Aggregation/Crystallization-Induced Emission in Naphthyridine-Based Carbazolyl-Modified Donor-Acceptor Boron Dyes Tunable by Fluorine Atoms.

Four donor-acceptor boron difluoride complexes based on the carbazole electron donor and the [1,3,5,2]oxadiazaborinino[3,4-a][1,8]naphthyridine acceptor were designed, synthesized, and systematically spectroscopically investigated in solutions, in dye-doped polymer films, and in the solid states. The dyes exhibit an intense blue to red solid-state emission with photoluminescence quantum yields of up to 59 % in pure dye samples and 86 % in poly(methyl methacrylate) films. All boron complexes show aggregation-induced emission and reversible mechanofluorochromism. The optical properties of these dyes and their solid state luminescence can be tuned by substitution pattern, i. e., the substituents at the naphthyridine unit. Exchange of CH3- for CF3-groups does not only increase the intramolecular charge transfer character, but also provides a crystallization-induced emission enhancement.

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se-NHC Adducts*.

N-heterocyclic carbene ligands (NHC) are widely utilized in catalysis and material science. They are characterized by their steric and electronic properties. Steric properties are usually quantified on the basis of their static structure, which can be determined by X-ray diffraction. The electronic properties are estimated in the liquid state; for example, via the 77 Se liquid state NMR of Se-NHC adducts. We demonstrate that 77 Se NMR crystallography can contribute to the characterization of the structural and electronic properties of NHC in solid and liquid states. Selected Se-NHC adducts are investigated via 77 Se solid state NMR and X-ray crystallography, supported by quantum chemical calculations. This investigation reveals a correlation between the molecular structure of adducts and NMR parameters, including not only isotropic chemical shifts but also the other chemical shift tensor components. Afterwards, the liquid state 77 Se NMR data is presented and interpreted in terms of the quantum chemistry modelling. The discrepancy between the structural and electronic properties, and in particular the π-accepting abilities of adducts in the solid and liquid states is discussed. Finally, the 13 C isotropic chemical shift from the liquid state NMR and the 13 C tensor components are also discussed, and compared with their 77 Se counterparts. 77 Se NMR crystallography can deliver valuable information about NHC ligands, and together with liquid state 77 Se NMR can provide an in-depth outlook on the properties of NHC ligands.

Controlling Emissive Properties by Intramolecular Hydrogen Bonds: Alkyl and Aryl meso-Substituted Porphycenes.

Porphycene, a porphyrin isomer, is an efficient fluorophore. However, four-fold meso substitution with alkyl groups decreases the fluorescence quantum yield by orders of magnitude. For aryl substituents, this effect is small. To explain this difference, we have synthesized and studied a mixed aryl-alkyl-substituted compound, 9,20-diphenyl-10,19-dimethylporphycene, as well as the 9,20-diphenyl and 9,20-dimethyl derivatives. Analysis of the structural, spectroscopic, and photophysical data of the six porphycenes, combined with quantum chemical calculations, shows a clear correlation between the strength of the intramolecular NH⋅⋅⋅N hydrogen bonds and the efficiency of the radiationless depopulation of the lowest-excited singlet state. This result led us to propose a model in which the delocalization of the inner protons in the cavity of the macrocycle is responsible for the nonradiative deactivation channel. The applicability of the model is confirmed by the literature data for other alkyl- or aryl-substituted porphycenes. The finding of a correlation between structural and emissive characteristics enables a rational design of porphycenes with desired photophysical properties.

Oxidative Functionalization of Trinor-18α-olean-17(22)-ene Derivatives. Annulation of the E-Ring by an Intramolecular Aldol Reaction.

cis-Dihydroxylation of trinor-18α-olean-17(22)-ene 2 with osmium tetroxide led to diol 9. Its cleavage with lead tetraacetate gave tetracyclic ketoaldehyde 10. By comparison, the ozonation of trinor-18α-olean-17(22)-ene 2 in the presence of p-toluenesulfonic acid gave the corresponding ketoacetal 12. Both products were subjected to an intramolecular aldol reaction under the acidic conditions and yielded unusual triterpenes bearing a bicyclo[4.3.1]decane fragment (22). Further manipulation of the protective groups afforded compounds useful in triterpene synthesis, especially in the preparation of potentially biologically active saponins based on a tetracyclic terpene core.

Rearrangements of the Betulin Core. Synthesis of Terpenoids Possessing the Bicyclo[3.3.1]nonane Fragment by Rearrangement of Lupane-Type Epoxides.

