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Title: Design and Exploration of Pi-Conjugated Donor-Acceptor Compounds for Potential Nonlinear Optional Properties
Authors: Khan, Muhammad Usman
Keywords: Physical Sciences
Applied Chemistry
Issue Date: 2020
Publisher: Government College University, Faisalabad
Abstract: The design and development of nonlinear optical (NLO) materials is an area of frontier research for both theoretical and experimental communities strengthened by extensive applications related to optics, optoelectronics, nanophotonics and photonics. Materials with NLO activity find use as electro-optic switching elements for optical information processing, telecommunication technologies and modern hi-tech applications. The NLO process requires materials that manipulate the amplitude, phase, polarization and frequency of optical beams. The variety of materials including inorganic, organometallic, organic and polymeric have been studied for their NLO activity. However, it is the organic materials which have been receiving the maximum attention. In addition to the advantage in synthesis, organic materials have ultra fast response time, photostability and large first hyperpolarizability (β) values. In particular, the exploration of π-conjugated systems linking a donor (D) and an acceptor (A) at opposite ends show large NLO response. Therefore, it has become one of the fascinating fields which have been receiving maximum attention and expedite the development of this new kind of materials. It has been considered that non-centrosymmetric D-π-A dipolar compounds are commonly used for achieving first hyperpolarizability or second-order nonlinear polarizability (β). Keeping in view the success of quantum chemical approaches, this thesis deals with the computational modeling of NLO materials with π-conjugated spacers between electron donor and an electron withdrawaing groups. This is based on the concept of charge transfer (CT) between D and A separated by π-conjugated spacers at molecular level. The CT in these molecules arises owing to the interaction of π orbitals of the D and A moiety through the π- bond. We have quantum chemically designed these D π-A compounds which can possess active electric, photophysical, linear and nonlinear optical properties. The organic materials based D-π-A framework has immense potential in the field of electronics, photonics, and information technology. We have found new insights into the properties of these functional materials. In this thesis, the quantum chemical approach has been carried using density functional theory (DFT) and time-dependent density functional theory (TDDFT) to compute the electronic properties including molecular orbital energy levels and natural bond orbital (NBO), UV–Visible absorption spectra, change of dipole moments (∆µ), average polarizability and NLO properties xix 1- The first research report is a valuable addition in the field of D-π-A NLO compounds and presented the vital confirmation for controlling the kinds of π conjugated linker is a significant approach for the design of new appealing NLO compounds. Series of triphenylamine-dicyanovinylene based donor-π-acceptor dyes have been designed by structural tailoring of π-conjugated linkers and theoretical descriptions of their molecular nonlinear optical (NLO) properties had been reported. DFT and TDDFT calculations were performed to shed light on how structural modification influences the NLO properties. The simulated absorption spectra results indicate that all dyes showed maximum absorbance wavelength in the visible region. The LUMO-HOMO energy gaps of all dyes have been found smaller which result in large NLO response. Calculation of NBO analysis reveals that electrons are successfully migrated from donor to acceptor via π-conjugated linkers and a charge separation state is formed. High NLO response reveals that this class of metal free organic dyes possesses eye-catching and remarkably large first hyperpolarizability values, especially D8 with highest α and βtot computed to be 771.80 (a.u) and 139075.05 (a.u) respectively. This theoretical framework also highlights the NLO properties of organic dyes that can be valuable for their uses in modern hi-tech applications. 