Spectral and thermal analyses of small structural shifts in some highly symmetric polycyclic aryl derivatives of cyclotriphosphazatriene

Abstract

Small structural shifts are induced in cyclotriphosphazatriene by symmetric substitution of its hexachloro derivative with analogous aryl derivatives, namely hexaphenyl, trisbiphenyl, trisnaphthyl and trisfluorene. These shifts are structurally probed by spectral analysis and thermally sensed by differential thermal analysis. Except for the fluorene derivative, which shows a coplanar ring structure, the other derivatives presume a molecular configuration in which the aryl rings are located in a plane perpendicular to the phosphazene ring. Such a perpendicular configuration facilitates strong intramolecular hydrogen bonding, which associates the aryl π-bonding electrons with the protonated phosphazene ring, which is not possible for the fluorene coplanar structure. The intramolecular association follows a typical order of the molecular resonance induced in these polycyclic phosphazenes: phenyl > biphenyl > naphthyl > fluorene. Various spectral evidence consistently support the proposed order of molecular resonance and intramolecular association. Also, various thermodynamic parameters of solid state and solid-liquid phase transitions consistently confirm that these polycyclic phosphazenes are structurally strained. However, the intramolecular association and molecular resonance play a significant role in stabilising the molecular structure against strain so that the thermal data reveal thermodynamic parameters reflecting the same order for the molecular thermal stability.