Photophysical Properties and Electronic Structure of Bacteriochlorin–Chalcones with Extended Near‐Infrared Absorption

Abstract

Synthetic bacteriochlorins enable systematic tailoring of substituents about the bacteriochlorin chromophore and thereby provide insights concerning the native bacteriochlorophylls of bacterial photosynthesis. Nine free-base bacteriochlorins (eight prepared previously and one prepared here) have been examined that bear diverse substituents at the 13- or 3,13-positions. The substituents include chalcone (3-phenylprop-2-en-1-onyl) derivatives with groups attached to the phenyl moiety, a "reverse chalcone" (3-phenyl-3-oxo-1-enyl), and extended chalcones (5-phenylpenta-2,4-dien-1-onyl, retinylidenonyl). The spectral and photophysical properties (τs, Φf, Φ(ic), Φ(isc), τT, k(f), k(ic), k(isc)) of the bacteriochlorins have been characterized. The bacteriochlorins absorb strongly in the 780-800 nm region and have fluorescence quantum yields (Φf) in the range 0.05-0.11 in toluene and dimethylsulfoxide. Light-induced electron promotions between orbitals with predominantly substituent or macrocycle character or both may give rise to some net macrocycle ↔ substituent charge-transfer character in the lowest and higher singlet excited states as indicated by density functional theory (DFT) and time-dependent DFT calculations. Such calculations indicated significant participation of molecular orbitals beyond those (HOMO - 1 to LUMO + 1) in the Gouterman four-orbital model. Taken together, the studies provide insight into the fundamental properties of bacteriochlorins and illustrate designs for tuning the spectral and photophysical features of these near-infrared-absorbing tetrapyrrole chromophores.

Integration of Cyanine, Merocyanine and Styryl Dye Motifs with Synthetic Bacteriochlorins

“…As can be seen from the spectral evolution and the inset kinetic trace, the S 1 lifetime is much longer than that observed for B‐1 and B‐2 . The time constants of 2.2 ns in DMF and 2.3 ns in DMSO are essentially the same as those of B‐Ch (and analogues ) and B‐rCh (Table ). The Φ isc for B‐3 , obtained from the spectra at the asymptote of the S 1 decay, is 0.15 in both DMF and DMSO.…”

“… *Data acquired at room temperature; †For B‐1 and B‐2 , the same spectral positions to within 1 nm are found in THF as in DMSO; ‡The B 1 band for B‐2 may be a shoulder at ~345 nm on the B 2 band; § B‐2 has a broad, weak fluorescence and the origin transition may be a shoulder at shorter wavelength than the maximum; ¶From reference ; #Shoulder; **From reference ; ††From reference . …”

“…The typical errors (percent of value) of the photophysical properties are as follows: τ S (±7%), Φ f (±5%), Φ isc (±15%), Φ ic (±20%), k f (±10%), k isc (±20%), k ic (±25%). The error bars for τ S , Φ f and Φ isc were determined from select repeat measurements, and those for the Φ ic , k f , k isc and k ic were obtained from propagation of errors; †From reference ; ‡From reference ; §From reference .…”

“… *Orbital energies obtained from DFT calculations. See Figures and for more orbitals; †The average Q y absorption energy for the bacteriochlorin in the solvents indicated in Table ; ‡The average Q y fluorescence energy for the bacteriochlorin in the solvents indicated in Table ; §Average of Q y absorption and emission energies in the prior two columns; ¶The difference in energy of the LUMO (Δ L ) or HOMO (Δ H ) or their gap (Δ L−H ) versus the value for reference bacteriochlorin B‐0 ; #From reference ; **From reference ; ††From reference . …”

Syntheses of Chalcone‐Type Chlorophyll Derivatives Possessing a Bacteriochlorin, Chlorin or Porphyrin π‐System and Their Optical Properties

“…Methyl pyrobacteriopheophorbide‐ a ( 1g ) bearing an acetyl group at the C3‐position and a bacteriochlorin π‐skeleton was semi‐synthesized by modifying natural BChl‐ a extracted from Rhodobacter sphaeroides as previously reported [steps (i) and (ii) in Scheme 1] . 3‐Acetyl‐bacteriochlorin 1g was treated with benzaldehyde in a 1:1 mixture of methanol and chloroform in the presence of 28% sodium methoxide in methanol under reflux for 1 h to give 3‐( trans ‐styryl)carbonyl‐bacteriochlorin 1a as the product of cross‐aldol (Claisen–Schmidt) condensation in 66% isolated yield [step (iii)]. Under the above refluxing conditions, ketone 1g reacted with p ‐anisaldehyde or p ‐tolualdehyde to give 3 3 ‐arylated 3‐acryloyl‐bacteriochlorin 1b (53%) or 1c (47%), respectively.…”