Excited-State Dynamics in Porphyrin Systems

Porphyrins are extended π-conjugated macrocycles renowned for their intense absorption bands and pivotal roles in light harvesting, sensing and catalysis. Upon photoexcitation into the Soret or Q bands, energy is rapidly redistributed through vibrational relaxation and internal conversion, populating lower‐lying singlet states within femtoseconds. A subsequent intersystem crossing transfers population to triplet manifolds on picosecond timescales, yielding long‐lived triplet states essential for photodynamic therapy, photocatalysis and molecular spintronics. The nature of the central metal, peripheral substituents and axial ligands can steer excited‐state pathways towards charge-transfer, exciplex or excimer formation, while oligomerisation and aggregation modulate exciton delocalisation. Advances in ultrafast spectroscopy, time-resolved electron paramagnetic resonance and quantum-chemical modelling have elucidated the interplay between molecular architecture, solvent environment and spin–orbit or spin–vibronic coupling. This detailed mechanistic insight enables rational design of porphyrin assemblies with tailored lifetimes, emission wavelengths and spin polarisation for next-generation photonic and energy-conversion technologies.

Research from Nature Portfolio

Femtosecond broadband transient absorption studies of copper(II) tetraphenylporphyrin reveal two competing relaxation channels: a sub-200 fs intersystem crossing to the triplet hypersurface and a ∼700 fs coordinative expansion to a five-coordinate exciplex in the presence of Lewis bases. Ligand-to-metal charge-transfer states determine branching ratios, with triplet lifetimes of ∼30 ps and exciplex lifetimes extending into the hundreds of picoseconds in O-coordinating solvents, underscoring the sensitivity of ultrafast dynamics to axial ligation and solvent coordination.

Excited-State Dynamics in Porphyrin Systems publication trend

The graph below shows the total number of articles in excited-state dynamics in porphyrin systems across all publications each year (not limited to Nature Index journals).

Technical terms

Excited state: A higher-energy electronic configuration populated upon photon absorption.

Intersystem crossing (ISC): A non-radiative transition between electronic states of different spin multiplicity, commonly singlet to triplet.

Triplet state: An electronic state with two unpaired electrons (total spin S = 1), often exhibiting extended lifetimes.

Exciplex/excimer: A transient complex formed between excited and ground-state species (exciplex) or two excited units (excimer), with distinct emission properties.

Soret and Q bands: Strong porphyrin absorption features in the near-UV (Soret) and visible (Q) regions arising from π–π* transitions.

Zero-field splitting (ZFS): The energy separation of triplet sublevels in the absence of an external magnetic field, measured by EPR.

Spin–vibronic coupling: Interaction between electronic spin states and nuclear vibrational modes that can facilitate intersystem crossing.

References

1. Excited-State Dynamics Leading Either to Triplet Formation or Coordinative Expansion following Photolysis of Cu(II)-Porphyrins: A DFT, TD-DFT, Luminescence and Femtosecond Time-Resolved Absorbance Study. Molecules (2023).
2. Tailoring the Intersystem Crossing and Triplet Dynamics of Free-Base Octaalkyl-β-oxo-Substituted Porphyrins: Competing Effects of Spin–Vibronic and NH Tautomerism Relaxation Channels. The Journal of Physical Chemistry A (2022).
3. The impact of spin–orbit coupling on fine-structure and spin polarisation in photoexcited porphyrin triplet states. Journal of Magnetic Resonance (2023).
4. Insights into the phototautomerism of free-base 5, 10, 15, 20-tetrakis(4-sulfonatophenyl) porphyrin. Photochemical & Photobiological Sciences (2023).
5. Transient EPR Reveals Triplet State Delocalization in a Series of Cyclic and Linear π‑Conjugated Porphyrin Oligomers. Journal of the American Chemical Society (2015).