| Functional groups containing nitrogen and boron atoms are present in a variety of naturally occurring and man made compounds. These functional groups impart different physical and chemical characteristics to these compounds. These groups are responsible for their unique chemical reactivity patterns and play crucial roles in the preparation of drugs, agrochemicals, dyes and molecules of life and nonlinear optical (NLO) materials and so on. There has been considerable success in the past decade at preparing compounds with large intrinsic nonlinear optical properties. Materials exhibiting NLO response are currently of great scientific and technological interest for applications as diverse as optical telecommunications, signal processing, data storage, image reconstruction, logic technologies, sensor protection, and optical computing. A large number of reports have been presented up till now on NLO materials with different conventional strategies to enhance NLO response. These strategies mainly include the use of the molecules with the extendedπ-electron systems, planer donor-πconjugated bridge-acceptor (D-π-A) model, bond length alternation (BLA) theory, twistedπ-electron systems, incorporation of ligated metal into the organic compounds, multi branches molecules with inceasing ability the charge transfer, and so forth. However, the intermolecular interaction effect of electronegative and electropositive atoms/ions on different acidic molecules containing nitrogen and boron atoms has not been studied from electro-optical point of view.In this thesis, we have selected first three alkali metals i.e. Li, Na, K and three halogens i.e. F, Cl, Br atoms/ions to dope them with different acidic molecules containing nitrogen and boron atoms. The interactions of these atoms/ions with different acidic molecules containing nitrogen and boron and their effects on the NLO properties of these molecules were investigated to find out a strategy, which is different from the above stated conventional methods for the improvement of significantly large NLO response. Our work will focus on the four different aspects in this regard:1. The effect of intermolecular interaction of a phthalimide based"On-Off"sensor with different halides ions (F): Tuning its efficiency and electro- optical properties The interaction between chemosensor, N-(2-Methyl-1,3-dioxo-indan-5-yl)-benzamide (1) and different halides ions (Fˉ, Clˉand Brˉ) has been investigated using density functional theory (DFT). A clear insight of the sensor anion binding process has been presented. Our calculations revealed that the observed colorimetric and fluorescent signals are induced due to the ground state deprotonation of the sensor molecule caused by Fˉwhich has two times higher binding affinity than other halide ions (Clˉand Brˉ). Derivatives of system 1 have been made to find a better sensor with higher binding affinity and longer wavelength of absorption. All the derivatives are better sensors than the parent 1 except 4-Methyl-N-(2-methyl-1,3-dioxo-indan-5-yl)- benzamide (2).2. In our second report, a new sequence has been presented for the reversible switching and modulation of NLO. The static first hyperpolarizabilities (β) of four dibenzoborole derivatives have been computed by DFT. Theβvalues of these dibenzoborole derivatives have shown a significant increase with the attachment of Fˉand/or one electron reduction. For example, 5-fluoro-5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bithienyl-5H-di-benzo[d,b] borole ion (3·Fˉ) and 5-fluoro-5-phenyl-3,7-bis-dinitrothienyl-5H-di-benzo[d,b]borole ion (4·Fˉ) both showedβvalues as large as 64×10-30 esu and 272×10-30 esu, that is about 12 times and 4 times larger than their counterparts 5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bithienyl-5H-dibenzo[d,b]borole (3) and 5-phenyl-3,7-bis-dinitrothienyl-5H-dibenzo[d,b]borole (4) (without Fˉ) respectively. Similarly, systems 3Red and 4Red (one electron reduced) also showed 47 times and 15 times largerβvalues than their neutral forms respectively. Interestingly, this NLO switching is two dimensional in characteristics, in which large off-diagonal hyperpolarizability tensors can be related to the charge-transfer transitions which are polarized perpendicular to the molecular dipolar axis. Density of states (DOS) and frontier molecular orbital (FMO) analysis show that the binding of Fˉat a boron atom and/or one electron reduction process turn off the Pz→π*conjugation of vacant p-orbital of boron atom in LUMOs, resulting in a higher extent of perpendicular charge transfer (CT) and lagerβvalues. The present investigation reveals a new idea and different means for multifunctional use of the present dibenzoborole