Rational Design of Fluorophores for In Vivo Applications - Progress in Molecular Biology and Translational Science - page 81

absorption wavelength shifted above 700 nm and extinction coefficient up
to 200,000 cm -1 M -1 ), and significant increase in fluorescence quantum
yield (up to 0.45 for naphthoporphyrin; see Chart 3.5 ). 171 Anthraporphyrins
(porphyrins with fused anthracene on the pyrrole units) absorb and emit
above 800 nm. 172 Benzo- and naphthoporphyrins upon complexation with
palladium (II) and platinum (II) show also an intense and long-lived near-IR
phosphorescence. 171 Phosphorescent benzoporphyrins, because of the long
lifetime of their phosphorescence, are prone to dynamic quenching by
oxygen; thus, palladium and, therefore, platinum benzoporphyrins find ap-
plication in sensing oxygen in vivo . 173,174

5.3. Strongly conjugated porphyrin arrays
An alternative strategy for increasing near-IR absorption and fluorescence of
porphyrins is an extension of the porphyrin conjugation by assembling sev-
eral porphyrin subunits into strongly coupled arrays, that is, arrays where the
porphyrin subunits are connected by a linker that provides strong electronic
coupling (e.g., acetylene linker, 175,178 or butadiyne linker 176,177 ) between
the subunits ( Chart 3.6 ). Such arrays composed of two, three, or five zinc
complexes of porphyrin connected by an acetylene linker exhibit a
progressive bathochromic shift of long-wavelength absorption and
emission bands (with emission up to 883 nm), increased extinction
coefficient, and good quantum yield of fluorescence. 178,179 The trimeric
arrays have been examined for imaging of B16 melanoma cells 180 and
labeling of the dendritic cells, 181 and their suitability for in vivo imaging
in living animals has also been demonstrated. 182

5.4. Chlorins and bacteriochlorins
The partial saturation of pyrrole units in porphyrins leads to formation of
new types of tetrapyrrolic macrocycles with distinctive spectral properties:
chlorins (with one partially saturated pyrrole ring), bacteriochlorins (two
partially saturated pyrrole rings on opposite sites), and isobacteriochlorins
(with two partially saturated pyrrole rings on the same site of macrocycle;
see Chart 3.7 ). In contrast to porphyrins, chlorins exhibit a strong absorption
in the red region (600-700 nm), 183 whereas bacteriochlorins absorb strongly
in the near-IR (700-800 nm). 184 Both classes of macrocycles show also
higher fluorescence quantum yields than the corresponding porphyrins
(average of

0.15 for bacteriochlorins 184 ). Iso-
bacteriochlorins exhibit strong absorption and emission (with quantum yield

0.25 for chlorins 185 and