PhotoBase

PhotoBase

PhotoBase is a user friendly software program for photographers who want to manage their images for personal or professional use. Photobase helps you to keep track of your photos or movies. It includes a comprehensive system for tracking and printing competition or client submissions, photo sales invoices, and contact information. You don’t have time for data entry? PhotoBase makes data entry a snap. The program automatically tracks and updates all information pertaining to each photo or movie clip, and finding just the right photo or movie is a breeze with PhotoBase’s easy to use internal search feature. New features include batch imports of photos or movies, and an enlarged photo view.

A Photo base generator is a compound which generates organic base such as amines upon irradiation of light in the UV range. The generated organic base accelerates an anionic UV curing of epoxy resin, sol-gel method, etc.

Photopolymerization is a powerful tool in materials science with many applications, including coatings, adhesives, inks, and 3D printing. Until now, the majority of photoinitiating systems have been suitable only for radical photopolymerization, which automatically excludes the use of light to trigger a great number of polymerization reactions. For instance, the preparation of polyurethanes via photopolymerization from isocyanates remains a real challenge since it requires a catalyst able to mediate nucleophilic substitution reactions. In this context, this study reports the successful synthesis of three new photobase generators based on a thioxanthone chromophore functionalized with a protonated 1,8-diazabicyclo[5.4.0]undec-7-ene as a latent base for the direct synthesis of polyurethanes from commercially available polyols and polyisocyanates. The catalytic activity of the photobase is modulated by introducing different functional groups at the α-position of the carboxylate which act as a photocleavable link between the chromophore and the latent base. A direct correlation between the steric hindrance of such groups and more efficient release of the base is observed by ¹H NMR. DFT studies have been performed to shed some light on the base release mechanism and to further confirm this evidence. To demonstrate their use, the ability of these photobases to mediate the nucleophilic substitution between isocyanates and alcohols has been proven by using bifunctional and trifunctional monomer mixtures by ¹H NMR, FTIR, and rheology experiments. To further exploit the full potential of the thioxanthone-based photobase generators, polyurethane coatings as well as 3D printed figures have been prepared at room temperature by using light as an external trigger.

Arrhenius photobases are of potential use for excited state hydroxide ion dissociation (ESHID), photo-induced pOH jump experiments, and base-catalyzed reactions. However, previously studied Arrhenius photobases have to be excited by UV light and undergo ESHID reactions only in protic solvents. These characteristics have become a disadvantage to their application in many fields of research. In this work, we have designed and synthesized a new Arrhenius photobase (NO₂-Acr-OH), the ESHID reaction of which is readily triggered by visible light excitation. In contrast to previously studied Arrhenius photobases, NO₂-Acr-OH undergoes ESHID reactions in protic solvents as well as in polar aprotic solvents. Solvent-dependent photo-induced reactions of NO₂-Acr-OH are comprehensively studied by time-resolved fluorescence spectroscopy. Molecular designs for visible light triggered acridinol-based Arrhenius photobases with a large ΔpK_b value are proposed.

In photocatalytic hydrogen generation the energy of an incident photon is utilized to drive the energetically up-hill water splitting reaction^1,2. The evolved hydrogen can then serve directly as an energy-rich fuel or as a reactant in chemical synthesis of carbon-based fuels, e.g. methane in the Sabatier or Fischer-Tropsch processes^3,4. This approach is an appealing alternative to harvest and store abundant solar energy, potentially providing a renewable solution for the global energy supply⁵. In general, the photocatalytic reaction involves a complex sequence of charge carrier (electron and hole) generation, separation and transfer steps, and typically requires a concurrent proton transfer. The efficiency of such proton-coupled electron transfer (PCET) processes is limited by its slowest component, which could be reactant diffusion⁶. Consequently, a major challenge in photocatalytic H₂ production is to ensure that all participants in the process (photoexcited charge carriers, protons, and possibly other molecules) are delivered to the reaction site at optimal time. Otherwise, the photoexcited carriers may recombine, photodegrade the catalysts or induce unwanted side reactions, in all cases reducing the activity.

Photobases are molecules that have higher proton affinity in the excited state than in the ground state. In other words, the pK_a in the excited state is much higher than in the ground state, so that upon photoexcitation the now stronger base abstracts a proton from the environment. Hence, such molecules offer an opportunity to control the proton transfer by light^7,8. For instance, quinoline photobases have been shown to form protonated species within tens of picoseconds of the light excitation⁹. Effectively, the photobase can ensure that the proton promptly and selectively arrives at the reduction site. This result can also be achieved by other means, for example by lowering the pH. Nonetheless, utilizing the excited state acid-base equilibria the same effect can conceivably be realized in a neutral medium, without the potentially corrosive acidic conditions. Therefore, for photocatalytic H₂ generation the photobases present an appealing approach, wherein the incident photons not only provide the energy for the water splitting, but also dynamically alter the photocatalyst to enhance the process efficiency¹⁰.

