Synthesis and photophysical properties of chlorins bearing 0-4 distinct meso-substituents

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.