TD-DFT Method Research Articles

Molecular self-assembly of 1D linear and 2D T-shape polyiodide arrangements in two organic ammonium triiodide salts: supramolecular structures and density functional theory-based optical properties

Schematic depiction of the σ MOs in the vicinity of the HOMO of I3−. The orbital distribution of HOMO-2 and LUMO for compounds 1 and 2 computed at the TD-DFT method.

Design of J-aggregates-like oligomers built from squaraine dyes exhibiting transparency in the visible regime and high fluorescence quantum yield in the NIR region.

New materials for transparent luminescent solar concentrators (TLSCs) are of large interest. Therefore, we investigated the optical properties of J-aggregates-like oligomers (hereinafter referred to as J-aggregates) based on covalently bound squaraine dyes in toluene solvent using DFT and TD-DFT methods. In addition, the rate constants needed for the prediction of fluorescence quantum yield (QY) have been calculated using Fermi's Golden rule and vertical harmonic approximation (VH) for ground and excited states. In the context of QY prediction, different broadening of the lineshape has also been employed. We found that J-aggregates based on squaraine dyes exhibit near-infrared (NIR) selective absorption and emission as well as high fluorescence QY. Comparison of the properties obtained for dimers, trimers and tetramers belonging to two classes (SQA and SQB) of J-aggregates allows us to select the tetramer of SQA J-aggregates as suitable for application. The scaling model for N ≥ 4 monomer subunits supports quantitative findings. Therefore, we propose J-aggregates containing N ≥ 4 subunits of SQA with a central squaric acid ring with two oxygen atoms in toluene solvent as a suitable candidate for TLSC application.

Theoretical studies of electronic and optical characteristics in donor-π-Acceptor (D-π-A) dyes: DFT and TD-DFT methods

Abstract This research focused on enhancing D-π-A organic dyes derived from coumarin and its derivatives, collectively referred to as D-CM-A dyes. The study aimed to improve these dyes by introducing various donors and acceptors to the coumarin structure. Six new coumarin dyes were evaluated, primarily for their potential application in dye-sensitized solar cells (DSSCs) to enhance energy efficiency. The analysis involved calculating the geometry, electronic properties, and optoelectronic characteristics of the dye molecules using DFT and TD-DFT methods with the B3LYP functional and the 6-311G basis set in both gas and solvent phases. The primary focus was to understand how modifications to the π-conjugated D-π-A organic dyes influenced their optoelectronic properties, including key factors such as maximum absorption wavelength (λmax), highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO), and energy gap (Egap). Additionally, the study explored the photovoltaic properties of these dyes. The findings highlighted D4-CM-A4 as a promising candidate with the narrowest energy gap, while D1-CM-A1 and D2-CM-A2 showed superior light-harvesting efficiencies (LHE) compared to other derivatives. In conclusion, this study suggests that D1-CM-A1 and D2-CM-A2 are favourable choices for enhancing the performance of DSSCs due to their promising optoelectronic properties.

The Pseudo Jahn-Teller effect and NBO analysis for untangling the symmetry breaking in the planar configurations of M2X4+ (M = Si, Ge and X = Cl, Br, I): effect on electronic structure and chemical properties.

The Pseudo Jahn- Teller effect is a significant tool for evaluating molecular distortion and symmetry breaking. The PJT effect associated with NBO analysis can be a powerful method for studying the structural properties variations arising from D2h → C2h distortions. The theoretical studies on Si2X4+ and Ge2X4+ radical cations have been rare. The calculations have shown that C2h non-planar structures are more stable than planar structures with D2h symmetry. The [Formula: see text] PJTE problem of M2X4+ compounds is a result of the coupling between the ground B3u state and the exited B1u state in the Qb2g direction causes. Also, the difference in M and X atoms can affect the PJT instability of compounds. The findings of this work show that the energy gap between the ground and excited states that have D2h symmetry decreases from M2Cl4+ to M2I4+ and increases from Si2X4+ to Ge2X4+. In fact, there is a significant relationship between instability of high-symmetry configurations, geometric parameters, electron delocalization, chemical hardness, electronegativity, electrophilicity index, and PJT stabilization energies. These results may serve to evaluate the distortion of similar systems. The structures of Si2X4+ and Ge2X4+ are optimized by LC-BLYP, M06-2X, and B3LYP methods with def2-TZVPP basis set in GAMESS software. The details of the excited states of compounds are studied by the TD-DFT method. NBO analysis for planar and non-planar structures is carried out at B3LYP/def2-TZVPP level by the NBO 5. G program that demonstrates HOMO, LUMO, ED, bonding and antibonding orbital occupancies, bond order, and E2.

