Lowest Singlet State Research Articles

Intersystem crossing and internal conversion dynamics with GAIMS-TeraChem: Excited state relaxation in 2-cyclopentenone.

Excited states relaxation in complex molecules often involves two types of nonradiative transitions, internal conversion (IC) and intersystem crossing (ISC). In the situations when the timescales of IC and ISC are comparable, an interplay between these two types of transitions can lead to complex nonadiabatic dynamics on multiple electronic states of different characters and spin multiplicities. We demonstrate that the generalized ab initio multiple spawning (GAIMS) method interfaced with the fast graphics processing unit-based TeraChem electronic structure code can be used to model such nonadiabatic dynamics involving both the IC and ISC transitions in molecules of moderate size. We carried out 1500fs GAIMS simulations leading to the creation of up to 2500 trajectory basis functions to study the excited states relaxation in 2-cyclopentenone. After a vertical excitation from the ground state to the bright S2 state, the molecule quickly relaxes to the S1 state via conical intersection. The following relaxation proceeds along two competing pathways: one involves IC to the ground state, and the other is dominated by ISC to the low-lying triplet states. The time constants describing the population transfer between the six lowest singlet and triplet states predicted by the GAIMS dynamics are in good agreement with the characteristic times of IC and ISC obtained from the analysis of the time-resolved photoelectron spectrum.

Diradicals or Zwitterions: The Chemical States of m -Benzoquinone and Structural Variation after Storage of Li Ions

Diradicals or Zwitterions: The Chemical States of <i>m</i> -Benzoquinone and Structural Variation after Storage of Li Ions

Persistent Organic White‐Emitting Afterglow from Ultralong Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence

AbstractThe current organic white light emission is unabiding and excited by UV light. This work presents the first ultralong organic room‐temperature white‐emitting crystal from 2,6‐di(carbazol‐9‐yl)benzonitrile that emits prompt blue light and delayed white light. Impressively, this white afterglow is nondiscoloring and exhibits high photoluminescence (PL) efficiency over 6% and ultralong lifetime over 900 ms and can be excited at wide wavelengths of 300–520 nm. Single‐crystal analysis and room‐ and low‐temperature PL reveal that this V‐type double D–A molecule can adopt different twisted conformations to pack together and form two kinds of ultralong room‐temperature afterglow crystals, and the vertical conformation can greatly reduce the energy gap between the lowest singlet and triplet states and prolong the reverse intersystem crossing to enable persistent and visible‐light excitable hybrid white‐emitting afterglow consisting of ultralong thermally activated delayed fluorescence and room‐temperature phosphorescence.

An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence

1,3,2,4-Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground- and excited-state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4-diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2 N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2 N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.

An Element‐Substituted Cyclobutadiene Exhibiting High‐Energy Blue Phosphorescence

Abstract1,3,2,4‐Diazadiboretidine, an isoelectronic heteroanalogue of cyclobutadiene, is an interesting chemical species in terms of comparison with the carbon system, whereas its properties have never been investigated experimentally. According to Baird's rule, Hückel antiaromatic cyclobutadiene acquires aromaticity in the lowest triplet state. Here we report experimental and theoretical studies on the ground‐ and excited‐state antiaromaticity/aromaticity as well as the photophysical properties of an isolable 1,3,2,4‐diazadiboretidine derivative. The crystal structure of the diazadiboretidine derivative revealed that the B2N2 ring adopts a planar rhombic geometry in the ground state. Yet, theoretical calculations showed that the B2N2 ring turns to a square geometry with a nonaromatic character in the lowest triplet state. Notably, the diazadiboretidine derivative has the lowest singlet and triplet states lying at close energy levels and displays blue phosphorescence.

