The stability and formation of a perovskite structure is dictated by the Goldschmidt tolerance factor as a general geometric guideline. The tolerance factor has limited the choice of cations (A) in 3D lead iodide perovskites (APbI 3 ), an intriguing class of semiconductors for high-performance photovoltaics and optoelectronics. Here, we show the tolerance factor requirement is relaxed in 2D Ruddlesden–Popper (RP) perovskites, enabling the incorporation of a variety of larger cations beyond the methylammonium (MA), formamidinium, and cesium ions in the lead iodide perovskite cages for the first time. This is unequivocally confirmed with the single-crystal X-ray structure of newly synthesized guanidinium (GA)-based ( n -C 6 H 13 NH 3 ) 2 (GA)Pb 2 I 7 , which exhibits significantly enlarged and distorted perovskite cage containing sterically constrained GA cation. Structural comparison with ( n -C 6 H 13 NH 3 ) 2 (MA)Pb 2 I 7 reveals that the structural stabilization originates from the mitigation of strain accumulation and self-adjustable strain-balancing in 2D RP structures. Furthermore, spectroscopic studies show a large A cation significantly influences carrier dynamics and exciton–phonon interactions through modulating the inorganic sublattice. These results enrich the diverse families of perovskite materials, provide new insights into the mechanistic role of A-site cations on their physical properties, and have implications to solar device studies using engineered perovskite thin films incorporating such large organic cations.

Reversible solid-state transformations are important in stimuli-responsive materials. The current understanding is limited on what kind of structure enables reversible transitions. We report that for molecular solids containing nitro groups, reversible phase transitions can occur if the nitro group is a hydrogen bond (HB) acceptor and has torsional freedom. We use the polymorphs of nifedipine (NIF) to illustrate this phenomenon taking advantage of their different molecular packing but identical chemical structure. NIF has six known polymorphs with four being kinetically stable at 100 K. Upon heating, two polymorphs undergo reversible phase transitions with large volume change, while the others do not. In the transforming structures, the nitro group is an HB acceptor and rotates to optimize HBs to offset loss of close packing, while in the inactive structures, the nitro group has similar torsional freedom but is not engaged in HBs. We test the generality of this phenomenon using all available systems in the Cambridge Structural Database, including NIF's derivative nisoldipine, and suggest possible applications in designing materials with controlled mechanical response.

The molecules at the surface of a liquid have different organization and dynamics from those in the bulk, potentially altering the rate of crystal nucleation and polymorphic selection, but this effect remains poorly understood. Here we demonstrate that nucleation at the surface of a pure liquid, d-arabitol, is vastly enhanced, by 12 orders of magnitude, and selects a different polymorph. The surface effect intensifies with cooling and can be inhibited by a dilute, surface-active second component. This phenomenon arises from the anisotropic molecular packing at the interface and its similarity to the surface-nucleating polymorph. Our finding is relevant for controlling the crystallization and polymorphism in any system with a significant interface such as nanodroplets and atmospheric water.

The unsymmetrical coordination of gold(I) by 2,2′bipyridine (bipy) in some planar, three-coordinate cations has been examined by crystallographic and computational studies. The salts [(Ph 3 P)Au(bipy)]XF 6 (X = P, As, Sb) form an isomorphic series in which the differences in Au−N distances range from 0.241(2) to 0.146(2) Å. A second polymorph of [(Ph 3 P)Au(bipy)]AsF 6 has also been found. Both polymorphs exhibit similar structures. The salts [(Et 3 P)Au(bipy)]XF 6 (X = P, As, Sb) form a second isostructural series. In this series the unsymmetrical coordination of the bipy ligand is maintained, but the gold ions are disordered over two unequally populated positions that produce very similar overall structures for the cations. Although many planar, threecoordinate gold(I) complexes are strongly luminescent, the salts [(R 3 P)-Au(bipy)]XF 6 (R = Ph or Et; X = P, As, Sb) are not luminescent as solids or in solution. Computational studies revealed that a fully symmetrical structure for [(Et 3 P)Au(bipy)] + is 7 kJ/mol higher in energy than the observed unsymmetrical structure and is best described as a transition state between the two limiting unsymmetrical geometries. The Au−N bonding has been examined by natural resonance theory (NRT) calculations using the "12 electron rule". The dominant Lewis structure is one with five lone pairs on Au and one bond to the P atom, which results in a saturated (12 electron) gold center and thereby inhibits the formation of any classical, 2 e − bonds between the gold and either of the bipy nitrogen atoms. The nitrogen atoms may instead donate a lone pair into an empty Au−P antibonding orbital, resulting in a three-center, four-electron (3c/4e) P−Au−N bond. The binuclear complex, [μ 2bipy(AuPPh 3 ) 2 ](PF 6 ) 2 , has also been prepared and shown to have an aurophillic interaction between the two gold ions, which are separated by 3.0747(3) Å. Despite the aurophillic interaction, this binuclear complex is not luminescent.

Mechanistic studies provide valuable insights into Cu-catalyzed N–N coupling reactions relevant to energy conversion and pharmaceutical synthesis.