Light-harvesting fullerene dyads as organic triplet photosensitizers for triplet-triplet annihilation upconversions

Visible light-harvesting C(60)-bodipy dyads were devised as universal organic triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion. The antennas in the dyad were used to harvest the excitation energy, and then the singlet excited state of C(60) will be populated via the intramolecular energy transfer from the antenna to C(60) unit. In turn with the intrinsic intersystem crossing (ISC) of the C(60), the triplet excited state of the C(60) will be produced. Thus, without any heavy atoms, the triplet excited states of organic dyads are populated upon photoexcitation. Different from C(60), the dyads show strong absorption of visible light at 515 nm (C-1, ε = 70400 M(-1) cm(-1)) or 590 nm (C-2, ε = 82500 M(-1) cm(-1)). Efficient intramolecular energy transfer from the bodipy moieties to C(60) unit and localization of the triplet excited state on C(60) were confirmed by steady-state and time-resolved spectroscopy as well as DFT calculations. The dyads were used as triplet photosensitizers for TTA upconversion, and an upconversion quantum yield up to 7.0% was observed. We propose that C(60)-organic chromophore dyads can be used as a general molecular structural motif for organic triplet photosensitizers, which can be used for photocatalysis, photodynamic therapy, and TTA upconversions.     

Rhenium(I) tricarbonyl polypyridine complexes showing strong absorption of visible light and long-lived triplet excited states as a triplet photosensitizer for triplet-triplet annihilation upconversion

The preparation of rhenium(I) tricarbonyl polypyridine complexes that show a strong absorption of visible light and long-lived triplet excited state and the application of these complexes as triplet photosensitizers for triplet-triplet annihilation (TTA) based upconversion are reported. Imidazole-fused phenanthroline was used as the N^N coordination ligand, on which different aryl groups were attached (Phenyl, Re-0; Coumarin, Re-1 and naphthyl, Re-2). Re-1 shows strong absorption of visible light (ε = 60,800 M(-1) cm(-1) at 473 nm). Both Re-1 and Re-2 show long-lived T(1) states (lifetime, τ(T), is up to 86.0 μs and 64.0 μs, respectively). These properties are in contrast to the weak absorption of visible light and short-lived triplet excited states of the normal rhenium(I) tricarbonyl polypyridine complexes, such as Re-0 (ε = 5100 M(-1) cm(-1) at 439 nm, τ(T) = 2.2 μs). The photophysical properties of the complexes were fully studied with steady state and time-resolved absorption and emission spectroscopes, as well as DFT calculations. The intra-ligand triplet excited state is proposed to be responsible for the exceptionally long-lived T(1) states of Re-1 and Re-2. The Re(I) complexes were used as triplet photosensitizers for TTA based upconversion and an upconversion quantum yield up to 17.0% was observed.     

Improvement of (bipy)Pt(XR)2 (X=O, S) type photosensitizers by covalent dye attachment

Irradiation into the dye-based absorption band of complexes ((t)Bu(2)bipy)Pt(SR)(2) and ((t)Bu(2)bipy)Pt(OR)(2) where R denotes a coumarine-based thiolate and alkoxolate substituent populates the same excited triplet state as is obtained by excitation into the much weaker (RX)(2)Pt→(t)Bu(2)bipy (X = O, S) charge-transfer band. This paves the way toward more efficient photosensitizers.     

Accessing the long-lived triplet excited states in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and applications as organic triplet photosensitizers for photooxidations

Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7-10). The compounds show red-shifted UV-vis absorption (530-580 nm; ε up to 50000 M(-1) cm(-1)) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (Φ(F) up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and Φ(F) as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7-9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7-10 upon photoexcitation; the lifetimes of the triplet excited states (τ(T)) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional [Ir(ppy)(2)(phen)][PF(6)] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds, and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.     

