Resonance Assisted Hydrogen Bonding Phenomenon Unveiled through Both Experiments and Theory: A New Family of Ethyl N‐Salicylideneglycinate Dyes

Abstract

Extensive experimental and theoretical investigations are reported on the nature of resonance-assisted hydrogen bonding phenomenon (RAHB) and its influenceo n photophysical properties of the newly designed dyes differing in donor-acceptor properties, namely ethyl N-salicylideneglycinate (1), ethyl N-(5-methoxysalicylidene)glycinate (2), ethyl N-(5-bromosalicylidene)glycinate (3)a nd ethyl N-(5-nitrosalicylidene)glycinate(4). All compounds are thermochromic in the solid state and they contain at ypical intramolecular OÀH•••N hydrogen bond formed between the hydroxyl hydrogen atom and the imine nitrogen atom, yielding the enol form in the solid state. It is unveiled, that the magnitude of RAHB effect fine tunes the strength of the OÀH•••N bondinga nd accordingly the relativep opulations of the enol, cis-keto and trans-keto formsl eading to variation of the photophysical properties of 1-4.I ti sd etermined, that the electron-withdrawing NO 2 in 4 amplifies the mostR AHB effectc ausing the breaking of the OÀH•••N hydrogenb ond and accordingly formation of the dominant cis-keto isomer in both the solid state and EtOH. To this end, the UV/Vis spectra of 1-3 in EtOH revealed the exclusive presence of the enol form, while the prevalent contribution of the cisketo form was found for 4.F urthermore, only compound 4 is emissive in the solids tate in ambient condition due to dual emissiona rising from the cis-keto* and trans-keto* forms,w hile 2 was found to be highly emissive in EtOH. It is revealed qualitativelya nd quantitatively,b ased on the ETS-NOCV charge and energy decomposition scheme and the EDDB population-based method, that RAHB is strongly a non-local phenomenon based on electrons pumping or suckingt hrough both the p-a nd s-channels, which accordingly exerts chemicalb ondingc hanges at both the phenyl ring and predominantly ad istant OÀH•••N area.

A Family of Ethyl N-Salicylideneglycinate Dyes Stabilized by Intramolecular Hydrogen Bonding: Photophysical Properties and Computational Study

“…Dyes 1 – 4 were produced according to the described synthetic procedure [ 18 ]. We first applied a Hirshfeld surface analysis [ 39 ] to study the intermolecular interactions in the reported crystal structures of 1 – 4 [ 17 ]. As a result, a set of 2D fingerprint plots [ 40 ] were generated using CrystalExplorer 3.1 [ 41 ].…”

“…The molecular surface of 3 is additionally characterized by a remarkable proportion of the H∙∙∙Br intermolecular contacts of 17.4%, with the shortest contacts at d e + d i ≈ 3.0 Å ( Figure S3 ). Notably, the molecular surface of 3 is further described by a distinct proportion of the C∙∙∙C contacts of 4.1%, which is due to intermolecular π⋯π stacking interactions between the phenylene rings [ 17 ]. These contacts are shown on the 2D plot as a typical area at d e = d i ≈ 1.7–2.0 Å ( Figure S3 ).…”

“…The H∙∙∙H and H∙∙∙C contacts occupy 30.0 and 11.5%, respectively. Notably, the structure of 4 , similar to 3 , also exhibits a distinct proportion of the C∙∙∙C contacts of 4.9% due to the formation of intermolecular π⋯π stacking interactions between the phenylene rings [ 17 ]. These contacts are also shown on the 2D plot as a typical area at d e = d i ≈ 1.7–2.0 Å ( Figure S4 ).…”

“…Schiff bases fabricated from salicylaldehyde derivatives ( N -salicylidene aniline derivatives) have been the main focus of our extensive studies due to their chromic properties and a broad color palette [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ]. Particularly, these compounds are known to a possible intramolecular transfer of the OH proton to the imine N-atom affording tautomerization between the enol-imine and keto-enamine forms [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ].…”

“…It should be noticed that we have recently initiated comprehensive studies of closely related N -salicylidene aniline derivatives 1 – 4 , derived from the ethyl ester of glycine ( Chart 1 ) [ 17 , 18 ]. We found that 1 – 4 in the solid state each exhibits an enol-imine form.…”