22 research outputs found
Fused Azulenes: Possible Organic Multiferroics
We present compelling theoretical results showing that fused azulene
molecules are strong candidates for exhibiting room temperature multiferroic
behavior, i.e., having both, ferroelectric and ferromagnetic properties. If
this is experimentally proved, these systems will be the first organic
multiferroic materials with important potential applications.Comment: 4 pages, 4 figure
Efficient Density Matrix Renormalization Group algorithm to study Y-Junctions with integer and half-integer spin
An efficient density matrix renormalization group (DMRG) algorithm is
presented and applied to Y-junctions, systems with three arms of sites that
meet at a central site. The accuracy is comparable to DMRG of chains. As in
chains, new sites are always bonded to the most recently added sites and the
superblock Hamiltonian contains only new or once renormalized operators.
Junctions of up to sites are studied with
antiferromagnetic (AF) Heisenberg exchange between nearest-neighbor spins
or electron transfer between nearest neighbors in half-filled Hubbard
models. Exchange or electron transfer is exclusively between sites in two
sublattices with . The ground state (GS) and spin densities at site are quite different for junctions with =
1/2, 1, 3/2 and 2. The GS has finite total spin for even (odd)
and for in the spin manifold, at sites
of the larger (smaller) sublattice. = 1/2 junctions have delocalized states
and decreasing spin densities with increasing . = 1 junctions have four
localized states at the end of each arm and centered on the
junction, consistent with localized states in = 1 chains with finite
Haldane gap. The GS of = 3/2 or 2 junctions of up to 500 spins is a spin
density wave (SDW) with increased amplitude at the ends of arms or near the
junction. Quantum fluctuations completely suppress AF order in = 1/2 or 1
junctions, as well as in half-filled Hubbard junctions, but reduce rather than
suppress AF order in = 3/2 or 2 junctions.Comment: 11 pages, 11 Figures and submitted to PR
Linear and nonlinear optical properties of indeno[2,1-b] fluorene and its structural isomers
Linear and non-linear optical properties of indeno[2,1-b]fluorene (1) and its structural isomers with 20 ??-electrons have been studied using many body methods. As compared to other ?? electron systems of similar conjugation length, the experimentally measured optical gap (vertical excitation energy) and the singlet-triplet gap of 1 are quite small. The diradical character calculated using the ab initio density matrix renormalization group (DMRG) of 1 is the largest among its isomers, which explains its lowest singlet-triplet gap. The optical gaps calculated by the DMRG method, employing the long-range interacting Pariser-Parr-Pople model, show good agreement with the experimental values as compared to those calculated by DFT, MP2, and CASPT2 methods. Using correlated Hamiltonian and the DMRG approach, the third harmonic generation coefficient is found to correlate well with the diradical character.close1
Nonlinear Optical Properties of Stacked Conjugated Systems
We study linear and nonlinear optical properties of two push-pull polyenes stacked in head to head (HtH) and head to tail (HtT) configurations, at different stacking angles within the Pariser-Parr-Pople model using exact diagonalization method. By varying the stacking angle between the polyenes, we find that the optical gap varies marginally, but transition dipoles show large variations. We find that the dominant first-order hyperpolarizability component beta(XXX) for HtH arrangement and beta(YYY) for HtT arrangement strongly depend on the distance of separation between molecules, while the other smaller component beta(XYY) for HtH arrangement and beta(XXY) for HtT arrangement) does not show this variation with distance. We find that the beta(XXX) for HtH configuration shows a maximum at an angle away from 0, in contrast with the oriented gas model. This angle varies with distance between the polyenes, and at large distance it falls to 0. The ratio of all components of beta of a dimer to monomer is less than two for HtH configuration for all angles. But for HtT configurations the ratio of the dominant beta component is greater than two at large angles. Our ZINDO study on two monomers (4-hydroxy-4'-nitroazobenzene) connected in a nonconjugative fashion shows a linear increase in vertical bar(beta) over right arrow (av)vertical bar without much red shift in optical gap. There is a linear increase in vertical bar(beta) over right arrow (av)vertical bar with increase in number of monomers connected nonconjugatively without resulting in a red shift in optical gap
Linear and nonlinear optical properties of expanded porphyrins: a DMRG study
We study absorption spectra and two photon absorption coefficient of expanded porphyrins (EPs) by the density matrix renormalization group (DMRG) technique. We employ the Pariser-Parr-Pople (PPP) Hamiltonian which includes long-range electron-electron interactions. We find that, in the 4n+2 EPs, there are two prominent low-lying one-photon excitations, while in 4n EPs, there is only one such excitation. We also find that 4n+2 EPs have large two-photon absorption cross sections compared to 4n EPs. The charge density rearrangement in the one-photon excited state is mostly at the pyrrole nitrogen site and at the meso carbon sites. In the two-photon states, the charge density rearrangement occurs mostly at the aza-ring sites. In the one-photon state, the C-C bond length in aza rings shows a tendency to become uniform. In the two-photon state, the bond distortions are on C-N bonds of the pyrrole ring and the adjoining C-C bonds which connect the pyrrole ring to the aza or meso carbon sites
In Search of a Two-Dimensional Material for DNA Sequencing
We analyze the transmission of narrow
semiconducting nanoribbons
designed from two-dimensional (2D) layered materials such as graphene,
silicene, hexagonal boron nitride (hBN), and molybdenum disulfide
(MoS<sub>2</sub>). The Fano resonance driven dips in the transmission,
when nucleobases stack with graphene nanoribbon, are known to be useful
for DNA sequencing. For graphene and hBN nanoribbons the transmission
dips are distinct for each nucleobase, but with a larger band gap
for the latter case. For silicene nanoribbon the dips due to different
nucleobases are somehow less clear. The transmission of the MoS<sub>2</sub> nanoribbon is unpromising for DNA sequencing as the dip in
the transmission is not useful to identify any of the nucleobase.
