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nanometer precision by relatively straightforward chemical modification
so that one common precursor can be used for the synthesis of a range of
derivatives with different emission bands. 193,198 Moreover, chlorins and
bacteriochlorins exhibit exceptionally narrow emission bands with full width
at half maxima (FWHM) of
20 nm for
bacteriochlorins, which are probably the narrowest emissions among organic
compounds. 183-185,198 Most of the chlorin and bacteriochlorin derivatives
exhibit also sufficiently high quantum yields of fluorescence and high
fluorescence lifetimes: 8-10 ns for chlorins 183 (which is of a magnitude
higher than those for typical organic fluorophores) and 4-6 ns for
bacteriochlorins. 184 Fluorescence lifetime can be also tuned to some extent
by substitution and metalation. 183 Their narrow and tunable emission bands
and long and tunable fluorescence lifetimes make them a superior choice for
spectral and lifetime in vivo multiplexing (see Section 6.1 ).
15 nm for chlorins and
5.5. Hydroporphyrin arrays for increased Stokes shift
and multicolor in vivo imaging
The inherent spectroscopic property of hydroporphyrins, both chlorins and
bacteriochlorins, is the small Stokes shift, which typically falls in the range
0-10 nm, regardless of the solvent, substitution pattern, and metalation state
of the macrocycle. 183,184 To overcome this limitation, which is critical for
in vivo imaging, hydroporphyrin energy-transfer dyads have been proposed.
Holten and coworkers have demonstrated that, in dyads comprising chlorin
and bacteriochlorin, efficient energy transfer from chlorin to bacteriochlorin
moieties occurs, and dyads behave as a single chromophore with excitation
wavelengths of chlorins (650 or 675 nm) and the emission wavelength of
bacteriochlorin (760 nm)
so that
the effective Stokes
shift can be
increased up to 110 nm ( Chart 3.8 ). 201,202
5.6. Water solubility and aggregation
The planar, aromatic, and highly hydrophobic structure of porphyrins and
hydroporphyrins causes difficulties in aqueous solubility. Additionally, por-
phyrins tend to aggregate in aqueous solution. 203 The aggregation behavior
of porphyrins, which is mainly driven by pp stacking interactions and hy-
drophobic forces, is quite complex and depends on the substitution pattern.
Introduction of the hydrophilic groups, such as carboxylates, sulfonic acids,
quarternary pyridinium, or ammonium groups, only partially solves the
problem because such porphyrins still tend to aggregate. 203 The aggregation
problem can be overcome by the introduction of a “swallow-tail”
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