Higher Excited States Research Articles

Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion.

In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the active catalyst formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion (IC) and vibrational relaxation (VR), also dissipate as much as 20-30% of the absorbed photon energy. Minimization of these energy losses, a holy grail in solar energy conversion and solar fuel production, is a challenging task because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here, we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for IC by means of inter-ligand electron transfer of around 2000 cm-1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligand charge transfer [MLCT(Lm)], and a 2,2'-bipyridine-centered MLCT [MLCT(bpy)], which lies 800-1400 cm-1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA. Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV. Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the corresponding Lm•--centered and bpy•--centered reduced photosensitizers, which involve different reducing abilities, that is, -0.79 and -0.93 V versus NHE for Lm•- and bpy•-, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant for energy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.

Spin-state energetics and magnetic anisotropy in penta-coordinated Fe(III) complexes with different axial and equatorial ligand environments.

The penta-coordinated trigonal-bi-pyramidal (TBP) Fe(III) complex (PMe2Ph)2FeCl3 shows a reduced magnetic anisotropy in its intermediate-spin (IS) state as compared to its methyl-analog (PMe3)2Fe(III)Cl3. In this work, the ligand environment in (PMe2Ph)2FeCl3 is systematically altered by replacing the axial -P with -N and -As, the equatorial -Cl with other halides, and the axial methyl group with an acetyl group. This has resulted in a series of Fe(III) TBP complexes modelled in their IS and high-spin (HS) states. Lighter ligands -N and -F stabilize the complex in the HS state, while the magnetically anisotropic IS state is stabilized by -P and -As at the axial site, and -Cl, -Br, and -I at the equatorial site. Larger magnetic anisotropies appear for complexes with nearly degenerate ground electronic states that are well separated from the higher excited states. This requirement, largely controlled by the d-orbital splitting pattern due to the changing ligand field, is achieved with a certain combination of axial and equatorial ligands, such as -P and -Br, -As and -Br, and -As and -I. In most cases, the acetyl group at the axial site enhances the magnetic anisotropy compared to its methyl counterpart. In contrast, the presence of -I at the equatorial site compromises the uniaxial type of anisotropy of the Fe(III) complex leading to an enhanced rate of quantum tunneling of magnetization.

Effective Field Theories for Dark Matter Pairs in the Early Universe

In this conference paper, we consider effective field theories of non-relativistic dark matter particles interacting with a light force mediator in the early expanding universe. We present a general framework, where to account in a systematic way for the relevant processes that may affect the dynamics during thermal freeze-out. In the temperature regime where near-threshold effects, most notably the formation of bound states and Sommerfeld enhancement, have a large impact on the dark matter relic density, we scrutinize possible contributions from higher excited states and radiative corrections in the annihilations and decays of dark-matter pairs.

Rational design of anti-Kasha photoemission from a biazulene core embedded in an antiaromatic/aromatic hybrid.

The violation of the Kasha photoemission rule in organic molecules has intrigued chemists since their discovery, being always of relevance given its connection with unique electronic properties of molecules. However, an understanding of the molecular structure-anti-Kasha property relationship in organic materials has not been well-established, possibly because of the few existing cases available, limiting their prospective exploration and ad hoc design. Here we introduce a novel strategy to design organic emitters from high excited states combining intramolecular J-coupling of anti-Kasha chromophores with the hindering of vibrationally-induced non-radiative decay channels by enforcing molecular rigidity. We apply our approach to the integration of two antiparallel azulene units bridged with one heptalene all inserted into a polycyclic conjugated hydrocarbon (PCH). With the help of quantum chemistry calculations, we identify a suitable PCH embedding structure and predict its anti-Kasha emission from the third high energy excited singlet state. Finally, steady fluorescence and transient absorption spectroscopy studies corroborate the photophysical properties in a recently synthesized chemical derivative with this pre-designed structure.

Prospects for useful fission from singlet states higher than S1 in aggregated organic chromophores.

