Molecular Aggregation and Luminescence Behavior of Bulk Poly(2,5,2‘,5‘-tetrahexyloxy-8,7‘-dicyano-di-p-phenylenevinylene)

Abstract

Structural evolution and its effect on the optical absorption/emission behavior of poly(2,5,2‘,5‘-tetrahexyloxy-8,7‘-dicyano-di-p-phenylenevinylene) (DH-CNPPV) upon short-term isothermal heat treatment at elevated temperatures were studied by means of a combination of X-ray diffraction, transmission electron microscopy, UV−vis spectroscopy, and photoluminescence spectroscopy. Results indicated kinetic preference for mesomorphic order in bulk DH-CNPPV, with the isotropization temperature (Ti) located in the vicinity of 210 °C. Upon heat treatment at elevated temperatures (Ta) below Ti, supramolecular self-assembly of DH-CNPPV chains results in lamellar structure ca. 2.0 nm in layer spacing. This lamellar phase exists in the form of nanodomains (ca. 10−20 nm in diameter) that agglomerate into wormlike features with DH-CNPPV backbones and the mesomorphic lamellae lying transverse to the worm's long axis. Details in the selected-area electron diffraction pattern of shear-oriented specimens prescribe a short intraboard spacing of 0.38 nm and longitudinal displacement by one-third of the repeating unit between neighboring backbones, such that adequate interaction between the cyano group and the phenylene ring is allowed for. Accompanying the thermally induced morphological changes, UV−vis light absorption behavior varies consistently with improvement or disruption of the lamellar order whereas the major emission in the photoexcited spectrum shifts from 590 to 640 nm upon emergence of mesomorphic order, which is directly excitable and hence attributable to either extended backbone conjugation or enhanced interchain delocalization in the mesomorphic state. This is in dramatic contrast to the closely related case of poly(2,5-di-n-octyl-oxy-1,4-phenylenevinylene) (DO-PPV) without cyano substitutions in the backbone (for which excimer emission is the preferred mode upon morphological aggregation), signifying the role of specific cyano−phenylene interaction in the ground state of aggregated chains in the mesomorphic phase.