The main objective of this action was to design novel heteroaromatic building blocks, which would allow to enhance TPA activity of given chromophores in the near-infrared (NIR) spectral region. This was achieved by using state-of-the-art quantum-chemical calculations of NLO properties for a large series of structurally diverse heteroaromatic systems. A systematic computer-aided modelling, focused primarily on calculations of TPA cross-sections, allowed us to avoid trial-and-error experimentation and to target at a set of highly efficient TPA dyes using various S,N-heteroarene platforms and substitution patterns.
In this regard, we found out that our concept of regioisomeric control of NLO properties proposed originally for a benzothiazole ring is valid generally and can be applied to a wide range of heteroaromatic scaffolds. This strategy (an appropriate arrangement of electron-donating/withdrawing substituents on the heteroaromatic core while keeping the molecular weight constant) offers thus an elegant way to maximize TPA activity within a confined chromophore space – the more so the larger is the “asymmetry” between two positions of the heteroaromatic scaffold considered for connection of donor and acceptor substituents. In addition, we also discovered, that this concept can be applied to centrosymmetric (quadrupolar) dyes with electron-rich and/or electron-defficient centres.
The most promising candidates identified by quantum-chemical calculations were synthesized in our laboratory and are currently under intensive photophysical characterizations, including steady UV-vis absorption/emission spectroscopy as well as laser measurements of TPA cross-sections in collaboration with our partners.
In the next stage, we developed efficient synthetic strategies for specific functional group transformations (one-pot reductive methylation of nitrobenzazoles, direct halogenations of deactived heteroaromatics, oxidative homocoupling of donor-substituted benzazoles etc.) and refined proposed mechanisms of some reactions, both facilitating the preparation of valuable building blocks and target TPA dyes. When attempting to prepare a reactive Pd species suitable for cross-coupling reactions, we characterized an unprecedented Pd7 nanocluster, whose electronic structure and aromatic character are attractive for a wide range of applications, including quantum computing or imaging.
The first samples were also subjected to two-photon excited fluorescence (TPEF) microscopy, showing application potential of our dyes in bioimaging, namely by mapping the atherosclerosis reflected in aorta size and changes in its structure. The good TPA response along with possibility to combine TPEF images with second-harmonic generation (SHG) microscopy for non-centrosymmetric push-pull dyes allows for higher spatial resolution than obtained solely by TPEF. This can lead to better understanding the plaque formation and mechanisms behind aorta narrowing using non-invasive NLO techniques and can be thus indirectly helpful in finding appropriate medication (note that vascular circulatory diseases are responsible for 35 % of all death each year and are related to hardening and narrowing of the aorta). To increase the selectivity of our TPA dyes to bind specifically to certain organelles or biomolecules, further modification of the pendant groups is required and it is ongoing in our laboratories.
In addition, we have shown that readily available 4,7-bis(arylethynyl)heteroarenes end-capped with NO2 groups display apart from large TPA activity also high quantum yields of excited triplet-state and singlet oxygen generation (50-70%). This along with their absorption/emission properties makes these dyes attractive as efficient metal-free photosensitisers in the photodynamic therapy (PDT) using both blue light irradiation (preferred in the treatment of skin cancer) and red light laser utilizing nonlinear two-photon excitation (PDT treatment, where a deep tissue penetration is required).
Completed works related to this action have been published in 7 peer-reviewed papers and another 5 manuscripts are expected to be submitted within next 12 months.
