A Multifunctional Photoswitch: 6π Electrocyclization versus ESIPT and Metalation

Abstract

A terthiazole-based molecular switch associating 6π electrocyclization, excited state intramolecular proton transfer (ESIPT), and strong metal binding capability was prepared. The photochemical and photophysical properties of this molecule and of the corresponding nickel and copper complexes were thoroughly investigated by steady-state and ultrafast absorption spectroscopy and rationalized by DFT/TDDFT calculations. The switch behaves as a biphotochrome with time-dependent photochemical outcome and displays efficient ESIPT-based fluorescence photoswitching. Both photochemical reactions are suppressed by nickel or copper metalation, and the main factors contributing to the quenching of the electrocyclization are discussed.

Light‐Controlled Release and Uptake of Zinc Ions in Solution by a Photochromic Terthiazole‐Based Ligand

“…The next electronic transitions investigated (from S 0 →S 2 to S 0 →S 8 ) contain important contributions of the LUMO+1 and LUMO+2 virtual orbitals, which show a suitable topology for the electrocyclization reaction, with a large electron density on the reactive carbons and a bonding character (Figure ). Additionally, it must be noted that all frontier molecular orbitals involved in the electronic transitions contain no contribution from the Zn 2+ cations, showing purely intra‐ligands transitions and no MLCT‐type transitions, contrary to the situation previously described in Ni 2+ and Cu 2+ complexes . This analysis is consistent with the observed photo‐reactivity of the Zn 2 ‐( 1 a ‐2 H) 2 complex.…”

“…Then, the back reaction 1 b to 1 a by visible irradiation at 600 nm in the presence of Zn(OH) 2 should allow the reformation of the initial complex Zn 2 ‐( 1 a ‐2 H) 2 . Finally, the tentative attribution of the absorption band at 462 nm to a form of zinc metalated 1 b is reasonable as both Cu‐( 1 b ‐2 H) and Ni‐( 1 b ‐2 H) are known to possess an absorption band around 450 nm …”

“…The fact that zinc metalated 1 a remains photochromic is not unexpected as, unlike its previously reported copper or nickel counterparts, the zinc complex lacks the MLCT band that could potentially deactivate the excited state and quench the photoreaction. Moreover, as revealed by the crystal structure, the two terminal thiazolyl groups of each ligand in the dimer are packed in the photoactive antiparallel conformation with respect to the central thiazolyl, and the distance between the two potentially photo‐reactive carbons within each ligand is approximately 3.71 Å (between C12 and C38) and 3.77 Å (between C28 and C52), which is much shorter than the generally admitted upper limit 4.2 Å to allow the photochemical reaction in the crystalline state or in solution …”

“…As am atter of fact, we have shown that when 1a was metalated by transition metal ions such as copper and nickel,t he resulting complexes, Cu-(1a-2 H) or Ni-(1a-2 H), did not exhibit any detectable photochromic behavior.S uch aq uenching was ascribed to the conjunction of two factors: lack of structure flexibility and metal-to-ligand charge transfer (MLCT)-related ultrafast deactivation of the excited state. [27] In this work, we report the synthesis of the zinc complex of 1a,Z n-(1a-2 H), its photochemical andp hotophysical properties, which reveal an interesting ands pectacular light-triggeredz inc ion release and uptake in solution (Scheme 1). DFT calculations were performed in an attempt to rationalize the difference in the affinity for zinc betweent he open form 1a and the closed form 1b.…”

“…[23][24][25][26] We have recently reported at erphenylthiazole-based multifunctional photo-switch that combines as alen-like cavity capable of bindingawide range of metal ions and two typeso f photochemical reactions:6 p electrocyclization/cycloreversion of the backbone and excited state intramolecular proton transfer (ESIPT),o ccurring between the hydrogen atom of one of the two hydroxyl groups and the nitrogen atom of the neighboring thiazolyl group (Scheme 1). [27] The ESIPT process confers av ery sensitive fluorescencet ot he open form 1a and the electrocyclization/cycloreversion allows its reversible photomodulation to al arge extent.…”