Hexa boron-dipyrromethene cyclotriphosphazenes: synthesis, crystal structure and photophysical properties

Abstract

We have synthesized four examples of a cyclotriphosphazene ring appended with six boron-dipyrromethene dyes N₃P₃(BODIPY)₆ by adopting two different methods. In method I, 1 equiv of N₃P₃Cl₆ was treated with 6 equiv of meso-(o- or m- or p-hydroxyphenyl)boron-dipyrromethene in tetrahydrofuran (THF) in the presence of cesium carbonate. This afforded N₃P₃ (BODIPY)₆ in yields ranging from 80 to 90 %. In method II, we first prepared hexakis(p-formylphenoxy)cyclotriphosphazene N₃P₃(CHO)₆ by treating 1 equiv of N₃P₃Cl₆ with 6 equiv of 4-hydroxybenzaldehyde in the presence of cesium carbonate in THF. In the second step, N₃P₃(CHO)₆ was condensed with excess of pyrrole in the presence of catalytic amount of trifluoroacetic acid (TFA) in CH₂Cl₂ at room temperature and afforded hexakis(p-phenoxy dipyrromethane)cyclotriphosphazene. In the last step, the hexakis(p-phenoxy dipyrromethane)cyclotriphosphazene was first oxidized with 6 equiv of DDQ in CH₂Cl₂ at room temperature for 1 h followed by neutralization with triethylamine and further reaction with excess BF₃•Et₂O afforded the target N₃P₃(BODIPY)₆ in 16% yield. The route II was used only for the synthesis of one target compound whereas the route I was used for the synthesis of all four target compounds. The four compounds were characterized by mass, NMR, absorption, electrochemical, and fluorescence techniques. The crystal structure solved for one of the compounds revealed that the P₃N₃ ring is slightly puckered and the six substituents were not interacting with each other and attained pseudo-axial and pseudo-equatorial positions. The photophysical studies in five different solvents indicated that the compounds exhibit large Stokes’ shifts unlike reference monomeric BODIPYs indicating that the compounds are promising for fluorescence bioassays. The quantum yields and lifetimes of compounds 1−4 depends on the type of BODIPY unit attached to the cyclotriphosphazene ring.