4 research outputs found
Origin of the Enhancement of the Second Hyperpolarizabilities in Open-Shell Singlet Transition-Metal Systems with Metal–Metal Multiple Bonds
Using the spin-unrestricted coupled-cluster method, we explore the origin of the second hyperpolarizabilities (γ) of singlet dichromium(II) and dimolybdenum(II) model systems with various bond lengths as a function of the diradical characters of the dσ, dπ, and dδ orbitals. Both systems exhibit enhanced γ values in the intermediate diradical character region, but by using a partitioning scheme, the dσ electrons are shown to play the essential role in contrast with the π-electrons of conventional organic π-conjugated systems. Then, in the equilibrium bond length region, the γ values are still governed by dσ electrons in the dichromium(II) system, although by dδ/dπ electrons in the dimolybdenum(II) system
Halide Ion Complexes of Decaborane (B<sub>10</sub>H<sub>14</sub>) and Their Derivatives: Noncovalent Charge Transfer Effect on Second-Order Nonlinear Optical Properties
Quantum molecular engineering has been performed to determine
the
second-order nonlinear optical (NLO) properties in different halo
complexes of decaborane (B<sub>10</sub>H<sub>14</sub>) and their derivatives
using the density functional theory (DFT) method. These decaborane
halo complexes of X<sup>–</sup>@B<sub>10</sub>H<sub>14</sub> (X = F, Cl, Br, and I) are found to possess noncovalent charge transfer
interactions. The static polarizability (α<sub>0</sub>) and
first hyperpolarizability (β<sub>0</sub>) among these complexes
increase by moving down the group from F to I, partly due to the increase
in size of their anionic radii and the decrease in their electron
affinities. A two-level approximation has been employed to investigate
the origin of β<sub>0</sub> values in these halo complexes,
which show very consistent results with those by the finite-field
method. Furthermore, in the same line, two experimentally existing
complexes, I<sup>–</sup>@B<sub>10</sub>H<sub>14</sub> and I<sup>–</sup>@2,4-I<sub>2</sub>B<sub>10</sub>H<sub>12</sub>, are
found to have considerably large β<sub>0</sub> values of 2859
and 3092 a.u., respectively, which are about three times larger than
a prototypical second-order NLO molecule of <i>p</i>-nitroaniline,
as reported by Soscun et al. [<i>Int. J. Quantum Chem.</i> <b>2006</b>, <i>106</i>, 1130–1137]. Besides
this, the special effects of solvent, counterion, and bottom substitutions
have also been investigated. Interestingly, 2,4-alkali metal-substituted
decaborane iodide complexes show remarkably large second-order NLO
response with β<sub>0</sub> amplitude as large as 62436 a.u.
for I<sup>–</sup>@2,4-K<sub>2</sub>B<sub>10</sub>H<sub>12</sub> complex, which are explained in terms of their transition energies,
frontier molecular orbitals and electron density difference plots.
Thus, the present investigation provides several new comparative insights
into the second-order NLO properties of halo complexes of decaborane,
which possess not only large first hyperpolarizabilities, but also
high tunability to get a robustly large second-order NLO response
by alkali metal substitution effects
Enhancement of the Third-Order Nonlinear Optical Properties in Open-Shell Singlet Transition-Metal Dinuclear Systems: Effects of the Group, of the Period, and of the Charge of the Metal Atom
Metal–metal multiply bonded complexes in their
singlet state
have been predicted to form a novel class of “σ-dominant”
third-order nonlinear optical compounds based on the results of dichromium(II)
and dimolybdenum(II) systems (H. Fukui et al. <i>J. Phys. Chem.
Lett.</i> <b>2011</b>, <i>2</i>, 2063) whose
second hyperpolarizabilities (γ) are enhanced by the contribution
of the dσ electrons with an intermediate diradical character.
In this study, using the spin-unrestricted coupled-cluster method
with singles and doubles as well as with perturbative triples, we
investigate the dependences of γ on the group and on the period
of the transition metals as well as on their atomic charges in several
open-shell singlet dimetallic systems. A significant enhancement of
γ is observed in those dimetallic systems composed of (i) transition
metals with a small group number, (ii) transition metals with a large
periodic number, and (iii) transition metals with a small positive
charge. From the decomposition of the γ values into the contributions
of dσ, dπ, and dδ electrons, the γ enhancements
are shown to originate from the dσ contribution, because it
corresponds to the intermediate diradical character region. Furthermore,
the amplitude of dσ contribution turns out to be related to
the size of the d<sub><i>z</i><sup>2</sup></sub> atomic
orbital of the transition metal, which accounts for the dependence
of γ on the group, on the period, and on the charge of the metal
atoms. These dependences provide a guideline for an effective molecular
design of highly efficient third-order nonlinear optical (NLO) systems
based on the metal–metal bonded systems
Open-Shell Character and Second Hyperpolarizabilities of One-Dimensional Chromium(II) Chains: Size Dependence and Bond-Length Alternation Effect
Using
the long-range corrected spin-unrestricted density functional
theory (LC-UBLYP) method, we have investigated the longitudinal third-order
nonlinear optical (NLO) properties, i.e., the second hyperpolarizabilities
γ of open-shell singlet one-dimensional (1D) extended metal
atom chain (EMAC) systems, chromium(II) chains Cr<sup>II</sup><sub>2<i>n</i></sub> (<i>n</i> = 1–4), with
different metal–metal bond lengths from the viewpoint of the
open-shell character dependences of each d orbital contribution (dσ,
dπ, dδ) to γ and of the chain-length dependence
of γ. It turns out that the chain length dependences of the
open-shell characters of dπ and dδ orbitals at any metal–metal
bond length are negligible, while the dσ open-shell character
decreases as a function of chain length. The systems display bell-shaped
behaviors of γ as a function of the metal–metal bond
length, in which the γ values attain maxima for intermediate
dσ open-shell character. The maximum γ value (γ<sub>max</sub>) exhibits remarkable enhancement as a function of chain
length. It is also found that the bond length alternation (BLA) significantly
affects the γ values and their chain-length dependences. The
present results provide the guiding principles for designing transition-metal
complexes with open-shell singlet 1D metallic chains exhibiting large
third-order optical nonlinearity