Thermal Properties

To elucidate the H-bonding effect on the thermal properties of the H-bonded dendrimers, all compounds were characterized by differential scanning calorimetry (DSC). Polarizing optical microscopy (POM) demonstrated that no phase separation took place between donors and acceptors in the H-bonded dendrimers. In addition, the third-generation (G3) of H-bonded dendrimers containing G3COOH have higher isotropization temperatures (Ti) than the lower generations of H-bonded dendrimers.

As shown in Table 4.1, it is observed that dendritic donors G1COOH, G2COOH, and G3COOH possess the glass transition temperatures (Tg) at 51, 63, and 72 °C, respectively. Due to higher molecular weights in the higher generations of dendrimers, the glass transition temperatures of the H-bonded dendrimers with higher generations were observed to increase monotonically. The behavior is consistent with the trends of the glass transition temperatures in earlier studied dendrimers.⁷⁰ As for the acceptor, PBP-OC8 exhibits a melting temperature at 137 °C without Tg, indicating the crystalline nature of the molecule. However, H-bonds have a strong effect on the thermal properties by the introduction of OXD dendritic donors (bearing benzoic acids) to the emitting acceptor PBP-OC8 via H-bonded self-assembly. Symmetric H-bonded dendrimers PBP-OC8/G1COOH, PBP-OC8/G2COOH, and PBP-OC8/G3COOH reveal distinct glass transition temperatures at 26, 48, and 65 °C, respectively, which are lower than their corresponding dendrimeric donors (owing to the formation of H-bonded dimers by two dendritic acids). Similar phenomena were observed in the other H-bonded dendrimers (see Table 4.1). The incorporation of OXD dendrons into the supramolecular structures shows no melting (only observed in POM) and crystallization peaks in DSC measurements, but Tg only. This clearly

Another interesting trend is that all generations of dendritic analogues have the same order of transition temperatures in Tg and Ti for the H-bonded dendrimers containing the following emitting acceptors: PBBBP-Me-OC8 > PBP-OC8 >

PBBCN-OC8 > PBBOC8-OC8. The result of higher Tg and Ti in symmetric dendritic supramolecules (H-bonded trimers) containing PBBBP-Me-OC8 and PBP-OC8 than asymmetric dendritic supramolecules (H-bonded dimers) containing PBBCN-OC8

and PBBOC8-OC8 may be explained by the higher molecular weights of H-bonded trimers in the symmetric dendritic supramolecules. Higher Tg and Ti in H-bonded dendrimer containing PBBBP-Me-OC8 than that containing PBP-OC8 is due to the longer central core of PBBBP-Me-OC8. Besides, higher Tg and Ti in H-bonded dendrimer containing PBBCN-OC8 than that containing PBBOC8-OC8 is because of the dipole-dipole interaction of CN groups in PBBCN-OC8. In the reverse H-bonded dendrimers (containing emitting shells), e.g. PBBBP-Me-OC8, PBP-OC8, PBBCN-OC8, and PBBOC8-OC8 were complexed with dendritic donor cores G1-C6-(COOH)6 and G1-C10-(COOH)6 in proper ratios, no phase transition peaks were found, which is probably due to the flexible chains of the polyfunctional cores in the H-bonded dendritic donors. Similar behavior in polypropylene dendrimers containing trialkoxybenzene wedges as mesogenic units was reported in the literature.⁷¹

Table 4.1. Transition Temperatures of H-Bonded Donors, Acceptors, and Dendritic

a Not observed in DSC measurements.

bThe isotropization temperatures were determined by polarizing optical microscopy (POM).

4.3.3 Optical Properties

The absorption and PL spectral data of the pure chromophores (in THF and solid films) and all H-bonded dendrimers in solid films are summarized in Table 4.2. The series of H-bonded dendrimers show very similar absorption and emission characteristics in the solid films. Figure 4.3 shows absorption spectra of uncomplexed H-bond donors and PBBCN-OC8 in THF solutions. Figure 4.4 shows various asymmetric H-bonded dendrimers containing single H-bonded acceptor emitter PBBCN-OC8 in solid films. In the asymmetric supramolecular complexes bearing dendritically mono-encapsulated chromophores, i.e. PBBCN-OC8/G1COOH, PBBCN-OC8/G2COOH, and PBBCN-OC8/G3COOH, the absorption peak at 305 nm was attributed to the transition absorption of OXD groups and the longer absorption peak at 423 nm was attributed to the characteristic absorption of emitter PBBCN-OC8. By increasing the generation of the dendritic donors in the H-bonded complexes, the absorbance of OXD units at 305 nm is proportional to the generation number of the dendrimers.