Predicting the Adsorption Behavior of 6-mercaptopurine Anticancer Drug upon Polyoxazoline Nanocarrier: A Theoretical Study

Abstract: Predicting the adsorption behavior of the 6-Mercaptopurine (MCP) anticancer drug upon the polyoxazolinenano-carrier was investigated using DFT and TD-DFT methods by B3LYP/6-31G* level in the gas phase and water solution. Based on the thermochemical parameters, the MCP/polymer complex in the solvent water is more stable rather than the gas phase. The adsorption energies of the MCP/polymer complex displayed that the adsorption process is exothermic. The UV/Vis absorption and IR spectra analysis were calculated to investigate the changes happening in the interaction of the MCP with the polymer. FMO analysis indicated that the energy gap (Eg) of the polymer decreased after the adsorption process. Electronic properties and MEP analysis were also studied. Based on NBO analysis and charge difference (ΔN), the charge transfer in MCP/polymer occurs essentially from the polymer to the MCP drug, which is consistent with the results of NBO analysis. It is predicted that the POZ polymer can be used as a drug delivery system for MCP drug.

Effect of Seleno-Thiophene π-Linkers on Electronic and Photovoltaic Properties of Boro-Phenothiazine Donors for DSSCs Application: TD-DFT and DFT Methods

Effect of Seleno-Thiophene π-Linkers on Electronic and Photovoltaic Properties of Boro-Phenothiazine Donors for DSSCs Application: TD-DFT and DFT Methods

Study of different interactions in piperazine benzoate and its effect on biological properties: Experimental and theoretical approach

Piperazine benzoate (PPB), an antibacterial molecule, was synthesized and characterized using FT-IR, FT-Raman, UV, and NMR spectra as well as biological research. The optimized geometry of the molecule was performed using Gaussian'09w programme at B3LYP with 6–311G++(d,p) basis set. Hyper conjugative interaction σ(CH) → σ*(OH) enhances NH…O interaction and C-H…O interaction arises mainly from charge transfer between the molecules. The electronic transitions in the molecule were exhibited by comparing the UV–vis spectrum with the observed spectrum using the TD-DFT method in gaseous phase. FMO analysis, natural charge analysis, and global chemical reactivity descriptors methods were investigated. PPB is a soft molecule with significant polarizability and chemical reactivity because of its low molecular orbital energy gap. Normal coordinate analysis (NCA) was used to determine the detailed interpretation of the fundamental modes. The chemical shifts of the molecule were determined from the recorded 13C and 1H NMR spectra using the gauge independent atomic orbital (GIAO) method. Using NBO analysis, concerns related to molecular stability and bond strength have been presented. The topological properties were studied using MEP, ELF, and LOL maps. From AIM findings, the PPB is defined by two BCPs, one each for an ON…H and CO…H type hydrogen bond. Through RDG investigation, the interactions inside the PPB molecule were examined. The interaction areas in the molecular structure of PPB are confirmed by the RDG graph data. PPB's absorption rate was 78.18 percent. This figure led to the conclusion that PPB had the highest percentage of absorption. Bacterial proteins were used for molecular docking, and docking parameters were derived. Based on Lipinski's rule of five, drug similarity was determined, and the ADMET variables were also predicted.

Computational study of electronic excitations properties in solvents, molecular interaction energies, topological and biological properties of Ethyl 6-bromoimidazo[1,2-a]pyridine-2-carboxylate

The current study emphasis on the structural characterization, solvation effects on electronic, thermodynamical properties and biological studies of Ethyl 6-bromoimidazo[1,2-a]pyridine-2-carboxylate (E6BP2C) employing DFT technique. Utilising 6–311++G(d,p) source set in DFT/B3LYP method, optimization of the structure & geometrical parameters were acquired. The resulting augmented geometrical factors are then utilized with an appropriate scaling factor to scale the probable vibrational wavenumbers. The MEP map disclosed the locations of electrophilic & nucleophilic regions. To describe the chemical reactivity, FMOs and Fukui function assessments have both been used. Multiwave function has been utilized to investigate topological analysis (ELF, LOL & RDG) and electron - hole dispersal for the excited states has also been pondered. The absorption of maximum wavelengths has been estimated with a variety of solvents utilizing the TD-DFT method and a virtual UV–Visible spectrum. Additionally, the chemical's optical and stability properties were ascertained using computational NBO and NLO investigations. 1.8765E-30 and 1.2688E-29 correspondingly are the 1st order hyperpolarizability values for gas and water phases. Thermodynamic parameters have also been calculated for variable solvents. ADMET along with drug likeness were used to determine the biologic characteristics.

