Ultrafast Intersystem Crossing and Structural Dynamics of [Pt(ppy)(µ-tBu2pz)]2.

Intersystem crossing (ISC) rates of transition-metal complexes are determined by the complex interplay of a molecule's electronic and structural dynamics. To broaden our understanding of these key factors, we investigate the case of the prototypical d8-d8 dimetal complex [Pt(ppy)(µ-tBu2pz)]2 using broad-band transient absorption anisotropy in combination with ultrafast fluorescence up-conversion and ab initio calculations. We find that, upon excitation of the molecule's metal-metal-to-ligand charge-transfer transition, ISC occurs in hundreds of femtoseconds from the lowest excited singlet state S1 to the triplet state T2, from where the energy relaxes to the lowest energy triplet state T1. ISC to the T2 state, rather than T1, is further rationalized through supporting arguments. Observed vibrational coherences along the Pt-Pt mode are attributed to the formation of nuclear wavepackets on the ground and excited electronic states that dephase prior to ISC because of the structural flexibility of the complex. Beyond demonstrating the relationship between the energy relaxation and structural dynamics of [Pt(ppy)(µ-tBu2pz)]2, our results provide new insights into the photoinduced dynamics of d8-d8 dimetal complexes more generally.

Tris-heteroleptic Iridium Complexes Based on Cyclometalated Ligands with Different Cores.

A series of tris-heteroleptic iridium complexes of the form [Ir(C^N1)(C^N2)(acac)] combining 2-phenylpyridine (ppy), 2-(2,4-difluorophenyl)pyridine (dFppy), 1-phenylpyrazole (ppz), and 1-(2,4-difluorophenyl)pyrazole (dFppz) as the C^N ligands have been synthesized and fully characterized by NMR, X-ray crystallography, UV-vis absorption and emission spectroscopy, and electrochemical methods. It is shown that "static properties" (e.g., absorption and emission spectra and redox potentials) are primarily dictated by the overall architecture of the complex, while "dynamic properties" (e.g., excited-state lifetime and radiative and nonradiative rate constants) are, in addition, sensitive to the specific positioning of the substituents. As a result, the two complexes [Ir(dFppy)(ppz)(acac)] and [Ir(ppy)(dFppz)(acac)] have the same emission maxima and redox potentials, but their radiative and nonradiative rate constants differ significantly by a factor ∼2. Then acetylacetonate (acac) was replaced by picolinate (pic), and two pairs of diastereoisomers were obtained. As expected, the use of pic as the ancillary ligand results in blue-shifted emission, stabilization of the oxidation potential, and improvement of the photoluminescence quantum yield, and only minor differences in the optoelectronic properties are found between the two diastereoisomers of each pair.

Synthesis and Characterization of a Series of Bis-homoleptic Cycloruthenates with Terdentate Ligands as a Family of Panchromatic Dyes.

A series of six homoleptic bis-cyclometalated ruthenium complexes, Ru(N^N^C)2, is reported where N^N^C is a 6-(2,4-difluoro-3-R3-phenyl)-4-R2-4'-R1-2,2'-bipyridine with R3 = -H or -CF3 and R2 and R1 = -COOEt or -CF3. An effective synthesis of the ligands and the complexes is described. The UV-visible absorption studies demonstrate that these complexes are panchromatic dyes absorbing up to 900 nm. Importantly, the onset of absorption depends only on the substitution on the metalated phenyl, whereas the intensity of absorption throughout the spectra is a function of substituents on both the phenyl and the bipyridine moieties. The same trend is observed in electrochemistry as the redox gap depends only on the substitution on the metalated phenyl, whereas the oxidation and reduction potentials are a function of substituents on both the phenyl and the bipyridine moieties. Preliminary tests as sensitizer for dye-sensitized solar cells demonstrate that the number of anchoring groups on the dye has a major influence on the device efficiency.

High performance optical oxygen sensors based on iridium complexes exhibiting interchromophore energy shuttling.

