An electroactive pentathiophene surfactant containing a phosphonic acid head group was designed and shown to provide strong binding to the surface of a CdSe nanocrystal and facilitate charge transfer between the nanocrystal and an organic semiconducting matrix (see Figure). Incorporation into organic–inorganic heterojunction solar cells could improve the efficiency of these promising devices.

A new approach to two-photon excited photodynamic therapy has been developed. A dendritic array of eight donor chromophores capable of two-photon absorption (TPA) was covalently attached to a central porphyrin acceptor. Steady-state fluorescence measurements demonstrated that the donor chromophores transfer excited-state energy to the porphyrin with 97% efficiency. Two-photon excitation of the donor chromophores at 780 nm resulted in a dramatic increase in porphyrin fluorescence relative to a porphyrin model compound. Enhanced singlet oxygen luminescence was observed from oxygen-saturated solutions of the target compound under two-photon excitation conditions.

C NMR spectra were recorded in deuterated solvents, such as CD 2 Cl 2 , on a Bruker DPX 250 and a Bruker DRX 500 spectrometer, using the proton or carbon signal of the solvent as an internal standard. The thermolysis reactions were carried out in sealed quartz tubes in a temperature controlled electromagnetic oven. SEM measurements were performed on a LEO 1530 field-emission scanning electron microscope. High-resolution TEM studies were conducted on a Philips Tecnai F30 analytical TEM at an operating voltage of 300 kV and on a TEM EM420 electron microscope at an operating voltage of 120 kV. The samples were dispersed in ethanol under ultrasonic irradiation and the suspension was dropped onto a TEM copper grid with a carbon film. TGA measurements were performed on a Mettler Toledo TS0801R0 device at a heating rate of 10°C min -1 between 0 and 900°C under an inert atmosphere. [1,2] share many common features with other types of organic solar cells, [3] which are collectively called "excitonic solar cells". [4] The operation of excitonic solar cells is fundamentally different from that of conventional solar cells in that the absorption of photons creates Frenkeltype excitons with a binding energy around 0.4-1 eV.[5] Because of the large binding energy, excitons can only efficiently dissociate at a heterojunction interface with favorably offset energy levels. The exciton diffusion length of most conjugated polymers is less than 10 nm, [6,7] which is much smaller than the optical absorption path length, even at the maximum absorption wavelength. To use materials with small exciton diffusion lengths and make reasonably efficient cells, electron donor and acceptor materials have been blended into bicontinuous structures known as bulk heterojunctions. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] In these cells, the size of each phase should be comparable to or less than the exciton diffusion length so that excitons can reach the interface before geminate recombination. However, for organic semiconductors that are highly immiscible, controlling COMMUNICATIONS 2960

Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organic-inorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. The bulk heterojunction concept [1,2] was designed to alleviate the problem of limited exciton migration by intimately blending the donor and acceptor phases on the nanometer length scale. In some polymer/fullerene systems, such as poly(2-methoxy,5-(3,7-dimethyloctyloxy)-1,4phenyl-enevinylene) (MDMO-PPV)/(6,6)-phenyl C61-butyric acid methyl ester (PCBM), time resolved spectroscopy shows the photoinduced formation of the radical anions and cations on femtosecond timescales. [3] This ultrafast formation of the polaron signature is only possible if every exciton is formed on a polymer chain segment that is immediately adjacent to one or more fullerene molecules. However, in other systems, very large domains prevail and, consequently, exciton harvesting is inefficient. [4] This has made the fabrication of efficient devices incorporating new materials difficult because each new material leads to a new morphology with its own characteristic length scale. The ordered bulk heterojunction architecture is intended to alleviate these issues by using a pre-patterned nanostructured scaffold [5][6][7] that has been engineered to have both straight pathways to the electrodes to ensure efficient carrier collection, and controlled domain size to ensure efficient exciton harvesting. Recently, chain alignment has been shown to be promoted for regioregular poly(3-hexyl thiophene) (RR-P3HT) in straight nanopores of anodic alumina leading to a 20-fold increase in hole-mobility in these structures.[8] For solar cell applications, higher mobilities reduce the effects of space charge [9] and increase the probability of separating the geminate pair formed immediately after exciton dissociation. [10][11][12][13] While the ordered bulk heterojunction architecture shows promise, existing structures have domains too large [14,15] for efficient exciton harvesting with singlet diffusion lengths only ca. 3-8 nm. [16][17][18] At this time, few alternatives other than nanostructuring have been proposed to increase exciton harvesting. Triplet excitons have been shown to have large diffusion lengths due to their long lifetimes, [18][19][20] but except in a few examples [21,22] this usually comes at the expense of a loss in energy of 0.4-0.8 eV associated with intersystem crossing between the photoexcited singlet to the first excited triplet. [23][24][25] At present, the direct engineering of singlet materials that have large diffusion lengths remains elusive due to the inherent disorder of most organic thin film materials. In this communication, we present theory and experiments that support a scheme to harvest singlet excitons over 25 nm away from the donor-acceptor int...

