Solvent Shifts Research Articles

Synthesis of Pyrrolo[2,3-c]isoquinolines via the Cycloaddition of Benzyne with Arylideneaminopyrroles: Photophysical and Crystallographic Study.

An efficient and quick access toward a series of (E)-2-arylideneaminopyrroles 6 and to their benzyne-promoted aza-Diels–Alder cycloaddition products is provided. These products are three pyrrolo[2,3-c]isoquinolines 8a–c substituted in position 5 with different electron-acceptor (A) or electron-donor (D) aryl groups. Intermediates and products were obtained in good yields (up to 78 and 84%, respectively), and their structures were determined on the basis of NMR measurements and HRMS analysis. Photophysical properties of 8a–c were investigated, finding good Stokes shift in different solvents, but only the product 8c showed appreciable fluorescence intensity since its 5-aryl group (2,4-Cl2Ph) could favor the twisted intramolecular charge transfer effect. In addition, a riveting relationship between solvent viscosity and fluorescence intensity was found. Structures of 6 and 8 were studied and confirmed by single-crystal X-ray diffraction, observing that their electronic distributions effect the supramolecular assembly but with only long-distance hydrophobic interactions. A CE-B3LYP model was used to study the energetic topology and understand the crystal architecture of compounds as well as find a connection with both the synthetic and photophysical results.

One- and two-photon absorption properties of quadrupolar thiophene-based dyes with acceptors of varying strengths.

The one-photon (1P) and two-photon (2P) absorption properties of three quadrupolar dyes, featuring thiophene as a donor and acceptors of varying strengths, are determined by a combination of experimental and computational methods employing the density functional theory (DFT). The emission shifts in different solvents are well reproduced by time-dependent DFT calculations with the linear response and state specific approaches in the framework of the polarizable continuum model. The calculations show that the energies of both 1P- and 2P-active states decrease with an increase of the strength of the acceptor. The 2P absorption cross-sections predicted by the response theory are accounted for by considering just one intermediate state (S1) in the sum-over-states formulation. For the chromophore featuring the stronger acceptor, the energetic positions of the 1P- and 2P-active states prevent the exploitation of the theoretically predicted very high 2P activity due to the competing 1P absorption into the S1 state.

Alkyl chain-dependent ESIPT luminescent switches of phenothazine derivatives in response to force

The response of organic materials to mechanical force has attracted significant attention because of its potential applications. Organic molecules with excited state intramolecular proton transfer (ESIPT) luminescence and responsive properties to force are rarely explored. Herein, three phenothiazine derivatives with ESIPT luminescence were synthesized and the influence of alkyl chain on their mechanochromic behaviors was investigated. The results indicate that their fluorescence spectra exhibited large bathochromic shift in polar solvents, meaning intramolecular charge transfer inherent characteristic. Moreover, the compounds with shorter alkyl chain possessed longer emission wavelength in pristine solids. All three compounds exhibited reversible mechanochromism and shifted their fluorescence band to lower energy region. XRD investigations suggested that mechanochromic process was accompanied with the phase transition between crystalline and amorphous states. Furthermore, fluorescence spectral shift values are dependent on alkyl chain length. The compound with ethyl group exhibited the smallest spectral shift (20 nm), whereas the compound with octyl group had the largest shift (31 nm). Solvent fuming and thermal annealing could promote fluorescence recovery. C2PAHN required high temperatures, and C8PAHN recovered its fluorescence at low temperatures, indicating that an alkyl chain length may regulate thermal recovery.

On the calculation of magnetic properties of nucleic acids in liquid water with the sequential QM/MM method

We present calculations of nuclear magnetic shieldings of the nucleic acids adenine, cytosine, guanine e thymine in liquid water environment in normal thermodynamic condition. The study is based on a sequential approach, where a classical Monte Carlo simulation of the solvated system is performed to generate the different solute–solvent configurations to be used in the following quantum mechanics calculations of magnetic properties. We find that the environment polarization effect shields the unprotonated nitrogens, with solvent shifts for the magnetic shieldings between +10.52 and +48.95 ppm. Protonated and amino nitrogens are less shielded by amounts of −17.77 and +0.22 ppm. The results obtained from supermolecule calculations show that protonated and amino nitrogen magnetic shieldings are particularly sensitive to the hydrogen bond interactions, with solvent shifts between −5.48 and −33.42 ppm, when the first microsolvation shell is explicitly considered. In addition, the supermolecular calculations also show a marked deshielding effect for the hydrogens of the protonated nitrogens, with solvent shifts between −2.43 and −5.12 ppm. There is an overall agreement between the theoretical predictions for the 1H and 15N chemical shifts and the available experimental results.