The few known reports and the likely prospects of finding new efficient routes to exciton fission from higher excited singlet states, Sn (n > 1), are reviewed. Aggregates of molecules that have large S2-S1 electronic energy spacings and/or emit measurable "contra-Kasha" emission may offer further opportunities. Among these, electronically excited molecular systems that exhibit known efficient (T1 + T1) triplet-triplet annihilation producing S2 could exhibit efficient singlet fission in aggregates when appropriately substituted to meet the necessary energy requirements. The potential problem of loss of triplet excitons via 2T1 → Tn>1 + S0 triplet-triplet annihilation following (S2 + S0) singlet fission is addressed. Aggregates of substituted azulenes and aliphatic thiones and dithiones are particularly attractive and are discussed in detail.

Exploring the potential of Eu3+ and Mn4+ activated LaAlO3 phosphors as red and far-red emitters for horticulture lighting.

The development of efficient red and far-red emitters, for efficient plant absorption in the Photosynthetically Active Radiation (PAR) region, holds significance in contemporary plant growth control. This study focuses on the synthesis and characterization of LaAlO3 as a host material, doped with Eu3+ and Mn4+ ions, using a solid-state reaction method. The investigation encompasses the creation and analysis of both single-doped and co-doped samples, employing techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. XRD analysis consistently confirmed the perovskite-like structure of all samples, devoid of detectable impurities or major structural changes due to doping. SEM images revealed a uniform distribution of regularly shaped particles for the co-doped sample. The PL spectroscopy showed that the doping led to strong photoluminescence, with the co-doped sample exhibiting the intensity of each of the ions independently neither exhibiting quenching nor energy transfer mechanisms. The excitation spectrum of Eu3+ exhibited a broad charge transfer band at approximately 328 nm, coupled with characteristic f-f excitation bands. On the other hand, the Mn4+ ion's excitation spectrum featured transitions from ground state (4A2g) electrons excited to higher excited states (4T1g, 2T2g, and 4T2g) centered at 350 nm and within the region 250-550 nm. The co-doped sample was excited at a common excitation wavelength of 460 nm and underwent an in-depth examination of its photoluminescent properties, including decay curves analysis and time dependence also. The results from this study suggest that the synthesized phosphor materials exhibit substantial potential for diverse applications, including but not limited to solid-state lighting for efficient plant growth.

TICT compounds by design: comparison of two naphthalimide-π-dimethylaniline conjugates of different lengths and ground state geometries.

Two new twisted intramolecular charge transfer (TICT) donor-π-acceptor compounds were designed by combining a well-known electron acceptor naphthalimide unit with a classic electron donor dimethylaniline through two types of different rigid linkers. The combined steady-state and time-resolved spectroscopy of molecules in solvents of different polarities in comparison to solid-state solvation experiments of doped polymer matrixes of different polarities allowed distinguishing between solvation and conformation determined processes. The photophysical measurements revealed that non-polar solutions possess high fluorescence quantum yields of up to 70% which is a property of pre-twisted/planar molecules in the excited charge transfer (CT) states. The increase of polarity allows tuning the Stokes shift through all the visible wavelength range up to 8601 cm-1 which is accompanied by a three orders of magnitude drop of fluorescence quantum yields. This is a result of the emerged TICT states as dimethylaniline twists to a perpendicular position against the naphthalimide core. The TICT reaction of molecules enables an additional non-radiative excitation decay channel, which is not present if the twisting is forbidden in a rigid polymer matrix. Transient absorption spectroscopy was employed to visualize the excited state dynamics and to obtain the excited state reaction constants, revealing that TICT may occur from both the Franck-Condon region and the solvated pre-twisted/planar CT states. Both molecules undergo the same photophysical processes, however, a longer linker and thus a higher excited state dipole moment determines the faster excited state reactions.

Intersystem crossing of perylene bisimide neutral, radical anion, and dianion derivatives compared via ultrafast spectroelectrochemistry.

Perylene bisimides are widely studied due to their various applications. Most research is carried out on neutral molecules but charged species are essential in the context of organic electronics and photovoltaics. In this study, we carry out ultrafast transient absorption spectroelectrochemistry and coherent two-dimensional electronic spectroelectrochemistry on three different neutral perylene bisimide derivatives as well as their singly and doubly reduced species. We compare an unsubstituted, flat parent perylene bisimide with two twisted ones that introduce spin-orbit coupling, leading to enhanced intersystem crossing. The internal conversion from higher excited states to the lowest electronically excited state occurs in the picosecond regime with lifetimes significantly shorter for the charged species compared to the neutral ones. Coherent oscillations of the transients for the flat dianion of parent perylene bisimide indicate the occurrence of Fermi resonance. The corresponding vibrational coupling and the superposition of the participating vibrational modes may lead to an enhanced charge separation and triplet formation.