When do tripdoublet states fluoresce? A theoretical study of copper(II) porphyrin.

Open-shell molecules rarely fluoresce, due to their typically faster non-radiative relaxation rates compared to closed-shell ones. Even rarer is the fluorescence from states that have two more unpaired electrons than the open-shell ground state, since they involve excitations from closed-shell orbitals to vacant-shell orbitals, which are typically higher in energy compared to excitations from or out of open-shell orbitals. States that are dominated by the former type of excitations are known as tripdoublet states when they can be described as a triplet excitation antiferromagnetically coupled to a doublet state, and their description by unrestricted single-reference methods (e.g., U-TDDFT) is notoriously inaccurate due to large spin contamination. In this work, we applied our spin-adapted TDDFT method, X-TDDFT, and the efficient and accurate static-dynamic-static second order perturbation theory (SDSPT2), to the study of the excited states as well as their relaxation pathways of copper(II) porphyrin; previous experimental works suggested that the photoluminescence of some substituted copper(II) porphyrins originate from a tripdoublet state, formed by a triplet ligand π → π* excitation antiferromagnetically coupled with the unpaired d electron. Our results demonstrated favorable agreement between the X-TDDFT, SDSPT2 and experimental excitation energies, and revealed noticeable improvements of X-TDDFT compared to U-TDDFT, not only for vertical excitation energies but also for adiabatic energy differences. These suggest that X-TDDFT is a reliable tool for the study of tripdoublet state fluorescence. Intriguingly, we showed that the aforementioned tripdoublet state is only slightly above the lowest doublet excited state and lies only slightly higher than the lowest quartet state, which suggests that the tripdoublet of copper(II) porphyrin is long-lived enough to fluoresce due to a lack of efficient non-radiative relaxation pathways; an explanation for this unusual state ordering is given. Indeed, thermal vibration correlation function (TVCF)-based calculations of internal conversion, intersystem crossing, and radiative transition rates confirm that copper(II) porphyrin emits thermally activated delayed fluorescence (TADF) and a small amount of phosphorescence at low temperature (83K), in accordance with experiment. The present contribution is concluded by a few possible approaches of designing new molecules that fluoresce from tripdoublet states.

Conformational stability, quantum computational (DFT), vibrational, electronic and non-covalent interactions (QTAIM, RDG and IGM) of antibacterial compound N-(1-naphthyl)ethylenediamine dihydrochloride

Naphthalene derivatives are heterocyclic compounds with significant applications in the pharmaceutical and biomedical industries. Several spectroscopy methods, including FT-IR, FT-Raman, and UV–Vis, were used in this study to examine the molecular structure of N-(1-Naphthyl) ethylenediamine dihydrochloride (NEDD). By employing DFT through the WB97XD with the level of 6–311++G(d, p) as the basis set, the optimized geometrical characteristics and full vibrational allocations of frequencies by the potential energy distribution were generated and matched to the experimental data. The delocalization of charge caused by intermolecular connections becomes apparent in the natural bond orbital (NBO) study. Additionally, frontier molecular orbital bandgap energy and electron excitation analyses were acquired to provide a deeper understanding of the structural characteristics of NEDD. Frontier molecular orbital energies revealed an energy band gap of 5.505 eV, demonstrating the existence of charge transfer within the molecule. Electronic transitions seen in the experimentally recorded UV–visible spectra were assigned using the TD-DFT method whereas transition density matrix (TDM) plots showed the ability to transmit charges. Considering wave function-derived parameters like the Laplacian of electron density (LED), local information entropy (LIE), the average localized ionization energy (ALIE) as well as non-covalent interactions, provides essential insights into the reactivity and stability traits of molecular systems. Local reactivity descriptions aided in understanding the interactions of physiologically relevant molecular systems, thereby facilitating the discovery of potential pharmaceutical compounds. Drug-likeness and ADMET assessment were achieved to assess biological properties of NEDD. Antibacterial assay confirmed its effectiveness against bacterial strains, while molecular docking studies. The stability of the best docked protein has been investigated using 70 ns molecular dynamics simulation followed by effective free energy calculation using MMPBSA.

