Oxazolochlorins. 9. meso-Tetraphenyl-2-oxabacteriochlorins and meso-tetraphenyl-2,12/13-dioxabacteriochlorins

The formal replacement of one or two pyrrole groups in meso-tetraphenylporphyrin by oxazole moieties is described, generating inter alia the bacteriochlorin-type chromophores oxazolobacteriochlorins (oxabacteriochlorins) and bisoxazolobacteriochlorins (dioxabacteriochlorins). The key step is the conversion of a β,β'-dihydroxy-functionalized pyrroline group into an oxazolone or (substituted) oxazole. Depending on the substitution pattern on the oxazole or oxazoline moieties, mono- and dioxabacteriochlorins may have chlorin- or bacteriochlorin-like spectra. The optical properties (as measured by UV-vis and fluorescence spectroscopies) of the novel oxa- and dioxabacteriochlorins are described and contrasted against benchmark chlorins and bacteriochlorins. The conformations of a representative number of mono- and dioxabacteriochlorins, as their free bases or Zn(II) complexes, were determined by single-crystal X-ray diffractometry. They proved to be essentially planar, showing that the modulation of their optical properties is primarily due to their intrinsic electronic structures and electronic substituent effects and are not largely affected by conformational effects. The mono- and bisoxazolobacteriochlorins are a novel class of readily prepared and oxidatively stable chlorin and bacteriochlorin analogues with tunable optical spectra that, in part, reach into the NIR.