Computational insights into the underlying mechanism of zinc ion-specificity of the fluorescent probe, BDA-1.

Ion specificity is crucial for developing fluorescence probes. Using a recently reported optical sensor (BDA-1) of Zn2+ as a representative, we carried out extensive quantum chemical calculations on its photophysical properties using density function theory. According to the calculated optimized geometries, excitation energies and transition oscillator strengths, the weak fluorescence of BDA-1 observed in experiments is attributed to the suppression of fluorescence emission by efficient internal conversion, rather than the previously proposed photoinduced electron transfer (PET) mechanism. With the addition of Zn2+ or Cd2+ ions, the tetradentate chelates [M:BDA-1-H+]+ (M=Zn, Cd) are produced. According to frontier molecular orbital and interfragment charge transfer analyses of these complexes, PET is preferentially confirmed to occur upon photo-excitation. Notably, as one coordination bond in the excited [Cd:BDA-1-H+]+ complex is significantly weakened in comparison to that of [Zn:BDA-1-H+]+, their molecular orbital compositions in the S1 state are completely different. As a result, absorption and radiation transitions of [Zn:BDA-1-H+]+ both have considerable oscillator strength, while fluorescence radiation from the excited [Cd:BDA-1-H+]+ is doubly suppressed. This difference causes that the fluorescence intensity of BDA-1 is sensitive to the addition of metal ions, and exhibits the zinc ion-specificity.

Density Functional Theory Calculations on Fluorescence-Enhanced Mechanisms of the Optical Sensor for Zinc Ions, ADPA.

N-(9-anthracenylmethyl)-N-(2-pyridinylmethyl)-2-pyridinemethanamine (ADPA) as a specific ion sensor for Zn2+ has been widely applied. Although the photo-induced electron transfer (PET) mechanism was proposed previously, its fluorescence-enhanced effect still remains somewhat ambiguous, according to unknown influences of non-radiative energy decay pathways, such as intersystem crossing and internal conversion. Herein, a thorough study using density functional theory has been performed for low-lying electronic states of the ADPA monomer and hydrated ADPA-Zn2+ complex. Based on interfragment charge transfer analyses, we quantitatively calculated the amount of transferred electrons in the monomer and complex, providing solid evidences for the PET mechanism and in line with the conclusion of frontier molecular orbital analyses. Moreover, the ISC process of S1→T2 was confirmed to play a considerable role in the excitation energy relaxation process of the ADPA monomer, but this influence was significantly suppressed in the hydrated ADPA-Zn2+ complex. These results provide additional clues for the design of new metal ion-specific fluorescence probes.

Carbon dioxide activation by discandium dioxide cations in the gas phase: a combined investigation of infrared photodissociation spectroscopy and DFT calculations.

We present a combined computational and experimental study of CO2 activation at the Sc2O2+ metal oxide ion center in the gas phase. Density functional theory calculations on the structures of [Sc2O2(CO2)n]+ (n = 1-4) ion-molecule complexes reveal a typical end-on binding motif as well as bidentate and tridentate carbonate-containing configurations. As the number of attached CO2 molecules increases, activated forms tend to dominate the isomeric populations. Distortion energies are unveiled to account for the conversion barriers from molecularly bound isomers to carbonate structures, and show a monotonically decreasing trend with successive CO2 ligand addition. The infrared photodissociation spectra of target ion-molecule complexes were recorded in the 2100-2500 cm-1 frequency region and interpreted by comparison with simulated IR spectra of low-lying isomers representing distinct configurations, demonstrating a high possibility of carbonate structure formation in current experiments.

Determinant Factor for Thermodynamic Stability of Sulfuric Acid-Amine Complexes.

Atmospheric amines are thought to play significant roles in the nucleation of sulfuric acid-mediated aerosol particles. Their enhancing effects on the stabilization of the related complexes have formerly been correlated with the amine base strength, but there are a few exceptions reported. In this work, the influence of seven alkylamines on the thermodynamic stability of sulfuric acid-amine complexes has been theoretically investigated, e.g., ethylamine, propylamine, isopropylamine, tert-butylamine, dimethylamine, ethylmethylamine, and trimethylamine. For all primary and secondary amine-mediated complexes, a dual hydrogen bond configuration is generally suggested in the most stable isomer. The stabilization of this special structure predicted by the electrostatic potential distribution on the molecular surface of amines exactly agrees with the base strength sequence, providing crucial evidence for the previous deduction of correlation between the base strength and the enhancing effect. Meanwhile, the considerable van der Waals interactions are found between the free hydroxyl of sulfuric acid and the ß-methyl group of amine, resulting in the extra stability for sulfuric acid-dimethylamine and sulfuric acid-ethylmethylamine complexes. Therefore, the electrostatic potential distribution of amines is the essential determinant factor for the thermodynamic stability of the relevant complexes. Our conclusions provide new insight into a way to evaluate the enhancing abilities of amines in aerosol particle nucleation.