Relationship between Metallophilic Interactions and Luminescent Properties in Pt(II) Complexes: TD-DFT Guide for the Molecular Design of Light-Responsive Materials

DFT/TD-DFT investigation has been performed on pyridyl triazolatoplatinum­(II) complexes with a systematic variation of the donor/acceptor properties of the ligand in order to illuminate its effect on the metallophilic intermolecular interaction in ground and excited states. The π-electronic properties of the pyridyl triazolate ligand were modified by the pyridine substituent: −N­(CH₃)₂, −H, −CHO, or −CHC­(CN)₂. The simulations reveal that the donor/acceptor strength of the substituent has a strong impact on the metallophilic interaction in the excited state and affects the emission properties at the supramolecular level. The theoretically derived structure–property relationships are corroborated by experimental data. Finally, it is proposed that the modification of the π-electronic character of the substituent (ligand field) can be applied in the molecular design of smart luminescent materials with light-driven metallophilic interactions

Phase Change Transformations with Dynamically Addressable Aminal Metallogels

Dynamic polymers assembled through hemiaminal and aminal functionalities reversibly fragment upon binding to trivalent metals. Gels produced with these dynamic polymers are broken down to liquids after the addition of metal salts. Nuclear magnetic resonance spectroscopy studies and density functional theory calculations of intermediates reveal that the presence of these metals causes shifts in the energetic landscape of the intermediates in the condensation pathway to render stable nonequilibrium products. These species remain stable in the liquid phase at room temperature but convert to gels upon heating. With thermal activation, the fragmented ligands transform catalytically into closed-ring hexahydrotriazine products, which are macroscopically observable as new gels with distinct physical properties. The interplay between equilibrium and nonequilibrium gels and liquids and the ligands responsible for these transformations has been observed rheologically, giving controlled gel times dictated by the thermodynamics and kinetics of the system. This constitutionally dynamic macromolecular system offers the possibility of harnessing an equilibrium/nonequilibrium system in tandem with its inherent self-healing properties and triggered release functionality

Molecular Design of pH-Sensitive Ru(II)–Polypyridyl Luminophores

Three new [Ru(bpy)2X]+ complex ions, where bpy represents bipyridyl ligand and X denotes pyridyl diazolate or pyrazinyl diazolate coordination site, have been computationally designed and synthesized as pH-sensitive molecules. The choice of pyridyl and pyrazinyl moieties allows for the nitrogen content to vary, whereas the influence of the protonation site is quantified by using 1,2-diazolate and 1,3-diazolate derivatives. The absorption and emission properties of the deprotonated and protonated complex ions were characterized by UV–vis and photoluminescence spectroscopy as well as by time-dependent density functional theory. Protonation causes (1) a strong blue shift in the lowest energy 3MLCT → S0 emission wavelengths, (2) a substantial increase in the emission intensity, and (3) a change in the character of the corresponding 3MLCT emitting states. The blue shift in the emission wavelength becomes less pronounced when the nitrogen content in the X-ligand increases and when going from 1,2- to 1,3-diazolate derivatives. The contrast in the emission intensity of the protonated/deprotonated forms is the highest for the complex ion, containing a 2-pyridyl derivative of the 1,2-diazolate. The complex ions are suggested as potential pH-responsive materials based on change in the color and intensity of the emitted radiation. The broad impact of the research demonstrates that the modification of the nitrogen content and position within the protonable ligands is an effective approach for modulation of the pH-optosensing properties of Ru–polypyridyl complexes

Theoretical and Structural Analysis of Long C−C Bonds in the Adducts of Polycyanoethylene and Anthracene Derivatives and Their Connection to the Reversibility of Diels–Alder Reactions

X-ray structure determinations on four Diels–Alder adducts derived from the reactions of cyano- and ester-substituted alkenes with anthracene and 9,10-dimethylanthracene have shown the bonds formed in the adduction to be particularly long. Their lengths range from 1.58 to 1.62 Å, some of the longest known for Diels–Alder adducts. Formation of the four adducts is detectably reversible at ambient temperature and is associated with free energies of reaction ranging from −2.5 to −40.6 kJ mol−1. The solution equilibria have been experimentally characterised by NMR spectroscopy. Density-functional-theory calculations at the MPW1K/6-31+G(d,p) level with PCM solvation agree with experiment with average errors of 6 kJ mol−1 in free energies of reaction and structural agreement in adduct bond lengths of 0.013 Å. To understand more fully the cause of the reversibility and its relationship to the long adduct bond lengths, natural-bond-orbital (NBO) analysis was applied to quantify donor–acceptor interactions within the molecules. Both electron donation into the σ*-anti-bonding orbital of the adduct bond and electron withdrawal from the σ-bonding orbital are found to be responsible for this bond elongation.

Engineering Tunable Single and Dual Optical Emission from Ru(II)–Polypyridyl Complexes through Excited State Design

Excited state design is an efficient approach toward new applications in molecular electronics spanning solar cells, artificial photosynthesis and biomedical diagnostics. Ruthenium­(II)–polypiridyl based complexes are an example of molecular building blocks with tunable single and dual wavelength emission that can be controlled by excited state engineering via selective ligand modification. Here we investigate three new heteroleptic [Ru­(bpy)₂X]⁺ complex ions, where X represents pyridinyl or pyrazinyl derivatives of diazolates, providing tunable emission in the visible and infrared region. The dual emission is shown to arise from the presence of two excited states consisting of a triplet metal-to-ligand charge transfer state localized on a bipyridine ligand, ³MLCT (bpy), and a state that either is entirely localized on the X ligand or is partially delocalized also spanning part of the bipyridine ligands, ³MLCT­(X). By a suitable choice of the X ligand, emission from ³MLCT­(bpy) and ³MLCT (X) states can be rationally varied between 743 and 865 nm and from 555 to 679 nm, respectively. An increase in the nitrogen content of the six-membered ring of the X ligand results in a blue shift of the ³MLCT­(bpy) emission but a red shift for the ³MLCT (X) emission. The wavelength difference between ³MLCT­(bpy) and ³MLCT (X) emissions can be tuned from 84 to 310 nm and is proportional to the difference in LUMO energies (reduction potentials) of the isolated ligands. Our study provides key information toward new routes for the design of optically active dual wavelength molecular emitters

CCDC 950609: Experimental Crystal Structure Determination

Related Article: A. K. H. Hirsch, P. Reutenauer, M. Le Moignan, S. Ulrich, P. J. Boul, J. M. Harrowfield, P. D. Jarowski, J.-M. Lehn|2014|Chem.-Eur.J.|20|1073|doi:10.1002/chem.201303276,An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.