Manipulating the relaxation pathways of excited states and understanding mechanisms of photochemical reactions present important challenges in chemistry. Here we report a unique zinc(II) complex exhibiting unprecedented interplay between the excitation-wavelength-dependent emission, thermally activated delayed fluorescence (TADF) and excited state intramolecular proton transfer (ESIPT). The ESIPT process in the complex is favoured by a short intramolecular OH⋅⋅⋅N hydrogen bond. Synergy between the excitation-wavelength-dependent emission and ESIPT arises due to heavy zinc atom favouring intersystem crossing (isc). Reverse intersystem crossing (risc) and TADF are favoured by a narrow singlet-triplet gap, ΔE ≈10 kJ mol . These results provide the first insight into how a proton-transfer system can be modified to show a synergy between the excitation-wavelength-dependent emission, ESIPT and TADF. This strategy offers new perspectives for designing ESIPT and TADF emitters exhibiting tunable excitation-wavelength-dependent luminescence.

Scheme 2. Proton transfer in imidazole-based ESIPT-fluorophores.

The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)(NO)·HO (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, T↑ = 337-345 K and T↓ = 316-313 K. In sealed ampoules the complex Fe(ATrz)(NO)·HO shows a narrow hysteresis (ca. 1-4 K), T↑ = 326-329 K and T↓ = 326-324 K. After adsorption of water the complex Fe(ATrz)(NO)·nHO (n = 1.25-1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)(NO)·nHO (n = 3.6-16.6) wide hysteresis (ca. 5-20 K) re-appears near room temperature (T↑ = 319-321 K and T↓ = 300-315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(ii) spin crossover complexes with 1,2,4-triazoles (ca. 1000-2000 kJ mol). The systems Fe(ATrz)(NO) and Fe(ATrz)(NO)·nHO show compensation effects (ΔH - ΔS, ln A - E). A correlation between the T↑, the ΔH values and the water content in the complex is observed: the highest ΔH values (27-29 kJ mol) and the lowest T↑ values (317-320 K) correspond to the samples with high water content, whereas the lowest ΔH values (19-23 kJ mol) and the highest T↑ values (337-345 K) correspond to water-free samples, Fe(ATrz)(NO). Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)(NO)·HO (ATrz = 4-amino-1,2,4-triazole).