π-Stacking in Conjugated Polymers and Oligomers: A Structural and Spectroscopic Study

Abstract

Regioregular, alkyl-substituted poly(heteroaromatics) tend to crystallize in π-stacked structures as opposed to the herringbone pattern characteristic of non-substituted, planar aromatic molecules, e.g. sexithiophene. π-Stacking, favored by the crystallization of the sidechains, induces planarity in the conjugated backbone. The increased planarity causes a large (50-100 nm) shift in λmax (abs.) compared to the solution phase in which the backbone is twisted out of coplanarity, π-Stacking is a form of ground state aggregation that causes “fine structure” on the absorption peak. Excitation into the aggregate states rapidly leads to the formation of excimers as shown by the excitation spectra. In -stacked polymers, photoluminescence is almost entirely from the excimer states.

Poly(nonylbisoxazole): A Member of a New Class of Conjugated Polymer

“…These conclusions are at slight variance with those reported in a recent paper by Bunz et al These authors report absorption and emission spectra of aggregated poly(phenylene-ethynylene)s (PPEs) that are strikingly similar to those exhibited here and elsewhere, but Bunz et al conclude that the low-energy shoulders in the absorption spectra can be adequately explained by a single-chain transition that becomes allowed as a result of the planarization of the main chain. Since the planarization is induced by aggregate formation, these authors describe the low energy shoulder as “aggregate induced” rather than an “aggregate absorption”.…”

“…The main basis for their conclusion is that the calculated energy shifts due to planarization (0.35 eV) are similar to the observed energy diference between λ max and the low-energy shoulder. We have shown elsewhere that the exciton bandwidth due to π-stacking and the Davydov splitting also have a magnitudes around 0.3 eV . Reference 24 contains further discussion of the relative effects of planarization vs aggregation.…”

“…The cause of the fine structure is less settled. Some authors have attributed the shoulders to differing, yet distinct, conjugation lengths, to vibronic coupling, to Davydov splitting (inequivalent sites) in the solid, and to a combination of Davydov splitting and exciton band structure . The latter can produce broad absorptions centered around vibronic peaks, as well as additional structure caused by translationally inequivalent site effects. , …”

Synthesis and Characterization of Regiorandom and Regioregular Poly(3-octylfuran)

“…The origin of the multiple peaks or “fine structure” on the main π−π* absorption peak is still a matter of some debate. ,19a, These peaks are only observed in regioregular, conjugated polymers in the solid state (or in colloidal aggregates that have molecular packing similar to that of the semicrystalline solids), and then only when the size of the side chains allows for close approach of the π-systems on neighboring chains (π-stacking). ,, The cause of the multiple peaks has been ascribed to absorption of polymer segments with different, yet discrete, effective conjugation lengths, 8a,d, to vibronic coupling, and, more recently, to Davydov splitting of the exciton levels by inequivalent sites in the solid, 14b,,, or to combinations of Davydov splitting and the effects of exciton band density-of-state structure . It is extremely unlikely that the peaks arise from chain segments with different conjugation lengths because the spectra of monodisperse, short-chain, crystalline, conjugated oligomers often show fine structure that is almost identical in form to that in Figure . ,, In these crystalline oligomers, all the molecules are planar, and all have the same conjugation length.…”

Structural Engineering in Polymer Semiconductors with Aromatic N-Heterocycles

“…These experimental results imply the reduced reactivity upon oligomer homologation of thiazole derivatives. This is likely the reason why reports about poly­(thiazole) are limited. − …”