J-Aggregate Behavior of Poly[(9,9- dioctyluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) in Crystal and Liquid Crystal Phases

Abstract

Poly­[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) in thermally annealed films exhibits red-shifted absorption spectra, which shows the typical feature of the J-aggregate and is confirmed by computational simulations. By controlling the annealing temperature, the aggregate behavior is explored in detail. It is found that the J-aggregate is formed only when the material is annealed at crystallization and liquid crystal temperatures. The microstructure and morphology of the aggregates show that the J-aggregates have tighter chain packing and a larger ordered domain. This work first demonstrates the J-aggregate behavior of the F8T2 material and establishes the relationship between optical spectra and the packing structure of F8T2.

Ultrafast electron dynamics in excited states of conjugated thiophene–fluorene organic polymer (pF8T2) thin films

“…Notably, there is no discernible intensity at this energy in the pristine emission spectrum. We conclude that for the asprepared sample the (0-0) peak is almost completely suppressed, in accordance with the observations of Ma et al 50 Spin-coated pF8T2 is presumed to form disordered H-like aggregates with low or suppressed (0-0) transition. 68,69 The emission spectrum was modelled using a modified Franck-Condon progression as outlined in eqn (S6) of the ESI.…”

“…72 A self-organized phase arising due to relaxion and re-ordering of the chains during the annealing process, in turn, promotes strong interchain coupling and resulting in longer exciton lifetimes, and was attributed to J-aggregates. 50 The fraction of the non-aggregated polymers present in the absorbance spectra in Fig. 1 is not observed in the emission spectra in Fig.…”

“…The vibronic broadening of conjugated polymers can typically be described by Franck-Condon fits depending on the Huang-Rhys factor. [50][51][52][53] For this purpose, we have isolated the main peak in Fig. 1b and subtracted With the annealing of the thin film in the UHV the absorption spectrum changes, see Fig.…”

“…55 According to Ma et al pF8T2 can undergo different phase transitions during annealing: a glassy state at 100 1C, an amorphous elastic state around 130 1C, crystallization between 150 and 200 1C, and an isotropic state above 310-330 1C. 41,50,56 While annealing between 100 and 200 1C improves the electronic and photovoltaic properties, higher temperatures seem to reduce crystallinity and increase disorder, which has a negative impact on these properties. 41 A new absorption peak due to annealing was also observed for the polymers P3HT and PFO, which is attributed either to the formation of a b-phase or to J-aggregates.…”

Insights into the origins of inverted circular dichroism in thin films of a chiral side chain polyfluorene

“…We studied the photophysical and chiroptical properties of cPFT2 in solution (Figure S1) and thin films (Figure 2) via UV/vis and CD spectroscopy. cPFT2 demonstrates spectral features reminiscent of achiral F8T2 36 . Both cPF and cPFT2 are CD silent in solution and demonstrate a weak optical activity in as‐cast films (Figure 2).…”