A doubly pyrene-grafted bis-cyclometallated iridium complex with engineered electronically excited states demonstrates reversible electronic energy transfer between adjacent chromophores giving rise to extremely long-lived red luminescence in solution (τ = 480 µs). Time-resolved spectroscopic studies afforded determination of pertinent photophysical parameters including rates of energy transfer and energy distribution between constituent chromophores in the equilibrated excited molecule (ca. 98% on the organic chromophores). Incorporation into a nanostructured metal-oxide matrix (AP200/19) gave highly sensitive O2 sensing films, as the detection sensitivity was 200-300% higher than with the commonly used PtTFPP and approaches the sensitivity of the best O2-sensing dyes reported to date.

Blue and Green Phosphorescent Liquid-Crystalline Iridium Complexes with High Hole Mobility.

Blue- and green-emitting cyclometalated liquid-crystalline iridium complexes are realized by using a modular strategy based on strongly mesogenic groups attached to an acetylacetonate ancillary ligand. The cyclometalated ligand dictates the photophysical properties of the materials, which are identical to those of the parent complexes. High hole mobilities, up to 0.004 cm(2) V(-1) s(-1), were achieved after thermal annealing, while amorphous materials show hole mobilities of only approximately 10(-7) -10(-6) cm(2) V(-1) s(-1), similar to simple iridium complexes. The design strategy allows the facile preparation of phosphorescent liquid-crystalline complexes with fine-tuned photophysical properties.

UV-visible Absorption Study of the Self-association of Non-ionic Chromonic Triphenylenes TP6EOnM (n = 2, 3, 4) in Dilute Aqueous Solutions: Impact of Chain Length on Aggregation.

A series of triphenylenes with oligoethoxy chains of various length, TP6EOnM with n = 2, 3, 4, has been synthesised and purified by HPLC. The self-association of these disc-shaped molecules in dilute aqueous solutions (∼10(-7) to ∼4 × 10(-4) M) has been studied by UV-visible absorption spectroscopy. The free energy of association decreases as the length of the chains increases. As a result, for a given concentration, the average size of aggregate diminishes as the chain length increases. While the absorption properties of the monomer are identical for the three molecules, the extinction coefficients of solutions of the three triphenylenes at a given concentration are significantly different and are directly linked to the average size of the aggregates. The change of epsilon values upon aggregation could explain the trend generally observed with dyes for solar cells substituted with chains of increasing length showing increasing extinction coefficient values.

Ligand-centred fluorescence and electronic relaxation cascade at vibrational time scales in transition-metal complexes.

Using femtosecond-resolved photoluminescence up-conversion, we report the observation of the fluorescence of the high-lying ligand-centered (LC) electronic state upon 266 nm excitation of an iridium complex, Ir(ppy)3, with a lifetime of 70 ± 10 fs. It is accompanied by a simultaneous emission of all lower-lying electronic states, except the lowest triplet metal-to-ligand charge-transfer ((3)MLCT) state that shows a rise on the same time scale. Thus, we observe the departure, the intermediate steps, and the arrival of the relaxation cascade spanning ∼1.6 eV from the (1)LC state to the lowest (3)MLCT state, which then yields the long-lived luminescence of the molecule. This represents the first measurement of the total relaxation time over an entire cascade of electronic states in a polyatomic molecule. We find that the relaxation cascade proceeds in ≤10 fs, which is faster than some of the highest-frequency modes of the system. We invoke the participation of the latter modes in conical intersections and their overdamping to low-frequency intramolecular modes. On the basis of literature, we also conclude that this behavior is not specific to transition-metal complexes but also applies to organic molecules.

Copper(I) complexes as alternatives to iridium(III) complexes for highly efficient oxygen sensing.

The complex [Cu(xantphos)(dmp)][PF6] (dmp = 2,9-dimethyl-1,10-phenanthroline) in a nanostructured metal oxyde matrix shows better sensitivity to oxygen (KSV = 9.74 ± 0.87 kPa(-1) between 0 and 1 kPa pO2 and 5.59 ± 0.15 kPa(-1) between 0 and 10 kPa pO2) than cyclometallated iridium complexes in the same conditions.

