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

Abstract

Rational design of ESIPT-capable metal complexes (ESIPT – Excited State Intramolecular Proton Transfer) requires two sites, namely an ESIPT site and a metal binding site, to be spatially separated into...

Reversible Switching Based on Truly Intramolecular Long-Range Proton Transfer─Turning the Theoretical Concept into Experimental Reality

“…A first working example of a tautomeric switch, where the switching is mediated by PT, was proposed by IBM Zurich Research Laboratory 15 years ago. , Using a low-temperature scanning tunneling microscope (STM), a voltage pulse at the STM tip can induce a change in the orientation of the hydrogen atom pair at the center of naphthalocyanine, leading to tautomeric switching between low and high conductance. , It sounds very exciting, indeed, and opens new horizons because most of the tautomeric compounds, where the proton(s) exchange happens over a short distance between two sites (as in naphthalocyanines or in similar macrocycles) along an intramolecular hydrogen bond, are switchable either at the conditions applied there, i.e., at the low temperature in a vacuum, or even at room temperature in solution under a variety of stimuli. , However, the relatively low PT barriers in such systems provide population of the on -state only while the external stimulus (light or voltage) is turned on. In this respect, a long-range PT potentially creates conditions for bistable switching.…”

“…23,24 It sounds very exciting, indeed, and opens new horizons because most of the tautomeric compounds, where the proton(s) exchange happens over a short distance between two sites (as in naphthalocyanines 21 or in similar macrocycles 23 ) along an intramolecular hydrogen bond, are switchable either at the conditions applied there, i.e., at the low temperature in a vacuum, or even at room temperature in solution under a variety of stimuli. 20,25 However, the relatively low PT barriers in such systems provide population of the onstate only while the external stimulus (light or voltage) is turned on. In this respect, a long-range PT potentially creates conditions for bistable switching.…”

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

“…The HL H ligand was rationally designed to accommodate a metal ion and the mobile H + ion, i.e., it has both the N,N -metal binding site and the O,N -ESIPT site. Such a design strategy allows one to obtain ESIPT-capable metal complexes, an extraordinarily rare class of ESIPT dyes (Scheme , top). ,,− , Recently, we synthesized and characterized HL , which differs from HL H by the number and position of CH 3 groups and by the position of hydroxyphenyl and pyrazole substituents to the central pyrimidine core. The reaction of HL with ZnCl 2 in equimolar ratio yielded an ESIPT-capable [Zn­(HL)­Cl 2 ] complex (Scheme , top) .…”

“…Due to the large variety of emissive excited states, the emission of ESIPT-capable molecules is sensitive to various stimuli. − The breaking of the intramolecular hydrogen bond due to the interaction of the ESIPT site of the molecule with other molecules or ions or due to deprotonation causes strong changes in the luminescence response of ESIPT-capable molecules. The protonation of ESIPT-capable molecules can proceed at the hydrogen bond-accepting atom switching the ESIPT reaction off or at other heteroatoms leading to modulating ESIPT and ESIPT-coupled luminescence. − Binding metal ions by ESIPT dyes which is accompanied by the deprotonation in most cases (only rare examples of ESIPT-capable metal complexes are known) ,,− also results in strong changes of emission. − This sensitivity allows one to switch the emission of ESIPT-capable molecules between various emission mechanisms and makes them a multivariate platform for the design of stimuli-responsive emissive materials, probes, and sensors.…”

“…Kinetic barriers between the minima of the normal and tautomeric forms in the excited state can partially or fully suppress the proton transfer process and lead to the emission of the normal form. If the barriers separating the normal and tautomeric forms in the excited state are surmountable, ESIPT-capable compounds can show dual emissions associated with the fluorescence of both forms. − Along with singlet-to-singlet transitions, these molecules can be converted to the ground state through triplet excited states and triplet-to-singlet transitions such as phosphorescence (including room temperature phosphorescence) ,,− and thermally activated delayed fluorescence. − Some ESIPT-capable compounds exhibit anti-Kasha emissions, which can be observed in molecules wherein the first and the second excited states are separated by large energy gaps. − …”