The real‐time dynamics of hydrogen‐atom‐transfer processes under collisionless conditions are studied using femtosecond depletion techniques. The experiments focus on the methyl salicylate system, which exhibits ultrafast hydrogen motion between two oxygen atoms due to molecular tautomerization, loosely referred to as intramolecular ‘‘proton’’ transfer. To test for tunneling and mass effects on the excited potential surface, we also studied deuterium and methyl‐group substitutions. We observe that the motion of the hydrogen, under collisionless conditions, takes place within 60 fs. At longer times, on the picosecond time scale, the hydrogen‐transferred form decays with a threshold of 15.5 kJ/mol; this decay behavior was observed up to a total vibrational energy of ∼7200 cm−1. The observed dynamics provide the global nature of the motion, which takes into account bonding before and after the motion, and the evolution of the wave packet from the initial nonequilibrium state to the transferred form along the O–H—O reaction coordinate. The vibrational periods (2π/ω) of the relevant modes range from 13 fs (the OH stretch) to 190 fs (the low‐frequency distortion) and the motion involves (in part) these coordinates. The intramolecular vibrational‐energy redistribution dynamics at longer times are important to the hydrogen‐bond dissociation and to the nonradiative decay of the hydrogen‐transferred form.

/pdf/femtosecond-real-time-probing-of-reactions-ix-hydrogen-atom-4309q6qr37.pdf

Femtosecond real‐time probing of reactions. IX. Hydrogen‐atom transfer

A comprehensive spectroscopic and photophysical study of the keto and leuco forms of indigo and three other ring-substituted derivatives in solution was performed. The characterization involves absorption, fluorescence, and triplet-triplet absorption spectra, making it possible to obtain the quantum yields for fluorescence (F), singlet-triplet intersystem crossing (ISC), internal conversion (IC), and lifetimes for fluorescence (UF) and triplet decay (UT). For the case of the keto forms, pulse radiolysis experiments have revealed the existence of a triplet acceptor (from energy transfer from different donors) for the indigo, purple, and indirubin compounds. It is shown that with the keto form the major deactivation pathway involves internal conversion from the lowest singlet excited state to the ground state whereas with the leuco form there is competition between internal conversion, triplet formation, and fluorescence deactivation processes. Furthermore, leuco forms present much higher Stokes shifts compared with keto ones, suggesting an excitedstate geometry different from the ground-state geometry, possibly involving rotational photoisomerization.

/pdf/photophysical-and-spectroscopic-studies-of-indigo-473s9suhqm.pdf

Photophysical and Spectroscopic Studies of Indigo Derivatives in Their Keto and Leuco Forms

The spectral parameters of the fluorescence emission, produced during the photoinduced intramolecular transfer of a proton (or hydrogen atom) in derivatives of 2-(2'-hydroxyphenyl)imidazole and 2-(2'-hydroxyphenyl)benzimidazole, were studied in several solvents at room temperature. In dry, non-polar solvents,only a single fluorescence band is observed, in the blue—green range, and is attributed to the tautomer produced by the shift of a hydrogen atom along the intramolecular hydrogen bond joining the phenolic oxygen and the imino N atom. In weak hydrogen-bonding solvents an additional UV (approximately 350 nm) band can be detected in both families of dyes. This is due to a small fraction of ground state open-enol form, which does not yield the excited state phototautomerization because the intramolecular hydrogen bond is missing. Several chromophores described here present good properties as “proton transfer” lasers, when pumped with pulses from an XeCl (308 nm) laser, e.g. the compound 4,5-dimethyl-2-(2'-hydroxyphenyl)imidazole, which shows an energy efficiency of 12%. The synthetic and anlytical details of all the dyes studied are described.

Proton transfer spectroscopy of 2-(2'-hydroxyphenyl)imidazole and 2-(2'-hydroxyphenyl)benzimidazole dyes

The photophysics of indigo as well as of bispyrroleindigo, the basic chromophore of indigo, has been investigated with ab initio electronic-structure calculations. Vertical electronic excitation energies and excited-state potential-energy profiles have been calculated with the CASSCF, CASPT2 and CC2 methods. The calculations reveal that indigo and bispyrroleindigo undergo intramolecular single-proton transfer between adjacent N–H and CO groups in the 1ππ* excited state. The nearly barrierless proton transfer provides the pathway for a very efficient deactivation of the 1ππ* state via a conical intersection with the ground state. While a low-lying S1–S0 conical intersection exists also after double-proton transfer, the latter reaction path exhibits a much higher barrier. The reaction path for trans → cisphotoisomerizationvia the twisting of the central CC bond has been investigated for bispyrroleindigo. It has been found that the twisting of the central CC bond is unlikely to play a role in the photochemistry of indigo, because of a large potential-energy barrier and a rather high energy of the S1–S0 conical intersection of the twisted structure. These findings indicate that the exceptional photostability of indigo is the result of rapid internal conversion via intramolecular single-proton transfer, combined with the absence of a low-barrier reaction path for the generation of the cis isomer via trans → cisphotoisomerization.

/pdf/molecular-mechanisms-of-the-photostability-of-indigo-vpo9j99flk.pdf

Molecular mechanisms of the photostability of indigo

Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle...

Excited-State Proton Transfer in Indigo

Picosecond spectroscopy of intramolecular hydrogen bonds in 4,4',7,7'-tetramethylindigo

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Picosecond spectroscopy of intramolecular hydrogen bonds in 4,4',7,7'-tetramethylindigo