FIrpic: archetypal blue phosphorescent emitter for electroluminescence.

FIrpic is the most investigated bis-cyclometallated iridium complex in particular in the context of organic light emitting diodes (OLEDs) because of its attractive sky-blue emission, high emission efficiency, and suitable energy levels. In this Perspective we review the synthesis, structural characterisations, and key properties of this emitter. We also survey the theoretical studies and summarise a series of selected monochromatic electroluminescent devices using FIrpic as the emitting dopant. Finally we highlight important shortcomings of FIrpic as an emitter for OLEDs. Despite the large body of work dedicated to this material, it is manifest that the understanding of photophysical and electrochemical processes are only broadly understood mainly because of the different environment in which these properties are measured, i.e., isolated molecules in solvent vs. device.

Direct observation of reversible electronic energy transfer involving an iridium center.

A cyclometalated iridium complex is reported where the core complex comprises naphthylpyridine as the main ligand and the ancillary 2,2'-bipyridine ligand is attached to a pyrene unit by a short alkyl bridge. To obtain the complex with satisfactory purity, it was necessary to modify the standard synthesis (direct reaction of the ancillary ligand with the chloro-bridged iridium dimer) to a method harnessing an intermediate tetramethylheptanolate-based complex, which was subjected to acid-promoted removal of the ancillary ligand and subsequent complexation. The photophysical behavior of the bichromophoric complex and a model complex without the pendant pyrene were studied using steady-state and time-resolved spectroscopies. Reversible electronic energy transfer (REET) is demonstrated, uniquely with an emissive cyclometalated iridium center and an adjacent organic chromophore. After excited-state equilibration is established (5 ns) as a result of REET, extremely long luminescence lifetimes of up to 225 µs result, compared to 8.3 µs for the model complex, without diminishing the emission quantum yield. As a result, remarkably high oxygen sensitivity is observed in both solution and polymeric matrices.

Structure-property relationships based on Hammett constants in cyclometalated iridium(III) complexes: their application to the design of a fluorine-free FIrPic-like emitter.

While phosphorescent cyclometalated iridium(iii) complexes have been widely studied, only correlations between oxidation potential EOX and Hammett constant σ, and between the redox gap (ΔEREDOX = EOX-ERED) and emission or absorption wavelength (λabs, λem) have been reported. We present now a quantitative model based on Hammett parameters that rationalizes the effect of the substituents on the properties of cyclometalated iridium(iii) complexes. This simple model allows predicting the apparent redox potentials as well as the electrochemical gap of homoleptic complexes based on phenylpyridine ligands with good accuracy. In particular, the model accounts for the unequal effect of the substituents on both the HOMO and the LUMO energy levels. Consequently, the model is used to anticipate the emission maxima of the corresponding complexes with improved reliability. We demonstrate in a series of phenylpyridine emitters that electron-donating groups can effectively replace electron-withdrawing substituents on the orthometallated phenyl to induce a blue shift of the emission. This result is in contrast with the common approach that uses fluorine to blue shift the emission maximum. Finally, as a proof of concept, we used electron-donating substituents to design a new fluorine-free complex, referred to as EB343, matching the various properties, namely oxidation and reduction potentials, electrochemical gap and emission profile, of the standard sky-blue emitter FIrPic.

Charged bis-cyclometalated iridium(III) complexes with carbene-based ancillary ligands.

Charged cyclometalated (C(^)N) iridium(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C(^)N ligands, in contrast to the classical design of charged complexes where N(^)N ancillary ligands with low-energy π* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the C(^)N ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One bis-carbene (:C(^)C:) and four different pyridine-carbene (N(^)C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C(^)C: complexes show much larger photoluminescence quantum yields (ΦPL) of ca. 30%, compared to the N(^)C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly(methyl methacrylate) matrix, ΦPL 30-60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N(^)C:-type complexes possess a low-lying triplet metal-centered ((3)MC) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C(^)C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N(^)C: and :C(^)C: complexes.

High performance optical sensing nanocomposites for low and ultra-low oxygen concentrations using phase-shift measurements.