Excited state proton transfer mechanism of a thiazolo [5,4-d] thiazole derivative in different solvents

In this paper, the excited state proton transfer mechanism of a thiazolo [5,4-d] thiazole derivative, 4-tert-butyl-2-(5-(5-tert-butyl-2-methoxyphenyl) thiazolo [5,4-d] thiazol-2-yl) phenol (MTTH), has been investigated in different polar solvents by using DFT and TDDFT methods. The hydrogen bond lengths, bond angles, and corresponding infrared vibrational frequencies of MTTH in three different polar solvents were compared, and the analytical results showed that the intramolecular hydrogen bonding of MTTH was enhanced in all S1 states. In addition, the simulated absorption and emission spectra values of MTTH are in good agreement with the experimental data, which further proves the rationality of the calculation method. Meanwhile, the electron density rearrangement between the proton acceptor and donor was found by analyzing the frontline molecular orbitals of the MTTH, which is favorable for the excited state proton transfer process. Finally, the potential energy curves of MTTH in three different solvents were calculated, and the results indicated that the weakly polar solvents were more favorable for the excited state intramolecular proton transfer of MTTH.

Spectroscopic and Computational pH Study of NiII and PdII Pyrrole-Imine Chelates with Human Serum Albumin.

Human serum albumin (HSA) efficiently transports drugs in vivo: most are organic. Therefore, it is important to delineate the binding of small molecules to HSA. Here, for the first time, we show that HSA binding depends not only on the identity of the d8 metal ion, NiII or PdII, of their complexes with bis(pyrrole-imine), H2PrPyrr, but on the pH level as well. Fluorescence quenching data for native and probe-bound HSA showed that sites close to Trp-214 (subdomain IIA) are targeted. The affinity constants, Ka, ranged from ~3.5 × 103 M-1 to ~1 × 106 M-1 at 37 °C, following the order Pd(PrPyrr) > Ni(PrPyrr) at pH levels of 4 and 7; but Ni(PrPyrr) > Pd(PrPyrr) at a pH level of 9. Ligand uptake is enthalpically driven, dependent mainly on London dispersion forces. The induced CD spectra for the protein-bound ligands could be simulated by hybrid QM:MM TD-DFT methods, allowing us to delineate the binding site of the ligands and to prove that the metal chelates neither decompose nor demetallate after uptake by HSA. The transport and delivery of the metal chelates by HSA in vivo is therefore feasible.

Optical monitoring of adsorption of chromium (VI) molecular anions, molecules and clusters on carbon nanomaterials

Search for efficient adsorbent materials for removal of hexavalent chromium compounds from environment is a topical technological problem. In this paper, the possibility of optical monitoring of the adsorption of chromium (VI) molecular compounds on the surface of carbon nanomaterials (CNM) is examined. Theoretical calculations of optical absorption spectra of CrO42- anions and molecular clusters of such anions with metals, n*K2CrO4 (n = 1, 2, 4) and MIICrO4 (MII = Zn, Cd, Pb) adsorbed on the surfaces of bare and doped carbon nanotubes and graphene are carried out by TD-DFT method. Results of calculations are compared with experimental data on optical transmission, diffuse reflection and photoluminescence of specially prepared samples containing chromate molecular compounds and particles adsorbed on CNM.It is shown that adsorption of CrO42- anions by CNM can be detected by the appearance of characteristic bands in diffuse reflectance spectra in 310–380 nm region. Nanoparticles of K2CrO4 crystal adsorbed on CNM reveal an additional luminescence band in 600–700 nm region, and it can be utilized in the monitoring of such adsorption. The effect of CNM surface on optical spectra of chromium (VI) molecular compounds is analyzed in terms of splitting of excited electronic states of quasi-tetrahedral CrO42- anions.

Chemical Tailoring Assisted non-TADF to TADF Switching in Carbazole-Benzophenone Emitter - An In-silico Investigation.

