Control of the Organization of 4,4'-bis(carbazole)-1,1'-biphenyl (CBP) Molecular Materials through Siloxane Functionalization.

We show that through the introduction of short dimethylsiloxane chains, it was possible to suppress the crystalline state of CBP in favor of various types of organization, transitioning from a soft crystal to a fluid liquid crystal mesophase, then to a liquid state. Characterized by X-ray scattering, all organizations reveal a similar layered configuration in which layers of edge-on lying CBP cores alternate with siloxane. The difference between all CBP organizations essentially lay on the regularity of the molecular packing that modulates the interactions of neighboring conjugated cores. As a result, the materials show quite different thin film absorption and emission properties, which could be correlated to the features of the chemical architectures and the molecular organizations.

Photoactive Organic/Inorganic Hybrid Materials with Nanosegregated Donor-Acceptor Arrays.

The synthesis of the first mesogenic donor-acceptor polyoxometalate (POM)-based hybrid is herein described. The structural and electronic properties of the hybrid compound were evaluated through combination of small- and wide-angle X-ray scattering, optical microscopy, electrochemistry and photoluminescence. In the solid state, the compound behaves as a birefringent solid, displaying a lamellar organization in which double-layers of POMs and bis(thiophene)thienothiophene organic donors alternate regularly. Noticeably, the sub-unit organizations in the composite are similar to that observed for the individual POM and organic donor precursors. Photophysical studies show that in the hybrid, the fluorescence of the organic donor unit is considerably quenched both in solution and in the solid state, which is attributed to occurrence of intramolecular charge-separated state.

Charge-transfer dynamics and nonlocal dielectric permittivity tuned with metamaterial structures as solvent analogues.

Charge transfer (CT) is a fundamental and ubiquitous mechanism in biology, physics and chemistry. Here, we evidence that CT dynamics can be altered by multi-layered hyperbolic metamaterial (HMM) substrates. Taking triphenylene:perylene diimide dyad supramolecular self-assemblies as a model system, we reveal longer-lived CT states in the presence of HMM structures, with both charge separation and recombination characteristic times increased by factors of 2.4 and 1.7-that is, relative variations of 140 and 73%, respectively. To rationalize these experimental results in terms of driving force, we successfully introduce image dipole interactions in Marcus theory. The non-local effect herein demonstrated is directly linked to the number of metal-dielectric pairs, can be formalized in the dielectric permittivity, and is presented as a solid analogue to local solvent polarity effects. This model and extra PH3T:PC60BM results show the generality of this non-local phenomenon and that a wide range of kinetic tailoring opportunities can arise from substrate engineering. This work paves the way toward the design of artificial substrates to control CT dynamics of interest for applications in optoelectronics and chemistry.

Directed Organization of Platinum Nanocrystals through Organic Supramolecular Nanoporous Templates.

We propose a novel approach to trap 2 nm Pt nanocrystals using nanoporous two-dimensional supramolecular networks for cavity-confined host-guest recognition process. This will be achieved by taking advantage of two features of supramolecular self-assembly at surfaces: First, its capability to allow the formation of complex 2D architectures, more particularly, nanoporous networks, through noncovalent interactions between organic molecular building-blocks; second, the ability of the nanopores to selectively host and immobilize a large variety of guest species. In this paper, for the first time, we will use isotropic honeycomb networks and anisotropic linear porous supramolecular networks to host 2 nm Pt nanocrystals.

Ordinary and Hot Electroluminescence from Single-Molecule Devices: Controlling the Emission Color by Chemical Engineering.

Single-molecule junctions specifically designed for their optical properties are operated as light-emitting devices using a cryogenic scanning tunneling microscope. They are composed of an emitting unit-a molecular chromophore-suspended between a Au(111) surface and the tip of the microscope by organic linkers. Tunneling electrons flowing through these junctions generate a narrow-line emission of light whose color is controlled by carefully selecting the chemical structure of the emitting unit. Besides the main emission line, red and blue-shifted vibronic features of low intensity are also detected. While the red-shifted features provide a spectroscopic fingerprint of the emitting unit, the blue-shifted ones are interpreted in terms of hot luminescence from vibrationally excited states of the molecule.

Narrow-Line Single-Molecule Transducer between Electronic Circuits and Surface Plasmons.

