Selective growth of green fluorescent metastable phase of perylene crystals

Synthesized new molecules have been extensively used in improving the functionality and properties of optoelectronic devices based on organic materials. Perylene, a simple aromatic molecule has been investigated for potential optoelectronic applications owing to its high charge-carrier mobility at low temperatures. Presently, stable α-phase and metastable β-phase are the well know crystal polymorphs of perylene and they can, therefore, be used to examine the correlation between the electronic structure and crystal morphology which also affects its fluorescence properties.

Since their accidental discovery by recrystallization, several methods have been proposed for the selective growth of metastable β-perylene. However, samples of β-perylene crystals obtained by these methods are unsuitable for the study of their optoelectronic properties due to impurity contamination that affects the stated properties. Thus, there is an urgent need to develop alternative methods.

To address this issue, Kenta Sato and Professor Ryuzi Katoh from Nihon University in Japan recently introduced the new concept of physical vapor transport method under atmospheric pressure. Using this technique, they selectively prepared metastable β-perylene crystals and evaluated their fluorescence properties: quantum yield, annihilation rate constant and lifetime. A time-resolved fluorescence spectrometer was used to measure the fluorescence decay profiles of the β-perylene crystals. Their work is published in the journal, Chemical Physics Letters.

The experiment was conducted in a large glass tube. During crystal growth under ultraviolet radiation, the authors observed various colors in the glass tube. α-perylene (source material) and gas-phase perylene produced orange and blue fluorescence respectively. However, only green fluorescent crystals were observed clearly at the downstream position of the tube. This was attributed to the high conversion rate from α-perylene to β-perylene.

The fluorescence lifetime was evaluated to be 12.3ns almost similar to the initially reported values of 13ns. Additionally, the difference in the decay profile at high and low excitation intensities was quite significant. Regardless, the annihilation of singlet excitons occurred at an annihilation constant rate of approximately 3×10^-10 cm³ s^-1 at high excitation density. This value represented that of a relaxed singlet exciton and was significantly smaller as compared to that of a free singlet exciton. In β-perylene crystals, it was worth noting that the annihilation free excitons became dormant at higher excitation density. Furthermore, the fluorescence decay was noted to become faster with the increase in the excitation light intensity probably due to the difference in the time-resolution and dynamic range measurements.

On the other hand, the fluorescence of the quantum yield of the β-perylene crystals being a function of the excitation wavelength was determined to be 0.6. At wavelengths less than 300 nm, the quantum yield decreased dramatically with a decrease in the wavelength indicating an occurrence of fission from a highly excited singlet state into two triplet states. However, no change in the quantum yield was observed at a wavelength of 400nm. The reported results, particularly the radiative lifetime that was determined to be 20 ns, confirmed that the fluorescence originated from excimer with a nonparallel configuration in the crystals.

In summary, Kenta Sato and Professor Ryuzi Katoh successfully analyzed the fluorescence properties of β-perylene crystals prepared by a physical vapor transport method under atmospheric pressure. In a statement to Advances in Engineering, Professor Ryuzi Katoh, the lead author, said that the physical vapor transport method can be scaled up for mass production and also be used in the preparation of other polymorphic crystals.