Implementation of state-averaged MCSCF method to RISM- and 3D-RISM-SCF schemes

The state-averaged multiconfiguration self-consistent field (MCSCF) method was implemented to the reference interaction site model and three-dimensional reference interaction site model (RISM and 3D-RISM) SCF schemes, where the electronic structures of multiple states and solvation structure are determined simultaneously by the state-averaged MCSCF method and state-specific RISM-SCF/3D-RISM-SCF scheme, respectively, in a single calculation. The method was applied to the potential energy curves of the low-lying states of NaCl in aqueous solution and solvation shifts of the excitation energy of formaldehyde and p-nitroaniline. The results showed good agreement with those of the state-specific MCSCF and/or experiments.

NLOphoric Triphenylamine Derived Donor-π-Acceptor-π-Donor Based Colorants: Synthesis, Spectroscopic, Density Functional Theory and Z-scan Studies.

Three Donor-π-Acceptor-π-Donor type styryl dyes (5a-c) with different secondary donors are synthesized and characterized to study their nonlinear and linear optical properties. The structure-property relationships of the dyes are described in the light of systematic photophysical and theoretical investigations. The photophysical characteristics of 5a-c are influenced by the polarity of the medium, with an appreciable bathochromic shift in emission (5b = 81 nm) and large Stoke shifts (5b = 104-173 nm) in polar solvents. 5a-c showed intramolecular charge transfer characteristics recognized with the help of emission solvatochromism, solvent polarity graphs, natural bond orbital analysis and HOMO-LUMO energy difference. The optimized geometry and frontier molecular orbitals reveal that the electron donation takes place from secondary donors and not from a fixed donor (triphenylamine) which is more twisted. The nonlinear optical properties obtained using solvent induced spectral shift and computational methods are found within the limiting values. Z-scan results reveal saturable kind of behavior for 5a, 5b and 5c, whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. The two-photon absorption cross section described by two-level approximation is highest for 5b (251-300 GM).

Synthesis of Fluorescent 1,7-Dipyridyl-bis-pyrazolo[3,4-b:4',3'-e]pyridines: Design of Reversible Chemosensors for Nanomolar Detection of Cu2.

An efficient access toward novel tridentate ligands based on 1,7-dipyridinyl-substituted bis-pyrazolo[3,4-b:4′,3′-e]pyridines (BPs) and their usefulness as fluorescent probes for cation detection is reported. The synthesis proceeds by a three-step sequence starting from 2-chloropyridine (1), all reactions were performed using microwave radiation under solvent-free conditions, and an overall yield of up to 63% was obtained. Photophysical properties of three representative 1,7-dipyridinyl-BPs (PBPs, 6a–6c) substituted at position 4 with different donor (D) or acceptor (A) groups were investigated. Compounds exhibited large Stokes shift in different solvents and strong blue light emission in both solution and solid state, and quantum yields were as high as 88% for some of them; thus, a twisted intramolecular charge transfer (TICT) fluorescence mechanism characteristic of the 1,4,7-triaryl-BPs was confirmed. The 4-phenyl-substituted probe (Ph-PBP, 6b) was used successfully in the detection of some metals (Cu2+, Co2+, Ni2+, and Hg2+) by fluorescence quenching phenomena, which could be reversed in the presence of ethylenediamine. This probe showed a greater sensitivity toward Cu2+ in concentrations as low as 26 nM, and in the process of “on–off–on” for this fluorescent molecular switch, only 1 equiv of the analyte was used.

State-specific electrostatic potential descriptors for estimating solvatochromic effects.

