Several neutral square planar complexes of formula [M(N-N)(X-X)] [where M is Pt(II), Pd(II) or Ni(II), N-N is 1,10-phenanthroline (phen), 4,7-diphenyl-1-10-phenanthroline (dpp) or 2,2′-biquinoline (biq), and X-X is dianion of catechol (CAT), 4-t-butyl-catechol (BCAT), 2,3-dihydroxynaphthalene (DHN), 3,4-dimercaptotoluene (DMT) or thiosalicylic acid (TSA)] have been prepared and characterized. They show a ligand-to-ligand charge-transfer (LLCT) band between 400 and 800 nm. [M(phenXX-X)] and [M(N-N)(DHN)] [where M is Pt(II) or Pd(II), N-N is 2,2′-bipyridine (bipy), phen, dpp or biq, and X-X is CAT, BCAT, DMT or TSA] photosensitize the photo-oxidation of 2,2,6,6-tetramethyl-4-piperidinol (NH) in the presence of molecular oxygen and light (300–800 nm) in dimethylformamide (DMF). This photo-oxidation involves the singlet molecular oxygen (¹O₂). However, [Pt(phen)(DMT)] and [Pt(phen)(TSA)] undergo self-sensitized photo-oxidation involving¹O₂; [Ni(phen)(DMT)] and [Ni(bipy)(DMT)] act as physical quenchers of¹O₂, and [Pd(phen)(DMT)] and [Pd(bipy)(DMT)] act as chemical quenchers of¹O₂.

Hematoporphyrin IX dimethyl ester (HPDME), tetraphenylporphyrin (TPP), tetra(4-methoxyphenyl)porphyrin (TMPP), tetra(3,4-dimethoxyphenyl)porphyrin (TDMPP), tetra(3,4,5-trimethoxyphenyl)porphyrin (TTMPP), tetraanthrylporphyrin (TAP) and tetraacridylporphyrin (TACP), and their Zn²⁺, SnX₂²⁺, Pd²⁺ and Pt²⁺ complexes have been prepared and characterized. The singlet molecular oxygen quantum yield (ϕ_Δ) values of the above porphyrins and their metal derivatives in N,N-dimethyl formamide (DMF) have been measured in the presence of 1,3-diphenylisobenzofuran (DPBF) as¹O₂ acceptor using steady state technique after correcting for the intensity of light absorbed by the photosensitizers. The ϕ_Δ values for the free base porphyrins are usually around 0.60. Hematoporphyrin IX dimethyl ester and its metal derivatives follow the order: HPDME (ϕ_Δ = 0.60) > Zn HPDME (0.40) > PdHPDME (0.34) > Sn(OH)₂ HPDME (0.28) > PtHPDME (0.24). Zinc(II) complexes of tetraarylporphyrins show about 65% efficiency in ϕ_Δ values are compared to the ϕ_Δ values of their corresponding free base porphyrins. These results can be explained on the basis of catalyzed intersystem crossing to the ground state. The ϕ_λ values of the above free base porphyrins and their metal complexes in DMF in presence of DPBF using single-pulsed laser excitation technique follow similar trends.