Theoretical Study On The Excited State Proton Transfer Of Nitrogen Heterocycles Compound

Excited-state proton transfer （ESPT） reactions can be dividedinto two large groups–intermolecular and intramolecular, and theexcited-state intramolecular proton transfer is often abbreviated inthe literature as ESIPT. ESPT plays an important role in physics,chemistry, biology and other many branches of science. It is alsoone of popular topics at the scientific research. In this paper, wetheoretically investigated the excited state proton transfer ofheterocyclic compounds containing N-H group using densityfunctional theory （DFT） and time-dependent density functionaltheory （TD-DFT）. The main results are outlined as follows:1. The ground-and excited-state structures, molecular orbitals,absorption and emission spectra of two dipyrrolyl derivatives （DPPand DPDCP） and their doubly hydrogen-bonded complexes with F-have been calculated by the DFT and TD-DFT methods, and theeffect of solvation on the optical properties have been discussed.Our study has revealed:（1） For hydrogen-bonded complex of dipyrrolyl derivatives,introduction of electron-withdrawing groups （-CN） promotesintramolecular charge transfer （ICT） and strengthens the doublehydrogen bond in the ground state. The stronger hydrogen bonds should be responsible to the more observable red-shift forelectronic spectra of DPDCP-F-complex.（2） After photoexcitation, introduction of electron-withdrawinggroups （-CN） promotes a ESPT process between receptor moleculesand F-, which can significantly lower the energy gap between the S₁and S₀states and facilitate the radiationless deactivation of theelectronic excited state via internal conversion （IC）. As a result,ESPT plays an important role for the fluorescence quenching inhydrogen-bonded complex. This provides a new theoretical basis forthe design of efficient anion fluorescence sensor.2. The ground state and first singlet excited state of fivetautomers of adenine, their monohydrated forms were studiedcomputationally at the B3LYP and TD-B3LYP levels using the6-311++G**basis set, respectively. We discussed intramolecularproton transfer from three reaction paths [I（AamN（9）H→AimN（9）H）,II（AamN（7）H→AimN（7）H）, III（AamN（9）H→AamN（3）H）].（1） The relative stability sequence of the isolated tautomers ofadenine in the ground state is AamN（9）H>AamN（7）H>AamN（3）H>AimN（9）H>AimN（7）H.（2） In these three paths of intramolecular proton transfer, theground state proton transfer of isolated adenine occurs through thetransition state with a four-member ring. We know that the energybarriers are1.99eV for path I （AamN（9）H→AimN（9）H）,2.06eV forpath II （AamN（7）H→AimN（7）H）, and0.93eV for path III （AamN（9）H→AamN（3）H）. Path III has a lowest barrier in these three paths and theproton transfer AamN（9）H→AamN（3）H is most likely to occur.（3） When one water molecule is involved in the proton transfer, proton transfer takes place through the transition state with asix-member ring. The forming of six-member hydrated transitionstate changes the proton transfer process, and the proton transferis one proton exchange process between water and adenine. Theinvolvement of a single water molecule in the proton transferreaction significantly reduces the reaction barrier.（4） For these three proton transfer paths, the excited state protontransfer of isolated adenine also occurs through the transition statewith a four-member ring. The barriers of the proton transfer in theexcited state are lower than those in the ground state. So thesinglet electronic excitation of adenine can facilitate the excitedstate proton transfer. In addition, the involvement of a single watermolecule lowers the barrier of Path I and III, but raises the barriersof Path II in the excited state.