The Lippert-Mataga equation is used widely to describe the solvatochromic effects of fluorescent molecules through the evaluation of solute-solvent interactions on the basis of the point-dipole model. A large dipole deviation of molecules in the ground-state and the lowest excited-state is a basic requirement for the design of a polarity-sensitive fluorescent probe. Some recently synthesized probes with center-symmetry have near zero dipole deviation while undergoing notably solvatochromic behaviors. Thus, it is necessary to find a new method beyond the Lippert-Mataga model to qualitatively estimate the molecular solvent shifts. To this end, a state-specific descriptor (SSD) based on molecular surface electrostatic potentials (ESP) is proposed to explain the solvatochromic behaviors of the well-studied coumarin C153 and center-symmetric DCB-1d. In contrast to the experimental solvent shifts and state-specific TD-DFT calculations, the SSD successfully explains the solvatochromic effect of C153 and DCB-1d molecules. In addition, the SSD was tested by using eight selected polarity-sensitive fluorescent molecules. The SSD was found to provide a good linear relationship with solvatochromism.

Specific and universal interactions in Benzo-[f]-Quinolinium Acetyl-Benzoyl Methylid (BQABM) solutions; excited state dipole moment of BQABM.

The zwitterionic molecule Benzo-[f]-Quinolinium Acetyl Benzoyl Methylid (BQABM) is studied in this paper from quantum mechanical and solvatochromic point of view. The nature and contribution of each type of intermolecular interaction to the spectral shifts in different solvents were estimated based on solvatochromic analyze of BQABM visible absorption band. The correlation coefficients for the dependence of the wavenumber in the maximum of BQABM visible band vs. the solvent parameters were computed by Linear Energy Relationships Simulation (LERS). The optimized geometry of BQABM and some physical and chemical parameters for isolated molecule in its ground electronic state were obtained with Spartan'14 programs. The results obtained in quantum mechanical analysis and also LERS correlation coefficients were used to determine the excited state dipole moment of BQABM by a Variational method in McRae and Kawschi hypothesis that the molecular polarizability does not change in the visible absorption process. An increase in the BQABM electric dipole moment by excitation resulted in our study.

Hydrogen-bonding-induced bathochromic effect of Si-coumarin and its use in monitoring adipogenic differentiation.

Introduction of a heteroatom into a fluorophore was carried out for coumarin through a replacement of its bridging oxygen atom with a silicon atom. The maximum-emission wavelength of Si-coumarin (SiC B) bathochromically shifted from 426 nm in cyclohexane to 626 nm in water. The adipogenic differentiation processes in mesenchymal stem cells were monitored using SiC B.

Microsolvation of porphine molecules in superfluid helium nanodroplets as revealed by optical line shape at the electronic origin.

We investigate the line shape at the electronic origin of single porphine molecules doped into superfluid helium droplets as a function of the droplet size. Helium droplets comprised of less than 105 atoms are generated from an expansion of gaseous helium, while droplets with more than 105 atoms originate from liquid helium. In contrast to our recent study on phthalocyanine, porphine is found to exhibit a solvent shift to the blue with respect to the gas-phase resonance frequency as well as a multiplet splitting. A comparison of the helium-induced features of phthalocyanine and porphine with those obtained in similar studies on tetracene and pentacene reveals that these occur chiefly as two kinds of excitations distinguished by their linewidths and their dependence on the droplet size. Moreover, at quasi-bulk conditions achieved with droplets in excess of 106 helium atoms, none of these four dopant species yields an optical spectrum that can be assigned to a plausible rotational band structure.

Crystal Structures and Optical Properties of Two Novel 1,3,5-Trisubstituted Pyrazoline Derivatives

Two novel 1,3,5-trisubstituted pyrazoline derivatives—1-acetyl-3-(4-methoxyphenyl)-5-(6-methoxy-2-naphthyl)-pyrazoline (2a) and 1-(4-nitrophenyl)-3-(4-methoxyphenyl)-5-(6-methoxy-2-naphtyl)-pyrazoline (2b)—were synthesized and their structures were determined by single crystal X-ray crystallography. Both of the two crystals exhibit twisted structures due to the large dihedral angles between the pyrazolinyl ring and the aromatic ring at the 5-position (88.09° for 2a and 71.26° for 2b). The optical–physical properties of the two compounds were investigated. The fluorescent emission of 2b arises from the 1,3-disubstituted pyrazoline chromophores and exhibits a red shift in polar solvents and solid-state, which could be attributed to photo-induced intramolecular charge transfer (ICT) from N1 to C3 in the pyrazoline moiety and the intermolecular interactions within the crystal. The fluorescent emissions of 2a (λmax 358–364 nm) in solvents and solid-state both come from 6-methoxy-2-naphthyl chromophores, which are fairly insensitive to the solvent polarity.

