Research Projects

The development of non-alternant congeners of alternant polycyclic hydrocarbons is of great interest in recent times. The classic example of such a polycyclic system is azulene, which is a non-alternant/non-benzenoid isomeric motif for naphthalene, having high HOMO and low LUMO energies that result in its unique polarized structure and redox activity, unlike alternant naphthalene and other acenes. The syntheses of non-alternant diradicaloid indacenes by Haley (Angew. Chem., Int. Ed., 2011, 50, 1127) and Tobe (Angew. Chem. Int. Ed., 2013, 52, 6076) as the fully conjugated antiaromatic indenofluorene isomers with aromatic pentacene-like topology are the two major inventions in the field of π-functional molecular materials. Since then, several contributions focusing on the search for stable antiaromatic scaffolds and novel diradicaloid molecules for applications in organic field-effect transistors, non-linear optics, thermoelectric, and singlet-fission studies have been noted. Indenofluorenes are of major interest due to their ‘delocalized diradical’ nature, which could compress the HOMO-LUMO energy gap, enabling redox amphotericity. Indenofluroene can be mapped onto fullerene, as indeno[1,2-b]fluorene is a fullerene-C₇₀ fragment. In this work, peri-indenophenalene (IP) has been designed as a new redox amphoteric, pro-aromatic fragment of fullerene-C₇₀. It has a lower molecular weight than indenofluorene, does not suffer from antiaromatic instability, absorbs in the UV-vis-NIR region covering the whole visible light, can reversibly accept and donate electrons, and its properties may be tunable by aryl-substitution and aryl-annulation approaches to modulate the diradical and zwitterionic-like contributions in the ground state. Notably, IP is a five-ringed non-benzenoid counterpart of olympicene/olympicenyl radical (JACS 2020, 142, 11022) with a smaller HOMO-LUMO gap, as seen in our preliminary studies stated in the proposal. The first objective is to synthesize and fully characterize the IP scaffold. The multi-step synthesis route involves the construction of an aldehyde intermediate by traditional Suzuki reaction, followed by Grignard reaction of aryl/acetynyl nucleophiles to aldehyde precursors, acid-mediated ring closure, and oxidative dehydrogenation to afford IP derivatives. The potential diradical character of the IP derivatives will be established by experimental (single-crystal XRD, VT-NMR, and VT-EPR) and theoretical approaches (DFT), including aromaticity analyses (NICS, HOMA, ACID ring-current). The aryl/acetynyl groups of varying electronic nature will be grafted suitably on the IP scaffold with the objective to further reduce the HOMO-LUMO gap and identify the zwitterionic diradical-like contribution in the ground state. A study showed the tunable single-molecule conductance nature of polycyclic hydrocarbons of different diradical character (JACS 2024, 146, 29977). Thus, benzo-fused IP isomers are designed with tunable diradical character. It is hypothesized that the Benzo-IP3 isomer may display a larger diradical character, and smaller HOMO-LUMO and singlet-triplet gaps relative to the other isomers Benzo-IP1 and Benzo-IP2. Another objective is to modulate the open-shell properties of IP by heteroatom-doping through the syntheses of novel heterocyclic-IPs (heteroatoms: S/SO2/-NR). Heteroatom-modification of the core not only could influence the molecular arrangement in the solid state but also affects the optoelectronic and diradical properties. Finally, cata and peri-fused IP-dimers are designed as open-shell non-alternant nanographenes. While the peri-IP-dimer is envisioned to show tetraradical character with an intermediate diradicaloid contribution, the cata-IP-dimer is envisaged to show only tetraradical character. The proposed molecules will be synthesized and characterized by analytical and computational techniques to finally test their charge-carrier mobilities and conducting properties (in collaboration).