The rearrangements of dihydrobetulin, dihydrobetulinic acid, and abeo-lupane epoxides under acidic conditions (HCl, montmorillonite K10, and BF3·Et2O) were studied. The treatment of dihydrobetulin with HCl or K10 produced abeo-lupane olefins. Their epoxidation afforded epoxides, which, in the presence of protic or Lewis acids, rearranged to dienes or lupanes bearing a bicyclo[3.3.1]nonane fragment. The structure of final products depended on the nature of the catalyst. The HCl promoted 1,4-elimination of water, whereas in the presence of BF3·Et2O bond migration took place preferentially. Montmorillonite K10 favored cyclization to bicyclononane.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism.

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

Organolithium-Mediated Postfunctionalization of Thiazolo[3,2-c][1,3,5,2]oxadiazaborinine Fluorescent Dyes.

An effective method for transition-metal-free postfunctionalization of thiazolo[3,2-c][1,3,5,2]oxadiazaborinine dyes via direct lithiation of the 1,3-thiazole ring was developed. The reaction allows valuable regioselective C-H modification of these N,O-chelated organoboron chromophores incorporating different groups, including C-, Hal-, Si-, S-, Se-, and Sn-substituents. As a result, a library of novel fluorescent 1,3-thiazole-based organoboron complexes has been synthesized and characterized. The influence of the donor/acceptor strength of the substituent E on the photophysical properties has been established. The compound with a bulky lipophilic substituent (SnBu3) exhibits a relatively high solid-state photoluminescence quantum yield of 44%.

Application of the Suzuki-Miyaura Reaction for the Postfunctionalization of the Benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinine Core: An Approach toward Fluorescent Dyes.

A fluorescent dye based on the 8-brominated benzo[4,5]thiazolo[3,2- c][1,3,5,2]oxadiazaborinine core was synthesized from benzo[ d]thiazol-2-amine. The new boron complex can be effectively modified by a palladium-catalyzed Suzuki-Miyaura cross-coupling reaction with (het)arylboronic acids. This reaction allows a valuable regioselective postfunctionalization of 1,3,5,2-oxadiazaborinine chromophores with different aromatic substituents. The solutions of obtained target complexes in organic solvents demonstrate high fluorescence quantum yields. The compound with a 4-cyanophenyl group at benzothiazole unit (Ar = 4-C6H4CN) exhibits a comparatively high fluorescence quantum yield of 0.31 in the solid state.

Antiaromatic or Nonaromatic? 21 H,61 H-2,6(2,5)-Dipyrrola-1,5(2,6)-dipyridinacyclooctaphane-3,7-diene: a Porphycene Derivative with 4 N π Electrons.

A porphycene-derived compound with a 20 π-electron skeleton has been obtained by replacing two pyrrolene units of porphycene by pyridine rings. NMR, electronic absorption and MCD spectra, and the lack of fluorescence are typical for 4 N cyclic π electron systems. The electronic structure and the differences with respect to porphycene can be rationalized by treating these compounds as perturbed, doubly positively charged [22]annulene and [20]annulene perimeters, respectively. Even though the spectroscopic and photophysical criteria proposed for antiaromatic systems are fulfilled, the molecule is very stable. We argue that the compound should be characterized as nonaromatic rather than antiaromatic. The perimeter model is recommended as a powerful tool for predicting the electronic structure and spectra and as a useful addition to other methods that probe the aromaticity.

In Depth Analysis of Chiroptical Properties of Enones Derived from Abietic Acid.

With the use of inexpensive commercially available abietic acid, a whole series of abietane enones were prepared in high yields. The structures of all the products obtained were determined by comprehensive spectroscopic analysis with particular emphasis on the use of advanced NMR techniques, comparison with previously reported data and, where possible, by single crystal X-ray diffraction. However, in cases where X-ray crystallography was not applicable or compounds tested were unstable, a final stereochemical assignment could be inferred only by electronic circular dichroism (ECD) supported by vibrational circular dichroism to increase credibility. To reveal the relationship between structure and chiroptical properties, we used combined experimental and theoretical analysis of geometries, structural parameters, and chiroptical properties of all enones synthesized. A thorough analysis of their conformational flexibility by examining the effect of solvent and temperature on the ECD spectra was also used to achieve desired objectives. As a result, the impact of substituents adjacent to the enone chromophore on the conformation was determined by demonstrating that even slight changes in the position of hydroxyl and isopropyl groups attached to carbon C13 may substantially affect ECD curves' pattern, leading in some cases to Cotton effects sign reversal.

Synthesis and Comprehensive Structural and Chiroptical Characterization of Enones Derived from (-)-α-Santonin by Experiment and Theory.

