Near-infrared molecular imaging probes based on chlorin-bacteriochlorin dyads
2010
Chlorin-bacteriochlorin dyads as a new class of near-infrared fluorophores were synthesized and
spectroscopically characterized. Each dyad is comprised of a chlorin macrocycle (free base or zinc chelate) as an energy
donor (and absorber) and a free base bacteriochlorin as an energy acceptor (and emitter). Excitation of the chlorin (λ=
650 nm, zinc chelate; 675 nm, free base) results in fast (5 ps) and nearly quantitative (>99%) energy transfer to the
adjacent bacteriochlorin moiety, and consequently bacteriochlorin fluorescence (λ= 760 nm). Thus, each chlorinbacteriochlorin
dyad behaves as a single chromophore, with a large effective Stokes shift (85 or 110 nm), a significant
fluorescence quantum yield (Φ f = 0.19), long excited-state lifetime (τ = 5.4 ns), narrow excitation and emission bands
(<20 nm), and high chemical stability.
Imaging experiments performed using phantoms show that the chlorin-bacteriochlorin dyads exhibit a range of
superior properties compare with commercially available imaging dyes. While the latter are six-fold brighter (comparing e•Φ f values), the chlorin-bacteriochlorin dyads exhibit narrower excitation and emission bands and larger Stokes shift,
therefore allowing more efficient and selective excitation and detection of fluorescence. The high selectivity is further
demonstrated with in vivo imaging studies using mice. This selectivity together with the tunability of absorption and
emission wavelengths using substituent effects under synthetic control make the chlorin-bacteriochlorin dyads ideal
candidates for multicolor imaging applications. In addition, the long fluorescence lifetimes make those probes suitable
for lifetime-imaging applications.
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