Energies Of Singlet States Research Articles

Investigation of the Electronic Structure in GaAs/AlxGa1‐xAs Quantum Dots with Four Electrons

AbstractIn this paper, a detailed analysis of the electronic structure of four‐electron quantum dots is performed with finite confinement potential by a modified variational optimization approach based mainly on the quantum genetic algorithm and the Hartree‐Fock‐Roothaan method. For the ground and higher excited configurations, our analysis covers a range of parameters like the average energies of ground and excited states, singlet and triplet state energies, orbital energies, and two‐electron Coulomb and exchange interaction energies. One‐electron kinetic energy, the Coulomb potential energy between electrons and impurity, the confinement potential energy for the electrons, and the probability of finding an electron inside or outside the quantum well are also studied. The results demonstrate that both spatial confinement and the height of the potential barrier have a pronounced effect on all energies in the strong and intermediate confinement regions, but this influence weakens significantly in large dot radii. The most substantial difference between singlet and triplet energies occurs in the 1s22s2p configurations, with this difference decreasing in higher configurations. Significant increases in the 1s and 2s orbital energies are observed at the dot radii where the 2p, 3d, and 4f electrons from the outermost orbit begin to penetrate the well.

Lowering of the singlet-triplet energy gap via intramolecular exciton-exciton coupling

Organic dyes typically have electronically excited states of both singlet and triplet multiplicity. Controlling the energy difference between these states is a key factor for making efficient organic light emitting diodes and triplet sensitizers, which fulfill essential functions in chemistry, physics, and medicine. Here, we propose a strategy to shift the singlet excited state of a known sensitizer to lower energies without shifting the energy of the triplet state, thus without compromising the ability of the sensitizer to do work. We covalently connect two to four sensitizers in such a way that their transition dipole moments are aligned in a head-to-tail fashion, but, through steric encumbrance, the delocalization is minimized between each moiety. Exciton coupling between the singlet excited states considerably lowers the first excited singlet state energy. However, the energy of the lowest triplet excited state is unperturbed because the exciton coupling strength depends on the magnitude of the transition dipole moments, which for triplets are very small. We expect that the presented strategy of designed intramolecular exciton coupling will be a useful concept in the design of both photosensitizers and emitters for organic light emitting diodes as both benefits from a small singlet-triplet energy gap.

Endo-Encapsulated Multi-Resonance Dendrimers with Through-Space Interactions for Efficient Narrowband Blue-Emitting Solution-Processed OLEDs.

Different from conventional luminescent dendrimers with fluorophore tethered outside to dendron, here we first developed endo-encapsulated luminescent dendrimers with multi-resonance (MR) fluorophore embedded inside of carbazole dendrons by growing dendrons through 1,8-positions of central carbazole moiety to create a cavity for accommodating the fluorophore. This endo-encapsulated structure not only shields the fluorophore to fully resist aggregation-caused spectral broadening, but also induce through-space interactions between dendron and fluorophore via intramolecular π-stacking, giving lowered singlet state energy and reduced singlet-triplet energy splitting to accelerate reverse intersystem crossing (RISC) from triplet to singlet states. The resultant dendrimer containing 1,8-linked second-generation carbazole dendrons and boron, sulfur-doped polycyclic MR fluorophore exhibits narrowband blue emission at 471 nm with FWHM kept at 34 nm even in neat film, together with ~4 times enhancement of RISC rate constant compared to its exo-tethered counterpart. Solution-processed OLEDs based on the endo-encapsulated dendrimer reveal efficient narrowband blue emissions with maximum external quantum efficiency of 22.6%, representing the best device efficiency for blue-emitting multi-resonance dendrimers so far.

