Panwang Zhou

Fluoride Anion Sensing Mechanism of 2-Ureido-4[1H]-pyrimidinone Quadruple Hydrogen-Bonded Supramolecular Assembly: Photoinduced Electron Transfer and Partial Configuration Change

The fluoride anion sensing mechanism of 6-methyl-5-(9-methylene-anthracene)-(2-butylureido-4[1H]-pyrimidinone) (AnUP) has been investigated using the DFT/TDDFT method. The theoretical results indicate that the proton of the N3-H3 group in pyrimidine moiety is captured by the added fluoride anion and then deprotonated. The calculated vertical excitation energies of AnUP-dimer and its deprotonated form agree well with the experimental results. The molecular orbital analysis demonstrates that the first excited state (S1) of AnUP-dimer is a local excited state with a π-π* transition, whereas for the deprotonated form, S1 is a completely charge-separation state and is responsible for the photoinduced electron transfer (PET) process. The PET process from anthracene to the pyrimidine moiety leads to the fluorescence quenching.

A DFT/TDDFT study of the excited state intramolecular proton transfer based sensing mechanism for the aqueous fluoride chemosensor BTTPB

The sensing mechanism of the aqueous fluoride chemosensor N-(3-(benzo[d]thiazol-2-yl)-4-(tert-butyldiphenyl silyloxy)phenyl)-benzamide (BTTPB) has been studied in detail by DFT/TDDFT methods. The desilylation reaction which has a moderate transition barrier of 17.6 kcal mol−1 and the excited state intramolecular proton transfer (ESIPT) of the desilylation reaction product (3-BTHPB) work together for the fluorescent sensing mechanism. The constructed potential energy curves among the optimized 3-BTHPB (enol form) and 3-BTHPB-e (keto form) geometries on the S0 and S1 states, indicated that the ESIPT is a low barrier process (0.1 kcal mol−1), and the energies of the optimized geometries showed that the ESIPT process is exothermic. The calculated vertical excitation energies in the ground state and the first singlet excited state reproduced the experimental UV-Vis absorbance and fluorescence emission spectra well.

New Insights into the Dual Fluorescence of Methyl Salicylate: Effects of Intermolecular Hydrogen Bonding and Solvation

In this paper, we propose a new and complete mechanism for dual fluorescence of methyl salicylate (MS) under different conditions using a combined experimental (i.e., steady-state absorption and emission spectra and time-resolved fluorescence spectra) and theoretical (i.e., time-dependent density function theory) study. First, our theoretical study indicates that the barrier height for excited state intramolecular proton transfer (ESIPT) reaction of ketoB depends on the solvent polarity. In nonpolar solvents, the ESIPT reaction of ketoB is barrierless; the barrier height will increase with increasing solvent polarity. Second, we found that, in alcoholic solvents, intermolecular hydrogen bonding plays a more important role. The ketoB form of MS can form two hydrogen bonds with alcoholic solvents; one will facilitate ESIPT and produce the emission band in the blue region; the other one precludes ESIPT and produces the emission band in the near-UV region. Our proposed new mechanism can well explain previous results as well as our new experimental results.

The invalidity of the photo-induced electron transfer mechanism for fluorescein derivatives

The ground and excited state geometries, the excitation and emission energies for a series of fluorescein derivatives in aqueous solutions have been investigated using time-dependent density functional theory (TD-DFT) with B3LYP and a long-range corrected CAM-B3LYP functional. The RI-CC2 method was employed to confirm the CAM-B3LYP results. As far as we know, the excited state geometries for a series of fluorescein derivatives were optimized for the first time, and the conformational changes upon photoexcitation were discussed. Importantly, the previous proposed photo induced electron transfer (PeT) mechanism for dictating the fluorescence quantum yield (Φ(fl)) of fluorescein derivatives was not fully supported by our calculations. Internal conversion may still be the most likely mechanism for dictating the Φ(fl) of fluorescein derivatives, which indicates a need for further experimental and theoretical studies on the excited state dynamics of fluorescein derivatives.

Experimental and theoretical study on the sensing mechanism of a fluorescence probe for hypochloric acid: a Se⋯N nonbonding interaction modulated twisting process

In this article, the sensing mechanism of a fluorescence probe for hypochloric acid, NI-Se, has been investigated using experimental and theoretical methods. Based on the results of the steady-state and time-resolved emission spectra of NI-Se and its oxidized form NI-SeO, we suggested that there was twist internal charge transfer (TICT) state with faint fluorescence in NI-Se. Subsequently, the ground and excited state minimum geometries of NI-Se and NI-SeO were optimized with DFT/TD-DFT methods. The results demonstrated there was a twisting process in the excited state of NI-Se and that this twist process was induced by the nonbonding interaction between the Se and N atoms. In addition, the calculated spectra and molecular orbitals confirmed the charge transfer character of the TICT state in NI-Se. To further investigate the driving force behind the twist in NI-Se, we synthesized NI-O, which has no Se···N nonbonding interaction, as a control sample. Herein, we also present the characterization, fluorescence properties and the optimized geometries of NI-O. Moreover, the results showed that Se···N nonbonding interaction plays a significant role in the twisting process of NI-Se.

