Oxazolochlorins. 9. <i>meso</i>-Tetraphenyl-2-oxabacteriochlorins and <i>meso</i>-Tetraphenyl-2,12/13-dioxabacteriochlorins
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Abstract
The
formal replacement of one or two pyrrole groups in <i>meso</i>-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<sup>II</sup> 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