Liquid light molecules

With suitable chemical modification, unprecedented room temperature liquid porphyrins could be used as optical limiters in optoelectronics and spectroscopic instrumentation, according to researchers in Poland.

Daniel Gryko, of the Polish Academy of Sciences, in Warsaw, and colleagues Agnieszka Nowak-Król and Dorota Gryko point out that porphyrins and related molecules have received a great deal of attention in the scientific community because of the remarkable role they play in the natural world at the heart of photosynthesis, enzymes, proteins, and vitamins. Porphyrins specifically have found many useful applications in a wide variety of areas from light therapy for cancer to energy-converting and catalytic systems that mimic photosynthesis and enzymes.

However, porphyrins have a major shortcoming when it comes to using them in research into certain applications. The molecules are essentially flat, aromatic rings that can readily form stacks and aggregates, which means they are commonly very insoluble, and readily form crystals with a high melting point.

Writing in Chemistry - An Asian Journal, Gryko and his colleagues recently described the design and synthesis of liquid porphyrin-based compounds that, with suitable structural modifications, have much lower melting points than their well-known natural counterparts. These novel materials may one day find applications as optical limiters.

The team focused on a related series of meso-substituted A4-porphyrins that have aromatic, aryl, groups bearing a long alkoxy chain. The alkyl chain length ranges from having 8 to 18 carbon atoms. The researchers synthesised these compounds using what they say is a high-yielding two-step process that begins with commercially available starting materials. The resulting porphyrins were all found to be very soluble in conventional organic solvents. The team used differential scanning calorimetry (DSC) to investigate the thermal properties of the new series of porphyrins.

"The behaviour of the compounds in the DSC thermograms is very sensitive to the alkoxy chain lengths present in the molecule" explains Gryko. He and his colleagues point out that a gradual decrease in the temperature at which the crystalline to isotropic liquid phase transition takes place is observed as the length of the alkoxy side chains increase. A minimum is observed for the decycloxy compound. A longer chain raises the melting point. The team points out that 5,10,15,20-Tetrakis[3,4,5-tri(undecyloxy)phenyl]porphyrin and its decyloxy analogue are both liquids at room temperature and are shown to have melting points of -24 and -55 Celsius, respectively. The team used proton and carbon-13 nuclear magnetic resonance spectroscopy to characterise their products and investigated their absorption with UV/Vis spectroscopy.

Liquid porphyrins, in contrast to porphyrins displaying liquid crystalline behavior, are unprecedented in the scientific literature. The detailed results provide significant insight into the possibilities for the design and synthesis of room temperature liquid porphyrins in a very simple and efficient way. The team suggests that the presence of the aryl and alkoxy appendages presumably disrupt the familiar pi-pi interactions that normally occur and lead to aggregation of flat porphyrins.

The researchers point out that despite the modifications, the liquid porphyrins retain the same spectroscopic features as their solid analogues. This discovery thus opens up not only a method for creating novel derivitised porphyrins with unique properties but could also have broader application in materials science for uses where access to high solubility or the liquid phase are needed for processing or the specific use.

"Although liquidity is a crucial pre-condition for optical limiting, 'simple' porphyrins have too low two-photon absoption cross-section to be used in this application," explains Gryko. "In other words, the published compounds cannot be used as optical limiters. We are in the process of modifing the core (pi-electron system) to increase non-linear properties (and maintaining the liquidity at RT) and hopefully we will be successful soon."