A molecular wire containing an emitting molecular center is controllably suspended between the plasmonic electrodes of a cryogenic scanning tunneling microscope. Passing current through this circuit generates an ultranarrow-line emission at an energy of ≈1.5 eV which is assigned to the fluorescence of the molecular center. Control over the linewidth is obtained by progressively detaching the emitting unit from the surface. The recorded spectra also reveal several vibronic peaks of low intensities that can be viewed as a fingerprint of the emitter. Surface plasmons localized at the tip-sample interface are shown to play a major role in both excitation and emission of the molecular excitons.

Surface-Confined Supramolecular Self-Assembly of Molecular Nanocranes for Chemically Lifting and Positioning C60 above a Conducting Substrate.

2D supramolecular self-assembly is a good way to form well-defined nanostructures on various substrates. One of the current challenges is to extend this approach to 3D functional building blocks. Here, we address this issue by providing a strategy for the controlled lifting and positioning of functional units above a graphitic substrate. This is the first time that multistory cyclophane-based 3D tectons incorporating C60 units have been designed and synthesized. Molecular modelling provides a description of the 3D geometries and evidences the flexible character of the building blocks. Despite this later feature, the supramolecular self-assembly of Janus tectons on HOPG yields well-ordered adlayers incorporating C60 arrays at well-defined mean distances from the surface. As our approach is not limited to C60 , the results reported here open-up possibilities for applications where the topological and electronic interactions between the substrate and the functional unit are of prime importance.

Electroluminescence of a polythiophene molecular wire suspended between a metallic surface and the tip of a scanning tunneling microscope.

The electroluminescence of a polythiophene wire suspended between a metallic surface and the tip of a scanning tunneling microscope is reported. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Both emission mechanism and polarity dependence are similar to what occurs in organic light emitting diodes (OLED) but at the level of a single molecular wire.

Emergence of chirality in hexagonally packed monolayers of hexapentyloxytriphenylene on Au(111): a joint experimental and theoretical study.

We investigate the expression of chirality in a monolayer formed spontaneously by 2,3,6,7,10,11-pentyloxytriphenylene (H5T) on Au(111). We resolve its interface morphology by combining scanning tunneling microscopy (STM) with theoretical calculations of intermolecular and interfacial interaction potentials. We observe two commensurate structures. While both of them belong to a hexagonal space group, analogical to the triangular symmetry of the molecule and the hexagonal symmetry of the substrate surface, they surprisingly reveal a 2D chiral character. The corresponding breaking of symmetry arises for two reasons. First it is due to the establishment of a large molecular density on the substrate, which leads to a rotation of the molecules with respect to the molecular network crystallographic axes to avoid steric repulsion between neighboring alkoxy chains. Second it is due to the molecule-substrate interactions, leading to commensurable large crystallographic cells associated with the large size of the molecule. As a consequence, molecular networks disoriented with respect to the high symmetry directions of the substrate are induced. The high simplicity of the intermolecular and molecule-substrate van der Waals interactions leading to these observations suggests a generic character for this kind of symmetry breaking. We demonstrate that, for similar molecular densities, only two kinds of molecular networks are stabilized by the molecule-substrate interactions. The most stable network favors the interfacial interactions between terminal alkoxy tails and Au(111). The metastable one favors a specific orientation of the triphenylene core with its symmetry axes collinear to the Au⟨110⟩. This specific orientation of the triphenylene cores with respect to Au(111) appears associated with an energy advantage larger by at least 0.26 eV with respect to the disoriented core.

Formation of hydroxyl-functionalized stilbenoid molecular sieves at the liquid/solid interface on top of a 1-decanol monolayer.

Specific molecular tectons can be designed to form molecular sieves through self-assembly at the solid-liquid interface. After demonstrating a model tecton bearing apolar alkyl chains, we then focus on a modified structure involving asymmetric functionalization of some alkyl chains with polar hydroxyl groups in order to get chemical selectivity in the sieving. As the formation of supramolecular self-assembled networks strongly depends on molecule-molecule, molecule-substrate and molecule-solvent interactions, we compared the tectons' self-assembly on graphite for two types of solvent. We demonstrate the possibility to create hydroxylated stilbenoid molecular sieves by using 1-decanol as a solvent. Interestingly, with this solvent, the porous network is developed on top of a 1-decanol monolayer.

