The synthetic macrocycle cucurbit[6]uril forms host-guest inclusion complexes with molecular dibromine and diiodine. As evidenced by their crystal structures, the encapsulated dihalogens adapt a tilted axial geometry and are held in place by two different types of halogen-bonding interactions, one with a water molecule (bond distances 2.83 Å for O···Br and 3.10 Å for O···I) and the other one with the ureido carbonyl groups of the molecular container itself (bond distances 3.33 Å for O···Br and 3.49 Å for O···I). While the former is of the conventional type, involving the lone electron pair of an oxygen donor, the latter is perpendicular, involving the π-system of the carbonyl oxygen (N-C═O···X dihedrals ca. 90°). Such perpendicular interactions resemble those observed in protein complexes of halogenated ligands. A statistical analysis of small-molecule crystal structural data, as well as quantum-chemical calculations with urea as a model (MP2/aug-cc-pVDZ-PP), demonstrates that halogen bonding with the π-system of the carbonyl oxygen can become competitive with the commonly favored lone-pair interaction whenever the carbonyl group carries electron-donating substitutents, specifically for ureas, amides, and esters, and particularly when the lone pairs are engaged in orthogonal hydrogen bonding (hX bonds). The calculations further demonstrate that the perpendicular interactions remain significantly attractive also for nonlinear distortions of the O···X-X angle to ca. 140°, the angle observed in the two reported crystal structures. The structural and theoretical data jointly support the assignment of the observed dihalogen-carbonyl contacts as genuine halogen bonds.

A series of nine borylated arylisoquinolines has been prepared with systematic variation in their electronic properties and their photophysical properties were investigated. The color of their fluorescence can be finely tuned by changing the properties of the aryl moiety, which is involved in internal-charge-transfer processes. For example, methoxy-substituted compound 5 showed an intense green emission, whereas dimethylamino-substituted compound 6 showed an orange-red emission. These new fluorophores were tested for their potential as molecular switches with external ionic stimuli, such as protons and fluoride ions. On the one hand, protonation of the isoquinoline moiety led to fluorescence enhancement for compounds that showed weak charge transfer and fluorescence quenching for compounds that showed strong charge transfer. On the other hand, the formation of ate complexes with fluoride led to strong fluorescence quenching in all of the investigated cases.

Borylated arylisoquinolines with redshifted internal charge-transfer (ICT) emission were prepared and characterized. Upon heating, significant fluorescence quenching was observed, which forms the basis for a molecular thermometer. In the investigated temperature range (283-323 K) an average sensitivity of -1.2 to -1.8% K(-1) was found for the variations in fluorescence quantum yield and lifetime. In the physiological temperature window (298-318 K) the average sensitivity even reaches values of up to -2.4% K(-1). The thermometer function is interpreted as the interplay between excited ICT states of different geometry. In addition, the formation of an intramolecular Lewis pair can be followed by (11)B NMR spectroscopy. This provides a handle to monitor temperature-dependent ground-state geometry changes of the dyes. The role of steric hindrance is addressed by the inclusion of a derivative that lacks the Lewis pair formation.

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