Results Of Time-dependent Density Functional Theory Calculations Research Articles

Substituent effects on the UV-visible spectrum and excited electronic states of dithiocarboxylates.

The absorption spectra of a series of dithiocarboxylates were investigated in the ultraviolet-visible region. Two questions that this study aimed to address were as follows: (1) What transitions give rise to the features in the electronic spectra? And (2) what are the long- and short-range substituent effects on the absorption spectra? A series of 11 dithiocarboxylates were prepared as organic soluble salts. Time-dependent density functional theory (TDDFT) was used to calculate excited state energies and oscillator strengths of electronic transitions. TDDFT at the CAM-B3LYP/def2-TZVPD level of theory predicts two low-energy n → π* transitions and two π → π* transitions at higher energy, consistent with the experimental spectra. This state ordering and density is in contrast to the better studied thiocarbonyls for which only two transitions within the singlet manifold appear in the UV-visible region. For derivatives of dithiobenzoate, the energy of the three lowest energy states are insensitive to changes to substituents para to the dithiocarboxylate group. In contrast, the energy of the highest ππ* state varies by 0.78eV. This work shows that the results of TDDFT calculations can be used to predict the electronic absorption spectra of dithiocarboxylates, providing a useful tool for designing dithiocarboxylate light absorbers.

2-(Benzothiazol-2-yl)pyren-1-ol, a new excited state intramolecular proton transfer-based fluorescent sensor for nitroaromatic compounds

2-(Benzothiazol-2-yl)pyren-1-ol (P3-NS) was developed as a excited-state intramolecular proton transfer (ESIPT)- based sensor for nitroaromatic compounds (NACs). Results of studies of its photophysical and optical properties show that P3-NS exists in various, solvent dependent isomeric forms arising from rotation about the CC bond connecting the benzothiazole and pyren-1-ol rings and the location of the pyrene-OH proton. The results of density functional theory (DFT) calculations enabled identification of the major structures of P3-NS dissolved in different solvents. Upon electronic excitation in nonpolar or weakly polar solvents, P3-NS undergoes an ESIPT reaction to form a product that only weakly fluoresces. In contrast, P3-NS exists in a strongly fluorescent deprotonated form in highly polar solvents. The results of time-dependent DFT calculations indicate that fluorescence quenching of the product generated by ESIPT reaction of P3-NS is caused by intersystem crossing at a conical intersection between S1 and T2 states. The anionic form of P3-NS, present in highly polar solvents, was shown to be a fluorescence sensor for nitroaromatic compounds (NACs). Fluorescence quenching by NACs occurs by protonation of the anionic form of P3-NS (static quenching) and by photoinduced electron transfer from the anionic form to NACs (dynamic quenching).

Structure, spectroscopic properties and catalytic activity for epoxide ring-opening of nickel methylxanthate

The structure and spectroscopic properties of nickel methylxanthate, Ni(S2COCH3)2, as isolated molecule and in crystalline phase, were investigated both experimentally and theoretically. Experimentally, single crystal X-ray diffraction, infrared and ultraviolet–visible spectroscopies have been used, while the applied theoretical approaches include density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. The isolated molecule of the complex under study was found to exist in two nearly degenerated conformers (cis and trans, with the methyl groups of the ligand molecules positioned the same side or in opposite sides of the molecule, respectively). The trans conformer is selected upon crystallization. The crystal structure (at 100 K) was solved and discussed, with emphasis given to the characterization of the main intermolecular interactions. The infrared spectrum (in the 3500-300 cm−1 range) of the crystalline material was assigned in details, also taking into account the results of the normal coordinate analysis performed for the isolated molecule, which in turn was made using the data resulting from the DFT calculations. The UV–visible spectrum of the compound in CHCl3 solution was also obtained and interpreted based on results of TD-DFT calculations, demonstrating the importance of the metal to ligand charge transfer in the studied complex. Finally, the catalytic activity of the complex for ring-opening reaction of styrene oxide resulting from nucleophilic attack of anilines was also evaluated, showing that the compound presents potential as catalyzer of this type of reactions.

ChemInform Abstract: Excited Electronic States and Internal Conversion in Cyanocobalamin

AbstractReview: 39 refs.

