Rajarshi Mondal

Carbodicarbenes (CDCs), featuring a formally zero‐valent carbon atom coordinated by two N‐heterocyclic carbene (NHC) units, are exceptionally strong σ‐donor ligands in molecular chemistry, yet their ability to functionalize and electronically modulate solid surfaces has remained unexplored. Here, we introduce CDCs as a new class of molecular surface ligands for nanoscale interface engineering and demonstrate their assembly into robust monolayers on Au(111). Scanning tunneling microscopy (STM) imaging, supported by simulations, resolve the nanoscale organization and adsorption geometry of the CDC monolayers, showing that CDCs adsorb predominantly in a tilted configuration and transform upon annealing into a flat‐lying geometry without long‐range surface diffusion. This behavior is consistent with strong ligand‐surface coupling and a high degree of interfacial stability. X‐ray photoelectron spectroscopy (XPS) and polarized near‐edge X‐ray absorption fine structure (NEXAFS) measurements further confirm the formation of chemically anchored, thermally stable CDC monolayers and the thermally‐induced shift in their adsorption geometry. Ultraviolet photoelectron spectroscopy (UPS) reveals that CDC adsorption decreases the Au work function by up to 1 eV, demonstrating substantial electronic modulation of the metal surface. These experimental findings, supported by Density functional theory (DFT) calculations, establish CDC as a surface‐ ligand for molecularly controlled work‐function modulation and interface engineering.

The ability to manipulate excited-state decay cascades using molecular structure is essential to the application of abundant-metal photosensitizers and chromophores. Ligand design has yielded some spectacular results elongating charge-transfer excited state lifetimes of Fe(II) coordination complexes, but triplet metal-centered (³MC) excited states─recently demonstrated to be critical to the photoactivity of isoelectronic Co(III) polypyridyls─have to date remained elusive, with temporally isolable examples limited to the picosecond regime. With this report, we show how strong-field donors and intramolecular π-stacking can conspire to stabilize a long-lived ³MC excited state for a remarkable 4.1 ± 0.3 ns in fluid solution at ambient temperature. Analysis of variable-temperature time-resolved absorption data using theoretical models ranging from Arrhenius to semiclassical Marcus theory, combined with computational modeling and X-ray crystallography, reveal a Jahn-Teller stabilized excited state with a high activation barrier for ground-state recovery. The net result is a chromophore with a ³MC excited-state lifetime that is orders of magnitude longer than anything yet observed for an Fe(II) complex.

Fluorinated self-assembled monolayers (SAMs) have been utilized in a variety of applications such as transistors and optoelectronic devices. However, in most SAMs the fluorinated groups could not be positioned in high proximity to the surface due to steric effects. This limitation hinders the direct analysis of the impact of the fluorination level on surface properties. Herein, fluorinated aromatic N-heterocyclic carbenes (NHCs), with 1-5 fluorine atoms, were self-assembled on a gold substrate. These NHCs enabled the positioning of fluorinated groups in high proximity to the metal surface to identify the influence of the fluorination level on surface properties. Experimental measurements and theoretical calculations identified that all fluorinated NHCs formed SAMs and adopted a flat-lying adsorption configuration while anchored to the metal surface via Au adatom. A higher fluorination level induced a stronger interaction of the fluorinated side groups with the Au surface. The stronger interaction and surface proximity of the fluorinated side groups deteriorated the overall binding energy of the NHC due to the less-optimized adsorption geometry of the carbene carbon. Ultraviolet photoelectron spectroscopy measurements revealed that fluorinated NHC monolayers lowered the surface work function by up to 1 eV and induced an increase of 15-20 degrees in the water contact angle. The impact on surface properties did not vary according to the fluorination level of NHCs, and similar values were measured for NHC with 1-5 fluorine atoms. It is therefore identified that dominant adsorbate-substrate interactions between the fluorinated side groups and the Au surface quenched the distinct impact of the fluorination level on surface functionality.

The incorporation of organic self‐assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self‐assembly process. In this work, selective deposition of N‐heterocyclic carbenes (NHCs) on specific electrodes within a two‐microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC‐coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes’ work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

A series of Fe and Ru coordination complexes supported by bidentate P^N ligands bearing π-extended phenanthridine (3,4-benzoquinoline; ^PhenL^R,R') or quinoline (^QuinL^H,Ph) N-heterocycles have been synthesized and fully characterized. Tetrahedral monomeric (P^N)FeBr₂ and trigonal bipyramidal dimeric ((P^N)FeBr)₂(μ-Br)₂ complexes were found to be high-spin paramagnets as solids (magnetic susceptibility balance measurements) and in solution (Evans method), while tris-ligated octahedral [(P^N)₃M]²⁺ cations (M = Fe, Ru) are low-spin diamagnets. The electronic environment about each Fe centre was further examined by Mössbauer spectroscopy. The impact of π-extension is evident when comparing the lowest energy absorption bands in the UV-Vis spectra. Contrary to conventional assumptions regarding π-extension, a small hypsochromic shift was observed for the more π-extended phenanthridinyl phosphine-supported Fe(II) complex compared to the analogous quinolinyl complex. Density functional theory (DFT), time-dependent DFT (TDDFT) and charge decomposition analysis (CDA) revealed benzannulation most significantly impacts the character of the lowest energy excitation, rather than appreciably affecting the energies of the frontier orbitals. The impact of electron-transfer on the electronic absorption properties of the π-extended pseudo-octahedral Fe complexes was further explored by spectroelectrochemistry.

