Studies on Novel Functionalized Nido-¬Carborane Derivatives : Synthesis and Their Thermally Activated Delayed Fluorescence(TADF) Properties

Abstract

The thesis entitled " Novel Functionalized Nido-¬Carborane Derivatives. Synthesis, Photophysical Properties, and its Potential Application in Thermally Activated Delayed Fluorescence (TADF) Emitters” presents the results obtained from the research work carried out on the synthesis, characterization, and exploration of the chemistry of nido-carboranes donors and organoboron-Acceptor based thermally activated delayed fluorescence (TADF) compounds. The work carried out has been broadly divided into four chapters. The first part includes the general introduction of the research work presented in the thesis. The first part briefly describes the all-important topic, which is discussing in the following chapters. The second part describes the synthesis and photophysical studies of nido-carborane-appended triarylboranes and its applications in TADF. In this work, a methyl group was introduced into the 4-position of the phenylene ring in the PhBMes2 acceptor moiety, and that the introduction of a steric group into the phenylene linker of nido-carboranetriarylborane DA dyads (nido-14) largely enhances their TADF properties both solution and rigid state. The crystal structure of 8-H substituted nido-1 reveals the existence of steric congestion around the cage, resulting in a highly twisted connectivity between the cage plane and the Ph ring ( = 85.7). All compounds with different 8-R groups (R = H, Me, iPr) exhibit strong TADF with long emission lifetimes (d = 4.89.7 s in THF) in both THF and PMMA film, which is supported by a very small singlet-triplet energy splitting (EST < 0.03 eV). Attaching an additional Me group to the ortho position of the BMes2 group further enhances the intensity of delayed fluorescence (nido-4). Theoretical studies show that although nido-14 reach their local energy minima at a perpendicular arrangement between the cage plane and the Ph ring, there exist large energy barriers to cage rotation. In contrast, the reference compound lacking a 4-Me group, which exhibits very weak delayed fluorescence in solution, has low rotational energy barriers. In the third part, a series of p-nido-carborane-triarylborane conjugates with a methyl‐substituted phenylene linker were prepared to investigate the positional effect on the photophysical and electroluminescent properties. The studies reveal the effect of 7,8-Dicarba-nido-undecaboranes having various 8-R groups (R = H, Me, i-Pr, Ph) in the meta or para position of the phenyl ring of a dimesitylphenylborane (PhBMes2) acceptor (nido-m1m3 and nido-p1p3). In the final chapter a series of o-nido-carborane donor and dibenzo oxaborinine acceptor based systems were designed and successfully synthesized. All compounds are well characterized using multinuclear NMR spectroscopy. The introduction of new dibenzo oxaborinine acceptor moiety to the ortho position of the phenylene linker of nido donor system leads formation of TADF system. All three nido-o1-o3 compounds shows good TADF activity along with AIE property. From the studies we successfully concluded that nido-carborane based donor can act as good candidate for TADF activity.