Micellization Turned on Dual Fluorescence and Room Temperature Phosphorescence by Pseudo-ESIPT in Thiadiazole Derivatives

Some luminophores cannot fluoresce from dilute solution, especially those that are highly flexible. By aggregation, the rigidity of these molecules increases allowing for photoluminescence. However, the enhanced rigidity and the intermolecular interactions can give rise to several high-energy excited states, and since they can fluoresce only from the lowest energy excited state, known as the Kasha rule, the energy surplus is lost to the surroundings. In the pursuit of developing efficient light harvesting strategies from the higher energy excited states, it is necessary to look upon the effects not only at the level of nanometric molecules themselves but also of their supramolecular assemblies. Herein, we pinpoint the impact of poor conjugated moieties over the chromophore units, as well as the optical phenomena emerging at the supramolecular level. The molecular design consists of the S-alkylation of 5-amino-1,3,4-thiadiazole-2-thiol with permethylated silicone or hydrocarbons. The synthesized compounds display excited state intermolecular proton transfer (pseudo-ESIPT), blue and green fluorescence, and near-infrared phosphorescence at room temperature.

Mutual neutralization in H++H− collisions: An improved theoretical model

The total and differential cross sections of mutual neutralization in ${\mathrm{H}}^{+}+{\mathrm{H}}^{\ensuremath{-}}$ collisions are calculated ab initio and fully quantum mechanically for energies between 0.001 and 600 eV. Effects which have not previously been considered in studies on mutual neutralization (MN) for this system, such as inclusion of rotational couplings and autoionization, are investigated. Adiabatic potential curves corresponding to the relevant states of $^{1}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$, $^{1}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$, $^{1}\mathrm{\ensuremath{\Pi}}_{g}$ and $^{1}\mathrm{\ensuremath{\Pi}}_{u}$ symmetries as well as radial and rotational nonadiabatic couplings are computed ab initio. A quasidiabatic model is developed and applied in order to investigate the importance of higher excited states as well as the inclusion of autoionization. Molecular data for the lowest electronic resonant state in each symmetry are obtained by performing electron scattering calculations. It is shown that rotational couplings cause a significant increase of the total MN cross section while autoionization plays a minor role as a loss mechanism. Additionally, a differential cross section is obtained that is symmetric around $\ensuremath{\theta}={90}^{\ensuremath{\circ}}$. This result is in disagreement with a previous theoretical calculation where it was found that the differential cross section is dominated by backwards scattering.

Relativistic ab initio study on the spectroscopic and radiative properties of the lowest states and modeling of the optical cycles for the LiFr molecule

The LiFr diatomic represents a promising candidate for indirect laser cooling that has not yet been investigated not theoretically or experimentally. The potential energy curves of the ground and low-lying excited states of the LiFr heteronuclear alkali metal dimer are calculated using the Fock-space relativistic coupled cluster theory for the first time. A number of properties such as the electronic term energies, equilibrium internuclear distances, transition and permanent dipole moments, sequences of vibrational energies, harmonic vibrational frequencies, Franck–Condon factors, and radiative lifetimes (including bound and free transitions) are predicted. The probabilities of the two-step schemes (optical cycles) for the transfer process of the LiFr molecules from high excited vibrational states to the ground vibronic state are also predicted. The data obtained would be useful for laser cooling and spectral experiments with LiFr molecules.

Tuning Phonon Energies in Lanthanide‐doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion

AbstractOptical applications of lanthanide‐doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size‐controlled synthesis of low‐phonon‐energy KPb2X5 (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm−1. KPb2Cl5 nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb2X5 nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche‐like emission from KPb2Cl5 : Nd3+ nanoparticles. The realization of nanoparticles with tunable, ultra‐low phonon energies facilitates the discovery of nanomaterials with phonon‐dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Tuning Phonon Energies in Lanthanide-doped Potassium Lead Halide Nanocrystals for Enhanced Nonlinearity and Upconversion.

