The effect of substitutents at alkylsulfanyl/arylsulfanyl non-peripherally substituted phthalocyanines: Spectral and photophysical properties, basicity and photostability

A series of magnesium, zinc and metal-free derivatives of non-peripherally substituted phthalocyanines (Pcs) bearing alkylsulfanyl or arylsulfanyl groups of different bulkiness was synthesized. Their spectral and photophysical properties including also the basicity of azomethine nitrogens and photostability were compared within the series as well as with similar peripherally substituted Pcs. Non-peripheral position of substituents led to the 70 nm red-shift of Q-band in comparison to the peripherally substituted Pcs. However, unexpected blue-shift of approximately 50 nm was observed in the series of non-peripherally substituted Pcs for the most bulky tert-butylsulfanyl derivative caused probably by extreme distortion of the macrocycle. The substitution had no effect on photophysical properties and compounds reached Phi(Delta) values 0.74-0.76 and Phi(F) 0.053-0.080 for zinc complexes, and Phi(Delta) 0.47-0.51 and Phi(F) 0.10-0.17 for magnesium complexes following the rule of heavy atom effect. Generally, non-peripherally substituted Pcs possessed improved singlet oxygen production in comparison to peripherally substituted ones. The photostability of the target compounds decreased with the red-shift of their absorption maxima with the arylsulfanyl derivatives being less photostable. The basicity of azomethine nitrogens was clearly dependent on the position and the character of substituent. Thus, non-peripherally substituted Pcs showed extraordinary increased basicity over the peripherally substituted ones with the most pronounced effect at alkylsulfanyl derivatives.