Multifunctional derivatives of donor-substituted perfluorobiphenyl for OLEDs and optical oxygen sensors

New donor-acceptor and donor-acceptor-donor compounds based on perfluorobiphenyl electron acceptor and diphenylamine or carbazole as electron-donating units were designed, synthesized and characterized. The influence of the nature and number of electron-donating units on thermal, electrochemical and photophysical properties was studied. The compounds exhibited photoluminescence ranging from near-UV to green region. The solid samples of the compounds demonstrated photoluminescence quantum yields in the range from 0.1 to 0.51.2,7-Dimethoxycarbazole-containing compound was characterized by negligible energy gaps between the lowest singlet and triplet states (0.1 eV) and subsequently showed thermally activated delayed fluorescence (TADF). Carbazolyl-substituted perfluorobiphenyl derivatives showed aggregation-induced emission enhancement and high values of the energies of triplet levels (up to 3.08 eV). The derivatives exhibited from moderate to high temperatures of 5% weight loss up to 356 °C. Electron mobility up to 1.2 × 10−4 cm2 V−1 s−1 at electric field of 7 × 105 V/cm was detected for the studied compounds by time-of-flight measurements. One compound with the best combination of the required properties for the OLED hosts was selected for the fabrication of green TADF-based OLED. The device showed maximum external quantum efficiency of 15.1%. Another compound exhibiting efficient TADF demonstrated oxygen sensitivity with non-linear Stern-Volmer curve which was well fitted by two Stern-Volmer constants of 6.54·10−4 and 7.17·10−6 ppm−1.

Adaptive Aromaticity in Metallasilapentalynes

Cyclic molecules with 4n + 2 or 4n electrons are aromatic in the lowest singlet state (S0) or the lowest triplet state (T1) according to Huckel and Baird’s rules. Thus, the design of aromatic speci...

Neural Network Representation of Three-State Quasidiabatic Hamiltonians Based on the Transformation Properties from a Valence Bond Model: Three Singlet States of H3+

A neural network (NN) approach was recently developed to construct accurate quasidiabatic Hamiltonians for two-state systems with conical intersections. Here, we derive the transformation properties of elements of 3 × 3 quasidiabatic Hamiltonians based on a valence bond (VB) model and extend the NN-based method to accurately diabatize the three lowest electronic singlet states of H3+, which exhibit the avoided crossing between the ground and first excited states and the conical intersection between the first and second excited states for equilateral triangle configurations (D3h). The current NN framework uses fundamental invariants (FIs) as the input vector and appropriate symmetry-adapted functions called covariant basis to account for the special symmetry of complete nuclear permutational inversion (CNPI). The resulting diabatic potential energy matrix (DPEM) can reproduce the ab initio adiabatic energies, energy gradients, and derivative couplings between adjacent states as well as the particular symmetry. The accuracy of DPEM is further validated by full-dimensional quantum dynamics calculations. The flexibility of the FI-NN approach based on the VB model shows great potential to resolve diabatization problems for many extended and multistate systems.

Adaptive aromaticity in 16-valence-electron metallazapentalenes.

According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S0) or in the lowest-lying triplet ππ* excited state (T1). Thus, species with aromaticity both in S0 and T1 states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S0 and T1 states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity.

Three states global fittings with improved long range: singlet and triplet states of H.

Full dimensional analytical fits of the coupled potential energy surfaces for the three lower singlet and triplet adiabatic states of H are developed, providing analytic derivatives and non-adiabatic coupling matrix elements. The fits are highly accurate and include an improved description of the long range interactions, including new terms for the description of the long range in the diatomic fits and the atom-diatom dissociation channels. The fits are based on the DIM formalism including three body terms in Hamiltonian matrix elements, each of them obeying S2 permutational symmetry, where the positive charge is placed in either of the three hydrogen atoms, but the full system obeys S3 permutational symmetry, invariant under all permutations of the nuclei. The ab initio points used in the fitting are obtained from a complete basis set extrapolation, made for all electronic states. Total root mean square errors of the fits are 27 and 12 cm-1, for the singlet and triplet states, respectively. The errors in the channels are lower than 2 cm-1 and 6 cm-1 for the H + H and H+ + H2 channels respectively. The new fits have been used to calculate the rovibrational bound states of the lowest singlet and lowest triplet states showing very good agreement with previous calculations in the literature.

Osmapentalyne and osmapentalene complexes containing boron monofluoride ligands: structure, bonding and adaptive aromaticity

Adaptive aromaticity, being aromatic in both the lowest singlet and triplet states, is found in a BF-containing osmapentalene complex.

Electron Affinity and Electronic Structure of Hexafluoroacetone (HFA) Revealed by Photodetaching the [HFA]•- Radical Anion.

