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The stability of merocyanine forms formed under UV irradiation of a solution of a spiropyran salt, in which an organic part acts as a cation and a compact bromide ion as an anion, their photophysical properties, and the formation mechanism are studied in this work using time-dependent density functional theory. Theoretical calculations show that TTC and CTT are the most stable open forms (the difference in stability energies is 10.5 and 12.0 kcal mol−1 relative to the formation energy of spiropyran, respectively). The simulated absorption bands in the UV spectrum of the merocyanine forms are observed both in the UV region at 308–366 nm and in the visible region at 544–757 nm due to n → π* and π → π* type transitions. We found that the isomerisation mechanism of spiropyran into merocyanine forms includes two key stages: the ring opening to form cisoid merocyanine forms (except unstable TCC) through conical intersection and their subsequent isomerisation to form stable transoid isomers. The length of the Cspiro–O bond is 1.97 Å and the C1′–C2′–C3′–C4′ angle is 70° in the structure close to conical intersection. The stage that determines the rate of this process is the isomerisation between transoid forms, as in the case of transformation of open merocyanine forms into spiropyran. [ABSTRACT FROM AUTHOR]

The present article details a novel approach for introducing strained 2,3-dimethylenebicyclo[2,2,0]hexane structures into the C60 fullerene molecule, based on the reaction of C60 with substituted propargylamines, promoted by the Ti(Oi-Pr)4 and EtMgBr system. The theoretical LUMO energies of the strained C60 derivatives calculated at the PBEPBE/6-311G(d,p) level of theory are from −3.93 to −4.09 eV, which indicates that these compounds seem to be promising as electron-acceptor materials for bulk heterojunction organic solar cells. [ABSTRACT FROM AUTHOR]

The conformational mobility of large heteroatomic cycles (＞6) in organic compounds studied in the liquid state is a major challenge. This is due to low energy barriers between the conformational isomers (1–2 kcal mol−1), which make it difficult to find the most important factors in their preference. Eleven single crystal tetraoxazaspirocycloalkanes are considered in this work with single-crystal X-ray diffraction, DFT, and AIM calculations. Natural bond orbital analysis at the B3LYP/6-31G(d,2p) level of theory and topological analysis of the electron density within Bader's theory of “Atoms in Molecules” showed that the tetraoxazocane cycle in spiro-adamantanetetraoxazocanes and tetraoxazaspiroalkanes possesses a twist–boat–chair conformation due to the lower interaction energy of LP(O6) →σ*(C5–N4) and the large contribution of the π-component to the C3–O2 bond. The tetraoxazocane cycle in isopropyl-methyl-tetraoxaza-spiro-tridecanes adopts a chair–chair conformation due to the higher interaction energy of LP(O6) →σ*(C5–N4) and the smaller contribution of the π-component. A key factor, which determines the large O1–O2–O6–O7 pseudo-torsion angles (102–112°) in spiro-adamantanetetraoxazocanes and tetraoxaza-spiro-alkanes, is the absence of intramolecular C–H…O and C–H…π contacts and vice versa, which provide smaller O1–O2–O6–O7 pseudo-torsion angles (0.6–4°) in isopropyl-methyl-tetraoxaza-spiro-tridecanes. The molecules of spiro-adamantanetetraoxazocanes and tetraoxazaspiroalkanes in single crystals are stabilized via C–H…O and C–H…H–C weak intermolecular interactions. The molecules of isopropyl-methyl-tetraoxaza-spiro-tridecanes are packed in crystals with C–H…O unusual bifurcated one-component and multicomponent contacts. [ABSTRACT FROM AUTHOR]

Crystals of (1,5,3-dithiazepan-3-yl)-alkanoic acids with achiral and chiral amino acid moieties have been obtained, and their structures were studied using a single crystal X-ray technique. Simple crystal systems, namely monoclinic, triclinic, and orthorhombic, were revealed in a series of the studied compounds. The reference 1,5,3-dithiazepan-3-ol forms head-to-head cyclic dimeric associates R22(6) via strong O–H…N intermolecular hydrogen bonds. The achiral 1,5,3-dithiazepanes form head-to-head cyclic dimers R22(8) between two carboxylic groups, whereas the co-crystals involve solvent molecules to constitute dimeric pairs R33(11) and R22(8) through O…H and N…H hydrogen bonds. These dimers further contribute to the aggregation of layers and stacks due to diverse H…H and S…H contacts. The chiral dithiazepanes form head-to-tail R12(6), R22(7), and R22(8) cyclic dimers between the carboxylic group of one molecule and the dithiazepane moiety of the other one through H…H, S…H, and O…H intermolecular contacts. Hirshfeld analysis has shown that S…H (9.2–19.8%) and O…H (5.4–18.2%) intermolecular hydrogen bonds as well as weak H…H contacts (40.2–64.0%) are predominant in all the compounds to form crystal packing. [ABSTRACT FROM AUTHOR]