a-6b) by time-resolved UV-vis spectroscopy. A triplet state appears as major transient, when 2-nitrobenzyl derivatives 1 are substituted by 4,5-dimethoxy (2) and 4,5-methylenedioxy (3/4) groups. This triplet of charge transfer character is, however, not part of the route via the aci-nitro into the 2-nitroso form. The activation energy and preexponential factor of the longest lifetime component (tau(aci)), i.e. the major part of the aci-nitro decay, were determined. The carboxylic acids as leaving groups have rather small effects on tau(aci). An additional nitrated phenyl ring in alpha-position (5) leads generally to shorter tau(aci) value. Otherwise, the photogeneration of nitroso products is similar. The quantum yield (Phi(d)) varies only moderately with structure, the yield of the aci-nitro form and Phi(d) are correlated and little affected by solvent properties.     

Effects of localized triplet exciton on reactivity of photoinduced omega-bond dissociation in naphthyl phenyl ketones having pi,pi* lowest triplet (T1) states studied by laser flash photolysis

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.     

Anion-directed formation and degradation of an interlocked metallohelicate

Although there are many examples of catenanes, those of more complex mechanically interlocked molecular architectures are rare. Additionally, little attention has been paid to the degradation of such interlocked systems into their starting complexes, although formation and degradation are complementary phenomena and are equally important. Interlocked metallohelicate, [(Pd(2)L(4))(2)](8+) (2(8+)), is a quadruply interlocked molecular architecture consisting of two mechanically interlocked monomers, [Pd(2)L(4)](4+) (1(4+)). 2(8+) has three internal cavities, each of which encapsulates one NO(3)(-) ion (1:3 host-guest complex, 2?(NO(3)|NO(3)|NO(3))(5+)) and is characterized by unusual thermodynamic stability. However, both the driving force for the dimerization and the origin of the thermodynamic stability remain unclear. To clarify these issues, BF(4)(-), PF(6)(-), and OTf(-) have been used to demonstrate that the dimerization is driven by the anion template effect. Interestingly, the stability of 2(8+) strongly depends on the encapsulated anions (2?(NO(3)|NO(3)|NO(3))(5+) ? 2?(BF(4)|BF(4)|BF(4))(5+)). The origins of this differing thermodynamic stability have been shown through detailed investigations to be due to the differences in the stabilization of the interlocked structure by the host-guest interaction and the size of the anion. We have found that 2-naphthalenesulfonate (ONs(-)) induces the monomerization of 2?(NO(3)|NO(3)|NO(3))(5+) via intermediate 2?(ONs|NO(3)|ONs)(5+), which is formed by anion exchange. On the basis of this finding, and using p-toluenesulfonate (OTs(-)), the physical separation of 2?(NO(3)|NO(3)|NO(3))(5+) and 1(4+) as OTs(-) salt was accomplished.     

Design, synthesis, and photophysical characterization of water-soluble chlorins

The use of chlorins as photosensitizers or fluorophores in a range of biological applications requires facile provisions for imparting high water solubility. Two free base chlorins have been prepared wherein each chlorin bears a geminal dimethyl group in the reduced ring and a water-solubilizing unit at the chlorin 10-position. In one design (FbC1-PO3H2), the water-solubilizing unit is a 1,5-diphosphonopent-3-yl ("swallowtail") unit, which has previously been used to good effect with porphyrins. In the other design (FbC2-PO3H2), the water-solubilizing unit is a 2,6-bis(phosphonomethoxy)phenyl unit. Two complementary routes were developed for preparing FbC2-PO3H2 that entail introduction of the protected phosphonate moieties either in the Eastern-half precursor to the chlorin or by derivatization of an intact chlorin. Water-solubilization is achieved in the last step of each synthesis upon removal of the phosphonate protecting groups. The chlorins FbC1-PO3H2 and FbC2-PO3H2 are highly water-soluble (>10 mM) as shown by 1H NMR spectroscopy (D2O) and UV-vis absorption spectroscopy. The photophysical properties of the water-soluble chlorins in phosphate-buffered saline solution (pH 7.4) at room temperature were investigated using static and time-resolved absorption and fluorescence spectroscopic techniques. Each chlorin exhibits dominant absorption bands in the blue and the red region (lambda = 398, 626 nm), a modest fluorescence yield (Phi f approximately 0.11), a long singlet excited-state lifetime (tau = 7.5 ns), and a high yield of intersystem crossing to give the triplet state (Phi isc = 0.9). The properties of the water-soluble chlorins in aqueous media are comparable to those of hydrophobic chlorins in toluene. The high aqueous solubility combined with the attractive photophysical properties make these compounds suitable for a wide range of biomedical applications.     

Cuprophilic interactions in luminescent copper(I) clusters with bridging bis(dicyclohexylphosphino)methane and iodide ligands: spectroscopic and structural investigations

Polynuclear copper(I) complexes with bridging bis(dicyclohexylphosphino)methane (dcpm) and iodide ligands, [Cu(2)(dcpm)(2)(CH(3)CN)(2)](BF(4))(2) (1), [Cu(2)(dcpm)(2)](BF(4))(2) (2), [(CuI)(3)(dcpm)(2)] (3), [(CuI)(4)(dcpm)(2)] (4), and [(CuI)(2)(dcpm)(2)] (5) were prepared and their structures determined by X-ray crystal analysis. The shortest Cu--Cu distance found in these complexes is 2.475(1) A for 3. Powdered samples of 1, 3, 4, and 5 display intense and long-lived phosphorescence with lambda(max) at 460, 626, 590, and 456 nm and emission quantum yields of 0.26, 0.11, 0.12, and 0.56 at room temperature, respectively. In the solid state, 2 displays both a weak emission at 377 and an intense one at 474 nm with an overall emission yield 0.42. The difference in emission properties among complexes 1-5 suggests that both Cu--Cu interaction and coordination around the copper(I) center affect the excited state properties. A degassed solution of 2 in acetone gives a bright red emission with lambda(max) at 625 nm at room temperature. The difference absorption spectra of the triplet excited states of 1-5 in acetonitrile show broad absorption peaks at 340-410 and 850-870 nm.     

