The molecular design concept and mechanism leading to highly efficient thermally activated delayed fluorescence are presented.

No abstract

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.

Interchain interaction, i.e., pi-pi stacking, can benefit the carrier transport in conjugated regio-regular poly(3-hexylthiophene) (P3HT) thin films. However, the existence of the insulating side hexyl chains in the surface region may be detrimental to the charge transfer between the polymer backbone and overlayer molecules. The control of the molecular orientation in the surface region is expected to alter the distribution of the pi electron density at the surface to solve such problems, which can be achieved by controlling the solvent removal rate during solidification. The evidence that the pi-electron density distribution at the outermost surface can be controlled is demonstrated by the investigation using the powerful combination of near edge X-ray absorption fine structure spectroscopy, ultraviolet photoelectron spectroscopy, and the most surface-sensitive technique: Penning ionization electron spectroscopy. From the spectroscopic studies, it can be deduced that the slower removal rate of the solvent makes the polymer chains even at the surface have sufficient time to adopt a more nearly equilibrium structure with edge-on conformation. Thus, the side hexyl chains extend outside the surface, which buries the pi-electron density contributed from the polymer backbone. Contrarily, the quench of obtaining a thermo-equilibrium structure in the surface region due to the faster removal of the solvent residual can lead to the surface chain conformation without persisting to the strong bulk orientation preference. Therefore, the face-on conformation of the polymer chain at the surface of thin films coated with high spin coating speed facilitate the electron density of the polymer backbone exposed outside the surface. Finally, thickness dependence of the surface electronic structure of P3HT thin films is also discussed.

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc = C32N8H16 −2 ) on Cu(111) using the X-ray standing wave technique, photoelectron spectroscopy, and quantum chemical calculations. We find that for sub-monolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a 'Cl-down' configuration by forming a covalent bond to the substrate. For bilayer coverages the XSW data indicate a co-existence of the 'Cl-down' and 'Cl-up' configuration on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.PACS numbers: 68.49. Uv, 68.43.Fg, 79.60.Fr The adsorption of organic semiconductor molecules has been in the focus of numerous experimental and theoretical investigations -many of them addressing the subtle interplay of electronic and structural properties. Early studies [1], which show that the energy levels of organic semiconductor/metal interfaces can exhibit large deviations from the Schottky-Mott relation, conveyed the significance of interface dipoles. Until today and despite the ubiquity of this concept in the field of organic materials, the origin of the interface dipole often remains vague.To establish a better understanding of the energy level alignment at the interface one should not neglect effects related to the molecular structure of organic adsorbates: Planar molecules such as F 16 CuPc [2], PTCDA [3,4] or pentacene derivatives [5], for example, can distort upon adsorption due to the interaction with the substrate and therefore exhibit an induced molecular dipole. Non-planar molecules such as TiOPc [6], SnPc [7][8][9], SubPc [10, 11] and VOPc [12], which may adsorb in different orientations, form layers with at least partially aligned dipole moments. Hence, for this class of systems the orientational order on the surface is a quantity which strongly influences the interface dipole. In particular, it has been shown that depending on the orientation a layer of molecular dipoles p with an area density N dip can shift the vacuum level (VL) in either direction and, therefore, increase or decrease the work function Φ of the sample according to [13] where ǫ is the effective dielectric constant of the monolayer. An experimentally and theoretically challenging model system of non-planar organic molecules with a significant dipole moment, for which these effects can be directly studied, is chlorogallium phthalocyanine (GaClPc, Fig. 1) [14].In this letter, we present a detailed study on the bonding and orientational ordering of GaClPc on Cu(111) surfaces using the X-ray standing wave (XSW) technique [15], ultra-violet photoelectron spectroscopy (UPS) and density functional theory (DFT) based calculations. While XSW data are taken to determine exact the atomic positions along the surface normal and, thereby, also the orientation of the molecul...

Designing molecular p-n heterojunction structures, i.e., electron donor-acceptor contacts, is one of the central challenges for further development of organic electronic devices. In the present study, a well-defined p-n heterojunction of two representative molecular semiconductors, pentacene and C60, formed on the single-crystal surface of pentacene is precisely investigated in terms of its growth behavior and crystallographic structure. C60 assembles into a (111)-oriented face-centered-cubic crystal structure with a specific epitaxial orientation on the (001) surface of the pentacene single crystal. The present experimental findings provide molecular scale insights into the formation mechanisms of the organic p-n heterojunction through an accurate structural analysis of the single-crystalline molecular contact.

We present a benchmark study for the adsorption of a large π -conjugated organic molecule on different noble metal surfaces, which is based on x-ray standing wave (XSW) measurements and density functional theory calculations with van der Waals (vdW) interactions. The bonding distances of diindenoperylene on Cu(111), Ag(111), and Au (111) surfaces (2.51, 3.01, and 3.10Å, respectively) determined with the normal-incidence XSW technique are compared with calculations. Excellent agreement with the experimental data, i.e., deviations less than 0.1Å, is achieved using the Perdew-Burke-Ernzerhof (PBE) functional with vdW interactions that include the collective response of substrate electrons (the PBE + vdW surf method). It is noteworthy that the calculations show that the vdW contribution to the adsorption energy increases in the order Au(111) < Ag(111) < Cu(111).