Abstract:Subject classification: 71.15.Mb; 78.40.Me; S12Time-dependent density functional theory (TD-DFT) calculations of the photoabsorption of molecules in the vacuum ultraviolet region have been performed in order to aid in the design of transparent materials for use as photoresists for F 2 lithography (157 nm). The method including an empirical equation for correcting the calculated transition energy is described. We have used the TD-DFT approach to predict the photoabsorption of substituted benzenes, and cycloalka… Show more
“…We are particularly interested in exploiting time-dependent DFT (TD-DFT) to predict the UV−visible spectra of a wide range of molecules from new photolithographic polymers for the production of semiconductors to biomolecules for neurotoxins . Such TD-DFT calculations have a strong dependence on the orbital energies.…”
Representative atomic and molecular systems, including various inorganic and organic molecules with covalent and ionic bonds, have been studied by using density functional theory. The calculations were done with the commonly used exchange-correlation functional B3LYP followed by a comprehensive analysis of the calculated highest-occupied and lowest-unoccupied Kohn-Sham orbital (HOMO and LUMO) energies. The basis set dependence of the DFT results shows that the economical 6-31+G* basis set is generally sufficient for calculating the HOMO and LUMO energies (if the calculated LUMO energies are negative) for use in correlating with molecular properties. The directly calculated ionization potential (IP), electron affinity (EA), electronegativity ( ), hardness (η), and first electron excitation energy (τ) are all in good agreement with the available experimental data. A generally applicable linear correlation relationship exists between the calculated HOMO energies and the experimental/calculated IPs. We have also found satisfactory linear correlation relationships between the calculated LUMO energies and experimental/calculated EAs (for the bound anionic states), between the calculated average HOMO/LUMO energies and values, between the calculated HOMO-LUMO energy gaps and η values, and between the calculated HOMO-LUMO energy gaps and experimental/ calculated first excitation energies. By using these linear correlation relationships, the calculated HOMO and LUMO energies can be employed to semiquantitatively estimate ionization potential, electron affinity, electronegativity, hardness, and first excitation energy.
“…We are particularly interested in exploiting time-dependent DFT (TD-DFT) to predict the UV−visible spectra of a wide range of molecules from new photolithographic polymers for the production of semiconductors to biomolecules for neurotoxins . Such TD-DFT calculations have a strong dependence on the orbital energies.…”
Representative atomic and molecular systems, including various inorganic and organic molecules with covalent and ionic bonds, have been studied by using density functional theory. The calculations were done with the commonly used exchange-correlation functional B3LYP followed by a comprehensive analysis of the calculated highest-occupied and lowest-unoccupied Kohn-Sham orbital (HOMO and LUMO) energies. The basis set dependence of the DFT results shows that the economical 6-31+G* basis set is generally sufficient for calculating the HOMO and LUMO energies (if the calculated LUMO energies are negative) for use in correlating with molecular properties. The directly calculated ionization potential (IP), electron affinity (EA), electronegativity ( ), hardness (η), and first electron excitation energy (τ) are all in good agreement with the available experimental data. A generally applicable linear correlation relationship exists between the calculated HOMO energies and the experimental/calculated IPs. We have also found satisfactory linear correlation relationships between the calculated LUMO energies and experimental/calculated EAs (for the bound anionic states), between the calculated average HOMO/LUMO energies and values, between the calculated HOMO-LUMO energy gaps and η values, and between the calculated HOMO-LUMO energy gaps and experimental/ calculated first excitation energies. By using these linear correlation relationships, the calculated HOMO and LUMO energies can be employed to semiquantitatively estimate ionization potential, electron affinity, electronegativity, hardness, and first excitation energy.
“…In contrast to the case of ground states, time-dependent density functional theory (TD-DFT) for treating excited-state properties − has only recently been applied to molecules, , although the theory itself was first proposed more than 20 years ago. Recent work has shown that TD-DFT can be used to reliably predict not only the location of the UV−vis excitation but also the oscillator strength (intensity of the transition). , In addition, Adamo and Barone have tested TD-DFT calculations in models of aqueous solutions, although the calculated results indicate that the solvent shifts of the calculated excitation energies are small compared to the accuracy required to qualitatively predict the color of a compound. 1 …”
First-principles electronic structure calculations have been performed to predict chromogenic properties of various candidate structures, including pyrrole monomers and dimers and their derivatives, of the chromophores formed from the reactions of 2,5-hexanedione (2,5-HD), a prototype of neurotoxic aliphatic γ-diketones, with NH 3 , amino acids, and proteins. The calculated results indicate that the pyrrole monomer structures and a previously proposed dimer structure do not have an absorption in the visible region (λ > ∼400 nm and < ∼700 nm), whereas a novel type of pyrrole dimer structure has absorptions (λ ) ∼400 to 420 nm) in the visible region if the methyl (CH 3 ) groups on the pyrrole rings are oxidized to CHO groups. The calculated results for the oxidized pyrrole dimer models for cross-linked proteins are consistent with all of the available experimental data for the chromogenic and neurotoxic effects of 2,5-HD. Our results strongly support the conclusion that the chromogenic effects of aliphatic γ-diketones are closely related to their neurotoxic effects and further predict that both the chromogenic and neurotoxic effects are associated with the same chemical reaction process. Such a reaction process most likely starts from the formation of the pyrrole-protein adducts followed by dimerization and further oxidization.
“…Further theoretical calculations are required to determine the validity of this assumption. 12,21 An important result of our investigation is recognizing the essential wavelength independence of the photolysis of the model compounds. This result establishes the possibility that exploratory experiments of the photochemistry of materials which absorb strongly in the deep UV may be examined with the much more convenient equipment that is available in the near UV.…”
The advance of 157 nm as the next photolithographic wavelength has created a need to for transparent and radiation durable polymers for the use as pellicles. The most promising materials for the pellicles are fluorinated polymers, but the currently available fluorinated polymers undergo photodegradation and/or photodarkening upon exposure to 157 nm irradiation. To understand the mechanism of the photodegradation and photodarkening of fluorinated polymers, mechanistic studies on the photolysis of liquid model fluorocarbons such as, perfluoro butylethyl ether and perfluoro-2H-3-oxa-heptane, were performed employing UV, NMR, FTIR, GC, and GC/MS analysis. All hydrogen containing compounds showed decreased photostability compared to the fully perfluorinated compounds. Irradiation in the presence of atmospheric oxygen showed reduced photodarkening compared to deoxygenated samples. Irradiations were performed at 157 nm, 172 nm, 185 nm, and 254 nm and showed only minor wavelength dependence. Mechanisms for photodegradation of the fluorocarbons were proposed, where Rydberg excited states are involved.
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