Influence of Interchain Interactions on the Absorption and Luminescence of Conjugated Oligomers and Polymers:  A Quantum-Chemical Characterization

Abstract

Correlated quantum-chemical calculations are used to investigate the influence of interchain interactions on the absorption and emission of π-conjugated chains. The results are discussed in relation to the utilization of conjugated materials as active elements in electro-optic devices; they provide guidelines on how to prevent a substantial decrease in luminescence yield in solid films. In high-symmetry cofacial configurations, interchain interactions lead to a blue shift of the lowest optical transition compared to that calculated for an isolated chain; the appearance of an additional red-shifted component is expected when positional disorder is considered. The absence of any significant oscillator strength in the transition between the ground state and the lowest excited state in highly symmetric complexes implies that the luminescence emission will be strongly quenched. This picture is, however, modified when one takes account of the relaxation processes which occur in the lowest excited state. The nature of the most stable photogenerated species and the role played by chemical impurities are also addressed.

The Nature of Interchain Excitations in Conjugated Polymers: Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

“…In the few regions where the interfacial emission shows the solvatochromic red shift, the fact that the excited state has a large dipole moment (∼9-13 D larger than that of the ground state) suggests strong excited-state charge separation. Because typical interchain distances in conjugated polymer films are ∼4 Å, 66 this magnitude of ∆µ e suggests nearly complete separation of unit charges between adjacent chains if the radius of the emitter R ) 5 Å, consistent with assignment to interchain polaron pairs. 6 This same dipole moment could be achieved for larger choices of the radius of the emitter if only fractional charges were separated, which still would be suggestive of interchain species comprised of highly charge-asymmetric exciplexes.…”

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

“…When the conjugation length is longer than the intermolecular separation, as is generally the case in polymers, quantumchemical calculations have shown that the exciton bandwidth decreases as the conjugation length increases. 1,46,47 This is because for long conjugation lengths the point dipole approximation breaks down and there is a tendency for positive and negative contributions to the atomic transition densities to cancel each other, leading to a reduction of the interaction energy/exciton bandwidth. Within this model we can now interpret the subtle variations in the absorption spectra as a function of MW in terms of variations of the intrachain conjugation length.…”

Nobel Lecture: Semiconducting and metallic polymers: The fourth generation of polymeric materials

“…The existence of an A g state or a triplet excited state below the 1B u state will favor nonradiative recombination (in both cases, direct radiative transitions to the ground state are forbidden). Interchain interactions in the excited state (''excimers'') also lead to nonradiative channels for decay (Jenekhe and Osaheni, 1994;Rothberg et al, 1996;Cornil et al, 1998).…”