Phosphorescent resonant energy transfer between iridium complexes

The mechanism for triplet energy transfer from the green-emitting fac-tris[2-(4'-tert-butylphenyl)pyridinato]iridium (Ir(tBu-ppy)3) complex to the red-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3')(acetylacetonato)iridium (Ir(btp)2(acac)) phosphor has been investigated using steady-state and time-resolved photoluminescence spectroscopy. [2,2';5,'2' ']Terthiophene (3T) was also used as triplet energy acceptor to differentiate between the two common mechanisms for energy transfer, i.e., the direct exchange of electrons (Dexter transfer) or the coupling of transition dipoles (F?rster transfer). Unlike Ir(btp)2(acac), 3T can only be active in Dexter energy transfer because it has a negligible ground state absorption to the 3(pi-pi*) state. The experiments demonstrate that in semidilute solution, the 3MLCT state of Ir(tBu-ppy)3 can transfer its triplet energy to the lower-lying 3(pi-pi*) states of both Ir(btp)2(acac) and 3T. For both acceptors, this transfer occurs via a diffusion-controlled reaction with a common rate constant (ken = 3.8 x 10(9) L mol-1 s-1). In a solid-state polymer matrix, the two acceptors, however, show entirely different behavior. The 3MLCT phosphorescence of Ir(tBu-ppy)3 is strongly quenched by Ir(btp)2(acac) but not by 3T. This reveals that under conditions where molecular diffusion is inhibited, triplet energy transfer only occurs via the F?rster mechanism, provided that the transition dipole moments involved on energy donor and acceptor are not negligible. With the use of the F?rster radius for triplet energy transfer from Ir(tBu-ppy)3 to Ir(btp)2(acac) of R0 = 3.02 nm, the experimentally observed quenching is found to agree quantitatively with a model for F?rster energy transfer that assumes a random distribution of acceptors in a rigid matrix.     

Intramolecular energy transfer reactions in polymetallic complexes

A series of polymetallic complexes are being developed for use in converting solar radiation to usable chemical potential energy. The system is made up of three components: 1) A highly-absorbing (antenna) metal center which absorbs visible light but is photochemically unreactive; 2) A second metal center which undergoes a useful chemical reaction from a non-spectroscopic excited state; and 3) A bridging ligand which couples the two metal fragments and facilitates intramolecular energy transfer from the antenna to the reactive fragment. This paper will focus on optimizing the three components of the system.     

Kinetics of energy transfer processes in C-phycocyanin complexes

The antenna system of algae for photosynthesis is a functional entity composed of various phycobiliproteins and the linker polypeptides. Up to now, high-resolution crystal structure data have been available only for the isolated phycobiliproteins. To have an understanding of the functional connection between different phycobiliproteins, it is necessary to study the complexes composed of different phycobiliproteins. The energy transfer processes in C-phycocyanin complexes were studied through computer simulation because it is difficult to be studied by conventional experimental methods. The main pathways of energy flow and the dynamic property of the energy transfer were obtained. A fast transfer process between two neighboring disks was observed through analyzing the distribution curves of excitation energy over time. According to the definition of the time constants for energy transfer in time-resolved spectrum techniques, for a complex with three C-phycoeyanin hexamer disks, a fluorescence-rising comp     

Intramolecular electronic energy transfer in bichromophoric macrocyclic complexes

Efficient intramolecular electronic energy transfer (EET) has been demonstrated for three novel bichromophoric compounds utilizing a macrocyclic spacer as the bridge between the electronic energy donor and acceptor fragments. As their free base forms, emission from the electronically excited donor is absent and the acceptor emission is reductively quenched via photoinduced oxidation of proximate amine lone pairs. As their Zn(II) complexes, excitation of the donor results in sensitization of the electronic acceptor emission.     

