Excited Triplet Electronic States Research Articles

Accurate absolute absorption cross-sections of the ozone Wulf bands at 1 µm range: measurements with high-resolution cw-CRDS laser techniques

Ozone plays a major role in atmospheric chemistry and radiative processes with an impact on climate and ecosystems being also considered as a possible biosignature species in exoplanetary observations. This makes important an extension of accurate laboratory data to various spectral ranges. The present work provides absolute absorption cross-sections of ozone accurate to about 3% in the near-IR range around 1 µm using for the first time a continuous-wave cavity-ring-down spectrometer (cw-CRDS) for recording the transitions above the first dissociation threshold of O3. This has permitted high-sensitivity and high-resolution measurements covering the region from 9745 to 10850 cm−1 corresponding to the transitions from the electronic ground state to spin-rovibronic levels of excited triplet electronic states 3A2 and 3B2 that form the Wulf band system. Two complementary methods based on the stationary recording with the quasi-continuous laser tuning and on the pressure ramp measurements at carefully chosen grids of wavenumbers were implemented and provided very well consistent results within 2% on average. The experimental integrated absorption intensity in the range under the study is evaluated as 1.393×10−20 (± 2%) cm/molecule. The comparison with the available published data of continuous recordings shows that our absorption cross-sections are much more accurate, particularly in the region with complex partly resolved spin-rovibronic spectral patterns. This region corresponds to a transparency near-IR window of the water vapor and is thus well suited for atmospheric applications. The theoretical modeling of the measured spectra using eight rovibronic bands is discussed. The experimental absorption cross-sections at the dense grid of wavenumbers are provided in the electronic supplementary files, which could be used as new reference data in the 9745 to 10850 cm−1 range.

Twisted Carotenoids Do Not Support Efficient Intramolecular Singlet Fission in the Orange Carotenoid Protein.

Singlet exciton fission is the spin-allowed generation of two triplet electronic excited states from a singlet state. Intramolecular singlet fission has been suggested to occur on individual carotenoid molecules within protein complexes provided that the conjugated backbone is twisted out of plane. However, this hypothesis has been forwarded only in protein complexes containing multiple carotenoids and bacteriochlorophylls in close contact. To test the hypothesis on twisted carotenoids in a "minimal" one-carotenoid system, we study the orange carotenoid protein (OCP). OCP exists in two forms: in its orange form (OCPo), the single bound carotenoid is twisted, whereas in its red form (OCPr), the carotenoid is planar. To enable room-temperature spectroscopy on canthaxanthin-binding OCPo and OCPr without laser-induced photoconversion, we trap them in a trehalose glass. Using transient absorption spectroscopy, we show that there is no evidence of long-lived triplet generation through intramolecular singlet fission despite the canthaxanthin twist in OCPo.

Spin–Orbit Couplings for Nonadiabatic Molecular Dynamics at the ΔSCF Level

A procedure for the calculation of spin-orbit coupling (SOC) at the delta self-consistent field (ΔSCF) level of theory is presented. Singlet and triplet excited electronic states obtained with the ΔSCF method are expanded into a linear combination of singly excited Slater determinants composed of ground electronic state Kohn-Sham orbitals. This alleviates the nonorthogonality between excited and ground electronic states and introduces a framework, similar to the auxiliary wave function at the time-dependent density functional theory (TD-DFT) level, for the calculation of observables. The ΔSCF observables of the formaldehyde system were compared to reference TD-DFT values. Our procedure gives all components (energies, gradients, nonadiabatic couplings, and SOC terms) at the ΔSCF level of theory for conducting efficient, full-atomistic nonadiabatic molecular dynamics with intersystem crossing, particularly in condensed phase systems.

Aggregation-Induced Radical of Donor-Acceptor Organic Semiconductors.

Narrow bandgap donor-acceptor organic semiconductors are generally considered to show a closed-shell singlet ground state, and their radicals are reported as impurities, defects, polarons, and charge transfer monoradicals. Herein, we systematically investigated the open-shell singlet diradical electronic ground state of two diketopyrrolopyrrole-based compounds via the combination of electron spin resonance (ESR), nuclear magnetic resonance, superconducting quantum interference device magnetometry, and theoretical calculations. It is widely known that the quinoidal character will be significantly enhanced in the aggregation state accompanied by improved planarity and enhanced delocalization. We proposed an aggregation-induced radical and captodative effect as the driving force for the formation and stabilization of the open-shell quinoid diradical based on the ESR test in different proportions of mixed solvents. Our results provided a novel view for understanding the intrinsic chemical structure of donor-acceptor organic semiconductors, the open-shell singlet and thermally excited triplet electronic states, and the unexpected physical processes between the ground state and the excited state.

