Excited-state proton transfer based fluorescence in Kaempferol powder and solutions with different concentrations

Kaempferol (KMP) is one of the most common flavonoids, currently being extensively studied for its numerous beneficial health effects. Here we study the fluorescence (FL) emission of KMP powder and of its solutions prepared using different types of solvents (polar and non-polar). In the spectra of KMP powder and KMP solutions with high concentration, the same FL peak with maximum at 1.9 eV is observed. Another FL peak, at higher energy of 2.45 eV, emerges in solutions, its relative intensity increases with decreasing solution concentration. The FL emission of solutions with lowest concentration displays only that peak. To calculate characteristic energies of absorption and emission of KMP molecule in vacuum and in solutions we use time-dependent density functional theory. Comparing the results of computations with measured FL spectra, we associate the FL band at 1.9 eV with the emission due to excited state intramolecular transfer of the proton of -OH5 hydroxyl group. The FL emission at 2.45 eV is related to the -OH3 proton transfer. We measure the FL spectra of KMP powder using two different excitation energies, 3.06 eV and 2.33 eV, and find that its FL spectrum depends on the excitation energy. To understand that dependence, we compare the FL spectra of KMP and Q monohydrate powders. We consider the excited state intermolecular transfer of the proton from -OH3' hydroxyl group to a neighboring molecule in Q crystal and calculate the energy corresponding to the emission of the resulted anion of Q molecule. The spectral feature at 1.69 eV observed only in the FL spectrum of Q hydrate is attributed to the Q anion FL emission.

Multi-analyte fluorescence sensing based on a post-synthetically functionalized two-dimensional Zn-MOF nanosheets featuring excited-state proton transfer process

Covalent post-synthetic modification of metal-organic frameworks (MOFs) represents an underexplored but promising avenue for allowing the addition of specific fluorescent recognition elements to produce the novel MOF-based sensory materials with multiple-analyte detection capability. Here, an excited-state proton transfer (ESPT) active sensor 2D-Zn-NS-P was designed and constructed by covalent post-synthetic incorporation of the excited-state tautomeric 2-hydroxypyridine moiety into the ultrasonically exfoliated amino-tagged 2D Zn-MOF nanosheets (2D-Zn-NS). The water-mediated ESPT process facilitates the highly accessible active sites incorporated on the surface of 2D-Zn-NS-P to specifically respond to the presence of water in common organic solvents via fluorescence turn-on behavior, and accurate quantification of trace amount of water in acetonitrile, acetone and ethanol was established using the as-synthesized nanosheet sensor with the detection sensitivity (<0.01% v/v) superior to the conventional Karl Fischer titration. Upon exposure to Fe3+ or Cr2O72-, the intense blue emission of the aqueous colloidal dispersion of 2D-Zn-NS-P was selectively quenched even in the coexistence of common inorganic interferents. The prohibition of the water-mediated ESPT process and local emission, induced by the coordination of ESPT fluorophore with Fe3+ or by Cr2O72- competitively absorbs the excitation energy, was proposed to responsible for the fluorescence turn-off sensing of the respective analytes. The present study offers the attractive prospect to develop the ESPT-based fluorescent MOF nanosheets by covalent post-synthetic modification strategy as multi-functional sensors for detection of target analytes.

Solvent assisted excited-state deactivation pathways in isolated 2,7-diazaindole-S1-3 (S = Water and Ammonia) complexes

The role of solvent molecules in the deactivation of photo-excited 2,7-diazaindole (DAI) - (H₂O)_1-3 and DAI - (NH₃)_1-3 complexes were computationally investigated. An excited-state proton transfer (ESPT) path from the solvent to the DAI molecule was followed using the TD-DFT-D4 (B3LYP) level of theory. The computed potential energy profile of ESPT process has shown intersection between ππ* and nπ* states facilitated via relative stabilization of the nπ* state with decreasing N(7)-H_b bond length. The ESPT process, starting from the DAI-S_n (ππ*) state, crosses through a barrier ranging from 27 to 36 kJmol^-1 for water complexes and 26-30 kJmol^-1 for ammonia complexes. The energy of the excited state was rapidly decreased with a shorter N(7)-H_b bond length. Subsequently, a significant trend of finding a second intersection between the ground and the excited state was observed for all the complexes. The results firmly suggested a significant deactivation channel of excited azaindole derivatives. In the present system, two competing channels, ESPT and ESHT, were found to be energetically accessible. The energy barriers associated with the ESPT barriers for DAI-(H₂O)_1-3 complexes are similar to the ESHT barrier, depicting equal dominance of both processes. The increased basicity of the N(7) atom in the excited state resulted a facile ESPT process from the water to N(7) of the DAI molecule. However, DAI-(NH₃)_1-3 complexes show clear preference for ESHT over ESPT process owing to its higher gas-phase pK_a value making it a poor proton donor. The above systems can be used as a model to computationally and experimentally investigate the competing radiative and deactivation pathways of photo-excited solvated complexes of N-H-bearing bio-relevant molecules via proton and hydrogen transfer reactions.

