Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee - 247667, India.
Dalton Trans. 2019 Oct 14;48(40):15002-15011. doi: 10.1039/c9dt02792k.
Mixed substitution at the β-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed β-octasubstituted porphyrins viz. MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co(ii), Ni(ii), Cu(ii), and Zn(ii)) have been synthesized and characterized by various spectroscopic techniques. The single crystal X-ray structure of H2TPP(NO2)(Ph)2Br5 showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H2TPP(Ph)2Br5X (X = NO2 or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the β-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H2TPP < H2TPP(NO2) < H2TPP(NO2)(Ph)2 < H2TPP(Ph)2Br6 < H2TPP(NO2)(Ph)2Br5. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibited a significant bathochromic shift (Δλmax = 53-61 nm) in the Soret band and (Δλmax = 90-95 nm) in the longest wavelength Qx(0,0) band as compared to H2TPP. Nonplanar conformations and electron withdrawing β-substituents induce higher protonation and deprotonation constants for H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 as compared to precursor porphyrins viz. H2TPP, H2TPP(NO2) and H2TPP(NO2)(Ph)2. The electronic spectral properties and redox potentials of MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co, Ni, Cu and Zn) are affected by β-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push-pull substituents at the β-position of the MTPP (M = 2H, CoII, NiII, CuII, and ZnII) skeleton of the macrocycle. CuTPP(Ph)2Br6 and CuTPP(NO2)(Ph)2Br5 exhibited a dramatically reduced HOMO-LUMO gap with a difference of 0.55 V and 0.62 V, respectively as compared to CuTPP due to the push-pull effect of β-substituents and nonplanarity of the porphyrin core.
β-位取代基的混合对卟啉的光物理和电化学氧化还原性质有影响。我们合成了两个新的系列不对称β-八取代卟啉，即 MTPP(Ph)2Br5X（X=NO2 或 Br，M=2H、Co(ii)、Ni(ii)、Cu(ii)和 Zn(ii)），并用各种光谱技术对其进行了表征。H2TPP(NO2)(Ph)2Br5 的单晶 X 射线结构显示出大环核的非平面鞍形构象。此外，完全优化的几何形状证实了 H2TPP(Ph)2Br5X（X=NO2 或 Br）的鞍形构象。电子光谱显示，在间四苯基卟啉骨架的β-位引入供电子和吸电子取代基，显著导致了红移，这反映了以下顺序 H2TPP<H2TPP(NO2)<H2TPP(NO2)(Ph)2<H2TPP(Ph)2Br6<H2TPP(NO2)(Ph)2Br5。与 H2TPP 相比，H2TPP(Ph)2Br5X（X=NO2 或 Br）在 Soret 带中显示出显著的红移（Δλmax=53-61nm），在最长波长 Qx(0,0)带中显示出显著的红移（Δλmax=90-95nm）。非平面构象和吸电子β-取代基导致 H2TPP(NO2)(Ph)2Br5 和 H2TPP(Ph)2Br6 的质子化和去质子化常数比前体卟啉 H2TPP、H2TPP(NO2)和 H2TPP(NO2)(Ph)2 更高。MTPP(Ph)2Br5X（X=NO2 或 Br，M=2H、Co、Ni、Cu 和 Zn）的电子光谱性质和氧化还原电位受卟啉核心外围β-取代基的影响。通过在大环的 MTPP（M=2H、CoII、NiII、CuII 和 ZnII）骨架的β-位引入推拉取代基，实现了氧化还原调谐。与 CuTPP 相比，CuTPP(Ph)2Br6 和 CuTPP(NO2)(Ph)2Br5 由于β-取代基的推拉效应和卟啉核的非平面性，HOMO-LUMO 能隙显著减小，分别为 0.55V 和 0.62V。