class, especially (already synthesized) 3·Fˉas two-dimensional NLO molecular switch with cartesian nonlinear anisotropy as large asη= 10.48.3. Different benzimidazole chromophores have been studied to tune their NLO properties with fluoride ions and on proton abstraction from acidic hydrogen atom. A novel sequence for reversible second-order NLO molecular switching with protonation/deprotonation has been achieved and tuned as well. The NLO switching with first hyperpolarizabilities (β0) as low as 14×10-30 esu (Off-phase) and as large as 1014×10-30 esu (On-phase) have been computed by using the density functional theory (DFT). Thisβ0 value has been tuned up to 2028×10-30 esu by effective substitutions in the derivatives of 1ˉ(1aˉ, 1bˉ, 1cˉ, and 1dˉ). Interestingly, the substituted compounds have illustrated robustly large off-on NLO switching with the difference ofβ0 values as 7, 63, 85 and 75 times larger than their neutral counterparts, respectively. TD-DFT calculations along with natural bond orbital (NBO), frontier molecular orbitals (FMOs) and molecular electrostatic potential (MEP) analyses depict that the abstraction of imido proton reverts the push-pull configurations resulting in a red shift for both absorption and emission spectra which leads to a high performance second-order NLO molecular switching. A similar trend of NLO switching in Fˉcompounds of these chromophores has also been observed with significantly largeβ0 values having analogous electro-optical properties like deprotonated anions. Furthermore, gas-phase acidity (GPA) calculations for the neutral molecule 1 and its derivatives (1a, 1b, 1c, and 1d) have also revealed that these are rationally potent nitrogen acids and can easily be dissociated to produce stable deprotonated anions.4. Polyhedral boranes, carboranes and borane derivatives have been used to study their interaction with different alkali metals. An innovative type of lithium decahydroborate (Li@B10H14) complex with a basket like complexant of decaborane (B10H14) has been designed using quantum mechanical methods. As Li atom binds in a handle fashion to terminal electrophilic boron atoms of decaborane basket, its NBO charge q (Li) is found to be 0.876, close to +1. This shows that the Li atom has been ionized to form a cation and an anion at the open end of B10H14. The most fascinating feature of this Li doping is its loosely bound valence electron, which has been pulled into the cavity of B10H14 basket and become diffuse by the electron deficient morphological features of the open end of B10H14 basket. Strikingly, the first hyperpolarizability (β0) of Li@B10H14 is about 340 times larger than B10H14, computed to be 23075 au (199×10-30 esu) and 68 au respectively. Besides this, the intercalation of Li atom to the B10H14 basket brings some distinctive changes in its Raman, 11B NMR and UV-Visible spectra along with its other electronic properties that might be used by the experimentalists to identify this novel kind of Li@B10H14 complex with large electro-optical response. This report may evoke the possibility to explore a new thriving area i.e. alkali metal-boranes for NLO application.5. Fluoro derivatives of B10H14 and Li@B10H14 baskets have been designed to achieve robustly large NLO response and thermal stability under simultaneous effects of conical-push and inward-pull, which have been reported discretely in previous lithium NLO complexes. Among the various derivatives, Li@6,9-F2B10H12, Li@1,3,6,9-F4B10H10 and Li@2,4,6,9-F4B10H10 have shown first hyperpolarizability (β0) values as large as 181124, 133199 and 32314 au along with vertical ionization potentials (VIPs) of 6.447, 6.302, 6.885 eV, respectively. The first hyperpolarizability values and VIPs are significantly larger than previously reported Li-doped fluorocarbon chains at the same MP2/6-31+G* level of theory (J.Am.Chem.Soc. 2007, 129, 2967). These values also exceed from our formerly designed Li@B10H14 basket (J.Am.Chem.Soc. 2009, 131, 11833). Further, the enthalpies of lithiation and fluorination reactions (ΔrHo) at 298 K are obtained to explore the thermal stability for the first time. The calculated enthalpies of lithiation reactions are -10.04, -11.29 and -13.18 Kcal/mol for B10H14, 6,9-F2B10H12, and 2,4-F2B10H12 respectively which demonstrate a higher probability of fluoro decaboranes for reaction with lithium. The obtained results not only explain the effect of positions and number dependence of substituted fluoro atom(s) in B10H14 and Li@B10H14 but also elucidate a unification of previous two benchmark strategies which have been used independently to polarize lithium excess electron for high NLO response and thermal stability.
|
PDF Full Text Request