The Quenching of Triplet States of Anthracene and Porphyrins by Heavy Metal Ions¹

Abstract

oscillations in "long-field" molecules compared with "round-field" molecule^^^^^^ may favor internal conversion. Should data on aromatic hydrocarbons embodying these geometries (coronene vs. perylene, for example) bear out this expectation, some clue will be afforded toward the understanding of internal conversion in complex molecules.Complexes of TPP and Transition Metals.-Entirely negative results were obtained in attempts to observe triplet states of copper and cobalt TPP complexes by flash-excitation. Even in the region adjoining the Soret band, and with oscillograms taken under very favorable scattered light conditions; no change in absorption was observed. Assuming the triplet absorption to be comparable to that of TPP or ZnTPP, a 5y0 conversion lasting 1 x A very short-lived phosphorescence and no observable fluorescence was iound by Becker and KashaIg in nickel and copper porphyrins in rigid EPA (ether-isopentane-alcohol) , compared with strong fluorescence and weak phosphorescence in non-paramagnetic complexes. This was attributed to enhancement of spin-orbit interactions by the paramagnetic ionz0 which both increased the yield of triplet by radiationless transition from the excited singlet and shortened the lifetime for radia- 3.second would have been detected readily.

A Flash Photolysis Study of Intermediates in Photochemical Reactions of Chlorophyll

“…Assuming that the temperature dependence of rate constants is determined only by the activation barrier from Arrhenius dependence we evaluate the effective activation energy of chlorophyll a + b triplet-triplet annihilation ET = 2.6 +-0.4 kcal and finally the decay rate constant K , = 10" e-2600'RT sec-'M-' [32]. It follows from the above data, that K , rate constants for pigments are in good agreement with the appropriate K,-values for the triplet states of fluorescein [25], eosin [26], acridine orange [27], anthracene [20,28] and chlorophyll [30]. The close similarity in K, obtained for different substances might be due to the diffusion-controlled nature of triplet-triplet annihilation.…”

Enhancing porphyrin photostability when locked in metal–organic frameworks

“…57 Similarly, Ni 2+ and Co 2+ were shown to quench the triplet state of porphyrins. 58 Zirconium (Zr 4+ ) has also been reported to efficiently quench the triplet excited state cyclopentadiene complexes. 59 As a result, we believe that a combination of structural configuration coupled with the physiochemical inhibition of intersystem crossing is responsible for the observed enhancement in the brightness and photostability of porphyrins.…”

Kinetics of photodegradation of the fungicide fenarimol in natural waters and in various salt solutions: salinity effects and mechanistic considerations

“…Quenching was observed with all the other salts, and the data obeyed the Stern±Volmer equation: Stern±Volmer constants (K SV ) were determined from the slopes of plots of¯uorescence intensity ratio against quencher concentration, and respectivē uorescence quenching rate constants (k q ) calculated from these using the¯uorescence lifetime of fenarimol measured in this solvent (Table 1). Quenching of excited states by halide anions may occur either by an electron transfer reaction (Beer Watkins, 1974;Shizuka et al, 1980;Najbar and Mac, 1991;Najbar et al, 1993) or an external heavy atom eect (Linschitz and Pekkarinen, 1960;Patterson and Rzad, 1975;Amorim et al, 1977;Koziar and Cowan, 1978). To test the possibility that quenching involves an electron transfer mechanism the logarithm of the quenching constants were plotted against the reduction potential (Stanbury, 1989) of the anions (X Á /X À ) (Fig.…”