Convergent Synthesis of Near‐Infrared Absorbing, “Push–Pull”, Bisthiophene‐Substituted,Zinc(II) Phthalocyanines and their Application in Dye‐Sensitized Solar Cells

Zinc(II) phthalocyanine dyes that contain triarylamine‐terminated bisthiophene and hexylbisthiophene groups have been synthesized by a convergent approach by using carboxytriiodo–ZnPc as a precursor. Further transformation of the iodo groups by a Pd‐catalyzed reaction allowed easy preparation of further extended π‐conjugated carboxy–ZnPcs. These dyes have been used as sensitizers in dye‐sensitized solar cells, which exhibit a panchromatic response and moderate overall efficiencies.     

Acid-induced degradation of phosphorescent dopants for OLEDs and its application to the synthesis of tris-heteroleptic iridium(III) bis-cyclometalated complexes

Investigations of blue phosphorescent organic light emitting diodes (OLEDs) based on [Ir(2-(2,4-difluorophenyl)pyridine)(2)(picolinate)] (FIrPic) have pointed to the cleavage of the picolinate as a possible reason for device instability. We reproduced the loss of picolinate and acetylacetonate ancillary ligands in solution by the addition of Br?nsted or Lewis acids. When hydrochloric acid is added to a solution of a [Ir(C^N)(2)(X^O)] complex (C^N = 2-phenylpyridine (ppy) or 2-(2,4-difluorophenyl)pyridine (diFppy) and X^O = picolinate (pic) or acetylacetonate (acac)), the cleavage of the ancillary ligand results in the direct formation of the chloro-bridged iridium(III) dimer [{Ir(C^N)(2)(μ-Cl)}(2)]. When triflic acid or boron trifluoride are used, a source of chloride (here tetrabutylammonium chloride) is added to obtain the same chloro-bridged iridium(III) dimer. Then, we advantageously used this degradation reaction for the efficient synthesis of tris-heteroleptic cyclometalated iridium(III) complexes [Ir(C^N(1))(C^N(2))(L)], a family of cyclometalated complexes otherwise challenging to prepare. We used an iridium(I) complex, [{Ir(COD)(μ-Cl)}(2)], and a stoichiometric amount of two different C^N ligands (C^N(1) = ppy; C^N(2) = diFppy) as starting materials for the swift preparation of the chloro-bridged iridium(III) dimers. After reacting the mixture with acetylacetonate and subsequent purification, the tris-heteroleptic complex [Ir(ppy)(diFppy)(acac)] could be isolated with good yield from the crude containing as well the bis-heteroleptic complexes [Ir(ppy)(2)(acac)] and [Ir(diFppy)(2)(acac)]. Reaction of the tris-heteroleptic acac complex with hydrochloric acid gives pure heteroleptic chloro-bridged iridium dimer [{Ir(ppy)(diFppy)(μ-Cl)}(2)], which can be used as starting material for the preparation of a new tris-heteroleptic iridium(III) complex based on these two C^N ligands. Finally, we use DFT/LR-TDDFT to rationalize the impact of the two different C^N ligands on the observed photophysical and electrochemical properties.     

Theory of sheath in a collisional multi-component plasma

The aim of this brief report is to study the behaviour of sheath structure in a multi-component plasma with dust-neutral collisions. The plasma consists of electrons, ions, micron size negatively charged dust particles and neutrals. The sheath-edge potential and sheath width are calculated for collisionally dominated sheath. Comparison of collisionless and collisionally dominated sheath are made.     

Highly phosphorescence iridium complexes and their application in organic light-emitting devices

A new series of iridium(III) mixed ligand complexes TBA[Ir(ppy)(2)(CN)(2)] (1), TBA[Ir(ppy)(2)(NCS)(2)] (2), TBA[Ir(ppy)(2)(NCO)(2)] (3), and [Ir(ppy)(2)(acac)] (4) (ppy = 2-phenylpyridine; acac = acetoylacetonate, TBA = tetrabutylammonium cation) have been developed and fully characterized by UV-vis, emission, IR, NMR, and cyclic voltammetric studies. The lowest energy MLCT transitions are tuned from 463 to 494 nm by tuning the energy of the HOMO levels. These complexes show emission maxima in the blue, green, and yellow region of the visible spectrum and exhibit unprecedented phosphorescence quantum yields, 97 +/- 3% with an excited-state lifetimes of 1-3 micros in dichloromethane solution at 298 K. The near-unity quantum yields of these complexes are related to an increased energy gap between the triplet emitting state and the deactivating e(g) level that have been achieved by meticulous selection of ligands having strong ligand field strength. Organic light-emitting devices were fabricated using the complex 4 doped into a purified 4,4'-bis(carbazol-9-yl)biphenyl host exhibiting a maximum of the external quantum efficiencies of 13.2% and a power efficiency of 37 lm/W for the 9 mol % doped system.     

