Detour to success: photoswitching via indirect excitation.

Photoswitchable molecules that undergo nanoscopic changes upon photoisomerisation can be harnessed to control macroscopic properties such as colour, solubility, shape, and motion of the systems they are incorporated into. These molecules find applications in various fields of chemistry, physics, biology, and materials science. Until recently, research efforts have focused on the design of efficient photoswitches responsive to low-energy (red or near-infrared) irradiation, which however may compromise other molecular properties such as thermal stability and robustness. Indirect isomerisation methods enable photoisomerisation with low-energy photons without altering the photoswitch core, and also open up new avenues in controlling the thermal switching mechanism. In this perspective, we present the state of the art of five indirect excitation methods: two-photon excitation, triplet sensitisation, photon upconversion, photoinduced electron transfer, and indirect thermal methods. Each impacts our understanding of the fundamental physicochemical properties of photochemical switches, and offers unique application prospects in biomedical technologies and beyond.

Far-red light-triggered cargo release from liposomes b ound to a photosensitizer-cellulose nanofiber hydrogel.

In our research we used the anionic nanofibrillar cellulose (ANFC) as a platform for far-red light-induced release of cargo from liposomes. In contrast to previous works, where photosensitizers are usually in the liposomal bilayers, we used a cellulose-binding dye. Our phthalocyanine derivative has been shown to bind very strongly to cellulose and cellulose nanofiber hydrogels, allowing us to place it outside of the liposomes. Both the sensitizer and cationic liposomes bind strongly to the ANFC after mixing, making the system easy to fabricate. Upon light activation, the photosensitizer generates reactive oxygen species (ROS) within the ANFC hydrogel, where the reactive oxygen species oxidize unsaturated lipids in the liposomal membrane, which makes the liposomes more permeable, resulting in on-demand cargo release. We were able to achieve ca. 70 % release of model hydrophilic cargo molecule calcein from the hydrogels with a relatively low dose of light (262 J/cm2) while employing the straightforward fabrication techniques. Our system was remarkably responsive to the far-red light (730 nm), enabling deep tissue penetration. Therefore, this very promising novel cellulose-immobilized photosensitizer liposomal platform could be used as a controlled drug delivery system, which can have applications in externally activated coatings or implants.

Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths.

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.

Disequilibrating azobenzenes by visible-light sensitization under confinement.

Photoisomerization of azobenzenes from their stable E isomer to the metastable Z state is the basis of numerous applications of these molecules. However, this reaction typically requires ultraviolet light, which limits applicability. In this study, we introduce disequilibration by sensitization under confinement (DESC), a supramolecular approach to induce the E-to-Z isomerization by using light of a desired color, including red. DESC relies on a combination of a macrocyclic host and a photosensitizer, which act together to selectively bind and sensitize E-azobenzenes for isomerization. The Z isomer lacks strong affinity for and is expelled from the host, which can then convert additional E-azobenzenes to the Z state. In this way, the host-photosensitizer complex converts photon energy into chemical energy in the form of out-of-equilibrium photostationary states, including ones that cannot be accessed through direct photoexcitation.

Unraveling an Alternative Mechanism in Polymer Self-Assemblies: An Order-Order Transition with Unusual Molecular Interactions between Hydrophilic and Hydrophobic Polymer Blocks.

Polymer self-assembly leading to cooling-induced hydrogel formation is relatively rare for synthetic polymers and typically relies on H-bonding between repeat units. Here, we describe a non-H-bonding mechanism for a cooling-induced reversible order-order (sphere-to-worm) transition and related thermogelation of solutions of polymer self-assemblies. A multitude of complementary analytical tools allowed us to reveal that a significant fraction of the hydrophobic and hydrophilic repeat units of the underlying block copolymer is in close proximity in the gel state. This unusual interaction between hydrophilic and hydrophobic blocks reduces the mobility of the hydrophilic block significantly by condensing the hydrophilic block onto the hydrophobic micelle core, thereby affecting the micelle packing parameter. This triggers the order-order transition from well-defined spherical micelles to long worm-like micelles, which ultimately results in the inverse thermogelation. Molecular dynamics modeling indicates that this unexpected condensation of the hydrophilic corona onto the hydrophobic core is due to particular interactions between amide groups in the hydrophilic repeat units and phenyl rings in the hydrophobic ones. Consequently, changes in the structure of the hydrophilic blocks affecting the strength of the interaction could be used to control macromolecular self-assembly, thus allowing for the tuning of gel characteristics such as strength, persistence, and gelation kinetics. We believe that this mechanism might be a relevant interaction pattern for other polymeric materials as well as their interaction in and with biological environments. For example, controlling the gel characteristics could be considered important for applications in drug delivery or biofabrication.

Phototoxicity of BODIPY in long-term imaging can be reduced by intramolecular motion.

