Excited-State Dynamics in Photophysical Systems

Excited-state dynamics encompass the sequence of events that follow the absorption of light by a molecular or nanoscale system. Upon photon absorption, a ground-state molecule is promoted to an electronically excited state, where it may undergo a variety of pathways: internal conversion to lower electronic states, intersystem crossing to a triplet manifold, radiative decay via fluorescence or phosphorescence, and non-radiative energy dissipation through vibrational relaxation or solvent interactions. The efficiency and timescale of each channel are governed by factors such as electronic structure, spin–orbit coupling, molecular conformation and environment. Time-resolved spectroscopies, including transient absorption and time-correlated single-photon counting, allow direct observation of ultrafast processes ranging from femtoseconds to microseconds. Understanding these pathways underpins applications in solar energy conversion, photocatalysis, optoelectronic devices and biological imaging. Control of excited-state lifetimes and branching ratios can optimise light harvesting, enhance singlet-oxygen generation for photodynamic therapy or improve the resolution of time-resolved fluorescence microscopy. Recent advances in theory, materials engineering and advanced detection schemes have deepened insight into how molecular design and microenvironment tune excited-state landscapes and intersystem crossing rates, thereby enabling more efficient photophysical systems.

Research from Nature Portfolio

Recent studies have characterised tetramethylalloxazines as versatile photosensitisers with high singlet-oxygen quantum yields and tunable excited-state lifetimes. Computational investigations using density functional theory and time-dependent DFT revealed closely spaced π,π* and n,π* transitions, which correlate with observed absorption and transient-absorption spectra in various solvents. Experimental measurements of fluorescence quantum yields, lifetimes and non-radiative rate constants in methanol, acetonitrile and dichloroethane demonstrated solvent-dependent modulation of intersystem crossing efficiencies. Transient absorption spectroscopy confirmed triplet-state formation and decay on the microsecond timescale. In vitro assays employing human red blood cells coupled with fluorescence lifetime imaging microscopy pinpointed intracellular changes under physiological and oxidative stress, illustrating the potential of these compounds as both redox-sensitive probes and singlet-oxygen generators.

Excited-State Dynamics in Photophysical Systems publication trend

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

Technical terms

Excited state: An electronic configuration of a molecule with higher energy than its ground state after photon absorption.

Intersystem crossing: A radiationless transition between electronic states of different spin multiplicity, typically singlet to triplet.

Quantum yield: The ratio of the number of events (for example, photons emitted or singlet oxygen molecules formed) to the number of photons absorbed.

Transient absorption spectroscopy: A pump-probe technique that tracks changes in absorbance over femtosecond to millisecond timescales to follow excited-state evolution.

Fluorescence lifetime imaging microscopy (FLIM): A imaging method that maps the spatial distribution of fluorescence decay rates in microscopic samples.

Singlet oxygen: A highly reactive form of molecular oxygen with paired electrons in an excited electronic state, often generated via energy transfer from triplet photosensitisers.

References

1. Photophysics of lumichrome in anionic and cationic micellar media. RSC Advances (2015).
2. 5-Deazaalloxazine as photosensitizer of singlet oxygen and potential redox-sensitive agent. Photochemical & Photobiological Sciences (2023).
3. Tetramethylalloxazines as efficient singlet oxygen photosensitizers and potential redox-sensitive agents. Scientific Reports (2023).