John O. Morley

Calculated hyperpolarisabilities of polythiophenes, polyfurans and polypyrroles

The calculated hyperpolarisabilities of substituted polythiophenes, polypyrroles and polyfurans containing an electron donor at one end of the molecule and an electron attractor at the other increase slowly with increasing number of heterocyclic rings. If the effective hyperpolarisability per unit volume is considered, the maximum values are obtained for the bifuran, the terpyrrole and the quaterthiophene. The largest hyperpolarisabilities are predicted for the polythiophenes, followed by the polypyrroles with the polyfurans showing the smallest values. Both the polythiophenes and polypyrroles have larger calculated hyperpolarisabilities than the similarly substituted polyphenyls but smaller than the corresponding polyenes.

Calculations of the electronic spectra and hyperpolarisabilities of selected dyes and pigments

A method has been developed for the calculation of the spectra and hyperpolarisability tensor for the linear electro-optic effect at the CNDO/S level of approximation using a sum-over-states procedure with singly excited states. A wide range of dyes and pigments have been studied to assess their potential as non-linear optical materials using both static-field and frequency-dependent values. Conjugated systems containing strong donors and acceptors positioned at either end of the conjugation path show the largest intrinsic effects. Substantial resonance enhancement effects occur for molecules such as azobenzenes and azoheterocycles which absorb in the red or infrared region of the spectrum.

Structure-activity relationships in 3-isothiazolones.

The biological activity of a series of structurally diverse 3-isothiazolones (1) has been assessed by evaluating the minimum inhibitory concentration required to inhibit the growth of E. Coli. The structure and electronic properties of these derivatives have been calculated using both semi-empirical and ab initio molecular orbital methods. Multi-linear regression analysis shows no correlation between the experimental activity of the 3-isothiazolones and either the calculated geometries, electronic properties, or the frontier orbital energies of these derivatives, but a reasonable relationship is found with other parameters including their calculated solvation energies, suggesting that diffusion may play an important role in their mode of action.

THEORETICAL STUDIES ON THE BIOCIDAL ACTIVITY OF 5-CHLORO-3-ISOTHIAZOLONE

Abstract The very high biocidal activity of 5-chloro-3-isothiazolone has been explored by calculating the structure and electronic properties of the tautomers which are thought to be produced in the cell by nucleophilic attack at the sulfur atom. The results obtained, both at the semi-empirical AM1/COSMO and PM3/SM3 levels and at the ab initio 4–31G level with polarisation functions on the sulfur and chlorine atoms, suggest that the high activity of this isothiazolone arises in part because of the preferential formation of a highly reactive thioacyl chloride following an initial reaction which cleaves the SN bond of the isothiazolone ring.

Theoretical studies on the electronic structure and spectra of the merocyanines

Abstract Molecular orbital calculations are reported on the structure and electronic properties of the simple merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (which can also exist as a zwitterion) using the AM1, CNDOVS and the ab initio STO-3G and 3-21G methods. The AM1 optimisation method invariably produces the lower energy quinone as the favoured structure, but the presence of ionic charges or sparkles positioned above and below the nitrogen and oxygen atoms of the merocyanine are sufficient to force the structure into a higher energy zwitterion form. Spectroscopic calculations on the AM1 structures in the gas phase using the CNDOVS method predict that the zwitterion will absorb at a longer wavelength (587 nm) than the quinone (470 nm), contradicting previous theoretical studies.

Studies on the biological activity of some nitrothiophenes

The biological activity of nineteen substituted thiophenes (3) have been assessed by evaluating the minimum inhibitory concentration required to inhibit the growth of E. coli, M. luteus and A. niger. The series displays a wide range of activities with 2-chloro-3,5-dinitrothiophene (3a) or 2-bromo-3,5-dinitrothiophene (3c) showing the highest activity against all three organisms, while the simplest compound of the series, 2-nitrothiophene (3s) shows the smallest activity in each case. The mode of action of 3a and 3c is thought to involve nucleophilic attack by intracellular thiols at the 2-position of the heterocyclic ring leading to displacement of halogen, but other active derivatives, such as 2,4-dinitrothiophene (3h) and 5-nitrothiophene-2-carbaldehyde (3d) which have no displaceable halogen or leaving group are thought to act by forming Meisenheimer complexes.

Theoretical calculations of the structure of a donor—acceptor stilbene, azobenzene and related molecules

Abstract The structures of a substituted stilbene, azobenzene, and two isomeric benzylidene anilines containing the 4-dimethyl-amino group at one end of the molecule and the 4′-nitro group at the other, have been calculated using the semiempirical MNDO, AM1, and PM3 methods and the ab initio HF/3-21G method. The results are compared with experimental data where available. The MNDO method gives a very poor account of the geometry of the stilbene, but both the AM1 and PM3 methods correctly predict the planarity of the substituted stilbene and nitrobenzylidene aniline but erroneously predict a non-planar structure for the azobenzene. In contrast, the ab initio method predicts planar structures for the substituted stilbene, azobenzene, and nitrobenzylidene aniline, and a twisted structure for the isomeric benzylidene nitroaniline, in line with crystallographic data. This loss of planarity for the benzylidene nitroaniline is almost certainly responsible for its smaller non-linear activity and for the hypsochromic shift observed in its low energy absorption band relative to the other molecules.

Theoretical investigation of the structure and spectra of donor–acceptor azobenzenes

Calculations are reported on the structure of 4-dimethylamino-4′-nitroazobenzene using the AM1, CNDO/2, STO-3G and 3-21G methods and the results are compared with crystallographic data on related structures. Subsequent spectroscopic calculations with a version of the CNDO/S method give transition energies which are highly dependent on the starting structure adopted. Similar trends are found with the calculated structure of the corresponding 4-dimethylamino-2′-nitroazobenzene, although here steric interactions are predicted by the AM1 and STO-3G methods to twist the aromatic rings and also force the nitro group into a non-planar conformation.

Theoretical investigation of the electronic properties of donor–acceptor N-benzylideneanilines and related molecules

The dipole moments, transition energies and hyperpolarizabilities of a substituted stilbene, azobenzene and two isomeric–benzylideneanilines containing the 4-dimethylamino-group at one end of the molecule and the 4-nitro-group at the other, have been calculated using a semi-empirical sum-over-states method on structures optimized with the 3-21G basis set. An excellent correlation is found between the calculated and experimental spectra. The azobenzene is predicted to have the largest hyperpolarizability of this series of molecules, followed by the stilbene and N-(nitrobenzylidene)aniline which have comparable values, with the non-planar N-(benzylidene)nitroaniline showing the smallest value.

Quadratic non-linear optical properties of some donor–acceptor substituted thiophenes

A series of thiophenes and bithiophenes containing combinations of dimethylamino and dimethylhydrazono π-donor and nitro and dicyanovinyl π-acceptor groups was synthesized in order to study their non-linear optical properties. Solution molecular hyperpolarizabilities were determined by a combination of experimental dipole moments and results from the electric field induced second harmonic generation experiment. Values obtained correlate well with those calculated by the CNDOVSB molecular orbital method. The main structure-property relationships deduced from the results are: (i) thiophene provides a more efficient electron delocalization pathway than benzene; (ii) this increase is proportionately even greater for bithiophene compared with biphenyl; (iii) dimethylhydrazono results in twice the hyperpolarizability of dimethylamino in a comparable molecule; and (iv) dicyanovinyl is a more effective electron acceptor group than nitro in the thiophenes synthesized and calculated.