Waruna Jinadasa R G, Thomas Michael B, Hu Yi, D'Souza Francis, Wang Hong
Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-5017, USA.
Phys Chem Chem Phys. 2017 May 24;19(20):13182-13188. doi: 10.1039/c7cp00024c.
A series of β-pyrrole functionalized push-pull porphyrins with amine push groups linked via an ethynylphenyl spacer, and cyclic imide or carboxylic esters as pull groups have been newly synthesized and characterized. The β-pyrrole functionalized ethynylphenyl spacer extends the conjugation of the porphyrin π-system, as reflected by their red-shifted absorbance and fluorescence spectra. The computed structures revealed no steric hindrance between the porphyrin π-system and the β-substituents. The calculated HOMO and LUMO of compounds WJ2 and WJ3 display significant segregation, where the electron density in the HOMO and LUMO is mainly located at the donor component and the acceptor component, respectively. The orbital segregation is likely attributed to the introduction of the electron-donating amine group at the porphyrin periphery. Electrochemical studies revealed the expected lower HOMO-LUMO gap as a result of the facile oxidation and reduction of the push-pull porphyrins. As a consequence of the push-pull effects, a reduction in fluorescence intensity and lifetime was observed, especially for compound WJ3 having two electron-donating amino groups and a strongly electron-withdrawing cyclic imide group. Femtosecond transient absorption spectral studies revealed the successful formation of the singlet excited state in all of these push-pull porphyrins. Due to the occurrence of intramolecular charge transfer-type interactions, relaxation of the singlet excited state was found to be faster in compound WJ3 compared to other two derivatives in polar solvent but not in nonpolar solvent. Such charge transfer-type interactions from the triplet excited state were also observed in the case of compound WJ3 in benzonitrile. The present findings bring out the importance of push-pull effects in governing the ground and excited (singlet and triplet) state properties of free-base porphyrins.
一系列通过乙炔基苯基间隔基连接胺推电子基团、以环状酰亚胺或羧酸酯作为拉电子基团的β-吡咯官能化推拉卟啉已被新合成并表征。β-吡咯官能化的乙炔基苯基间隔基扩展了卟啉π体系的共轭，这在它们红移的吸收光谱和荧光光谱中得到体现。计算得到的结构表明在卟啉π体系和β取代基之间不存在空间位阻。化合物WJ2和WJ3计算得到的最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）显示出显著的分离，其中HOMO和LUMO中的电子密度分别主要位于供体组分和受体组分。轨道分离可能归因于在卟啉外围引入了给电子胺基。电化学研究表明，由于推拉卟啉易于氧化和还原，预期会有较低的HOMO-LUMO能隙。由于推拉效应，观察到荧光强度和寿命降低，特别是对于具有两个给电子氨基和一个强吸电子环状酰亚胺基团的化合物WJ3。飞秒瞬态吸收光谱研究表明，所有这些推拉卟啉都成功形成了单重激发态。由于分子内电荷转移型相互作用的发生，发现在极性溶剂中，化合物WJ3中比其他两种衍生物的单重激发态弛豫更快，但在非极性溶剂中并非如此。在苯甲腈中的化合物WJ3的情况下也观察到了来自三重激发态的这种电荷转移型相互作用。本研究结果揭示了推拉效应在控制游离碱卟啉的基态和激发态（单重态和三重态）性质方面的重要性。
