Two-photon absorption and two-photon-induced isomerization of azobenzene compounds

Abstract

The process of two-photon-induced isomerization occurring in various organic molecules, among which azobenzene derivatives hold a prominent position, offers a wide range of functionalities, which can be used in both material and life sciences. This review provides a comprehensive description of nonlinear optical (NLO) properties of azobenzene (AB) derivatives whose geometries can be switched through two-photon absorption (TPA). Employing the nonlinear excitation process allows for deeper penetration of light into the tissues and provides opportunities to regulate biological systems in a non-invasive manner. At the same time, the tight focus of the beam needed to induce nonlinear absorption helps to improve the spatial resolution of the photoinduced structures. Since near-infrared (NIR) wavelengths are employed, the lower photon energies compared to usual one-photon excitation (typically, the azobenzene geometry change from trans to cis form requires the use of UV photons) cause less damage to the biological samples. Herein, we present an overview of the strategies for optimizing azobenzene-based photoswitches for efficient two-photon excitation (TPE) and the potential applications of two-photon-induced isomerization of azobenzenes in biological systems: control of ion flow in ion channels or control of drug release, as well as in materials science, to fabricate data storage media, optical filters, diffraction elements etc., based on phenomena like photoinduced anisotropy, mass transport and phase transition. The extant challenges in the field of two-photon switchable azomolecules are discussed.

Fig. 1. Molecular orbitals for trans and cis isomers of AB with corresponding energies in eV. The arrows indicate that the isomerization process can be driven by light for trans–cis transition and light or heat in the cis–trans case.

Fig. 2. Modification of azobenzene molecule at ortho position can influence the energy of the n → π* transition of isomers. (A) Trans–cis isomerization can be driven with green light (530–560 nm), cis–trans isomerization could be then induced by using blue light (450–460 nm). (B) In the case of ortho-fluorinated azobenzene n → π* transitions of isomers are separated by up to 50 nm, so trans–cis isomerization can be driven with λ > 500 nm light and the reverse process can be performed with 410 nm.

Fig. 3. Schematic depiction of basic structural motifs of multi-photon absorbing chromophores.

Fig. 4. The scheme of a Z-scan setup. OA – open aperture for measurement of nonlinear absorption; CA – closed aperture for measurement of nonlinear refractive index.

Fig. 5. Two-photon absorption cross-section of AB can be enhanced by donor/acceptor substituents. The values of the two-photon absorption cross section were determined by Mendonça and co-workers using the Z-scan technique with femtosecond laser pulses at 775 nm.

Fig. 6. Representative examples of azobenzene derivatives with increased length of π-conjugation bridge. The values of the two-photon absorption cross section were determined using the Z-scan technique with femtosecond laser pulses at 780 nm (A) or 600 nm (B).

Fig. 7. (A) Structure of the salen complexes tethered with two azo dye units. The values of the two-photon absorption cross section were determined from 700 nm to 1550 nm using the Z-scan technique with femtosecond laser pulses. The TPA maxima were found at approximately 1000 nm for the investigated molecules 8a and 8b. (B) Structural example of azo metal chelate dye.

Fig. 8. Two-photon induced isomerization triggers interconversion between two switching forms (A) and (B). (A) Direct photoexcitation, (B) energy transfer from antenna (donor) to the switch. S₀: ground state, S₁: first singlet excited state, RET: resonance energy transfer.

Fig. 9. Structure of bistable azobenzene derivatives switchable in the nonlinear regime. The value of the two-photon absorption cross section was determined from bleach signal.

Fig. 10. Illustration of the operating principle of indirect switching based on FRET process. Reproduced from ref. 108 with permission from John Wiley and Sons.

Fig. 11. Overlapping between the LSPR band of plasmonic nanoparticles (blue) and absorption bands of azobenzene (black, here: n → π* band of azo-based ligand depicted above the spectra) may contribute to the selective enhancement of photoswitching (here only cis → trans two-photon induced isomerization is promoted). Reproduced from ref. 104 with permission from Royal Society of Chemistry.

Fig. 12. TPE with NIR light induces glutamate recognition and the channel is opening by trans–cis isomerization, which leads to ion flow across the membrane. Irradiation with Vis light or thermal relaxation of AB revert the process. Reproduced from ref. 107 with permission from American Chemical Society.

Fig. 13. Structures of the photoswitchable tethered ligands applied to the two-photon control of LiGluR (light-gated glutamate receptors).

Fig. 14. Structures of the photoswitchable tethered ligands applied to the two-photon control of LiGluR.

Fig. 15. Schematic representation of the two-photon-triggered multifunctional nanovalve (A). The two-photon irradiation triggers the release of the camptothecin anticancer drugs (E), via the photo-isomerization of azobenzene moieties (C) grafted on the surface of so-called CF fluorophore (D) modified MSN, and bonded with β-cyclodextrin (B). Reproduced from ref. 129 with permission from John Wiley and Sons.

Fig. 16. Schematic depiction of azo unit photoisomerization induced by (A) ultraviolet (UV) light, and (B) NIR light via two-photon (TP)-excited FRET of a conjugated polymer. (C) The mechanism of the NIR-triggered photoisomerization of azo by TP-FRET. (D) The preparation of supramolecular conjugated HCP-PEG unimicelles and their NIR-triggered drug release in cancer cells. Reproduced from ref. 130 with permission from Royal Society of Chemistry.

Fig. 17. Volumetric birefringence pattern recorded via two-photon absorption. (a) Schematic representation of optical storage experimental setup. (b) When the sample orientation is set at 45° with respect to the polarizer axis the pattern can be visualized. (c) When the sample is placed parallel to the polarizer axis the picture cannot be seen. Based on the ref. 141.

Fig. 18. Structures of photoswitches used as guest molecules in polymer films. DR = Disperse Red, DO = Disperse Orange.

Fig. 19. Optical microscopy images of the copolymer containing bis-azobenzene thin film after recording (a)–(c), after erasing (d), and after rewriting (e)–(f) in the same region. Reproduced from ref. 146 with permission from Elsevier.

Fig. 20. Optically induced spontaneous formation of periodic two-dimensional patterns on newly synthesized polymers based on Y-shape azobenzene molecules. Reproduced from ref. 24 with permission from American Chemical Society.

Marta Dudek

Nina Tarnowicz-Staniak

Marco Deiana

Ziemowit Pokladek

Marek Samoć

Katarzyna Matczyszyn