Photoinduced Quantum Interference Antiresonances inπ-Conjugated Polymers

Abstract

We observed photoinduced quantum interference antiresonances between several discrete infrared-active vibrations and the lower-polaron continuous absorption band in a series of pi-conjugated polymer films having superior planar orders, where the polaron transition energy is relatively small. The photoinduced Fano-type antiresonances are well explained by extending the amplitude mode model beyond the adiabatic limit. The agreement between the data and the model confirms the presence of a continuous electronic band above the polaron state. We show that high frequency modes are strongly coupled to electrons, with implications for superconductivity.

Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers

“…6,50 This interpretation of the CMS spectra was based on the theoretical work by Blackman and Sabra who predicted that for interchain coupling strength on the order of 0.15 eV the 1D polaron abruptly delocalizes, 51 and was confirmed by quantum chemical calculations of the chargeinduced absorption spectra of molecular aggregates. 52 Similar results were obtained by Österbacka et al based on photo-induced absorption ͑PIA͒ measurements on P3HT, [53][54][55][56] who showed that increased interchain coupling leads to a splitting of the single chain polaron energy levels by 2⌬ and a red shift of the lower energy polaron band ͑called DP 1 in Ref. 53͒.…”

Giant infrared absorption bands of electrons and holes in conjugated molecules

“…5. The mechanism giving 'massive' charge displacements for solitons 37,38 and for polarons [20][21][22]39 is pictorially described in Fig. 5, and may resemble 'the changes in electronic structures induced by specific vibrations' mentioned by Sakamoto et al 4 Figure 5 gives us a picture of polarons as objects that are not static.…”

“…The fact that large CH bands are observed despite weak expected and computed dipole derivatives might be understood on the basis of enhanced IR intensities due to vibronic coupling 4,17,20,24,25,39 to low-energy electronic excited states. Nafie's 25 description for the transition dipole of an IR band at frequency o a multiplies the contributions of dipole ARTICLE derivatives to transition moments by a term…”

“…This behaviour and related anti-resonances 20 can be understood in terms of amplitude theory 18,[20][21][22]39 or the theory of Nafie 25 , which predicts intensity enhancement from a larger positive dipole matrix element when the low-lying electronic state is above the vibrational excited state (o eg 0 4o a ) but a negative matrix element when o eg 0 oo a . If both occur simultaneously, interference (cancellation) can occur, but simultaneity is important because, as Nafie notes, the dipole strength is the 'absolute square of the electric-dipole transition moment, always a positive quantity.'…”

“…These are plausibly strong, but absolute intensities were not determined. The IRAV bands were interpreted with an amplitude theory 18,19 that has been further developed 20 along with related theories [21][22][23] including descriptions with a molecular viewpoint 24,25 . The present work takes advantage of ready determination of absolute IR absorption (extinction) coefficients in solution to find coefficients 10 times larger than that of Sakamoto et al 4 Results IR spectra of anions.…”