Excited electronic states and internal conversion in cyanocobalamin

Abstract Cyanocobalamin (CNCbl) is a paradigm system for the study of excited electronic states and biological cofactors including the B 12 vitamers. The photophysics of CNCbl has been thoroughly investigated using both ultrafast spectroscopy and time dependent density functional theory (TD-DFT). Here we review the spectroscopic and theoretical investigations of CNCbl with emphasis on the nature of S 1 , the lowest excited electronic state, and extend the spectroscopic measurements to include the ultraviolet region of the spectrum. Ultrafast transient absorption measurements in the visible αβ band region and in the mid-infrared led to assignment of the S 1 state to a ligand-to-metal charge transfer (LMCT) with lengthened axial bonds and a ∼3 kcal/mol barrier for internal conversion to the ground state. The present measurements encompassing the γ band region of the spectrum provide further support for the assignment of the S 1 state. The experiments are in good agreement with the results of TD-DFT calculations which confirm the expected lengthening of the axial bonds in S 1 and account for the observed barrier for internal conversion back to the ground state.

Vibration and Fluorescence Spectra of Porphyrin- CoredBis(methylol)-propionic Acid Dendrimers

Bis-MPA dendron-coated free-base tetraphenylporphyrin and zinc-tetraphenyl-porphyrin (TPPH2 and TPPZn) were studied in comparison with simple porphyrins (H2P, ZnP) by theoretical simulation of their infrared, Raman and electronic absorption spectra, as well as fluorescense emission. Infrared and fluorescence spectra of the dendrimers were measured and interpreted along with time-resolved measurements of the fluorescence. The 0–1 emission band of the dendron substituted TPPZn was found to experience a “heavy substitution”-effect. The 0–1 vibronic emission signal is associated with a longer decay time (approx. 7 - 8 ns) than the 0-0 emission (approx. 1 - 1.5 ns). The former contributed with more relative emission yield for larger dendron substituents, in agreement with the appearance of steady-state emission spectra showing increased contribution from the 0–1 vibronic fluorescence band at 650 nm. No such substitution effect was observed in the electronic or vibrational spectra of the substituted free-base variant, TPPH2. Vibration spectra of the parent porphyrins (H2P, ZnP, TPPH2 and TPPZn) were calculated by density functional theory (DFT) using the B3LYP/6-31G** approximation and a detailed analysis of the most active vibration modes was made based on both literature and our own experimental data. Based on the results of theoretical calculations the wide vibronic bands in the visible region were assigned. The vibronic structure also gave a qualitative interpretation of bands in the electronic absorption spectra as well as in fluorescence emission depending on the size of dendrimer substitution. From the results of time-dependent DFT calculations it is suggested that the TPPZn-cored dendrimers indicate strong vibronic interaction and increased Jahn-Teller distortion of the prophyrin core for larger dendrimer generations. Specifically, this leads to the entirely different behaviour of the emission spectra upon substitution of the TPPH2 and TPPZn variants, which was also experimentally observed. Since TPPH2 is originally of lower symmetry the specific distortion upon dendron substitution is not expected to the same extent, which also was in agreement with the experimental findings.

Evidence for Significant Through‐Space and Through‐Bond Electronic Coupling in the 1,4‐Diphenylcyclohexane‐1,4‐diyl Radical Cation Gained by Absorption Spectroscopy and DFT Calculations

Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.

Theoretical analysis of electronic absorption spectra of vitamin B12 models

Time-dependent density-functional theory (TD-DFT) is applied to analyze the electronic absorption spectra of vitamin B12. To accomplish this two model systems were considered: CN-[CoIII-corrin]-CN (dicyanocobinamide, DCC) and imidazole-[CoIII-corrin]-CN (cyanocobalamin, ImCC). For both models 30 lowest excited states were calculated together with transition dipole moments. When the results of TD-DFT calculations were directly compared with experiment it was found that the theoretical values systematically overestimate experimental data by approximately 0.5 eV. The uniform adjustment of the calculated transition energies allowed detailed analysis of electronic absorption spectra of vitamin B12 models. All absorption bands in spectral range 2.0–5.0 eV were readily assigned. In particular, TD-DFT calculations were able to explain the origin of the shift of the lowest absorption band caused by replacement of the–CN axial ligand by imidazole.