Research

Atomically precise nanographenes and graphene nanoribbons (GNRs) can be constructed with suitable polycyclic aromatic hydrocarbon (PAH) building blocks. Advantageously, such a rational bottom-up protocol can tune the energy gaps through rendering the molecular size, edge structure, or incorporating appropriate heteroatoms. Among the PAHs, n-peri-acenes (PAs) are considered as potential building blocks for organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), spintronics, etc. Higher-order members of PAs possess singlet open-shell (radical) character at ground state due to the formation of additional Clar π-electron sextets and the spin-polarization hampers the stability. Recently, along with co-workers, I demonstrated the synthesis and characterization of the hitherto unknown peri-tetracene (4-PA) and peri-heptacene (7-PA) by a rational strategy in which steric protection of the zigzag edges playing the pivotal role. The obtained PAs possessed singlet open-shell character (y0 = 0.72 – 0.99) and exhibited remarkable persistent stability. Moreover, the bay regions of 4-PA enable the efficient 2-fold Diels–Alder reaction, yielding a novel full zigzag-edged circumanthracene (J. Am. Chem. Soc., 2018, 140, 6240; & Angew. Chem. Int. Ed., 2021, 60, 13853).

Importantly, organic radicals based self-assembled monolayers (SAMs) have emerged as potential building units for spintronic materials, which could be applied for downscaling the memory devices. I have gained the art of making SAMs of rationally designed organic molecules (radicals) on various substrates such as indium tin oxide (ITO) coated glass, Au, Ag, etc. The obtained robust r-SAMs were well characterized by X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS), cyclic voltammetry (CV), and electron spin resonance spectroscopy (ESR) [Chem. Sci., 2016, 7, 4940, RSC Adv., 2017, 7, 20076 & J. Phys. Chem. Lett., 2020, 11, 3897].

I also pursued the possibility of performing single-electron transfer (SET) reactions of highly electron-deficient molecules with suitable donor molecules/anions. Moreover, we applied a kinetically selective SET from cyanide to deep-lying LUMO orbitals of TCNQ to generate a persistently stable radical anion (TCNQ•–), under ambient condition. There, we demonstrated the fabrication of a practical electronic device with TCNQ as a detector. The device generates multi-order enhancement in current with cyanide because of the formation of the conductive TCNQ•– [ACS Appl. Mater. Interfaces, 2013, 5, 6996 (Cover Article)]. 

Interestingly, the differential response of naphthalene diimide (NDI) to fluoride anions induced intramolecular charge transfer (ICT), single/double electron transfer (SET/DET) leading to a set of combinational logic gates for the first time with a NDI moiety [Chem. Commun., 2013, 49, 7684 (Cover Article)].  Besides these projects, I have been involved in constructing nanostructures [Chem. Commun., 2015, 51, 15237 (Cover Article)], organic field-effect transistors [Adv. Mater. Technol., 2016, 1, 1600090], etc.

Several novel aromatic diimides, especially NDIs, were synthesized and pursued their single and double electron transfer (ET) reactions from organic amines, anions and cations. The electronic properties of the NDIs were tuned by axial- and core-substitution methods and used for the generation of persistent and stable radical ions (anions and cations). The rich optoelectronic and magnetic properties of these radical ions used for the recognition of various analytes (such as amine, cyanide and Cu2+), establishing logic-gate operations, etc. [Chem. Commun., 2009, 3702 (Cover Article); Org. Lett., 2010, 12, 2646].

Organic radicals with ambient stability would be crucial for organic spintronics. The multifaceted features of a phosphonium group were utilized to achieve the isolation of the first NDI radical ion as single crystals and an ultra-electron-deficient NDI having the lowest LUMO level recorded for an NDI [J. Am. Chem. Soc., 2014, 136, 12004]. As well as, the generation of the first persistent radical cation of NDI upon oxidation with Cu2+/Fe3+under ambient conditions has been demonstrated [Org. Lett., 2012, 14, 4822]. A switchable molecular construct (c-NDI) that can harness Na+/K+ driven thermal ET from a specific anion like CN− was rationally designed. This enabled us to opto-electronically and magnetically differentiate ions and delineate benign and lethal ion-pairs. Most importantly, supramolecular cation-gated effect was found to kinetically regulate the thermal ET from closely related ion-pairs. We have applied this phenomenon to demonstrate for the first time the optical, electronic and paramagnetic discrimination of NaCN, KCN and TBACN.