Theoretical insights into the interplay between metal-organic and covalent bonding in single-layer molecular networks formed by halogen dissociation.

Synthesis via dehalogenative coupling due to thermal annealing is one of the most common routes of growing metal-organic and covalent polymer networks on catalytic metal surfaces. We present a computational approach taking into account both metal-coordinated and covalent C-C bonding interactions, which drive the self-assembly of tetrabrominated polyarene molecules into single-layer ordered and disordered nanostructures. The proposed coarse-grained lattice model is simulated using the Monte Carlo method. We investigate the annealing effect in ensembles of nearly and fully dehalogenated molecules, accordingly decreasing the concentration of dissociated (chemisorbed) halogen atoms, to account for the desorption process. The results suggest that dissociated halogens may be at least partially responsible for fragmentation of metal-organic networks on the Cu and Au surfaces. The simulations also show that fragmented covalent networks are mostly disordered or characterized by short-range glass-like order, but larger domains of these phases can be obtained after removing the split off Br atoms. We additionally examine the potential formation of fragments with a hybrid structure consisting of oligomer chains linked side-to-side by metal adatoms.

Ordering of monomers, dimers and polymers of deposited Br2I2Py molecules: a modeling study.

We propose a lattice model describing the ordering of 1,6-dibromo-3,8-diiodopyrene (Br2I2Py) molecules on the Au(111) surface into two-dimensional structures and correlated one dimensional rows. Our model employs three (intact, singly and doubly deiodinated) types of Br2I2Py molecules and mimics the situation which occurs with increasing temperature, where the majority of intact molecules form ordered two-dimensional networks, while most of the doubly deiodinated molecules assemble into long organometallic polymeric rows. We use DFT calculations to determine the values of intermolecular interactions for intact molecules and propose a strategy for estimating the interactions for deiodinated molecules, where the organometallic interaction with Au atoms plays the dominant role. Our model is solved using Monte Carlo calculations and allows us to obtain the monomeric structure of intact molecules, the dimeric structure of singly deiodinated molecules and the polymeric row structure of (mostly) doubly deiodinated molecules. We obtain the coexistence of ordered intact Br2I2Py molecules and organometallic dimers, as well as their separation at large values of intermolecular interaction with Au. Similar results are obtained by studying mixtures of singly and doubly deiodinated molecules: dimer rows can be either incorporated into the two dimensional pattern of correlated polymeric chains or separated into their own dimeric structures.

Exploring the Antipolar Nature of Methylammonium Lead Halides: A Monte Carlo and Pyrocurrent Study.

The high power conversion efficiency of the hybrid CH3NH3PbX3 (where X = I, Br, Cl) solar cells is believed to be tightly related to the dynamics and arrangement of the methylammonium cations. In this Letter, we propose a statistical phase transition model which accurately describes the ordering of the CH3NH3+ cations and the whole phase transition sequence of the CH3NH3PbI3 perovskite. The model is based on the available structural information and involves the short-range strain-mediated and long-range dipolar interactions between the cations. It is solved using Monte Carlo simulations on a three-dimensional lattice allowing us to study the heat capacity and electric polarization of the CH3NH3+ cations. The temperature dependence of the polarization indicates the antiferroelectric nature of these perovskites. We support this result by performing pyrocurrent measurements of CH3NH3PbX3 (X = I, Br, Cl) single crystals. We also address the possible occurrence of the multidomain phase and the ordering entropy of our model.

Structural phase transition in perovskite metal-formate frameworks: a Potts-type model with dipolar interactions.

We propose a combined experimental and numerical study to describe an order-disorder structural phase transition in perovskite-based [(CH3)2NH2][M(HCOO)3] (M = Zn(2+), Mn(2+), Fe(2+), Co(2+) and Ni(2+)) dense metal-organic frameworks (MOFs). The three-fold degenerate orientation of the molecular (CH3)2NH2(+) (DMA(+)) cation implies a selection of the statistical three-state model of the Potts type. It is constructed on a simple cubic lattice where each lattice point can be occupied by a DMA(+) cation in one of the available states. In our model the main interaction is the nearest-neighbor Potts-type interaction, which effectively accounts for the H-bonding between DMA(+) cations and M(HCOO)3(-) cages. The model is modified by accounting for the dipolar interactions which are evaluated for the real monoclinic lattice using density functional theory. We employ the Monte Carlo method to numerically study the model. The calculations are supplemented with the experimental measurements of electric polarization. The obtained results indicate that the three-state Potts model correctly describes the phase transition order in these MOFs, while dipolar interactions are necessary to obtain better agreement with the experimental polarization. We show that in our model with substantial dipolar interactions the ground state changes from uniform to the layers with alternating polarization directions.