A great deal of effort has been focused on developing a metal-free catalytic system for epoxidation of unreactive alkenes. Fluoroketones are thought as remarkably promising catalysts for epoxidation reactions. The combination of fluorinated alcohols and catalytic amounts of hexafluoroacetone (HFA) gives a versatile and effective medium for epoxidation of various olefins with hydrogen peroxide. However, the fundamental physicochemical properties of HFA remained largely unclear, although they were very important to understand the related interactions. Here, we performed a joint study on the electron affinity and electronic structure of HFA employing negative ion photoelectron (NIPE) spectroscopy and quantum chemistry calculations. Two distinct bands with complicated vibrational progressions were observed in the 193 nm NIPE spectrum. The adiabatic/vertical detachment energies (ADE/VDE) were derived to be 1.42/2.06 and 4.43/4.86 eV for the ground singlet state and excited triplet state, respectively. Using the optimized geometries and vibrational frequencies of the anion and the neutral, the Franck-Condon factors were calculated for electron detachments to produce HFA in its lowest singlet and triplet states. Good agreements are obtained hereby for both bands between the experimental and calculated NIPE spectra, when taking into account combination vibrational excitations, unequivocally revealing that HFA possesses a singlet ground state with a giant singlet-triplet energy difference (ΔEST). The electron affinity (EA) and ΔEST of HFA were therefore determined to be EA = 1.42 ± 0.02 eV and ΔEST = -3.01 eV.

Neural Network Based Quasi-diabatic Representation for S0 and S1 States of Formaldehyde.

A neural network based quasi-diabatic potential energy matrix Hd that describes the photodissociation of formaldehyde involving the two lowest singlet states S0 and S1 is constructed. It has strict complete nuclear permutation inversion symmetry encoded and can reproduce high level ab initio electronic structure data, including energies, energy gradients, and derivative couplings, with excellent accuracy. It has been fully saturated in the configuration space to cover all possible reaction pathways with a trajectory-guided point sampling approach. This Hd will not only enable the accurate full-dimensional dynamic simulations of the photodissociation of formaldehyde involving S0 and S1 but also provide a crucial ingredient for incorporating spin-orbit couplings into a diabatic framework, thus ultimately enabling the study of both internal conversion and intersystem crossing in formaldehyde on the same footing.

A model electronic Hamiltonian to describe low-lying d-d and metal-to-ligand charge-transfer excited states of [Fe(bpy)3 ]2.

A simple practical method to compute both d-d and metal-to-ligand charge-transfer (MLCT) excited states of iron(II) polypyridyl complexes is proposed for use in simulation studies. Specifically, a model electronic Hamiltonian developed previously for d-d excited states of [Fe(bpy)3 ]2+ is extended to deal with low-lying MLCT excited states simultaneously by including the MLCT electronic configurations into the basis functions of the model Hamiltonian. As a first attempt, parameters in the model Hamiltonian matrix elements are determined by using density functional theory (DFT) and time-dependent (TD-)DFT calculation results as benchmarks. To examine the performance of the model Hamiltonian, the potential energy curves along the interpolation between the lowest singlet and quintet state structures are compared to those from the (TD-)DFT calculations and to those from CASPT2 calculations in literature. The electronic absorption spectrum computed through molecular dynamics simulation is compared to the experimental spectrum. The spin-orbit couplings at the ground state structure are also compared to those from wavefunction-based ab initio electronic structure calculations. The results indicate that the constructed model Hamiltonian provides reasonable information on both the low-lying d-d and MLCT excited states of [Fe(bpy)3 ]2+ .

Observation of “Outlaw” Dual Aromaticity in Unexpectedly Stable Open-Shell Metal Clusters Caused by Near-Degenerate Molecular Orbital Coupling

The Huckel’s rule, Baird’s rule, and electronic shell closure model are classical and well-established concepts in chemistry, which have long been employed in rationalizing the aromaticity/antiarom...

The vacuum ultraviolet spectrum of cyclohepta-1, 3, 5-triene: Analysis of the singlet and triplet excited states by ab initio and density functional methods.

