Platinum(II) Complexes of Nonsymmetrical NCN-Coordinating Ligands: Unimolecular and Excimeric Luminescence Properties and Comparison with Symmetrical Analogues

A series of seven new platinum(II) complexes PtLnCl have been prepared, where Ln is an NCN-coordinating ligand comprising a benzene ring 1,3-disubstituted with two different azaheterocycles. In PtL1–5Cl, one heterocycle is a simple pyridine ring, while the other is an isoquinoline, a quinoline, a pyrimidine (L1, L2, L3), or a p-CF3- or p-OMe-substituted pyridine (L4 and L5). PtL6Cl incorporates both a p-CF3 and a p-OMe-substituted pyridine. The synthesis of the requisite proligands HLn is achieved using Pd-catalyzed cross-coupling methodology. The molecular structures of six of the Pt(II) complexes have been determined by X-ray diffraction. All the complexes are brightly luminescent in deoxygenated solution at room temperature. The absorption and emission properties are compared with those of the corresponding symmetrical complexes featuring two identical heterocycles, PtLnsymCl, and of the parent Pt(dpyb)Cl containing two unsubstituted pyridines [dpybH = 1,3-di(2-pyridyl)benzene]. While the absorption spectra of the nonsymmetrical complexes show features of both PtLnsymCl and Pt(dpyb)Cl, the emission generally resembles that of whichever of the corresponding symmetrical complexes has the lower-energy emission. PtL1Cl differs in that─at room temperature but not at 77 K─it displays emission bands that can be attributed to excited states involving both the pyridine and the isoquinoline rings, despite the latter being unequivocally lower in energy. This unusual behavior is attributed to thermally activated repopulation of the former excited state from the latter, facilitated by the very long-lived nature of the isoquinoline-based excited state. At elevated concentrations, all the complexes show an additional red-shifted emission band attributable to excimers. For PtL1Cl, the excimer strikingly dominates the emission spectra at all but the lowest concentrations (<10–5 M). Trends in the energies of the excimers and their propensity to form are compared with those of the symmetrical analogues

Near-infrared electroluminescence beyond 940 nm in Pt(N^C^N)X complexes: influencing aggregation with the ancillary ligand X

We present a study of aggregate excited states formed by complexes of the type Pt(N^C^N)X, where N^C^N represents a tridentate cyclometallating ligand, and X = SCN or I. These materials display near-infrared (NIR) photoluminescence in film and electroluminescence in NIR OLEDs with λmaxEL = 720–944 nm. We demonstrate that the use of X = SCN or I modulates aggregate formation compared to the parent complexes where X = Cl. While the identity of the monodentate ligand affects the energy of Pt–Pt excimers in solution in only a subtle way, it strongly influences aggregation in film. Detailed calculations on aggregates of different sizes support the experimental conclusions from steady-state and time-resolved luminescence studies at variable temperatures. The use of X = I appears to limit aggregation to the formation of dimers, while X = SCN promotes the formation of larger aggregates, such as tetramers and pentamers, leading in turn to NIR photo- and electroluminescence > 850 nm. A possible explanation for the contrasting influence of the monodentate ligands is the lesser steric hindrance associated with the SCN group compared to the bulkier I ligand. By exploiting the propensity of the SCN complexes to form extended aggregates, we have prepared an NIR-emitting OLED that shows very long wavelength electroluminescence, with λmaxEL = 944 nm and a maximum EQE = 0.3 ± 0.1%. Such data appear to be unprecedented for a device relying on a Pt(II) complex aggregate as the emitter

Rigidly linked dinuclear platinum( ii ) complexes showing intense, excimer-like, near-infrared luminescence

