The characterization and application of hydroxyphenyltriphenylporphyrin-silica phases.

Abstract

The porphyrin derivative, [5-(p-hydroxyphenyl)-10,15,20-triphenyl]porphyrin (HPTPP), was immobilized on a silica gel substrate using glycidoxypropyltrimethoxysilane (GPTS). The resulting HPTPP-silica stationary phases were investigated for use in high-performance liquid chromatographic (HPLC) separations of fullerenes and polycyclic aromatic hydrocarbons (PAHs). Columns packed with HPTPP-silica are studied for their ability to separate fullerenes under a variety of mobile phase conditions. HPTPP-silica phases exhibit unmatched fullerene selectivity using mobile phases in which the fullerenes are most soluble (alphaC₇₀/C₆₀ = 7.0 using 100% toluene). The use of mobile phases such as <italic>o</italic>-dichlorobenzene enables the improved preparative scale separation of higher molecular weight fullerene isomers on HPTPP-silica. Studies aimed at fully characterizing the immobilization chemistry indicate that the phenol group of HPTPP reacts with the methoxy group of GPTS in solution. The resulting HPTPP derivative is immobilized via coupling through one of the remaining methoxy groups of GPTS to the silica substrate. It is suggested that this immobilized HPTPP derivative is the primary interaction site for fullerenes on HPTPP-silica. By immobilizing HPTPP on silica in this manner, HPTPP is bound via a very short tether arm. This restricts the mobility of the HPTPP ligand and provides for a more ordered stationary phase. The enhanced shape selectivity of HPTPP-silica vs. previously reported tetraphenylporphyrin-silica materials is mainly attributed to this increased phase order. This immobilization method also decreases the mean thickness of the immobilized HPTPP-silica phases as compared to porphyrin-silica phases immobilized using traditional methods. The lower mean thickness of the stationary phase improves the mass transfer of solute between the mobile and stationary phase, and accounts for the improved solute band efficiencies seen when using columns packed with HPTPP-silica (N &ge; 5,000 plates/m for C₆₀ and C₇₀ using a 100% toluene mobile phase). The separation of PAHs on HPTPP-silica using various mobile phase conditions is also examined. The planar vs. nonplanar shape selectivity of HPTPP-silica phases is assessed by using the solute pair triphenylene and <italic>o</italic>-terphenyl as well as by the separation of Standard Reference Material 869a on HPTPP-silica. The HPTPP-silica materials are shown to exhibit shape selectivities for PAHs that are comparable to other tetraphenylporphyrin-silica phases.