Theoretical Investigation On Excited State Intermolecular Proton Transfer Of Hydrogen Bonding Dynamics

The excited state intermolecular proton transfer process is a hot issue in the fieldof atomic and molecular. These molecules can be used to develop organiclight-emitting material. Moreover, the excited state intermolecular proton transferreaction has a wide range of applications in physics, chemistry and bioengineering.In the present paper, we have studied several molecular systems in the groundand excited states of intermolecular proton transfer reaction using quantum chemicalcalculation method. The main content summarized as follows:(1)4′-dimethylaminoflavonol (DMAF) can not only make intramolecularhydrogen bond by itself, but also form intermolecular hydrogen bond via molecules ofsolvent. We have investigated the excited state proton transfer reaction of the DMAFmonomer and DMAF–EtOH complex. It was found that intramolecular andintermolecular proton transfer reaction can occur in the excited state. Moreover, weconfirmed the relation between twisted intramolecular charge-transfer (TICT) processand the excited state intermolecular proton transfer process, and re-identified thefluorescence peak of DMAF–EtOH complex in previous experiment. In this part ofthe investigation, we revealed that the intermolecular hydrogen bonding interactioncan change the excited state dynamic mechanism of DMAF molecular system. That is to say, intramolecular charge transfer turns into TICT, and intramolecular protontransfer becomes intermolecular proton transfer.(2) We have studied the multiple proton transfer reaction of3-hydroxypyridine(3HP) molecule in aqueous solution. The multiple proton transfer can occur smoothlyof3HP-(H2O)3complex in the ground and excited states. Moreover, these reactionsgenerate a zwitterionic complex---3Py-(H2O)3. Our results confirmed for the firsttime that the stepwise mechanism of multiple proton transfer reactions is the mostreasonable mode in the ground and excited states. At the same time, the water wire(H2O…H2O…H2O) can drastically lower the energy barrier for the proton transferreaction in the ground and excited states, and provide a nice channel of the protontransfer process for the3HP-(H2O)3complex. A complicated cycle process between3HP-(H2O)3and3Py-(H2O)3in the ground and excited states has been presented. Thiscycle process clearly explains that the fluorescence of the excited state3HP isprevented in previous experiment. Furthermore, intermolecular hydrogen bonds of the3HP-(H2O)3complex are strengthened in the excited state in comparison with those inground state, whereas the hydrogen bonds interactions of3Py-(H2O)3complex areweakened in the excited state.(3) The excited-state multiple proton transfer (ESMPT) reaction has beeninvestigated for tautomerization of7-Hydroxy-4-methylcoumarin (NH3)3cluster bymeans of time-dependent density functional theory. We demonstrated that the ESMPTreaction can take place, and a sequential occurrence of the ESMPT reaction isrevealed for the7H4MC (NH3)3cluster. Moreover, a transient species (A (NH+4)3) isfound on the potential energy profiles of the7H4MC (NH3)3cluster, and the nature ofA (NH+4)3conformer has obtained. In addition, the influence of different typehydrogen bonded wires (ammonia and water) on the ESMPT reaction of the7H4MChas also been discussed. We found that the ESMPT reaction of the7H4MC moleculeis Eâ†'Aâ†'K mechanism, due to the hydrogen-bonded ammonia wire. However, theESMPT process of the7H4MC (H2O)3cluster is Eâ†'K mechanism. Simultaneously,we proved that the hydrogen-bonded ammonia wire is more beneficial to the ESMPTreaction in comparison with the hydrogen-bonded water wire for the7H4MC molecule, since that hydrogen-bonded ammonia and water wires have different protonaffinity.