Optical Properties

The absorption and PL spectral data of all luminescent H-acceptor polymers

PBT1-PBT3 (14-16) and PBOT1-PBOT3 (17-19) (in both THF solutions and solid films) as well as all H-bonded polymer complexes (in solid films) are summarized in Tables 4-5. The PL quantum yields (ΦPL) of polymers PBT1-PBT3 (14-16) and

PBOT1-PBOT3 (17-19) in solutions were excited at the maximum absorption peak as listed in Table 2.4. As shown in Figure 2.9(a), the maximum absorption peaks of

PBT and PBOT series are 350 and 384 (322) nm, respectively. The absorption bands

of PBT1-PBT3 (14-16) in THF solutions at c.a. 294, 330, and 344 nm are originated from the combined absorption bands of CAZ pendent groups. The additional

1.0 1.5 2.0 2.5 3.0

the n-π* transition⁸² contributed from the lateral methoxy groups in conjugated chromophores. Similarly, PBOT1-PBOT3 (17-19) have the same tendency by increasing the content of CAZ units. In Figure 2.9(b), the PL spectra of H-acceptor polymers PBT1-PBT3 (14-16) emitted blue light c.a. 431-440 nm in THF solutions.

In comparison with luminescent homopolymer PBT1 (14), the slightly blue-shifted PL spectra of copolymers PBT2 and PBT3 (15 and 16) can be explained by the dilution effect of the incorporated CAZ units to reduce the aggregation of the pyridyl chromophores, which also can enhance PL quantum yields (ΦPL = 40-59%) by copolymerization with CAZ units. Correspondingly, similar blue-shifted PL spectra (λPL,sol = 445-449 nm) and enhanced PL quantum yields (ΦPL = 49-63%) were observed in THF solutions of analogous H-acceptor polymers PBOT2-PBOT3

(18-19 with lateral methoxy groups) due to the dilution effects of CAZ units in

copolymers. Furthermore, in contrast to polymers PBT1-PBT3 (14-16), polymers

PBOT1-PBOT3 (17-19) have more red-shifted PL emissions due to the stronger electron donating effect of lateral methoxy groups, which induce smaller energy band gaps in chromophores. In Figure 2.9(c), comparing polymers PBT1-PBT3 (14-16 with lateral methyl groups) and PBOT1-PBOT3 (17-19 with lateral methoxy groups), the PL spectra in solid films are more red-shifted than those in THF solutions, which indicate that more serious π-π stacking and molecular aggregation occur in solid films.

Additionally, due to the larger separation of chromophores by the larger size of lateral methoxy groups in PBOT1-PBOT3 (17-19), they have higher PL quantum yields (ΦPL = 49-63%) than PBT1-PBT3 (14-16) with lateral methyl groups (ΦPL = 40-59%), respectively.

Table 2.4 Absorption and Photoluminescence Spectral Data of H-Acceptor Polymers

a Absorption and PL emission spectra were recorded in dilute THF solutions at room temperature.

b PL quantum yield in THF and 9,10-diphenylanthrance is the reference of quantum yield.

(a)

(b)

(c)

Figure 2.9 (a) Absorption spectra and (b) PL spectra (excited at the maximum absorption wavelengths) in THF solutions (c) normalized PL spectra (excited at the maximum absorption wavelengths) of H-acceptor polymers 14-19 in solid films.

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As shown in Table 2.5, the H-donor acids play an important role to induce the PL emission shift of light-emitting H-acceptor polymers in H-bonded polymer complexes because of their different acidities being able to tune the emission colors (λmax) by H-bonds. The proton donors in the H-bonded polymer complexes do not have PL properties due to lacking of conjugated structures, so they only offer the solid solvent environments with different pKa values (OBA (20): pKa ~ 4.21: ONA (21): pKa ~ 4.17; THDA (22): pKa ~ 3.49). Thus, different degrees of H-bonding occur in H-bonded polymer complexes for various acids H-bonded with light-emitting H-acceptor polymers, i.e., different electron densities and energy band-gaps of light-emitting H-bonded polymer complexes are induced by the H-bonding of distinct solid H-donors. In Table 2.5, compared with H-acceptor polymers PBT1-PBT3

(14-16), their H-bonded polymer complexes cangenerate 30-40 nm of red-shifted PL emissions in λmax as H-bonded to the asymmetric mono-functional H-donors OBA (20) and ONA (21), and up to 74-78 nm of red-shifted PL emissions in λmax as H-bonded to the symmetric bi-functional H-donor THDA (22). The redder-shifted PL emissions are originated from the stronger H-bonded effect of H-donor acids with smaller pKa values and thus to generate stronger H-bonding in corresponding H-bonded polymer complexes. Similarly, the PL emission peaks of the H-bonded polymer complexes containing H-acceptor polymers PBOT1-PBOT3 (17-19) are red-shifted about 43-51

nm as complexed with OBA (20) and ONA (21), and red-shifted 86-93 nm as complexed with THDA (22). For instance, compared with H-acceptor polymers

PBT2 (15) and PBOT2 (18), different extents of red-shifted PL emissions occurred in

solid films of their H-bonded polymer complexes PBT2/OBA-PBT2/THDA

(15/20-15/22) and PBOT2/OBA-PBOT2/THDA (18/20-18/22) (were excited at the maximum absorption wavelengths) in Figure 2.10. In general, by decreasing pKa values of proton donors, more red-shifted wavelengths of PL emissions of H-bonded polymer complexes were observed. In comparison with H-bonded polymer complexes containing PBT1-PBT3 (14-16), those containing H-acceptor polymers

PBOT1-PBOT3 (17-19) possess larger red-shifted PL emissions by the formation of H-bonded polymer complexes due to their stronger electron donating effect of lateral methoxy groups. Besides, H-bonded polymer complexes containing H-acceptor polymers with different CAZ contents appear to have similar degrees of red-shifted PL emissions in analogous H-bonded polymer complexes. Hence, the pKa values of H-donors are more important than the steric effect of CAZ contents in the H-bonded polymer complexes. Consequently,the results demonstrate that more red-shifted PL emissions happen in theH-bonded polymer complexes as H-donors with smaller pKa values are H-bonded to the light-emitting H-acceptor polymers. Therefore, PL emission colors, i.e., λmax values, of H-bonded polymer complexes can be tuned not

only by adjusting the light-emitting conjugated pyridyl cores but also by changing the non-emitting H-donors with different pKa values.

Table 2.5 Photophysical Properties of H-Acceptor Polymers and H-Bonded Polymer Complexes in Solid Films

H-acceptor polymer or H-bonded polymer complex

a The difference of PL emissions between the H-acceptor polymer and its H-bonded polymer complex.

(a)

(b)

Figure 2.10 Normalized PL spectra (excited at the maximum absorption wavelengths) of (a) H-acceptor polymer PBT2 (15) and its H-bonded polymer complexes PBT2/OBA (15/20), PBT2/ONA (15/21), and PBT2/THDA (15/22) in solid films;

(b) H-acceptor polymer PBOT2 (18) and its H-bonded polymer complexes PBOT2/OBA (18/20), PBOT2/ONA (18/21), and PBOT2/THDA (18/22) in solid films.