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Research on photonic materials has intensified
during the past decade in quest of developing new optically active materials
that exhibit interesting nonlinear optical (NLO) properties. Compared with
inorganic materials, organic molecular materials have many advantages owing to
their versatility and possibility of tuning material properties for particular
applications such as optical data storage, frequency doubling and in
telecommunications. The present project plans to achieve the objectives using a
dual approach comprising of experimental and computational techniques to
investigate the electronic structure, optical and nonlinear optical properties
of indigenously synthesized and efficient organic materials. Experimentally, we
will synthesize new coumarin appended organic materials for NLO
properties.  The synthesized compounds
will be characterized using different spectroscopic and computational methods.
The investigation on their structural, electronic, photo-physical as well as
nonlinear optical properties will be performed computationally and results will
be compared with experimental findings wherever possible. The findings of this
project will address the gap between the need of efficient organic NLO
materials and its technological applications.

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Tremendous growth in the field of NLO material has
been observed right from the discovery of first function laser. The NLO
materials are exploited for several applications of NLO materials in modern
hi-tech era. The NLO material has the ability to interact with intense laser
light to generate new electromagnetic fields with different phases and
frequencies, which leads to its novel applications in frequency doubling and mixing,
and telecommunication optical data storage, holographic imaging, etc 1, 2. In the past,
several design rules were used to guide synthetic explorations NLO materials.
In addition to these rules, the synthetic chemistry of present era has used the
intuitive of reliable computational techniques to synthesize many interesting
NLO-phores 3. Over the few
decades, a verity of materials has been explored for NLO applications that
include organic, inorganic, organometallic and organic-inorganic hybrid
materials 4-10. Organic
molecules containing electron-donating and accepting groups that are separated
by a conjugated bridge are identified as promising candidate for NLO materials
as their properties can be tuned and tailored for particular applications.
Based on these findings, herein we focus on chalcone derivatives to exploit
them as novel NLO materials. Recently, several theoretical and experimental
studies have been devoted to organic NLO-phores because of their easy in
fabrication, variety of structures and relatively low cost. 11, 12, 2 These
are essential NLO phenomena (electro-optical modulation, frequency mixing and
doubling etc.) in modern advanced communication and photonics media converters.
The proposed work involves the synthesis of novel D-?-A type organic materials
towards the NLO application for their prospective use in modern advanced
communication and other NLO applications. The promising chalcone derivatives
will be synthesized, characterized and calculated for their NLO properties.

The first step involves design and synthesis of
novel donor-acceptor type organic materials. The synthesized organic materials
will be characterized by using UV-vis, FT-IR and NMR techniques. The
photo-physical properties of the synthesized compounds will be explored by
applying steady state emission techniques. The stability of the NLO phores will
tested experimental and computationally. The NLO efficiency of indigenously
designed compounds will be studied by calculating their polarizability and
first hyperpolarizability.

The main objective of the proposal as follows:

To design and synthesize novel chalcone derivatives
as efficient NLO materials for their prospective use in modern advanced
communication and photonics media converters

To perform the characterization and stability tests
(experimental and computational) for indigenously designed NLO compounds

To study the NLO efficiency of indigenously designed
compounds by calculating their polarizability and first hyperpolarizability

Exchange of scientific knowledge with KKU students
about NLO materials and quantum computational methods

Publishing of research work in IS impact journals

Synthetic organic compounds displaying NLO
properties have great advantages over widely used inorganic materials in its
implicational level 13,14. Unlike several
inorganic compounds where ionic arrangements inside crystalline structure
causes electro-optical responses, the organic molecules show electro-optical
behavior due to intra and inter charge transfer effects 15. Owing to
larger nonlinear amplitudes and optical thresholds as well as lower dielectric
constants, several conjugated organic molecules containing
donor-pi-conjugation-acceptor framework are promising candidates for particular
needs of applications as it can be synthetically tailored for specific NLO
properties 16.

The substitution strategy involving the use of suitable substituents is
very important for organic molecules to explore and modify their optical
spectra as well as the NLO properties.17 Apart from the strong donor–acceptor intermolecular
interaction and delocalized ?-electron system, the increase of the conjugation
length also plays a vital role in the NLO behavior of these materials.18 Of the several organic compounds known to exhibit NLO properties,
chalcones and coumarins derivatives are 
seen to show novel electro-optical and nonlinear optical properties 19. Recent study suggests that coumarin-based chromophores
containing a chalcone moiety by fusing of chalcone moiety to the coumarin ring
will surprisingly increase the NLO propreties.

 

Here, the first step involves the synthesis of
substituted acetyl coumarins by the reaction between 2-Hydroxy naphthaldehyde
and Ethyl acetoacetate. In the next step, acetyl coumarins are reacted with
substituted aldehydes in presence of piperidine to yield coumarinyl
chalcones.  The synthesized compounds
will be characterized by means spectroscopic techniques like, IR, UV, 1HNMR, 13CNMR,
Mass spectroscopy etc.

Optical properties

The optical absorption (Uv-Visible spectra) and
emission spectra of the synthesized compounds will be determined experimentally
through time-dependent density functional theory (TD-DFT) methods. For example,
the popular exchange-correlation functionals including B3LYP (Becke,
three-parameter, Lee-Yang-Parr) and Perdew-Burke-Ernzerhof (PBE) have often
been employed to describe organic NLO compounds. These computational methods
have provided reasonably good agreements with experiments. Different DFT
functionals will be used to model and compare the experimental absorption and
emission spectra of newly grown single crystals.

Computational Methodology

Many simulation methodologies will be used to
investigate the geometrical electronic structures as well as the optical and
nonlinear optical behavior of newly grown single crystals. For the
characterization of newly grown single crystals, following major
electro-optical properties will be calculated using quantum chemical
methodologies.

Calculation of First Hyperpolarizability

A material with good nonlinear optical response
(first hyperpolarizability) has the ability to interact with laser light to
generate new frequencies and phases usually termed as second harmonic generation
(SHG). The charge transfer inside the crystallographic unit plays an important
role to originate first hyperpolarizability (?). The nonlinear optical
polarizability of designed materials will be calculated using DFT combined with
finite field (FF) method. A static electric field (F) is applied in FF approach
and the energy (E) of the molecule is given by following Eq.

In the absence of an electronic field, the total
energy of molecule is represented by E(0), is the dipole moment, is the polarizability,  and  are the first and second
hyperpolarizabilities, respectively.

Computational analysis and data visualization

This is the most advantageous aspect of
computational chemistry where the computed results are visualized into
pictorials or graphical representations. For example, locations of molecular
orbitals in newly synthesized compounds, their density of states, spin
densities, molecular electrostatic potentials and charges etc.  Besides this, several possible
crystal-packing symmetries can be simulated and checked for their stability.
Such types of computational techniques are very helpful to get preliminarily
crystal date for new crystal materials whose structures are not reported or
their crystals are not grown till now.

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