A twisted tale: measuring viscosity and temperature of microenvironments using molecular rotors

Measuring viscosity and temperature on the microscale is a challening yet very important task, in materials sciences and in biology alike. In this perpsective we review and discuss fluorescent microviscosity sensors, termed molecular rotors', that offer a convenient way of measuring microscopic viscosity and sometimes may even be used to measure microscopic temperature in addition to viscosity. We discuss how temperature in combination with various solvent properties can affect microviscosity measurements and we review possible action mechanisms that make molecular rotors sensitive to multiple parameters of their environment. Overall, we reveal a complicated, yet exciting, behaviour of molecular rotors at different viscosity, temperature and solvent properties on the microscale and how this behaviour can be explained and exploited.