Bacteriochlorins with a Twist: Discovery of a Unique Mechanism to Red-Shift the Optical Spectra of Bacteriochlorins

Abstract

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 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-C-C-C 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.

Molecular Photoacoustic Contrast Agents: Design Principles & Applications

“…While each of these dyes belong to a family of chromophores already classified above ( i.e . tetrapyrrole, squaraine, and xanthene), along with Brückner's prior synthetic approach to MPAC design , this represented a paradigm shift in how the scientific community was approaching the topic of MPAC choice, in contrast to earlier empirical approaches by simply choosing a poor fluorophore. Using phantom studies Ho et al .…”

β‐meso‐Annulated meso‐Tetraarylchlorins: A Study of the Effect of Ring Fusion on Chlorin Conformation and Optical Spectra

“…Single crystal X‐ray diffractometric analysis illustrated high degree of ruffling and deviation from planarity for both the mono and bis‐morpholino bacteriochlorins, whereas the β‐to‐ ortho ‐phenyl‐linkage induced planarization of phenyl fused morpholine portion in the mono‐fused morpholinochlorin 51 . The authors have argued that in addition to macrocycle ruffling, the C β −C α −C α −C β dihedral angle inside the morpholine ring plays a crucial role in red‐shifting of the optical spectra . They established through experimental and computational studies that a highly twisted torsion angle inside the morpholine ring would correspond to the lowest energy conformation in the non‐fused morpholino bacteriochlorins 49 and 51 .…”

Crystal structure of 2,3-dimethoxy-meso-tetrakis(pentafluorophenyl)morpholinochlorin methylene chloride 0.44-solvate

“…The twisted (ruffled) conformation of helimeric chirality of the morpholinochlorins was found to be affected by the size and number of alkoxy substituents, the presence of covalent links between the morpholine unit and the flanking aryl group, and the presence and type of central metal (Daniell & Brü ckner, 2004;Brü ckner et al, 2011;Sharma et al, 2017). Porphyrinoids containing two morpholine moieties are known (Daniell & Brü ckner, 2004;Guberman-Pfeffer et al, 2017), as well as other porphyrinoids containing morpholine building blocks (Lara et al, 2005;Samankumara et al, 2015;Akhigbe et al, 2016). The modulation of the conformation of the porphyrinic -system also affects their electronic properties; morpholinochlorins are more red-shifted than a corresponding chlorin (Brü ckner et al, 2011;Guberman-Pfeffer et al, 2017).…”

“…Porphyrinoids containing two morpholine moieties are known (Daniell & Brü ckner, 2004;Guberman-Pfeffer et al, 2017), as well as other porphyrinoids containing morpholine building blocks (Lara et al, 2005;Samankumara et al, 2015;Akhigbe et al, 2016). The modulation of the conformation of the porphyrinic -system also affects their electronic properties; morpholinochlorins are more red-shifted than a corresponding chlorin (Brü ckner et al, 2011;Guberman-Pfeffer et al, 2017). The influence of the meso-substituents on the conformation and electronics of the morpholinochlorins has not been investigated.…”