Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

Abstract

Photoisomerization of 3,4-di­(meth­oxy­carbonyl)-enediyne linker in hydroporphyrin (chlorin or bacteriochlorin) dyads leads to thermally stable cis isomers, where macrocycles adopt a slipped cofacial mutual geometry with an edge-to-edge distance of ∼3.6 Å (determined by density functional theory (DFT) calculations). Absorption spectra exhibit a significant splitting of the long-wavelength Qy band, which indicates a strong electronic coupling with a strength of V = ∼477 cm–1 that increases to 725 cm–1 upon metalation of hydroporphyrins. Each dyad features a broad, structureless emission band, with large Stokes shift, which is indicative of excimer formation. DFT calculations for dyads show both strong through-bond electronic coupling and through-space electronic interactions, due to the overlap of π-orbitals. Overall, geometry, electronic structure, strength of electronic interactions, and optical properties of reported dyads closely resemble those observed for photosynthetic special pairs. Dyads reported here represent a novel type of photoactive arrays with various modes of electronic interactions between chromophores. Combining through-bond and through-space coupling appears to be a viable strategy to engineer novel optical and photochemical properties in organic conjugated materials.

Meso bromination and derivatization of synthetic bacteriochlorins

“…Arrays containing Z2 bacteriochlorins have slowly begun to emerge. [25][26][27][28][29][30][31][32][33][34] A general synthetic requirement is the ability to install a single synthetic handle on the bacteriochlorin; a generally desirable molecular design feature is the ability to tune the long-wavelength absorption band over a defined range given that the position of the band sets an upper bound on the energy of resulting photochemical processes. For preparation of arrays, we sought bacteriochlorin building blocks bearing a 4-ethynylphenyl group at a meso-position.…”

Synthesis of bacteriochlorins bearing diverse β-substituents

“…Numerous gem -dimethyl-substituted bacteriochlorins bearing substituents at the β-pyrrolic positions have been prepared. 16,24–36 The placement of auxochromes at the β-pyrrolic positions enables tuning of the position of the long-wavelength (Q y ) absorption band, and the focus on the 3,13-positions stems from facile substitution of the site in the initial pyrrole precursor ( e.g. , substituent A in the Eastern–Western route, Scheme 1) destined for location at the 3,13-position.…”

“…Numerous gem-dimethyl-substituted bacteriochlorins bearing substituents at the b-pyrrolic positions have been prepared. 16,[24][25][26][27][28][29][30][31][32][33][34][35][36] The placement of auxochromes at the bpyrrolic positions enables tuning of the position of the long-wavelength (Q y ) absorption band, and the focus on the 3,13-positions stems from facile substitution of the site in the initial pyrrole precursor (e.g., substituent A in the Eastern-Western route, Scheme 1) destined for location at the 3,13position. For subsequent elaboration of the bacteriochlorins, the presence of b-pyrrolic substituents may be limiting (Chart 2): (a) in 3,13-disubstituted bacteriochlorins, the 3,13substituents can hinder the adjacent meso- (5,15) and 2,12positions, whereas the 10,20-positions are hindered by the gem-dimethyl groups at the adjacent (8,8,18,18) positions; and (b) in 2,12-disubstituted bacteriochlorins, the 10,20-positions are hindered by two flanking substituents, the 3,13-positions are hindered by only the flanking 2,12-substituents, but the 5,15-Bacteriochlorins bearing bromo/carboalkoxy groups at the b-pyrrole positions A candidate bacteriochlorin bearing 2,12-dicarboethoxy substituents (BC-1) was previously synthesized via the Northern-Southern route (Scheme 2).…”

Photophysical Properties and Electronic Structure of Hydroporphyrin Dyads Exhibiting Strong Through-Space and Through-Bond Electronic Interactions

“…Examples of other porphyrinic systems include dyads with directly linked units , or systems that have cofacial, − slipped cofacial, , or edge-to-edge arrangements that employ ethynyl or butadienyl linkers . Synthetic arrays have been prepared in which hydroporphyrins (chlorins or bacteriochlorins) are directly attached , or held in a cofacial arrangement via various linkers. − A few examples of dyads from our prior studies that involve the joining of two chlorins or two bacteriochlorins via a vinylene (i.e., 1,2-ethenediyl), ethynyl, or butadiynyl linker are shown in Chart . − Also shown is the dyad denoted trans- C1 in which two chlorins are joined via a linker comprised of a 3,4-dimethoxycarbonyl-hexa-3-en-1,5-diyne linker that contains a triple–double–triple-bond (enediyne) motif . Such enediyne linkers have been employed to construct a variety of arrays. − The dyads shown in Chart that employ a vinylene, ethynyl, or butadiynyl linker show split frontier MOs, split redox waves, split optical bands, and altered excited-state properties compared to the monomeric chlorins or bacteriochlorins. − The subunits have TB interactions and potentially also short-range and long-range TS interactions depending on the tetrapyrrole, linker type, and attachment sites.…”