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Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations

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A special issue of Molecules (ISSN 1420-3049). This special issue belongs to the section "Computational and Theoretical Chemistry".

Deadline for manuscript submissions: closed (31 December 2023) | Viewed by 13117

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Dr. Evangelos Miliordos

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Department of Chemistry and Biochemistry, Auburn University, Auburn, AL 36849, USA
Interests: quantum chemical calculations; excited electronic states; transition metal chemistry; diffuse electrons; weakly bound molecular complexes; molecular catalysis; solvated electrons

Special Issue Information

Dear Colleagues,

The excited electronic states of molecules and materials play a major role in chemistry and physics. Light-matter interactions observed in the various spectroscopic techniques (electronic spectroscopy, photo-electron spectroscopy and others) are a direct demonstration of accessing excited electronic states. Promoting molecular species or materials to their excited electronic states opens up often reaction channels unavailable in the ground state. Astrophysics, atmospheric chemistry, and photo-catalysis are some areas involving heavily chemistry of excited electronic states. A more subtle importance of excited states is that even “ground-state” chemistry is often driven by excited states via crossings between the potential energy surface of the ground and excited states. Moreover, the involvement of excited states of atoms can explain the formation of exotic chemical bonds. The theory is nowadays a sine-qua-non companion of experiments making possible the explanation of the experimental observations and suggesting directions for future experimental work. Due to the continuous development of quantum chemical methods, the improvement of the capabilities of the supercomputer, but also the difficulties encountered in increasingly more complex experiments, the theory is turning to the protagonist. This issue aspires to be a showcase of this trend.

Dr. Evangelos Miliordos
Guest Editor

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Keywords

excited electronic states
chemical reactions
spectroscopy
light-matter interactions
density functional theory
ab initio calculations
molecular systems
materials

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Published Papers (6 papers)

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Research

11 pages, 6034 KiB

Open AccessCommunication

Theoretical Investigation of a Coumarin Fluorescent Probe for Distinguishing the Detection of Small-Molecule Biothiols

by Yue Deng, He Huang, Jian Feng, Yongjin Peng and Yuling Liu

Molecules 2024, 29(3), 554; https://doi.org/10.3390/molecules29030554 - 23 Jan 2024

Cited by 1 | Viewed by 1674

Abstract

Monitoring the level of biothiols in organisms would be beneficial for health inspections. Recently, 3-(2′-nitro vinyl)-4-phenylselenyl coumarin as a fluorescent probe for distinguishing the detection of the small-molecule biothiols cysteine/homocysteine (Cys/Hcy) and glutathione (GSH) was developed. By introducing 4-phenyselenium as the active site, the probe CouSeNO₂/CouSNO₂ was capable of detecting Cys/Hcy and GSH in dual fluorescence channels. Theoretical insights into the fluorescence sensing mechanism of the probe were provided in this work. The details of the electron excitation process in the probe and sensing products under optical excitation and the fluorescent character were analyzed using the quantum mechanical method. All these theoretical results would provide insight and pave the way for the molecular design of fluorescent probes for the detection of biothiols. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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12 pages, 2088 KiB

Open AccessArticle

How Aqueous Solvation Impacts the Frequencies and Intensities of Infrared Absorption Bands in Flavin: The Quest for a Suitable Solvent Model

by D. P. Ngan Le, Gary Hastings and Samer Gozem

Molecules 2024, 29(2), 520; https://doi.org/10.3390/molecules29020520 - 20 Jan 2024

Cited by 2 | Viewed by 2246

Abstract

FTIR spectroscopy accompanied by quantum chemical simulations can reveal important information about molecular structure and intermolecular interactions in the condensed phase. Simulations typically account for the solvent either through cluster quantum mechanical (QM) models, polarizable continuum models (PCM), or hybrid quantum mechanical/molecular mechanical (QM/MM) models. Recently, we studied the effect of aqueous solvent interactions on the vibrational frequencies of lumiflavin, a minimal flavin model, using cluster QM and PCM models. Those models successfully reproduced the relative frequencies of four prominent stretching modes of flavin’s isoalloxazine ring in the diagnostic 1450–1750 cm⁻¹ range but poorly reproduced the relative band intensities. Here, we extend our studies on this system and account for solvation through a series of increasingly sophisticated models. Only by combining elements of QM clusters, QM/MM, and PCM approaches do we obtain an improved agreement with the experiment. The study sheds light more generally on factors that can impact the computed frequencies and intensities of IR bands in solution. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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13 pages, 2298 KiB

Open AccessArticle

Organoboron Complexes as Thermally Activated Delayed Fluorescence (TADF) Materials for Organic Light-Emitting Diodes (OLEDs): A Computational Study

by Jamilah A. Asiri, Walid M. I. Hasan, Abdesslem Jedidi, Shaaban A. Elroby, Saadullah G. Aziz and Osman I. Osman

Molecules 2023, 28(19), 6952; https://doi.org/10.3390/molecules28196952 - 6 Oct 2023

