MOLECULAR RECOGNITION .16. MOLECULAR RECOGNITION OF QUINONES - 2-POINT HYDROGEN-BONDING STRATEGY FOR THE CONSTRUCTION OF FACE-TO-FACE PORPHYRIN QUINONE ARCHITECTURES

5,15-cis-Bis(2-hydroxy-1-naphthyl)octaethylporphyrin (1) in chloroform selectively binds p-quinones via a two-point hydrogen-bonding interaction between the two, convergent hydroxyl groups of host 1 and the two carbonyl moieties of a guest quinone. Upon complexation, the H-1 NMR resonance and IR absorption for the OH groups undergo a characteristic downfield shift and a shift to lower wavenumber, respectively (DELTA-delta-comp(OH) = 1.38-2.85 ppm and DELTA-nu-comp(OH) = 30-102 cm-1, depending on the basicities of quinones). The binding constants (K) evaluated by H-1 NMR titration at 298 K decrease in the order anthraquinone (17; 2.3 x 10(2) M-1) > naphthoquinone (15; 1.7 x 10(2)) > benzoquinone (5; 5.5 x 10): Those for benzoquinone derivatives are duroquinone (12; 4.2 x 10(2)) > chloranil (6; 4.0 x 10(2)) > fluoranil (7; 3.7 x 10(2)) > 2,5-dichlorobenzoquinone (8; 2.2 x 10(2)) > 2-chlorobenzoquinone (9; 1.2 x 10(2)) > 2,5-dimethylbenzoquinone (11; 1.1 x 10(2)) > 2-methylbenzoquinone (10; 8.8 x 10) > benzoquinone (5; 5.5 x 10) > 2,3-dimethoxy-5-methylbenzoquinone (13; 3.5 x 10) > tetramethoxybenzoquinone (14; 7.8). The variation in K's suggests that the strength of hydrogen bonds, direct porphyrin-quinone interaction apparently of a charge-transfer type, and the steric effects of methoxy substituents are important factors. Reference guests such as anthrone (18; K = 4.2 M-1), o-naphthoquinone (16; 8.7), and 1,4-cyclohexanedione (19; 1.0 x 10) show significantly lower affinities to host 1 than those for the corresponding p-quinone counterparts. The resulting 1-quinone adducts have an estimated face-to-face separation of as short as 3 angstrom and exhibit porphyrin-quinone pi-pi interaction as revealed by UV/visible spectroscopy; the longest wavelength absorption band of 1 undergoes either a blue shift or a red shift depending on the electronic property of bound quinone. In addition, the adducts are rendered completely nonfluorescent as a result of an efficient, intracomplex electron transfer from photoexcited 1 to bound quinone. Thus, the abilities of quinones to quench fluorescence of porphyrin 1 are related not with their redox properties but with their abilities to bind to 1;6 congruent-to 12 > 17 > 8 > 11 > 5. The present host-guest complexation is discussed from the viewpoint of noncovalent strategy for the construction of biologically significant structures.