The structural and photophysical properties of three sets of luminescent copper complexes of the form [(P^N)₂Cu]X are presented. Here, P^N represents a bidentate ligand based on phenanthridine (3,4-benzoquinoline) incorporating a phosphine unit at the 4-position, of which three examples are investigated, namely 4-(diphenylphosphino)phenanthridine (L1), 4-(diphenylphosphino)-2-methylphenanthridine (L2) and 2,6-dimethyl-4-(diphenylphosphino)phenanthridine (L3). For each P^N-coordinating ligand, the corresponding homoleptic copper(I) complex (1X, 2X, 3X) has been prepared as both the hexafluorophosphate and tetraphenylborate salt (X = PF₆ ⁻ or BPh₄ ⁻). The identity of the counterion has a profound and unexpected impact on the emission properties of the powder samples – but only for complexes of ligands bearing methyl substituents close to the metal (3X). The synergistic effect of combining inter-ion interactions and ligand design enables emission tuning from orange to yellow, in the opposite (hypsochromic) direction compared to employing either strategy on its own. These effects can be attributed to differences in molecular packing, in particular to the combined impact of ligand structure and inter-ionic interactions on distortions in the excited state relative to the ground state. The results have been interpreted with the help of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

Benzannulated bidentate pyridine/phosphine (P^N) ligands bearing quinoline or phenanthridine (3,4-benzoquinoline) units have been prepared, along with their halide-bridged, dimeric Cu(I) complexes of the form [(P^N)Cu]₂(μ-X)₂. The copper complexes are phosphorescent in the orange-red region of the spectrum in the solid-state under ambient conditions. Structural characterization in solution and the solid-state reveals a flexible conformational landscape, with both diamond-like and butterfly motifs available to the Cu₂X₂ cores. Comparing the photophysical properties of complexes of (quinolinyl)phosphine ligands with those of π-extended (phenanthridinyl)phosphines has revealed a counterintuitive impact of site-selective benzannulation. Contrary to conventional assumptions regarding π-extension and a bathochromic shift in the lowest energy absorption maxima, a blue shift of nearly 40 nm in the emission wavelength is observed for the complexes with larger ligand π-systems, which is assigned as phosphorescence on the basis of emission energies and lifetimes. Comparison of the ground-state and triplet excited state structures optimized from DFT and TD-DFT calculations allows attribution of this effect to a greater rigidity for the benzannulated complexes resulting in a higher energy emissive triplet state, rather than significant perturbation of orbital energies. This study reveals that ligand structure can impact photophysical properties for emissive molecules by influencing their structural rigidity, in addition to their electronic structure.

Five new mixed ligand coordination polymers, {[Co_0.5(H₂O)(1,4-bdc)(3-bpdb)Co_0.5(H₂O)₂]·(H₂O)₃}_n (1), [Co(3-bpdh)(1,4-bdc)(H₂O)₂]_n (2), [Zn(3-bpdb)(1,4-bdc)]_n (3), {[Zn(3-bpdh)(1,4-bdc)]·(3-bpdh)_0.5}_n (4) and [Cd(3-bpdb)(1,4-bdc)(H₂O)]_n (5) [where 3-bpdb = 1,4-bis-(3-pyridyl)-2,3-diaza-1,3-butadiene; 3-bpdh = 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene and 1,4-bdc = benzene-1,4-dicarboxylate] were synthesized at room temperature. The structures of 1–5 were determined by single crystal X-ray diffraction analysis and were further characterized by elemental analysis, infrared spectroscopy (IR) and powder X-ray diffraction (PXRD). Compound 1 exhibits a one-dimensional (1D) chain structure with monodentate pendant 1,4-bdc ligands which further extend to a three-dimensional (3D) supramolecular structure by H-bonding and π⋯π interactions. Compound 2 displays a 4-connected two-dimensional (2D) framework with the point symbol {4⁴·6²}. Moreover, the 2D structure of 2 is also ultimately packed into 3D supramolecular frameworks through H-bonding and π–π stacking interactions. Compound 3 shows a 5-fold interpenetrated diamondoid net with 6⁶ topology. Compound 4 is a 2D 3-connected net having a point symbol {6³} and is extended to a supramolecular 3D structure through C–H⋯π and π⋯π interactions with lattice 3-bpdh ligands. Compound 5 features a 3-fold interpenetrated diamondoid net with 6⁶ topology. The thermal stabilities and luminescent properties of 3–5 were also studied in detail. The complexes exhibit ligands based photoluminescence properties at room temperature.