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Figure 4.3 UV-vis absorption spectra of uncomplexed H-bond donors and PBBCN-OC8 in THF solutions.

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Figure 4.4 UV-vis absorption spectra of PBBCN-OC8 and its H-bonded dendrimers in solid films, which are normalized at the absorption peak of PBBCN-OC8 at 423 nm.

In view of Figure 4.5, the PL emission spectrum of PBBCN-OC8 showed a characteristic peak at 470 nm in THF, which is red-shifted to 512 nm in solid films due to the formation of π–π stacking and molecular aggregation. In contrast to the single H-bonded acceptor emitter PBBCN-OC8 alone, the asymmetric supramolecular dendrimers PBBCN-OC8/G1COOH, PBBCN-OC8/G2COOH, and PBBCN-OC8/G3COOH exhibit red-shifted PL emissions with values of λmax at 554, 541, and 531 nm excited at 423 nm, respectively. This result is similar to our previous work,⁶⁶ where the photoluminescent H-bonded trimers containing bifunctional bis-pyridyl acceptors complexed with monofunctional carboxylic acids show red-shifted PL spectra as the acceptor emitters are H-bonded to donors with smaller pKa values. As well known that pH values of aqueous solutions may affect photoluminescence properties of polyelectrolytes containing pyridine units. Therefore, the H-bonded dendritic donors G1COOH, G2COOH, and G3COOH bearing benzoic acid groups can be regarded as acidic solvents, so the PL spectra reveal red-shifted PL emissions compared with that of emitter PBBCN-OC8 in THF.

Interestingly, different generations of asymmetric H-bonded dendrimers containing single H-bonded acceptor emitter PBBCN-OC8 appeared to have different degrees of red-shifted PL emissions. The red-shifts of PL emissions in asymmetric H-bonded dendrimers PBBCN-OC8/G1COOH, PBBCN-OC8/G2COOH, and PBBCN-OC8/G3COOH are 84, 71, and 61 nm, respectively, where the higher generations of the H-bonded dendrimers have smaller red-shifted PL emissions than the lower generations of the H-bonded dendrimers. It clearly indicates that larger dendritic wedges have higher site-isolation or dendron dilution effect than smaller dendritic ones, so the higher generations of dendrimers efficiently reduce the aggregation extent of the emitting cores. The opposite type of H-bonded dendritic complexes containing poly(alkyl aryl ether) dendrimer (dendritic donor core)

complexed with single H-bonded acceptor emitter PBBCN-OC8 at the periphery, i.e.

PBBCN-OC8/G1-C6-(COOH)6 and PBBCN-OC8/G1-C10-(COOH)6, also both exhibit red-shifted emission peaks at 556 and 550 nm, respectively, compared with that of PBBCN-OC8 in THF. With respect to PBBCN-OC8/G1-C6-(COOH)6,the relative 6 nm blue-shift of PL emission in PBBCN-OC8/G1-C10-(COOH)6 may be explained by that the longer flexible alkyl chains (as solid solvents) in the dendritic donor cores result in higher dilution effect for emitting acceptors. It is worthy noticing that the PL emission spectra of PBBCN-OC8/G1-C6-(COOH)6 and PBBCN-OC8/G1-C10-(COOH)6 are similar to that of PBBCN-OC8/G1COOH, which means that poly(alkyl aryl ether) dendritic acid cores in the reverse form of the previous H-bonded dendrimers affect PL emission behavior of PBBCN-OC8 in a similar way as the lower generation of G1COOH.

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PBBCN-OC8 , solution PBBCN-OC8 , solid film

Figure 4.5 Normalized PL spectra of mono-pyridyl (single H-bonded) acceptor emitter PBBCN-OC and its H-bonded dendrimers.

Figure 4.6 shows the PL emission characteristics of emitter PBP-OC8 and its double H-bonded dendrimers with symmetric structures. All PL emission data are summarized in Table 4.2, which demonstrate similar trends as those of single H-bonded acceptor emitter PBBCN-OC8 and its asymmetric H-bonded dendrimers.

