F HIGHLIGHTS (1)
F F
F F
F
F F
(2)
NC NC
(3) CN CN
Organic chemistry Figure 1
G. RICHARD STEPHENSON University of East Anglia, UK
A competitive intermolecular interaction strategy for white light emission Enabling organic molecules to line up accurately in the solid state is difficult, but certain pairs of structures are known to be good at this type of cocrystallisation. It has now been discovered that combining such pairs can provide a method to tune light-harvesting/light emitting systems (Y. Sun, Y. Lei, L. Liao, W. Hu; Angew. Chem. Int. Ed., 2017, 56, 10352). Molecules that can do this are
often flat (Figure 1) and in this case, the two pairs, pyrene (1) – octafluoronaphthalene (2) and pyrene (1) -1,2,4,5-tetracyanobenzene (3), exhibit different types of intermolecular interactions: [arene- perfluoroarene (AP) and charge- transfer (CT) interactions, respectively. Although 1 and 2 together emit blue light, in contrast to 1 and 3, which together emit orange light, these systems can be combined in the solid state to produce a white light emitting material.
Accessing a fluorescent folded conformer An example of another way to manipulate intermolecular stacking interactions has been reported recently (G. H. Aryal, K. I. Assaf, K. W. Hunter, W. M. Nau, L. Huang; Chem. Commun., 2017, 53, 9242). The perylene dye (4) forms non-
fluorescent aggregates in which the flat polycyclic arenes stack and the charged side-chains with their quaternary ammonium ions take up positions around the periphery. These two components, however, can both fit together within the tube-like binding pocket of cucurbit[8]uril. Consequently, when cucurbit[8]
F
F F
(1) F F
N O
O Flat section Charged section
Flexible link (4)
Scope for aggregation Flat section
Scope for aggregation Charged section Charged section (4a)
Flat section (4b)
Figure 2 The conformationally mobile dye molecule 4 contains a flat perylene section and a charged quaternary ammonium ion section which can interconvert between linear and folded conformations (4a and 4b, respectively). The tube-like binding pocket of cucurbit[8]uril is represented schematically (box) to show how 4b can be accommodated inside the tube
Figure 2
uril is added to a sample of 4, the side-chain holding the quaternary ammonium ion folds back (Figure 2) and the U-shaped conformer that is produced fits tightly inside the cucurbit[8]uril (see box in Figure 2) and as a consequence, the perylene aggregation is broken up. This switches on the fluorescence of the perylene.
Sensing viscosity with twisting fluorophores Intramolecular conformational effects can also exploit twisting motions. A series of conjugated porphyrin dimers have been prepared to apply this effect to the sensing of viscosity
F
F F
(2)
NC NC
(3) CN CN
Figure 1 Examples of molecules with flat
π-systems which stack well together
and temperature (A. Vyšniauskas, D. Ding, M. Qurashi, I. Boczarow, M. Balaz, H. L. Anderson, M. K. Kuimova; Chem. Eur. J., 2017, 23, 11001). The conjugated linker between the
two porphyrin rings is a diyne and consequently can offer the possibility of different conjugated pathways between the rings in different conformations. Rotation along the -C≡C-C≡C- axis within this central section is possible, but is influenced by the viscosity of the solvent, ie it will be affected by the choice of solvent and the temperature. The changes in fluorescence
properties of these compounds are a consequence of the local solvation environment, not the macroscopic properties of the bulk solvent, allowing approaches to be developed to explore the physical properties of microenvironments. The results are interpreted in
the context of the influences of substitution patterns on the energy barriers between planar and twisted conformations (Scheme 1).
R1
Charged section Flat section
Tube-like binding pocket N
44 08 | 2017 Figure 1
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