Kim Y H, Jeong D H, Kim D, Jeoung S C, Cho H S, Kim S K, Aratani N, Osuka A
Center for Ultrafast Optical Characteristics Control, Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
J Am Chem Soc. 2001 Jan 10;123(1):76-86. doi: 10.1021/ja0009976.
The molecular design of directly meso-meso-linked porphyrin arrays as a new model of light-harvesting antenna as well as a molecular photonic wire was envisaged to bring the porphyrin units closer for rapid energy transfer. For this purpose, zinc(II) 5,15-bis(3,5-bis(octyloxy)phenyl)porphyrin (Z1) and its directly meso-meso-linked porphyrin arrays up to Z128 (Zn, n represents the number of porphyrins) were synthesized. The absorption spectra of these porphyrin arrays change in a systematic manner with an increase in the number of porphyrins; the high-energy Soret bands remain at nearly the same wavelength (413-414 nm), while the low-energy exciton split Soret bands are gradually red-shifted, resulting in a progressive increase in the exciton splitting energy. The exciton splitting is nicely correlated with the values of cos[pi/(N + 1)] according to Kasha's exciton coupling theory, providing a value of 4250 cm(-1) for the exciton coupling energy in the S(2) state. The increasing red-shifts for the Q-bands are rather modest. The fluorescence excitation anisotropy spectra of the porphyrin arrays show that the photoexcitation of the high-energy Soret bands exhibits a large angle difference between absorption and emission dipoles in contrast with the photoexcitation of the low-energy exciton split Soret and Q-bands. This result indicates that the high-energy Soret bands are characteristic of the summation of the individual monomeric transitions with its overall dipole moment deviated from the array chain direction, while the low-energy Soret bands result from the exciton splitting between the monomeric transition dipoles in line with the array chain direction. From the fluorescence quantum yields and fluorescence lifetime measurements, the radiative coherent length was estimated to be 6-8 porphyrin units in the porphyrin arrays. Ultrafast fluorescence decay measurements show that the S(2) --> S(1) internal conversion process occurs in less than 1 ps in the porphyrin arrays due to the existence of exciton split band as a ladder-type deactivation channel, while this process is relatively slow in Z1 (approximately 1.6 ps). The rate of this process seems to follow the energy gap law, which is mainly determined by the energy gap between the two Soret bands of the porphyrin arrays.
设计直接中-中连接的卟啉阵列的分子结构，将其作为一种新型的光捕获天线模型以及分子光子线，旨在使卟啉单元更紧密排列以实现快速能量转移。为此，合成了锌（II）5,15-双（3,5-双（辛氧基）苯基）卟啉（Z1）及其直接中-中连接的卟啉阵列，直至Z128（Zn，n代表卟啉的数量）。这些卟啉阵列的吸收光谱随着卟啉数量的增加而有系统地变化；高能Soret带保持在几乎相同的波长（413 - 414 nm），而低能激子分裂Soret带逐渐红移，导致激子分裂能逐渐增加。根据卡沙激子耦合理论，激子分裂与cos[π/(N + 1)]的值有很好的相关性，在S(2)态下激子耦合能的值为4250 cm⁻¹。Q带的红移增加较为适度。卟啉阵列的荧光激发各向异性光谱表明，与低能激子分裂Soret带和Q带的光激发相比，高能Soret带的光激发在吸收和发射偶极子之间呈现出较大的角度差。这一结果表明，高能Soret带是各个单体跃迁总和的特征，其总偶极矩偏离阵列链方向，而低能Soret带是由与阵列链方向一致的单体跃迁偶极子之间的激子分裂产生的。通过荧光量子产率和荧光寿命测量，估计卟啉阵列中的辐射相干长度为6 - 8个卟啉单元。超快荧光衰减测量表明，由于存在激子分裂带作为阶梯型失活通道，在卟啉阵列中S(2)→S(1)内转换过程发生在不到1 ps的时间内，而在Z1中这个过程相对较慢（约1.6 ps）。这个过程的速率似乎遵循能隙定律，这主要由卟啉阵列的两个Soret带之间的能隙决定。
