Molecularly Resolved Electronic Landscapes of Differing Acceptor–Donor Interface Geometries

Organic semiconductors are a promising class of materials for numerous electronic and optoelectronic applications, including solar cells. However, these materials tend to be extremely sensitive to the local environment and surrounding molecular geometry, causing the energy levels near boundaries and interfaces essential to device function to differ from those of the bulk. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STM/STS) have the ability to examine both the structural and electronic properties of these interfaces on the molecular and submolecular scales. Here, we investigate the prototypical acceptor–donor system, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA)/copper­(II) phthalocyanine (CuPc) using submolecularly resolved pixel-by-pixel STS to demonstrate the importance of subtle changes in interface geometry of prototypical solar cell materials. PTCDA and CuPc were sequentially deposited on NaCl bilayers to create lateral heterojunctions that were decoupled from the underlying substrate. Donor and acceptor states were observed to shift in opposite directions, suggesting an equilibrium charge transfer between the two. Narrowing of the gap energy compared to isolated molecules on the same surface is indicative of the influence of the local dielectric environment. Further, we find that the electronic state energies of both acceptor and donor are strongly dependent on the ratio and positioning of both molecules in larger clusters. This molecular-scale structural dependence of the electronic states of both interfacial acceptor and donor has significant implications for device design, where level alignment strongly correlates to device performance

Design of High-Performance Thermally Activated Delayed Fluorescence Emitters Containing <i>s</i>‑Triazine and <i>s</i>‑Heptazine with Molecular Orbital Visualization by STM

Materials exhibiting thermally activated delayed fluorescence (TADF) are now key components of some of the most advanced organic light-emitting diodes, photocatalysts, and bioimaging probes. Designing a TADF emitter requires a precise understanding of its frontier molecular orbitals (FMOs), yet rarely are these orbitals visualized experimentally. Here, we use scanning tunneling microscopy on Ag(111) to probe the electronic structures of high-performance TADF materials with different orbital landscapes based on s-triazine and s-heptazine acceptors. These materials exhibit room-temperature phosphorescence or thermally activated delayed fluorescence, deep-blue (452 nm) to red (615 nm) emission, near-unity photoluminescence quantum yields, exceptional photostability, and two-photon absorption cross sections as high as 2098 GM. Overall, this work demonstrates the potential of s-heptazines as optoelectronic materials, as well as the utility of direct FMO visualization in material design

Designing Optoelectronic Properties by On-Surface Synthesis: Formation and Electronic Structure of an Iron–Terpyridine Macromolecular Complex

Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic-scale control over organic–inorganic interface structures. In this approach, adsorbate–surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal–organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms and intraligand conformational changes lead to Fe–tpy coordination and formation of these nanochains. We used low-temperature scanning tunneling microscopy and density functional theory to elucidate the atomic-scale morphology of the system, suggesting a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning tunneling spectroscopy reveals the highest occupied orbitals, with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photoinduced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthetic methods and is mediated by the bottom-up on-surface approach used here, offering pathways to engineer the optoelectronic properties and reactivity of metal–organic nanostructures

Polymer Dots with Enhanced Photostability, Quantum Yield, and Two-Photon Cross-Section using Structurally Constrained Deep-Blue Fluorophores

Semiconducting polymer dots (Pdots) have emerged as versatile probes for bioanalysis and imaging at the single-particle level. Despite their utility in multiplexed analysis, deep blue Pdots remain rare due to their need for high-energy excitation and sensitivity to photobleaching. Here, we describe the design of deep blue fluorophores using structural constraints to improve resistance to photobleaching, two-photon absorption cross sections, and fluorescence quantum yields using the hexamethylazatriangulene motif. Scanning tunneling microscopy was used to characterize the electronic structure of these chromophores on the atomic scale as well as their intrinsic stability. The most promising fluorophore was functionalized with a polymerizable acrylate handle and used to give deep-blue fluorescent acrylic polymers with Mn > 18 kDa and Đ < 1.2. Nanoprecipitation with amphiphilic polystyrene-graft-(carboxylate-terminated poly­(ethylene glycol)) gave water-soluble Pdots with blue fluorescence, quantum yields of 0.81, and molar absorption coefficients of (4 ± 2) × 108 M–1 cm–1. This high brightness facilitated single-particle visualization with dramatically improved signal-to-noise ratio and photobleaching resistance versus an unencapsulated dye. The Pdots were then conjugated with antibodies for immunolabeling of SK-BR3 human breast cancer cells, which were imaged using deep blue fluorescence in both one- and two-photon excitation modes