Poly(fluorenes)

In the past decade, fluorene-based conjugated polymers have emerged as a very outstanding class of blue-light emitting materials because of their high photoluminescence (PL) and electroluminescence (EL) quantum efficiencies, thermal stability, good solubility, high hole mobility, and facile functionalization at the C-9 position of fluorene.²³ However, devices are restricted by their tendency to form aggregates,²⁴ excimers,²⁵ or ketone defects²⁶ during either annealing or passage of

current, leading to red-shifted and less efficient emissions. To avoid this detrimental behavior, many studies have been carried out to solve the aggregation-related problems by increasing the structural hindrances of PFs, and thus to reduce their self-aggregation tendency in the solid state. For example, they can be distinguished and classified into one of the following several parts:

1. PFs containing long (or bulky) and branched side chains.²⁷

Figure 1.10 Example of poly(fluorene) containing long side chains.

2. Copolymerization techniques. The carbazole or thiophene units can be incorporated to polyfluorene derivatives. The introduction of a “kinked” disorder on the conjugated polymer chain to depress the aggregation phenomena and its effect on the excimer formation.²⁸

Figure 1.11 Examples of poly(fluorene)-based copolymers

3. The encapsulation of PF backbones into dendritic envelopes. The synthesis of a

promising properties. It was shown that the chemical stability and the shape persistence of Fréchet or Müllen-type dendrimers allows an effective shielding of the polyfluorene backbone.²⁹

Figure 1.12 Examples of poly(fluorene) with dendrons as side chains

4. The end-capping of PFs with bulky groups. The end-capping of the main chain of a PF homopolymer with hole-transporting moieties (HTM) opens a way to blue LEDs with high efficiency and excellent color stability, without altering the electronic properties of the conjugated polymer backbone. Furthermore, the end-capping with HTM does not disturb the LC properties or the orientational abilities of the PF polymer.³⁰

Figure 1.13 Example of end-capping of poly(fluorene) with bulky groups

6. Hyperbranched light-emitting polymers. It was shown that the introduction of branching unit into the PF to make the polymers with hyperbranched structure to avoid the excimer formation and to improve the charge transport-balance properties of PFs. The resulting hyperbranched polyfluorenes show stable blue light emission even in the air at the elevated temperatures.³¹

Figure 1.14 Examples of poly(fluorene)-based hyperbranched copolymers

On the other hand, another problem of PFs is their large band gaps between the LUMO and HOMO energy levels.³² The high energy-barrier between the emissive layer and the electrodes as well as the imbalance between the hole and electron transporting properties in the emissive layer are also possible to cause poor performance of a light-emitting diode (LED) device. In this regard, it is well recognized that the charge injection and transport can be facilitated by sandwiching the emissive layer between a hole-injecting/transporting layer (HTL) above the anode and an electron-injecting/transporting layer (ETL) under the cathode. An alternative approach addressing these issues is to vary the chemical structures of PFs. For example, the incorporation of electron-withdrawing or electron-donating groups into the PF main chains or side chains can influence the electron- or hole-injecting/-transporting capabilities of the polymers.^{22, 33}

Chapter 2

Synthesis and Characterization of

Poly(fluorene-co-alt-phenylene) Containing 1,3,4-Oxadiazole Dendritic Pendants

2.1 Introduction

Organic light-emitting diodes (OLEDs) have the potential to achieve low-cost and full-color flat panel displays due to their merits of high brightness, easy fabrication, and wide ranges of emission colors. However, some important and fundamental challenges remain unsolved, including maximization of external quantum efficiencies (EQEs), design and synthesis of new materials with purer colors, and modes of addressing devices for a full-color display with optimized resolution. A major factor responsible for low device EQEs is that the charge (electron and hole) injection and transportation in emissive materials are generally unbalanced. In general, most of the electroluminescent (EL) polymers inject and transport holes more efficiently than electrons due to their inherent richness of π-electrons. One approach for improving electron transporting properties of polymers is to incorporate electron deficient segments into main chains³⁴ or as pendants attached to the backbones.^{22, 35} Oxadiazole (OXD) units are among the most widely investigated electron transporting structures for OLEDs, which are due to the high electron affinity of the OXD segments in OLED molecules. Several OXD-containing light-emitting polymers have also been prepared in recent years.³⁶

