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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 (Φf) 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.

Understanding the effects of substituents on spectral properties is essential for the rational design of tailored bacteriochlorins for light-harvesting and other applications. Toward this goal, three new bacteriochlorins containing previously unexplored conjugating substituents have been prepared and characterized. The conjugating substituents include two positively charged species, 2-(N-ethyl 2-quinolinium)vinyl- (B-1) and 2-(N-ethyl 4-pyridinium)vinyl- (B-2), and a neutral group, acroleinyl- (B-3); the charged species resemble cyanine (or styryl) dye motifs whereas the neutral unit resembles a merocyanine dye motif. The three bacteriochlorins are examined by static and time-resolved absorption and emission spectroscopy and density functional theoretical calculations. B-1 and B-2 have Qy absorption bathochromically shifted well into the NIR region (822 and 852 nm), farther than B-3 (793 nm) and other 3,13-disubstituted bacteriochlorins studied previously. B-1 and B-2 have broad Qy absorption and fluorescence features with large peak separation (Stokes shift), low fluorescence yields, and shortened S1 (Qy ) excited-state lifetimes (~700 ps and ~100 ps). More typical spectra and S1 lifetime (~2.3 ns) are found for B-3. The combined photophysical and molecular-orbital characteristics suggest the altered spectra and enhanced nonradiative S1 decay of B-1 and B-2 derive from excited-state configurations in which electron density is shifted between the macrocycle and the substituents.

Six new bacteriochlorins expanding the range of the strong near-infrared (NIR) absorption (Qy band) to both shorter and longer wavelengths (∼690 to ∼900 nm) have been synthesized and characterized. The architectures include bacteriochlorin-bisimides that have six-membered imide rings spanning the 3,5- and 13,15-macrocycle positions or five-membered imide rings spanning the β-pyrrolic 2,3- and 12,13-positions. Both bisimide types absorb at significantly longer wavelength than the bacteriochlorin precursors (no fused rings), whereas oxo-groups at the 7- or 7,17-positions shift the Qy band to a new short wavelength limit. Surprisingly, bacteriochlorin-bisimides with five-membered β-pyrrolic-centered imide rings have a Qy band closer to that of six-membered bacteriochlorin-monoimides. However, the five-membered bisimides (versus the six-membered bacteriochlorin-monoimides) have significantly enhanced absorption intensity that is paralleled by an ∼2-fold higher fluorescence yield (∼0.16 vs ∼0.07) and longer singlet excited-state lifetime (∼4 ns vs ∼2 ns). The photophysical enhancements derive in part from mixing of the lowest unoccupied frontier molecular orbitals of the five-membered imide ring with those of the bacteriochlorin framework. In general, all of the new bacteriochlorins have excited-state lifetimes (1-4 ns) that are sufficiently long for use in molecular-based systems for photochemical applications.

Efficient light harvesting for molecular-based solar-conversion systems requires absorbers that span the photon-rich red and near-infrared (NIR) regions of the solar spectrum. Reported herein are the photophysical properties of a set of six chlorin-imides and nine synthetic chlorin analogues that extend the absorption deeper (624–714 nm) into these key spectral regions. These absorbers help bridge the gap between typical chlorins and bacteriochlorins. The new compounds have high fluorescence quantum yields (0.15–0.34) and long singlet excited-state lifetimes (4.2–10.9 ns). The bathochromic shift in Qy absorption is driven by substituent-based stabilization of the lowest unoccupied molecular orbital, with the largest shifts for chlorins that bear an electron-withdrawing, conjugative group at the 3-position in combination with a 13,15-imide ring.