Panchromatic chromophore-tetrapyrrole light-harvesting arrays constructed from Bodipy, perylene, terrylene, porphyrin, chlorin, and bacteriochlorin building blocks

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.