Synthesis and photochemistry of free base and zinc tetraaryl porphyrins mono-substituted with tungsten pentacarbonyl via a pyridine linker

Abstract

A series of free base and zinc porphyrins, mono meso-substituted with a pyridyl group via an amide link, have been prepared by stepwise functionalisation of free base tetraphenylporphyrin, H2tpp. Four [W(CO)5L] (L = pyridyl-porphyrin) complexes have been synthesised; [M{tpp-NHCO-py-W(CO)5}], where M = 2H or Zn, one phenyl group is substituted at the 4-position and the pyridine is substituted at the 3- or 4-position. The spectroscopic data of the [W(CO)5L] complexes have been compared with those of the porphyrin ligands alone and with those of the model compounds, [W(CO)5(3-H2NCO-py)] and [W(CO)5(4-H2NCO-py)]. The UV-Vis spectra of the model compounds show that the MLCT and LF absorption bands overlap and no emission is observed at room temperature in solution. The UV-Vis absorption and emission spectra of the porphyrins with peripheral W(CO)5 are dominated by porphyrin bands. Time-resolved emission was observed from the porphyrin moiety in the [W(CO)5L] complexes and fitted to mono-exponential kinetics to give lifetimes of ≤10 ns for the zinc porphyrins and of ca. 22 ns for the free base porphyrins. Transients characteristic of the porphyrin 3(π–π*) excited state have been observed by time-resolved absorption and their decay kinetics have been fitted to an appropriate rate law. Long wavelength photolysis of [W(CO)5(3-H2NCO-py)] and [W(CO)5(4-H2NCO-py)] in THF resulted in conversion to [W(CO)5(THF)]. The [W(CO)5L] complexes behaved similarly with photolysis at λ > 395 nm but were photostable with λ > 495 nm.