Photoluminescenct (PL) and Electroluminescent (EL) Properties

The photophysical properties of all luminescent compounds were studied by photoluminescence (PL) and UV-visible absorption spectra in dilute chloroform solutions and thin films. The optical properties of all compounds are summarized in Table 2.2. Due to the identical rigid cores, all synthesized materials in solutions have almost the same maximum absorption wavelength around 400nm in solutions and 390nm in films. Similar to the absorption spectra, the maximum PL wavelength of all compounds are around 448 nm in solutions and 496 nm in films, respectively.

Compared with the maximum PL wavelength in solutions, the materials in thin films exhibit red-shifted PL emission owing to the π- π* aggregation of the rigid cores. It is unexpected that the maximum absorption wavelength in solid state (390nm) is more blue-shifted than in solution (400nm). In terms of the onset wavelengths (λonset) of UV-visible absorption spectra, it exhibits that the λonset is around 469 nm in films and 446 nm in solutions. Therefore, the optical band gaps (calculated by the equation:

E^onset = 1240 / λonset) in film are smaller than in solutions and this is because of the π- π* aggregation of the rigid cores in films. Figure 2.4(a) shows an example of the UV-Visible and PL spectra of the rod-coil molecule, FOC8PEO44. In figure 2.4(b), the influence of the flexible chain lengths in PL spectra are compared. It shows that the intensity of the shorter wavelength peak at 464 nm grows as the flexible chains increase because the flexible chains of poly(ethylene oxide)s seem to be as solid solvents to insulate the intermolecular aggregation. Moreover, in order to realize the relation of two peaks between 464 nm and 496 nm, figure 2.5(a) shows the normalized photoluminescence excitation (PLE) spectra of FOC8PEO17 in the same film monitored the emissions at 464 nm and 496 nm. The patterns of PLE spectra monitored at 464 nm and 496 nm almost coincide with each other. It demonstrated that the shorter wavelength peak at 464 nm is a vibronic peak but not the

non-aggregated molecular emission. The joined flexible chains of poly(ethylene oxide)s in molecule change the state of the whole molecular aggregation which lead to change the distribution of the emission energy in PL spectra. Therefore, we suppose that the two peaks in PL spectra at 464 nm and 496 nm are the nature emission of the materials but not the co-existence of aggregated and non-aggregated emissions.

Besides the insulation of the molecules, we believe that bulky poly(ethylene oxide) chains would influence the conformation of the rigid cyclic backbone.²⁶ For these reasons, we might predict that the peak at 464nm will continue growing and the peak at 496nm decreasing as keeping on poly(ethylene oxide) length increase. Finally, it will be a blue-shifted spectrum. Furthermore, this prediction might be proved by the EL spectra of our PLED devices (the emitter were doped in PVK which behaves as a solid solvent) in figure 2.6(a). Figure 2.6(a) shows the EL spectra of analogous compounds and the maximum EL wavelengths are around 462 nm which correspond to the growing peak at 464nm and decreasing peak at 496nm in figure 2.4(b). Figure 2.5(b) displays the PL spectra of FOC8PEO44 in solid state and in Colh phase (50 ).℃ At mesomorphic temperature, the molecule, FOC8PEO44, was self-assembled to highly order Colh phase and it lead to the aggregation of the rigid cores. Hence, the PL spectra in solid state are red-shifted and the shorter wavelength peak at 464 nm became a shoulder.

Table 2.2 Absorption and Photoluminescence Spectral Data of Synthesized Molecules

absorption λ_max (nm) PL emission λ_max (nm)

Sample solution^a film solution^a film Φ (solution)^b Φ (film)^c

FOC8 (8) 401 382 447 496 0.69 0.36

FOC16 (10) 400 382 448 498 0.70 0.38

FOC8PEO17 (17) 400 387 448 496 0.71 0.43

FOC16PEO17 (18) 400 386 448 497 0.70 0.42

FOC8PEO44 (19) 400 393 448 496 0.72 0.47

FOC16PEO44 (20) 401 390 448 497 0.71 0.47

a Absorption and PL emission spectra were recorded in dilute CHCl3 solutions at room temperature.

b PL quantum yield in CHCl3 and 9,10-Diphenylanthrance is the reference of quantum yield.

c Solid film of 9,10-Diphenylanthrance blended in PMMA is the reference of quantum yield.