Dendronized polymers may exhibit superior properties in future applications as

compared to their non-dendronized analogues for a number of reasons. The dendritic side chains acting as solubilizers can enhance processsability without the loss of mechanical or thermal stability provided by the rigid polymer backbones. The dense dendron decorations may provide efficient shields, such as protection against chemical reactants and prevention of molecular aggregation. In addition, the dendrons may serve, depending on their chemical nature, as light harvesting antennae, charge carriers, and so on.³⁷

In hopes of combining both excellent electron affinities and energy transfer (light antenna) properties into polymers, a new family of polyfluorene-co-alt-phenylenes carrying peripheral oxdiazole (OXD) functional dendrons attached to the 2- and 5-positions of the phenylene rings will be presented in this report. Thus, the dendritic wedges play both roles of efficient site isolation and excellent electron affinities.

Especially, the enhanced PL emission properties of dendronized polymers were observed in contrast to those of analogous conjugated main-chain without dendritic pendants. Therefore, the light harvest of chromophores from the luminescence of the dendritic pendants by the light antenna design is more efficient than the direct excitation at the maximum absorption of light-emitting segments by an external light source.

2.2 Experimental Section

1H NMR spectra were recorded on a Varian unity 300M Hz spectrometer using CDCl3 solvent. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Monomers 7-9 were characterized by ¹H NMR, elemental analyses, and FAB (or MALDI-TOF) mass spectroscopy. Transition temperatures were determined by differential scanning calorimetry (DSC, Perkin Elmer, Model:

Diamond) with a heating and cooling rate of 10 °C/min. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer under a heating rate of 20 °C/min. Gel permeation chromatography (GPC) analysis was conducted on a Water 1515 separation module using polystyrene as a standard and THF as an eluant. UV-visible absorption spectra were recorded in dilute THF solutions (10^-6 M) on a HP G1103A spectrophotometer, and fluorescence spectra were obtained on a Hitachi F-4500 spectrophotometer.

Polymer solid films were spin-coated on quartz substrates from THF solutions with a concentration of 10 mg/ml. The relative photoluminescent quantum yields (ΦPL) of the compounds in THF were determined using a solution of 9,10-diphenylanthracene as a standard (cyclohexane, ΦPL = 0.90). Dilute sample solutions were used for the determinations (absorbance < 0.1). Values are calculated according to the equation, Φunk = Φstd(Iunk/Aunk)(Astd/Istd)( Πunk/Πstd)², where Φunk is the fluorescence quantum yield of the sample, Φstd is the fluorescence quantum yield of the standard, Iunk and Istd

are the integrated emission intensities of the sample and the standard, respectively, Aunk and Astd are the absorbances of the sample and the standard at the excitation wavelength, respectively, and Πunk and Πstd are the refractive indexes of the corresponding solutions (pure solvents were assumed). Cyclic voltammetry measurements of polymer films were performed on a BAS 100 B/W electrochemical

analyzer in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV/s. A polymer film on a glassy carbon disk electrode was used as the working electrode. A platinum wire was used as the counter electrode and a silver wire was used as the reference electrode.

All preparations and measurements were carried out at room temperature under nitrogen. The potentials were measured against an Ag/Ag⁺ (0.01M AgNO3) reference electrode with ferrocene as the internal standard. The onset potentials were determined by the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram.

2.2.2 Materials.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10), was synthesized according to literature procedures.³⁸ Chemicals and solvents were reagent grades and purchased from Aldrich, ARCROS, TCI, and Lancaster Chemical Co. Dichloromathane and tetrahydrofuran (THF) were distilled to keep anhydrous before use. The other chemicals were used without further purification.

