Our research focuses on the synthesis of fluorescent compounds. Some fluorescent compounds and materials are also electroluminescent. If you want to read more about electroluminescence, here are some recent books on the subject. In general, there are two main types of electroluminescent materials: conjugated polymers and small organometallic complexes, which could be used as emitters or dopants:
Conjugated polymers Small organometallic complexes
Our group specializes in the synthesis of small organometallic complexes. We utilize our expertise in organic and heterocyclic chemistry to design and prepare various electroluminophores (chromophores that are electroluminescent) based on modified 8-hydroxyquinoline ligands. This is very important area of research, because the main utility of electroluminophores is in fabrication of Organic Light-Emtting Diodes (OLEDs) for full-color displays.
We are particularly interested in three distinct areas of organometallic chemistry of electroluminescent materials:
1. We investigate the effect of ligand structure on emission wavelength of the electroluminescent complex. We believe that properly chosen substituents will allow us to shift the emission wavelength (color of emitted light). This rational approach to what is called color-tuning is important for preparation of full color displays that require Red, Blue and Green emitting pigments.
The electron-donating groups (EDG) and electron-withdrawing groups (EWG) when introduced to the hydroxyquinoline ligand moieties of organometallic electrolumophore such as AlQ3 cause change in electronic properties of the ligand involved in the emission of light.
The introduction of functional groups may therefore be used to fine-tune the luminescence of the electroluminophore. This approach may be successfully used to change a color of OLED elements. For example, the green emission of AlQ3 complex turns to red when an electron-donating substituents are introduced. Likewise, introduction of an electron withdrawing substituent may change the emission of the complex to blue. Yeah, we wish it is so easy like we write it here. In fact, the preparation of such materials is actually quite difficult. However, our current results suggest that we may have discovered how to do that. Next figure shows the photoluminescence in solution (illuminated with black light ~360 nm) of four different AlQ3 derivatives that differ "only" in their substituents on C5 (in the para-position to phenolate oxygen):
Next figure shows the actual OLED devices fabricated using the same compounds. One can clearly see that the Light-color tuning observed in the solution (above) is also observed in the solid state devices (although some small changes in the are observed). Again, the blue-emitting OLED on the left comprises strongly EWG, while the red-orange emitting OLED on the right contains EDG in C-5 position. Our success in this field was recently hailed by an article and a cover art in the Chemistry - A European Journal 2006.
Our current efforts focus on synthesizing of AlQ3 derivatives with true-blue emission in the solid state. This is quite a challenge, but every 10-15 nm blue shift results in a dramatic improvement in the color purity. This is particularly noticeable in the spectra (and OLED photographs) below. The graph shows EL spectra of three devices employing various AlQ3 derivatives prepared by our group. The photos show the two devices corresponding to the EL maxima of 460 nm (the blue OLED) and 468 nm (the green OLED). While one can see that the EL maxima do not differ significantly (~8 nm), the shoulder in the EL spectra (region 500-600 nm) differs a lot. Because this region (500-600 nm) is responsible for green-yellow-orange color, the broader emission band of the green device results in a mixed color (blue+green+yellow), which averages to a resulting green. On the other hand, the narrower spectrum of the blue device results in much more pure blue color. The width of the emission bands in solution and in solid state are difficult to predict, as they are affected by aluminum complex geometry, molecular arrangement in the solid state (OLED), temperature during fabrication, etc. Obviously, these features require significant research effort, which is what our group is known for.
2. Second area of our OLED-oriented research is focused on light harvesting and energy transfer in hybrid electroluminescent nanostructured materials. Click here for more details.