The fluorescence (or more generally the photoluminescence) lifetime is an intrinsic characteristic of a luminescent species that can provide insight into the species excited state dynamics. TRPL is the tool of choice for studying fast electronic deactivation processes that result in the emission of photons, a process called fluorescence. The lifetime of a molecule in its lowest excited singlet state usually ranges from a few picoseconds up to nanoseconds. This fluorescence lifetime can be influenced by the molecular environment (e.g., solvent, presence of quenchers (O2), or temperature) as well as interactions with other molecules. Processes like Förster Resonance Energy Transfer (FRET), quenching, solvation dynamics, or molecular rotation also have an effect on the decay kinetics. Lifetime changes can therefore provide information about the local chemical environment or insights into reaction mechanisms.
Some species such as metal-organic complexes, inorganic crystal structures, semiconductors and new types of hybrid materials have emission lifetimes ranging from nano- to micro- or even up to milliseconds. In this case the luminescent species relaxes from its lowest excited triplet state by emitting a photon in a process called phosphorescence.
Time-Correlated Single Photon Counting (TCSPC) is a popular method for carrying out TRPL measurements. TCSPC works by measuring the time between sample excitation by a laser pulse and the arrival of the emitted photon at the detector. TCSPC requires a defined “start”, provided by the electronics steering the laser pulse or a photo diode, and a defined “stop” signal, realized by detection with single-photon sensitive detectors. The measurement of this time delay is repeated many times to account for the statistical nature of the fluorophores emission. The detected events are then sorted into a histogram according to their arrival time which allows reconstruction of the photoluminescence decay.
