Periodic Reporting for period 1 - FLUID (Functional Low-Intensity Light Upconverting Inks for Everyday Applications)
Período documentado: 2018-02-15 hasta 2020-02-14
Almost everybody in the world has come across luminescence like fluorescence or phosphorescence, e.g. the hands of the good old analog watches. This is a phenomenon of a typical spectral downconversion of visible or UV-light whereby high-energy light is transformed into low-energy light by dissipation of energy. The opposite of this phenomenon is called upconversion whereby low-energy light is transformed into high-energy light by the combination of at least two photons into one photon of higher energy. This effect has barely been investigated as it is by far less likely to happen and its efficiency is usually low. However, solid-state materials that efficiently upconvert light at low intensities (< 100mW·cm-2) are slowly emerging as interesting applications like novel security inks offer a unique selling proposition. These upconversion phenomena could be achieved by several techniques like energy transfer upconversion (ETU), excited state absorption (ESA), both of which involving intermediary energy levels, but also simultaneous two-photon absorption (STRA) and second harmony light generation (SHG),which do not involve those intermediary energy levels. However, all of these mentioned techniques usually require high incident light densities which interfere with a common broad practical use. Another upconversion technique apart from the formerly mentioned is represented by triplet-triplet annihilation (TTA-UC), which can utilize low ambient solar irradiance, one requirement for a broader use in daily life applications, and was therefore, the chosen approach for this project. Efficient upconversion processes and a broad applicability of formulations and materials bear potential for energy conversion like photovoltaics and chemical reactions reducing the consumption and dependence of humanity on fossil-based usually CO2 emitting and non-renewable energy resources. However, access to such “easy to apply” and “easy to handle” upconverting formulations and materials is limited.
The aim of this project was to investigate, develop and prepare functional ink formulations capable of upconverting light by triplet-triplet annihilation (TTA-UC) therefore enabling unique new optical effects such as fluorescent patterns under near-infrared irradiation and a better applicability of upconversion in daily life. In order to obtain multicolor, designable and high efficiency upconverting functional inks, the goals of this action included investigations to find suitable printing techniques such as inkjet printing, developing coatable, printable and patternable solid-state light upconverting materials for ink formulations, such as modified rubbers or molecular glasses, but also analyzing the optical properties from the resulting specimens and develop new custom-made solid-state upconverter concepts for printing.
After the final period of the project the following conclusions can be drawn:
i) A stable and printable formulation capable of upconverting light by triplet-triplet annihilation (TTA-UC) can be obtained.
ii) The developed formulations can be used for screen-printing or bar coating and finally cured to obtain solid materials that show upconversion.
iii) Characterization and long-term stability tests of these specimen have shown that the upconversion is detectable for weeks but overall, the stability as well as the upconversion efficiencies are not sufficient to transfer it into an applicable product.
The aim of this project was to investigate, develop and prepare functional ink formulations capable of upconverting light by triplet-triplet annihilation (TTA-UC) therefore enabling unique new optical effects such as fluorescent patterns under near-infrared irradiation and a better applicability of upconversion in daily life. In order to obtain multicolor, designable and high efficiency upconverting functional inks, the goals of this action included investigations to find suitable printing techniques such as inkjet printing, developing coatable, printable and patternable solid-state light upconverting materials for ink formulations, such as modified rubbers or molecular glasses, but also analyzing the optical properties from the resulting specimens and develop new custom-made solid-state upconverter concepts for printing.
After the final period of the project the following conclusions can be drawn:
i) A stable and printable formulation capable of upconverting light by triplet-triplet annihilation (TTA-UC) can be obtained.
ii) The developed formulations can be used for screen-printing or bar coating and finally cured to obtain solid materials that show upconversion.
iii) Characterization and long-term stability tests of these specimen have shown that the upconversion is detectable for weeks but overall, the stability as well as the upconversion efficiencies are not sufficient to transfer it into an applicable product.
In light of achieving the aim of this project the focus lay one the following items: i) Systems containing (metal-)organic dyes facilitate good upconversion efficiencies at relatively low doses of irradiation. ii) For high efficiencies one must aim to maximize the amount of incorporated dye without causing detrimental quenching effects. Besides, transparency is a desirable property. To achieve both, multiphasic systems with a complex material distribution into nanoscopic domains were targeted.
