NEW ORGANIC AND HYBRID ORGANIC/INORGANIC MATERIALS WITH INTRIGUING SOLID STATE OPTOELECTRONIC PROPERTIES.

In the last decades the development of new technologies is requiring new materials with enhanced emissive properties in the solid state to be applied in different fields spanning from OLED to biological applications. Research in this context is historically limited by the observation that luminogens possess better properties in dispersed system than in the condensed phase due to the well-known Aggregation Caused Quencing (ACQ) phenomenon. However, in 2001 Tang and co-workers reported some pioneering works on luminogens displaying opposite properties, being highly emissive in the condensed phase but not in diluted solution, a behaviour named Aggregation Induced Emission (AIE) [1]. Different mechanisms may explain the AIE phenomenon, the most frequent one being the inhibition of molecular motions (vibrations and rotations) which works as nonradiative deactivation channels for the molecule but are somehow locked in the aggregated state. In parallel, strong efforts have been devoted to the search of organic molecules with long-lived excited states that enable exciton migration over long distances for increased production of free charges. Usually, transitions from singlet (short-lived) to triplet (long-lived) states are facilitated by the presence of metals or specific organic moieties (i.e. aromatic aldehyde, heavy halogen atoms, heteroatoms with lone pairs). In addition, since triplet excitons generated in organic molecules are highly sensitive to oxygen and temperature, stringent conditions are required to observe long-lived phosphorescence from pure organic molecules. However, very recently, An et al. reported ultralong phosphorescent emission features in structures of planar organic molecules coupled in H-aggregates, which provide an effective means of stabilizing and protecting triplet excitons formed through intersystem crossing (ISC) [2]. The stabilized excited state, which functions as an energy trap at a lower energy level, may delocalize on several neighbouring molecules, offering suppressed radiative and nonradiative deactivation decay rates in favour of long-lived excited states and room temperature ultralong phosphorescence (RTUP). In this regard, during my research work, I have been involved in the synthesis and characterization of triimidazo[1,2-a:1',2'-c:1'',2''-e][1,3,5]triazine (TT or cyclic triimidazole) and of its derivatives. In fact, TT is an intriguing solid state luminogen characterized by AIE and RTUP properties. Diluted solutions of TT in DCM display weak emission (Ф = 2%) even under inert atmosphere, while crystalline powders (Ф = 30%) show an intense fluorescent emission at 425 nm and an ultralong emission at 520 nm with decay time close to 1 s. This behaviour, in agreement with works by An et al. [2] arises from the formation of H-aggregates in the crystal structure of the compound as revealed by XRD studies [3]. The main goal of my project has been the synthesis and characterization of organic and hybrid inorganicorganic 1. Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z., Aggregation-induced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chemical Communications 2001, (18), 1740-1741. 2. An, Z.; Zheng, C.; Tao, Y.; Chen, R.; Shi, H.; Chen, T.; Wang, Z.; Li, H.; Deng, R.; Liu, X.; Huang, W., Stabilizing triplet excited states for ultralong organic phosphorescence. Nature Materials 2015, 14 (7), 685-690. 3. Lucenti, E.; Forni, A.; Botta, C.; Carlucci, L.; Giannini, C.; Marinotto, D.; Previtali, A.; Righetto, S.; Cariati, E., H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule. The Journal of Physical Chemistry Letters 2017, 8 (8), 1894-1898.