Stereodifferentiation in the decay of triplets and biradicals involved in intramolecular hydrogen transfer from phenols or indoles to pi,pi aromatic ketones

Laser flash photolysis studies on (R,S) and (S,S) diastereoisomers of the bichromophoric compounds 1-6 have been used to investigate the possible chiral discrimination in the quenching of triplet excited ketones, resulting in formal hydrogen abstraction. Deuterium isotopic effects show that triplet deactivation in these bichromophores is dominated by hydrogen atom transfer. A remarkable stereodifferentiation is found in the intramolecular quenching of the ketone triplets of 1-3 and 5 by the phenolic or indolic moieties, either in methanol or acetonitrile as solvents. This indicates the existence of specific structural requirements for hydrogen transfer. On the other hand, the lifetimes of the generated biradicals show large solvent dependence; solvation appears to slow their reversion to the starting ketone. The considerable stereodifferentiation observed for the biradical lifetimes suggests that the kinetics of biradical decay is faster when the approach of the two radical termini becomes easier.     

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.     

具有强可见光吸收的富勒烯-Bodipy衍生物作为光催化剂在硫醚光催化氧化中的应用

以氟硼吡咯染料（Bodipy）做为光吸收天线,富勒烯作为分子内自旋转换单元,分别利用前者的强吸光以及后者的高效系间窜越的优点,制备了吸收波长灵活可调、无重原子、具有强可见光吸收能力、长寿命三重激发态（92.1 μs）的二元（Dyad）、三元化合物（Triad）做为有机三重态光敏剂;其中Triad具有宽谱带可见光吸收能力,提高了光敏剂参与的分子间能量转移或电子转移过程的效率。与传统的Ru（Ⅱ）配合物等三重态光敏剂相比,光催化硫醚氧化的反应时间大大缩短。通过吉布斯自由能的计算、活性氧物种的捕获实验、电子顺磁共振等方法,证明了在光催化氧化硫醚的反应过程中,同时存在超氧负离子自由基和单线态氧两个活性物种,从而加快了光催化氧化反应速率。该研究结果将对新型有机三重态光敏剂的分子结构设计以及在光催化有机合成反应中的应用起到一定的促进作用。     

Room-temperature long-lived triplet excited states of naphthalenediimides and their applications as organic triplet photosensitizers for photooxidation and triplet-triplet annihilation upconversions

Naphthalenediimide (NDI) derivatives with 2,6- or 2,3,6,7-tetrabromo or amino substituents were prepared. N,N'-dialkyl-2,6-dibromo NDI (compound 2) and N,N'-dialkyl-2,3,6,7-tetrabromo NDI (compound 4) show phosphorescence emission at 610 or 667 nm, respectively. Phosphorescence was never observed for NDI derivatives. Conversely, N,N'-dialkyl-2,6-dibromo-3,7-diamino NDI (compound 5) shows strong absorption at 526 nm and fluorescence at 551 nm, and no phosphorescence was observed. However, nanosecond time-resolved transient difference absorption spectroscopy confirmed that the triplet excited state of 5 was populated upon photoexcitation. 2,3,6,7-Tetraamino NDI (6) shows fluorescence, and no triplet excited state was populated upon excitation. The compounds were used as singlet oxygen ((1)O(2)) photosensitizers for the photooxidation of 1,5-dihydroxylnaphthalene (DHN). We found that 5 is more efficient than the conventional photosensitizer, such as Ir(ppy)(2)(bpy)[PF(6)]. The compounds were also used as organic triplet photosensitizers for triplet-triplet annihilation based upconversions. An upconversion quantum yield up to 18.5% was observed.     