The accurate and real-time measurement of low and ultra-low concentrations of oxygen using non-invasive methods is a necessity for a multitude of applications, from brewing beer to developing encapsulating barriers for optoelectronic devices. Current optical methods and sensing materials often lack the necessary sensitivity, signal intensity, or stability for practical applications. In this report we present a new optical sensing nanocomposite resulting in an outstanding overall performance when combined with the phase-shift measurement method (determination of luminescence lifetime in the frequency domain). For the first time we have incorporated the standard PtTFPP dye (PtTFPP = platinum(II) 5,10,15,20-meso-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin) into AP200/19, a nanostructured aluminium oxide-hydroxide solid support. This sensing film shows an excellent sensitivity between 0 and 1% O2 (KSV = 3102 ± 132 bar⁻¹) and between 0 and 10% O2 (KSV = 2568 ± 614 bar⁻¹) as well as Δτ0.05% (62.53 ± 3.66%), which makes it 62 times more sensitive than PtTFPP immobilized in polystyrene and also 8 times more sensitive than PtTFPP immobilized on silica beads. Furthermore the phase-shift measurement method results in a significant improvement (about 23 times) in stability compared to the use of intensity recording methods. The film also displays full reversibility, long shelf stability (no change observed after 12 months), and it is not affected by humidity. To establish this sensing methodology and develop sensors over the full range of the visible light, we also studied three other dye-AP200/19 nanocomposites based on phosphorescent cyclometalated iridium(III) complexes.

Tuning the photophysical properties of cationic iridium(III) complexes containing cyclometallated 1-(2,4-difluorophenyl)-1H-pyrazole through functionalized 2,2'-bipyridine ligands: blue but not blue enough.

Four new heteroleptic iridium(III) complexes in the family [Ir(dfppz)(2)(N^N)](+), where Hdfppz = 1-(2,4-difluorophenyl)-1H-pyrazole and N^N = 6-phenyl-2,2'-bipyridine (1), 4,4'-(di-tert-butyl)-6-phenyl-2,2'-bipyridine (2), 4,4'-(di-tert-butyl)-6,6'-diphenyl-2,2'-bipyridine (3) and 4,4'-bis(dimethylamino)-2,2'-bipyridine (4), have been synthesized as the hexafluoridophosphate salts and fully characterized. Single crystal structures of ligand 3 and the precursor [Ir(2)(dfppz)(4)(µ-Cl)(2)] have been determined, along with the structures of the complexes 4{[Ir(dfppz)(2)(1)][PF(6)]}·3CH(2)Cl(2), [Ir(dfppz)(2)(3)][PF(6)]·CH(2)Cl(2) and [Ir(dfppz)(2)(4)][PF(6)]·CH(2)Cl(2). The role of inter- and intramolecular face-to-face π-stacking in the solid state is discussed. In the [Ir(dfppz)(2)(N^N)](+) (N^N = 1-3) cations, the phenyl substituent in ligands 1, 2 or 3 undergoes hindered rotation on the NMR timescale at 298 K in solution and the systems have been studied by variable temperature NMR spectroscopy. Acetonitrile solutions of [Ir(dfppz)(2)(N^N)][PF(6)] (N^N = 1-3) exhibit similar absorption spectra arising from ligand-based transitions; absorption intensity is enhanced on going to [Ir(dfppz)(2)(4)][PF(6)] and the spectrum extends further into the visible region. Acetonitrile solutions of the complexes are blue emitters with λ(em) = 517, 505, 501 and 493 nm for N^N = 1, 2, 3 and 4, respectively (λ(exc) = 280-310 nm). The redox behaviours of [Ir(dfppz)(2)(N^N)][PF(6)] (N^N = 1-3) are similar, and the introduction of the electron-donating NMe(2) substituents onto the N^N ligand shifts the metal-centred oxidation to less positive potentials. Theoretical calculations predict a mixed metal-to-ligand/ligand-to-ligand charge transfer (MLCT/LLCT) character for the emitting triplet state in agreement with the broad and unstructured character of the emission bands. The NMe(2) substituents enlarge the HOMO-LUMO gap and blue-shifts the emission of [Ir(dfppz)(2)(4)](+) that is centred on the ancillary ligand. These complexes, when processed into a thin film and sandwiched between two electrodes, lead to very low voltage operating electroluminescent devices. No additional components are needed, which demonstrates their electron and hole transport abilities in conjunction with the luminescent properties.

