This account aims at providing an understanding of singlet fission, i.e., the photophysical process of a singlet state ( S1 ) splitting into two triplet states (2 × T1 ) in molecular chromophores. Since its discovery 50 years ago, the field of singlet fission has enjoyed rapid expansion in the past 8 years. However, there have been lingering confusion and debates on the nature of the all-important triplet pair intermediate states and the definition of singlet fission rates. Here we clarify the confusion from both theoretical and experimental perspectives. We distinguish the triplet pair state that maintains electronic coherence between the two constituent triplets, 1(TT) , from one which does not, 1(T···T) . Only the rate of formation of 1(T···T) is defined as that of singlet fission. We present distinct experimental evidence for 1(TT) , whose formation may occur via incoherent and/or vibronic coherent mechanisms. We discuss the challenges in treating singlet fission beyond the dimer approximation, in understanding the often neglected roles of delocalization on singlet fission rates, and in realizing the much lauded goal of increasing solar energy conversion efficiencies with singlet fission chromophores.

It has been known for more than 200 years that the maximum static friction force between two solid surfaces is usually greater than the kinetic friction force. In contrast to solid-solid friction, there is a lack of understanding of liquid-solid friction, i.e. the forces that impede the lateral motion of a drop of liquid on a solid surface. Here, we report that the lateral adhesion force between a liquid drop and a solid can be divided into a static and a kinetic regime. This striking analogy with solid-solid friction is a generic phenomenon that holds for liquids of different polarities and surface tensions on smooth, rough and structured surfaces.When two solid objects are brought into contact, a threshold force FTHRD must be overcome in order for one of the objects to slide 1-3 . This phenomenon can be visualised in a typical classroom experiment where a solid block attached to a spring is pulled over a solid surface (Fig. 1a). The static force FS is applied to the stationary block and then increased until it exceeds FTHRD, upon which the block begins to slide. After that, a lower kinetic force FKIN is required to maintain the block's motion 3 . However, it is not clear whether these forces develop in a comparable manner when a drop of liquid resting on a solid surface starts to slide. This gap in our understanding is astonishing, given the fact that liquid drops are omnipresent in our lives and their motion is relevant for numerous applications, including microfluidics 4 , printing 5 , condensation 6,7 , and water collection 8,9 . Hence insight on the behaviour of drops that start sliding over solid surfaces is needed.A sessile drop of liquid is usually in molecular contact with the supporting solid surface. In contrast, two solid bodies are in direct contact only at asperities owing to surface roughness 10,11 . Thus, the real contact area of a solid-solid contact is much smaller than the apparent contact area. Consequently the sliding of drops might be fundamentally different.However, by simply observing a drop of water on a pivot window pane, we know that also sessile drops start sliding when a critical tilt angle is reached, i.e. when the gravitational force acting on the drop overcomes the lateral adhesion force. The question may therefore be raised whether a static and a kinetic regime are also present for sessile drops. The general questions is: How do drops start sliding over solid surfaces and how do the forces develop while the drops slide?

Library of Congress Cataloging•in•Publication Data. Atoms in strong magnetic fields: quantum mechanical treatment and applications in astrophysics and quantum chaos 1 H. Ruder ... [et al.]. p. cm. Includes bibliographical references and index. \. Atomic transition probabilities. 2. Stars-Magnetic fields. 3. White dwarfs. 4. Neutron stars. 5. Quantum chaos.

High‐density cell microarrays based on superhydrophilic microspots separated by superhydrophobic barriers have been realized. The microspots absorb water solutions, while the barriers prevent cross‐contamination, thus allowing the spots to be used as reservoirs for transfection mixtures and preventing cell proliferation and cell migration between the microspots. The picture shows four cell types after two days of culturing on the microarray.