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Solutions for the analysis of organic EL materials

Electroluminescent materials (EL) can be commonly found in nightlights, watch illuminations, flat wall decorative illuminations, medical tool display screens, computer monitors and billboards. Though analyzing organic electroluminescence elements can be difficult, it can be achieved by Waters’ impurity profiling as well as separation and analysis of EML organic materials.

Construction of organic EL elements

Fig.1 Schematic illustration of organic EL elements

As shown in Fig.1, the basic construction of organic electroluminescence (EL) elements comprises a light-emitting layer (EML) sandwiched between the anode and cathode electrodes. To emit light from an EML, a transparent electrode (indium tin oxide: ITO), including tin-doped indium oxide, is used as an anode. A variety of EML constructions, from a single-layer to a multi-layer, have been applied for patents, and in the multi-layer construction, each layer plays a specific role. The construction shown in Fig.1 comprises the three layers: from the cathode side, an electron transport layer (ETL), an EML, and a hole transport layer (HTL). In response to the electric field, holes and electrons are injected from the anode and the cathode, respectively, and the binding energy between the holes and electrons is produced in the EML. The energy excites the fluorescent organic material, which is the center of the light emission, and light is emitted. Therefore, to develop an element with higher brightness and efficiency, a balanced injection of holes and electrons from both carriers into the EML is required. The organic molecules used in the EML are classified into low molecules and high molecules.

Light-emission mechanisms of organic compounds (fluorescence and phosphorescence)

Fig.2 Jablonski diagram

Figure 2 shows a Jablonski diagram illustrating the energy state transitions of aromatic ring compounds. S0, S1, S2, and T1 show the ground state, the first excited singlet state, the second excited singlet state, and the triplet state, respectively. During the process of radiative transition, compounds emit light when the compounds excited by excessive energy shift to a low-energy state (stable ground state). The emission induced during transition from the first excited singlet state to the ground state is defined as “fluorescence,” and the emission induced during transition from the triplet state is defined as “phosphorescence.” Fluorescence is an emission with the electron transition within the same spin multiplicity state, and the emission is extremely short because the state of electron spin is maintained. In contrast, phosphorescence is an emission with the electron transition among different spin multiplicity states, and the states of the spin may change. This, basically, is a forbidden transition, and it takes a longer time to emit through this transition. Thus, the lifetime of emission might be prolonged in phosphorescence.

Selected EML organic materials used in organic EL elements

An example of an analysis of EML materials
An example of the structural elucidation of coumarin 6
Showing an example of the measurement of coumarin 6 using the SYNAPT G2-Si

Fig.3 MS spectra of coumarin 6

  • MS spectrum
    The molecular weight information is obtained in single Tof MS mode. Coumarin 6 was ionized by electrospray ionization (ESI) and detected as protonated ion [M+H]+. Tof MS provides accurate mass with the unit of mDa and the information of patterns of stable isotopes (Fig.3).
  • Elemental composition using accurate mass measurement
    The screen shown in Fig.4 indicates the results from the elemental composition using data from MS spectra. The numbers in the green frame exhibit the accuracy of accurate mass (differences between the actual values and the calculated values). i-Fit is used to narrow candidate components based on the coincidence between the patterns of stable isotopes and the accuracy of accurate mass, and is extremely useful toward analyzing the components of unknown chemicals (shown in the red frame in Fig.4).

Fig.4 Results of a component analysis based on data from MS spectra

  • MS/MS with dual-collision cells
    To obtain information about fragments in a chemical structure using MS/MS, higher-energy collision-induced dissociation (CID) is required, especially for chemicals with a rigid structure. Information about the fragments from a chemical with low molecular weight can be obtained by increasing the energy level. However, the production of fragments and the loss of ions are simultaneously enhanced by increasing the energy level. Therefore, this procedure is not always optimal in terms of the sensitivity and accuracy of the accurate mass of the fragment ions. In the SYNAPT G2-Si, two-step fragmentation is available with a dual-collision cell, which enables efficient and detailed information acquisition (Fig.5).

Fig.5 MS spectra of coumarin 6
In the figure (from top left):With a dual-collision cell / With a single-collision cell

System solution for the separation and analysis of EML organic materials
UPLC/FLR (fluorescence detector)

  • High-sensitivity fluorescence detector
  • Scanning in the fluorescence detection
  • Hyper-separation capability, optimal for the separation of analogues and isomers
  • Mobile phase Auto Blend technology, optimal for the consideration of separation conditions
  • A large line of columns
  • High reproducibility, optimal for yield measurement
  • Hyper-speed separation, optimal for response monitoring

System solution for the impurity profiling of EML organic materials
UPLC/SYNAPT G2-Si (UPLC/quadrupole, IMS, time-of-flight mass spectrometer)

  • Dual-collision cell, efficient for the analysis of chemicals with a rigid structure
  • With ion mobility, efficient for isomer separation
  • TAP fragmentation, efficient for the structural elucidation of complex molecules (assignment of fragment ions)
  • Auto-calibration by IntelliStart
  • Precise accurate mass measurement by LockSpray
  • Precise elemental composition analysis by i-Fit
  • Applicable to ultra-performance liquid chromatography (UPLC)
  • Applicable to the ESCi, APCI/APPI, ASAP, APGC, MALDI
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