title: Chlorin-bacteriochlorin energy-transfer dyads as prototypes for near-infrared molecular imaging probes: controlling charge-transfer and fluorescence properties in polar media.
abstract: The photophysical properties of two energy-transfer dyads that are potential candidates for near-infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC-FbB and ZnC-FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2-dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of approximately (5-10 ps)(-1) and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Phif=0.19) and singlet excited-state lifetimes (tau approximately 5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited-state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC-FbB than for ZnC-FbB in a given solvent. For example, the Phif and tau values for FbC-FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC-FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge-transfer states, as assessed by ground-state redox potentials and supported by molecular-orbital energies derived from density functional theory calculations. Controlling the extent of excited-state quenching in polar media will allow the favorable photophysical properties of the chlorin-bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC-FbB, 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (lambdaf=760 nm), long bacteriochlorin excited-state lifetime (approximately 5.5 ns), and narrow (<or=20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity- and lifetime-imaging techniques.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, MO, USA.
authors.forename: Hooi Ling
authors.initials: HL
authors.lastname: Kee
authors.forename: James R
authors.initials: JR
authors.lastname: Diers
authors.forename: Marcin
authors.initials: M
authors.lastname: Ptaszek
authors.forename: Chinnasamy
authors.initials: C
authors.lastname: Muthiah
authors.forename: Dazhong
authors.initials: D
authors.lastname: Fan
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Lindsey
authors.forename: David F
authors.initials: DF
authors.lastname: Bocian
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
mesh_terms: D004735:Energy Transfer; D005453:Fluorescence; D015335:Molecular Probes; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry*; D011789:Quantum Theory; D019265:Spectroscopy, Near-Infrared
chemical_list: D015335:Molecular Probes; D011166:Porphyrins; C006969:chlorin