Compared with single H-bonded dendrimers, no further red-shifted PL emissions were observed in the double H-bonded dendrimers due to the weaker H-bonded effect of the second H-bonds on the conjugated structures of double H-bonded acceptors and the double dilution effect from double amount of donor acids.

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0.0 0.2 0.4 0.6 0.8 1.0

PL Intensity (a.u.)

Wavelength (nm)

PBP-OC8, solution PBP-OC8, solid film PBP-OC8/G1COOH PBP-OC8/G2COOH PBP-OC8/G3COOH PBP-OC8/G1-C6-(COOH)6

PBP-OC8/G1-C10-(COOH)6

Figure 4.6 Normalized PL spectra of bis-pyridyl (double H-bonded) acceptor emitter PBP-OC8 and its H-bonded dendrimers.

300 350 400 450 500 550 PBBCN-OC8, absorption PBP-OC8, absorption PBBBP-Me-OC8, absorption

Figure 4.7 UV-vis absorption spectra of (single/double H-bonded) acceptor emitters PBBOC8-OC8, PBBCN-OC8, PBP-OC8, and PBBBP-Me-OC8 and PL spectrum of compound 2 (containing an OXD unit) in THF. It indicates that an overlap exists between the emission band of the donor and the absorption bands of the acceptor emitters, resulting in energy transfer from OXD units to the emitting core.

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PBPOC8/G1COOH, absorption PBPOC8/G1COOH, excitation PBPOC8/G2COOH, absorption PBPOC8/G2COOH, excitation PBPOC8/G3COOH, absorption PBPOC8/G3COOH, excitation

Figure 4.8 UV-vis absorption and corrected excitation spectra of symmetric H-bonded dendrimers (with different generations of donor dendrimers) PBP-OC8/G1COOH, PBP-OC8/G2COOH, and PBP-OC8/G3COOH monitored at

Due to significant overlap in the absorption spectra of the representative chromophores (H-bonded acceptors) and the emission spectrum of model compound 2 in Figure 4.7, the energy transfer from the OXD dendritic groups to the central emitters can be expected, which was also probed by photoluminescent excitation (PLE) spectra of H-bonded dendrimers containing PBP-OC8 as shown in Figure 4.8.

Similar spectral features of PLE spectra appear to match those of the corresponding absorption spectra, where both peaks (ca. 300 and 425 nm) exists, indicating that the peripheral OXD units in such supramolecular dendrimers possess light-harvesting capability. Hence, the functionalized OXD dendron units or the emitting core PBP-OC8 can be independently addressed by changing the excitation wavelength. By excitation of the dendrons and the cores selectively, it provides a window to study the photoinduced energy transfer between H-bonded donors and acceptors.

As shown in Figure 4.9, when the dendritic OXD groups were excited at 305 nm, the symmetric H-bonded dendrimers containing PBP-OC8, i.e. PBP-OC8/G1COOH, PBP-OC8/G2COOH, and PBP-OC8/G3COOH, emit fluorescence at wavelengths of 545, 539, and 522 nm, respectively. The photoexcitation of H-bonded dendrimers at the maximum absorption wavelength of OXD units apparently generated identical predominant emission peaks as those excited at the maximum absorption of PBP-OC8. Whereas, no luminescence was detected from the major emission of OXD dendron, and thus the energy transfer of OXD emission from the dendritic wedges to the central emitting cores is confirmed. In addition, the values of relative fluorescent intensities (RFI) were calculated from the intensity ratio of core emissions by respective excitations at the maximum absorption peaks of the OXD dendrons and the emitting cores (ca. 300 and 425 nm, respectively). The values of RFI from H-bonded G1 to G3 dendrimers are 1.27, 2.33, and 4.50, respectively. The results indicate that the intensity of the sensitized emission (by energy transfer from OXD dendritic

absorption at 300 nm) is even stronger than that of the direct core emission (by core absorption at 425 nm) in the H-bonded dendrimers. Therefore, sensitization by the telechelic antennae is more efficient than direct excitation at the maximum absorption of the chromophore. The increasing tendency from H-bonded G1 to G3 dendrimers unambiguously suggests that the more number of grafted OXD units (higher generations) in the dendrons, the higher capability of light-harvest. However, in contrast to symmetric H-bonded (G1-G3) dendrimers containing double H-bonded acceptor emitter PBP-OC8, where all RFI values are larger than 1 in all generations of H-bonded dendrimers, the enhancement of the core emission in the asymmetric H-bonded dendrimers containing single H-bonded acceptor emitter PBBCN-OC8 (as excited at the maximum absorption of OXD dendron) only occurred in the third generation of H-bonded dendrimers PBBCN-OC8/G3COOH as shown in Figure 4.10.