pH modulated luminescent switching and discriminative detection of amino acid based on metal-organic framework

The detection of glutamic (Glu) or aspartic (Asp) acids is vital for human nutrition and diagnosis of disease. Herein, the dht ligand containing hydroxy group (-OH) is used to design and synthesize a 2D luminescent [Cd₂(idc)(dht)(H₂O)₄] (1); H₂idc = 4,5-imidazoledicarboxylic acid and H₂dht = 2,5-dihydroxyterephthalic acid for sensing amino acids. The compound 1 can discriminatively detect Asp and Glu among other amino acids through blue-shifted emission (yellow → green). The dual sensing mechanism may be attributed to the intermolecular excited-state proton transfer between MOF and water to produce keto form along with the subsequent switching of keto form to enol form by protonation causing the increased band gap energy. This material can serve several benefits in terms of high selectivity, fast response (30s), good reproducibility and low LOD value of 11.34 μM which is less than the harmful concentration of Glu for human health (>400 μM). In addition, 1 shows the broad range detection of Glu covering in safe and unsafe levels. For on-site detection of Glu, MOF-based paper is devised and can be applied through color-scanning application in smartphone. Besides, this sensor can serve to detect Glu in real samples with good recovery.

The change in excited-state proton transfer kinetics of 1-naphthol i micelles upon the binding of polymers: The influence of hyaluronan hydration

The fast deprotonation of 1-naphthol was studied in aqueous solution and in polymer-cetyltrimethylammonium bromide (CTAB) mixtures using picosecond fluorescence spectroscopy to study the influence of hyaluronan hydration in polymer-surfactant interactions. The aqueous micelle solution showed the expected change of proton transfer rate around the reported critical micelle concentration (∼1 mM). The proton transfer rate dependence on CTAB concentration in the hyaluronan-CTAB and polystyrenesulfonate-CTAB systems differed significantly from that in the aqueous micelle solution. The dynamic study of excited state proton transfer (ESPT) revealed the significant influence of hyaluronan hydration in CTAB micelles. When hyaluronan-CTAB aggregates were formed at the CTAB concentration of 0.5mM, a tenfold decrease in the rate of deprotonation was observed when compared to polystyrenesulfonate-CTAB aggregates due to hyaluronan hydration. In 2mM CTAB-hyaluronan aggregates, the rate of deprotonation was found to be almost two times faster than in the 2mM CTAB or polystyrenesulfonate-CTAB system. Furthermore, the study of excited-state proton transfer of 1-naphthol confirmed that hyaluronan hydration layer penetrates into the micelle and changes the emission characteristics of 1-naphthol.

Excited-state proton transfer and phototautomerism in nucleobase and nucleoside analogs: a mini-review

Intermolecular excited-state proton transfer (ESPT) has been observed in several fluorescent nucleobase and/or nucleoside analogs. In the present work, some new examples of ESPT in this class of compounds are presented together with a brief recapitulation of the previously published data. The nucleobases, nucleosides, and their analogs contain many basic and acidic centers and therefore their ESPT behavior may be complex. To interpret the complex data, it is usually necessary to determine the microscopic pK* values for each (or most) of the possible ESPT centers. Typical approach to solve this problem is by analysis of the alkyl derivatives, in which the possibility of the ESPT is reduced. Of particular interest are examples of "phototautomerization via the cation," observed in several systems, which in the neutral media do not undergo ESPT. Protonation of the molecule in the ground state facilitates the two-step phototautomerism in several systems, including formycin A and 2-amino-8-azadenine. Fluorescence of the nucleobase and nucleoside analogs undergoing ESPT is usually solvent-, isotope-, and buffer-ion sensitive, and in some systems the ESPT can be promoted by environmental factors, e.g., the presence of buffer ions. This sensitivity to the microenvironment parameters makes the ESPT systems potentially useful for biological applications.

TDDFT study of the polarity controlled ion-pair separation in an excited-state proton transfer reaction

6-Hydroxyquinoline (6HQ) is an ideal photoacid system for exploring excited-state proton transfer (ESPT) reactions. We have previously (Mahata et al. (2002)) shown that the ESPT reaction between 6HQ and trimethylamine (TMA) leads to an "unusual" emission in the 440-450 nm range, containing two decay components (∼5 ns and ∼12 ns). The observed results suggest the presence of a contact ion-pair and a solvent separated ion-pair. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) have been employed to study the nature of the contact ion-pair formed between 6HQ and TMA and to determine why the decay component ∼12 ns is absent in a non-polar solvent. Calculations of the hydrogen-bonded complexes formed between 6HQ and TMA and its ESPT reaction product, namely 6HQ-TMA and 6HQ-TMA-PT, respectively, have been carried out, both in the electronic ground and excited states. Moreover, by using the CPCM model, different dielectric constants have been introduced into the calculations. On increasing the dielectric constant, the hydrogen bond in 6HQ-TMA-PT becomes weaker and the hydrogen bond length becomes larger; this effectively facilitates the proton transfer reaction and formation of separated ion-pair. Thus, the separation and diffusion of the contact ion-pair can be controlled by changing the polarity of the surroundings.