Synthesis and characterization of some new C2 symmetric chiral bisamide ligands derived from chiral Feist's acid

The hemilabile chiral C2 symmetrical bidentate substituted amide ligands (1R,2R)-5(a-d) and (1S,2S)-6(a-d) were synthesized in quantitative yield from (1R,2R)-(+)-3-methylenecyclo-propane-1,2-dicarboxylic acid (1R,2R)-3 and (1S,2S)-(-)-3-methylene-cyclopropane-1,2-dicarboxylic acid (1S,2S)-3, in two steps, respectively. The chiral Feist's acids (1R,2R)-3 and (1S,2S)-3 were obtained in good isomeric purity by resolution of trans-(±)-3-methylene-cyclopropane-1,2-dicarboxylic acid from an 8:2 mixture of tert-butanol and water, using (R)-(+)-α-methylbenzyl amine as a chiral reagent. This process is reproducible on a large scale. All these new synthesized chiral ligands were characterized by 1H-NMR, 13C-NMR, IR, and mass spectrometry, as well as elemental analysis and their specific rotations were measured. These new classes of C2 symmetric chiral bisamide ligands could be of special interest in asymmetric transformations.     

分子光化学贮存太阳能 Ⅲ.双环[2,2,1]-2,5-庚二烯的光诱导电子转移敏化异构化反应

以N,N,N′,N′-四甲基联苯二胺、2,6-二甲氧基萘和2,7-二甲氧基萘为光敏剂,在正己烷溶液中实现了双环[2,2,1]-2,5-庚二烯到四环[2,2,1,0^2,6,0^3,5]庚烷的异构化。测定了反应的量子产率。讨论了反应机理。通过激发态的光敏剂与二烯之间的电子转移反应,形成单重态和三重态处于平衡状态的离子自由基对中间体。处于溶剂笼中的三重态离子自由基对经电子反传,产生激发三重态二烯。最后该激发态二烯经分子内[2+2]环合加成反应异构化为四环烷。     

Enantioselective synthesis of primary 1-(aryl)alkylamines by nucleophilic 1,2-addition of organolithium reagents to hydroxyoxime ethers and application to asymmetric synthesis of G-protein-coupled receptor ligands

(E)-Arylaldehyde oxime ethers bearing a (1S)-2-hydroxy-1-phenylethyl or (2R)-1-hydroxy-2-phenylethyl group as a chiral auxiliary, both derived from a single precursor, methyl (R)-mandelate, underwent nucleophilic addition with organolithium reagents via six-membered chelates to give the diastereomerically enriched (R)- and (S)-adducts, respectively, which, after chiral auxiliary removal by reductive N-O bond cleavage, led to the corresponding (R)- and (S)-1-(aryl)ethylamines. This organolithium addition protocol using methyllithium was applied in an enantiodivergent fashion to the preparation of both enantiomers of 1-(2-hydroxyphenyl)ethylamine, which has been previously used as an efficient chiral auxiliary for the synthesis of natural products in this laboratory. The synthetic utility of this methodology involving diastereoselective methyl addition was demonstrated by further application to the asymmetric synthesis of a new type of calcium receptor agonist (calcimimetics), (R)-(+)-NPS R-568 and its thio analogue. Furthermore, diastereoselective vinylation was accomplished by application of the hydroxy oxime ether-based protocol using vinyllithium, which allowed the development of the enantioselective synthesis of the NK-1 receptor antagonists, (+)-CP-99,994 and (+)-CP-122,721.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

Involvement of triplet excited states and olefin radical cations in electron-transfer cycloreversion of four-membered ring compounds photosensitized by (thia)pyrylium salts

Cycloreversion of 1,2,3,4-tetraphenylcyclobutanes 1a,b and oxetane 2 is achieved using (thia)pyrylium salts as electron-transfer photosensitizers. Radical cation intermediates involved in the electron-transfer process have been detected using laser flash photolysis. The experimental results are consistent with the reaction taking place from the triplet excited state of the sensitizer.     

