INTERMOLECULAR INTERACTION BETWEEN MULTIFUNCTIONAL PORPHYRIN AND UBIQUINONE ANALOGS

Face-to-face complexation between meso-tetra(2-hydroxynaphthyl)porphyrin (1) and p-benzoquinone derivatives via hydrogen bonds has been clarified by spectroscopic measurements. Formation of quinone-porphyrin complex depends upon orientation of hydroxyl groups above and below porphyrin ring. Among them, alpha,alpha,alpha,alpha-atropisomer (1a) having four convergent hydroxyl groups shows most efficient multipoint interaction with ubiquinone analogues. The binding constants (K(a)) and thermodynamic parameters (DELTAG-degrees, DELTAH-degrees, TDELTAS-degrees) of quinones with 1 largely depend on the number and position of methoxy substituents on quinone ring. Tetramethoxy-p-benzoquinone (2e) is most favorable guest among ubiquinone analogues and its binding constant is determined as K(a) = 2.0 x 10(4) M-1 at 298 K in CHCl3. The present porphyrinquinone pair, which is mainly governed by specific hydrogen bonding fixation, is quite difference from the system of two-point fixation by 5,15-cis-bis(2-hydroxynaphthyl)-octaethylporphyrin (3) reported in previous work.