Synthesis and Photophysical Characterization of Stable Indium Bacteriochlorins

Abstract

Bacteriochlorins have wide potential in photochemistry because of their strong absorption of near-infrared light, yet metallobacteriochlorins traditionally have been accessed with difficulty. Established acid-catalysis conditions [BF(3)·OEt(2) in CH(3)CN or TMSOTf/2,6-di-tert-butylpyridine in CH(2)Cl(2)] for the self-condensation of dihydrodipyrrin-acetals (bearing a geminal dimethyl group in the pyrroline ring) afford stable free base bacteriochlorins. Here, InBr(3) in CH(3)CN at room temperature was found to give directly the corresponding indium bacteriochlorin. Application of the new acid catalysis conditions has afforded four indium bacteriochlorins bearing aryl, alkyl/ester, or no substituents at the β-pyrrolic positions. The indium bacteriochlorins exhibit (i) a long-wavelength absorption band in the 741-782 nm range, which is shifted bathochromically by 22-32 nm versus the analogous free base species, (ii) fluorescence quantum yields (0.011-0.026) and average singlet lifetime (270 ps) diminished by an order of magnitude versus that (0.13-0.25; 4.0 ns) for the free base analogues, and (iii) higher average yield (0.9 versus 0.5) yet shorter average lifetime (30 vs 105 μs) of the lowest triplet excited state compared to the free base compounds. The differences in the excited-state properties of the indium chelates versus free base bacteriochlorins derive primarily from a 30-fold greater rate constant for S(1) → T(1) intersystem crossing, which stems from the heavy-atom effect on spin-orbit coupling. The trends in optical properties of the indium bacteriochlorins versus free base analogues, and the effects of 5-OMe versus 5-H substituents, correlate well with frontier molecular-orbital energies and energy gaps derived from density functional theory calculations. Collectively the synthesis, photophysical properties, and electronic characteristics of the indium bacteriochlorins and free base analogues reported herein should aid in the further design of such chromophores for diverse applications.