Polymorph crystal packing effects on charge transfer emission in the solid state

Xiaoyan He; Andrew C Benniston; Hanna Saarenpää; Helge Lemmetyinen; Nikolia V Tkachenko; Ulrich Baisch

doi:10.1039/c5sc01151e

Polymorph crystal packing effects on charge transfer emission in the solid state

Chem Sci. 2015 Jun 1;6(6):3525-3532. doi: 10.1039/c5sc01151e. Epub 2015 Apr 20.

Authors

Xiaoyan He¹, Andrew C Benniston¹, Hanna Saarenpää², Helge Lemmetyinen², Nikolia V Tkachenko², Ulrich Baisch^{1

3}

Affiliations

¹ Molecular Photonics Laboratory , School of Chemistry , Newcastle University , Newcastle upon Tyne , NE1 7RU , UK.
² Department of Chemistry and Bioengineering , Tampere University of Technology , Tampere , Finland.
³ Department of Chemistry , University of Malta , Msida , MSD 2080 , Malta.

Abstract

Condensation of 1,8-naphthalic anhydride with N,N-(dimethylamino)aniline produced the donor-acceptor compound DMIM, which crystallised from a chloroform-diethyl ether mixture to afford two different coloured crystal polymorphs. Crystals for one polymorph are small and green, whereas the other crystals are orange and needle-like. X-ray crystal structures for both polymorphs were determined. The donor N,N-dimethylaniline and acceptor naphthalimide groups are twisted with respect to each other; the degree of twist is marginally different for the two structures. The orange crystal polymorph crystallises in the monoclinic space group C2/c and contains two slightly different molecular conformers in the unit cell (calculated density is 1.410 g cm^-3). The green crystal polymorph crystallises in the triclinic space group P1 and contains only one type of molecule in the unit cell (calculated density is 1.401 g cm^-3). The crystal packing motifs for the two polymorphs are subtly different, explaining the small variance in the observed densities. Very weak room temperature emission was observed for DMIM in a CHCl₃ solution, but crystals deposited on a glass slide glowed when irradiated at 488 nm using a fluorescence microscope. Disparate solid-state emission spectra and lifetimes for the two polymorphic crystal forms are observed for the dyad. The emission is assigned to charge recombination fluorescence from a charge transfer state.