The Role of Triplet States in the Photodissociation of a Platinum Azide Complex by a Density Matrix Renormalization Group Method.

Platinum azide complexes are appealing anticancer photochemotherapy drug candidates because they release cytotoxic azide radicals upon light irradiation. Here we present a density matrix renormalization group self-consistent field (DMRG-SCF) study of the azide photodissociation mechanism of trans,trans,trans-[Pt(N3)2(OH)2(NH3)2], including spin–orbit coupling. We find a complex interplay of singlet and triplet electronic excited states that falls into three different dissociation channels at well-separated energies. These channels can be accessed either via direct excitation into barrierless dissociative states or via intermediate doorway states from which the system undergoes non-radiative internal conversion and intersystem crossing. The high density of states, particularly of spin-mixed states, is key to aid non-radiative population transfer and enhance photodissociation along the lowest electronic excited states.

Excited-State Aromaticity Reversals in Möbius Annulenes.

It is suggested that Möbius annulenes follow a rule similar to Baird's rule such that the 4n and 4n + 2 criteria for Möbius electronic ground-state aromaticity and antiaromaticity are reversed in the lowest triplet and first singlet excited electronic states. Support comes from an investigation of aromaticity in the ground (S0), lowest triplet (T1), and first singlet excited (S1) electronic states of the Möbius-aromatic cyclononatetraenyl cation, C9H9+, using isotropic magnetic shielding isosurfaces calculated with state-optimized complete-active-space self-consistent field wave functions constructed from gauge-including atomic orbitals. Examination of these isosurfaces demonstrates that while the S0 state of C9H9+ is aromatic, the T1 and S1 states are antiaromatic.

Triplet Population Dynamics of Single Conjugated Polymer Molecules and Nanoscale Assemblies

The formation of long-lived triplet excited electronic states has important ramifications for conjugated organic materials used in optoelectronic devices. In the case of polymers, unravelling vario...

Effect of spin-orbit interaction on recombination luminescence of dye in films of halogen-containing derivative poly-N-epoxypropylcarbazole

A photoluminescence kinetics of a cationic polymethine dye in composite polymer films based on halogen-containing derivatives poly-N-epoxypropylcarbazole (PEPC) has been studied. The observed luminescence kinetics is formed by the fluorescence of the dye and recombination luminescence associated with the formation and recombination of electron-hole pairs (EHP). It was shown, that measuring the external magnetic field effect on the kinetics of dye luminescence in a wide time range made it possible to determine the characteristic times of the recombination processes with the involvement of EHP formed from singlet and triplet excited electronic states of the dye molecules and main channels of electron energy transformation in composite films. An enhancement of the spin-orbit interaction in polymers with different heavy atoms leads to an increase in a value of the magnetic effect (g(B)) in the nanosecond time range and to a decrease in g(B) value in the microsecond time range.

The Influence of the Electronic Structure Method on Intersystem Crossing Dynamics. The Case of Thioformaldehyde.

