Two novel fluorescent probes based on anthracene (AAC) and naphthalene (ANC) have been synthesised and characterised. Both the AAC and ANC derivative show green fluorescence at basic pH, but at acidic pH AAC is non-fluorescent, while ANC emits red light. This is attributed primarily to the FRET on–off processes. ANC has better pH discrimination abilities than AAC and can be used to distinguish between different pH environments inside a living cell for the first time.