Phenothiazine-carbazole-based bis oxime esters (PCBOEs) for visible light polymerization

In this study, a series of seven PCBOEs was innovatively designed and successfully synthesized for the first time by bridging a phenothiazine and a carbazole unit by mean of an acetylene spacer and bearing oxime ester groups as end groups on both sides. Structures of the different difunctional oxime esters were confirmed through NMR and HRMS. All PCBOEs exhibited excellent light absorption properties in the visible range. Upon excitation with a 405 nm LED for 60 s, a complete photolysis of the oxime esters could be achieved. Parameters such as the singlet state energy and the triplet state energy demonstrated the facile generation of the corresponding radicals upon light absorption by PCBOEs. Photopolymerization kinetics in TMPTA confirmed the homolytic cleavage of oxime esters and the generation of radicals. CO2 absorption peaks were detected when the PCBOEs/TMPTA systems were excited with a 405 nm LED. Based on the positive results, the photochemical mechanisms of polymerization with PCBOEs was proposed. PCBOE3 was identified as the best candidate of the series and this bifunctional oxime ester was successfully used for the 3D-printing of the “IS2M” logo via DLW. Experimental results from DSC also confirmed that PCBOEs not only serve as photoinitiators but also possess high reactivity as thermal initiators.

Photolysis of dinotefuran and nitenpyram in water and ice phase: Influence mechanism of temperature over photolysis

Neonicotinoids are widely used pesticides around the world, but the photolysis of neonicotinoids in cold agricultural region are still in blank. This paper aimed to study the influence of cold temperature over photolysis of neonicotinoids. To this end, the photolysis rates and photoproducts of dinotefuran and nitenpyram in water, ice and freeze-thawing condition were determined. Coupled with quantum chemistry calculation, the influence mechanisms of temperature and medium were investigated. The results showed the photolysis rates of neonicotinoids in water condition slightly declined with the lowered temperature due to the photolysis reactions were endothermic reactions. However, the photolysis rates increased by 89.8 %, 59.2 %, 49.4 % and 9.5 % for dinotefuran and nitenpyram in ice and thawing condition, respectively. This phenomenon was posed by the concentration-enhancing effect and change of photo-chemical properties of neonicotinoids in ice condition, which included lowered bond cleavage energy, lowered first excited singlet state energy and expanded light absorption range. The photolysis pathways of the two neonicotinoids did not change in different medium, but the concentration of carboxyl products was relatively higher than that of water condition due to the more amounts of reactive oxygen species in ice medium, which might increase the secondary pollution risk after ice-off in spring due to the higher ecotoxicity to nontarget organism of these photoproducts. The influence of cold temperature and medium change should be considered for the environmental fate and risk assessment of neonicotinoids in cold agricultural region.

Synthesis of MeBICAAC Supported Si(III) Radicals and a Silylone via Reduction Route From MeBICAAC-Si(IV) Precursors.

Past one decade has witnessed a tremendous growth in the field of carbene stabilized low-valent silicon compounds unravelling very exciting properties of these molecules. Herein, we have employed a bicyclic (alkyl)(amino)carbene, (MeBICAAC) to explore the low-valent chemistry of silicon. The reduction of bicyclic (alkyl)(amino)carbene-SiCl4 complex, [(MeBICAAC)SiCl4] (1) with KC8 afforded low-valent Si complexes, including Si(III) radical [(MeBICAAC)SiCl3] (2) and a complex with silicon center in a formal zero-valent state, [(MeBICAAC)2Si] (3). Similarly, the reduction of in-situ generated MeBICAAC adduct of Me2SiCl2 with one equivalent of KC8 led to the formation of [(MeBICAAC)SiMe2Cl] (4) complex having an unpaired electron. These complexes have been characterized by IR, UV-Vis., NMR, HRMS, EPR and their solid-state structures were also elucidated by single crystal X-ray crystallography. Further, DFT calculations revealed the lower energy singlet state for complexes 1, 3 and doublet state for complexes 2, 4.

Modulation of lanthanide luminescence by carbamoylmethylphosphine oxide ligand: A theoretical study

