Two-Photon Absorption Phenomena in Molecular Systems
Summary
Two-photon absorption (TPA) is a non-linear optical phenomenon in which a molecule simultaneously absorbs two photons, promoting it from the ground to an excited electronic state. Dependent on the square of the incident light intensity, this process enables confined excitation volumes with intrinsic three-dimensional resolution, underpinning applications such as deep-tissue bioimaging, photodynamic therapy, optical data storage and non-linear lithography. At the molecular level, TPA cross-sections are governed by electronic structure factors including donor–acceptor substitution, π-conjugation length, planarity and the involvement of higher-lying excited states. Synthetic advances have produced photochromic switches, push–pull chromophores and fluorescent proteins tailored for enhanced biphotonic response, while ultrafast laser technologies and advanced microscopy techniques have refined the measurement of TPA spectra. Complementary theoretical approaches—ranging from time-dependent density functional theory to coupled-cluster response methods—offer insight into transition dipoles, excited-state dipole moments and the impact of local environments. Studies of solvent and protein encapsulation reveal significant tunability of non-linear properties via electrostatic interactions and hydrogen bonding. Together, these experimental and computational developments are guiding the rational design of molecular systems with bespoke two-photon activity for materials science, chemical sensing and biomedical imaging.
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Two-Photon Absorption Phenomena in Molecular Systems publication trend
The graph below shows the total number of articles in two-photon absorption phenomena in molecular systems across all publications each year (not limited to Nature Index journals).
Technical terms
Two-photon absorption (TPA): Simultaneous absorption of two photons to reach an excited electronic state.
Cross-section: Measure of the probability that an absorption event will occur under given conditions.
Non-linear optical process: Optical response that scales non-linearly with the intensity of incident light.
Chromophore: Molecular moiety responsible for light absorption and subsequent photochemical events.
Density functional theory (DFT): Computational quantum mechanical method for predicting electronic structure.
Meta-generalized gradient approximation: Type of exchange–correlation functional incorporating kinetic energy density for improved accuracy.
Femtosecond pulses: Ultrashort laser bursts of approximately 10⁻¹⁵ seconds duration used for time-resolved spectroscopy.
References
- Two-photon isomerization properties of donor–acceptor Stenhouse adducts. Chemical Science (2023).
- Impact of the Protein Environment on Two-Photon Absorption Cross-Sections of the GFP Chromophore Anion Resolved at the XMCQDPT2 Level of Theory. International Journal of Molecular Sciences (2023).
- Choosing Bad versus Worse: Predictions of Two-Photon-Absorption Strengths Based on Popular Density Functional Approximations. Journal of Chemical Theory and Computation (2022).
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