Optical Properties

The absorption and photoluminescence (PL) spectra of H-acceptor monomer M1 (PBB) and polymers P1-P5 were measured in both solution and solid states, and their photophysical properties are summarized in Table 2.3. As shown in Figure 2.4, the absorption intensities of copolymers P2-P4 at 294, 328, and 342 nm (in THF solutions) increase dramatically by raising the CAZ content, which are attributed to the absorption bands of the CAZ moieties. The additional absorption bands of polymers P1-P4 at ca. 320 and 385 nm are assigned to the light-emitting PBB segments of the H-acceptor moieties. The copolymers do not demonstrate any new (or shifted) bands in the absorption spectra, indicating no interaction between the CAZ and PBB moieties in the ground state.

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Figure 2.4 Absorption spectra of H-acceptor polymers P1–P4 in THF solutions,

normalized at the maximum absorption peak of the light-emitting PBB segments at 385 nm.

Table 2.3 Absorption and PL Emission Spectral Data of Polymers (P1–P5) in THF Solutions and Solid Films

polymer λabs,sola (nm) λPL,sola,b (nm) λPL,filmb (nm) ΦPL,solb,c

a Measured in dilute THF solutions.

b Excited at the maximum absorption of PBB units.

c Solution fluorescence quantum efficiencies were measured in THF, relative to 9,10-diphenylanthracene (ΦPL = 0.90).

The absorption spectra of the polymers in solid films are similar except for 5-7 nm of red shifts in contrast to those in THF solutions (see Appendix A4). The PL emission spectra of polymers P1-P4 in solid films show 22-47 nm of significant red shifts, i.e., P1 (47 nm) > P2 (40 nm) > P3 (24 nm) > P4 (22 nm), in comparison with their corresponding dilute solutions (see Table 2.3), which is due to the formation of π-π stacking and molecular aggregation in solid state. Furthermore, compared with copolymers P2-P4 in solid films as shown in Figure 2.5, homopolymer P1 exhibits the largest red shift, i.e., 47 nm, in PL spectra. With the increase of CAZ contents in copolymers P2-P4, the PL peaks are blue-shifted from 487 nm in P2 to 466 nm in P4.

This result clearly indicates that the dilution effect by the CAZ moieties can be incorporated into the copolymers to suppress the intermolecular π-π stacking and the aggregation of the light-emitting PBB units in polymers and thus to reduce the red shifts (by aggregation) in the PL emission spectra. As listed in Table 2.3, the PL quantum yields (ΦPL) of polymers P1-P4 in solutions were in the order of P1 < P2 <

P3 < P4, where P4 has the largest quantum yield (ΦPL = 0.94) due to the highest content of CAZ moieties. This consequence indicates that the dilution effect of CAZ units in the copolymers will reduce the aggregation and self-quenching of the PBB segments to acquire higher PL quantum yields.

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Figure 2.5 Normalized PL spectra of H-acceptor polymers P1–P4 excited at the

maximum absorption (397 nm) of the light-emitting PBB segments in solid films.

In order to investigate the generation effect of OXD dendrons on the absorption spectra of H-bonded side-chain dendritic complexes (in solid films), the absorption spectra of model compound 1 (containing an OXD unit, which is illustrated in the Appendix A5) in THF solution and H-acceptor polymer P3 in solid state are compared.⁵⁴ As shown in the absorption spectra of Figure 2.6, the maximum absorption wavelength λmax,abs of model compound 1 in THF solution is ca. 305 nm (from OXD units) and that of H-acceptor polymer P3 in solid state is ca. 296 nm (from CAZ units). Nevertheless, the major absorption bands of H-bonded side-chain dendritic complexes containing P3 in solid films are dominated at ca. 296 nm (from