The ability of different electronic structure methods to correctly describe intersystem crossing dynamics is evaluated, using thioformaldehyde as a test case. Mischievously, all methods considered-ranging from the multireference methods MRCISD, MS-CASPT2, or SA-CASSCF, to the single-reference methods ADC(2), CC2, and TDDFT in different flavors-provide the same state ordering and energies of the low-lying singlet and triplet electronic excited states within an acceptable error of 0.2-0.3 eV. However, the outcome of the nonadiabatic simulations after excitation to the lowest S1 (1 nπ*) state are dramatically different. While MS-CASPT2, ADC(2), BP86, and PBE do not transfer population to the triplet states within 500 fs-consistent with experimental evidence-SA-CASSCF, B3LYP, and BHHLYP predict intersystem crossing yields between 3% and 21% within the same time. The different excited state dynamics can be rationalized by inspecting potential energy profiles along the C-S bond stretch mode and single-triplet energy gaps. It is found that already at a C-S bond length of 1.9 Å, all the single-reference methods struggle to describe the correct asymptotic behavior of the potentials. Moreover, some methods, including SA-CASSCF, obtain incorrect 1 nπ*-3 ππ* energy gaps, leading to compensation of errors (ADC(2), BP86, PBE), or wrong dynamics (SA-CASSCF, B3LYP, BHHLYP). Only the accurate MRCISD and MS-CASPT2 methods are able to describe the C-S bond correctly and thus able to deliver the correct potential energy surfaces and dynamics for the right reason. A correlation with the amount of Hartree-Fock exchange in the density functional and the ease to access the 3 ππ* state from the 1 nπ* are able to explain the different behavior observed for GGA and hybrid functionals. It is thus illustrated that even in the case of a simple molecule, like CH2S, the sole assessment of vertical excitation energies as reliability predictors for nonadiabatic dynamics is inadequate. The reason is that ISC does not occur at the FC geometry, but rather at distorted geometries where the singlet-triplet gaps become small. Hence, a characterization of the potential energy surfaces beyond the Franck-Condon region is mandatory.

QM/MM nonadiabatic dynamics simulation on ultrafast excited-state relaxation in osmium(II) compounds in solution

Photoinduced excited-state relaxation dynamics of organometallic compounds, due to their potential applications in solar cells, organic light-emitting diodes, photocatalysis, etc., have been recently explored by a wealth of ultrafast experimental techniques. However, the corresponding nonadiabatic dynamics simulations are rarely reported. In this work, the early-time excited-state relaxation dynamics of two Os(II) compounds i.e. Os(bpy)3 and Os(bpy)2(dpp) (Os1 and Os2) have been investigated by our recently implemented TD-DFT based generalized surface-hopping dynamics simulation method in combination with the quantum mechanics/molecular mechanics (QM/MM) approach. The metal-to-ligand charge transfer (MLCT) excited singlet states from the Os atom to the bpy and dpp ligands are first populated in the Franck-Condon region. These initially populated excited singlet states will be converted through a series of ultrafast internal conversion and intersystem crossing processes to the lowest triplet state as a result of significant nonadiabatic and spin-orbit couplings among singlet and triplet electronic excited states. The intersystem crossing rates are estimated to be 72 and 53 fs for Os1 and Os2, respectively, which are consistent with experimental measured 100 fs and less than 50 fs. The time-dependent transition density analysis reveals that the MLCT excited states are dominant in the entire relaxation dynamics for either Os1 or Os2; however, the MLCT character changes in these processes. In Os1, the initially populated MLCT state is mainly from the Os atom to one bpy ligand, which is changed to the other two MLCT states. In Os2, the MLCT state that is mainly from the Os atom to the two bpy ligands is converted to the MLCT state from the Os to the dpp ligand. Finally, we have found that inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics of Os1 and Os2; while, the hole transfer is minor. Our results also show that the ligand properties have significant influence on the excited-state relaxation dynamics of organometallic compounds. The insights gained in the present study provide atomistic insights for early-time excited-state relaxation dynamics of Osmium(II) compounds.

Triplet state promoted reaction of SO2 with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes.

The SO2 + H2O reaction is proposed to be the starting process for the oxidation of sulfur dioxide to sulfate in liquid water, although the thermal reaction displays a high activation barrier. Recent studies have suggested that the reaction can be promoted by light absorption in the near UV. We report ab initio calculations showing that the SO2 excited triplet state is unstable in water, as it immediately reacts with H2O through a water-assisted proton coupled electron transfer mechanism forming OH and HOSO radicals. The work provides new insights for a general class of excited-state promoted reactions of related YXY compounds with water, where Y is a chalcogen atom and X is either an atom or a functional group, which opens up interesting chemical perspectives in technological applications of photoinduced H-transfer.

Structures and conformational dynamics of trans- and cis-crotonaldehydes in excited electronic states: A quantum-chemical study

For trans- and cis-crotonaldehydes CH3CHCHCHO in the triplet and singlet lowest excited electronic states of n,π* and π,π* types, the geometries of conformers, conformer energy differences, and conformational dynamics have been addressed using ab initio multi-reference techniques (CASSCF and CASPT2). The conformers of investigated molecules under study have been shown to possess a symmetry plane in 1,3(n,π*) states and to be significantly non-planar in 1,3(π,π*) states. Couplings of asymmetric internal rotations around CC and CC bonds with others large amplitude molecular motions such as methyl torsion, non-planar distortions of CCHO, CCHC, and CCH(CH3) fragments are discussed. The type of coupling between large-amplitude motions has been shown to be connected with the localization of electronic excitation.

