Excited-state hydrogen bonding and deprotonation of esculetin in solution: a DFT/TDDFT study

The role played by solvent polarity in regulating the competitive mechanism between ESIPT and TICT of coumarin (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one)

Excited-state intramolecular proton transfer (ESIPT) and twist intramolecular charge transfer (TICT) are the two most fundamental dynamic processes, ubiquitous in biological and chemical reactions. The excited-state properties of (E-8-((4-dimethylamino-phenylimino)-methyl)-7-hydroxy-4-methyl-2H-chromen-2-one (CDPA) in various solvents with different polarities were investigated by using steady-state and femtosecond transient absorption spectroscopy combined with DFT/TDDFT calculations. The results demonstrated that CDPA exhibited low fluorescence in polar acetonitrile (ACN) due to ESIPT but high fluorescence in nonpolar n-Hexane was attributed to intramolecular rotation blocking ESIPT. TDDFT calculations confirmed that the dramatic phenyl group torsional of CDPA in Hexane, whereas a near planar conformation in ACN solvent. The ESIPT barrier decreases regularly with the increase of solvent polarity from n-Hexane, tetrahydrofuran to ACN solvent. These results demonstrated that the ESIPT and TICT processes of CDPA are competitive mechanisms. Our work revealed the effect solvent polarity on the emission behavior and excited-state deactivation mechanism of CDPA, which could help to design and develop new polarity probe in the microenvironments.

Solvent viscosity induces twisted intramolecular charge transfer state lifetime tunable of Thioflavin-T

Excited-state deactivation dynamics of Thioflavin-T (ThT) in gradual viscosity solvents were investigated. Femtosecond transient absorption spectra and dynamic decay curves both present significant distinction of ThT in different volume ratios binary mixtures solvents. Dynamics fitting lifetime of twisted intramolecular charge transfer (TICT) state is strongly dependent on solvents viscosity. Compared to rotation corresponding time of ThT in low viscosity solvent (0.6 cp) experimentally coincident well with Stokes-Einstein-Debye (SED) equation, the relation between rotation corresponding time and relatively high viscosity (5.9 cp to 1091.2 cp) is more consistent with fractional SED equation. Combined with optimized geometric structures of ThT by density functional theory and time-dependent density functional theory, further understand TICT state lifetime increases with increasing solvents viscosity. Our work provides a comprehensive understanding of fluorescence molecular rotor (FMR) deactivation process in different viscosity solvents and is helpful to design new FMR.

TDDFT study of twisted intramolecular charge transfer and intermolecular double proton transfer in the excited state of 4'-dimethylaminoflavonol in ethanol solvent

Time-dependent density functional theory method at the def-TZVP/B3LYP level was employed to investigate the intramolecular and intermolecular hydrogen bonding dynamics in the first excited (S1) state of 4'-dimethylaminoflavonol (DMAF) monomer and in ethanol solution. In the DMAF monomer, we demonstrated that the intramolecular charge transfer (ICT) takes place in the S1 state. This excited state ICT process was followed by intramolecular proton transfer. Our calculated results are in good agreement with the mechanism proposed in experimental work. For the hydrogen-bonded DMAF-EtOH complex, it was demonstrated that the intermolecular hydrogen bonds can induce the formation of the twisted intramolecular charge transfer (TICT) state and the conformational twisting is along the C3-C4 bond. Moreover, the intermolecular hydrogen bonds can also facilitate the intermolecular double proton transfer in the TICT state. A stepwise intermolecular double proton transfer process was revealed. Therefore, the intermolecular hydrogen bonds can alter the mechanism of intramolecular charge transfer and proton transfer in the excited state for the DMAF molecule.

On the nature of non-covalent interactions in isomers of 2,5-dichloro-1,4-benzoquinone dimers – ground- and excited-state properties

Impact of intermolecular NCIs (C–H⋯O, C–H⋯Cl, C–Cl⋯O, C–Cl⋯Cl–C, C–O⋯C, C–Cl⋯C and C–O⋯π) on chirality arrangement in the isomers of the 2,5-dichloro-1,4-benzoquinone (DCBQ) dimer.

A DFT/TDDFT study on the excited-state hydrogen bonding dynamics of 6-aminocoumarin in water solution

Intermolecular hydrogen bonding, as a site-specific solute-solvent interaction, plays an extremely important role on the spectra of chemical and biological molecules. A detailed theoretical study on the hydrogen bonds is extremely necessary for understanding the unusual spectra blue-shift for some molecules. In the present paper, the hydrogen bonding dynamics of 6-aminocoumarin (6AC) in the water was investigated by the time-dependent density functional theory (TDDFT) and density functional theory (DFT) methods. According to our results, it is demonstrated that C6-N1⋯H9 bond (AHB) is the strongest hydrogen bonds among three ones formed between 6AC and water molecules in the ground state. Upon the photoexcitation, due the photo-induced charge transfer from N1 to C6, AHB is proved to be extremely weakened which is significantly greater than the strengthening of C9=O2⋯H11 bond (BHB) and N1-H6⋯O5 bond (CHB) in S1 states. This is expected to be theoretical explanation for the unusual blue-shift of a long-wavelength band in absorption spectra for 6AC in the water observed in the experiment.