Absolute structures of some naturally occurring isopropenyldihydrobenzofurans,remirol, remiridiol,angenomalin, and isoangenomalin

The absolute structures of some naturally occurring chiral 2-isopropenyl-2,3-dihydrobenzofurans, (+)-remirol (1a), (+)-remiridiol (1b), (+)-angenomalin (2), and (+)-isoangenomalin (3), were studied by respective chiral synthesis. Kinetic resolutions of racemic 2-isopropenyl-2,3-dihydrobenzofurans, 2-isopropenyl-4,6-dimethoxy-2,3-dihydrobenzofuran (4), 4-hydroxy-2-isopropenyl-2,3-dihydrobenzofuran-5-carbaldehyde (8), and 2-isopropenyl-6-(MOM)oxy-2,3-dihydrobenzofuran-5-carbaldehyde (11c), by Sharpless dihydroxylation using (DHQ)(2)AQN or (DHQD)(2)AQN gave the corresponding chiral 2-isopropenyl-2,3-dihydrobenzofurans. Chiral (S)-(+)-4 (99% ee, using (DHQD)(2)AQN) was converted to natural remirol (S)-(+)-1a and then to natural remiridiol (S)-(+)-1b. (S)-(+)-8 (97% ee, using (DHQD)(2)AQN) was converted to natural angenomalin (S)-(+)-2. (R)-(-)-11c (>99% ee, using (DHQ)(2)AQN), was converted to natural isoangenomalin (R)-(+)-3. Thus, the absolute structures of natural remirol (+)-1a and remiridiol (+)-1b and angenomalin (+)-2 were determined to be S, and the structure of natural isoangenomalin (+)-3 was R.     

Long-lived room-temperature deep-red-emissive intraligand triplet excited state of naphthalimide in cyclometalated Ir(III) complexes and its application in triplet-triplet annihilation-based upconversion

Cyclometalated Ir(III) complexes with acetylide ppy and bpy ligands were prepared (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridine) in which naphthal (Ir-2) and naphthalimide (NI) were attached onto the ppy (Ir-3) and bpy ligands (Ir-4) through acetylide bonds. [Ir(ppy)(3)] (Ir-1) was also prepared as a model complex. Room-temperature phosphorescence was observed for the complexes; both neutral and cationic complexes Ir-3 and Ir-4 showed strong absorption in the visible range (ε=39,600 M(-1) cm(-1) at 402?nm and ε=25,100 M(-1) cm(-1) at 404?nm, respectively), long-lived triplet excited states (τ(T)=9.30?μs and 16.45?μs) and room-temperature red emission (λ(em)=640?nm, Φ(p)=1.4?% and λ(em)=627?nm, Φ(p)=0.3?%; cf. Ir-1: ε=16,600 M(-1) cm(-1) at 382?nm, τ(em)=1.16 μs, Φ(p)=72.6?%). Ir-3 was strongly phosphorescent in non-polar solvent (i.e., toluene), but the emission was completely quenched in polar solvents (MeCN). Ir-4 gave an opposite response to the solvent polarity, that is, stronger phosphorescence in polar solvents than in non-polar solvents. Emission of Ir-1 and Ir-2 was not solvent-polarity-dependent. The T(1) excited states of Ir-2, Ir-3, and Ir-4 were identified as mainly intraligand triplet excited states ((3)IL) by their small thermally induced Stokes shifts (ΔE(s)), nanosecond time-resolved transient difference absorption spectroscopy, and spin-density analysis. The complexes were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion and quantum yields of 7.1?% and 14.4?% were observed for Ir-2 and Ir-3, respectively, whereas the upconversion was negligible for Ir-1 and Ir-4. These results will be useful for designing visible-light-harvesting transition-metal complexes and for their applications as triplet photosensitizers for photocatalysis, photovoltaics, TTA upconversion, etc.     

