Synthesis and Excited‐state Photodynamics of a Chlorin–Bacteriochlorin Dyad—Through‐space Versus Through‐bond Energy Transfer in Tetrapyrrole Arrays

Abstract

Understanding energy transfer among hydroporphyrins is of fundamental interest and essential for a wide variety of photochemical applications. Toward this goal, a synthetic free base ethynylphenylchlorin has been coupled with a synthetic free base bromobacteriochlorin to give a phenylethyne-linked chlorin-bacteriochlorin dyad (FbC-pe-FbB). The chlorin and bacteriochlorin are each stable toward adventitious oxidation because of the presence of a geminal dimethyl group in each reduced pyrrole ring. A combination of static and transient optical spectroscopic studies indicate that excitation into the Qy band of the chlorin constituent (675 nm) of FbC-pe-FbB in toluene results in rapid energy transfer to the bacteriochlorin constituent with a rate of approximately (5 ps)(-1) and efficiency of >99%. The excited bacteriochlorin resulting from the energy-transfer process in FbC-pe-FbB has essentially the same fluorescence characteristics as an isolated monomeric reference compound, namely a narrow (12 nm fwhm) fluorescence emission band at 760 nm and a long-lived (5.4 ns) Qy excited state that exhibits a significant fluorescence quantum yield (Phif=0.19). Förster calculations are consistent with energy transfer in FbC-pe-FbB occurring predominantly by a through-space mechanism. The energy-transfer characteristics of FbC-pe-FbB are compared with those previously obtained for analogous phenylethyne-linked dyads consisting of two porphyrins or two oxochlorins. The comparisons among the sets of dyads are facilitated by density functional theory calculations that elucidate the molecular-orbital characteristics of the energy donor and acceptor constituents. The electron-density distributions in the frontier molecular orbitals provide insights into the through-bond electronic interactions that can also contribute to the energy-transfer process in the different types of dyads.

De NovoSynthesis of Gem-Dialkyl Chlorophyll Analogues for Probing and Emulating Our Green World

“…Additional notable chlorin building blocks are shown in Scheme 94. The set includes the following: (1) chlorins with a single halo or ethyne ( 217 , 292 218 , 272 219 , 442 220 , 442 221 , 256 and 222 256 ); (2) oxochlorins with a single halo or ethyne ( 223 , 259 224 258 ); (3) chlorins with two halo or two ethynes ( 225 , 284 226 284 ); (4) chlorins with one halo and one ethyne ( 227 , 284 228 299 ); and (5) a chlorin with two bromo substituents and one ethyne ( 229 278 ).…”

“…Iodophenyl/ethynylphenylchlorin 228 is an analogue of 147 (Scheme 56) and was prepared in similar fashion, where the iodophenyl and ethynylphenyl functional groups were installed upon formation of the pyrrole precursors. 299 Finally, chlorin 229 was prepared from bromo-Western half 152 and Eastern half 278 120g-Br 8,9 (analogue of 120c-Br 8,9 , Scheme 57); the Eastern half contained a 4-(TIPS-ethynyl)phenyl group at the dipyrromethane meso-position and the requisite 8,9-dibromo and 1-formyl groups.…”

“…Chlorin–bacteriochlorin dyads with a free base chlorin ( Dyad-6a ) 278 or zinc chlorin ( Dyad-6b ) 280 undergo fast and efficient excited-state energy transfer from chlorin to bacteriochlorin. 281 The 3,13-bis(phenylethynyl) substituents on the chlorin impart a bathochromic shift of the longwavelength absorption band.…”

Progress towards synthetic chlorins with graded polarity, conjugatable substituents, and wavelength tunability

“…The motivation for preparing both classes of chlorins reflects the dual desires to gain a deeper understanding of chlorin electronic/photophysical properties and to understand how to design chlorins with deep-red (or near-IR) absorption and (preferably intense) fluorescent properties. Such chlorins could be used as fluorophores for in vivo bioimaging applications (e.g., fluorescence-guided surgery [23,24]) or as the energy donor in energy-transfer arrays for artificial photosynthesis or in vivo multicolor bioimaging [18,20,25,26]. …”

“…For example, 3,13-bis(phenylethynyl)chlorin C-28 (Chart 2) exhibits a significant bathochromic shift of absorption and emission (λ abs = 676 nm, λ em = 678 nm), and large increase in fluorescence quantum yield (Φ f = 0.37, in toluene), compared to the analogous chlorin that lacks substituents at the 3,13-positions [18]. …”