Surface-confined self-assembled Janus tectons: a versatile platform towards the noncovalent functionalization of graphene.

A general strategy for simultaneously generating surface-based supramolecular architectures on flat sp(2) -hybridized carbon supports and independently exposing on demand off-plane functionality with controlled lateral order is highly desirable for the noncovalent functionalization of graphene. Here, we address this issue by providing a versatile molecular platform based on a library of new 3D Janus tectons that form surface-confined supramolecular adlayers in which it is possible to simultaneously steer the 2D self-assembly on flat C(sp(2))-based substrates and tailor the external interface above the substrate by exposure to a wide variety of small terminal chemical groups and functional moieties. This approach is validated throughout by scanning tunneling microscopy (STM) at the liquid-solid interface and molecular mechanics modeling studies. The successful self-assembly on graphene, together with the possibility to transfer the graphene monolayer onto various substrates, should considerably extend the application of our functionalization strategy.

A donor-acceptor cage for circularly polarized TADF emission.

Herein, we report the first example of chiral donor-acceptor cage DA-2 displaying efficient circularly polarized thermally activated delayed fluorescence (CP-TADF) with |glum| values up to 2.1 × 10-3 and PLQY of 32%. A small ΔEST of 0.051 eV and quasi-parallel (θ = 6°) transition electric and magnetic dipole moments were realized from the through-space charge transfer interaction between the parallelly aligned donor and acceptor in DA-2. This D-A cage configuration has provided a novel design strategy for discovering potential efficient CP-TADF emitters.

Oligothiophene nanorings as electron resonators for whispering gallery modes.

Structural and electronic properties of oligothiophene nanowires and rings synthesized on a Au(111) surface are investigated by scanning tunneling microscopy. The spectroscopic data of the linear and cyclic oligomers show remarkable differences which, to a first approximation, can be accounted by considering electronic state confinement to one-dimensional boxes having, respectively, fixed and periodic boundary conditions. A more detailed analysis shows that polythiophene must be treated as a ribbon (i.e., having an effective width) rather than a purely 1D structure. A fascinating consequence is that the molecular nanorings act as whispering gallery mode resonators for electrons, opening the way for new applications in quantum electronics.

D-π-A-Type Pyrazolo[1,5-a]pyrimidine-Based Hole-Transporting Materials for Perovskite Solar Cells: Effect of the Functionalization Position.

Donor−acceptor (D−A) small molecules are regarded as promising hole-transporting materials for perovskite solar cells (PSCs) due to their tunable optoelectronic properties. This paper reports the design, synthesis and characterization of three novel isomeric D-π-A small molecules PY1, PY2 and PY3. The chemical structures of the molecules consist of a pyrazolo[1,5-a]pyrimidine acceptor core functionalized with one 3,6-bis(4,4'-dimethoxydiphenylamino)carbazole (3,6-CzDMPA) donor moiety via a phenyl π-spacer at the 3, 5 and 7 positions, respectively. The isolated compounds possess suitable energy levels, sufficient thermal stability (Td > 400 °C), molecular glass behavior with Tg values in the range of 127−136 °C slightly higher than that of the reference material Spiro-OMeTAD (126 °C) and acceptable hydrophobicity. Undoped PY1 demonstrates the highest hole mobility (3 × 10−6 cm2 V−1 s−1) compared to PY2 and PY3 (1.3 × 10−6 cm2 V−1 s−1). The whole isomers were incorporated as doped HTMs in planar n-i-p PSCs based on double cation perovskite FA0.85Cs0.15Pb(I0.85Br0.15)3. The non-optimized device fabricated using PY1 exhibited a power conversion efficiency (PCE) of 12.41%, similar to that obtained using the reference, Spiro-OMeTAD, which demonstrated a maximum PCE of 12.58% under the same conditions. The PY2 and PY3 materials demonstrated slightly lower performance in device configuration, with relatively moderate PCEs of 10.21% and 10.82%, respectively, and slight hysteresis behavior (−0.01 and 0.02). The preliminary stability testing of PSCs is also described. The PY1-based device exhibited better stability than the device using Spiro-OMeTAD, which could be related to its slightly superior hydrophobic character preventing water diffusion into the perovskite layer.

