Luminescence of a Zinc(II) Complex with a Protonated 1‐Hydroxy‐1H‐imidazole ESIPT Ligand: Direct Excitation of a Tautomeric Form

Abstract

The complex (HL)2[ZnCl4] (L=2‐(1‐hydroxy‐4,5‐dimethyl‐1H‐imidazol‐2‐yl)pyridine) featuring a short intramolecular hydrogen bond O−H⋅⋅⋅N (O⋅⋅⋅N 2.61 Å) in the HL+ cation is a rare example of protonated ESIPT‐emitters (ESIPT=excited state intramolecular proton transfer). Photochemistry of (HL)2[ZnCl4] is dominated by the ESIPT tautomerization in HL+. In the solid state, (HL)2[ZnCl4] fluoresces in the sky‐blue region (S1tautomeric→S0tautomeric, λmax=500 nm). In MeCN solution, two minima on the S1 potential energy curve lead to dual emission of (HL)2[ZnCl4] (S1normal→S0normal and S1tautomeric→S0tautomeric, λmax=368 and 417 nm). Energy barriers in the ground state allow trapping and the excitation of the tautomeric form of HL+.

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(ii) complexes with 1-hydroxy-1H-imidazole-based ligands

“…[113][114][115][116][117][118] The ability of 1-hydroxy-1H-imidazoles to form N-hydroxy and N-oxide tautomers had not been used in the engineering of ESIPT-fluorophores until our recent works. 105,106,119 It is well known that the proton transfer abilities and emission properties of ESIPT-capable molecules can be tuned by substitution on the proton-donating and protonaccepting cores, as well as by the degree of π-conjugation. [120][121][122][123][124][125][126][127][128][129][130][131][132][133][134][135][136] As far as the degree of π-conjugation is concerned, extending the π-conjugated system on the protondonating part of the molecule can frustrate the ESIPT process, since the excited state is located predominantly on the ESIPT-incapable part of the molecule and involves the ESIPT site only to a small extent.…”

“…113–118 The ability of 1-hydroxy-1 H -imidazoles to form N -hydroxy and N -oxide tautomers had not been used in the engineering of ESIPT-fluorophores until our recent works. 105,106,119…”

ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

“…Normally, when decorated with such proton-donating groups as unsubstituted or substituted 2-hydroxyphenyl groups in the α-position to aza-atoms, their free nitrogen atoms act as proton acceptors during the ESIPT process [ 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 , 82 ]. Recently we proposed a new approach in the design of imidazole-based ESIPT-fluorophores in which we switched the role of the imidazole cycle to the one of a proton donor by introducing the hydroxy group in the position 1 and the pyridin-2-yl group in the position 2 of the imidazole ring [ 83 , 84 , 85 , 86 , 87 ]. Importantly, both roles of the imidazole ring in ESIPT-fluorophores, i.e., the proton acceptor and the proton donor ones, can be combined in a single molecule if we introduce the proton-donating 2-hydroxyphenyl group in the position 4 and the proton accepting pyridin-2-yl group in the position 2 of the 1-hydroxy-1 H -imidazole moiety.…”

Luminescence of ESIPT-capable zinc(ii) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety

“…The emission of ESIPT-capable dyes is highly sensitive to solvents, 43–50 substitution in the ESIPT-dye core, 51–71 protonation/deprotonation, 72–80 and coordination of metal ions. 81–96 Normally, the latter process is coupled with the deprotonation of ESIPT-dyes.…”