Spectral and photophysical properties of intramolecular charge transfer fluorescence probe: 4'-dimethylamino-2,5-dihydroxychalcone

The spectral and photophysical properties of a new intramolecular charge transfer (ICT) probe, namely 4′-dimethylamino-2,5-dihydroxychalcone (DMADHC) were studied in different solvents by using steady-state absorption and emission spectroscopy. Whereas the absorption spectrum undergoes minor change with increasing polarity of the solvents, the fluorescence spectrum experiences a distinct bathochromic shift in the band position and the fluorescence quantum yield increases reaching a maximum before decrease with increasing the solvent polarity. The magnitude of change in the dipole moment was calculated based on the Lippert–Mataga equation. These results give the evidence about the intramolecular charge transfer character in the emitting singlet state of this compound.     

Spectral properties of intramolecular charge transfer fluorescence probe 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene

The photophysical properties of a new compound 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene in various solvents at room temperature were characterized by the absorption and steady-state fluorescence technique. The bathochromic shift on the emission spectra, the broad halfwidth of the fluorescence band and the increase in the excited state dipole moment occurred. These results gave the evidence about the intramolecular charge transfer (ICT) character in the emitting singlet state of the compound.     

Theoretical investigation on dual fluorescence and intramolecular charge transfer of 5-phenyl-5H-phenanthridin-6-one

Quantum-chemical calculations with the time-dependent density function theory (TDDFT) have been carried out for 5-phenyl-5H-phenanthridin-6-one (PP). For this molecule, dual fluorescence and in- tramolecular charge transfer (ICT) were experimentally observed. The B3LYP functional with 6-311 G (2d, p) basis set has been used for the theoretical calculations. The solvent effects have been described within the polarizable continuum model (PCM). Ground-state geometry optimization reveals that the phenyl/phenanthridinone dihedral angle equals 90.0°, a nearly perpendicular structure. Vertical ab- sorption energy calculations characterize the lower singlet excited states both in gas phase and in solvents. It can be found that the lower excited states have locally excitation (LE) feature. Through constructing the potential energy curves of both isolated and solvated systems describing the LE→ICT reaction and fluorescence emission, we obtain the enthalpy difference ΔH between the LE and ICT states, energy barrier Ea, and energy difference δEFC, indicating the structural changes taking place during the ICT reaction. Potential curve and calculated emission energies for both isolated and sol- vated systems show a dual fluorescence phenomenon, consisting of a LE emission band and a red-shifted ICT band. Our calculations including the solvent effects indicate that the dual fluorescence is brought about by the change in molecular structure connected with the planarization of the twisted N-phenylphenanthridinone during the ICT reaction.     

A red emissive donor-acceptor fluorophore as protein sensor: Synthesis,characterization and binding study

The rational design of environmentally sensitive small molecule fluorophores with superior photophysical properties is critical for fluorimetry based biosensing. Herein, we have developed a new donor-acceptor fluorophore for quantitative detection of Human Serum Albumin (HSA) in aqueous samples. The fluorophore was easily prepared by Knoevenagel condensation, and showed excellent photophysical properties and positive solvatochromism. The design of the fluorophore was based on a nitrogen donor—π-conjugation—nitrile acceptors (D—π—A) to preserve efficient intramolecular charge transfer and long-wavelength emission. The fluorophore showed remarkable “turn-on” fluorescence in presence of HSA, which led to quantitative determination of the protein in aqueous buffer samples. Structure and electronic properties of the fluorophore played important roles on the superior HSA sensing ability. The findings indicate that minor changes in design strategy can be advantageous while developing long-wavelength (far red or near infrared) emitting fluorophores for biosensing and bioimaging.     

Crystal and molecular structure of three biologically active nitroindazoles

3-Bromo-1-methyl-7-nitro-1H-indazole (1), 3-bromo-2-methyl-7-nitro-2H-indazole (2) and 3,7-dinitro-1(2)H-indazole (3) have been synthesized and characterized by X-ray diffraction, ¹³C and ¹⁵N NMR spectroscopy in solution and in solid-state. The dihedral angles obtained in the crystal structures are in good agreement with the molecular parameters calculated using DFT B3LYP calculations employing the 6-311++G(d,p) basis set. Compounds 1 and 2 present intermolecular halogen bonds between the bromine and the oxygen atoms of the nitro group and in compound 3 inter- and intramolecular hydrogen bonding exists.     

