Highest Occupied Molecular Orbital Energy Level Research Articles

From Blue to Red: Syntheses, Structures, Electronic and Electroluminescent Properties of Tunable Luminescent N,N Chelate Boron Complexes

AbstractA comprehensive study of a series four‐coordinate boron compounds with the general formula of BPh2(N,N), where N,N are bidentate chelate ligands containing both neutral and negatively charged nitrogen donor atoms has been conducted. The structures of the boron complexes were examined via single‐crystal X‐ray diffraction. The series of molecules display bright luminescence with emission maxima λmax ranging from blue to red, depending on the nature of the N,N chelate ligand. The electronic effects and their consequences on the luminescent properties of the complexes due to the CH replacement of the chelate ligand by a nitrogen atom, the increase of conjugation, or the change of substituents on the chelate ligand have been examined using electrochemical analysis, UV‐visible, and fluorescence spectroscopic methods, and by molecular orbital calculations (Gaussian 98). Experimental data and MO calculation results established that the emission of this class of compounds is caused by π–π* transitions centered on the chelate ligand. Furthermore, the experimental and theoretical results consistently and conclusively established that electron withdrawing groups on the negatively charged N‐donor portion of the chelate ligand causes a decrease in the highest occupied molecular orbital (HOMO) energy level, thus increasing the energy gap. The CH replacement by a nitrogen atom on the negatively charged portion of the chelate ligand causes a dramatic decrease of the HOMO energy level, and the increase of conjugation in the chelate ligand significantly decreases the energy gap. Blue and red electroluminescent (EL) devices were fabricated successfully using two representative boron compounds from the series. The new boron compounds have been found to be able to function as both emitters and electron transport materials in EL devices.

Electrochemical and quantum chemical studies of some sulphur-containing organic compounds as inhibitors for the acid corrosion of mild steel

The influence of thioacetamide (TAcA), thiourea (TU) and thiobenzamide (TBA) on the corrosion of mild steel in 0.1 M H 2SO 4 solution has been studied in relation to the concentrations. The CS NH 2 group is the common main group for these compounds and they differ only having different groups on the other side of the thiocarbonyl group. Electrochemical impedance spectroscopy (EIS) and polarization resistance measurement ( R p) techniques have been used to obtain experimental data. The results showed that these compounds revealed a good corrosion inhibition, thiobenzamide being the most efficient and thioacetamide the least. The correlation between the molecular structures and corrosion inhibition efficiencies of TAcA, TU and TBA has been investigated using ab initio quantum chemical calculations. The structures of these compounds have been optimized, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, LUMO–HOMO energy gap and molecular orbital densities have been computed. The relations between the inhibition efficiency and quantum chemical parameters have been discussed. It was seen that there is a clear relation between the increase in corrosion inhibition and the increase of HOMO energy level. The highest values of the HOMO densities were found in the vicinity of the sulphur atom indicating it as most probable adsorption center.

Synthesis and characterization of new poly(aryl ether)s with isolated fluorophores

AbstractFour novel poly(aryl ether)s (P1–P4) consisting of alternate isolated electron‐transporting (3,3″′‐bis‐trifluoromethyl‐p‐quaterphenyl for P1, P3 or 3,3″′‐dicyano‐p‐quaterphenyl for P2, P4) and hole‐transporting fluorophores [N‐(2‐ethylhexyl)‐3,6‐bis(styryl)carbazole for P1, P2 or 9,9‐dihexyl‐2,7‐bis(styryl)fluorene for P3, P4] were synthesized and characterized. These poly(aryl ether)s can be dissolved in organic solvents and exhibited good thermal stability with 5% weight‐loss temperature above 500 °C in nitrogen atmosphere. The photoluminescent (PL) spectra of the films of these polymers showed maximum peaks at around 442–452 nm. The PL spectral results revealed that the emission of polymers was dominated by the fluorophores with longer emissive wavelength via the energy transfer from p‐quaterphenyl to 3,6‐bis(styryl)carbazole or 2,7‐bis(styryl)fluorene segments. Therefore, the p‐quaterphenyl segments function only as the electron‐transporting/hole‐blocking units in these polymers, and the other segments are the emissive centers and hole‐transporting units. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital energy levels of these polymers were measured by cyclic voltammetry. The electron‐donating nitrogen atom on carbazole resulted in the higher HOMO energy levels of P1 and P2 than those of P3 and P4. The single‐layer light‐emitting diodes (LED) of Al/poly(aryl ether)s (P1–P4)/ITO glass were fabricated. P1, P2, and P4 revealed blue electroluminescence, but P3 emitted yellow light as a result of the excimer emission. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2215–2224, 2002

Comparative Study of Photoluminescence Dynamics of Tris(8-hydroxyquinoline) Aluminum-Based Organic Multilayer Structures with Different Types of Energy Lineups

Photoluminescence (PL) dynamics of aluminumquinoline (Alq3; tris(8-hydroxyquinoline) aluminum)-based organic multilayer structures, i.e., Alq3/oxadiazole (t-BuPBD; (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole), Alq3/cyclopentadiene (PPCP; 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene), and Alq3/diamine (TPD; N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine), which possess different types of energy lineups (equivalent to types I and II of inorganic semiconductor multilayer structures) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, were investigated and compared. The fast energy transfer followed by strong PL in Alq3 was proved for Alq3/PBD which is of type-I energy lineup, while the tendency of separation of excitons, i.e., electrons and holes generated in the excited molecules, was demonstrated for Alq3/PPCP and Alq3/TPD which are of type II. These results indicate that the energy lineup with respect to HOMO and LUMO energy levels can be a guideline for luminescence dynamics in organic multilayer structures.

