Quantum Spin Sensors for Molecular Recognition

Abstract:
A single unpaired electron in an organic molecule residing in the singly occupied molecular orbital (SOMO) renders it an organic radical. It incorporates exchange splitting in the frontier occupied and unoccupied orbitals, separating the α- and β-orbitals. This fact enormously impacts the electron transport properties in organic radicals by promoting spin-polarized current and significantly enhancing conductance compared to their closed-shell counterparts. Exploring these phenomena, several monoradicals have been investigated through molecular spintronic experiments and theories. In this work, we addressed the impact of an increasing number of radical centers on the transport properties of multiradical molecular species by considering di- and triradicals based on a stable Blatter’s radical. With an increasing number of radical centers, the number of SOMOs increases. Does the increased number of frontier SOMOs provide larger exchange splitting and better transport properties? Here, we observed that the spatial distributions of SOMOs and their coupling with electrodes play a decisive role compared with the presence of multiple unpaired electrons in the molecular systems.

Abstract:

Abstract:
Zigzag graphene nanoribbons (ZGNRs) are known to possess spin moments at the hydrogen-terminated edge carbon atoms; thus, spin-polarized electron transmission is expected, while the current is longitudinally passed through the ZGNRs. However, in pristine ZGNRs, spin-polarized transmission is not observed due to symmetric anti-parallel distributions of the spin densities between the edges. Here, the hypothesis is that any physical or chemical process that breaks such anti-parallel spin symmetry can induce spin-polarized transmission in ZGNRs. In this work, we have established this proof-of-concept by depositing the trimethylenemethane (TMM) radical on 6ZGNRH and investigating the quantum transport properties by employing density functional theory in conjunction with the nonequilibrium Green’s function method. Although TMM has a high magnetic moment (2µB), it does not induce magnetization in 6ZGNRH when TMM is physisorbed. However, during the chemisorption of TMM, it forms the π − π bond with the 6ZGNRH in a particular geometric configuration, where the pz orbitals of carbon atoms of TMM have maximum overlap with the pz orbitals of carbon atoms of 6ZGNRH. The chemisorption of TMM transfers the spin moment to 6ZGNRH, which breaks the edge spin symmetry of pristine 6ZGNRH. The adsorption of the TMM radical results in transmission dips in the transmission spectra due to interference between localized states of TMM and 6ZGNRH states. This induces spin-polarized transmission with 60% spin-filtering efficiency at zero bias, which can further be enhanced up to 92% by applying a bias voltage of 1.0 V.

Abstract:
Robust organic triradicals with high-spin quartet ground states provide promising applications in molecular magnets, spintronics, etc. In this context, a triradical based on Blatter’s radical has been synthesized recently, having two low-lying non-degenerate doublet states with a quartet ground state. The traditional broken-symmetry (BS)-DFT computed doublet–quartet energy gaps are reported to be somewhat overestimated in comparison to the experimentally observed values. In this work, we have employed different ab initio methods on this prototypical system to obtain more accurate doublet–quartet energy gaps for this triradical. The spin-constraint broken-symmetry (CBS)-DFT method has been used to reduce the overestimation of energy gaps from BS-DFT. To address the issues of spin-contamination and the multireference nature of low-spin states affecting the DFT methods, we have computed the energy gaps using appropriately state-averaged CASSCF and NEVPT2 computations. Using a series of active spaces, our calculations are shown to provide quite accurate values in concordance with the experimentally observed results. Furthermore, we have proposed and modeled another two triradicals based on Blatter’s radical, which are of interest for experimental synthesis and characterization. Our computations show that all these triradicals also have a quartet ground state with a similar energy difference between the excited doublet states.  

Abstract:
The development of stimuli responsive systems that can switch between two distinct spin states under the application of an external stimulus has always remained an elusive challenge. Here, we report a stimuli-based spin filter by utilizing a photo-responsive endoperoxide (EPO) based single molecule device. Photo-irradiation of EPO triggers the homolytic cleavage of the peroxide O–O bond generating a diradical intermediate centered on two O-atoms which facilitates high spin filtering efficiency when placed between gold electrodes. The broken conjugated scenario due to the peroxide bridge of EPO hinders the propagation of de Broglie waves across the molecular skeleton. On the other hand, the diradical intermediate of EPO yields high conductance for one of the spin configurations. The transmission characteristics of various photoproducts along the photochemical reaction pathway of EPO are also investigated using density functional theory in combination with the non-equilibrium Green's function (NEGF-DFT) technique. We demonstrate the key role played by quantum interference (QI) effects in the dramatic modulation of conductance arising due to different degrees of conjugation along the reaction pathway of EPO.