Photophysical Properties of Porphyrin-Based Systems

Porphyrins, a class of tetrapyrrolic macrocycles, exhibit distinctive photophysical behaviour that underpins their versatility in fields ranging from photodynamic therapy to solar energy conversion. Their characteristic absorption features include a strong Soret band in the near-ultraviolet region and multiple Q-bands in the visible spectrum. Upon photoexcitation to the first singlet excited state, porphyrins may return to the ground state by fluorescence, undergo intersystem crossing to populate the triplet state, or engage in energy and charge transfer with adjacent molecules. Central metal ions and peripheral substituents modulate both the energy levels and lifetimes of these states, while solvent polarity and aggregation (H- and J-type) further influence spectral shifts and quantum yields. Understanding these parameters enables judicious design of porphyrin derivatives for applications such as photocatalysis, fluorescence imaging, sensitisation of singlet oxygen, and construction of light‐harvesting assemblies. Recent advances also highlight the role of supramolecular organisation and nanoparticle interfaces in tuning emission efficiency and excited‐state dynamics.

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Technical terms

Soret band: A strong absorption band near 400 nm arising from π–π* transitions in porphyrins.

Q-bands: Weaker visible absorption bands between 500 nm and 650 nm associated with lower-energy electronic transitions.

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

Intersystem crossing: A non-radiative transition between singlet and triplet electronic states.

Singlet and triplet states: Excited electronic configurations where electron spins are paired (singlet) or parallel (triplet), affecting lifetime and reactivity.

Fluorescence resonance energy transfer (FRET): A distance-dependent energy transfer process between a donor fluorophore and an acceptor molecule.

H- and J-aggregates: Molecular assemblies that cause blue (H) or red (J) shifts in absorption spectra due to exciton coupling.

References

1. Supramolecular porphyrin as an improved photocatalyst for chloroform decomposition. RSC Advances (2023).
2. Effects of serum albumin on the photophysical characteristics of synthetic and endogenous protoporphyrin IX. Brazilian Journal of Medical and Biological Research (2022).
3. Enhanced energy transfer from diolefinic laser dyes to meso-tetrakis (4-sulfonatophenyl) porphyrin immobilized on silver nanoparticles: DFT, TD-DFT and spectroscopic studies. Journal of Saudi Chemical Society (2022).