Synthesis and photophysical properties of chlorins bearing 0-4 distinct meso-substituentsElectronic supplementary information (ESI) available: Synthesis of a meso-aryl-substituted Western half. See DOI: 10.1039/c3pp50240f
The presence of substituents at designated sites about the chlorin macrocycle can alter the spectral properties, a phenomenon that can be probed through synthesis. Prior syntheses have provided access to chlorins bearing distinct aryl substituents (individually or collectively) at the 5, 10, and 15-...
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Main Authors | , , , , , , , |
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Format | Journal Article |
Language | English |
Published |
13.11.2013
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Online Access | Get full text |
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Summary: | The presence of substituents at designated sites about the chlorin macrocycle can alter the spectral properties, a phenomenon that can be probed through synthesis. Prior syntheses have provided access to chlorins bearing distinct aryl substituents (individually or collectively) at the 5, 10, and 15-positions, but not the 20-position. A new Western half (5-phenyl-2,3,4,5-tetrahydro-1,3,3-trimethyldipyrrin) has been employed in condensation with an Eastern half (9-bromodipyrromethane-1-carboxaldehyde) followed by oxidative cyclization to give (5% yield) the zinc(
ii
) 20-phenylchlorin. Condensation of the same Western half and a diaryl-substituted Eastern half provided (11% yield) the zinc(
ii
) 5,10,20-triarylchlorin; demetalation with TFA followed by 15-bromination and Suzuki coupling gave the free base 5,10,15,20-tetraarylchlorin. Altogether, 10 new synthetic chlorins have been prepared. The near-UV (B) absorption band of the free base chlorins shifts bathochromically from 389 to 429 nm and that for the zinc chlorins from 398 to 420 nm as the number of
meso
-aryl rings is increased stepwise from 0-4. The long-wavelength (Q
y
) absorption band undergoes a bathochromic and hypochromic shift upon increase in number of
meso
-aryl groups. Regardless of the number and positions of the
meso
-aryl substituents (including "walking a phenyl group around the ring"), the respective fluorescence quantum yields (0.17 to 0.27) and singlet excited-state lifetimes (9.4 to 13.1 ns) are comparable among the free base chlorins and the same is true for the zinc chelates (0.057 to 0.080; 1.2 to 1.6 ns). Density functional theory calculations show that of the frontier molecular orbitals of the chlorin, the energy of the HOMO−1 is the most affected by
meso
-aryl substituents, undergoing progressive destabilization as the number of
meso
-aryl groups is increased. The availability of chlorins with 0-4 distinct
meso
-aryl substituents provides the individual stepping-stones to bridge the known unsubstituted chlorin and the
meso
-tetraarylchlorins.
An increase in number of
meso
-substituents in synthetic chlorins causes a bathochromic and hypochromic effect on the long-wavelength absorption band. |
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Bibliography: | Electronic supplementary information (ESI) available: Synthesis of a 10.1039/c3pp50240f meso aryl-substituted Western half. See DOI |
ISSN: | 1474-905X 1474-9092 |
DOI: | 10.1039/c3pp50240f |