Fluorescence quenching phenomena facilitated by excited-state hydrogen bond strengthening for fluorenone derivatives in alcohols

Study on photophysical and aggregation induced emission recognition of 1,8-naphthalimide probe for casein by spectroscopic method

A novel water-soluble 1,8-naphthalimide derivative 1, bearing two acetic carboxylic groups, exhibited fluorescent turn-on recognition for casein based on the aggregation induced emission (AIE) character. The photophysical properties of 1 consisting of donor and acceptor units were investigated in different solutions. The fluorescence intensity decreased through taking advantage of twisted intramolecular charge transfer (TICT) and self-association emission with increasing solvent polarity. Moreover, the spectral red-shift and intensity quench in protic solvents were caused by the excited-state hydrogen bond strengthening effect. Density Functional Theory (DFT) calculations revealed that 1 exhibited a strong TICT character. The AIE mechanism of 1 with casein was due to 1 docked in the hydrophobic cavity between sub-micelles and bound with Tyr and Trp residues, resulting in the aggregation of 1 on the casein surface and emission enhancement. Based on this, a novel casein assay method was developed. The proposed exhibited a good linear range from 0.1 to 22 μg mL(-1), with the detection limit of 2.8 ng mL(-1). Satisfactory reproducibility, reversibility and a short response time were realized. This method was applied to the determination of casein in milk powder samples and the results were in good agreement with the result of Biuret method.

Fluorescence of tryptophan in aqueous solution

In this work, the absorption and emission spectra of Tryptophan (Trp) in aqueous solution were studied. Moreover, a hydrogen-bonded zwitterionic Trp(H2O)9 model was proposed and its ground-state and excited-state properties were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, respectively. All spectroscopic data in our experiments can be well explained by the hydrogen bond strengthening in the excited state of the model complex. The delocalization of electron density between indole moiety and neighboring H2O molecules in fluorescent state was proposed to be facilitated by the strengthened hydrogen-bond chain, and thus resulting in the large red-shift fluorescence of Trp in aqueous solution.

Effects of intermolecular hydrogen bonding on the excited-state proton transfer properties of the cinnamonitrile–methanol complex

The time-dependent density functional theory (TD-DFT) method was used to study the excited-state proton transfer (ESPT) properties of the hydrogen-bonded cinnamonitrile (3TPAN)–methanol (MeOH) complex (3TPAN–MeOH). The intermolecular hydrogen bonds N1···H11 in both the ground state S0 and the excited state S1 were demonstrated by the optimized geometric structures of the hydrogen-bonded 3TPAN–MeOH complex. While in the excited state S3, a new hydrogen bond H11···O1 was formed after the ESPT took place from the hydrogen-bonded MeOH molecule to the 3TPAN moiety. It was demonstrated that the electronic transitions of the S1 states for both the 3TPAN monomer (including the S3 state) and the hydrogen-bonded 3TPAN–MeOH complex should be of a localized-excited (LE) nature on the 3TPAN molecule, while the S3 state of the hydrogen-bonded 3TPAN–MeOH complex should be of charge transfer (CT) character from the hydrogen-bonded MeOH molecule (through O1···H11) to the 3TPAN moiety. The S3-state proton transfer and charge transfer due to the intermolecular hydrogen-bonding interaction should be the reasons for the remarkable redshift (0.91 eV) of the S3-state electronic energy for the hydrogen-bonded 3TPAN–MeOH complex compared with that of the 3TPAN monomer.

A theoretical study of hydration effects on structural stability and hydrogen-bonding dynamics of 2′-deoxyguanosine 5′-monophosphate with different negative charges

The effects of hydration on the ground-state structural stability and excited-state hydrogen-bonding dynamics of 2′-deoxyguanosine 5′-monophosphate (dGMP) carrying different negative charges were investigated with B3LYP/6–31+G(d,p) using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, respectively. Particularly, we not only considered the solvent effects by the polarizible continuum model (PCM), but also the first solvation shell was included explicitly. We demonstrated that the intramolecular hydrogen bond O2–H1···O3 will be weakened with the strengthening of the hydration. From the view of bond length, we can make a valid presumption that the site of negative charge will be the more preferable site of the hydration, and the preferable site may be changed because of the presence of other hydrogen bonds. Furthermore, we found that the first solvation shell had very little effect on the geometric structures except for the hydrogen bond P–O5···H5. By comparing the excitation energies, one important finding is that the changes in different electronic states are not obvious with the increase in n value when considering the PCM. Another finding is that the average interactions of hydrogen bonds may be strengthened with an increase of negative charge because of a decrease in excitation energies.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON THE ELECTRONIC EXCITED-STATE HYDROGEN BONDING DYNAMICS OF METHYL ACETATE IN AQUEOUS SOLUTION