Optical applications of lanthanide-doped nanoparticles require materials with low phonon energies to minimize nonradiative relaxation and promote nonlinear processes like upconversion. Heavy halide hosts offer low phonon energies but are challenging to synthesize as nanocrystals. Here, we demonstrate the size-controlled synthesis of low-phonon-energy KPb2 X5 (X=Cl, Br) nanoparticles and the ability to tune nanocrystal phonon energies as low as 128 cm-1 . KPb2 Cl5 nanoparticles are moisture resistant and can be efficiently doped with lighter lanthanides. The low phonon energies of KPb2 X5 nanoparticles promote upconversion luminescence from higher lanthanide excited states and enable highly nonlinear, avalanche-like emission from KPb2 Cl5 : Nd3+ nanoparticles. The realization of nanoparticles with tunable, ultra-low phonon energies facilitates the discovery of nanomaterials with phonon-dependent properties, precisely engineered for applications in nanoscale imaging, sensing, luminescence thermometry and energy conversion.

Few-Femtosecond C2H4+ Internal Relaxation Dynamics Accessed by Selective Excitation.

Dissociation of the ethylene cation is a prototypical multistep pathway in which the exact mechanisms leading to internal energy conversions are not fully known. For example, it is still unclear how the energy is exactly redistributed among the internal modes and which step is rate-determining. Here we use few-femtosecond extreme-ultraviolet pulses of tunable energy to excite a different superposition of the four lowest states of C2H4+ and probe the subsequent fast relaxation with a short infrared pulse. Our results demonstrate that the infrared pulse photoexcites the cationic ground state (GS) to higher excited states, producing a hot GS upon relaxation, which enhances the fragmentation yield. As the photoexcitation probability of the GS strongly depends on the molecular geometry, the probing by the IR pulse provides information about the ultrafast excited-state dynamics and the type of conical intersection (planar or twisted) involved in the first 20 fs of the nonradiative relaxation.

Ultrafast Dynamics of Fatty Acid Photodecarboxylase in Anionic Semiquinone State.

Fatty acid photodecarboxylase is a newly identified blue-light driven photoenzyme that catalyzes decarboxylation of fatty acids. The catalytic reaction involves a transient anionic semiquinone of flavin cofactor (FAD•-) as an intermediate, but photochemical properties of this anionic radical are largely unknown. Here, we have anaerobically produced the wild-type FAP in the FAD•- state and conducted femtosecond-resolved fluorescence and absorption measurements. We have observed the multiphasic deactivation dynamics of excited states on multiple time scales from a few picoseconds even to a few nanoseconds through conical intersections between various electronic states. Interestingly, the nanosecond components can only be observed from higher electronic excited states. Our results show the complexity of the energy landscapes of various excited states and rule out the occurrence of electron or proton transfer with nearby residue(s) in the active site.

Dual emissive cyclometallated iridium complexes: Synthesis, structure and photophysical properties

Herein, we present a new set of dual emissive heteroleptic cationic cyclometallated iridium complexeDepartment of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal By-pass Road, Bhauri, Bhopal, 462 066, Madhya Pradesh, Indias [(L)2Ir(N^N)]+ ([1]+ - [3]+) using extended π-conjugated 1-isopentyl-2-phenyl-1H-naphtho [2,3-d] imidazole (L) as C^N chelating ligand. The complexes were characterized using various analytical techniques including single crystal X-ray crystallography. The complexes exhibit dual emission behavior (560 nm, 610 and 670 nm) at ambient condition (298 K). The photophysical properties along with DFT results indicate that the origin of emission bands is not only from the lowest energy T1 state but also there is involvement of other higher energy excited triplet states. The DFT study further reveals that the lowest excited triplet state (T1) has 3MLLCT character, whereas the closely lying other higher excited triplet states are predominately of 3LLCT/3ILCT character which could be responsible for such dual emission.

A Thermally Activated Delayed Fluorescence Emitter Investigated by Time-Resolved Near-Infrared Spectroscopy.