Photophysical properties of glucoconjugated chlorins and porphyrins and their associations with cyclodextrins

Glucoconjugated analogues of the meta-hydroxyphenyl porphyrin (m-THPP) and meta-hydroxyphenyl chlorin (m-THPC) has been recently synthesized. The characteristics of their triplet states have been determined with regard to their involvement in the photodynamic (PDT) efficiency. In the case of porphyrin derivatives, triplet quantum yields (Phi(T)) were ranging from 0.42 to 0.55 and triplet life times (tau(T)) from 1 to 5 micros. High reaction rate constants (k(q)) with molecular oxygen (k(q): 1.2-1.6 x 10(9)s(-1)) have been found. The triplet lifetimes of chlorin derivatives were about four times higher than those of porphyrins whereas the Phi(T) and k(q) values remained quite similar. Singlet oxygen yields of glucosylated and non-glucosylated porphyrins and chlorins were not significantly different within experimental errors (Phi(Delta)((1)O(2)): 0.41-0.58). Furthermore, it has been shown that glucoconjugated photosensitizers could undergo associations with the methyl-beta-cyclodextrin (Me-beta-CD) which exhibit high triplet lifetimes and singlet oxygen yields ranging from 0.27 to 0.48.     

Functional monomers and polymers. LXI. Photochemical reactions on the synthetic polymers containing thymine bases

Photodimerization of N-2-isobutyloxyethyl thymine (T-M), bis[2-(5-methyl-1-primidinyl)-ethyl] glutarate (T-T), poly(N-2-methacryloyloxyethyl thymine) (polyMAOT), and poly(N-2-acryloyloxyethyl thymine) (poly AOT) was studied in dimethylformamide solution. The rate of intramolecular photodimerization of polyMAOT and polyAOT was 5.7 and 7.2 times faster, respectively, than that of T-T. In T-M, however, intermolecular photodimerization did not occur under the reaction condition used. Quenching studies with isoprene suggest that the photodimerization of T-T occurs predominantly in an excited triplet state, whereas that of polyMAOT and polyAOT occur in singlet and triplet states. It was concluded that the photodimers of T-T, polyMAOT, and polyAOT are two syn-fused cyclobutane-type dimers (cis-syn and trans-syn). Quantum efficiencies of thymine base disappearance were determined by ultraviolet (UV) analysis to be 0.0012 for T-T, 0.0084 for polyMAOT, and 0.010 for polyAOT.     

Competition between singlet state photoreduction and triplet state photodimerization of dimethyl 3-dehydrogibberellenate

Photolysis of dilute solutions (10^-4M) of dimethyl 3-dehydrogibberellenate 1, in MeOH, EtOH, i-PrOH, t-BuOH and 2, 2, 2-trifluoroethanol, showed that while the last two solutions underwent photodimerization reactions only; the other solutions gave photoreduction products 2 and 3, besides some photodimerization product. It is further shown that while photodimerization proceeded through triplet excited state, photo-reduction, surprisingly, proceeded only through singlet excited state.     

MOLECULAR STRUCTURE AND PHOTODECARBONYLATION OF 2,3,4,5- TETRAPHENYLCYCLOPENTANONE

Clemmensen reduction of tetracyclone yields r-2,t-3,t-4,c-5-tetraphenylcyclopentanone, the structure of which was established by x-ray crystallography. Photodecarbonylation of this molecule results in the formation of cis- and trans-stilbenes and minor amounts of tetraphenykyclobutane. The stilbenes are initially formed in comparable yield, but secondary photoisomerization results in cw-rich mixtures. Stepwise photodecarbonylation is proposed to yield triplet meso- 1,2,3,4-tetraphenylbutane-diyl which undergoes nonstereoselective elimination in preference to cyclization. The intermediacyof the same biradical in the photodimerization stilbene can be effectively eliminated on the basis of stereochemical evidence.     

PHOTOCHEMISTRY OF URACIL. INTERSYSTEM CROSSING AND DIMERIZATION IN AQUEOUS SOLUTION

Abstract— Ultraviolet irradiation of ¹⁴C-uracil in aqueous solution results in the formation of hydrate and dimer photoproducts. The rate of dimerization increases with increasing uracil concentration, and decreases with increasing concentration of oxygen in solution. The kinetics are in agreement with a model previously proposed to account for the reactions, in which dimerization occurs by a reaction involving the triplet state of uracil, but hydration occurs from an excited singlet state. Oxygen reduces dimer formation by quenching the triplet. The quantum yield for intersystem crossing (ISC) to the triplet depends on the irradiation wavelength, increasing from 0.0014 at 280 nm to 0.016 at 230 nm. The ratio of rate constants for reaction of the triplet with oxygen and for dimerization is 1.1; the ratio of rate constants for triplet decay and for dimerization is 5.9 × 10^-5 M. The increase in ISC with photon energy suggests that ISC is favoured from excited vibrational levels. The quantum yield for hydration is about 0.002 at pH 4.5 for all wavelengths, but increases as the pH is decreased.