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

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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 1,4‐dihalonaphthalene series, 1,4‐dichloronaphthalene and 1‐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 1‐bromo‐4‐chloronaphthalene stable for all composition, but 1‐bromo‐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 nonhalogen modes. In the 1‐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 1‐bromo‐4‐chloronaphthalene crystal form to that of 1,4‐dibromonaphthalene 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 1‐bromo‐4‐chloronaphthalene crystal form to that of 1,4‐dibromonaphthalene. 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 1,4‐dibromonaphthalene may also be present in the other two compounds.