300 350 400 450 500 550 600 650 700 750

PL intensity (a.u.)

Absorption (a. u.)

wavelength (nm)

FOC8PEO 44(19) UV-solution UV-film PL-solution PL-film

(a)

400 450 500 550 600 650 700 750

464 496

PL Intensity (a.u.)

wavelength (nm)

FOC8 (8) (film) FOC8PEO

17(17) (film) FOC8PEO44 (19) (film)

(b)

Figure 2.4 (a) Absorption and PL spectra of FOC8PEO44 (19) in solutions (CHCl3 as solvent) and films. (b) PL spectra of films of materials contain different flexible chains.

200 300 400 500 600 700

PLE intensity (a. u.)

wavelength (nm)

FOC8PEO

17 (17) (film) FOC8PEO17 (464nm)

FOC8^PEO17 ^(496nm)

(a)

400 450 500 550 600 650 700

wavelength (nm)

PL intensity (a. u.)

FOC8PEO44 (19) solid state

at LC phase (50^oC)

(b)

Figure 2.5 (a) PLE spectra of FOC8PEO17 (17) film monitored at 464 nm and 496 nm respectively (normalized at 412 nm). (b) PL spectra of FOC8PEO44 (19) in solid state and at 50℃ (Colh phase).

To fit the energy band structures of PLED devices, it is necessary to determine the

energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in each component, which were carried out by CV measurements to investigate the redox behavior of the molecules in solutions (CH2Cl2 as solvent). The potential values estimated here were based on the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation:²⁷ E^HOMO/E^LUMO = [-(E^onset – 0.45) -4.8] eV. The onset potentials were determined from the intersection of two tangents drawn at the rising and background currents of the cyclic voltammogram. A crude estimation of the LUMO levels of reduction compounds were deduced from the HOMO values and the optical band gaps.

The HOMO and LUMO energies are summarized in Table 2.3. As expected, HOMO and LUMO energies are almost similar on account of the identical rigid cores.

Table 2.3 HOMO and LUMO Energies, and Electrochemical Properties

Sample E^ox/onset (eV) E^HOMO(eV) ^a E^LUMO(eV) ^b E^opt. (eV) ^c

FOC8 (8) 0.83 -5.49 -2.71 2.78

FOC8PEO17 (17) 0.83 -5.5 -2.72 2.78

FOC8PEO44 (20) 0.81 -5.48 -2.7 2.78

a HOMO energies were obtained from the cyclic voltammetry.

b LUMO energies were deduced from HOMO values and optical band gaps.

c Optical band gaps were obtained from the UV-Visible spectra.

Due to the poor film quality of the synthesized molecules, the emitters were doped into PVK to fabricate PLED devices. A series of double-layered EL devices with the configuration of PVK:emitters(100:8 by weight)/TPBI(30 nm)/MgAg(50 nm)/Ag(100 nm) were made by spin-coating of PVK blended with the synthesized emitters (100:8 by weight) onto indium-tin oxide (ITO) glass substrates, and their EL data are demonstrated in Table 2.4. In Figure 2.5(a), It is reasonable to expect that the maximum EL wavelengths around 462 nm (they are not optimized devices) because

the emitters are doped into PVK which behaves as a solid solvent. The current-voltage and luminescence-voltage, i.e. EL response, curves of one typical PLED device (PVK:FOC8(100:8 by weight)/TPBI /MgAg/Ag ) is displayed in Figure 2.5(b). All these devices show turn-on voltages for current and turn-on voltages for light from 8 to 10 V, and their attainable maximum luminances are from 1045 to 2871 cd/m².

Table 2.4 EL Data of PLED Devices^a

Sample λ_{max, EL}

a PVK:emitters(100:8 by weight)/TPBI(30 nm)/MgAg(50 nm)/Ag(100 nm).

bVon is the turn on voltage of current.

c Von is the turn on voltage of light.

300 400 500 600 700 800

1200 Current Density (mA/cm²) Luminance (cd/m²)

Figure 2.6 (a) Normalized EL spectra of PLED devices, PVK:emitters(100:8 by weight)/TPBI(30 nm)/MgAg(50 nm)/Ag(100 nm). (b) Current-voltage and luminescence-voltage characteristics of the PLED device, PVK:FOC8(100:8 by weight)/TPBI (30 nm)/MgAg (50 nm)/Ag (100 nm).