On the Z-E photoisomerization of chiral 2-pentenoate esters: stationary irradiations, laser-flash photolysis studies, and theoretical calculations

Chiral pentenoates 1-3 in both Z and E isomeric forms underwent stationary irradiations in several solvents and in the presence of different photosensitizers. The photostationary-state ratio has been determined for each Z/E couple showing a predominance of the thermodynamically more stable isomer for 1 and 3. Moreover, transient species were generated by pulsed laser excitation and detected by their characteristic ultraviolet absorptions, being the first time that enoate-originated triplets are detected. Stern-Volmer quenching studies afforded a quantitative measure for the efficiency of the photosensitization processes induced by benzophenone or acetophenone and allowed the determination of the corresponding quenching rate constants. Density functional calculations permitted the determination of the geometries and the energies of the diastereomeric excited states. Two diastereomeric orthogonal and two diastereomeric planar structures result as a consequence of the presence of a chiral substituent. The orthogonal triplets are the energy minima in all cases, whereas the planar triplets are the transition states linking these orthogonal structures, the corresponding energy barriers being 8-10 kcal mol(-1) for enoates 1-3. The computed S(0) to T(1) excitation energies show a trend which is consistent with the quenching rate constants. On the other hand, the triplet lifetimes determined for 1 and 2 are unusually long (1-20 micros) if compared with the data already described for several enones, in the range of nanoseconds. This fact has been rationalized from calculations of spin-orbit coupling at several points of the T(1) potential energy surface. This coupling is maximum for structures with a torsional angle close to 45 degrees, which are 4-5 kcal mol(-1) above the minima of T(1). Calculations done on the hypothetical aldehyde 4 and methyl vinyl ketone show much lower energy barriers, thus accounting for the shorter lifetimes reported for enone triplets.     

Structures, spectroscopic properties and redox potentials of quaterpyridyl Ru(II) photosensitizer and its derivatives for solar energy cell: a density functional study

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru((R)L)(NCS)(2)] ((R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-2,2' : 6',2' : 6',2'-quaterpyridine, R = H (1), Me (2), (t)Bu (3) and COOH (4); (R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-cycloquaterpyridine, R = COOH (5)). The geometries of the molecular ground, univalent cationic and triplet excited states of 1-5 were optimized. In complexes 1-4, the quaterpyridine ligand retains its planarity in the molecular, cationic and excited states, although the C≡N-Ru angle representing the SCN → Ru coordination approaches 180° in the univalent cationic and triplet excited states. The theoretically designed complex 5 displays a curved cycloquaterpyridine ligand with significantly distorted SCN → Ru coordination. The electron spin density distributions reveal that one electron is removed from the Ru/NCS moieties upon oxidation and the triplet excited state is due to the Ru/NCS → polypyridine charge transfer (MLCT/L'LCT). The experimental absorption spectra were well reproduced by the time-dependent DFT calculations. In the visible region, two MLCT/L'LCT absorption bands were calculated to be at 652 and 506 nm for 3, agreeing with experimental values of 637 and 515 nm, respectively. The replacement of the R- group with -COOH stabilizes the lower-energy unoccupied orbitals of π* character in the quaterpyridine ligand in 4. This results in a large red shift for these two MLCT/L'LCT bands. In contrast, the lower-energy MLCT/L'LCT peak of 5 nearly disappears due to the introduction of cycloquaterpyridine ligand. The higher energy bands in 5 however become broader and more intense. As far as absorption in the visible region is concerned, the theoretically designed 5 may be a very promising sensitizer for DSSC. In addition, the redox potentials of 1-5 were calculated and discussed, in conjunction with photosensitizers such as cis-[Ru(L(1))(2)(X)(2)] (L(1) = 4,4'-bis(carboxylic acid)-2,2'-bipyridine; X = NCS(-) (6), Cl(-) (7) and CN(-) (8)), cis-[Ru(L(1)')(2)(NCS)(2)] (L(1)' = 4,7-bis(carboxylic acid)-1,10-phenanthroline, 9), [NH(4)][Ru(L(2))(NCS)(3)] (L(2) = 4,4',4'-tris(carboxylic acid)-2,2' : 6',2'-terpyridine, 10) and [Ru(L(2))(NCS)(3)](-) (11).     

