Department of Chemistry, North Carolina State University, Raleigh, NC 27695-8204, USA.
Photochem Photobiol Sci. 2013 Dec;12(12):2089-109. 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-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.
叶绿素大环上指定位置取代基的存在可以改变光谱性质，这一现象可以通过合成来探测。先前的合成已经提供了具有不同芳基取代基（单独或集体）在 5、10 和 15-位置的叶绿素，但不是 20-位置。一个新的西部半部分（5-苯基-2,3,4,5-四氢-1,3,3-三甲基二吡咯）已被用于与东方半部分（9-溴二吡咯甲烷-1-醛）缩合，然后进行氧化环化，得到（5%产率）锌（II）20-苯基叶绿素。相同的西部半部分和二芳基取代的东方半部分的缩合得到（11%产率）锌（II）5,10,20-三芳基叶绿素; 用 TFA 脱金属化，然后进行 15-溴化和 Suzuki 偶联，得到游离碱 5,10,15,20-四芳基叶绿素。总共制备了 10 种新的合成叶绿素。游离碱叶绿素的近紫外（B）吸收带从 389nm 移至 429nm，锌叶绿素的吸收带从 398nm 移至 420nm，随着间芳环数从 0 到 4 逐步增加。长波长（Q(y)）吸收带随着间芳基基团数的增加发生红移和蓝移。无论间芳基取代基的数量和位置（包括“在环周围行走一个苯基基团”）如何，游离碱叶绿素的相应荧光量子产率（0.17 至 0.27）和单重激发态寿命（9.4 至 13.1ns）是可比的，锌配合物也是如此（0.057 至 0.080; 1.2 至 1.6ns）。密度泛函理论计算表明，叶绿素的前线分子轨道中，HOMO-1 的能量受间芳基取代基的影响最大，随着间芳基基团数量的增加，其能量逐渐不稳定。具有 0-4 个不同间芳基取代基的叶绿素的可用性为桥接已知的未取代叶绿素和间四芳基叶绿素提供了单个的垫脚石。
