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Owing to their intense near infrared absorption and emission properties, to the ability to photogenerate singlet oxygen, or to act as photoacoustic imaging agents within the optical window of tissue, bacteriochlorins (2,3,12,13-tetrahydroporphyrins) promise to be of utility in many biomedical and technical applications. The ability to fine-tune the electronic properties of synthetic bacteriochlorins is important for these purposes. In this vein, we report the synthesis, structure determination, optical properties, and theoretical analysis of the electronic structure of a family of expanded bacteriochlorin analogues. The stepwise expansion of both pyrroline moieties in near-planar meso-tetraarylbacteriochlorins to morpholine moieties yields ruffled mono- and bismorpholinobacteriochlorins with broadened and up to 90 nm bathochromically shifted bacteriochlorin-like optical spectra. Intramolecular ring-closure reactions of the morpholine moiety with the flanking meso-aryl groups leads to a sharpened, blue-shifted wavelength λ max band, bucking the general red-shifting trend expected for such linkages. A conformational origin of the optical modulations was previously proposed, but discrepancies between the solid state conformations and the corresponding solution state optical spectra defy simple structure-optical property correlations. Using density functional theory and excited state methods, we derive the molecular origins of the spectral modulations. About half of the modulation is due to ruffling of the bacteriochlorin chromophore. Surprisingly, the other half originates in the localized twisting of the C β -C α -C α -C β dihedral angle within the morpholine moieties. Our calculations suggest a predictable and large spectral shift (2.0 nm/deg twist) for morpholine deformations within these fairly flexible moieties. This morpholine moiety deformation can take place largely independently from the overall macrocycle conformation. The morpholinobacteriochlorins are thus excellent models for localized bacteriochlorin chromophore deformations that are suggested to also be responsible for the optical modulation of naturally occurring bacteriochlorophylls. We propose the use of morpholinobacteriochlorins as mechanochromic dyes in engineering and materials science applications.

Six free base tetrapyrrolic chromophores, three quinoline-annulated porphyrins and three morpholinobacteriochlorins, that absorb light in the near-IR range and possess, in comparison to regular porphyrins, unusually low fluorescence emission and (1)O2 quantum yields were tested with respect to their efficacy as novel molecular photo-acoustic imaging contrast agents in a tissue phantom, providing an up to ∼2.5-fold contrast enhancement over that of the benchmark contrast agent ICG. The testing protocol compares the photoacoustic signal output strength upon absorption of approximately the same light energy. Some relationships between photophysical parameters of the dyes and the resulting photoacoustic signal strength could be derived.

The refined syntheses, modification, and first X-ray structural characterization of meso-tetraarylporphyrin-derived beta-tetraolbacteriochlorins are described. These investigations assign the relative stereochemistry of their two isomers (both cis-vic-diol pairs on the same or opposite sides of the porphyrin plane), an assignment that could not be provided by NMR, UV-vis or fluorescence spectroscopy, or mass spectrometry. Moreover, the first crystal structures of a 2-hydroxychlorin and a 2,3-dihydroxychlorin, as its dimethylether, are reported. Dihydroxylation and diimide reduction of the dimethoxychlorin result in the formation of stable mixed-functionality bacteriochlorins. The photophysical properties (UV-vis absorption and fluorescence emission) of all chromophores are contrasted against each other, delineating the electronic effects of diol substitution and conformational modulation. Lastly, the acid-induced dehydration/demethoxylation of the tetraol-, dioldimethoxy-, and tetramethoxybacteriochlorins to provide chlorins is delineated.

Indaphyrins, pyrrole-modified porphyrins containing a cleaved pyrrole β,β'-bond and two annulated indanone moieties, possess unusually broadened and redshifted UV/Vis spectra because of their π-expanded chromophores. The parent free base indaphyrin has been crystallographically characterized, highlighting its strongly ruffled conformation incorporating a helimeric twist. It was shown to be susceptible to regiospecific derivatizations at the opposite side of the ring-cleaved pyrrole (dihydroxylation, followed by functional group transformations of the resulting diol functionality), generating indaphyrin-based chlorin analogues, indachlorins, that incorporate a dihydroxypyrroline, pyrrolindione, oxazolone, or a morpholine moiety. Structural modifications resulted in further broadening and hyper- and bathochromic shifts of the optical spectra, some of which possess a nearly panchromatic absorption between 300 to well above 900 nm. The extents to which these modifications affect their solid-state conformations were analyzed. [ABSTRACT FROM AUTHOR]

Abstract: The reaction of free base or Ni(II) complex secochlorin bisaldehydes 4H2 and 4Ni regenerates the ultimate starting material of the bisaldehydes, meso-tetraphenylporphyrin 2H2 and 2Ni, respectively. Depending on the reaction conditions employed (hydrazine hydrate in pyridine at reflux or hydrazine hydrate activated with sulfur in the presence of aqueous NaOH at ambient temperature), either porphyrin 2H2 is formed together with known dihydroxymorpholinochlorin 9H2 or known 2-hydroxychlorin 8H2 . Two different reaction pathways for the hydrazine reaction can be derived, either involving the formation of a meso-tetraphenyl-1,4,5-triazepinoporphyrin that loses spontaneously N2 or a Wolff–Kishner-type pathway that also involves an intramolecular aldol-type reaction. Neither reaction is synthetically useful but both highlight in an impressive fashion the high thermodynamic stability of porphyrins. They also bring the ‘breaking and mending of porphyrin’ strategy to its ultimate conclusion by regenerating the starting porphyrin. [Copyright &y& Elsevier]