Self-templating polythiophene derivatives: electronic decoupling of conjugated strands through staggered packing.

Whereas molecular electronics needs well-controlled 3D geometries for decoupling or interconnecting individual molecules, conjugated polymers form disordered structures when deposited on a substrate. We show that this trend can be overcome in polythiophene derivatives designed so as to exploit weak sulfur-bromine interactions. A self-template effect follows, leading to staggered organizations of well-aligned electronically decoupled conjugated strands, as observed in situ by scanning tunneling microscopy and spectroscopy on graphite.

Solution-growth kinetics and thermodynamics of nanoporous self-assembled molecular monolayers.

The temperature and concentration dependences of the self-assembly onto graphite from solution of a series of molecular building blocks able to form nanoporous structures are analyzed experimentally by in situ scanning tunneling microscopy. It is shown that the commonly observed coexistence of dense and nanoporous domains results from kinetic blockades rather than a thermodynamic equilibrium. The ripening can be favored by high densities of domain boundaries, which can be obtained by cooling the substrate before the nucleation and growth. Then ripening at higher-temperature yields large defect-free domains of a single structure. This thermodynamically stable structure can be either the dense or the nanoporous one, depending on the tecton concentration in the supernatant solution. A sharp phase transition from dense to honeycomb structures is observed at a critical concentration. This collective phenomenon is explained by introducing interactions between adsorbed molecules in the thermodynamic description of the whole system.

Effect of the electron donating group on the excited-state electronic nature and epsilon-near-zero properties of curcuminoid-borondifluoride dyes.

Epsilon-near-zero (ENZ) properties have been reported in organic molecular films. In particular, cyanine and squaraine films have been shown to exhibit ENZ properties in the visible spectral region with a strong 3rd order nonlinear optical response near the ENZ spectral region. Noting both cyanine and squaraine belong to the polymethine family, a series of six curcuminoid borondifluoride (Curc) derivatives were developed to examine whether such a polymethine character is positively correlated with the ENZ property of the organic films. Those Curc derivatives possess a Donor-Acceptor-Donor (D-A-D) architecture with acceptor, AcacBF2, located at the molecular center. The backbone of Curc is designed such that the donor strength can be tuned to transit between charge transfer (CT) and polymethine character. This balance between CT and polymethine character of the Curc series is examined based on the Lippert-Mataga plot. As donor strength in the D-A-D structure increases, CT character is less marked resulting in a more dominant polymethine character. The structural and optical properties of the Curc films with a thickness in the order of 30 nm were examined to correlate the polymethine character with the ENZ response. The results obtained in isotropic Curc thin films demonstrate that an increase of polymethine character associated with a stronger donor strength leads to an appearance/enhancement of the ENZ property in the visible spectrum range from 500 to 670 nm. Overall, this study provides useful guidelines to engineer new organic materials showing ENZ properties in a desired spectral range.

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors.

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s-1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons.

Diethynylbenzene-based liquid crystalline semiconductor for solution-processable organic thin-film transistors.

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility of 4.5 x 10(-5) cm2/Vs, was showed after annealing of the films at liquid crystalline temperature. Time-of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5 x 10(-6) cm2/Vs for both positive and negative carriers. These results indicate that the liquid crystallinity helps to improve the molecular packing and enhance charge mobility for P1. These advantages can be applicable to design and construct solution-processable OTFT materials for electronic applications.

Dithia[3.3]paracyclophane Core: A Versatile Platform for Triplet State Fine-Tuning and Through-Space TADF Emission.

Thermally activated delayed fluorescence (TADF) based on through-space donor and acceptor interactions constitute a recent and promising approach to develop efficient TADF emitters. Novel TADF isomers using a dithia[3.3]-paracyclophane building block as a versatile 3D platform to promote through-space interactions are presented. Such a 3D platform allows to bring together the D and A units into close proximity and to probe the effect of their orientation, contact site and distance on their TADF emission properties. This study provides evidence that the dithia[3.3]paracyclophane core is a promising platform to control intramolecular through-space interactions and obtain an efficient TADF emission with short reverse-intersystem crossing (RISC) lifetimes. In addition, this study demonstrates that this design can tune the energy levels of the triplet states and leads to an upconversion from 3 CT to 3 LE that promotes faster and more efficient RISC to the 1 CT singlet state.