Many luminescent platinum(ii) complexes undergo face-to-face interactions between neighbouring molecules, leading to bimolecular excited states that may emit at lower energy (dimers and/or excimers). Detailed photophysical studies are reported on dinuclear complexes, in which two NCN-coordinated Pt(ii) units are covalently linked by a xanthene such that intramolecular formation of such dimeric or excimeric states is possible. These complexes display strong excimer-like photoluminescence at low concentrations where their monometallic analogues do not. However, a striking difference emerges between complexes where the Pt(NCN) units are directly connected to the xanthene through the tridentate ligand (denoted Class a) and a new class of compounds reported here (Class b) in which the attachment is through a monodentate acetylide ligand. The former require a substantial geometrical rearrangement to move the metal centres of the Pt(NCN) units to a distance short enough to form excimer-like states. The latter require only a small deformation. Consequently, Class a compounds display negligible excimer-like emission in solid films, as the rigid environment hinders the requisite geometric rearrangement. Class b complexes, in contrast, display strong excimer-like emission in film, even at very low loadings. The new dinuclear molecular architecture may thus offer new opportunities in the quest for efficient NIR-emitting devices

Electronic Conductance and Thermopower of Cross-Conjugated and Skipped-Conjugated Molecules in Single-Molecule Junctions

We report a combined experimental and theoretical study of a series of thiomethyl (SMe) anchored cross-conjugated molecules featuring an acyclic central bridging ketone and their analogous skipped-conjugated alcohol derivatives. Studies of these molecules in a gold|single-molecule|gold junction using scanning tunneling microscopy-break junction techniques reveal a similar conductance (G) value for both the cross-conjugated molecules and their skipped-conjugated partners. Theoretical studies based on density functional theory of the molecules in their optimum geometries in the junction reveal the reason for this similarity in conductance, as the predicted conductance for the alcohol series of compounds varies more with the tilt angle. Thermopower measurements reveal a higher Seebeck coefficient (S) for the cross-conjugated ketone molecules relative to the alcohol derivatives, with a particularly high S for the biphenyl derivative 3a (−15.6 μV/K), an increase of threefold compared to its alcohol analog. The predicted behavior of the quantum interference (QI) in this series of cross-conjugated molecules is found to be constructive, though the appearance of a destructive QI feature for 3a is due to the degeneracy of the HOMO orbital and may explain the enhancement of the value of S for this molecule

Exploring the Impact of the HOMO–LUMO Gap on Molecular Thermoelectric Properties: A Comparative Study of Conjugated Aromatic, Quinoidal, and Donor–Acceptor Core Systems

Thermoelectric materials have garnered significant interest for their potential to efficiently convert waste heat into electrical energy at room temperature without moving parts or harmful emissions. This study investigated the impact of the HOMO–LUMO (H-L) gap on the thermoelectric properties of three distinct classes of organic compounds: conjugated aromatics (isoindigos (IIGs)), quinoidal molecules (benzodipyrrolidones (BDPs)), and donor–acceptor systems (bis­(pyrrol-2-yl)­squaraines (BPSs)). These compounds were chosen for their structural simplicity and linear π-conjugated conductance paths, which promote high electrical conductance and minimize complications from quantum interference. Single-molecule thermoelectric measurements revealed that despite their low H-L gaps, the Seebeck coefficients of these compounds remain low. The alignment of the frontier orbitals relative to the Fermi energy was found to play a crucial role in determining the Seebeck coefficients, as exemplified by the BDP compounds. Theoretical calculations support these findings and suggest that anchor group selection could further enhance the thermoelectric behavior of these types of molecules

