Tuning Photochemical and Photophysical Properties of P(V) Phthalocyanines

Abstract

The ability of P(V) phthalocyanines (Pcs) for efficient singlet oxygen (SO) generation was demonstrated for the first time by the example of unsubstituted and α- and β-octabutoxy-substituted P(V)Pcs with hydroxy, methoxy and phenoxy ligands in the apical positions of the octahedral P centre. Variation of substituents in Pc ring and P(V) axial ligands allows careful tuning of photophysical and photochemical properties. Indeed, a combination of BuO groups in the β-positions of the Pc ring and PhO groups as axial ligands provides significant SO generation quantum yields up to 90%; meanwhile, the values of SO generation quantum yields for others investigated compounds vary from 27 to 55%. All the complexes, except α-substituted P(V)Pc, demonstrate fluorescence with moderate quantum yields (10-16%). The introduction of electron-donating butoxy groups, especially in the α-position, increases the photostability of P(V)Pcs. Moreover, it has been shown in the example of β-BuO-substituted P(V) that the photostability depends on the nature of axial ligands and increases in the next row: OPh < OMe < OH. The presence of oxy/hydroxy axial ligands on the P(V) atom makes it possible to switch the photochemical and photophysical properties of P(V)Pcs by changing the acidity of the media.

Keywords: fluorescence; phosphorus; photostability; phthalocyanine; singlet oxygen.

Scheme 1

Synthesis of P(V)Pcs investigated in this study. Highlighted in color are the substituents that bring the most noteworthy properties to a P(V)Pc molecule. * Not isolated intermediate products.

Figure 1

Normalized UV–vis spectra of P(V)Pcs with different type of substitution in DMSO.

Figure 2

UV–vis spectra of different forms of compound 7 in DMSO.

Figure 3

Normalized UV–vis spectra of P(V)Pcs with different axial ligands in DMSO.

Figure 4

³¹P{1H} NMR spectra of different forms of compound 7 in CHCl₃.

Figure 5

Normalized adsorption (orange) and emission (green) (λ_ex = 350 nm) spectra of 5 in DMSO.

Figure 6

Normalized emission spectra of different forms of compound 7 in DMSO.

Figure 7

Boundary orbitals calculated for model complex (β-OMe)₈PcP(OPh)₂. Note that neither of the axial phenyl rings holds any of the four boundary orbitals, hence no appreciable relaxation through charge transfer processes.