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

The palladium complex of 1,6,10,15,19,24,28,33-octabutoxynaphthalocyan ine, PdNc(OBu)8,is shown to undergo reversible addition of O2 in benze ne 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 ele ctronic absorption spectral evidence, the photoadduct is postulated to be an endoperoxide which has molecular oxygen added to one of the ben zene rings at the sites alpha to the point of fusion with the tetrapyr role ring, i.e., the positions bearing butoxy substituents. The format ion of this product probably involves O2(1DELTA(g)). The photoadduct d ecomposes 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. Th is reaction is accelerated by light. Laser flash photolysis results sh ow that the quantum efficiency for the formation of the photoadduct in O2-saturated benzene is <0.01. The quantum efficiency for decompositi on 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 tetra pyrrole to form a metastable endoperoxide, and the latter compound app ears to be the first example of an endoperoxide that is subject to pho tocycloreversion with visible light. It is also demonstrated that ener gy 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 ph osphoresces in fluid solution. This behavior suggests that the platinu m group metal complexes of octaalkoxynaphthalocyanine may prove to be useful as luminescence probes for improving the detection limit of O2( 1DELTA(g)).