The electronic structure and optical properties of fused S,N-heteroacenes (SNn, SN5–SN10) have been studied theoretically. The calculations reveal that, bond length alteration approaches zero with increasing number of heterocyclic rings in the conjugated molecules. As a general trend of optical property, the absorption maximum is red-shifted with increasing conjugation length, achieved through increasing the degree of polymerization or by incorporating strong electron withdrawing groups at the two ends of the molecules. However, an even–odd relationship is observed during electronic excitation followed by exciton dissociation. Thus, SN5 and SN9 experience better charge separation than SN6 and SN10, respectively. The theoretical results interpret the experimental finding where SN5 is reported to offer better photocurrent efficiency. To compare the photovoltaic performances of the materials, we compute the rate of charge recombination and charge transfer for the composites consisting of some SNn and a well reputed acceptor PC61BM.

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Inspired by recent experimental synthesis of two-dimensional cobalt bis(dithioline) (CoBHT) metal−organic surface (J. Am. Chem. Soc., 2015, 137, 118−121), herein, using first principle calculations, we have investigated the electronic and magnetic properties of two-dimensional BHT-based metal (M = Co, Fe, Mn, Cr)-organic frameworks (MBHT). Our detailed theoretical calculations predict that CoBHT, FeBHT, and MnBHT are planar ferromagnetic (FM) half-metals, whereas CrBHT is planar spinfrustrated kagome lattice antiferromagnetic (AFM) semimetal, a new state of matter referred as spin-liquid. These planar ferromagnetic half-metal materials are promising candidates for spintronic devices. Further, polluting gas such as CO can be detected by these MOFs as there is remarkable variation of electronic and magnetic properties after gas adsorption. Interestingly, these properties have been found to be a function of coordination environment of central metal atom, which is actually a coordinatively unsaturated (CUS) center. Square planar central metal atom changes its coordination geometry to distorted square pyramidal geometry (as metal atom protrude out of the square plane) to octahedral, after single and both side full coveraged gas adsorption, respectively. Especially, for their practical purposes in gas sensing, we have calculated the transport properties, taking the cobalt bis(dithioline) (Cobdt) molecule as an example. Unadsorbed (Cobdt) and coordinatively saturated bis-CO [Cobdt(CO) 2 ] adsorbed molecule provide remarkably distinct I−V responses, which becomes a signal for detection of CO gas. Moreover, Cobdt molecule shows the spin-filtering effect as it is half-metallic, thus Cobdt can be used as novel material for spintronic device fabrication. Therefore, the MOFs we have studied and Cobdt molecule can be used as potential materials for spintronic devices and for polluting gas sensing.

In a very recent article, atomically thin two-dimensional MoS2xSe2(1-x) nanosheets have been synthesized with complete composition tunability using a temperature gradient assisted chemical vapor deposition technique [J. Am. Chem. Soc., 2014, 136, 3756]. To have a better understanding of the composition dependent tunability of the properties of this class of materials we here perform first principles calculations on the detailed electronic structure of single layered transition metal dichalcogenides MoS2(1-x)Se2x and MoS2(1-x)Te2x. The positive value of mixing energy of both MoS2(1-x)Se2x and MoS2(1-x)Te2x 2D sheets at various composition confirms their formation at some energy cost. The analysis of the composition dependent band structure and the density of states of these 2D sheets reveals certain interesting features. The band gap variation of the MoS2(1-x)Se2x nanosheets is almost linear with composition while that of MoS2(1-x)Te2x deviates slightly from linearity. We have also calculated the optical absorption spectrum of these nanosheets as a function of composition and found that the optical transitions are mainly metal d-d type spin forbidden transitions.

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