The antenna capacity of Phenacyldiphenylphosphine oxide (Phenac), combining C=O and P=O donor groups was established by theoretical examination of its chromophore properties, binding ability to encapsulate and stabilize lanthanide complexes and potential to sensitize the EuIII and TbIII luminescence in Eu(Phenac)2(NO3)3(H2O) (1), Eu(Phenac)2(NO3)3 (2), Eu(Phenac)3(NO3)3 (3) and Tb(Phenac)2(NO3)3(H2O) (4) complexes in vacuum, solution and solid phase. The ligand-centered singlet and triplet excited state energies, the rates and lifetimes for the rival nonradiative (S1→T1,2 intersystem crossing (ISC)) and radiative and nonradiative S1→S0 and T1→S0 relaxation were predicted by means of DFT/TDDFT/ωB97XD calculations. Similar relaxation paths were found for all complexes. The absorbed energy by Sn > 1(π-π*) states relaxed to T1min by major ISC mechanism S1→T2→T1. The long radiative lifetime of S1→S0 due to n(p(C)(O))π-π*,p(P) character facilitated the forward ISC process. The environment, Phenac's number and LnIII size affect the T1 energy ∼0.1 eV. The calculated (Phenac→EuIII) energy transfer rates suggested the most likely T1 → 5D1/5D0 channels for a population of emitting metal-centered states. The small Φ of EuIII and larger luminescence of TbIII were discussed and explained. The importance of the P=O and C=O groups for the coordination ability and absorption properties of Phenac has been elucidated.

Thermally activated delayed fluorescence emitters for efficient sensitization of europium(III).

We demonstrate for the first time a unique approach to efficiently sensitize lanthanides(III) using photosensitizer ligands that show thermally activated delayed fluorescence (TADF). TADF ligands have very small singlet (S1) and triplet (T1) excited state energy splitting and S1/T1 energy levels are in optimum energy to the acceptor level of Eu(III) to enable high energy transfer efficiency. The synthesized Eu(III) coordination polymers with TADF ligands showed bright red luminescence with an outstanding sensitization efficiency of 90-94% and Φtot of 79-85% in poly(methyl methacrylate) encapsulated films. This rational approach of efficiently sensitizing lanthanides with TADF ligands demonstrates their great potential for imaging and optical communications applications.

Unraveling Triplet Formation Mechanisms in Acenothiophene Chromophores.

The evolution of molecular platforms for singlet fission (SF) chromophores has fueled the quest for new compounds capable of generating triplets quantitatively at fast time scales. As the exploration of molecular motifs for SF has diversified, a key challenge has emerged in identifying when the criteria for SF have been satisfied. Here, we show how covalently bound molecular dimers uniquely provide a set of characteristic optical markers that can be used to distinguish triplet pair formation from processes that generate an individual triplet. These markers are contained within (i) triplet charge-transfer excited state absorption features, (ii) kinetic signatures of triplet-triplet annihilation processes, and (iii) the modulation of triplet formation rates using bridging moieties between chromophores. Our assignments are verified by time-resolved electron paramagnetic resonance (EPR) measurements, which directly identify triplet pairs by their electron spin and polarization patterns. We apply these diagnostic criteria to dimers of acenothiophene derivatives in solution that were recently reported to undergo efficient intermolecular SF in condensed media. While the electronic structure of these heteroatom-containing chromophores can be broadly tuned, the effect of their enhanced spin-orbit coupling and low-energy nonbonding orbitals on their SF dynamics has not been fully determined. We find that SF is fast and efficient in tetracenothiophene but that anthradithiophene exhibits fast intersystem crossing due to modifications of the singlet and triplet excited state energies upon functionalization of the heterocycle. We conclude that it is not sufficient to assign SF based on comparisons of the triplet formation kinetics between monomer and multichromophore systems.

Planar hexacoordinate phosphorous and arsenic

Here the first planar hexacoordinate phosphorous and arsenic (phP and phAs) atoms are reported. Neutral 18 valence electron EBe3Li3H6 clusters (E = P, As) adopt planar D3h symmetry in their lowest energy singlet state. Chemical bonding reveals partial double bond character in P(As)-Be bonds and single bond character in P(As)-Li bonds. Born-Oppenheimer molecular dynamics simulation as well as their reluctance towards decomposition reveals high kinetic stability of these clusters.

Tuning photophysical properties of bicarbazole-based blue TADF emitters with AIE feature: Trade-off between small singlet-triplet energy splitting and high photoluminescence quantum yield

