A new class of collective excitations: Exciton strings

Grinding-induced supramolecular charge-transfer assemblies with switchable vapochromism toward haloalkane isomers

Synthetic macrocycles have proved to be of great application value in functional charge-transfer systems in the solid state in recent years. Here we show a switchable on-off type vapochromic system toward 1-/2-bromoalkane isomers by constructing solid-state charge-transfer complexes between electron-rich perethylated pillar[5]arene and electron-deficient aromatic acceptors including 4-nitrobenzonitrile and 1,4-dinitrobenzene. These charge-transfer complexes with different colors show opposite color changes upon exposure to the vapors of 1-bromoalkanes (fading) and 2-bromoalkanes (deepening). Single-crystal structures incorporating X-ray powder diffraction and spectral analyses demonstrate that this on-off type vapochromic behavior is mainly attributed to the destruction (off) and reconstruction (on) of the charge-transfer interactions between perethylated pillar[5]arene and the acceptors, for which the competitive host-guest binding of 1-bromoalkanes and the solid-state structural transformation triggered by 2-bromoalkanes are respectively responsible. This work provides a simple colorimetric method for distinguishing positional isomers with similar physical and chemical properties.

Stark effect of intrinsic and extrinsic charge-transfer excitons in a linear donor-acceptor stack: anthracene-pyromellitic dianhydride

Anthracene-PMDA single crystals display at 2K about 70 meV below a well-known intrinsic charge-transfer exciton three narrow absorption lines, which are attributed to CT excitons bound to defects of a few 10(-5) concentration. All excitons respond very sensitively to electric fields along the molecular stack because of the large dipole moment, about 2 eA, of an ionized donor-acceptor pair, but only intrinsic excitons observe an optical selection rule. Although the triclinic unit cell contains only one pair of molecules, excitons appear in field-modulated spectra as near-degenerate doublets of different parity with very small splitting. The line shape of the EA spectra and selection rules with respect to the polarization of light and orientation of the field are consistent with the inversion symmetry of the lattice and the molecules. The simple crystal structure enables identification of the defects that are responsible for extrinsic excitons. Symmetry consideration based on translation invariance lead to a new interpretation of intrinsic excitons as true crystal states with charge transfer from the donor to the acceptor sublattice.