Defense of thesis Mariam Tlaiss

Defense of thesis Mariam Tlaiss - laboratory LASIRe

Abstract : 

This PhD work investigates how molecular environment and structural confinement influence the ultrafast photoinduced processes of diarylethene (DAE) systems. Diarylethenes are reversible photochromic molecules that switch between an open (OF) and a closed (CF) form through a light-driven carbon-carbon electrocyclization. These transformations, known as photocyclization and photoreversion, occur on femtosecond-picosecond timescales and lead to pronounced changes in optical and electronic properties. Owing to their high thermal stability, fatigue resistance, and bistability, DAEs are considered reliable molecular photoswitches for optical data storage, molecular electronics, and photonic devices.

The main goal of this thesis is to understand how structure and environment modulate DAE excited-state dynamics. To achieve this, a multidisciplinary approach combining steady-state spectroscopy, femtosecond transient absorption spectroscopy (fs-TAS), spectroelectrochemistry, and time-dependent density functional theory (TD-DFT) calculations was employed. Three structural levels of increasing complexity were studied: (i) free DAE molecules, (ii) supramolecular capsules incorporating polyoxometalate (POM) clusters, and (iii) porphyrin-based conjugated polymers.

At the molecular level, the comparison between two bis-pyridyl derivatives, DAE1 and DAE2, revealed that the addition of a phenyl spacer in DAE2 enhances the photocyclization efficiency, accelerates the dynamics, and introduces a new emissive pathway localized on its pyridyl-phenyl-thiophene branch.

In the supramolecular assemblies, DAEs integrated within POM cages exhibit new excited-state behaviors arising from confinement and electronic coupling. Femtosecond spectroscopy revealed ultrafast charge-transfer (CT) processes between DAE and POM units, showing that the inorganic cluster acts as an active redox partner rather than an inert scaffold.

At the macromolecular scale, DAE units were electropolymerized with isoporphyrin radicals to form the conjugated polymers P1 and P2. fs-TAS, combined with spectroelectrochemistry, revealed a strong interplay between photoinduced electron transfer and photochromism. In solution, both direct and PET-assisted photocyclization pathways coexist, while in the solid state, the stabilization of the oxidized isoporphyrin (IsoP⁺) species suppresses photoactivity.

Overall, this thesis establishes clear structure-dynamics relationships across molecular, supramolecular, and polymeric scales. Confinement simplifies dynamics by fixing reactive conformers, whereas coupling to redox-active frameworks introduces new channels. Beyond fundamental understanding, these findings provide design principles for next-generation photoresponsive materials. By tuning rigidity, DAE-acceptor distance, or incorporating redox-active units such as POMs and porphyrins, one can control the balance between photoisomerization, charge separation, and recombination. These systems open promising perspectives for molecular memories, light-driven devices, and photocatalytic energy conversion.

Keywords : Organic Photochemistry, Time-resolved spectroscopy, Photochromism, Diarylethenes