Molecular motions and lattice stability of a disordered organic alloy: Binary solid solutions of 1,4-dihalonaphthalenes

Abstract

Molecular motions of 1,4-dihalonaphthalenes and their binary solid solutions are studied by Raman spectroscopy to understand the effect of chemical perturbation of the lattice stability. In the 1,4dihalonaphthalene series, 1,4-dichloronaphthalene and I-bromo-4-chloronaphthalene are isomorphous but the additional chemical perturbation by another bromine atom creates a lattice instability which results in a different crystal structure for 1,4-dibromonaphthalene. Not only is the binary organic alloy (solid solution) of 1,4-dichloronaphthalene with l-bromo-4-chloronaphthalene stable for all composition, but 1bromo-4-chloronaphthalene shows large solubility with 1,4-dibromonaphthalene as well. In both alloys, all phonons are amalgamated and so are some internal vibrations which are predominantly non halogen modes. In the l-bromo-4-chloronaphthalene alloy with 1,4-dibromonaphthalene, the lattice is unstable in the critical concentration region with mole fraction of 1,4-dibromonaphthalene between 0.24 and 0.44. In this region, the lattice can be driven from the l-bromo-4-chloronaphthalene crystal form to that of 1,4dibromonaphthalene by additional chemical perturbation. Certain features of the phonon motions and exciton splittings are retained across this critical concentration region, indicating that the lattice instability encountered is not along all molecular degrees of freedom and that certain intermolecular interactions do not change in going from l-bromo-4-chloronaphthalene crystal form to that of 1,4dibromonaphthalene. A close inspection of their crystal structures shows that in each case the molecules stack to form linear chains and that some of the relationships between two stacks remain unchanged. The structural similarity suggests that the one dimensional triplet exciton transport found in 1,4dibromonaphthalene may also be present in the other two compounds.5802

Dynamics of singlet-triplet transitions in 1,4-dihalonaphthalene neat crystals and solid solutions

“…2 The situation provides an opportunity for further investigation of the relative roles of the intrastack interactions and the interstack interactions in relation to the triplet exciton dynamics, (iii) Although DCN and BCN are isomorphic, BCN shows spectroscopic manifestations of structural disorder presumably with respect to chlorine and bromine positions. 3 The dynamics of energy states in disordered systems is a subject of current interest. A comparative study of DCN (not disordered) and BCN S0 -*• Tx transitions can be used to investigate the effect of structural disorder on exciton dynamics and optical dephasing.…”

“…(iv) In this system large mutual solubilities between DCN and BCN as well as between BCN and DBN are observed. 3 The most unusual feature is the large mutual solubility of BCN and DBN despite the difference in their crystal structures. 3 The BCN crystal structure prevails up to 24 mol % of DBN.…”

Chemical perturbation and lattice instability in molecular crystals: crystal structures and molecular motions of 1,4-dihalonaphthalenes

“…To facilitate the discussion, we would like to summarize the chief results of the Raman study of the dihalonaphthalenes (Bellows & Prasad, 1977). (1) DCN and BCN are isomorphous, but line-broadening indicated that BCN is orientationally disordered on the microscopic scale.…”

“…We discuss here the effect of chemical perturbation on the crystal structure in the series 1,4-dichloronaphthalene (DCN), l-bromo-4-chloronaphthalene (BCN), and 1,4-dibromonaphthalene (DBN) by combining the present X-ray diffraction study with the Raman spectra of these compounds and their mixed crystals, presented elsewhere (Bellows & Prasad, 1977). The three-dimensional crystal and molecular structures of DCN and DBN have been solved from single-crystal X-ray intensity measurements.…”

Six-Component Molecular Solids: ABC[D1–(x+y)ExFy]2

“…A priori, it is difficult to introduce more than one molecule into a stoichiometric molecular crystal. Nonstoichiometric multicomponent crystals, or in other words, solid solutions or organic alloys, − are more easily made but even here, there are chemical boundary conditions and many crystallization outcomes are not easily predicted.…”