Microscopy studies of non-linear optical materials.

摘要

A central challenge in the development of organic non-linear optical materials is the translation of molecules demonstrating appreciable hyperpolarizability into materials possessing substantial electro-optical activity. An important step in this translation is developing strategies to improve chromophore ordering in the material. In chromophore-polymer composite materials, ordering is induced through the interaction of the chromophore dipole moment with an external electric field, a process referred to as "poling". To provide insight into the molecular details of the poling process under conditions representative of device construction, the rotational dynamics of single DCM (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4 H-pyran) molecules in poly(methyl acrylate) (PMA) at T = Tg + 11° C are investigated using single-molecule confocal fluorescence microscopy. The rotational dynamics of DCM are found to be weakly perturbed in the presence of a 50 V/mum electric field, typical of the field strengths employed in device construction. In addition to the single-molecule studies, second harmonic generation (SHG) and two-photon fluorescence (TPF) microscopy are used to explore bulk reorientation dynamics operative in the poling process for DCM and TP-DBA1 (4-[2-(5-pyrrolidine-1-yl-thiophene-2-yl)-vinyl]-benzaldehyde) in PMA (Tg = 9° C), poly(vinyl acetate) (PVA, Tg = 30° C), and poly(methyl methacrylate) (PMMA, Tg = 86° C). SHG provides a measure of chi (2), which is dependent on the extent of chromophore ordering. The polymer hosts were chosen such that ambient temperature is above, below, or well below Tg. These bulk studies demonstrate that the rotational mobility of the chromophore is strongly dependent on temperature relative to the Tg of the polymer host. The relevance of these findings with respect to current models of the poling process is discussed.