H.B. Tripathi

Bio: H.B. Tripathi is an academic researcher from Kumaun University. The author has contributed to research in topics: Excited state & Emission spectrum. The author has an hindex of 19, co-authored 34 publications receiving 927 citations.

Ground and Excited State Intramolecular Proton Transfer in Salicylic Acid: an Ab Initio Electronic Structure Investigation

Abstract: Energetics of the ground and excited state intramolecular proton transfer in salicylic acid have been studied by ab initio molecular orbital calculations using the 6-31G** basis set at the restricted Hartree -Fock (RHF) and configuration interaction-single excitation (CIS) levels and also using the semiempirical method AM1 at the RHF level as well as with single and pair doubles excitation configuration interaction spanning eight frontier orbitals (PECI ) 8). The ab initio potential energy profile for intramolecular proton transfer in the ground state reveals a single minimum corresponding to the primary form. In the first excited singlet state, however, there are two minima corresponding to the primary and tautomeric forms, separated by a barrier of 6 kcal/mol, thus accounting for dual emission in salicylic acid. Electron density changes with electronic excitation and tautomerism indicate no zwitterion formation. Changes in spectral characteristics with change in pH, due to protonation and deprotonation of salicylic acid, are also accounted for, qualitatively. Although the AM1 calculations suggest a substantial barrier for proton transfer in the ground as well as the first excited state of SA, it predicts the transition wavelength in near quantitative accord with the experimental results for salicylic acid and its protonated and deprotonated forms.

Fluorescence Studies of Salicylic Acid Doped Poly(vinyl alcohol) Film as a Water/Humidity Sensor

Abstract: Photoluminescence of salicylic acid (SA) and its sodium salt in poly(vinyl alcohol) (PVA) film and its quenching by water/moisture has been studied by steady-state and time domain fluorescence measurements. The results suggest that salicylic acid is completely ionized and present as a monoanion in PVA film, having a molar extinction coefficient (max) of 3545 M -1 cm -1 , and its emission shows a large Stokes shifted (8300 cm -1 ) fluorescence band with a quantum yield (f) of 0.34 and a decay time (Uf) of 6.7 ns. This emission band is due to excited-state intramolecular proton transfer (ESIPT) and is found to be sensitive to moisture and water contents in organic solvents. The films dipped in organic solvents, viz. dioxane, ethanol, and acetonitrile, containing water shows Stern-Volmer type fluorescence quenching. It is shown that a linear quenching of fluorescence intensity and decay time of SA doped PVA film in the presence of moisture or water in the organic solvents can be useful as an optical sensor for determination of humidity in the range 5%-85% and water contents in these solvents in the range 1%-60% with an accuracy of (0.2%. The response time of the sensor film is about 2 min, and recovery time is less than 1 min.

Photophysics and photochemistry of salicylic acid revisited

Abstract: The spectral and photophysical properties of salicylic acid in various solvents have been studied by steady state and time resolved experiments. Various emitting species have been identified. In alcohols, e.g. methanol, both the zwitterion and monoanion show emission. The addition of a small amount of acid results only in zwitterionic emission, whereas in the presence of a small amount of alkali, the monoanion is produced. Only the monoanion shows emission in water. In concentrated sulfuric acid, emission from the cation is observed, whereas in concentrated KOH solution, the dianion is the emitting species. In hydrogen bonding solvents, e.g. diethyl ether, hydrogen bonded monomers are present. In polymer matrix poly(methyl methacrylate) only monomeric emission is observed. The deactivation of the cation is similar to that of the zwitterion, indicating that the presence of an extra proton in the carboxylic group increases the nonradiative process. Hydrogen bonding as well as viscosity/rigidity of the matrix have also been found to decrease the deactivation rate in the zwitterion.

Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process

Abstract: Recently, organic fluorescent molecules harnessing the excited-state intramolecular proton transfer (ESIPT) process are drawing great attention due to their unique photophysical properties which facilitate novel optoelectronic applications. After a brief introduction to the ESIPT process and related photo-physical properties, molecular design strategies towards tailored emission are discussed in relation to their theoretical aspects. Subsequently, recent studies on advanced ESIPT molecules and their optoelectronic applications are surveyed, particularly focusing on chemical sensors, fluorescence imaging, proton transfer lasers, and organic light-emitting diodes (OLEDs).

Site-selective photoinduced electron transfer from alcoholic solvents to the chromophore facilitated by hydrogen bonding : A new fluorescence quenching mechanism

Abstract: Solute−solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750−(alcohol)n complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the ...

Excited-state proton transfer reactions I. Fundamentals and intermolecular reactions

Abstract: Theoretical models that have been proposed and applied to proton transfer reactions are reviewed in this work. Simple models, like the Eigen model, Marcus theory and the intersecting state model, are applied to excited-state intermolecular proton transfers. The kinetics and thermodynamics of proton transfers occuring in the singlet states of aromatic molecules with OH, NH3+, NH2 and CO substituents are reviewed.

Heptamethine cyanine dyes with a large stokes shift and strong fluorescence: a paradigm for excited-state intramolecular charge transfer.

Abstract: New heptamethine cyanine dyes with an alkylamino group at the central position were found to exhibit a large Stokes shift (>140 nm) and strong fluorescence. They were suggested to be a new paradigm for excited-state intramolecular charge transfer (ICT). The configuration change of the bridgehead amine accompanying ICT was investigated in different viscosity and pH media.