Chiral palladium template promoted asymmetric hydrophosphination reaction between diphenylphosphine and vinylphosphines

An organopalladium complex containing ortho-metalated (S)-(1-(dimethylamino)ethyl)naphthalene as the chiral auxiliary has been used to promote the asymmetric hydrophosphination reactions between diphenylphosphine and (E)- or (Z)-diphenyl-1-propenylphosphine in high regio- and stereoselectivities under mild conditions. Hydrophosphination of (Z)-diphenyl-1-propenylphosphine with diphenylphosphine gave (S)-(-)-prophos as the major product. Using the same chiral metal template, the corresponding hydrophosphination reaction with (E)-diphenyl-1-propenylphosphine gave (R)-(+)-prophos predominantly. The hydrophosphination reactions generated the asymmetric diphosphines as bidentate chelates on the chiral naphthylamine palladium templates. The template products obtained undergo cis-trans isomerization in solution to form an equilibrium mixture of regioisomers. X-ray analysis of the major template products obtained from the hydrophosphination of (Z)-diphenyl-1-propenylphosphine reveals that the two regioisomers are cocrystallized in a 1:1 ratio. The naphthylamine auxiliary could be removed chemoselectively from the template products by treatment with concentrated hydrochloric acid to form the corresponding optically pure neutral complexes [(R)- or (S)-(prophos)PdCl(2)]. Subsequently, the (R)- and (S)-dichloro complexes undergo ligand displacement with aqueous cyanide to generate the corresponding optically pure diphosphine ligands in high yields. Mechanistic pathways explaining the stereoselectivity of the chiral organopalladium template promoted hydrophosphination reactions are also proposed.     

Reductive side of water splitting in artificial photosynthesis: new homogeneous photosystems of great activity and mechanistic insight

Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co(III)(dmgH)(2)(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H(2)/mol PS/h for O, S, and Se derivatives, respectively (Φ(H(2)) = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (～2.6 ns) and shorter for the Se analog (～0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS(-) that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.     

A photochemical study of (E,E,E,E)-2-[9-(2-hydroxyethyl)imino-3,7-dimethyl-1,3,5,7-decatrien-1-yl]- 1,3,3-trimethylcyclohexene, a derivative of all-trans-retinal and ethanolamine

A derivative of all-trans-retinal (RAL) and ethanolamine, A2-E, is the main fluorescent component of human retinal lipofuscin. The accumulation of lipofuscin has been correlated with exposure to ambient radiation and loss of photoreceptors. A possible precursor to A2-E is the imine formed from RAL and ethanolamine. This compound, (E,E,E,E)-2-[9-(2-hydroxyethyl)imino-3,7-dimethyl-1,3,5,7- decatrien-1-yl]-1,3,3-trimethylcyclohexene (HIDD), has been synthesized and structurally characterized. The photophysical and photochemical properties of HIDD and its protonated form, HIDD-H+, have been investigated using steady-state and time-resolved methods. Both HIDD and HIDD-H+ are weakly fluorescent, and the fluorescence lifetime and quantum yield for HIDD are ca 0.6 ns and 4.0 +/- 0.5 x 10(-4), respectively. HIDD forms a triplet excited state on direct excitation that decays with kd = 3.4 x 10(4) s-1. The extinction coefficient and quantum yield of intersystem crossing for the HIDD triplet are measured as 7.6 +/- 1.3 x 10(4) M-1 cm-1 and 0.055 +/- 0.006, respectively. The triplet excited state of HIDD-H+ can be sensitized by triplet energy transfer and has a decay rate constant of 1.6 x 10(4) s-1. The lifetime of the HIDD triplet excited state is observed to decrease with increasing concentration of the well-known electron or hydrogen atom donors, 2,3,5,6-tetramethyl-1,4-phenylenediamine and 2,3,5-trimethylhydroquinone, and the bimolecular rate constants for these reactions are approximately 5.4 x 10(6) M-1 s-1 and 1.7 x 10(8) M-1 s-1, respectively. These types of reactions may model photooxidative mechanisms of damage in the retina.