4-Chloromethyl-benzoic acid N'-[4-(2-ethyl-hexyloxy)-benzoyl]-hydrazide (1).

0.57 g (3.02 mmol) of 4-chloromethyl-benzoyl chloride was added into a solution containing 0.40 g (1.51 mmol) of 4-(2-ethyl-hexyloxy)-benzoic acid hydrazide and 0.25 mL (3.02 mmol) of pyridine in 10 mL of N-methyl-2-pyrrolidinone (NMP). The reaction mixture was stirred for 12 h and then poured into water. Finally, the product was filtered and crystallized from methanol. Yield: 87%. ¹H NMR (ppm, CDCl3): δ 0.90-0.97 (m, 6H), 1.33-1.56 (m, 8H), 1.80 (m, 1H), 3.99 (d, J = 5.7 Hz, 2H), 4.60 (s, 2H), 6.89 (d, J = 9 Hz, 2H), 7.43 (d, J = 8.1 Hz, 2H), 7.81 (d, J = 9 Hz, 2H), 7.85 (d, J

(2). 0.43 g (1.02 mmol) of 1 was dissolved in 10 mL of POCl3. The reaction mixture was heated to reflux overnight and then cooled to room temperature. Most POCl3 in reaction mixture was removed at reduced pressure. Later, water was added, and the aqueous layer was extracted with CH2Cl2. After drying of the organic layer and removal of the solvent under reduced pressure, the crude product was purified by column chromatography with CH2Cl2 to get a white solid. Yield: 76%. ¹H NMR (ppm, CDCl3): δ 0.90-0.97 (m, 6H), 1.33-1.56 (m, 8H), 1.76 (m, 1H), 3.92 (d, J = 5.7 Hz, 2H), 4.64 (s, 2H), 7.03 (d, J = 9 Hz, 2H), 7.55 (d, J =7.8 Hz, 2H), 8.06 (d, J = 8.7 Hz, 2H), 8.14 (d, J = 7.8 Hz, 2H).

General Synthetic Procedures of Dendritic Benzyl Alcohols (3 and 5). A mixture of 2 or 4 (2.1 equiv), 3,5-dihydroxybenzyl alcohol (1 equiv), K2CO3 (2.5 equiv), and 18-crown-6 (0.2 equiv) in dry THF was heated to reflux and stirred under nitrogen for 24 h. The mixture was evaporated to dry under reduced pressure, and the residue was partitioned between water and CH2Cl2. Then, the aqueous layer was extracted with CH2Cl2, and the organic layer was dried over MgSO4. Finally, the crude products 3 and 5 were collected without further purification.

General Synthetic Procedures of Dendritic Benzyl Bromides (4 and 6).

(OXD)2-(G-1)-OH (3) or (OXD)2-(G-2)-OH (5) was dissolved in THF under nitrogen, PBr3 (1 equiv) was added dropwise via an addition funnel over 10 min at 0 °C. The mixture was heated to reflux for 30 min and evaporated to dryness under reduced pressure. The residue was partitioned between water and CH2Cl2. Then, the aqueous layer was extracted with CH2Cl2, and the organic layers dried over MgSO4. Consequently, the crude products 4 and 6 were purified as outlined in the following text.

The First Generation of Benzyl Bromide (4). The (OXD)2-(G-1)-Br (4) was purified by column chromatography with EA/CH2Cl2 (1:10) to get a white solid. Yield:

83%. ¹H NMR (ppm, CDCl3): δ 0.90-0.98 (m, 12H), 1.24-1.56 (m, 16H), 1.75-1.79 (m, 2H), 3.93 (d, J = 5.7 Hz, 4H), 4.43 (s, 2H), 5.13 (s, 4H), 6.57 (s, 1H), 6.68 (s,2H), 7.03 (d, J = 8.7 Hz, 4H), 7.58 (d, J = 8.7 Hz, 4H), 8.06 (d, J = 8.4 Hz, 4H), 8.15 (d, J

= 8.4 Hz, 4H).