CAZ units) with a shoulder at ca. 315 nm (from OXD units), which are originated

from the combined absorption band of the OXD and CAZ moieties. The longer absorption band at ca. 397 nm is attributed to the characteristic absorption of the light-emitting PBB units. By increasing the generation number of the dendritic H-donors in the H-bonded side-chain dendritic complexes, the intensity of the absorption band in the OXD moieties at ca. 305-315 nm is proportional to the generation number of the H-donor dendrimers. The photophysical properties of H-bonded side-chain dendritic complexes containing H-acceptor monomer M1 (PBB) and polymers P2-P4 in solid films are summarized in Table 2.4, where M1/G3COOH and P2/G3COOH are omitted due to the possible unstabilized H-bonded structures originated from the highest steric hindrance between the highest generation of dendrimer G3COOH and the denser H-acceptor units in M1 and P2. From the results of Table 2.4, the least red-shifted PL emissions of ΔλPL in P2/G1COOH and P2/G2COOH have the evidence of the possible unstabilized H-bonded structures of

complexes made from P2. Therefore, the possible unstabilized H-bonded structures of complexes made from P2 will be similar to those made from P1, in which the H-bonded structures are possibly unsteady due to the steric hindrance between the denser H-acceptor units in P1 (as well as P2) and the higher generation of dendrimers, especially for the highest generation of H-bonded complexes containing G3COOH.

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Figure 2.6 Absorption spectra of H-acceptor polymer P3 and its H-bonded

side-chain dendritic complexes in solid films normalized at the maximum absorption (397 nm) of the light-emitting PBB cores along with model compound 1 (containing an OXD unit with the maximum absorption around 305 nm in THF solution).

Table 2.4 Photophysical Properties of H-Boned Side-Chain Dendritic Complexes Containing H-Acceptor Polymers P3 and P4

H-bonded complex λPL, film (nm) △λPLa (nm) RFI^b

a Different degrees of red-shifted PL emissions between the H-boned side-chain dendrimers and their corresponding acceptor copolymers.

b Relative fluorescent intensities (RFI) were calculated by the ratios of the core emission intensities excited at the absorption peaks of the OXD units (305 nm) and the PBB cores (397 nm).

The photophysical properties of both series of H-bonded sidechain dendritic complexes containing H-acceptor polymers P3 and P4 in solid films (see Table 2.4) are evaluated accordingly. Compared with H-acceptor polymer P3 (PBB-CAZ5) in Figure 2.7, the supramolecular side-chain dendritic complexes P3/G1COOH-G3COOH (excited at the maximum absorption of the light-emitting

PBB units) exhibit red-shifted PL emissions with λmax values at 530, 520, and 509

nm, respectively. This result is similar to our previous work⁶⁸ that red shifts of PL emissions are expected in the H-bonded structures, where the nonphotoluminescent H-donors bearing benzoic acids (as solid solvents) were H-bonded to the photoluminescent H-acceptors containing pyridyl groups. Therefore, analogous H-bonded sidechain dendritic complexes containing different generations of dendritic H-donors appeared to have different degrees of red shifted PL emissions in comparison with H-acceptor polymer P3. The red shifts of PL emissions in H-bonded side-chain dendritic complexes P3/G1COOH-G3COOH are 61, 51, and 40 nm, respectively, where the higher generation of the H-bonded side-chain dendritic complex has a smaller red-shifted PL emission than the lower generation of the H-bonded sidechain dendritic complex. It clearly indicates that the larger dendritic wedges on the side chains have a larger site isolation or dendritic dilution effect than the smaller dendritic ones, so the higher generations of dendrimers efficiently

minimize the interchain interaction and lower the aggregation extent between the light-emitting PBB units.

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0.0 0.2 0.4 0.6 0.8 1.0

PL Intensity (a.u.)

Wavelength (nm)

P3 (PBB-CAZ₅) P3/G1COOH P3/G2COOH P3/G3COOH

Figure 2.7 Normalized PL spectra of H-acceptor polymer P3 and its H-bonded

side-chain dendritic complexes excited at the maximum absorption (397 nm) of the light-emitting PBB cores in solid films.

The PL emission data of H-bonded dendritic dendrimers containing P4 are also summarized in Table 2.4, which demonstrate similar trends as those of H-bonded dendritic dendrimers containing P3. However, in contrast to the same generation of OXD dendron, H-bonded dendritic complexes containing P4 have smaller degrees of

red-shifted PL emissions than H-bonded dendritic complexes containing P3 (see Table 4). This might be due to the larger dilution effect of CAZ units in P4 (PBB-CAZ9) with higher CAZ contents than that in P3 (PBB-CAZ5), where a higher CAZ molar ratio of P4 reduces the interchain interaction between the light-emitting PBB units in these H-bonded side-chain dendritic complexes.