For long-term live-cell fluorescence imaging and biosensing, it is crucial to work with a dye that has high fluorescence quantum yield and photostability without being detrimental to the cells. In this paper, we demonstrate that neutral boron-dipyrromethene (BODIPY)-based molecular rotors have great properties for high-light-dosage demanding live-cell fluorescence imaging applications that require repetitive illuminations. In molecular rotors, an intramolecular rotation (IMR) allows an alternative route for the decay of the singlet excited state (S1) via the formation of an intramolecular charge transfer state (CT). The occurrence of IMR reduces the probability of the formation of a triplet state (T1) which could further react with molecular oxygen (3O2) to form cytotoxic reactive oxygen species, e.g., singlet oxygen (1O2). We demonstrate that the oxygen-related nature of the phototoxicity for BODIPY derivatives can be significantly reduced if a neutral molecular rotor is used as a probe. The studied neutral molecular rotor probe shows remarkably lower phototoxicity when compared with both the non-rotating BODIPY derivative and the cationic BODIPY-based molecular rotor in different light dosages and dye concentrations. It is also evident that the charge and localization of the fluorescent probe are as significant as the IMR in terms of the phototoxicity in a long-term live-cell imaging.

Microfluidic oxygen tolerability screening of nanocarriers for triplet fusion photon upconversion.

The full potential of triplet fusion photon upconversion (TF-UC) of providing high-energy photons locally with low-energy excitation is limited in biomedicine and life sciences by its oxygen sensitivity. This hampers the applicability of TF-UC systems in sensors, imaging, optogenetics and drug release. Despite the advances in improving the oxygen tolerability of TF-UC systems, the evaluation of oxygen tolerability is based on comparing the performance at completely deoxygenated (0% oxygen) and ambient (20-21%) conditions, leaving the physiological oxygen levels (0.3-13.5%) neglected. This oversight is not deliberate and is only the result of the lack of simple and predictable methods to obtain and maintain these physiological oxygen levels in an optical setup. Herein, we demonstrate the use of microfluidic chips made of oxygen depleting materials to study the oxygen tolerability of four different micellar nanocarriers made of FDA-approved materials with various oxygen scavenging capabilities by screening their TF-UC performance over physiological oxygen levels. All nanocarriers were capable of efficient TF-UC even in ambient conditions. However, utilizing oxygen scavengers in the oil phase of the nanocarrier improves the oxygen tolerability considerably. For example, at the mean tumour oxygen level (1.4%), nanocarriers made of surfactants and oil phase both capable of oxygen scavenging retained remarkably 80% of their TF-UC emission. This microfluidic concept enables faster, simpler and more realistic evaluation of, not only TF-UC, but any micro or nanoscale oxygen-sensitive system and facilitates their development and implementation in biomedical and life science applications.

Azobenzene Photoswitching with Near-Infrared Light Mediated by Molecular Oxygen.

Efficient photoisomerization between the cis and the trans states of azobenzenes using low-energy light is desirable for a range of applications in, e.g., photobiology yet challenging to accomplish directly with modified azobenzenes. Herein, we utilize molecular iodine as a photocatalyst to induce indirect cis-to-trans isomerization of 4,4'-dimethoxyazobenzene with 770 nm near-infrared light, showing robustness during more than 1000 cycles in ambient conditions. Intriguingly, the catalysis is mediated by molecular oxygen, and we demonstrate that other singlet-oxygen-generating photosensitizers besides iodine, i.e., palladium phthalocyanine, catalyze the isomerization as well. Thus, we envision that the approach can be further improved by employing other catalysts with suitable photoelectrochemical properties. Further studies are needed to explore the applicability of the approach with other azobenzene derivatives.

Expanding excitation wavelengths for azobenzene photoswitching into the near-infrared range via endothermic triplet energy transfer.

Developing azobenzene photoswitches capable of selective and efficient photoisomerization by long-wavelength excitation is an enduring challenge. Herein, rapid isomerization from the Z- to E-state of two ortho-functionalized bistable azobenzenes with near-unity photoconversion efficiency was driven by triplet energy transfer upon red and near-infrared (up to 770 nm) excitation of porphyrin photosensitizers in catalytic micromolar concentrations. We show that the process of triplet-sensitized isomerization is efficient even when the sensitizer triplet energy is substantially lower (>200 meV) than that of the azobenzene used. This makes the approach applicable for a wide variety of sensitizer-azobenzene combinations and enables the expansion of excitation wavelengths into the near-infrared spectral range. Therefore, indirect excitation via endothermic triplet energy transfer provides efficient and precise means for photoswitching upon 770 nm near-infared light illumination with no chemical modification of the azobenzene chromophore, a desirable feature in photocontrollable biomaterials.

Taste compound - Nanocellulose interaction assessment by fluorescence indicator displacement assay.

Interactions between taste compounds and nanofibrillar cellulose were studied. For this, a new fluorescent indicator displacement method was developed. Two fluorescent indicators, namely, Calcofluor white and Congo red, were chosen because of their specific binding to cellulose and intrinsic fluorescence. Seven taste compounds with different structures were successfully measured together with nanofibrillar cellulose (NFC) and ranked according to their binding constants. The most pronounced interactions were found between quinine and NFC (1.4 × 104 M-1), whereas sucrose, aspartame and glutamic acid did not bind at all. Naringin showed moderate binding while stevioside and caffeine exhibited low binding. The comparison with microcrystalline cellulose indicates that the larger surface area of nanofibrillated cellulose enables stronger binding between the binder and macromolecules. The developed method can be further utilized to study interactions of different compound classes with nanocellulose materials in food, pharmaceutical and dye applications, using a conventional plate reader in a high-throughput manner.