The Second Generation of Benzyl Bromide (6). The (OXD)4-(G-2)-Br (6) was purified by column chromatography with THF/CH2Cl2 (1:10) to get a white solid.

Yield: 85%. ¹H NMR (ppm, CDCl3): δ 0.89-0.97 (m, 24H), 1.26-1.57 (m, 32H), 1.74-1.78 (m, 4H), 3.91 (d, J = 5.4 Hz, 8H), 4.40(s, 2H), 4.98 (s, 4H), 5.11 (s, 8H), 6.48 (s, 1H), 6.57(s, 2H), 6.60 (s,2H), 6.67(s, 4H), 7.01 (d, J = 8.7 Hz, 8H), 7.55 (d, J

= 8.4 Hz, 8H), 8.04 (d, J = 8.7 Hz, 8H), 8.11 (d, J = 8.1 Hz, 8H).

General Synthetic Procedure of Dendronized Monomers (7-9). A mixture of corresponding compound 2, 4, or 6 (2.1 equiv), 2,5-dibromo-benzene-1,4-diol (1 equiv), K2CO3 (2.5 equiv), and 18-crown-6 (0.2 equiv) in dry THF was heated to reflux and stirred under nitrogen for 48 h. The mixture was evaporated to dryness under reduced pressure, and the residue was partitioned between water and CH2Cl2. The aqueous layer was extracted with CH2Cl2, and the organic layers dried over MgSO4. The crude product was purified as outlined in the following text.

Monomer 7. Monomer 7 was purified by column chromatography with EA/CH2Cl2

(1:10) to get a white solid. Yield: 75%. ¹H NMR (ppm, CDCl3): 0.90-0.97 (m, 12H), 1.34-1.54 (m, 16H), 1.75-1.77 (m, 2H), 3.92 (d, 4H), 5.16 (s, 4H), 7.03 (d, 4H), 7.21 (s, 2H), 7.63 (d, 4H), 8.06 (d, 4H), 8.16 (d, 4H). Anal. Calcd for C52H56Br2N4O6: C, 62.91; H, 5.69; N, 5.64. Found: C, 63.01; H, 5.73; N, 5.64. MS (FAB): m/z [M⁺] 993.20, calcd m/z 992.83.

Monomer 8. Monomer 8 was purified by column chromatography with EA/CH2Cl2

(s, 2H), 6.70 (s, 4H), 7.01 (d, 8H), 7.09 (s, 2H), 7.57 (d, 8H), 8.05 (d, 8H), 8.13 (d, 8H). Anal. Calcd for C112H120Br2N8O14: C, 68.56; H, 6.16; N, 5.71. Found: C, 68.91;

H, 6.27; N, 5.80. MS (MALDI-TOF): m/z [M⁺ ] 1962.84, calcd m/z 1962.00.

Monomer 9. Monomer 9 was purified by column chromatography with EA/CH2Cl2

(1:12) gradually increasing to EA/CH2Cl2 (1:8) to get a white solid. Yield: 65%. ¹H NMR (ppm, CDCl3): 0.91-0.96 (m, 48H), 1.25-1.54 (m, 64H), 1.70-1.76 (m, 8H), 3.89 (d, 16H), 4.96 (s, 4H), 4.99 (s, 8H) , 5.08 (s, 16H), 6.50 (s, 2H), 6.54 (s, 4H), 6.65 (s, 4H), 6.66 (s, 8H), 6.99 (d, 16H), 7.06 (s, 2H), 7.53 (d, 16H), 8.02 (d, 16H), 8.08 (d, 16H). Anal. Calcd for C232H248Br2N16O30: C, 71.44; H, 6.41; N, 5.75. Found:

C, 71.34; H, 6.50; N, 5.76. MS (MALDI-TOF): m/z [M⁺] 3900.28, calcd m/z 3900.35.