The low-lying triplet electronic excited states of TiO2 and ZrO2: A symmetry adapted cluster–configuration interaction (SAC-CI) study

The vertical excitation energies and electron detachment energies to the low-lying triplet excited states of TiO2 and ZrO2 were calculated at the SAC-CI/aug-cc-pVTZ levels. The vertical electron detachment energies to the triplet excited states of TiO2 were compared with previous photoelectron spectra. Possible candidate states for the unassigned bands in the previous photoelectron spectrum were found. Several triplet excited states of TiO2 and ZrO2 were optimized at the SAC-CI/aug-cc-pVDZ and SAC-CI/def2-SVPD levels, respectively. The adiabatic excitation energies and electron detachment energies to the selected triplet states were obtained by single-point calculations at the SAC-CI/def2-TZVPPD level. Density functional theory (BPW91/aug-cc-pVTZ) was also applied to study some of the triplet states for comparison. The calculated electron detachment energies and optimized geometries of the triplet excited states should be useful for studying the photoelectron spectra of TiO2− and ZrO2−.

Photoprotection through ultrafast charge recombination in photochemical reaction centres under oxidizing conditions.

Engineering natural photosynthesis to address predicted shortfalls in food and energy supply requires a detailed understanding of its molecular basis and the intrinsic photoprotective mechanisms that operate under fluctuating environmental conditions. Long-lived triplet or singlet excited electronic states have the potential to cause photodamage, particularly in the presence of oxygen, and so a variety of mechanisms exist to prevent formation of such states or safely dissipate their energy. Here, we report a dramatic difference in spectral evolution in fully reduced and partially oxidized Rhodobacter sphaeroides reaction centres (RCs) following excitation of the monomeric bacteriochlorophyll (BChl) cofactors at 805 nm. Three types of preparation were studied, including RCs purified as protein/lipid nanodiscs using the copolymer styrene maleic acid. In fully reduced RCs such excitation produces membrane-spanning charge separation. In preparations of partially oxidized RCs the spectroscopic signature of this charge separation is replaced by that of an energy dissipation process, including in the majority sub-population of reduced RCs. This process, which appears to take place on both cofactor branches, involves formation of a BChl+/bacteriopheophytin- radical pair that dissipates energy via recombination to a vibrationally hot ground state. The possible physiological role of this dissipative process under mildly oxidizing conditions is considered.This article is part of the themed issue 'Enhancing photosynthesis in crop plants: targets for improvement'.

Electronic structure of SrO molecules with dipole moments and ro-vibrational calculations

Ab initio calculations have been applied to investigate the electronic structure of the SrO molecule. The potential energy curves have been computed in the Born–Oppenheimer approximation for the ground, and the 29 low-lying singlet and triplet excited electronic states. Complete active space self-consistent field, multi-reference configuration interaction have been utilized to investigate these states. The harmonic frequency ωe, the internuclear distance Re, the rotational constant Be, and the electronic energy with respect to the ground state Te have been calculated by using two different techniques of calculation with different bases. The transition dipole moment between some doublet states is used to determine the Einstein spontaneous A21 and induced emission coefficients as well as the spontaneous radiative lifetime τspon, emission wavelength λ21 and oscillator strength f21. The eigenvalues Ev, the rotational constant Bv, the centrifugal distortion constant Dv, and the abscissas of the turning points Rmin and Rmax have been calculated by using the canonical functions approach. The comparison between the values of the present work and those available in the literature for several electronic states shows a very good agreement. Twenty-four new electronic states have been studied here for the first time.

Laser control of the RbCs bond

We show numerically that it is possible to control the bond distance of the RbCs dimer and create laser-induced oscillations using a single chirped pulse. The implementation of the scheme involves the creation of a superposition of the 1 3 Σ + and 2 3 Σ + excited triplet electronic states. We assume that the molecule is aligned and the dynamics is initiated in the bound 2 3 Σ + state. The chirped pulse allows full population transfer and dynamic vibrational trapping in the 1 3 Σ + state. In the adiabatic regime the position of the vibrational wave packet can be fully controlled by the carrier frequency of the field, so that one can determine and fix the bond length at any desired value, or impose particular trajectories, depending on the choice of the chirp.

