Structure, Emissive Properties, and Reporting Abilities of Conjugated Polymers - Advanced Fluorescence Reporters in Chemistry and Biology - page 361

room temperature lead to reduced delocalization and conjugation lengths. The
p

-bond system can contain double or triple bonds and aromatic rings. It should be
noted that there are examples of emissive

-conjugated polymers with Si and Ge
backbones used in sensing [ 28 - 30 ], but these are relatively rare and this chapter will
focus on carbon-conjugated polymers.
Delocalization of

a

electrons leads to the absorbance and emission properties of
the conjugated polymer. These optical properties are analogous to those of small-
molecule fluorophores; however, the extended physical structures of the polymers
lead to more varied and complicated excited state relaxation and energy transfer
pathways than in small-molecule fluorophores. Molecular orbital (MO) theory and
valence band gap models have been used to describe the electronic states and
transitions of conjugated polymers. Br´das and co-workers have reviewed model-
ing conjugated polymers with MO theory [ 31 , 32 ] and describe how Huckel theory
and configuration interaction analysis (CI) are used for modeling oligomers.
Quantum mechanical predictions for conjugated polymers are achieved by per-
forming calculations on oligomer series and extrapolating the results to an extended
polymer; interchain interactions have also been modeled [ 31 , 33 ]. Comparisons
between theoretical predictions and experimental results, and discussion of the
possible pitfalls in these techniques, have been reviewed by Gierschner et al. [ 33 ].
Conjugated polymers have also been described using valence band gap theory,
developed for infinite solids, with the filled

p

-orbitals making up a valence band
with an energy gap to a conduction band. When the polymer is excited via photon
absorption, an electron is promoted from the valence to the conduction band
creating a positively charged hole in the valence band paired with the electron in
the conduction band. The electron/hole pair forms an exciton, which can migrate
along the conjugated backbone; the exciton is destroyed when it encounters a
perturbation in the valence gap and the electron returns to the ground state via a
radiative or nonradiative process to recombine with the hole (Fig. 3 ). In some
polymers, the hole and electron decouple and the electron migrates as a free-carrier
creating charge separation - this usually results in nonradiative return of the

p

-

h ν′

-

+

h ν

band gap

OR

+

-

R R R R

heat

n

+

R R R R

Fig. 3 Cartoon of exciton formation, migration, and recombination (radiative and nonradiative)