Concerted versus stepwise proton transfer reactions in the [2, 2'-bipyridyl]-3-3'-diol molecule: A static and dynamic ab-initio investigation

The double proton transfer (PT) reaction has been investigated in the [2,2'-bipyridyl]-3-3'-diol, a complex molecule where the proton movements is coupled to significant rearrangement of the electronic structure. Moreover, the reaction could be concerted, that is the two protons are exchanged simultaneously, or stepwise, where the two protons are transferred sequentially. To this end, a static exploration of the potential energy surface (PES) was carried together with the analysis of the free-energy surface (FES), both surfaces being evaluated at density functional theory level and different exchange-correlation functionals. While the concerted mechanism has been clearly discharged, the characteristics of the stepwise PT significantly depends on the chosen functionals, some suggesting a clear stepwise mechanism characterized by a stable reaction intermediates and two transitions states, whereas other approaches propend for a asynchronous PT, with a single TS. These features appear on both PES and FES, albeit some differences appears due to their different nature.

A Deep Understanding on the Effective Generation of Twisted Intramolecular Charge Transfer by Protonation in Thiazolo[5,4-d]thiazole Derivatives

The exploration of the intrinsic relationship between the phototautomerization and photoelectric properties is of great significance for the application of the emerging novel organic materials, such as the (bi)heterocyclic thiazolo[5,4-d]thiazole derivatives (TzTz). Here, by introducing the chemical-controlling protonation, a barrierless spontaneous rotation movement of the designed TzTz derivative (2,5-diyl-amino-thiazolo[5,4-d]thiazole, DA-TzTz) was ensured through the facilitation of the excited-state intramolecular proton transfer (ESIPT) triggered twisted intramolecular charge transfer (TICT) process by the enhancement of the intramolecular hydrogen bonds, steric hindrance effect, and conjugative effect. It is further verified that the hetero S atoms could mostly effect the proton accepting ability of -N═ through comparing with the influences to the intramolecular H-bond between the protonated/nonprotonated amino groups and the -N═ atoms brought by the replacement of them with N or O atoms. As a result, the dissociation and rearrangement of the π conjugation in DA-TzTz accompanying with the variation of the optoelectronic characteristics was benefited from the establishment of the preferential charge-transfer and separation. We expect this tentative study could establish a new concept of designing an efficient charge-transfer and separation method, paving the way for the development of the TzTz derivatives and other optoelectronic organic materials.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn²⁺ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λ_ex = 300 nm and 27% at λ_ex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S₁ state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL₂][Zn(OAc)₂]₂·2H₂O and [ZnL₂][ZnCl₂]₂·4H₂O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Regulation of excited-state intramolecular proton transfer process and photophysical properties for benzoxazole isothiocyanate fluorescent dyes by changing atomic electronegativity

Excited-state intramolecular proton transfer (ESIPT) is favored by researchers because of its unique optical properties. However, there are relatively few systematic studies on the effects of changing the electronegativity of atoms on the ESIPT process and photophysical properties. Therefore, we selected a series of benzoxazole isothiocyanate fluorescent dyes (2-HOB, 2-HSB, and 2-HSeB) by theoretical methods, and systematically studied the ESIPT process and photophysical properties by changing the electronegativity of chalcogen atoms. The calculated bond angle, bond length, energy gap, and infrared spectrum analysis show that the order of the strength of intramolecular hydrogen bonding of the three molecules is 2-HOB<2-HSB<2-HSeB. Correspondingly, the magnitude of the energy barrier of the potential energy curve is 2-HOB>2-HSB>2-HSeB. In addition, the calculated electronic spectrum shows that as the atomic electronegativity decreases, the emission spectrum has a redshift. Therefore, this work will offer certain theoretical guidance for the synthesis and application of new dyes based on ESIPT properties.