Studies on Quinoidal Diketopyrrolopyrrole Derivatives and their Applications
Quinoidal π-conjugated molecules have endowed significant attention over the years especially due to inherent high electron affinity and amphiprotic redox behavior, establishing their application as versatile active material in organic field-effect transistors, photonic and optoelectronic devices. Such exceptional properties in these systems confer them with high ambient stability, near-infrared (NIR) absorption as well as nonlinear optical responses. However, a persistent diradicaloid character observed in several π-conjugated quinoidal systems has gained an appreciable deal of current interest due to their low band gaps and magnetic properties evolving through the open-shell character. Such an unanticipated diradicaloid configuration in these systems is abeted by an increase in conjugation length of oligomeric chains as a result of closed shell (Kekule) and diradical (open-shell) resonance. Considering the inherent reactivity of open-shell diradical forms in ambient atmosphere, the stability of these species can be tuned by separate control of effective conjugation length and electron withdrawing substituents at strategic positions in the conjugated backbone. The small HOMO-LUMO gaps and the gained aromatic resonance energy is the primary driving force for the possible cross-over from closed-shell form (for short oligomeric chains) to open-shell form (for extended systems), leading to moderate diradical contribution to the ground state electronic structure. These molecules with diradicaloid character possess various potential applications in the vast field of singlet fission, spintronics and quantum information science. Therefore, understanding the nature of diradicaloids is critical for harnessing them in optoelectronic or spintronic applications. The work presented in this thesis is focused to provide a detailed insight into the diradicaloid nature of spin states and also the nature of the doped species post n-doping with inorganic dopant. In addition, we have also explored the n-type doping of organic materials in ambient conditions using air-stable molecular dopant and investigated the underlying fundamental processes during molecular doping of DPP derivatives. The thesis is organized in five chapters, apart from the introduction, there are four chapters. A brief discussion on the content of individual chapters is provided below. Chapter 1 provides a short description of Quinoidal molecules, Diradicaloids and highlights different approaches (reported in literature) for stabilization of extended diradical systems. Furthermore, the recent development in n-type molecular doping of organic semiconductors is concisely illustrated. Chapter 2 projects a comprehensive study on exploration of diradicaloid nature of DPP quinoidal molecules. The importance of pull-pull (placing electron-deficient units at terminal positions) approach and extended conjugation in governing the ground-state electronic properties as well as evolving diradicaloid character, have been elucidated. This chapter as well describes the influence of chalcogen based different donor groups and alkyl substituents on electronic and solid state packing behaviour of Quinoidal DPP based derivatives. The crossover from aromatic to quinoid through terminal cyano-substitution was confirmed by UV-Vis-NIR doping studies, ESR spectroscopy and DFT calculations. Inclusion of hexyl side chains instead of branched C20 in quinoidal DPP molecules improved the molecular interactions with strong π-π stacking in the crystals. Chapter 3 reports the air stability and dopant optimization for n-doped host semiconductors through several spectroscopic experiments supported by theoretical calculations. Enhanced photoconductivity of N-DMBI doped films has been observed, suggesting these host and dopant system combination as promising active materials for optoelectronic devices. The nature of electronic transport is thermally activated with evident signatures of Fermi level pinning by donor band. Chapter 4 explores the quinoidal form of DPP (TDPP-CN4) as the suitable candidate for ETL material in p-i-n perovskite solar cells. Systematic spectroscopic studies with perovskite absorbing layer and TDPP-CN4 as ETL followed by device fabrication documents that air stable electron transport organic semiconductors are excellent alternatives to PCBM in inverted perovskite devices and opens the avenues to many more studies in the field of emerging photovoltaic materials. Chapter 5 reports the application of Quinoidal DPP derivative TDPP-Hex-CN4 as an anolyte in non-aqueous organic redox flow batteries. Inspired by the excellent thermodynamic and kinetic reversibility in organic solvents, flow cells were assembled utilizing TDPP-Hex-CN4 and commercially available Unisol blue dye/DBBB as redox active electrolytes. The proof-of-concept flow cells with an observed electrochemical potential around 1.2 V delivered significant capacity retention upto 100 cycles. The exact cause of capacity decay needs further post-test analysis.