The Charge Transfer Problem in Density Functional Theory Calculations of Aqueously Solvated Molecules

Recent advances in algorithms and computational hardware have enabled the calculation of excited states with time-dependent density functional theory (TDDFT) for large systems of <i>O</i>(<i>1000</i>) atoms. Unfortunately, the aqueous charge transfer problem in TDDFT (whereby many spuriously low-lying charge transfer excited states are predicted) seems to become more severe as the system size is increased. In this work, we concentrate on the common case where a chromophore is embedded in aqueous solvent. We examine the role of exchange-correlation functionals, basis set effects, ground state geometries, and the treatment of the external environment in order to assess the root cause of this problem. We conclude that the problem rests largely on water molecules at the boundary of a finite cluster model, i.e., “edge waters.” We also demonstrate how the TDDFT problem can be related directly to ground state problems. These findings demand caution in the commonly employed strategy that rests on “snapshot” cutout geometries taken from ground state dynamics with molecular mechanics. We also find that the problem is largely ameliorated when the range-separated hybrid functional LC-ωPBEh is used

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 Brønsted 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^∧N¹)­(C^∧N²)­(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^∧N¹ = ppy; C^∧N² = 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

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Influence of Halogen Atoms on a Homologous Series of Bis-Cyclometalated Iridium(III) Complexes

A series of homologous bis-cyclometalated iridium­(III) complexes Ir­(2,4-di-X-phenyl-pyridine)₂(picolinate) (X = H, F, Cl, Br) <b>HIrPic</b>, <b>FIrPic</b>, <b>ClIrPic</b>, and <b>BrIrPic</b> has been synthesized and characterized by NMR, X-ray crystallography, UV–vis absorption and emission spectroscopy, and electrochemical methods. The addition of halogen substituents results in the emission being localized on the main cyclometalated ligand. In addition, halogen substitution induces a blue shift of the emission maxima, especially in the case of the fluoro-based analogue but less pronounced for chlorine and bromine substituents. Supported by ground and excited state theoretical calculations, we rationalized this effect in a simple manner by taking into account the σp and σm Hammett constants on both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. Furthermore, in comparison with <b>FIrPic</b> and <b>ClIrPic</b>, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of <b>BrIrPic</b> and switches the corresponding lifetime from mono to biexponential decay. We performed theoretical calculations based on linear-response time-dependent density functional theory (LR-TDDFT) including spin–orbit coupling (SOC), and unrestricted DFT (U-DFT) to obtain information about the absorption and emission processes and to gain insight into the reasons behind this remarkable change in photophysical properties along the homologous series of complexes. According to theoretical geometries for the lowest triplet state, the large halogen substituents contribute to sizable distortions of specific phenylpyridine ligands for <b>ClIrPic</b> and <b>BrIrPic</b>, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect

Unravelling the Potential for Dithienopyrrole Sensitizers in Dye-Sensitized Solar Cells

Two D−π–A dyes based on the dithieno­[3,2-<i>b</i>:2′,3′-<i>d</i>]­pyrrole π-bridge (DTP) were synthesized, characterized using UV–vis absorption spectroscopy and electrochemistry, modeled using quantum chemical calculations, and used as sensitizers in dye-sensitized solar cells (DSCs). The photoelectrochemical properties and DSC performance are thoroughly compared with their cyclopenta­[1,2-<i>b</i>:5,4-<i>b</i>′]­dithiophene (CPDT) analogues. The use of DTP results in a small increase in the zero–zero transition energy reflecting the higher lying lowest unoccupied molecular orbital that is commonly reported for DTP relative to CPDT systems. This increased optical gap manifests in slightly blue-shifted incident photon-to-collected electron conversion efficiency (IPCE) responses; however, increased open-circuit photovoltage values and improved charge-transfer kinetics relative to the CPDT systems result in comparable power conversion efficiencies. The present report highlights the potential of DTP for the development of tailored sensitizers employing stronger acceptors

Molecular Engineering of a Fluorene Donor for Dye-Sensitized Solar Cells

To improve their efficiency beyond the state-of-the-art, D−π–A dyes must display increased spectral breadth and account for the physical limitations observed in the dye-sensitized solar cells. In particular, they should be designed to control the electron-transfer processes that ensure efficient dye-regeneration and prevent undesired electron recombination. In this article, the electronic and steric properties of a fluorene donor are engineered to meet all these requirements. This elegant donor is featured along with a cyclopentadithiophene bridge and a cyanoacrylic acid acceptor in <b>JF419</b>. A thorough comparison with <b>Y123</b> and <b>C218</b> demonstrates the relevance of the design. Relative to conventional donors, the fluorene construct described here enhances the light-harvesting properties, because of its exceptional electron-donating character. The functionalities used to induce the electronic push through the D−π–A structure also provide the dye with favorable steric properties. Indeed, the substitution around the fluorene core adequately insulates the TiO₂ surface from the electrolyte, which prevents back-recombination and prolongs the electron lifetime in the semiconductor. Furthermore, compared to analogous dyes, <b>JF419</b> maintains nearly quantitative regeneration efficiency, despite the lower regeneration driving force. The root of this observation is contributed to a significantly more delocalized hole in the photo-oxidized <b>JF419</b>^{• <b>+</b>}, which is highlighted through transient absorption spectroscopy and quantum chemical calculations. The design principles established are relevant to the development of more comprehensive sensitizers, as evidenced by the 10.3% efficiency obtained in cobalt-based liquid dye-sensitized solar cells

Nanocomposites Containing Neutral Blue Emitting Cyclometalated Iridium(III) Emitters for Oxygen Sensing

The behavior toward oxygen sensing of nanocomposites made of the aluminum oxide-hydroxide nanostructured solid support (AP200/19) and neutral blue emitting cyclometalated iridium­(III) complexes was studied. The results are compared with the same dyes immobilized in polystyrene films. Since the photoluminescence of the complexes is totally quenched for oxygen concentrations just over 10%, these systems using the blue region of the visible spectrum are promising for oxygen detection at low concentration. In particular, dyes supported into the AP200/19 provide the best sensitivity to oxygen concentration, with the possibility to detect oxygen below 1% O₂ in gas (0.01 bar)

Nanocomposites Containing Neutral Blue Emitting Cyclometalated Iridium(III) Emitters for Oxygen Sensing

The behavior toward oxygen sensing of nanocomposites made of the aluminum oxide-hydroxide nanostructured solid support (AP200/19) and neutral blue emitting cyclometalated iridium­(III) complexes was studied. The results are compared with the same dyes immobilized in polystyrene films. Since the photoluminescence of the complexes is totally quenched for oxygen concentrations just over 10%, these systems using the blue region of the visible spectrum are promising for oxygen detection at low concentration. In particular, dyes supported into the AP200/19 provide the best sensitivity to oxygen concentration, with the possibility to detect oxygen below 1% O₂ in gas (0.01 bar)