Suppressing Energy Loss due to Triplet Exciton Formation in Organic Solar Cells: The Role of Chemical Structures and Molecular Packing

In the most efficient solar cells based on blends of a conjugated polymer (electron donor) and a fullerene derivative (electron acceptor),ultrafast formation of charge‐transfer (CT) electronic states at the donor‐acceptor interfaces and efficient separation of these CT states into free charges, lead to internal quantum efficiencies near 100%. However, there occur substantial energy losses due to the non‐radiative recombinations of the charges, mediated by the loweset‐energy (singlet and triplet) CT states; for example, such recombinations can lead to the formation of triplet excited electronic states on the polymer chains, which do not generate free charges. This issue remains a major factor limiting the power conversion efficiencies (PCE) of these devices. The recombination rates are, however, difficult to quantify experimentally. To shed light on these issues, here, an integrated multi‐scale theoretical approach that combines molecular dynamics simulations with quantum chemistry calculations is employed in order to establish the relationships among chemical structures, molecular packing, and non‐radiative recombination losses mediated by the lowest‐energy charge‐transfer states.

Electronic structure with rovibrational and dipole moment calculation of the diatomic molecules AsBr and AsI

CASSCF/MRCI calculations have been performed for the ground and the low-lying singlet and triplet excited electronic states, in the representation 2s+1Λ(+/−) of the diatomic molecules AsBr and AsI. Twenty and seventeen excited electronic states have been investigated respectively for AsBr AsI molecules. The potential energy curves, the electronic energy with respect to the ground state Te, the harmonic frequency ωe, the internuclear distance Re, the rotational constant Be, and the static and transition dipole moment have been investigated for the considered electronic states. The comparison between these constants of the four As-halides has been done along with the energy levels Te for the different electronic states. By using the canonical functions approach, the eigenvalue Ev, the rotational constant Bv, and the abscissas of the turning points Rmin and Rmax have been calculated for the considered electronic states up to the vibrational level v=100. The comparison between the values of the present work with the experimental data available in the literature shows a very good agreement. This study represents the first theoretical investigation of the electronic structure of both AsBr and AsI molecules.

Studies on GeF4 Valence and Rydberg States by Electron Impact Spectroscopy and Ab Initio Calculations.

Electron energy loss (EEL) spectra of GeF4 have been measured with incident electrons at 100 eV for 5° scattering angle and at 30 eV for 30° scattering angle, while sweeping the energy loss over the range 7.0-18.0 eV. Low-lying excited triplet, singlet, valence, and Rydberg states are investigated and the assignments supported by quantum chemical ab initio calculations. This provides the first comprehensive investigation of all singlet and triplet excited electronic states of germanium tetrafluoride up to the first ionization energy. The Rydberg series converging to the (lowest) ionization energy limits of GeF4 are also identified according to the magnitude of the quantum defects (δ).

Photoelectron spectroscopy and density functional theory studies of N-rich energetic materials.

The geometric and electronic structures of molecular anionic energetic materials (EMs) DAAF (3,3'-diamino-4,4'-azoxyfurazan), FOX-7 (1,1-diamino-2,2-dinitroethene), 5,5'-BT (5,5'-bistetrazole), and 1,5'-BT (1,5'-bistetrazole) are explored employing anionic photoelectron spectroscopy and density functional theory calculations. The electron binding energies of the observed anionic, energetic material related species are determined and their corresponding anionic structures are assigned. Decomposition reactions for negatively charged EMs can proceed with different energy barriers, and thus mechanisms, from those for their related neutral EMs. Reactivity based on the anionic initial fragments of these EM species further reinforces their respective highly reactive and explosive nature. Fragment ions of the form EM--H-X (X = N2, N2+NH, …) are additionally observed. Detection of such species suggests that EM--H could serve as promising new candidates for EMs, assuming that such species are synthetically available, perhaps as energetic salts. Vertical detachment energies for transitions to the ground and first triplet electronic excited states of neutral matrix dye anion DCM- are additionally determined.