Intramolecular energy transfer involving heisenberg spin-coupled dinuclear iron-oxo complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe(2)O(O(2)CCH(2)-C(10)H(7))(tren)(2)](BPh(4))(NO(3))(2) (8), [Fe(2)O(O(2)CCH(2)-C(10)H(7))(TPA)(2)](ClO(4))(3) (9), Fe(2)O(O(2)CCH(2)-C(10)H(7))(2)(Tp)(2) (10), and Fe(2)O((O(2)CCH(2)CH(2))(2)-C(10)H(6))(Tp)(2) (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm(-)(1) for H = -2JS(1).S(2)). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH(3)CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) (13, where TACN-Me(3) is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with tau(obs)(S)()1 = 36 +/- 2, 32 +/- 4, 30 +/- 5, and 39 +/- 3 ns for compounds 8-11, respectively. The decays are assigned to the S(1) --> S(0) fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (tau(obs)(S)()1 = 45 +/- 2 ns), indicating quenching of the S(1) state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) reveal a feature at lambda(max) = 420 nm that can be assigned as the T(1) --> T(n) absorption of the naphthalene triplet; the rise time of 50 +/- 10 ns corresponds to an intersystem crossing rate of 2 x 10(7) s(-1). A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T(1) lifetime of the pendant naphthalene for all of the iron-oxo complexes (tau(obs)(T)1 = 5 +/- 2 micros vs 90 +/- 10 micros for [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4))) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced Fe(II)Fe(III) species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S(1) and T(1) states. Spectral overlap considerations strongly support a F?rster (i.e., dipolar) mechanism for energy transfer from the S(1) state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed.     

Excitonic energy transfer in light-harvesting complexes in purple bacteria

Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting (LH) systems in purple bacteria. It is found that the inclusion of long-range dipolar interactions in the two methods results in significant increase in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal an interesting role of dipolar interactions in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (4 ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, non-nearest neighbor interactions can induce symmetry breaking, which leads to global and local minima of the average trapping time in the presence of a non-zero dephasing rate, suggesting that environment dephasing helps preserve quantum coherent energy transfer when the perfect circular symmetry in the hypothetic system is broken. This study reveals that dipolar coupling between chromophores may play an important role in the high energy transfer efficiency in the LH systems of purple bacteria and many other natural photosynthetic systems.     

Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

The metal-to-metal charge transfer (MMCT) transitions of a series of Class II mixed valence dinuclear complexes bearing cyano bridging ligands may be varied systematically by variations to either the hexacyanometallate(II) donor or Co(III) acceptor moieties. Specifically, the new dinuclear species trans-[L(14S)CoNCFe(CN)(5)](-)(L(14S)= 6-methyl-1,11-diaza-4,8-dithia-cyclotetradecane-6-amine) and trans-[L(14)CoNCRu(CN)(5)](-)(L(14)= 6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine) have been prepared and their spectroscopic and electrochemical properties are compared with the relative trans-[L(14)CoNCFe(CN)(5)](-). The crystal structures of Na(trans-[L(14S)CoNCFe(CN)(5)]).51/2H(2)O.1/2EtOH, Na(trans-[L(14)CoNCRu(CN)(5)]).3H(2)O and Na(trans-[L(14)CoNCRu(CN)(5)]).8H(2)O are also reported. The ensuing changes to the MMCT energy have been examined within the framework of Hush theory, and it was found that the free energy change between the redox isomers was the dominant effect in altering the energy of the MMCT transition.     

Light-harvesting energy transfer and subsequent electron transfer of cationic porphyrin complexes on clay surfaces

A novel energy-transfer system involving nonaggregated cationic porphyrins adsorbed on an anionic-type clay surface and the electron-transfer reaction that occurs after light harvesting are described. In the clay-porphyrin complexes, photochemical energy transfer from excited singlet zinc porphyrins to free-base porphyrins proceeds. The photochemical electron-transfer reaction from an electron donor in solution (hydroquinone) to the adsorbed porphyrin in the excited singlet state was also examined. Because the electron-transfer rate from the hydroquinone to the excited singlet free-base porphyrin is larger than that to the excited singlet zinc porphyrin, we conclude that the energy transfer accelerates the overall electron-transfer reaction.     