The aim of the present work is to explain the causes of the observed deviations from sector and helicity rules to determine the absolute configuration of optically active α,ß-unsaturated ketones by means of electronic circular dichroism (ECD). To this end, a series of model compounds with a common decahydronaphthalene skeleton representing both cisoid and transoid enones were synthesized. In the framework of this work, detailed dichroic studies supported by single crystal X-ray analysis were performed where possible. To assist the achievement of the desired objectives the conformational flexibility of the selected cis-enones through the dependence of solvent and temperature on the ECD spectra were examined. All experimental studies were supplemented by detailed DFT calculations. A notable result of the study is assessing the applicability of the enone sector and helicity rules in dichroic studies and potential restrictions. To this end, a number of factors that could determine the signs of the individual Cotton effects has been considered. Among these nonminimum structure effects, i.e., twisting of the enone chromophore and nonplanarity of the enone double bond can be mentioned.

Synthesis of 28a-homoselenolupanes and 28a-homoselenolupane saponins.

A practical synthesis of 28a-homo-28a-selenolupane triterpenes and the corresponding selenosaponins containing d-mannose, l-arabinose, l-rhamnose, and d-idose moieties is described. Selenium containing triterpenes were obtained from the readily available 3-O-allyl-homobetulin mesylate by nucleophilic substitution with the selenocyanate ion which upon reduction of the -SeCN group afforded the free selenol. Glycosylation using classical Schmidt donors gave 1,2-trans selenosaponins as the main product as well as minute amounts of 1,2-cis isomers. This is one of the very few examples of the synthesis of selenoglycosides by direct glycosylation of free selenols. The studied selenol showed high resistance to air oxidation resulting in good stability during the synthesis of selenolupane derivatives. Cytotoxic activities of new homoselenolupane derivatives were also evaluated in vitro and revealed that some triterpenes exhibited an interesting profile against human cancer cell lines.

Arresting consecutive steps of a photochromic reaction: studies of ß-thioxoketones combining laser photolysis with NMR detection.

Photochromism of monothiodibenzoylmethane has been studied in a number of environments at different temperatures. Direct laser irradiation of a sample located in the NMR magnet allowed in situ monitoring of the phototransformation products, determining their structure, and measuring the kinetics of the back reaction. These observations, along with the data obtained using electronic and vibrational spectroscopies for rare gas matrix-isolated samples, glasses, polymers, and solutions, as well as the results of quantum-chemical calculations, provide insight into the stepwise mechanism of the photochromism in ß-thioxoketones. At low temperature in rigid matrices the electronic excitation leads to the formation of the -SH exorotamer of the (Z)-enethiol tautomer. In solutions, further steps are possible, producing a mixture of two other non-chelated enethiol forms. Photoconversion efficiency strongly depends on the excitation wavelength. Analysis of the mechanisms of the photochromic processes indicates a state-specific precursor: chelated thione-enol form in the excited S2(ππ*) electronic state. The results show the potential of using laser photolysis coupled with NMR detection for the identification of phototransformation products.

Comprehensive chiroptical study of proline-containing diamide compounds.

The continuously growing interest in the understanding of peptide folding led to the conformational investigation of methylamides of N-acetyl-amino acids as diamide models. Here we report the results of detailed conformational analysis on Ac-Pro-NHMe and Ac-ß-HPro-NHMe diamides. These compounds were analyzed by experimental and computational methods, the conformational distributions obtained by Density Functional Theory (DFT) calculations for isolated and solvated diamide compounds are discussed. The conformational preference of proline-containing diamide compounds as a function of the ambience was observed by a number of chiroptical spectroscopic techniques, such as vibrational circular dichroism (VCD), electronic circular dichroism (ECD), Raman optical activity (ROA) spectroscopy, and additionally by single crystal X-ray diffraction analyses. Based on a comparison between Ac-Pro-NHMe and Ac-ß-HPro-NHMe, one can conclude that due to the greater conformational freedom of the ß-HPro derivative, Ac-ß-HPro-NHMe shows different behavior in solid- and solution-phase, as well. Ac-ß-HPro-NHMe tends to form cis Ac-ß-HPro amide conformation in water, dichloromethane, and acetonitrile in contrast to its α-Pro analog. On the other hand, the crystal structure of the ß-HPro compound cannot be related to any of the conformers obtained in vacuum and solution while the X-ray structure of Ac-Pro-NHMe was identified as tα(L)-, which is a trans Ac-Pro amide containing conformer also predominant in polar solvents.

Selective transformations of friedelanes isolated from cork smoker wash solids.

Friedelin (1) and 3-acetoxyfriedel-3-en-2-one (4), commonly known as friedelane triterpenoids, have been isolated from cork smoker wash solids (also known as black wax) on a multi-gram scale. These compounds are valuable starting materials for the synthesis of new friedelane derivatives. Stereoselective reduction of friedelin by treatment with LiAlH4, sodium, or catalytic hydrogenation results in the formation of both isomers of friedelinol (5 and 7) in excellent yields. Similarly, the reduction of 3-acetoxyfriedel-3-en-2-one gave epi-cerin (14) and a series of isomeric 2,3-diols or α-hydroxyketones. These transformations provide the most straightforward and convenient methods for the synthesis of A-ring functionalised friedelane derivatives using easily accessible starting materials.