Subnanometer Ga2O3 tunnelling layer by atomic layer deposition to achieve 1.1 V open-circuit potential in dye-sensitized solar cells.

Herein, we present the first use of a gallium oxide tunnelling layer to significantly reduce electron recombination in dye-sensitized solar cells (DSC). The subnanometer coating is achieved using atomic layer deposition (ALD) and leading to a new DSC record open-circuit potential of 1.1 V with state-of-the-art organic D-π-A sensitizer and cobalt redox mediator. After ALD of only a few angstroms of Ga(2)O(3), the electron back reaction is reduced by more than an order of magnitude, while charge collection efficiency and fill factor are increased by 30% and 15%, respectively. The photogenerated exciton separation processes of electron injection into the TiO(2) conduction band and the hole injection into the electrolyte are characterized in detail.

A new generation of platinum and iodine free efficient dye-sensitized solar cells.

We report a series of cobalt complexes with various polypyridyl ligands, where the oxidation potential is tuned from 0.17 to 0.34 V vs. ferrocene. The highest occupied molecular orbitals (HOMO) of the cobalt complexes were stabilized by adding electron acceptor groups on pyridyl or replacing pyridyl by pyrazole. These complexes are then used as one-electron redox mediators in dye sensitized solar cells (DSSCs) together with polymer based cathode resulting in an excellent performance. The performance of DSSCs using the molecularly engineered cobalt redox shuttle and poly(3,4-alkylthiophenes) based cathode is better than the triiodide/iodide redox shuttle with platinized cathode. The use of high surface area poly(3,4-propylenedioxythiophene) based nanoporous layers allows higher catalytic activity thus minimizing the electrode-electrolyte interface issues.

A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials.

Dye-sensitized solar cells are a promising alternative to traditional inorganic semiconductor-based solar cells. Here we report an open-circuit voltage of over 1,000 mV in mesoscopic dye-sensitized solar cells incorporating a molecularly engineered cobalt complex as redox mediator. Cobalt complexes have negligible absorption in the visible region of the solar spectrum, and their redox properties can be tuned in a controlled fashion by selecting suitable donor/acceptor substituents on the ligand. This approach offers an attractive alternate to the traditional I(3)(-)/I(-) redox shuttle used in dye-sensitized solar cells. A cobalt complex using tridendate ligands [Co(bpy-pz)(2)](3+/2+)(PF(6))(3/2) as redox mediator in combination with a cyclopentadithiophene-bridged donor-acceptor dye (Y123), adsorbed on TiO(2), yielded a power conversion efficiency of over 10% at 100 mW cm(-2). This result indicates that the molecularly engineered cobalt redox shuttle is a legitimate alternative to the commonly used I(3)(-)/I(-) redox shuttle.

Bright blue phosphorescence from cationic bis-cyclometalated iridium(III) isocyanide complexes.

We report new bis-cyclometalated cationic iridium(III) complexes [(C(^)N)(2)Ir(CN-tert-Bu)(2)](CF(3)SO(3)) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4'-fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or5-trifluoromethyl) as C(^)N ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58-3.83 V. They emit blue or blue-green phosphorescence in liquid/solid solutions from a cyclometalating-ligand-centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440-459 nm. The corresponding quantum yields and observed excited-state lifetimes are up to 76% and 46 µs, respectively, and the calculated radiative lifetimes are in the range of 46-82 µs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.

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) HIrPic, FIrPic, ClIrPic, and BrIrPic 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 FIrPic and ClIrPic, the impact of the large bromine atom remarkably decreases the photoluminescence quantum yield of BrIrPic 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 ClIrPic and BrIrPic, which are likely to play a role in the emissive and nonradiative properties when coupled with the heavy-atom effect.