Negative photochromism triggers unusual nonlinear optical responses

Research Project, 2026 – 2030

Multiphoton processes are crucial in various scientific and technological fields due to their unique ability to interact with matter in ways that one-photon processes cannot. For example, in fluorescence applications relying on one-photon excitation, the intensity of the emitted fluorescence is directly proportional to the intensity of the excitation light. Multiphoton absorption implies that there is instead a nonlinear relation between the excitation intensity and the intensity of the emitted photons. If focused excitation sources are used (e.g., in fluorescence microscopy), this implies that the spatial resolution is improved. However, multiphoton excitation requires extreme light intensities implying the use of expensive and hard-to-access laser systems, which is a barrier for widespread adoption. This proposal addresses this issue and shows that the incorporation of T-type negative photochromic molecules into fluorescent probes triggers strongly nonlinear responses in fluorescence intensity, albeit one-photon excitation was used, dramatically improving the spatial resolution (by 2-4 times). These photochromic probes, that unify several photophysical assets, will be developed (year 1-2) and their performance will be critically validated in a multi-angle work-flow, including spectroscopic characterization (year 1-4), and proof-of-principle microscopy experiments in model systems (year 3-4).