Calculating Optical Rotatory Dispersion Spectra in Solution Using a Smooth Dielectric Model.

The calculation of specific rotation of molecules in solution is probed at the coupled cluster (CC) level utilizing a continuum dielectric model based on a definition of the dielectric permittivity as a smooth function of electron density. Solvation effects are captured through polarization of Hartree-Fock (HF) molecular orbitals before subsequent calculations with the coupled cluster singles and doubles (CCSD) method. For the challenging ( S)-methyloxirane molecule, CCSD specific rotations yield an incorrect sign for the rotation in water, and the continuum model is unable to predict the wide variations in the optical rotatory dispersion (ORD) curves seen for nonpolar solvents of similar dielectric constant. In two molecules, (1 S,4 S)-norbornenone and ( S)-2-chloropropionitrile, specific rotations computed with CCSD in conjunction with implicit solvent fail to provide solvent shifts of the correct order of magnitude, indicating that the solvent response is a major contribution to the overall solvation effect.

Physicochemical Investigation of HDDP Azomethine Dye as Turn-On Fluorescent Chemosensor for High Selectivity and Sensitivity of Al3+ Ions

4-[(2-Hydroxy-naphthalen-1-ylmethylene)-amino]-1,5-dimethyl-2-phenyl-1,2-dihydro-pyrazol-3-one (HDDP) was synthesized by the reaction of 4-amino-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one with 2-hydroxy-l-naphthaldehyde. The structure was confirmed by the IR, 1H-NMR, 13C-NMR, and EI-MS spectra and elemental analysis. Physicochemical parameters of the HDDP such as extinction coefficient, oscillator strength, transition dipole moment, Stokes shift, and fluorescence quantum yield in different solvents were studied on the basis of polarities. The interactions of various metal ions with HDDP were also studied using steady state fluorescence measurements. The emission profile reveals that it acts as off–on type fluorescent chemosensor for selective and sensitive detection of Al3+ ions. Complexation stoichiometry and mechanism of enhancement were determined from a Benesi–Hildebrand plot.

The Influence of Polymers on the Supersaturation Potential of Poor and Good Glass Formers.

The increasing number of poorly water-soluble drug candidates in pharmaceutical development is a major challenge. Enabling techniques such as amorphization of the crystalline drug can result in supersaturation with respect to the thermodynamically most stable form of the drug, thereby possibly increasing its bioavailability after oral administration. The ease with which such crystalline drugs can be amorphized is known as their glass forming ability (GFA) and is commonly described by the critical cooling rate. In this study, the supersaturation potential, i.e., the maximum apparent degree of supersaturation, of poor and good glass formers is investigated in the absence or presence of either hypromellose acetate succinate L-grade (HPMCAS-L) or vinylpyrrolidine-vinyl acetate copolymer (PVPVA64) in fasted state simulated intestinal fluid (FaSSIF). The GFA of cinnarizine, itraconazole, ketoconazole, naproxen, phenytoin, and probenecid was determined by melt quenching the crystalline drugs to determine their respective critical cooling rate. The inherent supersaturation potential of the drugs in FaSSIF was determined by a solvent shift method where the respective drugs were dissolved in dimethyl sulfoxide and then added to FaSSIF. This study showed that the poor glass formers naproxen, phenytoin, and probenecid could not supersaturate on their own, however for some drug:polymer combinations of naproxen and phenytoin, supersaturation of the drug was enabled by the polymer. In contrast, all of the good glass formers—cinnarizine, itraconazole, and ketoconazole—could supersaturate on their own. Furthermore, the maximum achievable concentration of the good glass formers was unaffected by the presence of a polymer.

