Resonance Assisted Hydrogen Bonding Phenomenon Unveiled from Both Experiment and Theory - an Example of New Family of Ethyl N-salicylideneglycinate Dyes.

Extensive experimental and theoretical investigations are reported on the nature of resonance-assisted hydrogen bonding phenomenon (RAHB) and its influence on photophysical properties of the newly designed dyes differing in donor-acceptor properties, namely ethylN-salicylideneglycinate (1), ethylN-(5-methoxysalicylidene)glycinate (2), ethylN-(5-bromosalicylidene)glycinate (3) and ethylN-(5-nitrosalicylidene)glycinate (4). All compounds are thermochromic in the solid state and they contain a typical intramolecular O-H...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 O-H...N bonding and accordingly the relative populations of the enol,cis-keto andtrans-keto forms leading to variation of the photophysical properties of1-4. It is determined, that the electron-withdrawing NO(2)in4amplifies the most RAHB effect causing the breaking of the O-H...N hydrogen bond and accordingly formation of the dominantcis-keto isomer in both the solid state and EtOH. To this end, the UV/Vis spectra of1-3in EtOH revealed the exclusive presence of the enol form, while the prevalent contribution of thecis-keto form was found for4. Furthermore, only compound 4is emissive in the solid state in ambient condition due to dual emission arising from thecis-keto* andtrans-keto* forms, while2was found to be highly emissive in EtOH. It is revealed qualitatively and quantitatively, based 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 sucking through both the pi- and sigma-channels, which accordingly exerts chemical bonding changes at both the phenyl ring and predominantly a distant O-H...N area.