Takahiro Ueba

The complex polymorphism and thermodynamic behavior of a seemingly simple system: naphthalene on Cu(111).

Naphthalene, C10H8, is a polycyclic aromatic hydrocarbon (PAH) consisting of two fused benzene rings. From previous studies, it is known to form three different commensurate structures in thin epitaxial films on Cu(111), depending on the preparation conditions. One of these structures even exhibits a chiral motif of molecular rotations within the unit cell. In an attempt to elucidate this polymorphism, we performed in situ low-energy electron diffraction (LEED) as a function of temperature and surface coverage, revealing an unexpected and extraordinarily complex structural and thermodynamic behavior. We present experimental evidence for a phase transition from a two-dimensional gas to a highly ordered molecular solid via an intermediate metastable phase with moderate order (extending over a few lattice constants only) which undergoes a reversible orientational shift upon temperature variation. At monolayer coverage and above, we find that two different point-on-line (POL) coincident epitaxial relations constitute the dominant structures. This is remarkable because, so far, POL structures of naphthalene on Cu(111) and other substrates have either not been recognized or not obtained under the respective experimental conditions. Our results are corroborated by the analysis of characteristic moiré patterns observed in scanning tunneling microscopy (STM), indicative of a noncommensurate epitaxial registry.

Self-Assembly of Tetraphenyldibenzoperiflanthene (DBP) Films on Ag(111) in the Monolayer Regime.

Tetraphenyldibenzoperiflanthene (DBP) is a promising candidate as a component of highly efficient organic photovoltaic cells and organic light-emitting diodes. The structural properties of thin films of this particular lander-type molecule on Ag(111) were investigated by complementary techniques. Highly ordered structures were obtained, and their mutual alignment was characterized by means of low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) images reveal two slightly different arrangements within the first monolayer (ML), both describable as specific herringbone patterns with two molecules per unit cell whose dibenzoperiflanthene framework is parallel to the surface. In contrast, single DBP molecules in the second ML were imaged with much higher intramolecular resolution, resembling the shape of the frontier orbitals in the gas phase as calculated by means of density functional theory (DFT). Further deposition leads to the growth of highly ordered bilayer islands on top of the first ML with identical unit cell dimensions and orientation but slightly inclined molecules. This suggests that the first ML acts as a template for the epitaxial growth of further layers. Simultaneously, a significant number of second-layer molecules mainly located at step edges or scattered over narrow terraces do not form highly ordered aggregates.

Electronic excitation and relaxation dynamics of the LUMO-derived level in rubrene thin films on graphite

Time resolved two-photon photoemission (TR-2PPE) spectroscopy has been performed for rubrene films on highly oriented pyrolytic graphite. When a second layer is formed on the first monolayer (ML), 2PPE intensity from the lowest unoccupied molecular orbital (LUMO)-derived level shows a clear resonance at a pump photon energy of 4.1 eV. In contrast, the resonance is very weak for sub-ML films. Substrate-molecule interaction blurs the intramolecular resonant transition for sub-ML films. The lifetime of electrons in the LUMO-derived level increases exponentially with increasing film thickness, for thickness up to 3 ML. The lifetime increase becomes more moderate for further increase in the film thickness. This change in the slope of the increase in lifetime suggests a transition in the relaxation mechanism, from electron tunneling to intramolecular relaxation medicated by the substrate. When ultraviolet photons of 4.45 eV are used to pump electrons to the LUMO-derived level, the decay profiles for films thicker than 1 ML deviate from a simple exponential decay. Such deviation is not significantly observed for sub-ML films. When visible photons of 2.97 eV are used for pumping, the decay profiles are well reproduced by a simple exponential decay, irrespective of the film thickness. The deviation from simple exponential decay is attributed to the relaxation of holes produced at deep occupied levels to the highest occupied molecular orbital-derived level.

Probing Triplet Excited States and Managing Blue Light Emission of Neutral Tetradentate Platinum(II) Complexes

The structural and photophysical properties of tetradentate Pt(ppzOppz), Pt(ppzOpopy), Pt(ppzOczpy), and Pt(czpyOczpy) have been experimentally and theoretically explored. Single-crystal diffraction measurements provided accurate structural information. Electrochemical and photophysical characterizations revealed internal electronic energy levels in ground and excited states. (Time-dependent) Density functional theory calculation revealed electron distributions in transition processes of S0 → S1 and S1 → T1 → S0. Electronic transition study indicated that Pt(ppzOppz) demonstrated mixed MLCT/LC states and Pt(czpyOczpy) showed MLCT-dominated states in S1 and T1. Both Pt(ppzOpopy) and Pt(ppzOczpy) presented strong delocalized spin transition (DST) during intersystem crossing. Upon frame modification of Pt(ppzOczpy), we found that their S1 and T1 can be independently manipulated. These blue emitters showed a tunable and narrow emission band (the narrowest fwhm was 19 nm) with luminescence efficiency as high as 86%. The findings of the DST transition mode in the neutral Pt(II) complexes provide guidance for rational design of novel phosphorescent materials.