A semi-empirical estimation of ground and excited state dipole moments of zinc phthalocyanine from solvatochromic shift data

A semi-empirical determination of ground and excited state dipole moments of zinc phthalocyanine (ZnPc) from solvatochromic shifts is hereby presented. The ratio of the excited- and ground-state dipole moments of ZnPc ( ) was estimated by a combination of the Bakshiev and the Kawski-Chamma-Viallet’s equations, while the difference in the excited- and ground-state dipole moments (Dm) was estimated usingthe molecular-microscopic solvent polarity parameters ( ), alongside the Stokes’ shifts (Dá¿¡) in the various solvents. The dipole moment of ZnPc is significantly higher in the excited singlet state (me = 3.12 D) than in the ground state (mg = 1.50 D). Obviously charge separation is greater in the excited state of ZnPc than in its ground state. Â

Ability of gelatin and BSA to stabilize the supersaturated state of poorly soluble drugs

Gelatin and bovine serum albumin (BSA), two readily available biopolymers, were examined for their effect on solubility and supersaturation of drugs because of their capacity to interact with drugs (e.g. via hydrogen bonding, van der Waals or electrostatic interactions, etc.). Carbamazepine, cinnarizine, diazepam, itraconazole, nifedipine, indomethacin, darunavir (ethanolate), ritonavir, fenofibrate, griseofulvin, ketoconazole and naproxen were selected accordingly as twelve structurally different model BCS Class II drugs. All selected drugs were evaluated for solubility and supersaturation in presence and absence of these two biopolymers in four media (purified water, FaSSIF, FaSSGF and FeSSIF) by means of the shake flask method for 48 h and solvent shift induced supersaturation, respectively. In ca. 75% of the supersaturation experiments with these two biopolymers, drug concentrations significantly different delete from solubility were observed with supersaturation factors (SF) varying between 1.28 and 7.89 (p ≤ 0.05) and between 1.16 and 20.51 (p ≤ 0.01). In order to make an estimation on the relevance of these results, a comparison with three commonly used (semi-) synthetic polymers (HPMC, PVP and PVPVA) was included in purified water. This showed that both biopolymers were at least as efficient as the (semi-) synthetic polymers in sustaining induced supersaturation as in ten out of twelve API comparable results were obtained.

Probing the effect of solvation on photoexcited 2-(2'-hydroxyphenyl)benzothiazole via ultrafast Raman loss spectroscopic studies.

2-(2'-Hydroxyphenyl)benzothiazole (HBT) molecule is known to exhibit efficient excited state intramolecular proton transfer. As a consequence, it shows fluorescence with a large Stokes shift (∼10 000 cm-1) in non-polar solvents. However, fluorescence in polar solvents has a dual-band which corresponds to the emission from both the enol* and the keto* forms. Also, the excited state lifetime significantly varies with the solvent polarity. Recently, Mohammed et al. [J. Phys. Chem. A 115, 7550 (2011)] have shown that the excited state of HBT in acetonitrile (ACN) relaxes back to its ground electronic state through two competitive decay pathways, i.e., intramolecular proton transfer and intramolecular twisting between hydroxyphenyl and benzothiazole units in contrast to its behavior when it is in tetrachloroethene, a non-polar solvent. Here, by following the time-evolution of vibrational features of excited state HBT in ACN through ultrafast Raman loss spectroscopy, we demonstrate a direct evidence for the involvement of torsional motion leading to an ultrashort lifetime of HBT. The time evolution of the C7-N vibrational frequency exhibited a red-shift in its peak position, clearly indicating the evolution of the initially planar cis-keto* form to the more twisted keto* form. Density functional theory calculations also well corroborate the experimental findings. Furthermore, wavepacket analysis of this mode reveals a strong correlation with the torsional motion in ACN.

3-Formylpyrazolo[1,5- a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores.

A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5- a]pyrimidines 4a-k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between β-enaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer-Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5- a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5- a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

Microsolvation of phthalocyanine molecules in superfluid helium nanodroplets as revealed by the optical line shape at electronic origin.

We investigate the solvent shift of phthalocyanine (Pc) doped into superfluid helium droplets and probed by optical spectroscopy at the electronic origin. Our present work complements extant studies and provides results that in part contradict previous conclusions. In particular, the solvent shift does not increase monotonously with droplet radius all the way up to the bulk limit, but exhibits just the reverse dependence instead. Moreover, a substructure is resolved, whose characteristics depend on the droplet size. This behavior can hardly be reconciled with that of a freely rotating Pc-helium complex.