Symmetrical, near-infrared absorbing bacteriochlorin dyads exhibit gradual reduction of their fluorescence (intensity and lifetime) and reactive oxygen species photosensitization efficiency (ROS) with increasing solvent dielectric constant ε. For the directly linked dyad, significant reduction is observed even in solvents of moderate ε, while for the dyad containing a 1,4-phenylene linker, reduction is more parallel to an increase in solvent ε. Bacteriochlorin dyads are promising candidates for development of environmentally responsive fluorophores and ROS sensitizers.

We report here synthesis and characterization of BODIPY-chlorin arrays containing a chlorin subunit, with tunable deep-red (641–685 nm) emission, and one or two BODIPY moieties, absorbing at 504 nm. Two types of arrays were examined: one where BODIPY moieties are attached through phenylacetylene linker at the 13- or 3,13-positions of chlorin, and a second type where BODIPY is attached at the 10-position of chlorin through an amide linker. Each of the examined arrays exhibits an efficient (≥ 0.80) energy transfer from BODIPY to chlorin moiety in both toluene and DMF and exhibits intense fluorescence of chlorin upon excitation of BODIPY at ~ 500 nm. Therefore, the effective Stokes shift in such arrays is in the range of 140–180 nm. Dyads with BODIPY attached at the 10-position of chlorin exhibit a bright fluorescence in a range of solvents with different polarities (i.e. toluene, MeOH, DMF, and DMSO). In contrast to this, some of the arrays in which BODIPY is attached at the 3- or at both 3,13-positons of chlorin exhibit significant reduction of fluorescence in polar solvents. Overall, dyads where BODIPY is attached at the 10-position of chlorin exhibit ~5-fold brighter fluorescence than corresponding chlorin monomers, upon excitation at 500 nm.

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.

BODIPY-hydroporphyrin energy transfer arrays allow for development of family of fluorophores featuring a common excitation band at 500 nm, tunable excitation band in deep red/near-infrared window, and tunable emission. Their biomedical applications are contingent upon retaining their optical properties in aqueous environment. Amphiphilic arrays containing PEG-substituted BODIPY and chlorins or bacteriochlorins were prepared and their optical and fluorescence properties were determined in organic solvents and aqueous surfactants. The first series of arrays contains BODIPYs with PEG substituents attached to the boron, whereas in the second series, PEG substituents are attached to the aryl at the meso positions of BODIPY. For both series of arrays, excitation of BODIPY at 500 nm results in an efficient energy transfer to and bright emission of hydroporphyrin in deep-red (640–660 nm) or near-infrared (740–760 nm) spectral windows. In aqueous non-ionic surfactants (Triton X-100 and Tween 20) arrays from the second series exhibit significant quenching of fluorescence, whereas properties of arrays from the first series are comparable to those observed in polar organic solvents. Reported arrays possess large effective Stokes shift (115 – 260 nm), multiple excitation wavelengths, and narrow, tunable deep-red/near-IR fluorescence in aqueous surfactants, and are promising candidates for a variety of biomedical-related applications.

Near infrared (NIR) fluorescent probes are attractive tools for biomedical in vivo imaging due to the relatively deeper tissue penetration and lower background autofluorescence. Activatable probes are turned on only after binding to their target, further improving target to background ratios. However, the number of available activatable NIR probes is limited. In this study, we introduce two types of activatable NIR fluorophores derived from bacteriochlorin; chlorin-bacteriochlorin energy-transfer dyads and boron-dipyrromethene (BODIPY)-bacteriochlorin energy-transfer dyads. These fluorophores are characterized by multiple narrow excitation bands with relatively strong emission in the NIR. Targeted bacteriochlorin-based antibody or peptide probes have been previously limited by aggregation after conjugation. Polyethylene glycol (PEG) chains were added to improve the hydrophilicity without altering pharmacokinetics of the targeting moieties. These PEGylated bacteriochlorin-based activatable fluorophores have potential as targeted activatable, multi-color NIR fluorescent probes for in vivo applications.