Dr. Krishnayan Basuroy
Department of Chemistry, University at Buffalo
New York, USA

Investigation of light-induced structural changes in purely organic molecular crystals, by means of in-house time resolved photocrystallography

Day, Date & Time :Tuesday, 5th September, 2017 at 4:00 PM

Schmidt and coworkers first showed that the nature of the products of a solid-state reaction is topochemically restricted by the geometry of the crystal structure.[1-2] This topochemical postulate opened a new field of solid-state chemistry, encouraged by its close association of crystallographic and chemical methods. Since that time, dramatic increase in the brightness of X-ray sources and highly sensitive detectors made it possible to perform time resolved pump-probe experiments with synchrotron sources, on transient species with lifetimes of ms, ms and less.[3-4] Purely organic luminescent molecules with long-lived excited states gained a certain amount of interest, due to their wide range of optoelectronic applications. The environment friendliness, non-toxicity and low cost also brought favourable attention over their inorganic counterparts. However, the benefit of using organic phosphorescent materials have been limited by their extreme sensitivity towards the surrounding environment (humidity and temperature) and fast non-radiative quenching. Nonetheless, a number of recent studies on purely organic phosphorescent molecules with unusually short Br…Br intermolecular contact distances have been reported.[5-8] We have also recently reported the excited state geometry of a Br containing purely organic phosphorescent molecule, by means of In-House Time Resolved X-Ray Diffraction.[ 9] We also observed an increment in the Br…Br intermolecular contact distance from 3.29Å (GS) to 3.38Å (ES), upon light induced excitation at 90K, in crystals (Figure a). The rapid improvement in the intensity of X-ray sources and the sensitivity of detectors, enables the use of laboratory equipment to explore short-lived excited state geometries in molecular crystals. The importance of this knowledge in order to tune the photo-physical properties for the above mentioned class of purely organic luminophore cannot be ignored.

[1] M. D. Cohen and G. M. J. Schmidt, J. Chem. Soc., 1964, 1996
[2] M. D. Cohen, Angew. Chem., 1975, 14, 386
[3] Carducci, M. D. et al. (1997). J. Am. Chem. Soc. 119, 2669–2678
[4] Kim, C. D. et al. (2002). Acta Cryst. A58, 133–137
[5] Yuan, W. Z. et al. (2010) J. Phys. Chem. C 114, 6090−6099
[6] Bolton, O. et al. (2011) Nat. Chem. 3, 205-210
[7] Maity, S. K. et al. (2013) Chem. Commun. 49, 9051-9053
[8] Shi, H. F. et al. (2016). Cryst. Growth Des. 16, 808-813
[9] Basuroy, K. et al. (2017) Struct. Dyn. 4(2): 024501