The Photoisomerization of 11-cis-Retinal Protonated Schiff Base in Gas Phase: Insight from Spin-Flip Density Functional Theory

This extensive theoretical study employed the spin‐flip density functional theory (SFDFT) method to investigate the photoisomerization of 11‐cis‐retinal protonated Schiff base (PSB11) and its minimal model tZt‐penta‐3,5‐dieniminium cation (PSB3). Our calculated results indicate that SFDFT can perform very well in describing the ground‐ and excited‐state geometries of PSB3 and PSB11. We located the conical intersection (CI) point and constructed the photoisomerization reaction path of PSB3 and PSB11 by using the SFDFT method. To further verify the SFDFT results, we computed the energy profiles along the constructed linearly interpolated internal coordinate (LIIC) pathways by using high‐level theoretical methods, such as the EOM‐CCSD, CR‐EOM‐CCSD(T), CASPT2, NEVPT2, and XMCQDPT2 methods. The SFDFT method predicts that the photoisomerization of PSB3 is barrierless, in accordance with previous complete‐active‐space self‐consistent‐field (CASSCF) results. However, an energy barrier is predicted along the LIIC pathways of PSB11. This finding is different from previous CASSCF results and may indicate that the photoisomerization of PSB11 in gas phase is similar to that in solution. However, the higher spin contamination of the SFDFT method in the vicinity of the CI point caused the located CI geometry to deviate from that of the real CI. In addition, the LIIC pathways are only approximations to the minimum energy path (MEP). Thus, further experimental and theoretical studies are needed to verify the existence of an energy barrier along the photoisomerization reaction path of PSB11 in gas phase.

Theoretical study on photoisomerization effect with a reversible nonlinear optical switch for dithiazolylarylene.

DFT and TDDFT methods have been performed to investigate the photoisomerization effect for dithiazolylarylene on solution. The weak S···N interaction and CH···N hydrogen bond restrain the rotation of the side-chain thiazolyl ring in open-isomer 1a, the higher stability of which prefers to show a high quantum yield of photoisomerization. The calculated UV-Vis spectrum at around 320 nm for open-isomer 1a is bathochromically shifted to 647 nm for closed-isomer 1b, in excellent agreement with the experimental photochromic phenomenon. The electron transition in ECD (electron circular dichroism) spectra for closed-isomer 1b with two chiral carbon atoms is dominated by ICT (intramolecular charge transition) and LE (local excitation) corresponding to one positive (440 nm) and one negative Cotton effect (650 nm), respectively, where the two chiral carbon atoms play a slight role in these transitions. The PES in the S(1) and S(0) states, respectively, indicates that the cyclization reaction from open-isomer 1a to closed-isomer 1b is allowed in the photoexcited state with high-conversion quantum efficiency, while it is forbidden in the thermodynamic process. In addition, the second-order nonlinear optical response for closed-isomer 1b is nearly six times larger than that for open-isomer 1a. It is also confirmed that the photoirradiation evokes the photoisomerization character to show dramatic difference in the second-order NLO response, which can be applied to designing photochromic materials and reversible NLO switches.

Non-adiabatic dynamics of isolated green fluorescent protein chromophore anion

On-the-fly ab initio molecular dynamics calculations have been performed to investigate the relaxation mechanism of green fluorescent protein chromophore anion under vacuum. The CASSCF surface hopping simulation method based on Zhu-Nakamura theory is applied to present the real-time conformational changes of the target molecule. The static calculations and dynamics simulation results suggest that not only the twisting motion around bridging bonds between imidazolinone and phenoxy groups but the strength mode of C=O and pyramidalization character of bridging atom are major factors on the ultrafast fluorescence quenching process of the isolated chromophore anion. The abovementioned factors bring the molecule to the vicinity of conical intersections on its potential energy surface and to finish the internal conversion process. A Hula-like twisting pattern is displayed during the relaxation process and the entire decay process disfavors a photoswitching pattern which corresponds to cis-trans photoisomerization.

Synthesis and Characterization of Phenothiazine-Based Platinum(II)-Acetylide Photosensitizers for Efficient Dye-Sensitized Solar Cells.

Three new unsymmetrical phenothiazine-based platinum(II) bis(acetylide) complexes PT1-PT3 with different electron-donating arylacetylide ligands were synthesized and characterized. Their photophysical, electrochemical, and photovoltaic properties have been fully investigated and the density functional theory (DFT) calculations have been carried out. Under AM 1.5 irradiation (100 mW cm(-2)), the PT1-based dye-sensitized solar cell (DSSC) device exhibited an attractive power conversion efficiency (η) up to 5.78 %, with a short-circuit photocurrent density (J(sc)) of 10.98 mA cm(-2), an open-circuit photovoltage (V(oc)) of 0.738 V, and a fill factor (ff) of 0.713. These findings provide strong evidence that platinum-acetylide complexes have great potential as promising photosensitizers in DSSC applications.

Solvent Effects on 3-Keto-1H-pyrido[3,2,1-kl]phenothiazine Fluorescence in Polar and Protic Solvents

Solvent effects on 3-keto-1H-pyrido[3,2,1-kl]phenothiazine (PTZ-5) fluorescence in aprotic and protic solvents were investigated using the time-dependent density functional theory method. Enhancement of PTZ-5 fluorescence in the polar aprotic solvent is caused by the decoupling of the two proximity low-lying excited singlet states. In polar protic solvents, aside from the solvent polarity effects, hydrogen-bonding effects are significant to the enhancement of fluorescence. Hydrogen bonds confine C-H out-of-plane bending and severely decouple the two low-lying excited singlet states, greatly decreasing nonradiative decay. The excited state intermolecular proton transfer model is considered a quenching reaction for excited PTZ-5; however, it is uncompetitive in excited-state dynamics.