The segmental ligand 2-(6-(N,N-diethylcarbamoyl)pyridin-2-yl)-1,1'-dimethyl-2'-(5-(N,N-diethylsulfonamido)-pyridin-2-yl)-5,5'-methylenebis[1H-benzimidazole] (L3) is synthesized via a multistep strategy that allows the selective introduction of an electron-withdrawing sulfonamide group into the ligand backbone and its subsequent hydrolysis to the hydrophilic sulfonate group. Compared to that of the methylated analogue L1, the affinity of the bidentate binding unit of L3 for H+ and for trivalent lanthanide ions (LnIII) in [Ln(L3)3]3+ and [Ln2(L3)3]6+ is reduced because the electron-withdrawing sulfonamide substituent weakens sigma-bonding, but improved retro-pi-bonding between the bidentate binding units of L3 and soft 3d-block ions (M(II) = FeII, ZnII) overcomes this effect and leads to homometallic complexes [Mn(L(i))m]2n+ (i = 1, 3) displaying similar stabilities. Theoretical ab initio calculations associate this dual effect with a global decrease in energy of pi and sigma orbitals when the sulfonamide group replaces the methyl group, with an extra stabilization for the LUMO (pi). The reaction of L3 with a mixture of LnIII and M(II) (M = Fe, Ni, Zn) in acetonitrile gives the noncovalent podates [LnM(L3)3]5+ in which LnIII is nine-coordinated by the three wrapped tridentate segments, while the bidentate binding units provide a facial pseudooctahedral site around M(II). The X-ray structure of [EuZn(L3)3](ClO4)4(PF6)(CH3NO2)3(H2O) reveals that the bulky sulfonamide group at the 5-position of the pyridine ring only slightly increases the Zn-N bond distances as a result of sigma/pi compensation effects. The introduction of spectroscopically and magnetically active FeII and NiII into the pseudooctahedral site allows the detailed investigation of the electronic structure of the bidentate segment. Absorption spectra, combined with electrochemical data, experimentally demonstrate the dual effect associated with the attachment of the sulfonamide group (decrease of the sigma-donating ability of the pyridine lone pair and increase of the pi-accepting properties of the coordinated bidentate binding unit). The influences on the ligand field strength and on tunable room-temperature FeII spin-crossover processes occurring in [LnFe(L3)3]5+ are discussed, together with the origin of the entropic control of the critical temperature in these thermal switches.

The segmental ligand 2-[6-(N,N-diethylcarbamoyl)pyridin-2-yl]-1,1'-dimethyl-5,5'-methylene-2'-(6-methylpyridine-2-yl)bis[1H-benzimidazole] (L3) reacts with a stoichiometric mixture of LnIII (Ln = La, Eu, Gd) and M(II) (M = Zn, Fe) in acetonitrile to produce selectively the heterodimetallic triple-stranded helicates (HHH)-[LnM(L3)3]5+. In these complexes, M(II) is pseudooctahedrally coordinated by the three wrapped bidentate binding units, thus forming a noncovalent tripod which organizes the three unsymmetrical tridentate segments to give ninefold coordination to LnIII. The introduction of a methyl group at the 6 position of the terminal pyridine in L3 sterically reduces the complexing ability of the bidentate segment for M(II). Spectroscopic (ESI-MS, UV/Vis/NIR, NMR), magnetic and electrochemical measurements show that 1) the head-to-head-to-head triple helical complexes (HHH)-[LnM(L3)3]5+ are quantitatively formed in solution only for ligand concentrations larger than 0.01 M, 2) FeII adopts a pure high-spin electronic configuration in (HHH)-[LnFe(L3)3]5+ and 3) the FeII/FeIII oxidation process is prevented by steric constraints. Detailed photophysical studies of (HHH)-[Eu-Zn(L3)3]5+ confirm that the pseudotricapped trigonal-prismatic lanthanide coordination site is not affected by the methyl groups bound to the terminal pyridine, thus leading to significant Eu-centered emission upon UV irradiation. In (HHH)-[EuFe(L3)3]5+, a resonant intramolecular Eu-->Fe(II)hs energy transfer partially quenches the Eu-centered luminescence; however, the residual red emission demonstrates that high-spin iron(II) is compatible with the sensitization of Eu(III) in heterodimetallic d-f complexes. The influence of the electronic configuration of Fe(II) on the efficiency of Eu(III)-->Fe(II) energy-transfer processes is discussed together with its consequence for the design of optically active spin-crossover supramolecular devices.

Geneva (1995Geneva ( -1998

No abstract