2- The second report not only describes a new paradigm for structural modification of linkers and acceptors to design new appealing NLO compounds with giant NLO response properties, but also provides new horizons to explore unexplored DSSCs materials for NLO applications. This theoretical framework also exposed the fluorene substituted organic chromophores as excellent NLO candidates that can be valuable for their uses in technologically relevant applications. In this report, first attempt had been made to utilize fluorene based dye-sensitized solar cell (DSSCs) dye JK-201 as potential NLO material and also for the quantum chemical designing of series of novel organic NLO chromophores JK-D1 to JK-D12. The effect of π-linkers and acceptors-steered modulation of JK-201 on molecular energy levels, electronic, photophysical average polarizability and NLO properties of JK-D1 to JK-D12 had been studied in detail. Results illustrate that studied compounds showed absorbance wavelength in the visible region. Computed λmax (484.74 nm) and experimentally calculated λmax (481 nm) value of JK-201 was found to be in very good agreement. The maximum red shifted absorption spectrum is observed in JK-D12 with 599.38 nm value. JK-D1 to JK-D12 showed narrow energy gap and broader absorption spectrum as compared to JK-201. NBO xx analysis confirms the formation of charge separation state due to robust range of electrons/charge transfer from donor to acceptor via π-bridge, hence, responsible for large NLO response. An eye-catching and giant NLO response has been observed in all investigated compounds. In particular, JK-D12 displayed appealingly large α and surprisingly large βtot computed to be 1376.74 (a.u) and 405731.84 (a.u) respectively. This work might be useful to design other organic dyes in the field of electro-optics. 3- Third report evident that controlling the kind of donor, π-bridge, acceptor and intramolecular charge transfer (ICT) process at molecular level could be a key to develop strong nonlinear optical (NLO) active materials for technological applications. In this work, NLO response of quinacridone (QA)-based dyes has been tuned from 0.00 (a.u) to 128082.15 (a.u). Theoretical designing of quinacridone (QA) dye and new class of QA based compounds (QA-1 to QA-9) was proposed by installing auxiliary donors (dimethylvinyl, methoxy, and N,N-dimethylamine), donor (diphenylamine) and acceptors (cyanoacrylic acid, CN and NO2) segments into fixed π-bridge QA. The QA-9 was found to be the most red shifted among all studied compounds with λmax=489.02 nm. QA-1 to QA-9 showed narrow HOMO-LUMO energy gap as compared to QA which results in enhanced NLO response. NBO analysis confirms the formation of charge separation state in QA-1 to QA-9 due to successful migration of electrons from auxiliary donor/donor to acceptors via π-bridge. The present research evokes the scientific interest regarding the development of QA based tempting NLO compounds that can be beneficial in modern hi-tech applications. 4- In this work, we designed series of indolo[3,2,1-jk]carbazole based donor-π acceptor organic dyes S1-S8 by modifying the π-bridge of experimentally synthesized dye IC-2 in order to evaluate their potential nonlinear optical (NLO) properties. Density functional theory (DFT) calculations with B3LYP/6-31+G(d,p) level of theory was used to shed light on how bridging core modification affect the charge transport, electronic and NLO properties of S1-S8? Time-dependent DFT (TDDFT) computations using B3LYP, CAM-B3LYP, ωB97XD and M062X functional were performed to examine photophysical properties. Results indicate that HOMO-LUMO energy gaps in S1-S8 (3.03-2.61 eV) were found smaller than IC2 (3.11 eV) which results in large NLO response and obvious red shifts than IC2. Natural bond orbital (NBO) analysis reveals that separation of charge is formed and electrons are successfully moved from donor to acceptor unit. S1-S8 showed absorbance wavelength in the visible region (388-436 nm) xxi and greater than IC2 (386 nm). An enhanced NLO response was observed in S1-S8 as compared to IC2. Interestingly, S7 exhibits appealingly large enhancement in NLO response properties with <α> and βtot computed 512 (a.u) and 39664 (a.u) respectively. Hopefully, this theoretical investigation will serve as a springboard for the future experimental studies of these NLO dyes. 5- The heterocyclic aromatic compounds have become an important pharmacophore due to promising applications in medicine and nonlinear optics (NLO) fields. In this report, keeping in view of the significance of 4-thiopyrimidines derivatives, we selected phenyl pyrimidine derivatives namely: ethyl 4-((3-chlorobenzyl)thio)-6-methyl-2- phenylpyrimidine-5-carboxylate (EMPPC), (4-((4-chlorobenzyl)thio)-6-methyl-2- phenylpyrimidin-5-yl)methanol (MPPM) and 4-((4-chlorobenzyl)thio)-5,6-dimethyl-2- phenylpyrimidine (DPP) for NLO study. From density functional theory (DFT) calculations, HOMO–LUMO energies and global reactivity parameters have been obtained. The photophysical properties of investigated molecules were estimated using time dependent DFT (TDDFT) at B3LYP/6-311G(d,p) level of theory. Nonlinear optical (NLO) analysis has also been carried out by DFT calculations with B3LYP/6-311G(d,p) functional. The NLO properties of EMPPC, MPPM and DPP were observed larger as compared to the standard molecule indicating the considerable NLO character of all molecules especially recommended the NLO activity of EMPPC for optoelectronic associated hi-tech applications.
Gov't Doc #: 23483
Appears in Collections:PhD Thesis of All Public / Private Sector Universities / DAIs.

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