Syntheses and properties of π-Extended indenofluorene derivatives

Abstract

Chemistry of alternant acenes have attracted tremendous interest due to their challenging synthesis and interesting optoelectronic properties. Pentacene derivatives had received more attention in this regard, but unfortunately, they are prone to oxidative degradation. This led to the development of five non-alternant antiaromatic indenofluorene (IF) isomers as alternative pentacene-like structures. Chapter 1 covers the synthesis of five antiaromatic IF derivatives, including π-extended variants, IF oligomers/polymers, and their applications in organic electronics. These fully conjugated IFs, along with their core-extended counterparts, have garnered significant interest because of their synthesis related challenges, (anti)aromaticity, diradicaloid properties, and optoelectronic applications. Among the five IF isomers, two para-quinodimethane embedded IFs, namely indeno[1,2-b]fluorene and indeno[2,1-c]fluorene have found applications in charge-transport and solar cell devices. In contrast, the application of indeno[2,1-a]fluorene is limited due to its instability. The traditional synthesis of IFs requires harsh conditions and doesn’t allow for introduction of unsymmetrical disubstituents to the apical carbons of the five-membered rings. In Chapter 2A, I report a mild synthetic approach to make mesityl disubstituted [2,1-c]IF, and further extended this approach to synthesize unsymmetrically disubstituted [2,1-c]IFs (Chapter 2B), as well as a (4n+2)π tetraradicaloid s-indacenodifluorene (s-IDF) (Chapter 2C) that exhibit small HOMO-LUMO and singlet-triplet energy gaps. Adopting a similar synthetic approach while utilizing the steric crowding of bulky aryl substituent, a steric promoted synthesis of symmetrical and unsymmetrical [1,2-b]IFs including benzo-extended [1,2-b]IFs were reported in Chapter 3. Notably, use of less bulky pentafluorophenyl group as one of the substituent afforded a mixture of IF regioisomers ([1,2-b]IF and [2,1-a]IF) which could be purified by column chromatography in air. Since the syntheses and properties of IF dimers remain largely unexplored, while their alternant analogue pentacene dimers are well-studied for optoelectronic applications, we directed our efforts towards the design and syntheses of three mesityl substituted IF dimers, presented as biindenofluorene (BIF) derivatives in Chapter 4. These dimers were formed by dimerizing formally antiaromatic IF units through the 2,2'- and 3,3'-carbons of [2,1-c]IF and [1,2-b]IF units, respectively. This design modulated the optoelectronic properties by significantly reducing the HOMO-LUMO energy gaps, in comparison to previously reported IF-based polymers. Both experimental and theoretical studies confirmed that all three dimers, in Chapter 4, exhibit a closed-shell configuration. In the quest for open-shell BIF, Chapter 5 explores the design of a [1,2-b]IF-based dimer, which was synthesized by linking [1,2-b]IF units through their 6,6'-carbons, and it displayed an open-shell singlet ground state with 26.8% diradical character.