Emitters for organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) require small singlet (S1 )-triplet (T1 ) energy gaps as well as fast intersystem crossing (ISC) transitions. These transitions can be mediated by vibronic mixing with higher excited states Sn and Tn (n=2, 3, 4, …). For a prototypical TADF emitter consisting of a triarylamine and a dicyanobenzene moiety (TAA-DCN) it is shown that these higher states can be located energetically by time-resolved near-infrared (NIR) spectroscopy.

Beyond Hard-Core Bosons in Transmon Arrays

Arrays of transmons have proven to be a viable medium for quantum information science and quantum simulations. Despite their widespread popularity as qubit arrays, there remains yet untapped potential beyond the two-level approximation or, equivalently, the hard-core boson model. With the higher excited levels included, coupled transmons naturally realize the attractive Bose-Hubbard model. The dynamics of the model has been difficult to study due to unfavorable scaling of the dimensionality of the Hilbert space with the system size. In this work, we present a framework for describing the effective unitary dynamics of highly-excited states of coupled transmons based on high-order degenerate perturbation theory. This allows us to describe various collective phenomena -- such as bosons stacked onto a single site behaving as a single particle, edge-localization, and effective longer-range interactions -- in a unified, compact and accurate manner. While our examples deal with one-dimensional chains of transmons for the sake of clarity, the theory can be readily applied to more general geometries.

Multistate electronic quenching: Nonadiabatic pathways in NOA2Σ+ + O2X 3Σg - scattering.

The quenching of NOA2Σ+ with O2 as a collisional partner is important for combustion and atmospheric processes. There is still a lack of theoretical understanding of this event, especially concerning the nature of the different quenching pathways. In this work, we provide potential energy surfaces (PESs) of 20 electronic states of this system. We computed the spin-doublet and spin-quartet PESs using SA-CASSCF and XMS-CASPT2. We find two potential quenching pathways. The first one (Q1) is a two-step orientation-specific process. The system first undergoes an electron transfer (NO+X1Σ+ + O2 -X 2Πg) at short distances, before crossing to lower neutral states, such as NOX2Π + O2a 1Δg, O2b 1Σg +, O2X 3Σg -, or even 2 O(3P). The second quenching pathway (Q2) is less orientation-dependent and should be sudden without requiring the proximity conditioning Q1. The Q2 cross section will be enhanced with increasing initial vibrational level in both O2 and NO. It is responsible for the production of NOX2Π with higher O2 excited states, such as O2c 1Σu -, A'3Δu, or A 3Σu +. Overall, this work provides a first detailed theoretical investigation of the quenching of NOA2Σ+ by O2X 3Σg - as well as introduces a weighting scheme generally applicable to multireference, open-shell bimolecular systems. The effect of spin-multiplicity on the different quenching pathways is also discussed.

Electronic Excitations and Low-Energy Electron-Induced Scattering Studies of Acrylonitrile (CH2CHCN).

We report here theoretical investigations on structural, spectroscopic, and electron scattering for acrylonitrile (CH2CHCN), a molecule of importance in astrochemistry as well as the chemical industry. Quantum chemical calculations for ground and excited states are performed using density functional theory (DFT) and time-dependent-DFT methods, respectively. The results of geometry optimization and vibrational frequencies agree well with data available in the literature, while vertical excited singlet-state energies are extended to higher excited states as compared to earlier studies, including Rydberg and valence excitations. Quantum defect analysis and comparison of the theoretically predicted energies with earlier reported experimental works led to the confirmation of some spectral assignments and the revision of a few assignments. Vibrational frequencies calculated for the cationic ground state are used to tentatively assign vibrational bands appearing along with the Rydberg transitions. Triplet excited-state energies for which no data is available in the literature are reported here for the first time. Low-energy (0.1 to 20 eV) electron scattering calculations are performed using the ab initio R-matrix method. Several types of resonances are predicted in the energy-dependent elastic cross-section, most of which are in good agreement with earlier experimental or theoretical works, while a few new resonances are found above 6 eV. Additionally, calculations of eigenphase sum, differential, momentum transfer, electronic excitation, ionization, and total cross-sections are being reported for the first time. This work represents a comprehensive theoretical study of the electronically excited states as well as low-energy electron scattering of acrylonitrile, which would be useful for understanding its chemistry in the interstellar medium as well as industrial applications.