Chemistry Reference
In-Depth Information
In this chapter we discuss applications of conjugated polymers in which the
readout is observed colorimetrically. We provide an overview of the platforms that
have been employed and the diversity of screened analytes. The chapter focuses pri-
marily on polydiacetylenes (PDAs), reflecting the large body of literature on PDA-
based molecular recognition and sensor development (Okada et al. 1998; Jelinek
and Kolusheva 2001). We also discuss polythiophene, a type of conjugated
polymer that is finding increasing use in sensor applications (McCullough 1998;
Leclerc 1999).
12.2. PDA
Diacetylenes undergo photochemical polymerization to provide a conjugated enyne
polymer, as shown in Figure 12.1. The polymerization has a topochemical require-
ment: the a-carbon of one diyne must be positioned in proximity to the d-carbon
of an adjacent diyne. The resulting PDAs are frequently colored a deep blue or
purple. The blue/purple PDAs can be converted to a red material by a variety of
external stimuli, including pH, irradiation time and intensity, solvent (solvatochro-
mism), and temperature (thermochromism). The specific mechanisms of the blue to
red chromatic transition are context dependent and have not been established unam-
biguously (Carpick et al. 2004; Schott 2006). These chromatic transitions have been
studied most thoroughly in the context of thermochromism, and they generally
involve a perturbation of one or both of the following two features: 1) head group
packing and orientation and 2) conformation of main chain rotamers along the
enyne backbone. These perturbations change the core chromophore of the PDA by
affecting the planarity of the enyne, which is manifested by the color change. The
chromatic transition can also be followed using fluorescence spectroscopy because
the red form of PDA is fluorescent whereas the blue form is not.
Early work in diyne polymerizations focused on the formation of PDA from
diynes that adopted the proper geometry in the crystalline state (Baughman 1972;
Wegner 1972; B¨ssler 1984). If the diacetylene is amphiphilic, a variety of self-
assembled structures, such as monolayers, vesicles, and fibrillar superstructures,
can form. Many of these self-assemblies orient the diyne in a geometry that allows
photochemical polymerization. The synthesis of phospholipid analogs in which the
naturally occurring fatty acids on the glycerol were replaced with diacetylenic acids
was reported in 1980 (Hub et al. 1980). These materials were assembled into lipo-
somes that were subsequently photochemically polymerized to yield blue or red sol-
utions, depending on the extent of irradiation. The efficiency of this polymerization
is correlated with vesicle size (Peek et al. 1994).
Figure 12.1 Topochemical polymerization of 1,3-diacetylenes.
