Synthesis and supramolecular properties of molecular clips with anthracene sidewalls.
Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.
Klärner
Frank-Gerrit
FG
Institut für Organische Chemie der Universität Duisburg-Essen, 45117 Essen, Germany. frank.klaerner@uni-essen.de
Kahlert
Björn
B
Boese
Roland
R
Bläser
Dieter
D
Juris
Alberto
A
Marchioni
Filippo
F
D000873:Anthracenes / Q000138:chemical synthesis* / Q000737:chemistry; D001555:Benzene Derivatives / Q000737:chemistry*; D018360:Crystallography, X-Ray; D005449:Fluorenes / Q000737:chemistry*; D008163:Luminescent Measurements; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009010:Monte Carlo Method; D009281:Naphthalenes / Q000138:chemical synthesis* / Q000737:chemistry; D009570:Nitriles / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
C412109:1,2,4,5-tetracyanobenzene; D000873:Anthracenes; D001555:Benzene Derivatives; D005449:Fluorenes; D009281:Naphthalenes; D009570:Nitriles; C005644:2,4,7-trinitrofluorenone