Research at CMMPE — Materials — Dyes

The research group has considerable interest in dye-doped liquid crystal and liquid crystalline polymers for both display and organic laser devices. For the purpose of low-power consumption reflective colour displays, CMMPE carries out studies on highly ordered dichroic dyes for use in nematic, smectic, and polymer-dispersed liquid crystal devices. By dispersing a dye into a liquid crystalline matrix the absorption can be controlled through electric fields as the director, and consequently the transition dipole moment of the dyes, can be reoriented (as shown in Figure 1). Specifically, the research carried out by the group focuses on all elements of the dyes ranging from the synthesis through to the resulting optical performance when tested in devices.

Figure 1. Absorption and emission in a dye-doped nematic for two different alignments: (left) homogenous and (right) homeotropic alignment.
In the left image, the light wave propagates in a direction perpendicular to the director and the optical field is aligned parallel to the director
corresponding to maximum absorbance and emission. In the right image, the situation is reversed.

Compounds have been developed whereby the dye is grafted onto a mesogenic unit so as to improve the miscibility and order in liquid crystalline structures. An example are the fluorescent perylene based structures that exhibit smectic C phases (Perylene-based Fluorescent Liquid Crystal Dye Guest-host Mixtures de Hondt, P., Perkins, S. and Coles, H.J. Mol.Cryst.Liq.Cryst., 366, 263-70, (2001)). In addition, dyes that exhibit photo-isomerising effects through the cis-trans conformation change have also been developed for use in flexoelectric and ferroelectric devices.

For the past ten years the group has carried out studies on small molecule organic laser dyes, such as the ubiquitous DCM laser dye, in chiral nematic, chiral smectic, and blue phase lasers. The research concentrates on the photo-physical properties, e.g. photoluminescence efficiency and fluorescence lifetime (carried out in collaboration with the Optoelectronics group at the Cavendish Laboratory), through to the resulting output characteristics that are observed when tested in liquid crystal lasers. The research also involves the development of new stable organic laser dyes that are highly miscible with liquid crystalline structures.

Figure 2. Chemical structure of the arylidene laser dye
4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4Hpyran (DCM).

Chemistry of an Organic Dye

In organic molecules, p-orbitals diffuse above and below the plane of the molecules leading to pi-bonds. The electrons in these bonds are responsible for pi-pi* excitations which absorb light in the UV or visible spectrum giving rise to a particular colour. The greater number of conjugated single-double bonds changes the colour of dye from UV to blue, blue to red and red to infra red. In a particular dye these conjugated bonds form a set of chromophore.

Figure 3. Beta-carotene

For the laser dyes there is one basic requirement; uniform distribution/ localisation of pi-electrons at one area of the molecule. In conventional dyes these electrons can excite at different energy levels so lead to different absorption-bands. Because the laser emission entirely depends upon the fluorescence band there are more chances of over- lapping of absorption and fluorescence bands. To overcome this issue, and to convert a dye to in to a laser one, conjugated systems are functionalised with either charged or electronegative groups of atoms as terminal functionalities. Our research group is currently focusing their efforts to explore non-ionic laser dyes either as a single component or as part of the mesogenic unit of the liquid crystal.