5-Aryl Substituted 2-(2-Methoxyphenyl)benzoxazoles with Large Stokes Shifts: Synthesis, Crystal Structures and Optical Properties

2-(2-methoxyphenyl)benzoxazole (MBO) displays similar photophysical properties to 2-(2-hydroxyphenyl)benzoxazole (HBO), but it lacks the large Stokes shifts necessary for fluorescence microscopy applications. In this paper, we report the 5-substitution of MBO with aryl groups to produce 2-(4-methoxy-[1,1'-biphenyl]-3-yl)benzoxazoles (MBBOs) with large Stokes shifts. The synthesis of MBBOs 6-15 was accomplished in moderate to good yields by microwave-assisted Sonogashira cross couplings. Crystal structures for compounds 6-9 and 15 are provided. MBBOs 6-10, 12, and 14 displayed considerably blue-shifted absorption maxima and slight bathochromic shifts of their fluorescence maxima relative to the unsubstituted MBO. Unlike 2-(2-hydroxyphenyl)benzoxales (HBOs), 2-(2-methoxyphenyl)benzoxazoles (MBOs) lack the sought-after large Stokes shifts of the former which are necessary in fluorescence microscopy. Herein, we demonstrate that the 5-aryl substitution of MBOs affords 2-(4-methoxy-[1,1'-biphenyl]-3-yl)benzoxazoles (MBBOs) dyes with large Stokes shifts. image