Combined TDDFT and AIM Insights into Photoinduced Excited State Intramolecular Proton Transfer (ESIPT) Mechanism in Hydroxyl- and Amino-Anthraquinone Solution

Revealing the synergetic interaction between amino and carbonyl functional groups and their effect on the electronic and optical properties of carbon dots

Nitrogen and oxygen-based functionalized carbon dots (CDs) surfaces have attracted significant attention due to their ability to tailor the optical and electronic properties of CDs.

Design of Large Stokes Shift Fluorescent Proteins Based on Excited State Proton Transfer of an Engineered Photobase

The incredible potential for fluorescent proteins to revolutionize biology has inspired the development of a variety of design strategies to address an equally broad range of photophysical characteristics, depending on potential applications. Of these, fluorescent proteins that simultaneously exhibit high quantum yield, red-shifted emission, and wide separation between excitation and emission wavelengths (Large Stokes Shift, LSS) are rare. The pursuit of LSS systems has led to the formation of a complex, obtained from the marriage of a rationally engineered protein (human cellular retinol binding protein II, hCRBPII) and different fluorogenic molecules, capable of supporting photobase activity. The large increase in basicity upon photoexcitation leads to protonation of the fluorophore in the excited state, dramatically red-shifting its emission, leading to an LSS protein/fluorophore complex. Essential for selective photobase activity is the intimate involvement of the target protein structure and sequence that enables Excited State Proton Transfer (ESPT). The potential power and usefulness of the strategy was demonstrated in live cell imaging of human cell lines.

Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV-Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K_T in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol^-1 of 4 and 0.63 kcal mol^-1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Complementary Effect of Intra- and Intermolecular Hydrogen Bonds on Electron Transfer in β-Hydroxy-Anthraquinone Derivatives

We have carried out an electrochemical and theoretical study on the relationship between electron transfer (ET) and hydrogen bonding in 11 different 9,10-anthraquinone (AQ) derivatives, including β-hydroxy AQs and their methoxylated analogs, in the presence of hydrogen donors in acetonitrile (ACN). The complementary effects of the intra- and intermolecular hydrogen bonds (HBs) on ET were studied by analyzing the complex formation constants derived from the intermolecular HB. Our results revealed that the inductive effect of the β substituent that indirectly controls the charge distribution on the AQ carbonyl oxygen, and steric hindrance at the β position, affect the complex formation constants. Furthermore, the analysis of ET in different isomeric dihydroxy AQs suggested that the position of the hydroxy groups affects the charge distribution and stabilizes structures through conjugation of the quinone moiety including the ipso ring, controlling the intra- and intermolecular HBs complementarily.

Effect of number and different types of proton donors on excited-state intramolecular single and double proton transfer in bipyridine derivatives: theoretical insights

Intra-HBs are strengthened upon photoexcitation, confirmed by red-shift in vibrational mode and topology analysis. Number and type of donors result in difference in photophysical properties. Occurrence of ESIPT depends on barrier and reaction energy.

UV synchrotron radiation linear dichroism spectroscopy of the anti-psoriatic drug anthralin

Anthralin (1,8-dihydroxyanthrone, 1,8-dihydroxy-9(10H)-anthracenone), also known as dithranol and cignolin, is one of the most efficient drugs in the treatment of psoriasis and other skin diseases. The precise mode of biochemical action is not fully understood, but the activity of the drug is increased by the influence of UV radiation. In the present investigation, the UV absorption of anthralin is studied by synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched polyethylene, covering the near and vacuum UV regions with wavenumbers ranging from 23,000 to 58,000 cm–1(430–170 nm). The observed polarization spectra are well predicted by quantum chemical calculations using time-dependent density functional theory (TD–DFT). About a dozen spectral features are assigned to computed electronic transitions. The calculations support interpretation of the anomalous fluorescence of anthralin as a result of barrier-less excited state intramolecular proton transfer (ESIPT) to the tautomer 8,9-dihydroxy-1(10H)-anthracenone.

Effect of substitution on the excited state photophysical and spectral properties of boron difluoride curcumin complex dye and their derivatives: A time dependent-DFT study

The optical, charge transport and electronic properties of boron difluoride curcumin (BFC) complex have been explored using the DFT (Density Functional Theory) method and B3LYP functional with the combination of 6-31 + G(d,p) as a basis set. The influence of substitution with various electron releasing and withdrawing groups on the above properties is analyzed and discussed in this work. The results reveal that the BFC complex on additional electron releasing substitution experiences redshifts in the optical transitions, and this is correlated with the dipole moment, NBO charges, HOMO-LUMO energy gap. Further, the absorption (λ_abs) and emission (λ_ems) spectra of substituted and unsubstituted BFCs are calculated using Time-Dependent Density Functional Theory (TD-DFT). The results show that the electron releasing groups strongly influence the absorption and emission spectra of BFC. Electron releasing groups in BFC derivatives generate the wavelength shift (Bathochromic), but the electron-withdrawing groups in BFC don't affect the λ_abs and λ_ems when compare to its original (parent) compound. The output of the research work strongly recommends that the amino, phenyl and N, N'-dimethylamino derivatives are potential candidates to act as fluorescent materials due to enhance the emission behavior of BFC and also can be used as an electron/charge transport material for organic light-emitting diodes (OLEDs).

Facile Synthesis of an Organic Solid State Near-Infrared-Emitter with Large Stokes Shift via Excited-State Intramolecular Proton Transfer

An organic solid-state near-infrared (NIR)-emitter (λ_em = 738 nm) exhibiting large Stokes shift (Δλ = 293 nm) through the excited-state intramolecular proton transfer phenomenon has been synthesized and characterized. The present discovery points to the possibility of achieving a new family of solid-state NIR emitters starting from simple aldehyde and amine precursors.