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COMPUTATIONAL STUDY ON POLYMERS OF UNSUBSTITUTED AND SOME SUBSTITUTED PYRROLES
Published 2015-09Subjects: “…Energy gap…”
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PHOTOPHYSICAL PROPERTIES OF SOME HETERO-AROMATIC AND CARBONYL COMPOUNDS
Published 2014-12Subjects: “…Energy gap…”
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- A large number of hetero-aromatic and carbonyl molecules are useful in pharmaceutical and dye industries. Studies of their transition dipole moment (∆μ) and transition polarizability (∆α) are important because they determine their solubility and usefulness. Experimental determination of these parameters and other electronic properties such as oscillator strength (f) using Solvatochromic Shift Equations (SSE) has proved to be unreliable. This is due to the large spread in their reported values and the overlapping of the electronic transition bands for a given molecule. This study was designed to modify the SSE in improving its reliability, and to computationally determine the photophysical properties of some hetero-aromatic and carbonyl compounds with a view to studying the extent of overlap of the electronic transition bands. The ultra-violet spectra of 2,3-diphenylcyclopropenone; 9,10-phenanthrenequinone; Furan-3,4-dicarboxylic acid; 3,4-diphenylthiophene; 3,4-dicarboxylic-2,5-diphenylthiophene and benzo[b]thiophene in the wavenumber range (25,000 - 52,631) cm-1, in solvents of different polarities at concentrations range of 10-6 - 10-5 M were measured at 25oC. These compounds were selected for the modified SSE due to the variation in their structures. The magnitudes of f in solution (fs) and in vapour phase (fv) were calculated using the Onsager-Abe reaction field model equations. The frequencies of electronic transitions in various solvents were used to characterise the observed bands. The SSE was modified by incorporating the molecular ground state polarizability (α), the molar refraction of each compound, Einstein coefficient and stark term. Estimation of the Δµ and Δα of these compounds were determined using the modified SSE. The electronic properties: f, number of transitions, the frontier orbitals energy gap (ΔELUMO-HOMO) and associated parameters such as: Ionization Potential (IP) and global hardness (ƞ) of the optimized structures of the molecules were calculated based on Time-Dependent Density Functional Theory using Becke’s three parameter with Lee-Yang-Parr modification and 6-31G* basis set. The spectra of these compounds gave a range of one to five bands designated as S0-S1, S0-S2, S0-S3, S0-S4 and S0-S5 in order of increasing energy. The magnitudes of fs and fv for the observed bands increased with increasing solvent polarities and were within the range 3.2x10-4 - 1.78 and 3.1x10-4 - 1.33 for hetero-aromatics; 1.4x10-3 - 1.52 and 1.3x10-3 - 1.23 for carbonyls. Similar trends observed for the experimental values of Δµ and Δα indicated that the more a transition is allowed, the greater the probability Δµ being larger than zero. The positive values of Δµ (5.7x10-3 - 1.73D) and Δα (8.0x10-5 - 5.5Å3) for hetero-aromatics; Δµ (3.7x10-2 - 1.23D) and Δα (4.8x10-4 - 0.95Å3) for carbonyls indicated substantial redistribution of the π-electron densities in more polar excited state than the ground state. The ΔELUMO-HOMO were (3.19 - 4.09eV) and (4.36 - 5.43eV) for carbonyls and hetero-aromatics respectively. The IP and ƞ increased as solvent polarity increased, suggesting high stability of these compounds in polar solvents. The modified equation is better in the estimation of transition dipole moment and transition polarizability. The energy gaps and associated parameters suggested strong activity of the molecules and minimal overlapping of the transition bands. 1 results 1
- Conjugated polymers 1 results 1
- Conjugated polymers which interact with biological systems have attracted interest due to their high conductivity, stability and electronic properties. Substituted polymers of 3-methyl pyrrole-4-carboxylic acid (MPCa) have been synthesised and used as components of biosensor, while unsubstituted polypyrroles are not effective for such application. However the mechanism of interaction, nature, the relative importance of dynamic and static electron correlation of the polymers are not completely understood. This research was designed using computational approach to study the molecular properties of substituted and unsubstittued pyrrole polymers with a view to understanding what make polymers of substituted pyrroles suitable components of biosensor. Structures of unsubstituted Pyrrole (Py); substituted pyrroles which include 3-methyl- pyrrole-4-carboxylic acid (MPCa), 3-methyl-pyrrole-4-carboxamide (MPCam), 3- methyl-pyrrole-4-sulfonic acid (MPSO3H), 3-methyl-pyrrole-1-carboxylic acid, (MPCb), 3-methyl-pyrrole-4-carbothioic acid (MPCOSH), 3-methyl-pyrrole-4- carbaldehyde (MPCHO) and their polymers were studied using quantum mechanical approach. The molecular properties investigated were Energy gap (Eg), Koopman’s reactivity descriptors, Fukui function, Lowest Unoccupied Molecular Orbital (LUMO), Highest Occupied Molecular Orbital (HOMO) and thermodynamic properties. These were calculated using restricted hybrid density functional theory with Becke three, Lee Yang and Parr at 6-31G(d) basis set. The calculated Eg were extrapolated to polymer through second order-degree polynomial equation. Spin-flip time density functional theory and coupled cluster single and double method with 6-311++G(d,p) basis set were used to calculate Coupled Cluster operator (T1) diagnostic and Vertical Singlet- Triplet (VST) gap to accurately determine polymers suitability as components of biosensor. All calculations were carried out using quantum mechanical software. The calculated Eg of the polymers decreased with increasing chain length and the nature of substituent. The order of Eg was MPCHO > Py > MPCb > MPCa > MPCam > MPSO3H > MPCOSH, with MPCOSH having the lowest value of 1.7 eV. Substituted polypyrroles except MPCHO have stronger electron-electron interactions since electron-electron interaction is more when the Eg is low (between 1.0 and 3.0 eV). Koopman’s reactivity descriptors were within the range of -3.9 to 2.4 eV (chemical i potential), 1.5 to 2.1 eV (chemical hardness) and 1.4 to 4.4 eV (electrophilicity index). Fukui function revealed a high electron density around the substituted functional groups and the LUMO and HOMO were extended over the C-C and C=C bonds. Thermodynamic parameters were enthalpy change (-4361.1 to -1045.7 kJmol-), entropy change (540.3 to 952.2 Jmol-1K-1) and free energy change (∆G0f) (-4361.2 to - 1045.8 kJmol-1) indicating spontaneous formation of the polymers. The T1 diagnostic of unsubstituted polypyrroles ranged from 0.0015 to 0.0013, while substituted polypyrroles ranged from 0.030 to 0.065. The T1 <0.02 indicated that unsubstituted polypyrrole had dynamic correlation with single reference (closed shell), while T1 >0.02 showed that substituted polypyrroles possessed static electron correlation with multireference (open shell) nature. The VST gap of unsubstituted polypyrroles ranged from 3.0 to 4.8 eV, while substituted polypyrroles ranged from 3.1 to 5.3 eV. The VST gap >0 revealed that all studied systems have a singlet ground state. The presence of substituents on polypyrrole decreased the energy gaps which led to the enhancement of their molecular properties making them suitable components of biosensor. 1 results 1
- Density functional theory 1 results 1
- Dipole moment 1 results 1
- Hetero-aromatic compounds 1 results 1
- Solvatochromic shift 1 results 1
- Substituted and unsubstituted polypyrrole 1 results 1
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