General Synthetic Procedure of Dendronized Polymers P1-P3. A mixture of

corresponding dendritic monomer 7, 8, or 9,

2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10), K2CO3, toluene, and H2O was degassed, and Pd{P(p-tolyl)3}3 was added under a nitrogen atmosphere. The reaction mixture was heated to reflux and stirred under nitrogen for 48 h. End group capping was performed by heating the solution under reflux for 6 h sequentially with phenylboronic acid and iodobenzene. After cooling, the crude polymers were dissolved in THF and purified by precipitation from methanol, and dendronized polymers P1-P3 were collected and dried under vacuum.

Dendronized Polymer P1.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10) (273.5 mg, 0.46 mmol), 7 (463 mg, 0.46 mmol), K2CO3 (1.1 g), Pd{P(p-tolyl)3}3 (8.00 mg), toluene (10 mL), and H2O (4 mL) were used in the reaction mixture. Polymer P1 was obtained as a slightly yellow solid. Yield: 75%. ¹H NMR (ppm, CDCl3): 0.62-0.97 (m, 18H), 1.26-1.75 (m, 34H), 1.97(broad, 4H), 3.90 (d, 4H), 5.16 (s, 4H), 7.00 (d, 4H), 7.20 (s, 2H), 7.49 (d, 4H), 7.63 (broad, 4H), 7.83 (d, 2H), 8.03 (d, 4H), 8.09 (d, 4H).

Anal. Calcd for [C77H88N4O6]n: C, 79.35; H, 7.61; N, 4.81. Found: C, 78.12; H, 7.56;

N, 4.45.

Dendronized Polymer P2.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10) (120 mg, 0.20 mmol), 8 (400 mg, 0.20 mmol), K2CO3 (1.1 g), Pd{P(p-tolyl)3}3 (8.00 mg), toluene (10 mL), and H2O (4 mL) were used in the reaction mixture. Polymer P2 was obtained as a slightly yellow solid. Yield: 68%. ¹H NMR (ppm, CDCl3): 0.63-0.97 (m, 30H), 1.31-1.73 (m, 52H), 2.05(broad, 4H), 3.84-3.92 (braod, 8H), 4.89 (s, 8H), 5.10 (broad, 4H), 6.46 (s, 2H), 6.61 (s, 4H), 6.93-7.03 (broad, 10H), 7.42 (d, 8H), 7.74 (broad, 6H), 7.97-8.14 (broad, 16H). Anal. Calcd for [C137H152N8O14]n: C, 77.08; H, 7.18; N, 5.25. Found: C, 75.99; H, 7.17; N, 4.67.

Dendronized Polymer P3.

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10) (60 mg, 0.10 mmol), 9 (400 mg, 0.10 mmol), K2CO3 (1.1 g), Pd{P(p-tolyl)3}3 (8.00 mg), toluene (10 mL), and H2O (4 mL) were used in the reaction mixture. Polymer P3 was obtained as a slightly yellow solid. Yield: 70%. ¹H NMR (ppm, CDCl3): 0.63-0.95 (m, 54H), 1.29-1.66 (m, 88H), 2.05(broad, 4H), 3.80-3.89 (braod, 16H), 4.88 (broad, 28H), 6.44-6.59 (broad, 18H), 6.87-6.98 (broad, 18H), 7.39-7.57 (broad, 22H), 7.88-8.07 (broad, 32H). Anal. Calcd for [C257H280N16O30]n: C, 75.78; H, 6.93; N, 5.50.

Found: C, 74.75; H, 6.91; N, 5.05.

Scheme 2.1 The synthetic route of dendritic benzyl bromides 4 and 6

Scheme 2.2 The synthetic route of monomers 7-9 and polymers P1-P3

2.3 Results and Discussion

The synthetic routes of monomers 7-9 and polymer P1-P3 are shown in Scheme 2.1 and Scheme 2.2. 2-(4-Chloromethyl-phenyl)-5-[4-(2-ethyl-hexyloxy)-phenyl]-[1,3,4]