Nanostructured photocatalysts, either inorganic, organic, or hybrid, rose to the forefront of the photocatalytic field because their composition, structure, and surface can be controlled to tailor the optoelectronic and morphological properties to the desired function^11,12,13. Carbon dots (CDs) are a recent entrant into photocatalysis that has attracted attention due to their versatility, photostability, absence of heavy metals, ease of preparation, and tunable properties^14,15. This flexibility derives from their complex internal structure, inherent disorder, and surface functionality. Effectively, small changes in the preparative procedure can result in large differences in the properties. While the application possibilities are impressive, there is still no detailed understanding of their structure and the mechanism of photocatalytic reactions^16,17,18. They are generally considered to consist of aromatic, sp²-hybridized domains immersed in a sp³-hybridized amorphous matrix with various functional groups on the surface^19,20. In this context, we have shown that the optical properties of CDs prepared from citric acid and ethylenediamine can be reproduced by a simple model of polycyclic hydrocarbons embedded in an amorphous polymer, poly(methyl methacrylate)²⁰. Heterocyclic compounds, including strong molecular fluorophores, form in the presence of nitrogen-containing precursors^21,22. Importantly, the position of the nitrogen atom dopant in the heterocyclic aromatic structure, controlled by the synthetic procedure, determines the functionality of the CDs. Whilst the graphitic nitrogen increases the photoluminescence (PL) quantum yield, pyridinic and pyrrolic nitrogen yields much higher photocatalytic H₂ generation activity²³. Pyridine and its larger analogs, such as quinoline or acridine, are weak bases that can be protonated at the nitrogen atom in the excited state, suggesting that such photocatalytically active CDs contain photobasic moieties in their structure. Interestingly, heptazine (tri-s-triazine), the building block of graphitic carbon nitrides (g-CN), also contains multiple N atoms at the edge sites of the aromatic structure and exhibits photobasic behavior^24,25. Recent calculations show that in the excited state heptazine molecules induce proton and electron transfer from hydrogen-bonded water molecules leading to heptazinyl radicals^26,27. These species can undergo further photolysis, recovering the heptazine and releasing hydrogen radicals that further react to form molecular H₂. In corroborating evidence, Electron Paramagnetic Resonance (EPR) experiments show that a long-lived radical forms in cyanamide-functionalized heptazine-based g-CN upon illumination that can be later, under dark conditions, used to produce H₂^28,29,30. It has been observed that protonation of the g-CN structure in acidic media increases the H₂ evolution rate^31,32,33. This further supports the argument that the proton transfer rate may be a limiting factor in the photocatalytic H₂ generation on g-CN. Overall, these experimental and computational results offer tantalizing hints that the photobasic effect of the constituent units in CDs and in g-CN is highly beneficial for the photocatalytic activity of these nanomaterials and at least partially responsible for their impressive H₂ formation rates.

In this paper, we show that the introduction of photobasic units into carbon dots indeed increases the H₂ generation rate, on the basis of time-resolved spectroscopy and photocatalytic activity measurements. Due to complexity of nitrogen-containing CDs, it is difficult to find an appropriate blank reference sample which exhibits no nitrogen-related acid-base activity. Therefore, we started with polyethylene glycol (PEG) derived nitrogen-free CDs and introduced acridine, a model photobase, into the photocatalyst. Acridine is an N-heterocyclic aromatic compound, resembling molecules likely present in the CDs²⁰, with pK_a = 5.5 in the ground state and pK_a* = 10.7 in the excited state^34,35,36. This means that at pH 7 it is protonated to a very small extent (3%), but becomes mostly protonated in the excited state (see details in the Supplementary Note 1). Calculations suggest that excitation of acridine in water induces proton-coupled electron transfer leading to acridinyl radical that can split under illumination to release a hydrogen radical³⁷. A common acridine-based derivative photobase, acridine orange, can abstract a proton from alcohols coupled with an electron transfer that can also result in acridinyl-based radicals^38,39. This further implied that acridine in its excited protonated state is prone to further reduction reactions (e.g. accepting an electron), making it suitable for studying H₂ generation from water¹⁰. These studies were focused on molecular interactions of acridine with a potential proton and electron donor (alcohol and water). In our work where acridine is integrated with CDs the process can be even more efficient. This is because the electron can also be readily transferred from the photoexcited CDs, which have a broader absorption range.

In short, we synthesize CDs which comprise a model photobase and we show that the photobasic effect can be a plausible element of the mechanism of photocatalytic hydrogen generation with CDs. The understanding of the mechanism demonstrates that careful design of the CDs enables control by light of the proton transfer rates at the photocatalyst surface to enhance the efficiency of H₂ production. In this way, we believe it opens a new avenue for modifying the functionality of CDs to bring further improvements in efficiency.