Convergent synthesis of nonsymmetric pi-stacked protophanes assembled with urea linkers.
A convergent approach has been developed for the preparation of nonsymmetric tertiary arylureas possessing two or three urea linkages. This approach has been used for the preparation of ureas possessing 1-naphthylenyl and 4-nitrophenyl end groups separated by either one or two phenylene diamine bridging units. These ureas were constructed as prototypes for donor-bridge-acceptor systems based on tertiary arylurea architecture. AM1 calculations indicate a preference of these arylureas for folded, protophane structures in which the aryl groups are loosely pi-stacked. Analysis of the (1)H NMR chemical shifts supports the assignment of folded structures in solution. The absorption and luminescence spectra of tertiary ureas possessing 1-naphthylenyl and/or 4-nitrophenyl are reported. The absence of fluorescence and appearance of structured phosphorescence at 77 K are attributed to nitrophenyl-localized lowest energy singlet and triplet states. Localization of excitation on the acceptor chromophore precludes investigation of charge transfer in these systems.
Lewis
Frederick D
FD
Department of Chemistry, Northwestern University, Evanston, IL 60208-3113, USA. lewis@chem.northwestern.edu
Delos Santos
Grace B
GB
Liu
Weizhong
W
D002626:Chemistry, Pharmaceutical / Q000379:methods*; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015394:Molecular Structure; D014508:Urea / Q000737:chemistry*
D007202:Indicators and Reagents; D014508:Urea