Structural distortions within the extensive family of organic/inorganic hybrid tin iodide perovskite semiconductors are correlated with their experimental exciton energies and calculated band gaps. The extent of the in- and out-of-plane angular distortion of the SnI4(2-) perovskite sheets is largely determined by the relative charge density and steric requirements of the organic cations. Variation of the in-plane Sn-I-Sn bond angle was demonstrated to have the greatest impact on the tuning of the band gap, and the equatorial Sn-I bond distances have a significant secondary influence. Extended Hückel tight-binding band calculations are employed to decipher the crystal orbital origins of the structural effects that fine-tune the band structure. The calculations suggest that it may be possible to tune the band gap by as much as 1 eV using the templating influence of the organic cation.

Direct measurement of chemical exchange events in the crystalline polycarbonate monomer 4,4‘- isopropylidenediphenol (bisphenol A, BPA) via 2D 13C solid-state NMR reveals slow, large-amplitude aromatic ring reorientations. X-ray diffraction, however, indicates a static crystalline structure. Experiments with multiple exchange times show that ring flips occur in all of the three unique conformers found in the crystalline unit cell of this compound, but in specific cases, two of the three unique molecules actually switch conformations. Kinetic analysis of the exchange data indicates that the average rate constant k ex = 0.01 s-1 for ring flips and conformer interchange at room temperature. Differential scanning calorimetry and variable-temperature powder diffraction studies indicate a systematic volume expansion that accompanies this motion but no first-order phase transition. All room-temperature exchange events may be quenched at 213 K, at least on the time scale (up to several seconds) probed in this work. Simulation of the potential-energy surface of BPA molecules reveals that the lowest-energy pathway for ring flips (maximum energy of 1.9 kcal/mol) involves +110° flips of one ring coincident with +70° flips of the second. The mechanism of the ring dynamics in the crystalline monomer is unique relative to those previously reported for the polycarbonate in that the collective motion of both rings in a monomer unit is 180°, whereas single-ring flips of 180° occur in the polymer.