Tuned Range-Separated Density Functional Theory and Dyson Orbital Formalism for Photoelectron Spectra

© 2018 American Chemical Society. Photoelectron spectroscopy represents a valuable tool to analyze structural and dynamical changes in molecular systems. Comprehensive interpretation of experimental data requires, however, involvement of reliable theoretical modeling. In this work, we present a protocol based on the combination of well-established linear-response time-dependent density functional theory and Dyson orbital formalism for the accurate prediction of both ionization energies and intensities. Essential here is the utilization of the optimally tuned range-separated hybrid density functionals, improving the ionization potentials not only of frontier but also of the deeper lying orbitals. In general, the protocol provides accurate results as illustrated by comparison to experiments for several gas-phase molecules, belonging to different classes. Further, we analyze possible pitfalls of this approach and, namely, discuss the ambiguities in the choice of optimal range-separation parameters, the influence of the stability of the ground state, and the spin contamination issues as possible sources of inaccuracies

Effective quenching and excited-state relaxation of a Cu(I) photosensitizer addressed by time-resolved spectroscopy and TDDFT calculations

© 2018 Elsevier B.V. Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance, a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen, an appearing absorption band is assigned to the oxidized copper complex demonstrating that an electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 30% of the quenching events result in an electron transfer while the other 70% experience deactivation indicating that the photocatalytic performance might suffer from geminate recombination

Tautomerism in large databases

We have used the Chemical Structure DataBase (CSDB) of the NCI CADD Group, an aggregated collection of over 150 small-molecule databases totaling 103.5 million structure records, to conduct tautomerism analyses on one of the largest currently existing sets of real (i.e. not computer-generated) compounds. This analysis was carried out using calculable chemical structure identifiers developed by the NCI CADD Group, based on hash codes available in the chemoinformatics toolkit CACTVS and a newly developed scoring scheme to define a canonical tautomer for any encountered structure. CACTVS’s tautomerism definition, a set of 21 transform rules expressed in SMIRKS line notation, was used, which takes a comprehensive stance as to the possible types of tautomeric interconversion included. Tautomerism was found to be possible for more than 2/3 of the unique structures in the CSDB. A total of 680 million tautomers were calculated from, and including, the original structure records. Tautomerism overlap within the same individual database (i.e. at least one other entry was present that was really only a different tautomeric representation of the same compound) was found at an average rate of 0.3% of the original structure records, with values as high as nearly 2% for some of the databases in CSDB. Projected onto the set of unique structures (by FICuS identifier), this still occurred in about 1.5% of the cases. Tautomeric overlap across all constituent databases in CSDB was found for nearly 10% of the records in the collection

Tuned Range-Separated Density Functional Theory and Dyson Orbital Formalism for Photoelectron Spectra

© 2018 American Chemical Society. Photoelectron spectroscopy represents a valuable tool to analyze structural and dynamical changes in molecular systems. Comprehensive interpretation of experimental data requires, however, involvement of reliable theoretical modeling. In this work, we present a protocol based on the combination of well-established linear-response time-dependent density functional theory and Dyson orbital formalism for the accurate prediction of both ionization energies and intensities. Essential here is the utilization of the optimally tuned range-separated hybrid density functionals, improving the ionization potentials not only of frontier but also of the deeper lying orbitals. In general, the protocol provides accurate results as illustrated by comparison to experiments for several gas-phase molecules, belonging to different classes. Further, we analyze possible pitfalls of this approach and, namely, discuss the ambiguities in the choice of optimal range-separation parameters, the influence of the stability of the ground state, and the spin contamination issues as possible sources of inaccuracies

The effect of N-heterocyclic carbene units on the absorption spectra of Fe(ii) complexes: A challenge for theory

© the Owner Societies. The absorption spectra of five Fe(ii) homoleptic and heteroleptic complexes containing strong sigma-donating N-heterocyclic carbene (NHC) and polypyridyl ligands have been theoretically characterized using a tuned range-separation functional. From a benchmark comparison of the obtained results against other functionals and a multiconfigurational reference, it is concluded that none of the methods is completely satisfactory to describe the absorption spectra. As a compromise using 20% exact exchange, the electronic excited states underlying the absorption spectra are analyzed. The low-lying energy band of all the compounds shows predominant metal-To-ligand charge transfer (MLCT) character while the triplet excited states have metal-centered (MC) nature, which becomes more pronounced with increasing the number of NHC-donor groups. Excited MC states with partial charge transfer to the NHC-donor groups are higher in energy than comparable states without these contributions. The presence of the low-lying MC states prevents the formation of long-lived MLCT states. This journal i