A quantitative structure–Activity relationship study of the skin irritation potential of phenols

Quantitative structure–activity relationships (QSARs) for skin irritation potential were studied using twenty-four phenols. Based on the hypothesis that skin irritation is induced by reaction of phenols with macromolecules present in epidermal and dermal levels of the skin, the following descriptors for QSAR were selected, the absolute hardness (N) calculated from HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) energy levels for reactivity, and logP (octanol–water partition coefficient) for permeability. Using these descriptors, we fitted a regression function to the set of skin irritation scores obtained from an in vivo study, which allowed derivation of equations (r=0.85). The equations were verified with six additional phenols, showing good correlations with the expected skin irritation scores. From the above findings, the equations can be considered useful for predicting the skin irritation potential of phenol compounds.

The role of electronic and steric factors in the fluorination of P(III) compounds by Ph 3BiF 2

The reactivity of P(III) compounds towards fluorination by Ph 3BiF 2 was studied from the viewpoint of frontier molecular orbitals theory. The highest occupied molecular orbital (HOMO) energies of phosphites and amidophosphites were calculated. A qualitative relationship between HOMO energy levels and reactivity was found. The difference in chemical behaviour of (Me 2N) 3P and (Et 2N) 3P was attributed to steric hindrance.

Kinetic and molecular orbital studies on the rate of oxidation of monosubstituted phenols and anilines by horseradish peroxidase compound II

The second-order rate constant (k4) for the oxidation of a series of aromatic donor molecules (monosubstituted phenols and anilines) by horseradish peroxidase (HRP) compound II was examined with a stopped-flow apparatus. The electronic states of these substrates were calculated by an ab initio molecular orbital method. It was found that in both phenols and anilines log k4 values correlate well with the highest occupied molecular orbital (HOMO) energy level and the lowest unoccupied molecular orbital (LUMO) energy level, but not with the net charge or frontier electron density on atoms of these molecules. The HOMO and LUMO energy levels of phenols and anilines further showed linear relationships with Hammett's sigma values with negative slopes. Similar results were obtained in the oxidation of substrates by HRP compound I, except that the rate of reaction was much higher than in the case of HRP compound II. In addition, the rates of oxidation of phenols by compound I or II were found to be about 1000 times higher than those of anilines with similar HOMO energy levels. On the basis of these results, the mechanism of electron transfer from the substrate to the heme iron of HRP compound II is discussed.

Molecular orbital studies of the action of thyroid hormone analogs: Effects on oxygen consumption of mitochondria and horseradish peroxidase-catalyzed NADH oxidation

The electronic structure of thyroxine and related compounds were calculated by semiempirical molecular orbital methods. When the quantum chemical indices obtained were compared with the structure-activity relationship obtained so far byin vivo andin vitro assays, it was found that HOMO (highest occupied molecular orbital) energy levels of thyroxine and its analogs are well correlated with the increase in oxygen consumption of rat kidney mitochondria determined byin vitro assay. This finding permits the hypothesis that these compounds may play a role in activating the electron transport system of mitochondria by mediating the oxidation-reduction of cytochromes. Furthermore, HOMO energy levels of thyroxine and phenol derivatives were found to correlate well with the stimulation of horseradish peroxidase-catalyzed oxidation of NADH. This suggests that the step of electron removal from these compounds by the enzyme system may be a rate-limiting step, confirming the view that phenoxy-radicals meditate the whole reaction.

The relation between Ames test data and electronic structure for a series of benzidine derivatives investigated by using cluster analysis.

The dose-response curves of benzidine derivatives calculated from Ames test data were analyzed by combining the least-squares method with cluster analysis. The calculations of the electronic structures of various benzidine derivatives have at the same time been performed by using the CNDO/2 method. The cluster analysis was carried out on various parameters concerning the electronic structure, such as total electron density, frontier electron density, the energy level of HOMO (highest occupied molecular orbital), and so on. It was found that the total electron density on the nitrogen atom gives a similar pattern of classification to that for mutagenic compounds in cluster analysis. In other words, it may be inferred that the nitrogen atoms of the benzidine derivatives play an important role in the metabolic activation of the compounds. Consequently, cluster analysis with a parameter of electronic structure should be a useful initial screening procedure for various analogous mutagenic compounds as well as carcinogenic compounds.