The vacuum ultraviolet (VUV) spectrum for cyclohepta-1,3,5-triene up to 10.8 eV shows several broad bands, which are compared with electron impact spectra. Local curve fitting exposed groups of sharp vibrational peaks, which are assigned to Rydberg states. The vertical excitation profile of the VUV spectrum, reproduced by time dependent density functional theory (TDDFT), gives a good interpretation of the principal regions of absorption. Fourth order Möller-Plessett perturbation theory, including single, double, and quadruple excitations, showed that the lowest singlet and triplet states retain CS symmetry. This contrasts with TDDFT where several low-lying excited states are planar. Detailed vibrational analysis of the first UV band was performed by Franck-Condon, Herzberg-Teller, and their combined methods. These show the dominance of mid-range frequencies, while the lowest frequency (75 cm-1) has negligible importance. In contrast, the second excited (Rydberg) state shows a major progression with separations of 115 (6) cm-1. This is interpreted by re-analysis of the X2A' ionic state at the anharmonic level. Extremely low exponent Gaussian functions enabled several low-lying Rydberg state energies to be determined theoretically; extrapolation of the 3s-, 4s-, and 5s-Rydberg state calculated energies gives the adiabatic ionization energy as 7.837 eV (4) with δ 0.964 (2). Similarly, extrapolation of the centroids of the observed Rydberg states gave the vertical ionization energy (VIE) as VIE1 = 8.675 ± 0.077, close to the photoelectron spectroscopy VIE value [8.55 (1) eV].

The energy transfer model of nonphotochemical quenching: Lessons from the minor CP29 antenna complex of plants

Antenna complexes in photosystems of plants and green algae are able to switch between a light-harvesting unquenched conformation and a quenched conformation so to avoid photodamage. When the switch is activated, nonphotochemical quenching (NPQ) mechanisms take place for an efficient deactivation of excess excitation energy. The molecular details of these mechanisms have not been fully clarified but different hypotheses have been proposed. Among them, a popular one involves excitation energy transfer (EET) from the singlet excited Chls to the lowest singlet state (S1) of carotenoids. In this work, we combine such model with μs-long molecular dynamics simulations of the CP29 minor antenna complex to investigate how conformational fluctuations affect the electronic couplings and the final EET quenching. The computational framework is applied to both CP29 embedding violaxanthin and zeaxantin in its L2 site. Our results demonstrate that the EET model is rather insensitive to physically reasonable variations in single chlorophyll-carotenoid couplings, and that very large conformational changes would be needed to see the large variation of the complex lifetime expected in the switch from light-harvesting to quenched state. We show, however, that a major role in regulating the EET quenching is played by the S1 energy of the carotenoid, in line with very recent spectroscopy experiments.

Achieving Adaptive Aromaticity in Cyclo[10]carbon by Screening Cyclo[n]carbon (n=8-24).

Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8-24), which is supported by nucleus-independent chemical shift (NICS), anisotropy of the current-induced density (ACID), π contribution of electron localization function (ELFπ ) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in-plane ππ* excitation. Thus, the major contribution to the aromaticity from out-of-plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out-of-plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon.

The longer timescale excited state dynamics of isolated benzene.

The excited state photodynamics of isolated benzene have been studied in the nanosecond range by two-step photoionization through various vibrations of the lowest singlet state, with imaging photoelectron spectroscopy detection. Photoelectron spectra are measured as a function of pump-probe delay time, and their time evolution is successfully compared to a biexponential decay function without regard to a particular kinetic model. The only reasonable kinetic model with only two exponentials is the one that involves an intersystem crossing from S1 to T1, although that model has previously been called into question by high-resolution studies that failed to find any singlet-triplet perturbations in Zeeman studies of the S1 spectrum. That contradiction remains unresolved.

Photonics of Molecular Oxygen in Aqueous Solutions

The main stages in the development of research in the field of molecular-oxygen photonics and the most important methodical approaches to investigation of the lowest singlet states of oxygen molecules in organic and aqueous solutions are briefly outlined. The contemporary views on the kinetic and radiative parameters of singlet oxygen in water and deuterium oxide as well as the results of most recent studies of the red and IR absorption bands of oxygen molecules dissolved in water under normal conditions are presented. The data are compared with the absorption and luminescence parameters of oxygen in the gas phase and in organic media. The importance of the results of these studies for specialists in various fields of knowledge from molecular photophysics and spectroscopy to photodynamic and laser medicine is emphasized.