For this purpose, 3-phase polymeric nanoglasses were investigated consisting of a photo- or thermocurable hydrophilic polymer phase, amphiphilic surfactants and hydrophobic oils carrying upconversion dyes. After initial experiments probing the impact of the oil phase on the rheological profiles and surface tension properties, it was found that inks suitable for bar coating, screen or inkjet printing can be obtained for oil concentrations in the range of 10%. To avoid fast photo or oxygen bleaching of the coated and cross-linked nanoglass films, different combinations of sensitizer and emissive dye were investigated. Once a sufficiently stable dye system and base formulation was established the work focused on evaluating the impact of the employed plasticizer and amphiphilic molecules. It was found that for good upconversion performance and mechanical film stability a certain amount of plasticizer was necessary. In general, copolymerizable plasticizers performed best as did combinations of interacting amphiphiles that were able to prevent aging due to phase separation. Best performance was reached with water-containing formulations that allowed to host antioxidative agents as well. This led to upconversion coating that optically stable for weeks or even months. Building on these investigations, protecting the dyes from oxidative stress via incorporation into microcapsules was explored in BASF internal research activities.
The results and their exploitation and dissemination can be summarized as follows:
i) The overall findings, especially regarding formulation development on efficient dye-pairs capable of low-energy TTA upconversion as well as antioxidants to sustain the upconversion for weeks have been one part of the discussions with Prof. Dr. Christoph Weder, Adolphe Merkle Institute, Fribourg and his team. Finally, these actions and investigations have been succeeded in two publications.
ii) Furthermore, the developed formulations have been pursued within and were part of a new BASF internal research project, which is still active.
iii) On several internal as well as external conferences and symposia the work has been discussed with several parties, leading to not only the BASF internal research project, but also contributing to two publications by F. Saenz et. al.
For this purpose, 3-phase polymeric nanoglasses were investigated consisting of a photo- or thermocurable hydrophilic polymer phase, amphiphilic surfactants and hydrophobic oils carrying upconversion dyes. After initial experiments probing the impact of the oil phase on the rheological profiles and surface tension properties, it was found that inks suitable for bar coating, screen or inkjet printing can be obtained for oil concentrations in the range of 10%. To avoid fast photo or oxygen bleaching of the coated and cross-linked nanoglass films, different combinations of sensitizer and emissive dye were investigated. Once a sufficiently stable dye system and base formulation was established the work focused on evaluating the impact of the employed plasticizer and amphiphilic molecules. It was found that for good upconversion performance and mechanical film stability a certain amount of plasticizer was necessary. In general, copolymerizable plasticizers performed best as did combinations of interacting amphiphiles that were able to prevent aging due to phase separation. Best performance was reached with water-containing formulations that allowed to host antioxidative agents as well. This led to upconversion coating that optically stable for weeks or even months. Building on these investigations, protecting the dyes from oxidative stress via incorporation into microcapsules was explored in BASF internal research activities.
The results and their exploitation and dissemination can be summarized as follows:
i) The overall findings, especially regarding formulation development on efficient dye-pairs capable of low-energy TTA upconversion as well as antioxidants to sustain the upconversion for weeks have been one part of the discussions with Prof. Dr. Christoph Weder, Adolphe Merkle Institute, Fribourg and his team. Finally, these actions and investigations have been succeeded in two publications.
ii) Furthermore, the developed formulations have been pursued within and were part of a new BASF internal research project, which is still active.
iii) On several internal as well as external conferences and symposia the work has been discussed with several parties, leading to not only the BASF internal research project, but also contributing to two publications by F. Saenz et. al.
Within this project it was demonstrated that complex formulations meeting the requirements for coating and printing techniques are accessible and capable of delivering materials that show stable upconversion over the course of months. These results pave the way to new materials prepared by screen or inkjet printing that - to best of our knowledge - have not been accessible by such commercialized techniques before. The introduction of new antioxidants has been one enabler to stabilize the upconversion over a longer period of time – an important requirement for broad application of this technique. The promising results obtained during this project resulted in a transfer of some findings to a BASF internal project aimed at further developing the upconversion technique into an applicable product. Furthermore, regular scientific exchange with researchers from the Adolphe Merkle Institute, University of Fribourg on the conducted works and obtained results fostered academic research that finally lead to two publications in peer-review journals.