Observation of the β‐Phase in Two Short‐Chain Oligofluorenes

Abstract

The polymer poly(9,9‐dioctylfluorene) (PF8) can be driven into a morphological form termed the β‐phase that has enhanced planarity and increased structural rigidity. We show that the β‐phase can be generated in two different fluorene oligomers; a fluorene pentamer, and a short (statistical) oligomer composed of chains having a maximum length of 19 monomer units. By comparing the energy of the zero‐zero phonon line in fluorescence from the β‐phase oligomers with that in the PF8 polymer, we show that the electronic conjugation length of the β‐phase is (30 ± 12) monomer‐units, a value consistent with the persistence length of the molecular chain.

Understanding the Nature of the States Responsible for the Green Emission in Oxidized Poly(9,9‐dialkylfluorene)s: Photophysics and Structural Studies of Linear Dialkylfluorene/Fluorenone Model Compounds

“…In particular, the spectral range may be split into two parts: a) l 400 nm and b) l > 400 nm. The absorption in region (a) originates from p-p à transitions similar to those observed in oligo-dialkylfluorenes [17][18][19] whereas the absorption in region (b) is believed to be due to p-p à transitions involving (but not wholly localized on) the fluorenone moiety. [2,14] The 370 nm p-p à absorption maximum reported for a pentadialkylfluorene [19] is close to the 368 nm measured here for the fluorenone-centered pentamer 5, consistent with calculations by Zojer et al [7] which show that the p-electron structure of an oligofluorene backbone is not significantly disturbed by the replacement of one fluorene moiety by a fluorenone moiety, at least in respect of the strongest long wavelength p-p à transition.…”

“…Each spectrum shows a broad and unstructured emission band, red-shifted from the typical vibronically strucwww.afm-journal.de tured emission spectra characteristic of corresponding length oligo-dialkylfluorenes. [17,18] The emission bands of fluorenoneterminated 2 and 3 peak at around 528 nm, blue-shifted by some 22 nm from the bands of the corresponding fluorenone-centered 4 and 5 which peak at around 550 nm. This is a significantly smaller energy shift (0.09 eV) than observed for the absorption spectra (0.19 eV), consistent with participation of a different excited state in the emission process than that directly reached by p-p à absorption.…”

Comparing morphology in dip-coated and spin-coated polyfluorene:fullerene films

“…The fluorescence spectra of the films as well as of the solutions (not shown), show three main emission regions: 18,19 The F8 α-phase emission between 410 and 420 nm, the β-phase emission from located between 440 and 500 nm, and a broad emission band between 500 to 550 nm. There are two possibilities for this red-shifted emission: Either it originates from an intramolecular F8 charge transfer state or it stems from a photo-oxidized keto-state of F8.…”

“…The α-phase has its emission maximum around 410 and 420 nm and shows no vibronic fine structure, while the emission from the β-phase is located between 440 and 500 nm and shows vibronic peaks. 18,19 The mechanism of the formation of the β-phase structure is poorly understood, but it has been found in both films and in solution. 20,21 There is also a third contribution to the emission spectra, a broad emission between 500 and 550 nm.…”

“…This emission is attributed to either a charge transfer state of F8 or to photo-oxidized F8 keto-states. [18][19][20][21][22][23][24][25] Finally, UV/VIS absorption spectroscopy was used to quantify the presence of the α-and β-phases by a method proposed by Cone et al 24 …”