Optical Properties

The spectroscopic properties of polymers P1-P3 were measured in both solutions (THF) and solid films. The optical properties of polymers P1-P3 are summarized in Table 2.2. The dendronized polymers show very similar absorption and emission characteristics in THF. As shown in Figure 2.2, polymers in THF solutions exhibit the absorption peak at ca. 300 nm whose intensities increase with the generation number, which are due to the absorption of the peripheral OXD moieties. This result further confirms the accomplishment of various generations of OXD dendrimers. The additional absorption peak at ca. 367 nm is assigned to the π-π^＊ transition contributed from the conjugated backbones of the polymers, which is not affected by the generation of dendronized polymers.

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Intensity (a.u.)

Wavelength (nm) P1, absorption P2, absorption P3, absorption Compound 2, emission

Figure 2.2 Normalized UV-vis absorption spectra of polymers P1-P3 and the PL emission spectrum of compound 2 in THF, the absorption spectra are normalized at the absorption peaks of polymer backbones ca. 367 nm.

Upon excitation of the backbones of polymers P1-P3 at 367 nm, the PL emission spectra display the value of λmax at ca. 414 nm. The PL spectra in THF are almost identical for polymers P1-P3 as shown in Figure 2.3. In contrast to dilute solutions (in THF), the absorption spectra of the polymers in solid films are similar (a little red-shifted) to those in solutions. Optical band gaps (Eg) determined from the absorption edges of the UV-vis spectra of polymers P1-P3 in solid films are found to be 3.03 eV. The PL emission spectra of polymers P1-P3 in solid films are only 4-7 nm bathochromically red shifted compared with those in solutions. Especially, due to the stronger aggregation in the solid films of the lowest generation polymer, P1 containing the smallest dendron size show a more obvious shoulder and followed by a long featureless tail (extending into the red region) than the higher generation polymers P2 and P3 (Figure 2.4). It can be concluded that the higher generation

poly[(9,9-dihexylfluorene)-alt-co-1,4-phenylene)] (PDHFP) were attached to the phenylene rings of the backbones and the full width at half-maximum (fwhm) values of these polymers strongly depend on the lengths of the attached alkoxy side chains.⁴³ The fwhm value decreases from 62 nm in PDHFP (without any side chain on the phenylene ring) to 46 nm in PDHFDDOP (longer side chains of -OC10 on the phenylene ring). It explained that the smaller fwhm value of the polymers with longer side chains were due to the emission resulting from more isolated main-chain fluorophores. Thus, the fwhm values of the emission curves in solid films of dendronized polymers P1-P3 are also quite narrow (44-45nm) to yield purer blue light emission due to their outstanding site-isolation effect. In addition, compared with PDHFP, the vibronic structures in PL emissions of P2 and P3 are remarkably reduced and no noticable spectral shoulder above 500 nm is observed.

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Intensity (a.u.)

Wavelength (nm) P1 P2 P3

Figure 2.3 Normalized PL spectra of polymers P1-P3 in THF.

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Intensity (a.u.)

Wavelength (nm) P1 P2 P3

Figure 2.4 Normalized PL spectra of polymers P1-P3 in solid film.

As comparing the PL emission spectrum of model compound 2 and the absorption spectra of dendronized polymers P1-P3 in THF (Figure 2.2), the overlap of OXD emission peak (363 nm) and P1-P3 backbones absorption peaks (ca. 367 nm) is extremely large. The large spectral overlap between the two interacting chromophores indicates that the probability of donor-acceptor energy transfer should be high. The energy transfer efficiency (ETE) of the dendritic wedge is estimated by the intensity ratio between the maximum absorption of OXD dendrons in the absorption spectrum and that in the fluorescence excitation spectrum.⁴⁴ According to this estimation, the energy transfer efficiencies of various generations of OXD dendrons are 59%, 43%, and 49% for P1, P2, and P3, respectively. It was consistently found that the energy transfer efficiency decreased as the generation (i.e. the size) of the surface-functionalized poly(benzyl ether)-type dendritic wedge increased. Upon excitation either at OXD dendrons or polymer backbones of polymers P1-P3 in THF, the obtained PL emission spectra are identical as shown in Figure 2.5(a). The almost complete disappearance of OXD emission at 363 nm indicates that energy transfer

emission intensity from the sensitized excitation (excited at the maximum absorption of OXD dendrons) and that from the direct excitation (excited at the maximum absorption of the polymer backbones) of dendronized polymers, P2 and P3 gain more intense PL backbone emissions by the sensitized excitation from the energy transfer of OXD dendrons than those by the direct excitation from the absorption of chromophore backbones. This indicates that the overall fluorescence of these dendronized polymers P1-P3 result not only from the contribution of the polymer backbones but also from that of the peripherally dendritic OXD units. In addition, the OXD-functionalized dendritic wedges are very efficient light-harvesting moieties for funneling energy to the polymer backbones. As shown in Figure 2.5(b), the excitations of the peripherally OXD moieties of dendronized polymers P1-P3 in solid films result in similar fluorescence patterns as those excited at the maximum absorption of polymer backbones alone. Especially, the PL emission intensities by the sensitized excitations of OXD dendrons in solid films of polymers P1-P3 are all stronger than those by the direct excitations of their polymer conjugated backbones.

Actually, the PL emission intensity of polymer P3 in solid film excited at peripheral OXD dendrons is about 86% higher than that excited at the backbones. In general, similar effects were commonly described for dye-labeled dendrimers.⁴⁵ However, the result demonstrates that the dendronized polymers may show stronger emissions through efficient energy transfer from peripheral dendrons to emitting backbones.

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Figure 2.5 PL Emission spectra of polymers P1-P3 (a) in solutions (THF) and (b) in solid films, which were excited at the maximum absorption of the polymer backbones (for solid symbols) and the polymer periphery OXD dendrons (for open symbols).

As listed in Table 2.2, the PL quantum yields (ΦF) of polymers P1-P3 in THF solutions were measured with 9,10-diphenylanthracene as a reference standard⁴⁶ (cyclohexane, ΦF = 0.9), and the highest quantum yield reaches 0.87. The PL quantum yields in solid films were measured using the same standard in poly-(methyl

backbones is also reflected on the emission efficiency of the dendronized polymers.

Although this relative method can only give an estimation of the fluorescence quantum yields of the polymers, the data still indicate that the fluorescence quantum yields of the polymers depend on the sizes of the attached dendrons, i.e. the larger the dendrons, the higher the PL quantum yields. Therefore, the second generation of dendronized polymer P3 in the solid film shows the highest quantum yield, which is attributed to the minimization of the self-quenching by attaching bulky pendent dendrons on the polymer backbones.

Table 2.2 Absorption and PL Emission Spectral Data of Polymers P1-P3 in THF and Solid Films

a Band gaps were calculated from the onsets of UV-visible absorption spectra of P1-P3 in solid films.

b Solution fluorescence quantum efficiency measured in THF, relative to 9,10-diphenylanthracene (ΦPL = 0.90).

c PL quantum efficiency estimated relative to 9,10-diphenylanthracene in poly(methyl methacrylate) as a standard (ΦPL = 0.83).