The present proposal aims to develop two novel classes of functional probes, which will enable the measurement of viscosity in cells. This important physiological parameter will be measured during normal cell function and during photodynamic therapy (PDT), a photoactivated cancer treatment of clinical importance. To achieve these objectives, we will characterise and use two types of tetrapyrrole macrocycles: porphycenes and porphyrazines as red/NIR emissive molecular rotors and PDT sensitizers and we will measure viscosity based on their fluorescence lifetime. The molecules will be used first in model systems such as vesicles, liposomes and gels and then in the complex environment of living cells. Protocols for the simultaneous detection of viscosity and other important physiological parameters such as pH will be developed. The novel methods will allow the quantification of viscosity in heterogeneous media, which is fundamental for understanding the complex dynamics of important biological, chemical and physical processes.