Low-Molecular-Weight Organogelators Based on N-dodecanoyl-L-amino Acids-Energy Frameworks and Supramolecular Synthons.

Lauric acid was used to synthesize the low-molecular-weight organogelators (LMOGs), derivatives of two endogenous (L)-alanine, (L)-leucine, and three exogenous (L)-valine, (L)-phenylalanine, and (L)-proline amino acids. The nature of processes responsible for the gel formation both in polar and in apolar solvents of such compounds is still under investigation. Knowing that the organization of surfactant molecules affects the properties of nano scale materials and gels, we decided to elucidate this problem using crystallographic diffraction and energy frameworks analysis. The single crystals of the mentioned compounds were produced successfully from heptane/tBuOMe mixture. The compounds form lamellar self-assemblies in crystals. The energetic landscapes of single crystals of a series of studied amphiphilic gelators have been analyzed to explore the gelling properties. The presented results may be used as model systems to understand which supramolecular interactions observed in the solid state and what energy contributions are desired in the designing of new low-molecular-weight organic gelators.

Ring-expanded bicyclic ß-lactams: a structure-chiroptical properties relationship investigation by experiment and calculations.

In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic ß-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the ß-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240-215 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic ß-lactams with the nonplanar amide chromophore only.

Fluorinated Porphycenes: Synthesis, Spectroscopy, Photophysics, and Tautomerism.

Six porphycenes have been synthesized, bearing one, two, or three fluorine atoms attached directly to the 18-π-electron system at the meso positions. These novel compounds have been characterized by structural, electrochemical, and spectral techniques, combined with quantum chemical calculations. In three fluoroporphycenes, the unsymmetric substitution pattern leads to the presence of two nonequivalent trans tautomeric forms. They have been identified using electronic absorption, emission, and magnetic circular dichroism spectroscopies. Their relative energies have been estimated for the ground and lowest excited electronic states. Tautomerization potential is quasi-symmetric in S0 , but becomes strongly nonsymmetric in S1 . Femtosecond transient absorption studies allowed determination of tautomerization rates, larger and similar for both directions of the double hydrogen transfer in S0 , lower and disparate in S1 . Fluoroporphycenes emerge as good candidates for detailed studies of mechanisms of double hydrogen transfer, as well as processes responsible for rapid radiationless excited state depopulation.

Synthesis and comprehensive structural and physicochemical characterization of dutasteride hydrochloride hydrate solvates.

Four crystalline dutasteride hydrochloride hydrate solvates containing respectively methanol, ethanol, acetone and acetonitrile molecules were obtained. All samples were characterized by extensive spectroscopic analysis with infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and 1H as well as 13C NMR techniques. For three solvates, i.e. methanol, ethanol and acetone solvates, the single crystal X-ray diffraction (SCXRD) experiments were possible, and their respective crystal and molecular structures were determined. The present study allowed to unambiguously establish the molecular composition of solvates as consisting of a dutasteride : hydrogen chloride : water : solvent in a molar ratio of 1:1:1:1 and confirm that they are isostructural. Beyond providing the full spectroscopic characteristic of the compounds, the results obtained have also allowed clarifying of some appearing inconsistencies in published literature regarding the appropriate attribution of IR absorption bands to the relevant molecular vibrations.

Comprehensive spectroscopic characterization of finasteride polymorphic forms. Does the form X exist?

The pure polymorphic forms I, II, and III of finasteride were prepared and their purity was confirmed by FTIR, differential scanning calorimetry, and X-ray powder diffraction measurements. The preparation experiments demonstrated that the desolvation process of some finasteride solvates does result not only in the formation of polymorphic forms I and II, but also in obtaining the pure form III. The (13)C cross-polarization magic angle spinning (CP-MAS) and the (15)N CP-MAS spectra can distinguish all three polymorphic forms of finasteride. Additionally, the data point to the presence of only one molecule in crystallographic asymmetric unit of polymorphic forms I and III and two molecules in the form II. The application of electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectroscopy for finasteride polymorphic forms shows that the three polymorphs could be distinguished by the characteristic shapes of their VCD spectra in the spectral range 1520-1440 cm(-1). The ECD spectral patterns of all these forms, however, are almost indistinguishable because of their close similarity. Comparison of the (13)C CP-MAS spectra of forms I, II, and III with those reported in the literature indicates that the so-called finasteride "form X" is identical to the previously known finasteride form III. On this basis, the existence of form X was excluded.