oxadiazole (2) was firstly prepared according to the literature procedure.³⁹ In the beginning, acylation of 4-(2-ethyl-hexyloxy)-benzoic acid hydrazide with 4-chloromethyl-benzoyl chloride was followed by cyclodehydration of acylated hydrazide in the presence of POCl3. Dendritic benzylic alcohol (3) of functionalized first-generation (G1) was prepared by the Williamson ether reaction of 3,5-dihydroxybenzyl alcohol with the corresponding functionalized benzyl chloride (2) in the presence of K2CO3 and 18-crown-6. A subsequent reaction of compound 3 with PBr3 afforded dendritic benzylic bromide (4) of functionalized G1. An iterative Williamson ether reaction of 3,5-dihydroxybenzyl alcohol with dendritic benzylic bromide (4) of functionalized G1 produced the corresponding dendritic benzylic alcohol (5) of functionalized second-generation (G2), which reacted with PBr3 togive the corresponding dendritic benzylic bromide (6) of functionalized G2. In the presence of K2CO3 in THF at reflux, 2,5-Dibromo-benzene-1,4-diol with the corresponding different generation dendrons was achieved to obtain the monomers

(7-9), which was then copolymerized with 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluorene (10) using

the Suzuki coupling. In the beginning, when Pd(PPh3)4 was used as a catalyst precursor, polymer P3 could not be obtained due to the shielding effect of monomer 9 on the reactive site, so Pd{P(p-tolyl)3}3 seems to be a superior choice in many SPC cases.⁴⁰ Freshly prepared Pd{P(p-tolyl)3}3 was used as the catalyst precursor⁴¹ with aliquate 336 as the phase-transfer reagent in a biphasic system (toluene/aqueous K2CO3), to give alternating copolymers P1-P3. All dendronized polymers could be

fully dissolved in common organic solvents, such as methylene chloride, chloroform, toluene, and THF. The molecular weights of polymers determined by GPC using polystyrene standards are summarized in Table 2.1. GPC analysis showed that the weight-average molecular weights (Mw) and polydispersity indexes (PDI) of the dendronized polymers are in the range of (3.3-4.4) ×10⁴ and 1.4-2.4, respectively. The data reveal that the degree of polymerization decreases as the size of the dendritic monomer increases. The same observation was also reported in the synthesis of similar dendronized polymers.²⁹ The thermal properties of the dendronized polymers were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and the results are presented in Table 2.1. All polymers exhibit good thermal stability, where less than 5% of weight-loss decompositions at ca.

310-353 °C under nitrogen but more than 50% of weight-loss decompositions at 450-460 °C (Figure 2.1) were observed. The glass transition temperatures (Tgs) of the dendronized polymers are in the range of 77-122 °C, which are declined by increasing the size of the attached dendrons. Compared with a previous report, Tgs of dendronized polymers P1-P3 are higher than that of poly[(9,9-dihexylfluorene)-alt-co-(2,5-dihexyl-2,5-phenylene)] (Tg = 50 °C) or poly[(9,9-dihexylfluorene)-alt-co-(2,5-dihexyloxy-1,4-phenylene)] (Tg = 72 °C).⁴²

0 100 200 300 400 500 600 700 800 900

Table 2.1 Molecular Weights and Thermal Properties of Polymers P1-P3 Polymer Mna Mw a PDI ^a Tg b (°C) Td c(°C)

P1 17500 42000 2.4 122 310

P2 24100 44300 1.8 94 341

P3 24000 33500 1.4 77 353

a Molecular weights determined by GPC in THF, based on polystyrene standards.

(PDI = Mw/Mn)

b Glass transition temperature (°C) determined by DSC at a heating rate of 10

°C min^-1.

c Temperature (°C) at 5% weight loss measured by TGA at a heating rate of 20

°C min^-1 under nitrogen.

2.3.2 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.

250 300 350 400 450 500 0.0

0.2 0.4 0.6 0.8 1.0

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.

350 400 450 500 550 600 650

0.0 0.2 0.4 0.6 0.8 1.0

Intensity (a.u.)

Wavelength (nm) P1 P2 P3

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

350 400 450 500 550 600 650 0.0

0.2 0.4 0.6 0.8 1.0

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

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