Organic light-emitting diodes (OLEDs) have become one of the most popular lighting technologies since they offer several advantages over conventional devices. In carbazole-benzophenone (CzBP) OLED devices, the polymeric form of the compound is previously reported to be Thermally Activated Delayed Fluorescence (TADF)-active (ΔEST ≈0.12 eV), while the monomer (CzBP) (ΔEST ≈0.39 eV) does not. The present study examines the effect of chemical tailoring on the optical and photophysical properties of CzBP using DFT and TDDFT methods. The introduction of a single -NO2 group or di-substitution (-NO2 , -COOH or -CN) in the selected LUMO region of the reference CzBP monomer significantly reduces ΔEST ≈0.01 eV, projecting these systems as potential TADF-active emitters. Furthermore, the chemical modification of CzBP-LUMO alters the two-step TADF mechanism (T1 →T2 →S1 ) in CzBP (ES₁ >ET2 >ET₁ ) to the Direct Singlet Harvesting (T1 →S1 ) mechanism (ET2 >ES₁ >ET₁ ), which has recently been identified in the fourth-generation OLED materials.

Synthesis, DFT, in silico anticancer, ADME and toxicity prediction study of (E)-2-(2-(3,4-dihydronaphthalen-1(2H)- ylidene)hydrazineyl)-4-(4-methoxyphenyl)thiazole

In the current paper, we describe computational molecular modeling to investigate the quantum chemical, spectroscopic, MESP and in-silico Anticancer, ADME and toxicity study of (E)-2-(2-(3,4-dihydronaphthalen-1(2H)-ylidene)hydrazineyl)-4-(4-methoxyphenyl)thiazole by one pot three component reaction. DFT method with B3LYP/6-311G(d,p) level was used to optimize the geometry, molecular electrostatic potential (MESP), Mulliken charges and vibrational assignments. The UV-Vis assignments, frontier molecular orbital (FMO), electronic parameters, and global descriptor were studied using the TD-DFT method with CAM-B3LYP/6-311G(d,p) level. In-silico anticancer study revealed favorable binding interaction with an excellent docking score comparable to the anticancer drug Dasatinib. The molecular docking study found that, target 4xt2 receptor’s amino acid residues VAL422B, LEU407D, MET426D, TYR327A, GLN421B, SER424B, and LEU423D play a vital role. The title compound has the requisite toxicity and ADME profiles, and it has the potential to be therapeutic.

Interaction of carbon nanotube with anti-cancer drugs Busulfan and Mercaptopurine: A DFT study

In this research, the interactions of busulfan and mercaptopurine with single-walled carbon nanotubes (CNT) are investigated. The interaction of the drugs busulfan and mercaptopurine with carbon nanotube has been studied by DFT method. The chosen drugs, CNT(6,6–6), CNT/drug complexes, and their electronic properties are studied using the B3LYP/6-31G(d,p) method in the gas phase. The electronic transition is also calculated with the use of TD-DFT method. NMR Chemical shift, NBO, FMO calculations are implemented. DOS plotsand MEPare obtained for every complex. Natural bond orbital (NBO) study indicates charge transfer have occurredbetween drug molecules and CNT(6,6–6). TD-DFT results revealed the HOMO and LUMO states energy gaps of busulfan and mercaptopurine are 8.94 eV and 5.27 eV respectively, but after interaction with carbon-nanotube they are reduced to 1.63 eV and 1.74 eV. These results revealed that both complexes are favorable. Busulfan strongly interacts with carbon nanotube compare to mercaptopurine.

Solvent impact on electronic, photochemical, molecular structure, topology studies, and the antihistamine activity of 2-(2-Benzylphenoxy)-N,N-dimethylethanamine

In the present study, Density functional theory (DFT) is used for the analysis of molecular structure of 2-(2-Benzylphenoxy)-N, N-dimethylethanamine (2BNDA). The optimization of the title compound were calculated by the method B3LYP/6–311++G (d,p) basis set in gas phase. Frequency assignments of the vibrational spectra were carried with potential energy distribution (PED). The electronic properties namely UV–Vis, HOMO-LUMO, MEP maps have been obtained by IEFPCM model with different solvation effects which is based TD-DFT method. The donor and acceptor charge transfer mechanism has been explained by the natural bond orbital investigation and its maximum for π* (C6 – C10) → π* (C3 – C9) with a value 216.33 Kcal/mol for the title compound. Furthermore, the electrophilic and nucleophilic regions have been shown by Fukui function analysis. The electron localization function (ELF), Localized orbital locator (LOL) with contour maps and RDG has been depicted to know the chemically active regions. Finally, a molecular docking study is carried out to investigate antihistamine activity of the title compound.