The dip positions in the transmission shift linearly with bias voltage.
This shift depends on the nanoribbon used and the orientation of the
DNA base. Hence, edge-modified hBN nanoribbons with a reduced band
gap could be an alternative to graphene nanoribbon (GNR) for DNA sequencing
and recognition of other adsorbents
Improving the Stability of Organic Semiconductors: Distortion Energy versus Aromaticity in Substituted Bistetracene
Polycyclic aromatic
hydrocarbons (PAHs) have been widely explored
as molecular semiconductors in organic electronic devices such as
field-effect transistors or solar cells. However, their tendency to
undergo photooxidation is a primary limitation to their practical
applications. Bistetracene derivatives have recently been demonstrated
to possess much larger photooxidation stability than the widely investigated
pentacene and rubrene, while maintaining high charge-carrier mobilities.
Here, using several levels of density functional theory, we identify
the origin of the increased stability of bistetracene with respect
to molecular oxygen by systematically investigating the [4 + 2] cycloaddition
(Diels–Alder) photooxidation reaction mechanism. Importantly,
our computational results indicate that endoperoxide formation in
bis(2-(trimethylsilyl)ethynyl) bistetracene (BT) occurs not
on the ring with least aromaticity, but rather on the ring with smallest
distortion energy. This feature was subsequently confirmed by experimental
NMR analyses. The oxidation activation barriers of bistetracene, pentacene,
and rubrene are found to be 17.7, 13.6, and 14.4 kcal/mol, respectively,
in agreement with the observed order of stability of these molecules
with respect to oxidation reactions in solution. In the cases of BT
and pentacene, the rates of electron transfer to create charged species
(PAH<sup>+</sup> and O<sub>2</sub><sup>–</sup>) are at least
two orders of magnitude lower than that of the charge recombination
process (back to PAH and O<sub>2</sub>); for rubrene, both of these
processes are calculated to be of the same order of magnitude, in
agreement with experimental electron paramagnetic resonance spectroscopy
observations
Design and Synthesis of Two-Dimensional Covalent Organic Frameworks with Four-Arm Cores: Prediction of Remarkable Ambipolar Charge-Transport Properties
We have considered three two-dimensional (2D)
π-conjugated polymer networks (i.e., covalent organic frameworks, COFs)
materials based on pyrene, porphyrin, and zinc-porphyrin cores connected via diacetylenic linkers. Their
electronic structures, investigated at the density functional theory
global-hybrid level, are indicative of valence and conduction bands that have
large widths, ranging between 1 and 2 eV. Using a molecular approach to derive
the electronic couplings between adjacent core units and the electron-vibration
couplings, the three π-conjugated 2D COFs are predicted to have ambipolar
charge-transport characteristics with electron and hole mobilities in the range
of 65-95 cm2V-1s-1. Such predicted values rank
these 2D COFs among the highest-mobility organic semiconductors. In addition,
we have synthesized the zinc-porphyrin based 2D COF and carried out structural
characterization via powder X-ray diffraction and surface area analysis, which
demonstrates the feasability of these electroactive networks.</p
A Thiazole–Naphthalene Diimide Based n-Channel Donor–Acceptor Conjugated Polymer
Conjugated monomers
and polymers containing 2,2′-bithiazole
(BTz) and naphthalene diimide (NDI) units in the main chain were prepared.
Polymer PNDI2Tz was obtained via palladium-catalyzed Stille polycondensation
of a dibromo-substituted NDI derivative with distannyl-2,2′-bithiazole.
The optical and electronic properties were investigated using UV–vis
absorption spectroscopy and ultraviolet photoelectron spectroscopy.
It was found that the polymers show very broad absorption bands in
the 540 nm region, and PNDI2Tz has an optical bandgap of 1.87 eV.
Computational analysis demonstrates that holes and electrons are mainly
localized on the 2,2′-bithiazole and NDI units, respectively.
Organic field-effect transistors (OFETs) fabricated with PNDI2Tz exhibit
unipolar n-channel characteristics with mobility as high as 0.05 cm2 V–1 s–1