Small band gap semiconducting polymers made from dye molecules: Polysquaraines

Journal of Physical Chemistry

Volumn 100, Issue 5, 1996, Pages 1838-1846

  Brocks, G ^a   Tol, A ^a  

^a PHILIPS RESEARCH LABORATORIES   (Netherlands)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0001470619     PISSN: 00223654     EISSN: None     Source Type: Journal    
DOI: 10.1021/jp952276c     Document Type: Article

References (47)

1. Band gaps in polymers are identified with the onset of optical absorption due to electronic excitations. An excitation energy of 2 (3.5) eV corresponds to an absorption wavelength of 620 (350) nm.
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41. Even for the dye molecule 8, this procedure works reasonably well. Dividing the calculated LDA HOMO/LUMO gap of 1.2 eV by 0.6 gives a fair agreement with experiment. See e.g.: Dirk, C. W.; et al. J. Am. Chem. Soc. 1995, 117, 2214.
42. Bear in mind that the out-of-plane torsion in polymer 5 increases the band gap as compared to completely planar structures; cf. ref 31.
43. In some cases we find in our calculation oxygen lone-pair-like n levels close to HOPO π level; cf. ref 21. In order to avoid complicating the discussion, we will only discuss states of π symmetry. Since the n-π* transition is forbidden, n states will not appear in the optical spectrum at a low energy. However, they could in principle act as trap states upon p-doping of the polymer.
44. For the helical symmetry group, see e.g.: Springborg, M. Int. Rev. Phys. Chem. 1993, 12, 241 and references therein.
45. x. The gap is then found at K = 0.
46. H is simply that the amplitude of the LUMO on the carbon atom of the fragment coupled to the nitrogen atom is much larger than of the HOMO.
47. In ref 16 it was speculated that the small band gaps of squaraine polymers could be the results of donor/acceptor properties of the individual fragments. That type of reasoning relies on charge transfer between the energy levels of the fragments, but one has to assume that the fragment states are not completely rehybridized (and thus remixed and reordered) in the polymer. However, as we have shown here, such hybridization effects are very large here and mask the charge transfer effects. Therefore, it seems that the donor/acceptor concept is not the key to understanding the electronic structure.