Mechanism of Excited State Double Proton Transfer in 2-Amino-3-Methoxypyridine and Acetic Acid Complex

The excited-state double-proton transfer （ESDPT） mechanism of 2-amino-3-methoxypyridine and acetic acid com- plex is studied by the density functional theory （DFT） and time-dependent DFT with CAM-B3LYP functional. The complex is connected through two different types of inter-molecular hydrogen bonds. After photo-excitation, both hydrogen bonds get strengthened, which can facilitate the ESDPT reaction. The scanned potential energy curve along the proton transfer coordinate indicates that the ESDPT reaction proceeds in a stepwise pattern.

Photochemistry of Potassium Ferrocyanide and its Reaction with Uridine 5′-monophosphate in Aqueous Solution under Ultraviolet Irradiation

The photochemical reaction of potassium ferrocyanide(K_(4)Fe(CN)_(6))exhibits excitation wavelength dependence and non-Kasha rule behavior.In this study,the excited-state dynamics of K_(4)Fe(CN)_(6) were studied by transient absorption spectroscopy.Excited state electron detachment(ESED)and photoaquation reactions were clarified by comparing the results of 260,320,340,and 350 nm excitations.ESED is the path to generate a hydrated electron(e^(−)_(aq)).ESED energy barrier varies with the excited state,and it occurs even at the first singlet excited state(^(1)T_(1g)).The ^(1)T_(1g) state shows∼0.2 ps lifetime and converts into triplet[Fe(CN)_(6)]4−by intersystem crossing.Subsequently,3Fe(CN)_(5)]^(3-)appears after one CN−ligand is ejected.In sequence,H2O attacksFe(CN)_(5)]^(3-)to generate[Fe(CN)_(5)H_(2)O]^(3−)with a time constant of approximately 20 ps.The ^(1)T_(1g) state and e−aq exhibit strong reducing power.The addition of uridine 5′-monophosphate(UMP)to the K_(4)Fe(CN)_(6) solution decrease the yield of e−aq and reduce the lifetimes of the e−aq and ^(1)T_(1g) state.The obtained reaction rate constant of ^(1)T_(1g) state and UMP is 1.7×10^(14)(mol/L)^(−1)·s^(−1),and the e−aq attachment to UMP is∼8×10^(9)(mol/L)^(−1)·s^(−1).Our results indicate that the reductive damage of K_(4)Fe(CN)_(6) solution to nucleic acids under ultraviolet irradiation cannot be neglected.

Hydrogen Bonding Effects on the Photophysical Properties of 2,3-dihydro-3-keto- 1H-pyrido[3,2,1-kl] phenothiazine

Time-dependent density functional theory （TDDFT） and femtosecond transient absorption spectroscopy were used to investigate the photophysical properties of 2,3-dihydro-3-keto-lH- pyrido[3,2,1-kl]phenothiazine （PTZ4） and 3-keto-lH-pyrido[3,2,1-kl]phenothiazine （PTZ5）. The calculated results obtained from TDDFT suggest that the red-shifts of the absorption spectra of these two fluorophores in methanol are due to the formation of hydrogen-bonded complexes at the ground state. Four conformers of PTZ4 were obtained by TDDFT. The two fluorescence peaks of PTZ4 in tetrahydrofuran （THF） came from the ICT states of the four conformers. The fluorescence of PTZ4 in THF showed a dependence on the excitation wavelength because of butterfly bending. The excited state dynamics of PTZ4 in THF and methanol were obtained by transient absorption spectroscopy. The lifetime of the excited PTZ4 in methanol was 53.8 ps, and its relaxation from the LE state to the ICT state was completed within several picoseconds. The short lifetime of excited PTZ4 in methanol was due to the formation of out-of-plane model hydrogen bonds between PTZ4 and methanol at the excited state.

Excess-Electron Attachment and Ionization of Aqueous Uridine Monophosphate Anion

We applied quantum mechanics/classical mechanics simulations to study excess-electron attachment and ionization of uridine monophosphate anion(d UMP-)in explicit aqueous solutions.We calculated vertical electron affinities(VEAs),adiabatic electron affinities(AEAs),vertical detachment energies(VDEs),vertical ionization energies(VIEs),and adiabatic ionization energies(AIEs)of the 40 structures obtained from molecular dynamic trajectory.The excess-electron and hole distributions were analyzed in electron attachment and ionization of aqueous d UMP^(-).The converged mean VEA(-0.31 e V)and AEA(2.13 e V)suggest that excess-electron can easily attach to d UMP^(-).The mean vertical(-0.50 e)and adiabatic(-0.62 e)excess-electron on uracil reveal that main excesselectrons are localized on nucleobases at the most snapshots.The distributions at several special snapshots demonstrate the excess-electron delocalization over nucleobases/ribose or ribose/phosphate group after the structural relaxations of d UMP^(2-)dianion.The VDE value(2.78 e V)indicates that d UMP2-dianion could be very stable.Moreover,the mean VIE is 8.13 eV which is in agreement with the previous calculation using solvation model.The hole distributions on uracil suggest that the nucleobases are easily ionized after the irradiation of high-energy rays.In vertical ionizations,the holes would be delocalized over uracil and ribose at several snapshots.Observing the adiabatic hole distributions,it can be found that electrons on phosphate group and holes on nucleobases can be transferred to ribose at the special snapshots in the structural relaxation of neutral species.

Semi-Quantitatively Designing Two-Photon High-Performance Fluorescent Probes for Glutathione S-Transferases

Glutathione S-transferases(GSTs),detoxification enzymes that catalyze the addition of glutathione(GSH)to diverse electrophilic molecules,are often overexpressed in various tumor cells.While fluorescent probes for GSTs have often adopted the 2,4-dinitrobenzenesulfonyl(DNs)group as the receptor unit,they usually suffer from considerable background reaction noise with GSH due to excessive electron deficiency.However,weakening this reactivity is generally accompanied by loss of sensitivity for GSTs,and therefore,finely turning down the reactivity while maintaining certain sensitivity is critical for developing a practical probe.Here,we report a rational semiquantitative strategy for designing such a practical two-photon probe by introducing a parameter adopted from the conceptual density functional theory(CDFT),the local electrophilicityω_(k),to characterize this reactivity.As expected,kinetic studies establishedω_(k)as efficient to predict the reactivity with GSH,and probe NI3 showing the best performance was successfully applied to detecting GST activities in live cells and tissue sections with high sensitivity and signal-to-noise ratio.Photoinduced electron transfer of naphthalimide-based probes,captured by femtosecond transient absorption for the first time and unraveled by theoretical calculations,also contributes to the negligible background noise.