Technical Note - Phosphorescence Lifetime Imaging Microscopy (PLIM) Measurements - Practical Aspects

Oct 26, 2026

Phosphorescence Lifetime Imaging Microscopy (PLIM) measurements are presented with a focus on practical aspects relevant to material characterization and chemical sensing. The note explains how PLIM contrast arises from excited-state lifetime differences between fluorophores, contrasting triplet-state phosphorescence with singlet fluorescence, and relates typical lifetime ranges (ns for fluorescence; hundreds of ns to ms for phosphorescence). It also reviews common phosphorescent compounds, including metal-ion complexes, semiconductor materials, and their applications such as oxygen imaging and energy-transfer studies, and outlines measurement requirements for TCSPC confocal setups.

LanguageEnglish

CategoryTechnology

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Pages10

Reading Time25 min

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Introduction

Phosphorescence Compounds and their Applications

Technical Note Phosphorescence Lifetime Imaging Microscopy (PLIM) Measurements: Practical Aspects Volker Buschmann, Sandra Orthaus, André Devaux, Rainer Erdmann PicoQuant GmbH, Rudower Chaussee 29, 12489 Berlin, Germany, [info@picoquant.com](info@picoquant.com) Introduction For many years, the characterization of luminescent materials including their luminescence lifetime has played an important role in the field of material science, especially in chemical sensing. Interest in characterization methods for phosphorescence imaging has been renewed over the past decade with the booming development of Organic Light Emitting Diode (OLED) technology, in which phosphorescent compounds are often a key component (“PHOLEDs”)[1] . In life sciences, typical phosphorescent probes include metal ions complexed with organic ligands, which can be used to, e.g. , image oxygen consumption in living tissue[2] or for energy transfer measure- ments. Furthermore, nanoparticles and quantum dots can also exhibit long fluorescence lifetime components and be thus imaged by Phosphorescence Lifetime Imaging Microscopy (PLIM)[3] . Analogous to Fluorescence Lifetime Imaging Microscopy (FLIM), the contrast in a PLIM image is based on excited state lifetime differences between individual fluorophores. This situation is schematically shown in the simplified Jablonski diagram in Fig. 1. A molecule in the electronic ground state (S0) can absorb (A) photons whose wavelength corresponds to the energy difference between the S0 and the excited singlet states (S1 , S2 , and so on) . In most cases, the excited molecule will relax via internal conversion (IC) to the Figure 1: Simplified Jablonski diagrams summarizing possible electronic transitions in a molecule upon absorption of a photon. On the left, a the scheme for a classic dye-molecule is shown. The right diagram displays the transition scheme of a Eu3+ -complex. In this example, light is absorbed by the ligand and transferred via an energy transfer to the Eu3+ ion of the complex[4]. This example shows that luminescence from metal ions or other non purely organic dyes can be more complex than in the classic scheme. In PLIM measurements, only the measurable lifetime of the excited state is relevant, not the exact mechanisms involved in the whole process. lowest emissive singlet state (i.e. S1) from where it can return to the ground state by emitting a photon. This process is called fluorescence (F) and is spin-allowed. While being in the S1 state, molecules can also either return to the ground state by transforming the excitation energy into vibrational energy (internal conversion, IC) or undergo an intersystem crossing (ISC) process. In this process, the excited electron transits into the energetically closest triplet state (T2 in the example shown in Fig. 1) . From there, the molecule will relax into the lowest triplet state via IC (T1) and can return to the S0 ground state by emitting a photon. In this case, the process is called phosphorescence (P) and is spin-forbidden. Therefore, the excited state lifetimes of the triplet state are much longer than those of the S1 state in classic fluorescence. The dominant relaxation pathway will depend on the material’s structure as well as on it’s environment. Efficient phosphorescence probes have a high intersystem crossing rate and a highly emissive T1 state. While fluorescence lifetimes are in the range of ns, phosphorescence lifetimes can range from hundreds of ns to ms. From a practical point of view, measurements of all lifetime transitions involving an emissive process with a lifetime >100 ns have similar requirements and are therefore covered here, even if the process in question may not be phosphorescence ina strict quantum mechanical sense. When using confocal set-ups with Time-Correlated Single Photon Counting (TCSPC), the requirements for performing PLIM are similar to classic FLIM, yet with slight differences in a few key points. This

FAQs

What determines the contrast in a PLIM image?

PLIM contrast is based on excited-state lifetime differences between individual fluorophores, analogous to Fluorescence Lifetime Imaging Microscopy (FLIM).

How does phosphorescence differ from fluorescence in terms of excited states?

Fluorescence involves relaxation from the singlet S1 state and is spin-allowed, while phosphorescence proceeds via intersystem crossing to a triplet state (e.g., T2 to T1) and is spin-forbidden, leading to much longer lifetimes.

What typical lifetime ranges are expected for fluorescence and phosphorescence?

Fluorescence lifetimes are typically in the nanosecond range, while phosphorescence lifetimes can extend from hundreds of nanoseconds to milliseconds.

Oct 26, 2026

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