Isomery of [Re6S6Br8] and [Re6S5Br9] Units in a Rhenium Cluster Thiobromide: Experimental and Theoretical Approaches

The rhenium cluster thiobromide Cs_1.95(1)Re₆S_5.82(3)Br_8.19(3), belonging to the solid solution Cs₂Re₆S₆Br₈–CsRe₆S₅Br₉, crystallizes in the trigonal system (P31c, a = 10.001(5) Å and c = 14.676(5) Å). It is built up from [Re₆L ₈ ⁱ ]Br ₆ ^a cluster units in which sulphur and bromine are randomly distributed on inner position (Lⁱ). From the structural refinement performed using single-crystal X-ray diffraction data, the isomers of the [Re₆Sⁱ ₆Br ₂ ⁱ ] and [Re₆S ₅ ⁱ Br ₃ ⁱ ] cluster cores present in the structure have been unambiguously determined, due to the non-centro symmetry of the structure. Density functional theory calculations have been performed for all possible di- and tri-substituted isomers in order to confirm experimental analyses. Slight differences between the stability of di-substituted and tri-substituted cluster unit isomers built from Mo₆ cluster and Re₆ clusters are evidenced.     

An experimental and theoretical study of dipole moments of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide

The excited state (S1) dipole moment of m-AMSA (1), an acridine derivative with antitumor activity, was determined from solvatochromic shifts of the lowest energy absorption band in several organic solvents. The effect of the solute shape and the values of polarizability on the determined change of dipole moment between ground and excited state was discussed. The dipole moments in S0 and S1 state were calculated in gas phase with semiempirical quantum-chemical and DFT and CIS methods and in solvents with SM5.4A solvation model and compared with values obtained experimentally. All the results show that the dipole moment of compound 1 in the excited state is higher than that in the ground state. These methods quite well predict the values of Deltamicro between two states of an investigated compound.     

On the mechanism of rhodium-catalyzed [6+2] cycloaddition of 2-vinylcyclobutanones and alkenes

The intramolecular [6+2] cycloaddition mechanism of 2-vinylcyclobutanones and alkenes catalyzed by the [Rh(CO)₂Cl]₂ rhodium dimer has been studied using density functional theory, comparing this multi-step process with the one-step reaction in the absence of catalyst. According to our results the calculated mechanism agrees with what was previously experimentally suggested. Calculations have also allowed to explain the reaction selectivity.     

1,6- and 1,7-regioisomers of dinitro- and diamino-substituted perylene bisimides: synthesis,photophysical and electrochemical properties

1,6- and 1,7-regioisomers of dinitro- (1,6-3 and 1,7-3) and diamino-substituted perylene bisimides (1,6-1 and 1,7-1) were synthesized. The regioisomers 1,6-3 and 1,7-3 were successfully separated by high performance liquid chromatography and characterized by 500 MHz ¹H NMR spectroscopy. Subsequently, the reduction of 1,6-3 and 1,7-3 afforded the corresponding diaminoperylene bisimides 1,6-1 and 1,7-1, respectively. This is the first time 1,6-regioisomers of dinitro- and diamino-substituted perylene bisimides are obtained in pure form. The photophysical and electrochemical properties of 1,6-3 and 1,7-3 were found to be almost the same. However, the regioisomers 1,6-1 and 1,7-1 exhibit significant differences in their optical characteristics. The absorption spectrum of 1,6-1 covers a larger part of the visible region compared to that of 1,7-1. Upon excitation, 1,6-1 also show larger dipole moment change than that of 1,7-1. Time-dependent density functional theory calculations are reported on these dyes in order to rationalize their electronic structure and absorption spectra.     