Theoretical investigations on structural, spectral, NBO, NLO and topology exploration (AIM, ELF, LOL, RDG) of piperazine-2,5-dione oxalic acid monohydrate

The present work investigates the structural stability and nonlinear optical (NLO) activity of piperazine-2,5‑dione oxalic acid monohydrate (PDOAH) using quantum chemical computational techniques. The optimized molecular structure and vibrational wavenumbers of the PDOAH molecule are determined by density functional theory (DFT) with B3LYP/6–311G(d,p) basis set. The vibrational wavenumbers, IR intensities, Raman activities and their corresponding PED values are obtained and analysed. Natural bond orbital (NBO) analysis is carried out to determine inter and intramolecular charge transfer interaction, intermolecular hydrogen bonding and hyperconjugative interactions that stabilize the structure. The charge transfer interaction, electrophilic, nucleophilic and chemical reactivity sites are identified with the aid of molecular electrostatic potential (MEP) and frontier molecular orbital (FMO) analysis. The TD-DFT method is utilized in UV–Vis spectral analysis for the molecule in the water solvent. RDG and IRI analysis is performed to spot the van der Waals interaction and steric effect within the molecule. The Hirshfeld surface analysis is carried out to scrutinize the intermolecular interactions. The nonlinear optical (NLO) parameters of PDOAH are evaluated through DFT calculation and compared with that of urea.

Adsorption behavior, different green solvent effect and surface enhanced Raman spectra (SERS) investigation on inhibition of SARS-CoV-2 by antineoplastic drug Carmofur with silver/gold/platinum loaded silica nanocomposites: A combined computational analysis and molecular modelling approach

In recent times, extensive research has been carried out to uncover effective drug treatments for coronavirus disease (COVID-19). The Carmofur (HCFU) molecule is primarily known for its antineoplastic properties and its use in cancer treatment. Nowadays, the antineoplastic drug Carmofur has shown its effectiveness in inhibiting the SARS-CoV-2 virus. The primary objective of this article is to utilize theoretical knowledge to explore the absorption mechanism and interaction between the HCFU molecule and the noble metal (Ag/Au/Pt)-loaded silica nanocomposites (HCFU + Ag/Au/Pt…SiO2). The optimized geometry, frequency, and intensity of the vibrational bands of the HCFU molecule and its complexes were obtained from the DFT method with the LANL2DZ basis set. The SERS technique was carried out to explore the adsorption of HCFU molecule with metal (Ag/Au/Pt)-loaded silica nanocomposites. The chemical reactivity and the charge distribution within the molecule were analysed with the help of HOMO-LUMO, Molecular Electrostatic Potential (MEP), Electron Localized Function (ELF), and Localized Orbital Locator (LOL) studies. The polar protic (ethanol, methanol, and water) and polar aprotic (DMSO) solvents were used to analyse the UV–Visible spectra of the molecules using the TD-DFT method. The drug-likeness properties of the molecules were calculated utilizing Lipinski’s rule, and finally, the molecular docking prediction was carried out for the Carmofur (HCFU) and its complexes with the selected proteins.

TDDFT study on a fluorescent probe for distinguishing analogous thiols based on smiles rearrangement

The complete excited-state sensing mechanism of a fluorescent probe capable of distinguishing cysteine/homocysteine and glutathione from analogous biological thiols has been investigated. Using a TDDFT method, the nature of the fluorescence differences in the detection of thiols by the probe has been explained at the molecular level. Calculation results imply that the probe undergoes photoinduced electron transfer (PET) from the fluorophore to the nitrobenzooxadiazole (NBD)-based acceptor in the excited state. In the presence of a thiol, the NBD moiety is cleaved and the red fluorescence emission of the fluorophore is enhanced through inhibition of the PET process. The sulfur-substituted NBD-thiol product is predisposed to undergo excited-state torsion, leading to fluorescence quenching. However, for cysteine and homocysteine, their appropriate distances lead to Smiles rearrangements with relatively low activation energies (26.60 kJ/mol and 42.94 kJ/mol, respectively) and the emission of a distinct green fluorescence at ambient temperature. It has been theoretically confirmed that the distance between two reactive sites, such as sulfhydryl and amino moieties, can be used to distinguish different thiols, thus providing rational support for the control of fluorescence activity and the design of probe molecules.