Dithienopyrrole-based materials: development of new polymeric derivatives

The possibility to control molar mass and termination of the growing chain is fundamental to create well-defined, reproducible materials. For this reason, in order to apply polydithienopyrrole (PDTP) as organic conjugated polymer, the possibility of controlled polymerization needs to be verified. Another aspect that is still not completely explored is bound to the optical activity of the PDTP, which bearing appropriate substituents may adopt a helical conformation. The configuration of the helix, built up from achiral co-monomers, can be established in an enantiopure way by using only a small percentage of the chiral monomer co-polymerized with achiral co-monomer. The effect, called “sergeants and soldiers effect”, is expressed by the nonlinear increase of the chiral response vs the ratio of the chiral co-monomer used for the polymerization. To date, this effect is still not completely explored for PDTP. In this framework the project will investigate, firstly, the possibility to obtain a controlled polymerization of PDTP. Then, monomers with different side chains and organometallic functions will be screened for a CTCP-type polymerization. Also a Lewis-acid based cationic polymerization will be performed. Moreover the chemical derivatization of dithienopyrrole DTP is explored: the research is going to concern also block copolymers, built up by DTP and monomers of different nature. The research will be extended also to the investigation of optically active derivates of PDTP, using a chiral monomer for the synthesis. The possibility to develop a supramolecular distribution of the polymeric chains, together with the “sergeants and soldiers effect” will be checked investigating a series of polymers with increasing amounts of chiral monomer.

Excited state energy surfaces of flexible emitters for thermally activated delayed fluorescence

Organic Light-Emitting Diodes (OLEDs) technology has matured over recent years, reaching the commercialization level and being used in various applications. The required efficiency can be achieved by transforming triplet excitons into singlet states via Reverse InterSystem Crossing (RISC), which a general mechanism called thermally activated delayed fluorescence (TADF). Two prototypical molecules in the field, 2CzBN and 4CzBN, Carbazole Benzonitrile (donor-acceptor) derivatives, possess similar energy gap between singlet and triplet (∆EST, a key parameter in the RISC rate), but different TADF performance. In this sense, other parameter must be considered to explain these different behaviors. In this work, we theoretically investigate 2CzBN and 4CzBN and address the problem of how flexible donor-acceptor (D-A) or donor-acceptor-donor (D-A-D) molecular architectures affect the nature of excited state, and the oscillator strength. Furthermore, we analyze the RISC rates as a function of the conformation of the carbazole side groups, considering the S0, S1, T1 and T2 states. The oscillator strength of 4CzBN is higher than of 2CzBN, which, in turn, is almost vanishing, resulting in only 4CzBN being a TADF active molecule. We also note the presence of a second triplet state T2 lower in energy than S1, and that the reorganization energies, associated to the RISC processes involving T1 and T2, are both important factor in differentiating the rates in 2CzBN and 4CzBN. However, the 4CzBN RISC rate from T2 to S1 is surprisingly high with respect to the one from T1 to S1, although, according to EL-Sayed rules, since T2 (CT/LE) is more similar to S1 (CT) than in 2CzBN (LE, CT), this transition should be less favored. These insights are important to understand the photophysics of the TADF process and to design novel TADF emitters based on the benzo-carbazole architecture.