The molecular scale charge transport has been investigated in a few recently synthesized molecular crystals of R-oligofuran via thermally activated hopping mechanism described by the semiclassical Marcus theory. The microscopic order parameters such as reorganization energy and hopping matrix elements, governing charge transfer phenomena, are estimated accurately using quantum chemical calculations. The dispersion corrected density functional calculations are carried out to capture the weak van der Waal interactions between the π-stacked molecules. The hopping matrix elements or charge transfer integrals are computed as the offdiagonal elements of Kohn-Sham matrix using fragment orbital approach which explicitly considers the spatial overlap between the molecular orbitals. Our study reveals that such oligofuran molecular crystals are excellent conductors for both charge carriers. However, the hole mobility is found to be slightly larger than electron mobility in smaller oligofuran molecular crystals, whereas the reverse holds true for larger molecule. Such ambipolar organic crystals with higher electron mobility show the possibility of sophisticated device fabrication in advanced electronics. In addition, we compare all our results with analogous oligothiophene crystals by performing the same level of calculations.

The hole and electron mobilities of the polymorphs of benzene and naphthalene crystals are estimated through quantum chemical calculations. The reorganization energy (lambda) and the charge-transfer matrix elements (Hmn) calculated for the two molecules reveal that these crystals can be used for dual applications, for both hole and electron conductance. The electron mobilities are five to eight times more than the hole mobilities for benzene while for naphthalene, the hole mobilities are almost an order magnitude more than the electron mobilities. The transfer matrices for both hole and electron conductance decrease monotonically with increase in the intermolecular distances. Calculations for various unique stacked dimers as determined from the radial distribution functions in both the crystals for the two molecules show strong dependence on the orientations of the rings and for similar intermolecular separations; Hmnhole is larger than Hmnelectron. The crystal mobilities are calculated from the weighted average over all the unique pair of molecules. The overall preference in a crystal for hole or electron mobility depends on the mutual competition of lambdahole/lambdaelectron and Hmnhole/Hmnelectron. From our microscopic understanding of essential parameters, specific dimers are identified from the crystalline solids of the two polymorphs and experimental strategies are suggested to enrich such pairs in aggregates for enhancing mobilities for these organic solids.

Computational studies are carried out on a and b polymorphs of perylene molecular crystals to estimate their hole and electron mobilities (m hole and m electron ). For both the polymorphs, it is found that the LUMO bandwidhs are larger than the HOMO bandwidth resulting in larger electron conductance, as observed experimentally and which has remained unexplained until now. For a microscopic understanding, we perform quantum chemical calculations for the hole and electron transfer matrix elements (H mn hole and H mn electron ) on some selected unique nearest neighbour pairs of molecules in the crystal geometries. This along with calculations for the hole and electron reorganization energies (l hole and l electron ) within the embedded cluster method reveal that for both the crystals, m electron exceeds m hole by two orders of magnitude. The electron mobility for the a-phase (m electron = 67.2 cm 2 V 21 s 21 ) is found to be three times that for the b-phase. The major driving force for preferential electron conductance in a-perylene is the slipped parallel p-stacking arrangement of the molecules at short intermolecular distances (d = 3.9 A ˚) in the crystal. We suggest that experimental strategies that further enhance the percentage of such specific p-stacking dimers in molecular assemblies have the potential to further increase m electron . The present theoretical calculations provide a unified understanding of the parameters that optimize an organic crystal for enhanced electron and hole mobilities.

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We study the properties of the ground state and the low-lying excited states of an alternating spin-1 and spin-1 2 antiferromagnetic chain with ferromagnetic next-nearest-neighbor coupling with a variety of methods, namely, spin-wave analysis, density-matrix renormalization-group ͑DMRG͒ method, and exact-diagonalization method. The ground state of this model with 2N spins is ferrimagnetic with the total spin S G = N / 2 for all parameter values. In the long chain limit, a gapless excitation is found with characteristics of goldstone mode of the ferromagnetic order with total spin S = S G − 1. The lowest gapped excitation, however, exists in the total spin S = S G + 1 sector. Interestingly, we find that with the increase in ferromagnetic coupling, spin-wave gapped excitation becomes lower than the massless mode for the large wave numbers. Correspondingly, from DMRG analysis we find that the excitation in the total spin S = S G + 1 is lower than that in the total spin S = S G − 1 when the size of the chain is short and the ferromagnetic interaction is large. Characteristics of these excitations are also reflected in the low temperature thermodynamic quantities, where the low-energy properties are strongly affected by the system size and the coupling strength.