Three aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) blue light-emitting materials were successfully synthesized using bicarbazole and phenylsulfonyl/benzophenone moieties as electron donors and acceptors, respectively, via Buchwald-Hartwig reaction. Density functional theory calculations showed that the three emitters have a moderate dihedral angle between the two carbazole planes and between the donor and acceptor, resulting in a good conjugated structure and a trade-off between the small ΔEST and high photoluminescence quantum yield of the BCzDPM molecule. The cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry tests demonstrated that the three materials had high electrochemical and thermal stability. The three emitters exhibited obvious intramolecular charge transfer characteristics in solution and film. According to the fluorescence and low-temperature phosphorescence spectra, the lowest singlet and triplet excited state energy levels of BCzDPM were 2.96 eV and 2.78 eV, respectively, with a ΔEST of 0.18 eV. Using these emitters as non-doped emissive layers, efficient blue electroluminescent devices were fabricated. The maximum current efficiency and external quantum efficiency reached 15.4 cd/A and 7.05% for the BCzDPM device, respectively, and the efficiency roll-off of BCzDPM was smaller than that of PSOBCz and PBCzPM, under high current density.

Probing ground and low-lying excited states for indole derived compounds of potential herbicide action

In this work, we present a study on a series of recently synthesized indole derived compounds that were previously suggested by experimental tests as having herbicidal action. Density functional theory (DFT) and its time-dependent formalism (TD-DFT) were used for investigating ground and excited state properties, respectively. The M06-2X and CAM-B3LYP exchange-correlation functionals were used with the aug-cc-pVTZ basis set. Computations were performed in the gas-phase, water, and dimethyl sulfoxide. Solvation was found to play minor effects on the ground state structure of any given compound. On the other hand, solvation was found to cause a decrease in the excitation energies of the lowest-lying excited singlet states of almost all the compounds. More importantly, all the indole derived compounds presented excitation energies that are considerably lower than those determined for their parent molecule. For instance, the lowest-lying excited singlet was probed at 3.45 eV in the case of 6-chloro-8-nitro-2,3,4,9-tetrahydro-1H-carbazole (compound 15a) versus 5.00 eV for the corresponding state of the indole molecule, both values determined at the TD-DFT/M06-2X/aug-cc-pVTZ level in water. In addition, compound 15a along with 5-chloro-2,3-dimethyl-7-nitro-1H-indole (15b) presented three excited states (each) that are likely to be accessed (having oscillator strength ≳0.1 at the TD-DFT/M06-2X/aug-cc-pVTZ level of theory in water) among the five respective lowest-lying excited singlets. The existence of various excited states accessible at lower energies may provide competing alternative paths to the chlorophyll-solar radiation interaction, in detrimental manner to the photosynthetic process; an observation in agreement to the previous experimental findings. Frontier orbital analysis suggested electron delocalization (advent from structural modifications) to be responsible for lowering the excitation energies.

Dynamics of the Ligand Excited States Relaxation in Novel β-Diketonates of Non-Luminescent Trivalent Metal Ions.

Complexes emitting in the blue spectral region are attractive materials for developing white-colored light sources. Here, we report the luminescence properties of novel coordination compounds based on the trivalent group 3, 13 metals, and the 1-phenyl-3-methyl-4-cyclohexylcarbonyl-pyrazol-5-onate (QCH) ligand. [M(QCH)3] (M = Al, Ga, and In), [M(QCH)3(H2O)] (M = Sc, Gd, and Lu), [Lu(QCH)3(DMSO)], and [La(QCH)3(H2O)(EtOH)] complexes were synthesized and structurally characterized by a single-crystal X-ray diffraction study. It has been found that the luminescence quantum yields of the ligand increase by one order of magnitude upon metal coordination. A significant correspondence between the energies of the ligand's excited states and the luminescence quantum yields to the metal ion's atomic numbers was found using molecular spectroscopy techniques. The replacement of the central ion with the heavier one leads to a monotonic increase in singlet state energy, while the energy of the triplet state is similar for all the complexes. Time-resolved measurements allowed us to estimate the intersystem crossing (ISC) rate constants. It was shown that replacing the Al3+ ion with the heavier diamagnetic Ga3+ and In3+ ions decreased the ISC rate, while the replacement with the paramagnetic Gd3+ ion increased the ISC rate, which resulted in a remarkably bright and room-temperature phosphorescence of [Gd(QCH)3(H2O)].