Optimized long-range corrected density functionals for electronic and optical properties of bare and ligated CdSe quantum dots

The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parametrized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered Cd33Se33 quantum dots are investigated using the optimally tuned LC-BLYP functional. The range separation parameter, which determines the switching on of the exact exchange contribution, is found to be 0.12 bohr-1 and 0.09 bohr-1 for the bare and covered quantum dot, respectively. It is shown that density functional optimization indeed yields optical and fundamental gaps and thus exciton binding energies, considerably different compared with standard functionals such as the popular PBE and B3LYP ones. This holds true, despite the well established fact that the leading transitions are localized on the quantum dot and do not show pronounced long-range charge transfer character. 2016 American Chemical SocietyGrant: This work was made possible by NPRP grant #NPRP 7-227-1- 034 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.Scopu

Effective quenching and excited-state relaxation of a Cu(I) photosensitizer addressed by time-resolved spectroscopy and TDDFT calculations

© 2018 Elsevier B.V. Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance, a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen, an appearing absorption band is assigned to the oxidized copper complex demonstrating that an electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 30% of the quenching events result in an electron transfer while the other 70% experience deactivation indicating that the photocatalytic performance might suffer from geminate recombination

Optimized long-range corrected density functionals for electronic and optical properties of bare and ligated CdSe quantum dots

The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parametrized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered Cd33Se33 quantum dots are investigated using the optimally tuned LC-BLYP functional. The range separation parameter, which determines the switching on of the exact exchange contribution, is found to be 0.12 bohr-1 and 0.09 bohr-1 for the bare and covered quantum dot, respectively. It is shown that density functional optimization indeed yields optical and fundamental gaps and thus exciton binding energies, considerably different compared with standard functionals such as the popular PBE and B3LYP ones. This holds true, despite the well established fact that the leading transitions are localized on the quantum dot and do not show pronounced long-range charge transfer character

Ground- And Excited-State Properties of Iron(II) Complexes Linked to Organic Chromophores

© Two new bichromophoric complexes, [Fe(bim-ant)2]2+ and [Fe(bim-pyr)2]2+ ([H2-bim]2+ = 1,1′-(pyridine-2,6-diyl)bis(3-methyl-1H-imidazol-3-ium); ant = 9-anthracenyl; pyr = 1-pyrenyl), are investigated to explore the possibility of tuning the excited-state behavior in photoactive iron(II) complexes to design substitutes for noble-metal compounds. The ground-state properties of both complexes are characterized thoroughly by electrochemical methods and optical absorption spectroscopy, complemented by time-dependent density functional theory calculations. The excited states are investigated by static and time-resolved luminescence and femtosecond transient absorption spectroscopy. Both complexes exhibit room temperature luminescence, which originates from singlet states dominated by the chromophore (1Chrom). In the cationic pro-ligands and in the iron(II) complexes, the emission is shifted to red by up to 110 nm (5780 cm-1). This offers the possibility of tuning the organic chromophore emission by metal-ion coordination. The fluorescence lifetimes of the complexes are in the nanosecond range, while triplet metal-to-ligand charge-transfer (3MLCT) lifetimes are around 14 ps. An antenna effect as in ruthenium(II) polypyridine complexes connected to an organic chromophore is found in the form of an internal conversion within 3.4 ns from the 1Chrom to the 1MLCT states. Because no singlet oxygen forms from triplet oxygen in the presence of the iron(II) complexes and light, efficient intersystem crossing to the triplet state of the organic chromophore (3Chrom) is not promoted in the iron(II) complexes

Distinct photodynamics of κ-N and κ-C pseudoisomeric iron(ii) complexes

Two closely related FeIIcomplexes with 2,6-bis(1-ethyl-1H-1,2,3-triazol-4yl)pyridine and 2,6-bis(1,2,3-triazol-5-ylidene)pyridine ligands are presented to gain new insights into the photophysics of bis(tridentate) iron(ii) complexes. The [Fe(N^N^N)2]2+pseudoisomer sensitizes singlet oxygen through a MC state with nanosecond lifetime after MLCT excitation, while the bis(tridentate) [Fe(C^N^C)2]2+pseudoisomer possesses a similar3MLCT lifetime as the tris(bidentate) [Fe(C^C)2(N^N)]2+complexes with four mesoionic carbenes