Giant infrared absorption bands of electrons and holes in conjugated molecules

Abstract

Infrared (IR) absorption bands often convey identifying information about molecules, but are usually weak, having molar absorption coefficients o200 M À 1 cm À 1 . Here we report observation of radical anions and cations of conjugated oligomers and polymers of fluorene and thiophene that possess intense mid-infrared absorption coefficients as large as 50,000 M À 1 cm À 1 , perhaps the largest known for molecular species. For anions of fluorene oligomers, F n , n ¼ 2-4, IR intensities increase almost linearly with n, but with a slope much larger than one, indicating that the absorptions are not extensive properties. Large intensities seem to arise from a mechanism known for charged solitons and polarons in conjugated polymers. In this mechanism, vibrations of ungerade symmetry drive substantial displacements of charge, creating the large dipole derivatives responsible for intense IR absorption. Both experiments and calculations find that pairing with counter-ions attenuates IR band intensities. The IR bands may be diagnostic for bound ion pairs and their escape to form free ions.

How intermolecular geometrical disorder affects the molecular doping of donor–acceptor copolymers

“…4b,c, it can be seen how the biggest atomic displacements of this mode happen to be in the region where the polaron is located. The enhancement in absorption intensity associated with this mode is consistent with previous results, which have argued that the oscillation of the polaron charge density concomitantly with a vibrational mode can result in an increase in the transition dipole moment 27 . Thus, for the case of PCPDT-BT, while the charge transfer occurs when F4-TCNQ is close to the D moiety, the bond vibrations involving the two nearest neighbours A moieties on which the polaron wavefunction delocalizes give an important contribution to the mode absorption intensity.…”

“…We attribute the intensity enhancement of absorption between 1,000 and 1,300 cm À 1 to the presence of dopantinduced hole polarons on the polymer chain. The high oscillator strength for vibrational absorption bands in presence of charged states has been extensively documented in conjugated molecules 27,33,34 . Figure 2b shows the infrared spectra on a larger spectral range up to 6,000 cm À 1 for the same PCPDT-BT samples presented above.…”

“…In PCPDT-BT, the cyclopenta-dithiophene (CPDT) moiety is D and the BT moiety is A. We used infrared spectroscopy to study microscopic changes induced by doping, as this technique is known to be sensitive to the presence of charged species in conjugated polymers through molecular vibrations [27][28][29] . An advantage of looking at vibrations is that their absorption resonances are less affected by inhomogeneous broadening, an effect which is instead markedly impacting the amount of information possibly extracted from electronic transitions in conjugated polymers 30,31 .…”

Structure-property relations in amorphous carbon for photovoltaics

“…We conclude that the value of the band gap is determined by the conjugation length of p states from sp 2 C atoms within the a-C network, similar to trends observed in conjugated molecules. [23][24][25] Our results show that lower densities in a-C lead to larger fractions of sp 2 C atoms, and thus to more extensive p conjugation and reduced band gaps. We observed similar trends in a-C:H, with the important difference that the disorder induced by the H atoms further increases the number of states in the Urbach tails (Figure 2(a)).…”

Electron Localization of Anions Probed by Nitrile Vibrations

“…It is known that electrons and holes in conjugated molecules exhibit intense IR bands, 8,9 especially (C=C) bands, and that their intensities are dependent upon delocalization of charges. 9 Infrared (IR) spectroscopy has previously been used to discern charge transfer states by means of spectral shifts of specific vibrational bands such as (C=O) bands [10][11][12][13][14][15] and (C=C) bands.…”