Calculation of the energy of a two-dimensional hydrogen molecule

ABSTRACT The equilibrium distance and energies of singlet and triplet states of a two-dimensional (2D) hydrogen molecule are calculated. Variational calculations were carried out using a multiparameter system of Gaussian functions with correlation multipliers. The results are compared with the calculations performed by other authors. Currently, the most accurate values of the singlet and triplet states energy of the 2D-molecule in two-dimensional systems have been obtained: Es = –5.2843Eh , Et = –2.9277Eh ; the equilibrium state corresponds to the distance Rm = 0.36614ao , where Eh is the Hartree energy, and ao is the Bohr radius. In the mathematical sense, a 2D hydrogen molecule is a direct analog of a pair of shallow hydrogen-like paramagnetic centres in covalent two-dimensional crystals. The energies of the singlet and triplet states of 2DH2 are significantly lower than the corresponding values of the three-dimensional hydrogen molecule (3DH2), while the singlet–triplet splitting is much higher than that of 3DH2. In the equilibrium state, 2DH2 protons are much closer to each other, which leads to a significant increase in correlation effects associated with the direct dependence of the wave function on the interelectron distance compared to 3DH2.

Singlet Fission in Perylene Monoimide Single Crystals and Polycrystalline Films

Singlet fission (SF) is a spin-allowed process in which a photogenerated singlet exciton down-converts into two triplet excitons. Perylene-3,4-dicarboximide (PMI) has singlet and triplet state energies of 2.4 and 1.1 eV, respectively; thus making SF slightly exoergic and providing triplet excitons that have sufficient energy to raise the efficiency of single-junction solar cells by reducing thermalization losses from hot excitons formed when absorbed photons have energies higher than the semiconductor bandgap. However, PMI SF in the solid state has not been studied previously. Here, we show that 2,5-diphenyl-N-(2-ethylhexyl)perylene-3,4-dicarboximide (dp-PMI) crystallizes into a slip-stacked intermolecular morphology favorable for SF. Transient absorption microscopy and spectroscopy show that dp-PMI SF occurs in ≤50 ps in both single crystals and polycrystalline thin films with a triplet yield of 150 ± 20%. Ultrafast SF in the solid state, the high triplet yield, and its photostability make dp-PMI an attractive candidate for SF-enhanced solar cells.

Electronic structure contributions to O-O bond cleavage reactions for MnIII-alkylperoxo complexes.

Synthetic manganese catalysts that activate hydrogen peroxide perform a variety of hydrocarbon oxidation reactions. The most commonly proposed mechanism for these catalysts involves the generation of a manganese(III)-hydroperoxo intermediate that decays via heterolytic O-O bond cleavage to generate a Mn(V)-oxo species that initiates substrate oxidation. Due to the paucity of well-defined MnIII-hydroperoxo complexes, MnIII-alkylperoxo complexes are often employed to understand the factors that affect the O-O cleavage reaction. Herein, we examine the decay pathways of the MnIII-alkylperoxo complexes [MnIII(OOtBu)(6Medpaq)]+ and [MnIII(OOtBu)(N4S)]+, which have distinct coordination environments (N5- and N4S-, respectively). Through the use of density functional theory (DFT) calculations and comparisons with published experimental data, we are able to rationalize the differences in the decay pathways of these complexes. For the [MnIII(OOtBu)(N4S)]+ system, O-O homolysis proceeds via a two-state mechanism that involves a crossing from the quintet reactant to a triplet state. A high energy singlet state discourages O-O heterolysis for this complex. In contrast, while quintet-triplet crossing is unfavorable for [MnIII(OOtBu)(6Medpaq)]+, a relatively low-energy single state accounts for the observation of both O-O homolysis and heterolysis products for this complex. The origins of these differences in decay pathways are linked to variations in the electronic structures of the MnIII-alkylperoxo complexes.

Degenerate and non-degenerate two-photon absorption of coumarin dyes.

