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The development of chromophores that absorb in the near-infrared (NIR) region beyond 1000 nm underpins numerous applications in medical and energy sciences, yet also presents substantial challenges to molecular design and chemical synthesis. Here, the core bacteriochlorin chromophore of nature's NIR absorbers, bacteriochlorophylls, has been adapted and tailored by annulation in an effort to achieve absorption in the NIR-II region. The resulting bacteriochlorin, Phen2,1-BC, contains two annulated naphthalene groups spanning meso,β-positions of the bacteriochlorin and the 1,2-positions of the naphthalene. Phen2,1-BC was prepared via a new synthetic route. Phen2,1-BC is an isomer of previously examined Phen-BC, which differs only in attachment via the 1,8-positions of the naphthalene. Despite identical π-systems, the two bacteriochlorins have distinct spectroscopic and photophysical features. Phen-BC has long-wavelength absorption maximum (912 nm), oscillator strength (1.0), and S1 excited-state lifetime (150 ps) much different than Phen2,1-BC (1292 nm, 0.23, and 0.4 ps, respectively). These two molecules and an analogue with intermediate characteristics bearing annulated phenyl rings have unexpected properties relative to those of non-annulated counterparts. Understanding the distinctions requires extending concepts beyond the four-orbital-model description of tetrapyrrole spectroscopic features. In particular, a reduction in symmetry resulting from annulation results in electronic mixing of x- and y-polarized transitions/states, as well as vibronic coupling that together reduce oscillator strength of the long-wavelength absorption manifold and shorten the S1 excited-state lifetime. Collectively, the results suggest a heuristic for the molecular design of tetrapyrrole chromophores for deep penetration into the relatively unutilized NIR-II region. [ABSTRACT FROM AUTHOR]

A new pentad array designed to exhibit panchromatic absorption and charge separation has been synthesized and characterized. The array is composed of a triad panchromatic absorber (a bis(perylene-monoimide)-porphyrin) to which are appended an electron acceptor (perylene-diimide) and an electron donor/hole acceptor (bacteriochlorin) in a crossbar arrangement. The motivation for incorporation of the bacteriochlorin versus a free-base or zinc chlorin utilized in prior constructs was to facilitate hole transfer to this terminal unit and thereby achieve a higher yield of charge separation across the array. The intense S0 → S1 (Qy) band of the bacteriochlorin also enhances absorption in the near-infrared spectral region. Due to synthetic constraints, a phenylethyne linker was used to join the bacteriochlorin to the core porphyrin of the panchromatic triad rather than the diphenylethyne linker employed for the prior chlorin-containing pentads. Static and time-resolved photophysical studies reveal enhanced excited-state quenching for the pentad in benzonitrile and dimethyl sulfoxide compared to the prior chlorin-containing analogues. Success was only partial, however, as a long-lived charge separated state was not observed despite the improved energetics for the final ground-state hole/electron-shift reaction. The apparent reason is more facile competing charge-recombination due to the shorter bacteriochlorin – porphyrin linker that increases electronic coupling for this process. The studies highlight design criteria for balancing panchromatic absorption and long-lived charge separation in molecular architectures for solar-energy conversion. [ABSTRACT FROM AUTHOR]

Bacteriochlorophylls, nature's near-infrared absorbers, play an essential role in energy transfer in photosynthetic antennas and reaction centers. To probe energy-transfer processes akin to those in photosynthetic systems, nine synthetic bacteriochlorin–bacteriochlorin dyads have been prepared wherein the constituent pigments are joined at the meso-positions by a phenylethyne linker. The phenylethyne linker is an unsymmetric auxochrome, which differentially shifts the excited-state energies of the phenyl- or ethynyl-attached bacteriochlorin constituents in the dyad. Molecular designs utilized known effects of macrocycle substituents to engineer bacteriochlorins with S0 → S1 (Qy) transitions spanning 725–788 nm. The design-predicted donor–acceptor excited-state energy gaps in the dyads agree well with those obtained from time dependent density functional theory calculations and with the measured range of 197–1089 cm−1. Similar trends with donor–acceptor excited-state energy gaps are found for (1) the measured ultrafast energy-transfer rates of (0.3–1.7 ps)−1, (2) the spectral overlap integral (J) in Förster energy-transfer theory, and (3) donor–acceptor electronic mixing manifested in the natural transition orbitals for the S0 → S1 transition. Subtle outcomes include the near orthogonal orientation of the π-planes of the bacteriochlorin macrocycles, and the substituent-induced shift in transition-dipole moment from the typical coincidence with the NH–NH axis; the two features together afforded the Förster orientation term κ2 ranging from 0.55–1.53 across the nine dyads, a value supportive of efficient excited-state energy transfer. The molecular design and collective insights on the dyads are valuable for studies relevant to artificial photosynthesis and other processes requiring ultrafast energy transfer. [ABSTRACT FROM AUTHOR]

Diverse dihydrodipyrrins are available as precursors in the de novo synthesis of bacteriochlorins and chlorins. Each dihydrodipyrrin contains one pyrrole and one pyrroline (3,4-dihydropyrrole) ring joined at the respective α-positions via a methylene unit as well as a geminal-dimethyl group at one of the pyrroline β-positions. Complexation of the dihydrodipyrrin ligands occurs smoothly upon treatment with Bu2B–OTf or BF3·OEt2 in dichloromethane containing triethylamine at room temperature. Six such dihydrodipyrrinatoboron complexes have been prepared and are examined here. The complexes with –BF2 or –BBu2 absorb in the blue region (λabs∼ 400 nm) and fluoresce (λem∼ 500 nm) with large Stokes shift (∼100–150 nm), almost no absorption-fluorescence spectral overlap, and high fluorescence quantum yield (Φf∼ 0.4–0.9). The spectral features are rather insensitive to substituents in the pyrrole nucleus (carboethoxy, bromo, and p-tolyl) and the presence of a 1-naphthalenyl group at the meso-position. In one case examined, the spectral properties including Φf value were almost identical in toluene and acetonitrile. The blue BODIPYs may be useful as broadband photosensitizers upon violet-laser excitation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]