Cited by 2 | Viewed by 2249

Abstract

We report on organoboron complexes characterized by very small energy gaps (ΔE_ST) between their singlet and triplet states, which allow for highly efficient harvesting of triplet excitons into singlet states for working as thermally activated delayed fluorescence (TADF) devices. Energy gaps ranging between 0.01 and 0.06 eV with dihedral angles of ca. 90° were registered. The spin–orbit couplings between the lowest excited S₁ and T₁ states yielded reversed intersystem crossing rate constants (K_RISC) of an average of 10⁵ s⁻¹. This setup accomplished radiative decay rates of ca. 10⁶ s⁻¹, indicating highly potent electroluminescent devices, and hence, being suitable for application as organic light-emitting diodes. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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12 pages, 1798 KiB

Open AccessArticle

Ab Initio Calculations on the Ground and Excited Electronic States of Thorium–Ammonia, Thorium–Aza-Crown, and Thorium–Crown Ether Complexes

by Zhongyuan Lu, Benjamin A. Jackson and Evangelos Miliordos

Molecules 2023, 28(12), 4712; https://doi.org/10.3390/molecules28124712 - 12 Jun 2023

Cited by 3 | Viewed by 2055

Abstract

Positively charged metal–ammonia complexes are known to host peripheral, diffuse electrons around their molecular skeleton. The resulting neutral species form materials known as expanded or liquid metals. Alkali, alkaline earth, and transition metals have been investigated previously in experimental and theoretical studies of both the gas and condensed phase. This work is the first ab initio exploration of an f-block metal–ammonia complex. The ground and excited states are calculated for Th^0–3+ complexes with ammonia, crown ethers, and aza-crown ethers. For Th³⁺ complexes, the one valence electron Th populates the metal’s 6d or 7f orbitals. For Th^0–2+, the additional electrons prefer occupation of the outer s- and p-type orbitals of the complex, except Th(NH₃)₁₀, which uniquely places all four electrons in outer orbitals of the complex. Although thorium coordinates up to ten ammonia ligands, octa-coordinated complexes are more stable. Crown ether complexes have a similar electronic spectrum to ammonia complexes, but excitations of electrons in the outer orbitals of the complex are higher in energy. Aza-crown ethers disfavor the orbitals perpendicular to the crowns, attributed to the N-H bonds pointing along the plane of the crowns. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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11 pages, 18081 KiB

Open AccessArticle

Shaped Microwave Field in a Three-Level Closed Loop Dense Atomic System

by Nadia Boutabba and Hazrat Ali

Molecules 2023, 28(5), 2096; https://doi.org/10.3390/molecules28052096 - 23 Feb 2023

Viewed by 1646

Abstract

In this work, we investigate the atomic properties of a three-level system under the effect of a shaped microwave field. The system is simultaneously driven by a powerful laser pulse and a weak constant probe that drives the ground state to an upper level. Meanwhile, an external microwave field drives the upper state to the middle transition with shaped waveforms. Hence, two situations are considered: one in which the atomic system is controlled by a strong laser pump and a classical constant microwave field, and another in which both the microwave and pump laser fields are shaped. Finally, for sake of comparison, we investigate the tanh-hyperbolic, the Gaussian and the power of the exponential microwave form in the system. Our results reveal that shaping the external microwave field has a significant impact on the absorption and dispersion coefficient dynamics. In comparison with the classical scenario, where usually the strong pump laser is considered to have a major role in controlling the absorption spectrum, we show that shaping the microwave field leads to distinct results. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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26 pages, 416 KiB

Open AccessArticle

Complete, Theoretical Rovibronic Spectral Characterization of the Carbon Monoxide, Water, and Formaldehyde Cations

by Megan C. Davis, Xinchuan Huang and Ryan C. Fortenberry

Molecules 2023, 28(4), 1782; https://doi.org/10.3390/molecules28041782 - 13 Feb 2023

Cited by 7 | Viewed by 2488

Abstract

New high-level ab initio quartic force field (QFF) methods are explored which provide spectroscopic data for the electronically excited states of the carbon monoxide, water, and formaldehyde cations, sentinel species for expanded, recent cometary spectral analysis. QFFs based on equation-of-motion ionization potential (EOM-IP) [...] Read more.

\tilde{A}

^{2}

_{1}

and

\tilde{B}

^{2}

_{1}

states of the formaldehyde cation; only three of these frequencies have experimental assignment available. Rotational constants corresponding to these vibrational excitations are also provided for the first time for all electronically excited states of both of these molecules. EOM-IP-CCSDT/CcC computations support tentative re-assignment of the

ν_{1}

and

ν_{3}

frequencies of the

\tilde{B}

^{2} B_{2}

state of the water cation to approximately 2409.3 cm

^{- 1}

and 1785.7 cm

^{- 1}

, respectively, due to significant disagreement between experimental assignment and all levels of theory computed herein, as well as work by previous authors. The EOM-IP-CCSDT/CcC QFF achieves agreement to within 12 cm

^{- 1}

for the fundamental vibrational frequencies of the electronic ground state of the water cation compared to experimental values and to the high-level theoretical benchmarks for variationally-accessible states. Less costly EOM-IP based approaches are also explored using approximate triples coupled cluster methods, as well as electronically excited state QFFs based on EOM-CC3 and the previous (T)+EOM approach. The novel data, including vibrationally corrected rotational constants for all states studied herein, provided by these computations should be useful in clarifying comet evolution or other remote sensing applications in addition to fundamental spectroscopy. Full article

(This article belongs to the Special Issue Insights in Chemistry and Spectroscopy of Excited Electronic States from Theoretical Calculations)

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