Triplet exciton state and related phenomena in the $\beta$-phase of poly(9,9-dioctyl)fluorene

Using both time-resolved emission and cw photoinduced absorption spectroscopy as a function of temperature, the aggregation phenomena ($\beta$-phase formation) observed in poly(9,9-dioctyl)fluorene is studied. All spectra of the $\beta$ phase, including absorption, prompt and delayed fluorescence, phosphorescence, and photoinduced triplet absorption feature very narrow linewidths, which are unique within the class of conjugated polymers. From the comparison of the latter data with amorphous polyfluorene, poly(9,9-diethylhexyl)fluorene, as well as with the fully planar ladder-type poly(paraphenylene), we conclude that the origin of the $\beta$ phase cannot simply be an extended intrachain conjugation, but interchain interactions are involved. Furthermore, the $\beta$ phase acts as an energetic trap for both singlet and triplet excitons initially created on amorphous chain segments. The delayed fluorescence kinetics of the $\beta$ phase were measured at different temperatures. From the analysis of these decays within the framework of dispersive triplet migration in a Gaussian density of states distribution, further evidence is provided that the delayed fluorescence originates from triplet-triplet annihilation. At room temperature, it is clear that triplet excitons migrate over large distances, exceeding that of singlet excitons. Also, the segregation time between dispersive triplet migration and classical thermally activated hopping, is in the case of $\beta$-phase containing samples, dependent on the separation of the $\beta$-phase domains.

Figure 1

Compendium of room temperature, solid-state absorption spectra including PFO, PF2/6, and MeLPPP. All spectra are normalized and offset for clarity. The coumarin 120 dye laser profile for selective excitation of the $\beta$ phase is shown as dashed line. The insets show the chemical structures of the polymers investigated $(R_1=n\text{−}{\mathrm{C}}_6{\mathrm{H}}_{13};R_2=1,4\text{−}{\mathrm{C}}_6\mathrm{H}4\text{−}n\text{−}{\mathrm{C}}_{10}{\mathrm{H}}_{12})$.

Figure 2

Normalized prompt fluorescence spectra of PFO, PF2/6, and MeLPPP obtained with thin film samples at $11\mathrm{K}$ using one nanosecond integration time. The PFO $\beta$ phase was selectively excited at $2.86\mathrm{eV}$. A three nanosecond delayed $\beta$-phase spectrum is shown as well.

Figure 3

Thin film phosphorescence spectra of the three polymers investigated. All spectra were obtained at $11\mathrm{K}$ upon integrating the delayed emission from $10\text{to}90\mathrm{ms}$; PFO was excited selectively in the $\beta$ phase at $2.86\mathrm{eV}$. For the ease of evaluation the data are shown normalized and offset.

Figure 4

Compendium of photoinduced quasi-cw transient absorption spectra. All data were measured on ${10}^{-5}\mathrm{wt.}\%$ solutions (solvent as indicated) at room temperature, excitation was made at $3.1\mathrm{eV}$; the chopping frequency was $72\mathrm{Hz}$.

Figure 5

$77\mathrm{K}$ photoinduced transient absorption spectrum of a PFO thin film sample. The sample was excited at $3.1\mathrm{eV}$; the chopping frequency was $72\mathrm{Hz}$. The area around $2\mathrm{eV}$ is shown on a magnified scale.

Figure 6

Normalized prompt (upper part) and one microsecond delayed (lower part) emission spectra of a PFO thin film sample, either obtained at ambient or at low temperature after excitation in the amorphous PFO absorption at $3.5\mathrm{eV}$.

Figure 7

Comparison of the PFO (directly excited in the $\beta$ phase at $2.86\mathrm{eV}$) and all-amorphous PF2/6 delayed fluorescence decay at $11\mathrm{K}$ using thin film samples. In this double logarithmic presentation the solid lines refer to power law decays, characterized by an exponent of $-1.04$. The exponential decay of the prompt fluorescence of PF2/6 is simulated using a lifetime of $300\mathrm{ps}$. Note, the relative emission intensity between both curves is arbitrary.

Figure 8

Decays of the spectrally integrated, room-temperature fluorescence of PFO $({\lambda }_{\mathrm{exc}}=2.86\mathrm{eV})$ and PF2/6 $({\lambda }_{\mathrm{exc}}=3.5\mathrm{eV})$ thin films. The solid lines refer to power law decay kinetics with slope of $-1.04$ and $-2$, respectively. The data are arbitrarily scaled to each other.