Theoretical study on molecular structures, intramolecular proton transfer reaction, and solvent effects of 1-phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one

The molecular structures and intramolecular proton transfer reaction of 1-phenyl-3-methyl-4-(6-hydro-4-amino-5-sulfo-2,3-pyrazine)-pyrazole-5-one have been investigated with both ab initio method and the density functional theory. The solvent effects are simulated using the self-consistent reaction field (SCRF) method within the framework of the polarizable continuum model (PCM). The results show that the computed geometrical parameters at the B3LYP levels are in better agreement with experimental values than those at the RHF levels, and the choice of functional in DFT plays an important role in describing the title compound. It is found that strong hydrogen bonds (O–H···N and O···H–N) exist in the title compound, and in the proton transfer process, the O–H bond is broken while the N–H bond is formed. In addition, the order of stability of the isomers remains the same in different solvents, while the barrier height of the proton transfer reaction and dipole moments for the title compound grow with the increase of the solvent polarity. Eventually, the NBO analysis shows that the strength of the hydrogen bond reduces with the increase of the solvent polarity.     

Effect of electron-withdrawing groups on photovoltaic performance of thiophene-vinyl-thiophene derivative and benzochalcogenadiazole based copolymers: A computational study

We report a density functional theory study of the effect of electron-withdrawing groups such as –F, –CN, –NO₂ on the geometrical, optoelectronic, intramolecular charge transfer (ICT), and photovoltaic properties of (E)-1,2-bis(5-alkyl-[2,3′-bithiophene]-2′-yl)ethene (TVT-T) based donor-acceptor (D-A) copolymers with different acceptor units, that is, benzo[c][1,2,5]thiadiazole, benzo[c][1,2,5]oxadiazole, and benzo[c][1,2,5]selenadiazole. The computed optical absorption spectra of the designed compounds lie in the visible and near-infrared regions. Of all the studied copolymers, -CN substituted and Se-based compound displays the lowest HOMO-LUMO (E _{H - L}) gap and optical band gap (E _opt). The exciton binding energy (E _b) is found to be smaller for O-incorporated compounds and -CN substituted copolymer as well, inferring more ICT. The electron-hole coherence concentrated over the D-A units is nearly the same for -CN and -NO₂ substituted compounds, but larger in -F derivatives, indicating weak electron-hole coupling in the formers. Comparatively larger dipole moment (6.421 Debye-9.829 Debye) and charge transfer length (D _CT) (1.976 Å-3.122 Å) for -CN derivatives lead to enhanced ICT properties. The designed donors yield good hole mobilities (0.127-6.61 cm² V⁻¹ s⁻¹) and the predicted power conversion efficiencies are calculated to be as high as ~6%-7% for –CN and –NO₂ substituted compounds.     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.     

D-A structured high efficiency solid luminogens with tunable emissions: Molecular design and photophysical properties

The combination of an electron-accepting unit with aggregation-induced emission features and varying electron-donating arylamines yields high efficiency solid luminogens with tunable emissions from green to red.     

Matrix-isolation FT-IR and theoretical investigation of the competitive intramolecular hydrogen bonding in 5-methyl-3-nitro-2-hydroxyacetophenone

The paper presents the conformational, vibrational and hydrogen bond characteristics of 5-methyl-3-nitro-2-hydroxyacetophenone studied with the combined matrix-isolation FT-IR spectroscopic and theoretical (DFT/B3LYP/6-31++G^**) technique. Theoretical calculations predict three stable conformations of the studied compound. Only two of these conformations could be identified experimentally using the matrix-isolation FT-IR technique. The conformation with the intramolecular hydrogen bond OH^…ON has been found to be more stable than the conformation with the OH^…OC type of hydrogen bond by 7.28 kJ/mol. The complete assignment of the experimental spectra could be performed based on the theoretical calculations including the normal coordinate analysis and isotopic substitution.     

Structure of the 2-isopropylaminoethanol isolated molecule: Conformational analysis and intramolecular interactions

In this paper, a systematic exploration of all the possible conformers of 2-isopropylaminoethanol (2-IPAE) was carried out using the Density Functional Theory (B3LYP) and the 6-311++G(d,p) basis set. At this level, 66 unique conformers within a Gibbs energy range of ca. 31 kJ mol⁻¹ were found in the potential energy surface and their geometrical and thermodynamic properties were determined and discussed. A significant molecular strain was evidenced by the dihedrals and distances between non-bonded hydrogen atoms. According to the geometrical parameters, a O–H···N hydrogen bond was found to be present in the three most stable conformers, representing 68% of the conformational composition at 298.15 K. The energetic and geometrical data derived from the DFT calculations were further complemented by a NBO analysis of the most stable conformers.     

