The alpha substitution effect on phthalocyanine aggregation

The dimerization and tendency to form higher aggregates of two isomeric metal-free phthalocyanine compounds with tetracumylphenoxy peripheral substitution at the alpha (1 or 4, 5 or 8, 9 or 12, 13 or 16) and beta (2 or 3, 6 or 7, 10 or 11, 14 or 15) positions has been analysed by spectroscopic measurements. The dimerization constants in chloroform solution are 1.3 x 10(1) M-1 and 7.0 x 10(3) M-1 respectively. At concentrations greater than 10(-3) M, the beta phthalocyanine isomer forms a spectroscopically resolvable higher aggregate. Q-band spectra of the dimer of each isomer and of the aggregate of the beta isomer in chloroform solution were generated. Comparison of these spectra with the corresponding thin film spectra of each isomer indicates that the stronger tendency toward aggregation of the beta isomer extends into the solid state. The difference in aggregating tendency of the two isomers was attributed to a steric constraint of the alpha isomer that restricts conformational freedom and, consequently, hinders formation of dimer and higher order complexes. (C) 1998 John Wiley & Sons, Ltd.