Excited-State Dynamics in Nitroaromatic Compounds

Nitroaromatic molecules, characterised by one or more nitro (–NO₂) groups attached to an aromatic ring, display rich excited-state behaviour of broad scientific and technological importance. Photoexcitation promotes these systems into a manifold of singlet states, followed by competing pathways of internal conversion and intersystem crossing. Rapid energy relaxation through conical intersections or spin–orbit coupling can lead to efficient formation of triplet states, proton transfer events and photochemical rearrangements. Solvent interactions and molecular structure modulate lifetimes spanning femtoseconds to nanoseconds and determine yields of photoproducts. Understanding these processes informs diverse applications, from environmental photochemistry of atmospheric aerosols and pollutant degradation to design of organic optoelectronic materials and trace detection of energetic compounds.

Research from Nature Portfolio

Excited-state intramolecular proton transfer in o-nitrophenol has been explored through on-the-fly surface-hopping simulations, revealing three distinct intersystem crossings and a conical intersection within low-lying singlet and triplet manifolds. The dominant non-hydrogen transfer channel (48 %) occurs within 300 fs, while tunnelling-assisted proton transfer (36 %) proceeds on a 10 ps timescale and direct hydrogen transfer (13 %) completes in 40 fs.

Advanced chemometric analyses of femtosecond transient absorption in ortho-nitroaniline, combined with quantum-mechanical modelling, have deconvoluted sequential excited-state decays. Photoexcitation to S₃ relaxes to S₂ within ≈0.9 ps, transitions to S₁ occur, and a long-lived intermediate S* state emerges in ≈6.4 ps, preceding intersystem crossing to T₁ in ≈19.6 ps. The triplet state exhibits lifetimes up to 2 ns, acting as a precursor to diverse photoproducts.

Excited-State Dynamics in Nitroaromatic Compounds publication trend

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

Technical terms

Intersystem crossing (ISC): nonradiative transition between electronic states of different spin multiplicity, often singlet to triplet, mediated by spin–orbit coupling.

Internal conversion (IC): nonradiative relaxation between states of the same spin multiplicity via conical intersections.

Conical intersection: intersection of two potential energy surfaces where electronic states of identical multiplicity become degenerate, enabling rapid nonadiabatic transitions.

Transient absorption spectroscopy: pump–probe technique measuring time-resolved changes in molecular absorption after photoexcitation to track excited-state dynamics.

Nonadiabatic molecular dynamics: simulation methodology combining nuclear motion and electronic transitions to model ultrafast photochemical processes.

References

1. Intersystem crossing-branched excited-state intramolecular proton transfer for o-nitrophenol: An ab initio on-the-fly nonadiabatic molecular dynamic simulation. Scientific Reports (2016).
2. A Combination of Chemometrics and Quantum Mechanics Methods Applied to Analysis of Femtosecond Transient Absorption Spectrum of Ortho-Nitroaniline. Scientific Reports (2016).
3. Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution. Journal of the American Chemical Society (2024).
4. The Role of Aqueous Solvation on the Intersystem Crossing of Nitrophenols. Journal of Chemical Theory and Computation (2024).
5. Nonadiabatic Dynamics Simulation Predict Intersystem Crossing in Nitroaromatic Molecules on a Picosecond Time Scale. ChemPhotoChem (2019).