title: 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 λmaxband, 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 Cβ-Cα-Cα-Cβdihedral 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.
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Guberman-Pfeffer
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
authors.forename: Jordan A
authors.initials: JA
authors.lastname: Greco
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
authors.forename: Lalith P
authors.initials: LP
authors.lastname: Samankumara
authors.affiliation: Department of Chemistry, Youngstown State University , One University Plaza, Youngstown, Ohio 44555-3663, United States.
authors.forename: Matthias
authors.initials: M
authors.lastname: Zeller
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.|Department of Molecular and Cell Biology, University of Connecticut , Storrs, Connecticut 06269-3125, United States.
authors.forename: Robert R
authors.initials: RR
authors.lastname: Birge
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
authors.forename: José A
authors.initials: JA
authors.lastname: Gascón
authors.affiliation: Department of Chemistry, University of Connecticut , Storrs, Connecticut 06269-3060, United States.
authors.forename: Christian
authors.identifier: 0000-0002-1560-7345
authors.initials: C
authors.lastname: Brückner
mesh_terms: D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008968:Molecular Conformation; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
chemical_list: D011166:Porphyrins; C452433:bacteriochlorin