The proton-controlled fluorescence of aminomethyltetraphenylporphyrin–tin(IV) derivatives

Abstract

The fluorescence quantum yield of the aminomethyltetraphenylporphyrin-tin(iv) compound, la-h is strongly influenced by pH in a bimodal manner owing to photoinduced electron transfer and axial ligand exchange processes. ~ Ion-controlled fluorescence in the visible region, especially 'on-off' switching, is of interest not only for ion sensing in the life sciences,2 but also for the construction of molecular optoelectronic devices.3 The use of porphyrin derivatives in photodynamic therapy4 and their intracellular location by fluorescence imaging,5 combined with the fact that intracellular pH is significantly different in normal and tumour cells,6 provided the impetus to the present work.The aminomethyltetraphenylporphyrin-tin(1v) series 1 was designed so that photoinduced electron transfer7 would take place from the amino group to the porphyrin moiety, resulting in fluorescence quenching. However, when the amino group is protonated fluorescence becomes an important deexcitation pathway. A related intermolecular situation is known.* Application of the Weller equation9 shows that the photoinduced electron transfer process is thermodynamically feasible (Table 1). The syntheses of la-h are summarised in Scheme 1.The fluorescence quantum yield (@)-pH profiles (Fig. 1) of 1 are bimodal, being composed of two sigmoidal curves of opposite gradients, and are due to two separate processes. The segment with the negative gradient is assignable to the photoinduced electron transfer process. The other is attributed to an axial ligand exchange process on the tin(1v) centre. The results (Table 1, Fig. 1) show that significant protoninduced fluorescence changes are exhibited by all members of the series la-h without alteration of fluorescence band position or vibrational fine structure of fluorescence. The pKa2 values determined by fluorescence titration and the use of eqn. (1) on the sigmoidal segment of the @-pH profile with negative gradient correlate well with the pKa,model values for the corresponding 'parent' methylamines, as demonstrated by the constant ApK, values in Table 1. Thus the sigmoidal segment results from protonation equilibria of the amino group in la-h. Furthermore, this correlation suggests a constant steric hindrance to solvation16 of the protonated amino group by the bulky porphyrin moiety across the series. It is also notable that the pKa2 values determined fluorimetrically are in close agreement with those obtained by the analysis, via eqn. (2), of the small pH-dependent alterations of the UV-VIS absorption spectrum for each member of series 1. This demonstrates the unusual result that the pKa2 values obtained by excited state experiments are thermodynamic (equilibrated) values. 17 This arises from the spatial isolation of the amino group from the photoantenna porphyrin moiety by the methylene spacer unit. The @max values are essentially constant across the series la-h and are identical to the value for the parent compound 2. The @min2 values correlate with the thermodynamic driving force for photoinduced el...

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