Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 360

O

O

O

O O

OH

NH 2

n

OH H

H OH

N
H

R

O

m

R =

H 2 N HO

O

O O OH

H

OP
OH

HO

Common chain lengths from 21 to 26
with the diacetylene at m,n:
2,4; 5,7; 6,8; or 10,12

HO

HN HN OH

O

O

HO HO

HO

OH

Fig. 2 Examples of diacetylene monomers showing diversity of head groups

phenyl groups with heteroatom aromatics are other types of conjugated sensing
polymers. Poly(thiophene)s (PTs) are the most common class but nitrogen-containing
conjugated polymers such as poly(benzoxazole)s [ 16 ] are known. A common
approach in developing polymers for ion detection is to copolymerize nitrogen-
containing aromatics with carbon aromatic units, examples include: polyfluorene
(PF) copolymers with oxadiazole [ 17 - 20 ] and benzothiadiazole (BT) [ 8 ] units,
PAEs incorporating quinoline [ 21 ] and quinoxaline [ 22 ] units, and PPVs with
phenothiazylene and pyridylene [ 23 ] groups in the backbone. As with PAEs, side
chains are added to affect solubility, chain structure and packing, as well as to add
binding functionality.
PDA has a backbone of alternating double and triple bonds with side chains
coming off the double bonds [ 5 ]. Diacetylene monomers can have a wide variety of
charged and polar head groups (Fig. 2 ). Their amphiphilic nature allows self-
assembly in water; the self-assembled structures are then photopolymerized to
create the polymer in situ . Early studies on PDA were performed on chains formed
in diacetylene crystals and then dissolved; however, for sensing applications, PDA
is commonly prepared as self-assembled (in water) 2-D (films) or 3-D (liposomes,
tubules, etc.) materials. For most PDA materials, the polymer backbone is isolated
from the target by multiple methylene units; the notable exceptions are materials
based on 2,4-diacetylenes. PDA was originally used for colorimetric biosensing
[ 24 ]; realization that there was a concomitant emission change, that could be
exploited for sensing, was developed later [ 25 , 26 ] and has been adopted gradually
by many groups in the PDA field [ 5 , 27 ].

3 OptoElectronics of Conjugated Polymers

The optical properties of conjugated polymers primarily arise from the -bonds in

p

the polymer backbone. Alignment of -bonds leads to electron delocalization, also

p

referred to as conjugation or -conjugation. Theoretically, backbone -electrons

p p

can be delocalized along the entire length of the polymer if the -bonds are all

aligned; however, twists in the backbone and defects in the polymer structure at

p