Synthesis and Photochromic Behaviour of Novel 2H‐1‐Benzopyrans (=2H‐Chromenes) Derived from Carbazololes

The synthesis and photochromic properties of new 2,2‐diphenyl‐2H‐1‐benzopyrans, fused to an indole moiety, are described. All compounds exhibit photochromic behaviour in solution at room temperature. The heteroanellation effects are variable and depend on the position and geometry of the fused indole moiety. A general bathochromic shift in the spectra of the open forms is observed. The presence of a N‐methyl group prevents the broadening of the absorption spectra and promotes the instability of some photoinduced forms of compounds with the indole moiety fused at the 5,6 positions of the 2H‐1‐benzopyran skeleton. The enhanced photocolouration efficiency in the near‐UV and the kinetics of thermal bleaching indicate that the novel compounds with an indole moiety fused at the 6,7 positions, particularly those with a linked thiophene moiety, are very interesting molecules for applications in the field of variable optical absorption systems.     

Photoinduced processes in dipyrrolyl-perfluoro-cyclopentenes

The photobehavior of five photochromic dipyrrolyl-perfluoro-cyclopentenes was studied by steady state and time-resolved absorption spectroscopy. The quantum yields of the UV-photoinduced ring-closing reaction (coloration) and the visible-stimulated cycloreversion reaction (bleaching) were measured. Kinetic and thermodynamic parameters of thermal bleaching were also determined. Nanosecond time-resolved experiments showed formation of a transient, which was not a precursor of the reaction photoproduct. This transient was tentatively assigned to a radical cation formed by direct photoionization through a short-lived triplet state. The nature of the transient species was supported by photoinduced electron transfer to electron acceptors.     

The structure of pyrazoles in the solid state: a combined CPMAS, NMR, and crystallographic study

The structures of six N-unsubstituted pyrazoles, three already known and three newly synthesized, have been studied by a combination of X-ray crystallography, multinuclear NMR (solution and solid state), and density functional theory (DFT) calculations. In those cases where crystal structure and CPMAS NMR were available, the agreement was almost perfect, allowing a prediction of the tautomer (with certitude) and the tetrameric structure (with high probability) in the case of 5-isopropyl-3-phenyl-1H-pyrazole without knowing the X-ray structure. In the case of the 5-(2-benzylphenyl)-3-trifluoromethyl-1H-pyrazole above represented, the DFT calculations at the B3LYP/6-31G level justify the great stability of this tautomer by the presence of an intramolecular N-H...pi interaction, present in solution.     

A Mechanistic Study of the Utilization of arachno‐Diruthenaborane [(Cp*RuCO)2B2H6] as an Active Alkyne‐Cyclotrimerization Catalyst

The reaction of nido‐[1,2‐(Cp*RuH)₂B₃H₇] ( 1 a , Cp*=η⁵‐C₅Me₅) with [Mo(CO)₃(CH₃CN)₃] under mild conditions yields the new metallaborane arachno‐[(Cp*RuCO)₂B₂H₆] ( 2 ). Compound 2 catalyzes the cyclotrimerization of a variety of internal‐ and terminal alkynes to yield mixtures of 1,3,5‐ and 1,2,4‐substituted benzenes. The reactivities of nido‐ 1 a and arachno‐ 2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne‐insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2 . The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne‐insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum‐chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne.     

Synthesis of new benzylpyridines as potential photochromic compoundsa

Photoinduced proton transfer systems such as 2‐(2′,4′‐dinitrobenzyl)pyridine derivatives have been developed in an attempt to apply the compounds to variable optical transmission materials. However their fatigue resistance property is a limitation for these applications. New benzylpyridines have been synthesized and tested. 2‐(2′,4′‐Diphenylsulfonybenzyl)pyridine was found to be photochromic in the crystalline state and in ethanolic solution (λ_max= 485 nm) upon UV flash photolysis.     

University timetabling by constraint-based reasoning: A case study

This paper presents a case-study which applies constraint-based reasoning to university timetable planning. The timetabling problem is formulated as a constraint satisfaction model and this model is solved using constrained-directed search algorithms with built-in forward checking and constraint propagation. The model and algorithms were tested with real data containing 536 lessons to be scheduled into 45 timeslots and 21 rooms. The solution to the problem was obtained with minimal computing effort and processing time. This study showed that modelling and remodelling could be carried out easily through a proposed parameterised model formulation. The proposed approach has also successfully maximised room utilisation and minimised the number of timeslots required to deliver all the lectures. This finding may facilitate the widespread implementation of automated timetabling systems to larger scale problems and a wider variety of application domains.     

Effect of complexation on spectral and kinetic properties of crown-substituted photochromic spironaphthoxazines

The insertion of benzo-15-crown-5 or benzo-18-crown-6 as a substituent at the 9'-position of a spironaphthoxazine molecule increases the lifetime of the open form of spironaphthoxazine by a factor ofca. 2. The complex formation between the crown groups and Mg²⁺ or Ba²⁺ ions changes the lifetime slightly. The addition of benzo-18-crown-6 at the 5'-position leads to an increase in the lifetime of the open form by an order of magnitude and the appearance of absorption with a maximum at 16420 cm^–1 under dark conditions, which testifies to the stabilization of the open form. The addition of Ba²⁺ ions to this compound results in a further increase in the lifetime byca. 35% due to complex formation. The luminescence of the benzo-15-crown-5-substituted compound is probably caused by both the complexation with Mg²⁺ and the photochemical reaction induced by photoactive light and is accompanied by the irreversible loss of photochromism.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2220–2224, September. 1996.     

Effect of the type of linkage between phenyl groups on the structure and photochemical properties of 2,2-diaryl-substituted pyridoannelated [2H]-chromenes

Three 2,2-diaryl-substituted pyridoannelated [2H]-chromenes have been studied by X-ray diffraction analysis. Bonding of the benzene rings through bridges of different nature and with different length affects substantially the orientation of the benzene rings, steric interactions at the C center, the conformation of the molecule, and the C−O bond length. A correlation between the photocolorability of chromenes under study and the orientation of the benzene rings with respect to the C−O bond, which provides different prerequisites to stabilization of the C(1)-centered, cation formed upon cleavage of the C−O bond, was established. The effect of the orientation of the benzene rings on the dark reaction of ring closure was found. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1129–1135, June, 1998.     

Étude structurale de spiropyrannes benzothiazoliniques par détermination expérimentale et théorique des moments dipolaires

For the conformational study of a series of benzothiazolinic spiropyrans, experimental and theoretical (E.H.T. method) dipole moments have been checked.The energetic data of the semi-empirical calculations for the eight possible spiropyrannic conformations (four limit conformations for each enantiomer) and the comparison between experimental and calculated dipole moments fit quite well, and allow an interesting approach to the conformation of these molecules, in spite of their complexity.