One of the major reasons should be attributed to that single H-bonded acceptor emitter PBBCN-OC8 is complexed only with one H-bonded donor (energy-transfer donor) in the molar ratio of 1:1 to result in the relatively lower enhancement of fluorescent intensity, as compared with double H-bonded acceptor emitter PBP-OC8

complexed with two H-bonded donor (energy-transfer donor) in the molar ratio of 1:2.

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Figure 4.9 PL spectra of symmetric dendritic supramolecules containing double H-bonded acceptor emitter PBP-OC8 in thin films, which were excited at the dendritic peripheral OXD units (at 305 nm for open symbols ) and at the maximum absorption of the emitting core PBP-OC8 in H-bonded G1-G3 dendrimers (at 418, 415, 408 nm, respectively, for solid symbols).

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Figure 4.10 PL spectra of asymmetric dendritic supramolecules containing single H-bonded acceptor emitter PBBCN-OC8 in thin films, which were excited at the dendritic peripheral OXD units (at 305 nm for open symbols) and at the maximum absorption of the emitting core PBBCN-OC8 in H-bonded G1-G3 dendrimers (at 424, 423, 419 nm, respectively, for solid symbols).

The possible behavior of supramolecular aggregation might be explained as shown in Figure 4.11. To balance the contribution of the number of OXD units in asymmetric/symmetric H-bonded dendrimers, the complexation of G1 donors with double H-bonded acceptor emitters should be comparable with the complexation of G2 donors with single H-bonded acceptor emitters as shown in Figure 4.11(b)-(d), which all possess 4 OXD units in each asymmetric/symmetric H-bonded dendrimer.

Interestingly, comparing asymmetric H-bonded dendrimers with 4 OXD units on one donor side of asymmetric ones and 2 OXD units on both donor sides of symmetric ones (or with 8 OXD units on one donor side of asymmetric ones and 4 OXD units on both donor sides of symmetric ones), both series of RFI values exhibit the trends in H-bonded dendrimers containing: PBP-OC8 > PBBOC8-OC8 > PBBCN-OC8 as the acceptor emitters PBP-OC8, PBBOC8-OC8, and PBBCN-OC8 were complexed with dendritic donors bearing the same number of OXD units. In terms of energy transfer, RFI values (with 4 OXD units in each H-bonded complex) are equal to 1.27, 1.14, and 0.97 in PBP-OC8/G1COOH, PBBOC8-OC8/G2COOH, and PBBCN-OC8/G2COOH, respectively; and RFI values (with 8 OXD units in each H-bonded complex) are 2.33, 2.16, and 1.97 in PBP-OC8/G2COOH, PBBOC8-OC8/G3COOH, and PBBCN-OC8/G3COOH, respectively. This should be attributed to the best acceptor emitter separation by donor dendrons on both sides of the symmetric H-bonded dendrimers containing double H-bonded acceptor emitter PBP-OC8. Moreover, comparing asymmetric H-bonded dendrimers containing single H-bonded acceptor emitters PBBOC8-OC8 and PBBCN-OC8, both acceptor emitters are encapsulated by single-side dendrons, but, as shown in Figure 4.1(d), PBBCN-OC8 in H-bonded dendrimers is further aggregated by the dipole-dipole

number of the OXD units but also the distance between OXD units and the central acceptor emitter is major concern for the energy transfer regarding the RFI value. In general, the energy transfer is highly dependent on a number of factors,⁷² including the extent of aggregation, the relative orientation of the transition dipoles, the extent of the spectral overlap, and the distance between the donor and acceptor moieties. For instance, the overlap extent between the absorption spectra of the representative chromophores PBP-OC8, PBBOC8-OC8, and PBBCN-OC8 and the emission spectrum of model compound 2 are approximately similar in Figure 4.7. In corresponding study of the H-bonded dendrimers containing double H-bonded acceptor emitter PBBBP-Me-OC8 with longer conjugation length (Figure 4.12), PBBBP-Me-OC8/G2COOH and PBBBP-Me-OC8/G3COOH excited at the peripheral dendritic OXD units exhibit higher fluorescent intensities than those excited at direct PBBBP-Me-OC8 absorption, except for the lowest generation of H-bonded dendrimer PBBBP-Me-OC8/G1COOH. Compared with H-bonded dendrimers containing acceptor emitter PBP-OC8, the less spectral overlap in the emission of OXD dendrons and the absorption of PBBBP-Me-OC8 in Figure 4.7 seems to explain the min reason for lower energy transfer (i.e. lower RFI values) of OXD units to chromophores in H-bonded counterparts containing acceptor emitter PBBBP-Me-OC8.