Two-photon absorption (2PA) spectroscopy is a robust bioimaging tool that depends on the determined cross-sections (σ2PA). The absorption of both photons occurs simultaneously with equivalent (degenerate) or different (non-degenerate) photon energies, D-2PA and ND-2PA, respectively. The former has been investigated experimentally and computationally for many systems, while the latter remains relatively unexplored computationally and limited experimentally. In this study, response theory using time-dependent density functional theory (TD-DFT) and the 2-state model (2SM) have been utilized to investigate σD-2PA and σND-2PA for the excitation to the lowest energy singlet state (S1) of coumarin, coumarin 6, coumarin 120, coumarin 307, and coumarin 343. Solvents involved were methanol (MeOH), chloroform (ClForm), and dimethylsulfoxide (DMSO), where the latter leads to the largest σ2PA. Values of σ2PA are largest for coumarin 6 and lowest for coumarin, which illustrates the effect of substituents. The 2SM clarifies how the largest cross-sections correspond to molecules with the largest transition dipole moments, μ01. In general, σD-2SM computations agree with σD-2PA. Moreover, σND-2SM are in qualitative agreement with σND-2PA with comparable enhancement relative to σD-2PA. Overall, σND-2PA are larger than σD-2PA where the increase is in the range of 22% to 49%, depending on the coumarin as well as the relative energies of the two photons. This work aids in future investigations into various fluorophores to understand their photophysical properties for ND-2PA.

Harnessing multiple generated excitons from intermolecular singlet fission of perylene–monoimides in a p-type dye-sensitized solar cell

A perylene-monoimide (PMI) monomer is functionalized with a benzoic acid to support immobilization on copper oxide (CuO) / nickel oxide (NiO) p-type semiconductors in order to construct p-type dye-sensitized solar cells (DSSCs). All devices are fully characterized by means of J-V, electrochemical impedance, and incident photon-to-current conversion efficiency (IPCE) measurements. Their optimization in terms of PMI uptake, iodine / Li+ addition and layer thickness lead to efficiencies as high as 0.07 %. For PMIs, the favorable relationship between their respective singlet and triplet excited state energies of 2.36 and 1.12 eV enable thermodynamically driven singlet fission (SF). The strong inter–PMI interactions on the semiconductor surface allow for an efficient and fast intermolecular SF. The formed correlated triplet pair ((T1)(T1)) is corroborated by means of time-resolved transient absorption spectroscopy (TAS) and partakes in an interfacially charge injection into the CuO / NiO semiconductors within the p-type DSSCs.

Impact of Connectivity on the Electronic Structure of N-Heterotriangulenes

N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer, both N-HTAs adopt a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) are assigned. The optical gap (S0 → S1 transition) is found to be 3.4 eV for N-HTA 550 and 2.5 eV for N-HTA 557. Thus, the introduction of the −C═C– double bond in N-HTA 557 resulted in a pronounced decrease of the optical gap size by 0.9 eV due to the larger π-conjugated electron system compared to N-HTA 550. Structural variations or substitution patterns in N-HTAs foster the opportunity for tailoring their electronic properties.

Assessment of State-Averaged Driven Similarity Renormalization Group on Vertical Excitation Energies: Optimal Flow Parameters and Applications to Nucleobases.

We present a comprehensive excited-state benchmark for the state-averaged (SA) driven similarity renormalization group (DSRG) [Li, C.; Evangelista, F. A. J. Chem. Phys. 2018, 148, 124106]. Following the QUEST database [Véril, M.; Scemama, A.; Caffarel, M.; Lipparini, F.; Boggio-Pasqua, M.; Jacquemin, D.; Loos, P.-F. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2021, 11, e1517], 280 vertical transition energies of 35 medium-sized molecules are computed using the SA-DSRG derived second- and third-order perturbation theories (PT2/PT3) along with a nonperturbative approach [sq-LDSRG(2)]. Comparing to the theoretical best estimates, the optimal flow parameter is found to be 0.35 and 2.0 for SA-DSRG-PT2 and SA-DSRG-PT3, respectively. For SA-sq-LDSRG(2), a flow parameter of 1.5 provides converged equations without compromising the accuracy. We then assess the accuracy of the SA-DSRG hierarchy using these parameters. The SA-DSRG-PT2 scheme outperforms the level-shifted CASPT2 by 0.10 eV in mean absolute error (MAE), yet this accuracy is slightly inferior than that of CASPT2 with the ionization-potential-electron-affinity shift. Both SA-DSRG-PT3 and SA-sq-LDSRG(2) yield a MAE of 0.10 eV, which is comparable to that of CASPT3 (0.09 eV). Finally, we compute vertical excitation energies of several low-lying singlet states of nucleobases. The SA-sq-LDSRG(2) approach provides highly accurate results for π → π* excitations, while n → π* transitions are better described by SA-DSRG-PT3.