Cognitive and Tactile Factors Affecting Human Haptic Performance in Later Life

Background: Vision and haptics are the key modalities by which humans perceive objects and interact with their environment in a target-oriented manner. Both modalities share higher-order neural resources and the mechanisms required for object exploration. Compared to vision, the understanding of haptic information processing is still rudimentary. Although it is known that haptic performance, similar to many other skills, decreases in old age, the underlying mechanisms are not clear. It is yet to be determined to what extent this decrease is related to the age-related loss of tactile acuity or cognitive capacity. Methodology/Principal Findings: We investigated the haptic performance of 81 older adults by means of a cross-modal object recognition test. Additionally, we assessed the subjects ’ tactile acuity with an apparatus-based two-point discrimination paradigm, and their cognitive performance by means of the non-verbal Raven-Standard-Progressive matrices test. As expected, there was a significant age-related decline in performance on all 3 tests. With the exception of tactile acuity, this decline was found to be more distinct in female subjects. Correlation analyses revealed a strong relationship between haptic and cognitive performance for all subjects. Tactile performance, on the contrary, was only significantly correlated with male subjects ’ haptic performance. Conclusions: Haptic object recognition is a demanding task in old age, especially when it comes to the exploration o

The Edinburgh Type 2 Diabetes Study:study protocol

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Exploratory Study of Executive Function Abilities Across the Adult Lifespan in Individuals Receiving an ASD Diagnosis in Adulthood

The few studies of autism spectrum disorder (ASD) across adulthood suggest different age-related associations in different aspects of executive function (EF). In this exploratory study we examined EF abilities and self-report autism traits in 134 adults (aged 18-75 years; mean=31 years) with abilities in the normal range, receiving a first diagnosis of ASD. Results suggest that in some EF relying on speed and sequencing (Trails A and B; Digit Symbol), late-diagnosed ASD individuals may demonstrate better performance than typical age-norms. On other EF (Digit Span, Hayling, Brixton tests) age-related correlations were similar to typical age-norms. Different domains of EF may demonstrate different trajectories for ageing with ASD, with patterns of slower, accelerated or equivalent age-related change observed across different measures

Exploratory Study of Executive Function Abilities Across the Adult Lifespan in Individuals Receiving an ASD Diagnosis in Adulthood

The few studies of autism spectrum disorder (ASD) across adulthood suggest different age-related associations in different aspects of executive function (EF). In this exploratory study we examined EF abilities and self-report autism traits in 134 adults (aged 18-75 years; mean=31 years) with abilities in the normal range, receiving a first diagnosis of ASD. Results suggest that in some EF relying on speed and sequencing (Trails A and B; Digit Symbol), late-diagnosed ASD individuals may demonstrate better performance than typical age-norms. On other EF (Digit Span, Hayling, Brixton tests) age-related correlations were similar to typical age-norms. Different domains of EF may demonstrate different trajectories for ageing with ASD, with patterns of slower, accelerated or equivalent age-related change observed across different measures

Excimer or aggregate? Near infrared electro- and photoluminescence from multimolecular excited states of N^C^N-coordinated platinum(ii) complexes

We present an experimental and theoretical study of aggregate excited states formed by complexes of the type Pt(N^C^N)Cl, which display near-infrared (NIR) photoluminescence in film and NIR electroluminescence in OLED devices. Here, N^C^N represents a tridentate cyclometallating ligand binding through a central benzene ring and two lateral N-heterocycles. A simple theoretical model for the bimolecular excited states of these complexes accounts for the observed behaviour in terms of metal–metal-to-ligand charge-transfer (MMLCT). Detailed photophysical study provides insight into the kinetic properties of Pt–Pt excimers in solution, in films processed from solution, and in vacuum-deposited films. A clear distinction between emission from dimers and higher oligomers (e.g., trimers and tetramers) is evident. We also demonstrate an alternative approach to the architecture of OLEDs based on aggregate states of Pt(II) complexes. An emissive layer (EML) as thin as 1 to 2 nm can be employed, without compromising the efficiency, while reducing the use of the precious-metal compound by >90% compared to typical devices using EMLs of ∼20–40 nm thickness. For example, OLEDs obtained with an EML of 2 nm thickness display an EQE of 1.2% with λEL = 817 nm and a maximum radiosity of 1.46 mW cm−2. As for the photoluminescence of films, the longest-wavelength electroluminescence from devices employing pristine films of emitter is also attributed to trimers and tetramers