Understanding how the complex interplay among excitonic interactions, vibronic couplings, and reorganization energy determines coherence-enabled transport mechanisms is a grand challenge with both foundational implications and potential payoffs for energy science. We use a combined experimental and theoretical approach to show how a modest change in structure may be used to modify the exciton delocalization, tune electronic and vibrational coherences, and alter the mechanism of exciton transfer in covalently linked cofacial Zn-porphyrin dimers (meso-beta linked AB_m-β and meso–meso linked AA_m-m). While both AB_m-β and AA_m-m feature zinc porphyrins linked by a 1,2-phenylene bridge, differences in the interporphyrin connectivity set the lateral shift between macrocycles, reducing electronic coupling in AB_m-β and resulting in a localized exciton. Pump–probe experiments show that the exciton dynamics is faster by almost an order of magnitude in the strongly coupled AA_m-m dimer, and two-dimensional electronic spectroscopy (2DES) identifies a vibronic coherence that is absent in AB_m-β. Theoretical studies indicate how the interchromophore interactions in these structures, and their system-bath couplings, influence excitonic delocalization and vibronic coherence-enabled rapid exciton transport dynamics. Real-time path integral calculations reproduce the exciton transfer kinetics observed experimentally and find that the linking-modulated exciton delocalization strongly enhances the contribution of vibronic coherences to the exciton transfer mechanism, and that this coherence accelerates the exciton transfer dynamics. These benchmark molecular design, 2DES, and theoretical studies provide a foundation for directed explorations of nonclassical effects on exciton dynamics in multiporphyrin assemblies.

Panchromatic absorbers are potentially valuable for use in molecular-based energy-conversion schemes. A prior dyad composed of a perylene-monoimide joined at the 9-position via an ethyne to the porphyrin meso-position afforded panchromatic absorption (350–700 nm). To explore the effects of structure on panchromaticity, five new arrays were synthesized from eight (four new) chromophore or tetrapyrrole building blocks. Two dyads contain a dipyrrinatoboron difluoride (bodipy) or terrylene chromophore in lieu of the perylene to assess the effects of the chromophore π-system on panchromaticity. The long-wavelength absorption band of sparsely substituted variants of the three chromophores (dipyrrinatoboron difluoride, perylene-monoimide, terrylene-monoimide) is generally in the range 480–550 nm, 490–540 nm and 600–660 nm. Three perylene–tetrapyrrole constructs were prepared wherein the perylene–ethyne is attached at the β-pyrrole position of a porphyrin, chlorin, or bacteriochlorin, versus the meso-site used previously. The latter set addresses the effect of linker connection site on the spectral properties, which is prompted by the differing electron densities at these positions of the three tetrapyrrole macrocycles. A new route is reported for the synthesis of a β-bromoporphyrin building block. Static absorption and emission spectra, fluorescence yields and singlet excited-state lifetimes were examined for all arrays and nine (five new) ethynyl-bearing benchmarks. Four guidelines emerge: (1) panchromaticity decreases in the order perylene > dipyrrinatoboron difluoride > terrylene; (2) the porphyrin meso-site affords far greater panchromaticity than the β-site; (3) panchromaticity for chlorins and bacteriochlorins is less affected by linker site; and (4) the chromophore–porphyrin dyad with perylene (versus boron-dipyrrin or terrylene) affords the most porphyrin-like and robust lowest singlet excited state for utilization of the harvested energy. Collectively, the new synthetic constructs (5 arrays, 5 benchmarks, 4 building blocks) have elicited heuristics for the de novo design of panchromatic light-harvesting architectures.

Fluorophores that absorb and emit in the red spectral region (600–700 nm) are of great interest in photochemistry and photomedicine. Eight new target chlorins (and 19 new chlorins altogether)—analogues of chlorophyll—of different polarities have been designed and synthesized for various applications; seven of the chlorins are equipped with a bioconjugatable tether. Hydrophobic or amphiphilic chlorins in a non-polar organic solvent (toluene), polar organic solvent (DMF), and aqueous or aqueous micellar media show a sharp emission band in the red region and modest fluorescence quantum yield (Φ_f = 0.2–0.3). A Poisson analysis implies most micelles are empty and few contain >1 chlorin. Water-soluble chlorins each bearing three PEG (oligoethyleneglycol) groups exhibit narrow emission bands (full-width-at-half maximum <25 nm). The lifetime of the lowest singlet excited state and the corresponding yields and rate constants for depopulation pathways (fluorescence, intersystem crossing, internal conversion) are generally little affected by the PEG groups or dissolution in aqueous or organic media. A set of chlorin–avidin conjugates revealed a 2-fold increase in Φ_f with increased average chlorin/avidin ratio (2.3–12). In summary, the chlorins of various polarities described herein are well suited as red-emitting fluorophores for applications in aqueous or organic media.