Photophysical and electrochemical properties of new bacteriochlorins and characterization of radical cation and radical anion species

The synthesis, photophysical, and photochemical properties of a series of stable bacteriochlorins containing a fused six-member anhydride or an imide ring are discussed. The Q_y band (a_1u → e_gx transition) in the near-infrared region (NIR) lies between 788 and 831 nm depending upon the macrocycle substituents. Compounds with such a long-wavelength absorption are highly promising for their potential use in photodynamic therapy. Fluorescence maxima are also observed in the long-wavelength region of the spectrum, between 804 and 842 nm, and have lifetimes between 1.1 and 1.4 ns. The phosphorescence maxima are red-shifted to 840-870 nm. The triplet-triplet transient absorption spectra are observed to have maxima between 570 and 640 nm with lifetimes between 72 and 150 μs. The triplet excited states are efficiently quenched by oxygen to produce singlet oxygen. The quantum yields of the generated singlet oxygen were determined to be in the range of 0.33-0.55. The bacteriochlorin derivatives are easy to oxidize by one electron, and reversible half-wave potentials range between 0.65 and 0.82 V vs SCE in benzonitrile containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The second oxidation is irreversible and occurs at a rather constant potential of 1.17-1.22 V independent of the macrocycle substituents. The bacteriochlorin derivatives are also easy to reduce, and the reversible first and second one-electron reduction potentials range between -0.53 and -0.80 V and between -0.95 and -1.28 V vs SCE, respectively. Spectroelectrochemical measurements reveal the expected π radical cation and π radical anion marker bands of the bacteriochlorin derivatives. The electron spin resonance (ESR) spectra of the radical cations and radical anions produced by the chemical oxidation and reduction are reported, and the experimental and calculated spin densities are compared to each other.