Because of the significant spectral overlap in the absorption spectrum of homopolymer P1 and the emission spectra of model compound 1 (containing an OXD unit) and homopolymer P5 (as shown in the Appendix A5), the energy transfer from the OXD dendritic wedges and CAZ pendant groups to the light-emitting PBB cores can be expected. This effect was also probed by photoluminescent excitation (PLE) and absorption spectra of H-bonded side-chain dendritic complexes containing P4 (see Figure 2.8). Similar spectral features of PLE spectra appear to match those of their corresponding absorption spectra, where PLE spectra were monitored at the corresponding maximum PL emission. This result indicates that the existing sites of CAZ pendent groups, peripheral OXD dendritic wedges, and light-emitting PBB

cores (ca. 296, 305, and 397 nm, respectively) in such supramolecular side-chain dendrimers provide the characteristics of light-harvesting capability, i.e., antenna effect. Since only the energy transfer of the dendritic OXD units in the H-bonded side-chain dendritic complexes is concerned, the excitation wavelength of 305 nm

was chosen at the maximum absorption of the dendritic OXD units.

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0.0

Figure 2.8 UV spectra and PLE spectra of H-acceptor polymer P4 and its

H-bonded side-chain dendritic complexes in solid films normalized at the light-emitting PBB units (397 nm), where PLE spectra were monitored at the corresponding maximum PL emissions.

As shown in Figure 2.9, different generations of H-bonded dendritic complexes containing P4 (P4/G1COOH-G3COOH) have PL emissions at wavelengths of 545, 539, and 522 nm, respectively. Upon excitation of the dendritic OXD units at 305 nm, H-bonded dendritic complexes apparently generated identical predominant emission peaks as those excited at the maximum absorption of the PBB cores. Whereas, no major PL emission from OXD dendrons was detected, and thus an efficient energy

transfer from the peripheral OXD units to the PBB cores is confirmed. Furthermore, the functionalized OXD dendritic units or the light-emitting PBB cores can be independently addressed by changing the excitation wavelengths in PL experiments.

By excitation of the electron-transporting dendrons and the lightemitting cores selectively, it provides a window to study the photoinduced energy transfer between proton donors and acceptors.

In addition, different generations of OXD dendritic H-donors were investigated to evaluate their antenna effect in the supramolecular side-chain dendrimers. Thus, the values of relative fluorescent intensities (RFI) in H-bonded side-chain dendritic complexes were calculated from the intensity ratios of their PBB core emissions by respective excitations at the maximum absorption peaks of the OXD dendrons (by the sensitized absorption at 305 nm and then energy transfer to PBB core emissions) and the maximum absorption peaks of the PBB cores (by the direct core absorption at 397 nm), respectively. The RFI values of H-bonded side-chain dendrimers containing H-acceptor copolymer P4 and H-donor dendrimers G1COOH to G3COOH are 2.09, 2.44, and 4.45, respectively (as shown in Figure 2.9 and Table 2.4), which indicates that the intensity of the sensitized emission (by the energy transfer from OXD dendritic absorption at 305 nm) is even stronger than that of the direct PBB core emission (by the direct core absorption at 397 nm) in the H-bonded side-chain

dendrimers. Therefore, the RFI values are much enhanced in the higher generations of H-bonded dendritic complexes (with the maximum values in both series of H-bonded side-chain dendritic complexes bearing the highest generation of H-donor dendrimer G3COOH), which is attributed to the higher absorptions by the larger numbers of

OXD units in the higher generations of dendritic wedges as well as the further

reduced aggregation of the light-emitting PBB cores by the more bulky sizes of dendrons in the higher generations of OXD H-donors.

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Figure 2.9 PL spectra of H-acceptor polymer P4 and its H-bonded side-chain

dendritic complexes in solid films, which were excited at the dendritic peripheral OXD units (at 305 nm for open symbols) and at the maximum absorption of the light-emitting PBB cores in H-bonded side-chain dendritic complexes containing dendritic H-donors (ca. 397 nm for solid symbols).