Absorption polyfluorenes

FIGURE 2.9 Typical absorption and emission spectra of polyfluorene in thin films (shown for poly(9,9-dioctylfluorene) 196). (From Gong, X., Iyer, P.K., Moses, D., Bazan, G.C., Heeger, A.J., and Xiao, S.S., Adv. Fund. Mater., 13, 325, 2003. With permission.)... 

H.-Z. Tang, M. Fujiki, and M. Motonaga, Alkyl side chain effects of optically active polyfluorenes on their chiroptical absorption and emission properties, Polymer, 43 6213-6220, 2002. 

Low polarisation ratios (<2 1) for absorption had also been found for amorphous PPV (1) deposited from solution by spin-coating on rubbed poly(tetrafluoroethylene) [PTFE]. It is evident that this could be improved on by making use of the high order parameter and self-organising properties of the nematic phase of liquid crystalline electroluminescent polymers such as those (16, 28 and 78-82) shown in Table 6.16. - 2 ° This was then found subsequently to be the case using thermotropic liquid crystalline polyfluorenes, such as 28 and 80 shown in Table 6.6 and segmented PPV derivatives, such as 81. The nematic phase exhibits the lowest viscosity of... 

Polyfluorene was first synthesized by Fukuda et al. via oxidative polymerization of huorene monomers using ferric chloride as a catalyst.2,11 Both mono- and dialkyl-substituted polyfluorenes were synthesized. Figure 10.1 shows the repeat unit of poly(9,9/-dialkyl-huorene-2,7-diyl). The polymers are soluble in common solvents such as chloroform, dichloromethane, and toluene. Figure 10.2 shows the absorption and fluorescence spectra of a solution of poly(9,9/-dihexylfluorene-2,7-diyl) (PDHF) in chloroform.11 The onset of the tt-tt absorption is at 420 nm, rising to a peak at 380 nm, yielding an optical gap of 2.95 eV. The fluorescence spectrum contains vibronic peaks at 417 and 440 nm and a shoulder at 470 480 nm. 

Measuring the steady state optical absorption and emission properties of any luminescent polymer is the most basic but fundamental photophysical measurement we can make. Figure 1 depicts both the absorption and emission spectra for a series of polyfluorene oligomers and poly[9,9-di- -(2-ethylhexyljfluorene] (PF2/6), along with the fully rigid ladder type MeLPPP. 

The transient triplet absorption for the common polyfluorene PF2/6 is shown in Fig. 9, the spectra was measured by quasi-cw photoinduced absorption. This lock-in technique allows the absorption of long-lived states to be measured with high sensitivity. It is possible to estimate the lifetime of the state using the quasi-cw experiment [46] however, more accurate methods in-... 

Fig. 10 Transient absorption spectrum of the polyfluorene charged state, in this case free carriers were generated by highly efficient singlet-singlet annihilation similar spectra are obtained by field induced quenching of polyfluorene singlet states...

Fig. 21 Pump-probe spectra for the prototypical polyfluorene PF2/6 at high excitation density, showing the photoinduced absorption feature of the singlet population (1.6 eV peak) being rapidly quenched, leading to the formation of charges and characterized by the absorption of the charged state at 2.6 eV...

Fig. 3 a Absorption and emission spectra of monoalkylated polyfluorenes in solution (dashed line) and in the solid state (solid line), b Absorption and emission spectra of dialkylated polyfluorenes in solution (dashed line) and in the solid state (solid line) (modified from [16,17])... 

In combination with a C70 fullerene derivative—PTPF70—the system yielded improved power conversion efficiencies of 0.7% due the increased absorption of the C70 derivative [208,210]. Using similar polyfluorene derivatives, power conversion efficiencies of about 0.9% (APFO-Green2) [212[ and 2.2% (APFO-Green5) [213] were achieved. Other low band gap polymers, with absorption spectra extending up to 1100 run, yielded efficiencies of aroimd 1% [214-217]. 

In Fig. 3.15 the fluorescence spectra of a polyfluorene (PF2/6) film recorded in time windows covering ca. 10 ps to ca. 2 ns are presented. The film was held at 80 K and excited with 3.35 eV photons, i. e. well above the S S() 0-0 absorption band [73, 74], The vibronic components show a gradual bathochromic shift of roughly 100 meV, indicative of spectral diffusion. Further insight into the relaxation process is obtained by plotting the emission spectra as a function of the time... 

Fig. 3.15 (Right) Time-gated fluorescence spectra of a film of polyfluorene PF2/6 after optical excitation at 3.35 eV at low temperature (15 K). The spectra were time integrated from 0-2, 8-10, 35-57, 134-136, 329-331, 822-824 and 1770-1850 ps, respectively. The arrow indicates the excitation energy. (Left) Low-temperature (80 l<) absorption spectrum of the film. The dashed line is a fit of a Gaussian curve to the red edge of absorption spectrum. Reprinted from [73], copyright 2001, with permission from Elsevier.

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