1. INTRODUCTION
During the past decades, people have paid great efforts to explore more
high-performance nonlinear optical (NLO) materials because of their wide
applications in the field of information processing, photoelectric,
optical data storage and many
others.1-7 Currently,
numerous types of molecules with large NLO response have been studied,
including structures of the donor-π bridge-acceptor
type,8,9the transition metal-ligand
structures,10,11and complexes with loosely bound excess
electron.12,13In 2004, Li et al. reported that the molecular cluster anions
(FH)2{e}(HF) and
(H2O)3{e} with excess electron
exhibited significantly large nonlinear optical
responses.14,15This opens up a new direction for designing novel compounds with
considerable NLO properties. These novel compounds are a class of
nontraditional ionic salts, named electrides, where the anionic sites
are occupied solely by
electrons.16 Studies
have shown that alkali metal doping is one of the most common methods
for designing
electrides.12 Recently,
the use of superalkalis instead of alkali atoms to design new electrides
with larger hyperpolarizabilities (β 0) has
attracted much
attention.17,18
Superalkalis19 are a
well-known class of
superatoms20 that
possess lower ionization potential (IP) values than alkali metal atoms,
thus their valence electrons are more likely to be polarized by ligands
and form loosely bound excess electrons. Therefore, the interaction of
superalkali with appropriate ligands can generate more diffuse excess
electron, leading to new superalkali-doped systems with larger NLO
response.
Although the excess electron generated by the polarization of organic
ligands can greatly increase the β 0 value of a
molecular system, the presence of the loosely bound excess electrons
makes the thermal stability of the molecule unsatisfactory. Besides, NLO
materials in the infrared and deep-ultraviolet (deep-UV) working
wavebands are currently the research
hotspots.3 Consequently,
finding a system with high stability, large hyperpolarizability and
working area in the infrared or deep-ultraviolet regions has become an
important research topic in the field of NLO.
Recently, new two-dimensional carbon allotrope-graphynes with special
structural features and thermal stability have attracted the interest of
many researchers from different areas. As early as 1987, Baughman et
al.21 first proposed
that graphynes (GYs) are a series of one atom thick carbon allotropes
composed of sp- and sp2-hybridized carbon atoms. Two
hybridization states of carbon atoms enable GYs to have many excellent
properties, including extended π-conjugation, uniformly distributed
pores, tunable electronic properties, good chemical stability and large
surface
area.22,23Four main types of graphyne namely α -, β -, γ -, and
6, 6, 12-graphynes have been
identified.21γ -graphyne is the most widely studied form of GYs, especially
graphdiyne (GDY), which is formed by the self-assembly of hexagonal
rings and acetylenic groups and has a largely delocalized π-conjugated
surface. Although many scientists have attempted to prepare GYs, it was
not until 2010 that Li’s group successfully synthesized large-area films
of graphdiyne on a copper substrate via a cross-coupling
reaction.24 Due to the
fascinating structures and particular electronic properties, graphyne
materials show promising applications in catalysts, hydrogen storage,
anode materials, optoelectronic devices, biomedicine and therapy,etc .22,23,25-29
More interestingly, the graphyne molecules have recently aroused
extensive attention of theoretical researchers in the field of nonlinear
optics. In 2016,
Chakraborti30theoretically investigated the NLO properties of donor-acceptor
substituted graphyne structures. In the same year, the
hyperpolarizabilities of graphdiyne functionalized by the alkali metal
atom etc . were
investigated.31 Very
recently, Li et al. designed a variety of promising novel GDY-based NLO
materials.32-35 Instead
of replacing hydrogen of GDY with alkali metal
atom,31 Li et al.
theoretically confirms that alkali atom doping is a viable approach to
increase the β 0 value of
GDY.32,33Besides, superalkaline-earth metal M3F (M = Li, Na, and
K), and superalkali Li3NM and M2X (M =
Li, Na, K and X = F, Cl, Br) can also be adsorbed on the GDY surface,
respectively, to produce new complexes with largeβ 0values.34-36 Thus, the
GDY and superalkaline-earth metal/superalkali would be a good
combination for designing novel NLO materials.
Considering characteristics of GYs including thermal stability,
deep-ultraviolet transparency, and large pores, we have designed a
series of superalkali salts of graphynes, namely
OM3+@GYs– (M = Li,
Na, and K; GYs include GY, GDY, and GTY) in the present work. Figure 1
shows the graphyne structure consists of hexagons connected by
acetylenic (–C≡C–) rather than cumulative (=C=C=)
linkages.37 According
to the number of acetylenic linkers (–C≡C–), GYs can be classified
into GY, GDY, and GTY. These representative structural models of
delocalized π-conjugated graphynes were chosen here to combine with
superalkalis (OLi3, ONa3, and
OK3) to generate a new series of complexes. Note that
these superalkalis have been experimentally
synthesized,38-40 and
they can serve as basic building blocks for new
complexes.41-44 The
evolution of their first hyperpolarizability with varying superalkali
atom and pore size of graphyne has been analyzed in order to explore new
high-performance NLO molecules. The structures, stability, and static
first hyperpolarizabilities of the investigated complexes are explored
by employing density functional theory (DFT). The frontier molecular
orbital and atomic charge analyses indicate that
OM3+@GYs– have
superalkali salt characteristics. Two influencing factors onβ 0 values, namely the atomic number of alkali
atom M and pore size of graphyne, are discussed in detail. Results show
that the combination of OLi3 and GTY (with a large pore)
forms a planar stable structure possessing the largestβ 0 up to 6.5×105 au. This work
proposes a new kind of high-performance deep-UV NLO molecules. It is
extremely expected that this research can attract more experimental
interest in designing novel carbon-based NLO materials in the near
future.