Photophysical Properties of Coumarin Dyes

Coumarin dyes constitute a versatile class of organic fluorophores characterised by a benzopyrone core that readily absorbs ultraviolet or visible light and re-emits at longer wavelengths. Their photophysical behaviour is governed by intramolecular charge transfer, the degree of π-conjugation and the nature of peripheral substituents, which together determine absorption maxima, fluorescence quantum yields and Stokes shifts. Solvent polarity and hydrogen-bonding interactions can induce pronounced solvatochromism, shifting emission bands and modulating non-radiative decay pathways. Phenomena such as excited-state intramolecular proton transfer and dual fluorescence from higher excited states illustrate deviations from classical Kasha’s rule, offering routes to tailor emission colour and lifetime. Recent advances exploit conformational control and aggregation-induced emission to enhance solid-state brightness, two-photon absorption cross-sections and photostability, paving the way for applications in bioimaging, optical materials and solar-energy conversion.

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Photophysical Properties of Coumarin Dyes publication trend

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Technical terms

Fluorescence quantum yield: The ratio of photons emitted to photons absorbed, indicating emission efficiency.

Stokes shift: The wavelength difference between absorption peak and emission peak, reflecting energy loss before fluorescence.

Intramolecular charge transfer (ICT): Electron redistribution within a molecule upon excitation, influencing emission wavelength and polarity sensitivity.

Solvatochromism: Change in a dye’s absorption or emission spectrum with solvent polarity, arising from differential stabilisation of electronic states.

Excited-state intramolecular proton transfer (ESIPT): Proton relocation within a molecule after excitation, often leading to large Stokes shifts and tautomeric emission.

Aggregation-induced emission (AIE): Enhanced fluorescence upon molecular aggregation, typically due to restriction of intramolecular motions that quench emission.

References

1. Conformation of the Ester Group Governs the Photophysics of Highly Polarized Benzo[g]coumarins. JACS Au (2023).
2. Polarized, V‑Shaped, and Conjoined Biscoumarins: From Lack of Dipole Moment Alignment to High Brightness. The Journal of Organic Chemistry (2022).
3. Janus-Type AIE Fluorophores: Synthesis and Properties of π-Extended Coumarin-Bearing Triskelions. Molecules (2022).