arXiv subjects

Alessandra Ronchi

Publications and source records attributed to Alessandra Ronchi.

3 recordsLinked to original sources

arXiv2026

Excited states engineering maximizes singlet generation by triplet fusion in conjugated systems

Photon upconverters are anti Stokes emitters capable of generating photons with higher energy than those absorbed. This behavior can be achieved through different mechanisms, which are extensively studied for applications in imaging, anticounterfeiting, information encryption and most importantly, solar energy technologies. Among these mechanisms, photon upconversion based on sensitized triplet-triplet annihilation (sTTA-UC) is particularly attractive because it operates under low intensity, incoherent light. In sTTA UC, two optically dark triplet states of a conjugated annihilator fuse upon collision to populate a higher energy fluorescent singlet, with the triplets initially generated via energy transfer from a lower energy sensitizer. Here we introduce a general molecular design strategy to maximize singlet generation through TTA. By selective substitution, we engineered a naphthalene derived annihilator with an excited state energy landscape that strongly favors singlet formation, achieving yields up to 0.83. When combined with an appropriate triplet sensitizer, the system delivers stable UV/visible upconversion peaking at 390 nm, with an absolute upconversion yield about 0.19 and an activation excitation intensity threshold lower than 0.1 suns under non-coherent broadband excitation fully compatible with the requirements of solar powered technologies.

physics.optics↗

arXiv2022

Diffusion-free photon upconversion driven by intramolecular triplet-triplet annihilation in engineered conjugated chromophores

The photon upconversion based on sensitized triplet-triplet annihilation (sTTA-UC) is a spin-flip mechanism exploited to recover the energy of dark triplet states in conjugated systems. In this process a high-energy fluorescent singlet is created through the collision and fusion of two low-energy triplets belonging to different diffusing molecules. Its excellent yield in solution under low excitation intensity and non-coherent light highlighted the huge potential of sTTA-UC to provide a breakthrough in solar technologies. However, its diffusion-limited bi-molecular nature limits its efficiency in the solid state. To overcome this issue, we designed a molecular systems able to host simultaneously more than one triplet, thus enabling a diffusion-free intramolecular TTA. We obtain the first direct demonstration of intramolecular triplet fusion by tailored photoluminescence spectroscopy experiments, thus opening the way to realize a new family of single molecule upconverters with huge potential in solar and lightening technologies by accessing the natural triplets energy reservoir.

cond-mat.mtrl-sci↗

arXiv2020

High Photon Upconversion Efficiency with Hybrid Triplet Sensitizers by Ultrafast Hole-Routing in Electronic-Doped Nanocrystals

Low power photon upconversion (UC) based on sensitized triplet-triplet annihilation (sTTA) is considered as the most promising upwards wavelength-shifting technique to enhance the light harvesting capability of solar devices by recovering the low-energy tail of the solar spectrum. Semiconductor nanocrystals (NCs) with conjugated organic ligands have been proposed as broadband sensitizers for extending the light-harvesting capability of sTTA-UC molecular absorbers. Key to their functioning is efficient energy transfer (ET) from the NC to the triplet state of the ligands that sensitizes the triplet state of free emitters, whose annihilation generates the upconverted emission. To date, the triplet sensitization efficiency in such systems is limited by parasitic processes, such as charge transfer (typically of the photohole) to the organic ligand due to the disadvantageous band alignment of typical NCs and organic moieties. Available strategies only partially mitigate such losses and intrinsically limit the ET yield. Here we demonstrate a new exciton-manipulation approach that enables loss-free ET without detrimental side-effects. Specifically, we use CdSe NCs doped with gold atoms featuring a hole-accepting state in the NC bandgap at a higher energy than the HOMO level of the ligand 9-anthracene acid. Upon photoexcitation, the NC photoholes are routed to the Au-state faster than their transfer to the ligand, producing a long-lived bound exciton in perfect resonance with its triplet state. This hinders hole-transfer losses and results in ~100% efficient ET, over 50-fold higher than in standard NCs. By combining our hybrid sensitizers with an annihilator moiety, we achieved an sTTA-UC efficiency of ~12% (~24% in the normalized definition), which is the highest value for hybrid upconverters based on sTTA reported to date and approaches optimized organic systems.

physics.app-ph↗