A growing theme in chemistry is the joining of multiple organic molecular building blocks to create functional molecules. Diverse derivatizable structures—here termed “scaffolds” comprised of “hubs”—provide the foundation for systematic covalent organization of a rich variety of building blocks. This review encompasses 30 tri- or tetra-armed molecular hubs (e.g., triazine, lysine, arenes, dyes) that are used directly or in combination to give linear, cyclic, or branched scaffolds. Each scaffold is categorized by graph theory into one of 31 trees to express the molecular connectivity and overall architecture. Rational chemistry with exacting numbers of derivatizable sites is emphasized. The incorporation of water-solubilization motifs, robust or self-immolative linkers, enzymatically cleavable groups and functional appendages affords immense (and often late-stage) diversification of the scaffolds. Altogether, 107 target molecules are reviewed along with 19 syntheses to illustrate the distinctive chemistries for creating and derivatizing scaffolds. The review covers the history of the field up through 2020, briefly touching on statistically derivatized carriers employed in immunology as counterpoints to the rationally assembled and derivatized scaffolds here, although most citations are from the past two decades. The scaffolds are used widely in fields ranging from pure chemistry to artificial photosynthesis and biomedical sciences.

Molecules with strong absorption in the near-infrared (NIR) spectral region are of interest for a variety of applications in the photosciences. Nature's NIR absorbers are bacteriochlorophylls, which contain a tetrahydroporphyrin chromophore accentuated by attached auxochromes, including a β,meso-bridging isocyclic ring. Modification of the isocyclic ring has provided a convenient means of wavelength tuning. In other tetrapyrroles, annulation at the β,meso-, β,β-, and β-meso-β-positions also has enabled wavelength tuning. Here, synthetic bacteriochlorins bearing a gem-dimethyl group in each pyrroline ring have provided the foundation for studies of annulation. Seven new free base bacteriochlorins have been prepared and seven others have been elaborated therefrom. Six distinct routes known among porphyrins were explored. Ultimately, annulation of phenaleno or benzo groups at the β,meso-sites (i.e., spanning the 3,5- and 13,15-positions) of bacteriochlorins was achieved by a new route that entails two successive Pd-mediated coupling reactions of a dibromobacteriochlorin with a bromoarylboronic acid. The corresponding Phen-BC or Benz-BC absorbs at 913 or 1033 nm, respectively. Examination by time-resolved spectroscopy revealed an excited-state lifetime of 150 ps or 7 ps, with commensurably low fluorescence quantum yield (Φ_f) values estimated to be ∼0.004 or 3 × 10⁻⁵, respectively. The origin of the short excited-state lifetimes due to β,meso-annulation is unknown, but is not simply a consequence of the energy-gap law for non-radiative decay.