Perovskite solar cells have the potential to revolutionize the world of photovoltaics, and their efficiency close to 23 % on a lab-scale recently certified this novel technology as the one with the most rapidly raising performance per year in the whole story of solar cells. With the aim of improving stability, reproducibility and spectral properties of the devices, in the last three years the scientific community strongly focused on Cs-doping for hybrid (typically, organolead) perovskites. In parallel, to further contrast hygroscopicity and reach thermal stability, research has also been carried out to achieve the development of all-inorganic perovskites based on caesium, the performances of which are rapidly increasing. The potential of caesium is further strengthened when it is used as a modifying agent of charge-carrier layers in solar cells, but also for the preparation of perovskites with peculiar optoelectronic properties for unconventional applications (e.g., in LEDs, photodetectors, sensors, etc.). This Review offers a 360-degree overview on how caesium can strongly tune the properties and performance of perovskites and relative perovskite-based devices.

Despite the wide applicability of enantioselective Brønsted acid catalysis, experimental insight into transition states is very rare, and most of the mechanistic knowledge is gained by theoretical calculations. Here, we present an alternative approach (decrypting transition state by light = DTS-hν), which enables the decryption of the transition states involved in chiral phosphoric acids catalyzed addition of nucleophiles to imines. Photoisomerization of double bonds is employed as a mechanistic tool. For this class of reactions four pathways (Type I Z, Type I E, Type II Z, Type II E) are possible, leading to different enantiomers depending on the imine configuration (E- or Z-imine) and on the nucleophilic attack site (top or bottom). We demonstrated that the imine double bond can be isomerized by light (365 nm LED) during the reaction leading to a characteristic fingerprint pattern of changes in reaction rate and enantioselectivity. This characteristic fingerprint pattern is directly correlated to the transition states involved in the transformation. Type I Z and Type II Z are demonstrated to be the competing pathways for the asymmetric transfer hydrogenation of ketimines, while in the nucleophilic addition of acetylacetone to N-Boc protected aldimines Type I E and Type II E are active. Accelerations on reaction rate up to 177% were observed for ketimines reduction. Our experimental findings are supported by quantum chemical calculations and noncovalent interaction analysis.

Can green chemistry be the right reading key to let organocatalyst design take a step forward towards sustainable catalysis? What if the intriguing chemistry promoted by more engineered organocatalysts was carried on by using renewable and naturally occurring molecular scaffolds, or at least synthetic catalysts more respectful towards the principles of green chemistry? Within the frame of these questions, this Review will tackle the most commonly occurring organic chiral catalysts from the perspective of their synthesis rather than their employment in chemical methodologies or processes. A classification of the catalyst scaffolds based on their E factor will be provided, and the global E factor (E G factor) will be proposed as a new green chemistry metric to consider, also, the synthetic route to the catalyst within a given organocatalytic process.

Natural substances such as pelletierine and its analogues have been prepared in up to 97% ee and good yield by a protective-group-free, biomimetic approach. Usage of benzonitrile or acetonitrile as solvents effectively prevents product racemization.

Over the years, the field of enantioselective organocatalysis has seen unparalleled growth in the development of novel synthetic applications with respect to mechanistic investigations. Reaction optimization appeared to be rather empirical than rational. This offset between synthetic development and mechanistic understanding was and is generally due to the difficulties in detecting reactive intermediates and the inability to experimentally evaluate transition states. Thus, the first key point for mechanistic studies is detecting elusive intermediates and characterizing them in terms of their structure, stability, formation pathways, and kinetic properties. The second key point is evaluating the importance of these intermediates and their properties in the transition state. In the past 7 years, our group has addressed the problems with detecting elusive intermediates in organocatalysis by means of NMR spectroscopy and eventually theoretical calculations. Two main activation modes were extensively investigated: secondary amine catalysis and, very recently, Brønsted acid catalysis. Using these examples, we discuss potential methods to stabilize intermediates via intermolecular interactions; to elucidate their structures, formation pathways and kinetics; to change the kinetics of the reactions; and to address their relevance in transition states. The elusive enamine in proline-catalyzed aldol reactions is used as an example of the stabilization of intermediates via inter- and intramolecular interactions; the determination of kinetics on its formation pathway is discussed. Classical structural characterization of intermediates is described using prolinol and prolinol ether enamines and dienamines. The Z/E dilemma for the second double bond of the dienamines shows how the kinetics of a reaction can be changed to allow for the detection of reaction intermediates. We recently started to investigate substrate-catalyst complexes in the field of Brønsted acid catalysis. These studies on imine/chiral phosphoric acid complexes show that an appropriate combination of highly developed NMR and theoretical methods can provide detailed insights into the complicated structures, exchange kinetics, and H-bonding properties of chiral ion pairs. Furthermore, the merging of these structural investigations and photoisomerization even allowed the active transition state combinations to be determined for the first time on the basis of experimental data only, which is the gold standard in mechanistic investigations and was previously thought to be exclusively the domain of theoretical calculations. Thus, this Account summarizes our recent mechanistic work in the field of organocatalysis and explains the potential methods for addressing the central questions in mechanistic studies: stabilization of intermediates, elucidation of structures and formation pathways, and addressing transition state combinations experimentally.