The time-dependent density functional theory (TDDFT) method has been carried out to investigate the hydrogen-bonding dynamics of methyl acetate ( CH 3 CO 2 CH 3) in hydrogen-donating water solvent. The ground-state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low-lying electronically-excited states for the isolated CH 3 CO 2 CH 3 and H2O monomers, the hydrogen-bonded CH3CO2CH3-(H2O)1, 2 complexes have been calculated using DFT and TDDFT methods respectively. One intermolecular hydrogen bond C=O⋯H–O is formed between CH3CO2CH3 and one water molecule in CH3CO2CH3-H2O dimer. Meanwhile, in CH3CO2CH3-(H2O)2 trimer, two intermolecular hydrogen bonds C=O⋯H–O are formed between CH3CO2CH3 and two water molecules. By theoretically monitoring the excitation energy changes among the CH3CO2CH3 monomer, the CH3CO2CH3-H2O dimer, and the CH3CO2CH3-(H2O)2 trimer, we have demonstrated interestingly that in some electronically-excited states, the intermolecular hydrogen bonds are strengthened inducing electronic spectral redshifts, while in others weakened with electronic spectral blueshifts. The phenomenon that hydrogen bonds are strengthened in some electronic states while weakened in others can arouse further probe into CH3CO2CH3-(H2O)1, 2 complexes.

Theoretical reconsideration on the hydrogen bonding and coordination interactions of chlorophyll a in aqueous solution

In the present work, explicit water molecule and solvent-field effects on the absorption spectrum of chlorophyll a have been studied using time-dependent density functional theory (TDDFT) method. Calculated results show that the one complex and two water coordinated complexes formed by concerted coordination and hydrogen-bonding interactions would be the most preferable conformations of chlorophyll a in aqueous surroundings. Moreover, four obvious absorption bands are assigned by comparing the theoretically simulated absorption spectra with the experimental ones. The theoretical study shows that the explicit water molecule interactions slightly influence the first absorption band. However, the water coordination and hydrogen-bonding interactions can significantly affect the second absorption band which has a strong red-shift. The solvent-field effect due to the polarity of water on absorptions in Q-bands is relatively smaller than that on absorptions in B-bands. As a consequence, our theoretical study on the absorption spectra in the 350–400 nm region presents that the absorption strength in this region was influenced by the explicit coordination and hydrogen bonding interactions from water molecules, significantly.

THEORETICAL STUDY OF THE HYDROGEN BOND CHARACTER BETWEEN THE FNO AND HO2 RADICAL

The hydrogen-bonding characters between FNO and HO2 radical are studied systematically with the MPWB1K method and 6-311++G (d, p), aug-cc-PVDZ as well as aug-cc-PVTZ basis sets. The relevant geometrical characteristics, energy properties, and the characters of the intramolecular hydrogen bonds have been reported in this work. In addition, the changes of electrostatic potential density are presented for further understanding the nature of hydrogen bonds and the preference of F atom as the hydrogen-bonding site.

Experimental and theoretical study of the exited state of aminostyryl terpyridine derivatives: hydrogen-bonding effects

The synthesis, structure and photophysical behavior of the aminostyryl terpyridine derivatives, named 4'-(4-{2-[4-(N,N-dimethylaniline)]vinyl}phenyl)-2,2':6',2″-terpyridine (M(1)) and the related model compounds 4'-(4-{2-[4-(N,N-diphenylammino)phenyl]vinyl}phenyl)-2,2':6',2″-terpyridine (M(2)), respectively, are reported. Large solvatochromic shifts of the first excited-state fluorescence maximum suggest the intramolecular charge transfer characters for both compounds. In addition, with N,N-dimethyl substituents, its fluorescence is quenched a lot in protic solvents. This is consisted with the decay of its S(1) state, through nonradiative internal conversion, to the ground state, which is facilitated by the formation of the hydrogen bond between M(1) and alcohols. Whereas, the introduction of N,N-diphenyl substituents has been proved to be a hydrogen-bond-free case with the unchanged Φ(f). Furthermore, the formation of TICT state via diffusive twisting motion of the dimethyl/phenylamino group is the major relaxation process, which is proved by the ultrafast relaxation dynamics experiment and theoretically conformational optimization of the first excited-state of both the compounds.