(a)

(b)

(c)

(d)

Figure 4.11 Schematic representation of double H-bonded encapsulation of symmetric dendrimers: (a) PBP-OC8/G2COOH and (b) PBP-OC8/G1COOH; and single H-bonded encapsulation of asymmetric dendrimers: (c) PBBOC8-OC8/G2COOH and (d) PBBCN-OC8/G2COOH.

300 400 500 600 700 PBBBP-Me-OC8/G1COOH PBBBP-Me-OC8/G2COOH PBBBP-Me-OC8/G2COOH PBBBP-Me-OC8/G3COOH PBBBP-Me-OC8/G3COOH

Figure 4.12 PL spectra of symmetric dendritic supramolecules containing double H-bonded acceptor emitter PBBBP-Me-OC8 in thin films as they were excited at the maximum absorption of the dendritic peripheral OXD units (at 305 nm for open symbols) and at the maximum absorption of the emitting core PBBBP-Me-OC8 in H-bonded G1-G3 dendrimers (at 450, 446, 439 nm, respectively, for solid symbols).

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Figure 4.13 PL spectra of the simple mixture (without H-bonds) of PBP-OC8/G1COOCH3, PBP-OC8/G2COOCH3, and PBP-OC8/G3COOCH3 in thin films excited at the dendritic peripheral OXD units (at 305 nm for open symbols) and at the maximum absorption of the emitting core PBP-OC8 in (at 425 nm for solid symbols).

In all H-bonded dendrimers containing emitting cores, the PL efficiencies are higher than pure chromophores and the values are much enhanced (the maximum value of 4 times larger) in the higher generation of H-bonded dendrimers, which is in accordance with the results obtained previously.⁷³ This improvement of PL efficiencies should be attributed to the bulky OXD dendrons refraining from the aggregation of chromophores. These results consistently indicate that the dendritic wedges play an important role of shielding/isolating effects among the cores.

However, the PL efficiencies are not obviously improved by the complexation of chromophores with functionalized poly(alkyl aryl ether) donor dendrimers containing peripheral carboxylic acid units to form exterior emitting shells (reverse to the previous system with emitting cores). Therefore, the supramolecular anchoring of chromophores on the dendritic surface by H-bonds to form exterior emitting shells seems to be inefficient to solve the intermolecular aggregation of chromophores.

To evaluate the H-bonding effect on the efficiency of energy transfer within H-bonded dendrimers, analogous dendritic mixtures without H-bonds were prepared by the esterification of the dendritic acids and then mixing with corresponding chromophores. Due to lacking of H-bonds between two components, the simple mixtures of PBP-OC8/G1COOCH3, PBP-OC8/G2COOCH3, and PBP-OC8/G3COOCH3 (molar ratio = 1:2, shown in Figure 4.13) showed similar emission spectra in contrast to PBP-OC8 in solid films (Figure 4.6). These mixed systems (without H-bonds) reveal lower ratio (the ratio of the core emissions excited at dendritic OXD units and at the core) of PL emission enhancement in comparison with those of H-bonded dendritic counterparts. Hence, PBPOC8/G1COOCH3, PBP-OC8/G2COOCH3, and PBP-OC8/G3COOCH3 have the relative fluorescent

counterparts. The results indicate that better energy-transfer properties and higher fluorescence quantum yields are obtained as the dendritic donors are H-bonded to the emitting acceptors, where the better energy transfer is caused by higher pairing ratio and closer molecular distance between donors and acceptors through H-bonds.

Table 4.2 Photophysical Properties of H-Bonded Acceptors and Dendritic Complexes

Compound or H-Bonded Complex λPL,sol

(nm)

aThe relative fluorescent intensities (RFI) were calculated by the ratio of the core emission intensities excited at the OXD unit and the core absorption peaks.