With organic light-emitting diodes (OLEDs) emerging in ever more applications, such as smart phones, televisions, and lighting, it is easy to forget that the present technology is based on a brilliantly simple patch to an inherent problem of fluorescent hydrocarbons: three quarters of the electrically generated energy is dissipated as heat by triplet excitons. Radiative decay from the triplet state via phosphorescence is generally very weak, and has only been resolved in transient spectroscopy at low temperatures in select organic semiconductors. [1] The solution to this problem has been to incorporate metal-organic emitters in OLEDs, [2] which mix spin by enhancing intersystem crossing through spin-orbit coupling: the heavy-atom effect. As this approach relies on the longevity of triplet excitons and the associated diffusion lengths, it is highly effective: in a suitably homogeneous environment even ppm concentrations of covalently bound metal atoms are sufficient to activate electrophosphorescence. [3] The second conceivable approach to harvesting energy from triplets is based on endothermic conversion [4] to a fluorescent singlet by reverse intersystem crossing. [5] This method necessitates control not only over spin-orbit coupling, requiring a heavy atom or a carefully engineered charge-transfer state, but also over the singlet-triplet exchange gap, which can be tuned by excitonic confinement. [6] Although progress has been made recently, conceptually it parallels the former approach: all excitations are converted to either triplets or singlets, thereby losing information on the underlying spin correlations of charge carriers. Evidence is emerging, however, that spin correlations in excitonic electron-hole precursor pairs can be used for exquisitely sensitive measurements of magnetic fields [7] and possibly even for quantum coherence phenomenology, [7b] with analogies to avian radical-pair photomagnetosensory processes. [8] To quantify such spin correlations, it is desirable to develop materials without heavy-atom spin mixing that show both intrinsic fluorescence and phosphorescence.The third approach to triplet harvesting has not been explored previously: tuning spin-orbit coupling without heavy atoms such that non-radiative internal conversion from the triplet excited state to the singlet ground state is suppressed and phosphorescence is the only remaining relaxation mechanism. Even in low-atomic-order-number compounds such as hydrocarbons, the orbital component of the wavefunction can give rise to substantial magnetic moments, leading to non-negligible spin-orbit energy terms. The effect is well-studied in carbon nanotubes and graphene, where zero-field splitting correlates with nanoscale curva

Molecular polygons with three to six sides and binary mixtures thereof form long-range ordered patterns at the TCB/HOPG interface. This includes also the 2D crystallization of pentagons. The results provide an insight into how the symmetry of molecules is translated into periodic structures.

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SummaryA series of dithienophenazines with different lengths of the oligomeric thiophene units (quaterthiophenes and sexithiophenes) was synthesized. The thiophene and phenazine units act as electron donors and acceptors, respectively, resulting in characteristic absorption spectra. The optical spectra were calculated using time-dependent density functional theory at the B3LYP/TZVP level and verify the experimental data. Adsorption of the dithienophenazines on highly ordered pyrolytic graphite (HOPG) was investigated by scanning tunneling microscopy, showing that one of the compounds forms highly organized self-assembled monolayers.

Dual singlet-triplet fluorescence-phosphorescence emitting compounds demonstrate that plasmonic surface enhancement is controlled solely by the underlying oscillator strength of a transition: metal-free compounds with weak spin-orbit coupling show no enhancement in phosphorescence efficiency even though fluorescence is amplified.Excitement in the area of plasmonics stems from the ability to detect processes that are otherwise undetectable. Collective electron oscillations at metal surfaces experience strong spatial localization, which results in extremely intense and localized electromagnetic fields. This phenomenon is at the heart of all surface-enhanced spectroscopic processes. 1 The intense local field can relax several selection rules, thereby allowing transitions that are otherwise forbidden, and are therefore inaccessible to regular spectroscopy. While observation of forbidden Raman modes in surface-enhanced Raman scattering (SERS) is well documented, 2 the same is not as common for electronic transitions. Particularly noteworthy is a report on the observation of dipole-forbidden, but quadrupole-allowed transitions in conjugated oligoenes near silver films. 3 A crucial question to explore is hence whether metal nanostructures can also enhance dipole-forbidden radiative recombination from triplet excited states, phosphorescence. The necessity arises because 75% of radical-pair recombination events in an organic light-emitting diode (OLED) lead to triplet excited states that generally have poor radiative recombination efficiency in the absence of heavy-metal atoms. There have been a few reports on plasmon-enhanced phosphorescence. 4 One aspect common to these studies is that the effect was investigated in materials that are strongly spin-orbit (SO) coupled and thus highly phosphorescent to begin with. Most hydrocarbon organic semiconductors are weakly phosphorescent. OLED electrodes provide a natural environment for surface enhancement. If surface enhancement were to apply to transitions involving pure triplet and singlet states, in the absence of SO-induced spin mixing, the mechanism could open a new intrinsic radiative channel in an OLED, changing the way triplet harvesting is achieved and removing present limitations on triplet emitters posed by organometallic chemistry.Unfortunately, as we demonstrate here, this approach will not succeed. Phosphorescence can only be enhanced by plasmonics when intersystem crossing (ISC) is already strong. Surprisingly, phosphorescence due to transitions between pure triplet and singlet states is not enhanced to any measurable extent by plasmonic effects, even though a strong increase in fluorescence is observed in dual singlet-triplet-emitting compounds.To assess the possibility of surface enhancement of phosphorescence, we need an independent observable to confirm the presence of an enhancement effect. This observable is given by the dipole-allowed singlet transition in the dual-emitting compounds shown in Fig. 1a. We chose four materials with variable triplet yield (...

Molecular self-assembly has been established as a key concept for the formation of functional monolayer patterns decorating solid surfaces. The field of two-dimensional (2D) crystal engineering relates the adlayer structure with the nature of the substrate and with the geometry and functionality of the adsorbed molecules. Shape-persistent molecules having a one-dimensional (1D; rigid rod oligomers) [1] or twodimensional (polycyclic aromatic hydrocarbons (PAHs) [2] as well as rigid macrocycles) [3] structure and carrying flexible alkyl/alkoxy side chains have attracted considerable attention recently. They are adjustable in size with atomic-scale precision and can contain various functionalities. Moreover, the geometries and lattice dimensions of the surface patterns are transferred to the functional unit arrangements, thus giving rise to potential applications in single-molecule-based devices. However, to address that functionality on a singlemolecule level by existing lithographic techniques, the amplitude of these structures has to have mesoscopic dimensions. A key problem is that the lattice parameters are generally close to the size of the molecules, and the time and effort required for the synthesis of defined molecules of that size is rather high.Several self-assembly approaches are therefore explored to obtain 2D superstructures with large (tens of nanometers) lattice constants, and the approaches are based on either the homo-or co-assembly of molecules. In most cases, strong and directional noncovalent intermolecular interactions such as hydrogen bonding, dipole-dipole interaction, or metal coordination are the driving forces for the attraction between the molecules adsorbed to a (metal)