5-Phenylazopyrimidines are structurally similar to the well explored photosensitive azobenzenes, where the photoswitching is based on a reversible trans-cis isomerization. Contrary to azobenzenes, in azopyrimidines, one phenyl ring is replaced by pyrimidine. Such molecules may obtain new prominent properties, such as keto/enol tautomerism, protonation sites, higher biocompatibility or intermolecular hydrogen bonding similar to those between natural bases in nucleic acids. In this work, we prepared a series of substituted 2-amino-5-phenylazopyrimidines without, with one and with two hydrogen bond donors in the neighboring positions (C4 and C6) with respect to the 5-phenylazo group. We studied the effect of intramolecular hydrogen bonds (IMHBs) on the photochemical behavior of these compounds by NMR spectroscopy with in situ irradiation,[1] where UV/visible light is guided via an optical fiber directly into the NMR spectrometer.[2] We found out that IMHBs didn’t lock cis isomer formation but destabilized it significantly. In compounds with two hydrogen bond donors, we uncovered unique photoswitchable IMHBs. At lower temperature, these compounds have two stable rotamers (both trans) differing only in the orientation of the phenylazo moiety (rotamers A/B), each with one IMHB. The rotamer ratio changes reversibly upon irradiation. This photoinduced process as well as thermal relaxation is strongly substituent-dependent. Electron donating substituent (OCH3) in para position of the phenyl ring allowed to see negligible change in rotamer ratio upon irradiation at –105 °C, which is probably caused by very low energy barrier between both rotamers (ΔGB-A). H and Cl derivatives behaved like a decent photoswitch; the rotamer ratio could be modulated by irradiation and the thermal relaxation occurred within several hours. The derivatives with electron-accepting substituents (CN, NO2) provided very stable non-equilibrium state (rotamer B as the predominant form) and the initial equilibrium was achieved after heating only. A mechanism of these unique photoswitching processes was proposed by DFT computations.
References
[1] a) C. Feldmeier, H. Bartling, E. Riedle, R. M. Gschwind, J. Magn. Reson. 2013, 232, 39-44; b) C. Wolff, J. Kind, H. Schenderlein, H. Bartling, C. Feldmeier, R. M. Gschwind, M. Biesalski, C. M. Thiele, Magn. Reson. Chem. 2016, 54, 485-491.
[2] J. Kind, L. Kaltschnee, M. Leyendecker, C. M. Thiele, Chem. Commun. 2016, 52, 12506-12509.
Acknowledgement
This work has been financially supported by the Czech Science Foundation (grant no. 15-11223S) and the German Research Council (TH1115/9-1).