Molecular control of recombination dynamics in dye-sensitized nanocrystalline TiO2 films: free energy vs distance dependence

In this paper we address the dependence of the charge recombination dynamics in dye-sensitized, nanocrystalline TiO2 films upon the properties of the sensitizer dye employed. In particular we focus upon dependence of the charge recombination kinetics upon the dye oxidation potential E0(D+/D), determined electrochemically, and the spatial separation r of the dye cation HOMO orbital from the metal oxide surface, determined by semiempirical calculations. Our studies employed a series of ruthenium bipyridyl dyes in addition to porphyrin and phthalocyanine dyes. A strong correlation is observed between the recombination dynamics and the spatial separation r, with variation in r by 3 A resulting in a more than 10-fold change in the recombination half-time t(50%). This correlation is found to be in agreement with electron tunneling theory, t(50%) proportional, variant exp(-betar) with beta = 0.95 +/- 0.2 A-1. In contrast, the recombination dynamics were found to be relatively insensitive to variations in E0D+/D), indicative of the recombination reaction lying near the peak of the Marcus free energy curve, DeltaG approximately lambda, and with lambda approximately 0.8 eV. A correlation is also observed between the recombination half-time and the temporal shape of the kinetics, with faster recombination dynamics being more dispersive (less monoexponential). Comparison with numerical Monte Carlo type simulations suggests this correlation is attributed to a shift from fast recombination dynamics primarily limited by dispersive electron transport within the metal oxide film to slower dynamics primarily limited by the interfacial electron-transfer reaction. We conclude that the primary factor controlling the charge recombination dynamics in dye-sensitized, nanocrystalline TiO2 films is the spatial separation of the dye cation from the electrode surface. In particular, we show that for the Ru(dcbpy)2NCS2 dye series, the use of X = NCS rather than X = CN results in a 2 A shift in the dye cation HOMO orbital away from the electrode surface, causing a 7-fold retardation of the recombination dynamics, resulting in the remarkably slow recombination dynamics observed for this sensitizer dye.     

Supramolecular control of charge-transfer dynamics on dye-sensitized nanocrystalline TiO2 films

A [Ru(dcbpy)(2)(NCS)(2)] dye has been chemically modified by the addition of a secondary electron donor moiety, N,N-(di-p-anisylamino)phenoxymethyl. Optical excitation of the modified dye adsorbed to nanocrystalline TiO(2) films shows a remarkably long-lived charge-separated state, with a decay half time of 0.7 s. Semiempirical calculations confirm that the HOMO of the modified dye molecule is localised on the electron donor group. The retardation of the recombination dynamics relative to the unmodified control dye is caused by the increase in the spatial separation of the HOMO orbital from the TiO(2) surface. The magnitude of the retardation is shown to be in agreement with that predicted from the non-adiabatic electron-tunnelling theory.     

Fast iterative methods for symmetric sinc-Galerkin systems

The symmetric sinc-Galerkin method developed by Lund, when appliedto the second-order self-adjoint boundary value problem, givesrise to a symmetric coefficient matrix has a special structureso that it can be advantageously used in solving the discretesystem. In this paper, we employ the preconditioned conjugategradient method with banded matrices as preconditioners. Weprove that the condition number of the preconditioned matrixis uniformly bounded by a constant independent of the size ofthe matrix. In particular, we show that the solution of an n-by-ndiscrete symmetric sinc-Galerkin system can be obtained in O(nlog n) operations. We also extend our method to the self-adjointelliptic partial differential equation. Numerical results aregiven to illustrate the effectiveness of our fast iterativesolvers.     

(e, 3e) Differential cross section of He (21 S) and He (23 S)

The angular distribution of the five-fold differential cross section for the electron impact double ionization of He (2¹ S) and He (2³ S) has been studied. The kinematical conditions for maxima/minima in the angular distribution for the two cases have been compared. The two-step process for the double ionization is found to contribute very little in the triplet case.     

The Influence of Substituent Orientation on the Photovoltaic Performance of Phthalocyanine‐Sensitized Solar Cells

Phthalocyanines (Pcs) are used as sensitizers in dye‐sensitized solar cells (DSSCs) because of their stability and intense absorption in the red and near‐IR regions. Impressive progress has been made in photovoltaic efficiencies by introduction of bulky peripheral substituents to help suppress macrocycle aggregation. To reach benchmark efficiencies reported for other related dyes, new designs need to be explored. Single carboxy‐ZnPc regioisomers substituted at the non‐peripheral positions by rigid aryl groups have now been studied, which has shed light on the influence of steric hindrance and/or orientation of the substituent around the anchoring group on the photovoltaic response. The regioisomer bearing the aryl group far away from the anchoring group produces a more effective sensitization of the TiO₂ films and higher short‐circuit photocurrent density (J_sc). Taking advantage of the good photovoltaic performance in the near‐IR region of this ZnPc, it was combined with another appropriate dye for panchromatic sensitization of the mesoporous photoelectrode and an increase of the overall device efficiency.