Theoretical study on the structures and electronic spectra of TCNE2-.

Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) complexes have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE2- has D2d symmetry in vacuum as well as in the solvents dichloromethane and acetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoretical results are compared to the experimental data and good agreement is achieved.     

Rearrangement of azirine intermediates to nitriles: Theoretical study of cleavage of 3,4-dihydro-1aH-azirine[2,3-c]pyrrol-2-one to cyanoketene–formaldimine complex

A detailed investigation of the reaction path for the thermal rearrangement of 3,4-dihydro-1aH-azirine[2,3-c]pyrrol-2-one to yield a cyanoketene–formaldimine complex is carried out at the MP2/6-31G* and B3LYP/6-31G* levels of theory. The ring opening of the five-membered pyrrolinone ring and the formation of the nitrile group takes place in a concerted manner, presenting a significant strain energy release and allowing for an electronic stabilization by coarctate conjugation of the transition structure (TS). These two factors make possible a moderate energy barrier. Although the structural features B3LYP/6-31G* theoretical levels, it is found that the MP2 energy barrier (28.8) CCSD(T)/6-31G*//MP2/6-31G* value (17.1 kcal/mol). The complex electronic rearrangement can be rationalized using the theory of coarctate transition structures developed by Herges as the evolution of an azirine structure without referring to a hypothetical vinyl nitrene intermediate. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 912–922, 1998     

Theoretical investigation of the tautomerism of isoorotic acid in gaseous and aqueous phases

In the present investigation, the tautomeric and conformational equilibrium of isoorotic acid have been studied using Møller–Plesset second‐order (MP2) and density functional theory (DFT) methods in the gas phase and aqueous solution (ε = 78.5) using the IPCM model. The relative energies of these tautomers have been calculated at the two levels of theory using 6‐311++G** basis set. Energetics and relative stabilities of the tautomers were compared and analyzed in both the gaseous and aqueous phases. The results indicate that the diketo tautomer (iso) is the most stable form in the gas phase and water. The carboxylic substitution in the uracil ring does not alter its relative stability order of the tautomers. The proton affinity of the oxygen atoms and the deprotonation enthalpy of the NH bonds of isoorotic acid have been compared with recent data of uracil. The relative stability of both syn‐ and anti‐conformations was investigated and the syn form was found to be more stable by 17.65 kcal/mol. It was determined in ab initio calculations that an electron can attach to isoorotic acid, forming a stable anion better than uracil. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical and experimental investigations of complexation with BF3.Et2O effects on electronic structures,energies and photophysical properties of Anil and tetraphenyl (hydroxyl) imidazol

The novel compounds (E)‐2‐(((4‐hydroxyphenyl)imino)methyl)phenol, Tetraphenyl (hydroxyl) imidazole and their corresponding Boron difluoride complexes were synthesized and characterized by spectroscopic techniques. Density functional theory calculations at B3LYP‐D3/6–311++G (d, p) level of theory were performed for the geometric parameters. The MEP surface studies were used to understand the behavior of molecules in terms of charge transfer and to determine how these molecules interact. We used the GIAO and the B3LYP‐D3 with a 6–311++ G (d, p) basis set to simulate the (¹H‐NMR and ¹⁹F‐NMR) and the IR spectra, respectively. The corresponding calculated results are in good agreement with the experimental data. The stability of the molecule arising from hyperconjugation interaction and charge delocalization were analyzed using NBO analysis. FMOs revealed the occurrence of charge transfer within the molecule. The complexation using BF₃.Et₂O was also found to have remarkable effects on the electrochemical properties of the studied molecules, where (b) and (d) present lower chemical stability, higher reactivity and higher polarizability than (a) and (c), respectively. Moreover, the energy gap of (a) and (c) decreased after complexation using BF₃.Et₂O, indicating the reliability of the electrochemical evaluation of LUMO and HOMO energy levels. These values are the factors explaining the possible charge transfer interaction within the molecule. The absorption and emission spectra of the model compound were also simulated and compared to experimental observations in the DMF solvent. The results of DFT calculations supported the structural and spectroscopic data and confirmed the structure modification of frontier molecular orbitals for BF₂ complexes as well as tunable potentials and energy levels.