Intra- and intermolecular energy transfer in highly excited ozone complexes

The energy transfer of highly excited ozone molecules is investigated by means of classical trajectories. Both intramolecular energy redistribution and the intermolecular energy transfer in collisions with argon atoms are considered. The sign and magnitude of the intramolecular energy flow between the vibrational and the rotational degrees of freedom crucially depend on the projection K(a) of the total angular momentum of ozone on the body-fixed a axis. The intermolecular energy transfer in single collisions between O(3) and Ar is dominated by transfer of the rotational energy. In accordance with previous theoretical predictions, the direct vibrational de-excitation is exceedingly small. Vibration-rotation relaxation in multiple Ar+O(3) collisions is also studied. It is found that the relaxation proceeds in two clearly distinguishable steps: (1) During the time between collisions, the vibrational degrees of freedom are "cooled" by transfer of energy to rotation; even at low pressure equilibration of the internal energy is slow compared to the time between collisions. (2) In collisions, mainly the rotational modes are "cool" by energy transfer to argon.     

Micellar effects upon electron transfer from ferrocenes

The oxidations of ferrocene (FcH) and n-butylferrocene (FcBu) by ferric salts (nitrate or bromide) are strongly inhibited by aqueous cetyltrimethylammonium bromide and nitrate (CTABr and CTANO₃, respectively). The kinetics of inhibition fit a model in which the substrates are distributed between water, and the micelles and binding constants K_s to the micelle can be estimated. The oxidations are strongly catalyzed by micelles of sodium lauryl sulfate (NaLS), and the kinetics can be fitted to a model in which the reaction rate depends upon the concentration of both reactants in the micellar pseudophase and the rate constants in that pseudophase, which for both substrates are very similar to those in water. Some added salts reduce the micellar catalysis by excluding ferric ions from the micelle. The oxidations of FcH and FcBu by ferricyanide ions are too fast to be followed in water, but they are inhibited by anionic micelles of NaLS. By analyzing the rate surfactant profiles using independently measured values of K_s the second-order rate constants in water have been estimated.     

Surprising photochemical reactivity and visible light-driven energy transfer in heterodimetallic complexes

The bis(bidentate) phosphine cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphino)cyclobutane (dppcb) has been used for the synthesis of a series of novel heterodimetallic complexes starting from [Ru(bpy)(2)(dppcb)]X(2) (1; X = PF(6), SbF(6)), so-called dyads, showing surprising photochemical reactivity. They consist of [Ru(bpy)(2)](2+)"antenna" sites absorbing light combined with reactive square-planar metal centres. Thus, irradiating [Ru(bpy)(2)(dppcb)MCl(2)]X(2) (M = Pt, 2; Pd, 3; X = PF(6), SbF(6)) dissolved in CH(3)CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH(3)CN)(2)(dppcb)MCl(2)]X(2) (M = Pt, 7; Pd, 8; X = PF(6), SbF(6)). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (5/6) lead to [Ru(bpy)(CH(3)CN)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (9), where only the RuP(2)N(4) moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb)NiCl(2)]X(2) (4; X = PF(6), SbF(6)) no clean photoreaction is observed. Interestingly, this difference in photochemical behaviour is completely in line with the related photophysical parameters, where 2, 3, and 5/6, but not 4, show long-lived excited states at ambient temperature necessary for this type of photoreaction. Furthermore, the photochemical as well as the photophysical properties of 2-4 are also in accordance with their single crystal X-ray structures presented in this work. It seems likely that differences in "steric pressure" play a major role for these properties. The unique complexes 7-9 are also fully characterized by single-crystal X-ray structure analyses, clearly showing that the stretching vibration modes of the ligand CH(3)CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH(3)CN)(2)](2+) chromophore of 9 acts as efficient "antenna" for visible light-driven energy transfer to the Os-centred "trap" site, resulting in k(en) ≥ 2 × 10(9) s(-1) for the energy transfer. Since electron transfer is made possible by the use of this intervening energy transfer, in dyads like 2-4 highly reactive M(0) species (M = Pt, Pd, Ni) could be generated. These species are not stable in water and M(II) hydride intermediates are usually formed, further reacting with H(+) to give H(2). Thus, derivatives of 3, namely [M(bpy)(2)(dppcb)Pd(bpy)](PF(6))(4) (M = Os, Ru) dissolved in 1:1 (v/v) H(2)O-CH(3)CN produce H(2) during photolysis with visible light.     

