Dye molecule

The overall performance of OLEDs can be dramatically improved by the use of dye molecules. These small molecules efficiently trap electrons and holes (ie carbanions and carbocations) and have high radiative recombination rates relative to non-radiative decay processes. One can quickly see why this would be effective in a device as the dye molecule can be optimized for recombination, while the HTL and ETL layers can be optimized for carrier transport, carrier injection and chemical stability without having to worry about radiative recombination. From the discussion in Section 9.2, it is known that many molecules exhibit low radiative recombination rates due to the symmetries of their HOMO and LUMO orbitals, resulting in disallowed optical recombination pathways. The solution to the problem is to dissolve dye molecules optimized for emission in one or more of the transport layers. 

Based on the above, criteria important in the selection or design of a dye molecule include  

The dye should trap free carbocation and carbanion defects from the surrounding matrix and efficiently convert these to excitons. 

These excitons should remain on a single dye molecule, rather than being distributed over multiple molecules. (The carbocation and carbanion should both be on the same molecule.) 

The excitons should decay radiatively. To accomplish this the exciton lifetime should be relatively short. 

---

Kuhn H 1958 Oscillator strength of absorption bands in dye molecules J. Chem. Phys. 29 958-9... 

Experimental investigations of the model system of dye molecules adsorbed onto surfaces of polystyrene spheres have finuly established the sensitivity and surface specificity of the SHG method even for particles of micrometre size [117]. The surface sensitivity of die SHG process has been exploited for probing molecular transport across the bilayer in liposomes [118], for measurement of electrostatic potentials at the surface of small particles [119] and for imaging... 

Zeisel D, Deckert V, Zenobi R and Vo-Dinh T 1998 Near-field surface-enhanced Raman spectroscopy of dye molecules adsorbed on silver island films Chem. Phys. Lett. 283 381... 

Murakami H, Kinoshita S, Hirata Y, Okada T and Mataga N 1992 Transient hole-burning and time-resolved fluorescence spectra of dye molecules in solution evidence for ground-state relaxation and hole-filling effect J. Chem. Phys. 97 7881-8... 

Weiner A M and Ippen E P 1985 Femtosecond excited state relaxation of dye molecules in solution Chem. Phys. Lett. 114 456-60... 

Basche T, Moerner W E, Orrit M and Talon FI 1992 Photon antibunching in the fluorescence of a single dye molecule trapped in a solid Phys. Rev. Lett. 69 1516-19... 

A dye molecule has one or more absorption bands in the visible region of the electromagnetic spectrum (approximately 350-700 nm). After absorbing photons, the electronically excited molecules transfer to a more stable (triplet) state, which eventually emits photons (fluoresces) at a longer wavelength (composing three-level system.) The delay allows an inverted population to build up. Sometimes there are more than three levels. For example, the europium complex (Figure 18.15) has a four-level system. 

Two typical dye molecules. The europium complex (a) transfers absorbed light to excited-state levels of the complexed Eu , from which lasing occurs. The perylene molecule (b) converts incident radiation into a triplet state, which decays slowly and so allows lasing to occur. 

Figure 9.18 shows a typical energy level diagram of a dye molecule including the lowest electronic states Sq, and S2 in the singlet manifold and and T2 in the triplet manifold. Associated with each of these states are vibrational and rotational sub-levels broadened to such an extent in the liquid that they form a continuum. As a result the absorption spectrum, such as that in Figure 9.17, is typical of a liquid phase spectrum showing almost no structure within the band system. 

Figure 9.1 8 Energy level scheme for a dye molecule showing nine processes important in laser action...

If dye molecules are embedded into an amorphous matrix, preferably transparent polymers, greatly and inbornogenously broadened spectral lines are observed. This broadening is caused by the energetic interaction of the dye molecules with the locally different environment in the polymer matrix. The ratio of the homogenous initial line width of the dye molecule T to the inhomogenous line width of the dye in the polymer T ranges from 1 10 to 1 10 . ... 

Fig. 3. Order parameter as a function of temperature for -methoxybeiizylidene-/) - -butylariiline (MBBA), a room temperature nematic Hquid crystal. S(T) is determined from tbe polarization of tbe absorption (dicbroism) of small quantities of a dye molecule of similar stmcture (/n / .f-dimetby1aminonitrosti1bene) wbicb bas been dissolved in tbe Hquid crystal bost (1).

Table 3. Donor and Acceptor Groups in Fluorescent Dye Molecules...

Fig. 1. Schematic energy-level diagram for a dye molecule. Electronic states Sq = ground singlet state = first excited singlet state S2 = second excited singlet state Tj = first excited triplet state T2 = second excited triplet state EVS = excited vibrational states. Transitions A = absorption excited states ...

In the process of excitation, the dye molecule absorbs a quantum of uv or visible radiation. The quantum has an energy E = hv, where b is Planck s constant and O is the frequency of the radiation. The higher the frequency of the quantum, the shorter the wavelength X, with u-A = c, where c is the velocity of light in a vacuum. 

Fluorescent photons can vary widely in energy, even if emission occurs from the same type of dye molecule. As a result, the emission spectmm of a typical dye is quite broad, frequently extending for 150 nm. 

In addition to the processes that can compete with fluorescence within the molecule itself, external actions can rob the molecule of excitation energy. Such an action or process is referred to as quenching. Quenching of fluorescence can occur because the dye system is too warm, which is a very common phenomenon. Solvents, particularly those that contain heavy atoms such as bromine or groups that ate detrimental to fluorescence in a dye molecule, eg, the nitro group, ate often capable of quenching fluorescence as ate nonfluorescent dye molecules. 

A nearby molecule with a conjugated system may rob the dye molecule of its electronic energy. On the other hand, a fluorescent dye can pick up electronic energy from such a substance, called a sensitizer, with increased fluorescence. 

Radiation, both in the uv and in the visible region, can have a highly destmctive effect by decomposing the dye molecule. Other substances, particularly water, can reinforce the photochemical effect of light. Once the dyed material fades, its original condition usually cannot be restored. 

As in chemical sensitization, spectral sensitization is usually done after precipitation but before coating, and usually is achieved by adsorbing certain organic dyes to the silver haUde surfaces (47,48,212—229). Once the dye molecule is adsorbed to the crystal surface, the effects of electromagnetic radiation absorbed by the dye can be transferred to the crystal. As a result of this transfer, mobile electrons are produced in the conduction band of the silver haUde grain. Once in the conduction band, the electrons are available to initiate latent-image formation. 

Fig. 11. Mechanism of electron transfer from an excited dye molecule to a silver haUde crystal where HOMO and LUMO are highest occupied and lowest...

Many spectral-sensitizing dyes can be classified according to molecular stmctures (228). The stmctural part of a dye molecule that enables the molecule to absorb visible or infrared radiation is called a chromophore. The resonance stmcture for three common chromophores is shown. 

---