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The study investigated the effects of symmetry relaxation on one- and two-photon absorption spectra of two bichromophore systems based on difluoroborate core linked by biphenylene or bianthracene moieties, and found that deviations from planarity of building blocks ensure maximum values of two-photon transition strengths.
A team of scientists from the Nicolaus Copernicus University in Poland and the Wroclaw University of Science and Technology in Poland have conducted a study on the potential applications of multiphoton absorbers. In their research, published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, the team focused on the symmetry-relaxation effects in one- and two-photon absorption spectra in two bichromophore systems based on difluoroborate core linked by biphenylene or bianthracene moieties (1).
Multiphoton absorbers are molecules that can absorb multiple photons of light simultaneously. They are used in various applications such as bioimaging, photodynamic therapy, and data storage. Compared to single-photon absorbers, multiphoton absorbers offer better resolution, deeper penetration, and less photodamage to biological tissues. They typically have large two-photon absorption cross sections, which means that they can be excited by two photons of lower energy instead of one photon of higher energy.
The team utilized a variety of experimental methods, including synthesis, one- and two-photon spectroscopy, and X-ray crystallography, along with computational methods to explore how symmetry relaxation, which occurs as a result of twisting building blocks, affects the one- and two-photon absorption of the two fluorescent dyes studied.
In this study, researchers examined the impact of symmetry relaxation on the absorption spectra of two fluorescent dyes, which are made up of difluoroborate core connected to biphenylene or bianthracene moieties. The study found that deviations from planarity, resulting from twisting of the building blocks, had a significant effect on the two-photon transition strengths. Specifically, the researchers discovered that the planarity of the central biphenyl moiety, as well as deviations from planarity up to 30-40 degrees, resulted in maximum values of two-photon transition strengths. On the other hand, the perpendicular arrangement of phenylene units in the biphenylene moiety resulted in a 20% reduction in the two-photon transition strengths. Overall, these findings suggest that the structure and geometry of bichromophore systems can play a crucial role in determining their optical properties.
Moreover, detailed studies of the equilibrium structures of both compounds in chloroform solution demonstrated that they have very different values of two-photon absorption cross sections at absorption band maxima. The biphenyl and bianthracene linkers exhibited two-photon absorption cross sections of 224 GM and 134 GM, respectively. The latter value was in good agreement with the experimental value obtained using the Z-scan method.
The difference in two-photon absorption cross section between both compounds can be explained based on equilibrium geometry differences, where the interplanar angle was found to be 35 deg and 91 deg in the case of the biphenylene and bianthracene moieties, respectively. The study suggests that it is not beneficial to introduce conformationally locked central linker based on bianthracene moiety.
Overall, the findings of this study shed light on the potential applications of bichromophores as multiphoton absorbers and highlight the importance of understanding the effects of symmetry relaxation on one- and two-photon absorption spectra.
(1) Petrusevich, E. F.; Glodek, M.; Antoniak, M. A.; Muziol, T.; Plazuk, D.; Siomra, A.; Nyk, M.; Osmialowski, B.; Zalesny, R.Difluoroborate-based bichromophores: Symmetry relaxation and two-photon absorption. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2023, 295, 122600. DOI: 10.1016/j.saa.2023.122600