PHOTOCHROMIC REACTIONS INVOLVING PALLADIUM(II) OCTABUTOXYNAPHTHALOCYANINE AND MOLECULAR-OXYGEN

The palladium complex of 1,6,10,15,19,24,28,33-octabutoxynaphthalocyanine, PdNc(OBu)8,is shown to undergo reversible addition of O2 in benzene solution at ambient temperature (22 +/- 1-degrees-C) during steady-state photolysis with an Ar ion laser. On the basis of H-1-NMR and electronic absorption spectral evidence, the photoadduct is postulated to be an endoperoxide which has molecular oxygen added to one of the benzene rings at the sites alpha to the point of fusion with the tetrapyrrole ring, i.e., the positions bearing butoxy substituents. The formation of this product probably involves O2(1DELTA(g)). The photoadduct decomposes mainly back into PdNc(OBu)8 by a thermal process whose first-order rate constant is (2.6 +/- 0.2) x 10(-4) s-1 at 22-degrees-C. This reaction is accelerated by light. Laser flash photolysis results show that the quantum efficiency for the formation of the photoadduct in O2-saturated benzene is <0.01. The quantum efficiency for decomposition of the adduct into PdNc(OBu)8 in Ar-saturated benzene is 0.035 +/- 0.010 with either 355- or 532-nm light, but the value is 0.0015 +/- 0.0005 with 683-nm light. PdNc(OBu)8 may be the first example of a tetrapyrrole to form a metastable endoperoxide, and the latter compound appears to be the first example of an endoperoxide that is subject to photocycloreversion with visible light. It is also demonstrated that energy transfer from O2(1DELTA(g)) to PdNc(OBu)8 occurs with a bimolecular rate constant of 9 X 10(9) M-1 s-1 to produce PdNc(OBu)8(T1) which phosphoresces in fluid solution. This behavior suggests that the platinum group metal complexes of octaalkoxynaphthalocyanine may prove to be useful as luminescence probes for improving the detection limit of O2(1DELTA(g)).