Solvent effect on the photogeneration of singlet molecular oxygen by copper tetra(tert-butyl)phthalocyanine

Abstract

The photogeneration of the singlet molecular oxygen ( 1 O 2 ) by copper tetra(4-tert-butyl) phthalocyanine in organic solvents of different nature was substantiated. The apparent quantum yields of 1 O 2 in benzene and pyridine were determined. The data obtained indicate an influence of the coordinating properties of the solvent on the 1 O 2 yield. However, this factor is not "critical" as in the case of photogeneration of 1 O 2 by copper tetra(5-tert-butylpyrazino)porphyrazine.The interest in the photogeneration of singlet molecular oxygen ( 1 O 2 , 1 Δ g ) by phthalocyanines is due to their application in a number of photochemical and photomedical technologies [1][2][3][4].A photogeneration of 1 O 2 by phthalocyanines proceeds as a quenching of their excited triplet states by O 2 ( 3 Σ g -) molecules through the mechanism of energy transfer [2,5], and depends on both intramolecular (the macroheterocycle structure, the central atom nature) and external (solvation, aggregation) factors. As to copper phthalocyanine, the published data on the effectiveness of this process are contradictory. Usually this situation is attributed to a tendency of copper phthalocyanine to aggregation (see, for example, [2]). However, in [6] by an example of the 1 O 2 photogeneration by copper tetra(5-tert-butylpyrazino)porphyrazine it has been shown that the important external factors affecting the yield of 1 O 2 may include the coordinating ability of the solvent. This effect is apparently due to the competitive quenching of excited triplet molecules of the sensitizer by the solvent due to its axial coordination [7-10].The purpose of this study is to investigate the influence of the coordinating ability of solvent on the photogeneration of 1 O 2 by copper phthalocyanine by an example of behavior in the process of copper tetra-(4-tert-butyl)phthalocyanine [(t-Bu) 4 PcCu], the most convenient object for this purpose owing to its good solubility in many organic media.The electron absorption spectrum of the (t-Bu) 4 PcCu solutions is typical of most metallphthalocyanines, with an intense Q-band at 670-680 nm [11,12]. The band position depends on the solvent. In benzene, the band maximum (λ Q ) occurs at 677.5 (±0.5) nm, and its decimal molar absorption coefficient is equal to 1.7×10 5 (±0.2×10 5 ) cm -1 mol -1 l. With the increase in the solvent refractive index, the λ Q value undergoes a red shift (Fig. 1). These data can be interpreted within the framework of universal solvation of (t-Bu) 4 PcCu [13].A good qualitative example of photochemical activity of (t-Bu) 4 PcCu is its ability to sensitize the photooxidation of the 1 O 2 acceptors. The results of one such experiment on the sensitized photobleaching of tetracene solution in pyridine are shown in Fig. 2. Similar results were obtained with the other, less basic, solvents. Accounting for the oxidative nature of this process, whose mechanism has been substantiated in detail in [14][15][16], these data allow a preliminary