分子内扭转电荷转移的胶束效应

研究了水溶液中对二甲氨基苯甲(DMABA)分子内扭转电荷转移(TICT)的胶束效应.胶束能促进DMABA的TICT过程,并导致DMABA的总荧光量子产率提高和TICT荧光峰的显著蓝移,DMABA的TICT荧光强度与正常荧光带的强度之比与DMABA的浓度之间有线性关系,胶束存在时该直线的斜率提高,认为DMABA的TICT激发态涉及两个DMABA分子,并且影响DMABA的TICT激发态相对布居的主要因素是其与相应的三重态和基态的能隙.讨论了DMABA的TICT激发态的可能结构及胶束效应的本质.     

Micellar catalysis on the electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfides-importance of hydrophobic interactions

The oxidation of organic sulfides with iron(III)-polypyridyl complexes [Fe(NN)3]3+ proceeds through an electron transfer mechanism and an increase in the methanol content in the methanol-water mixture favors the reaction. The reaction is catalyzed by both the anionic surfactant, sodium dodecyl sulfate (SDS) and the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The micellar catalysis in the presence of SDS is accounted for in terms of strong binding of the cationic oxidant with the anionic surfactant and the development of positive charge on sulfur center of substrate in the transition state. The micellar catalysis observed on the reaction involving a trication, [Fe(NN)3]3+, in the presence of CTAB indicates the importance of hydrophobic interaction between the micelle and hydrophobic ligand of [Fe(NN)3]3+. The micellar catalysis is explained in terms of a pseudophase ion exchange model.     

Investigation of the photoinduced energy transfer in the supramolecular complexes of styryl dyes

Self-assembly of monostyryl dye of benzothiazole series with symmetric and asymmetric bisstyryl dyes based on dibenzo-18-crown-6 ether has been studied by optical and NMR spectroscopies. The photoinduced energy transfer between monoand bisstyryl dyes in supramolecular assembly has been analyzed. The cascade energy transfer in the supramolecular complex of monostyryl dye with asymmetric bisstyryl dye has been suggested.     

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.     

Triplet energy back transfer in conjugated polymers with pendant phosphorescent iridium complexes

The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)(2)(acac)] fragment (where btp is 2-(2'-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH(2))(8)- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 A between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH(2))(8)- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.     

Photoluminescence of europium(III) dithiocarbamate complexes: electronic structure, charge transfer and energy transfer

For the first time, we observed photoluminescence in Eu(III) dithiocarbamate complexes at room temperature -- more specifically in [Eu(Et(2)NCS(2))(3)phen], [Eu(Et(2)NCS(2))(3)bpy] and the novel [Eu(Ph(2)NCS(2))(3)phen], where phen stands for 1,10-phenanthroline and bpy for 2,2'-bipyridine. Correlations between the electronic structure of the dithiocarbamate ligands on one hand, and covalency, intensity, and ligand field spectroscopic parameters on the other, could be established. Moreover, the relative values of the emission quantum efficiencies obtained for these complexes, as well as their dependence with temperature, could be satisfactorily described by a theoretical methodology recently developed.     

Communication: Exciton-phonon information flow in the energy transfer process of photosynthetic complexes

Non-Markovian and nonequilibrium phonon effects are believed to be key ingredients in the energy transfer in photosynthetic complexes, especially in complexes which exhibit a regime of intermediate exciton-phonon coupling. In this work, we utilize a recently developed measure for non-Markovianity to elucidate the exciton-phonon dynamics in terms of the information flow between electronic and vibrational degrees of freedom. We study the measure in the hierarchical equation of motion approach which captures strong coupling effects and nonequilibrium molecular reorganization. We propose an additional trace distance measure for the information flow that could be extended to other master equations. We find that for a model dimer system and for the Fenna-Matthews-Olson complex the non-Markovianity is significant under physiological conditions.     

Electronic excitation energy transfer between molecules of carbocyanine dyes in complexes with DNA

Electronic excitation energy transfer has been carried out between molecules of carbocyanine dyes bound noncovalently to DNA. 3,3′,9-Triethyl-5,5′-dimethyloxacarbocyanine iodide was used as an energy donor and 3,3′-diethylthiacarbocyanine iodide as an acceptor dye. In this process, the band belonging to the donor is observed in the fluorescence excitation spectrum of the acceptor. Donor fluorescence quenching by the acceptor in the presence of DNA was studied. The results of the experiments are discussed in terms of the Dye-DNA stoichiometric complex formation and with respect to concentrating the dyes in the microphase (pseudophase) of the biopolymer.