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Oxazolones or azlactones are small heterocyclic systems that can be easily functionalized with a double bond directly attached to their structure. In addition, they can be substituted on the opposite side of the heterocycle with styrenyl groups to extend their conjugation. This allows for versatile tuning of their linear and non‐linear optical properties. In this work, eight doubly substituted oxazolones including different push‐pull or pull‐pull designs were synthesized, and their linear and non‐linear photophysical properties were studied. As we show, the two‐photon activity of these systems is dependent on the donor‐acceptor pattern, where a methoxy substitution on the styrene end of the molecule and an acetyl group on the other end shows two‐photon Z‐E conversion properties with an important two photon absorption cross section of 308 GM. Most oxazolones shown in this study undergo picosecond relaxation to the electronic ground state, releasing the biphotonic energy to their surrounding environment. The results obtained in this work are indicative of potential applications of these molecules in schemes where high spatial patterning or thermalization is required. [ABSTRACT FROM AUTHOR]

We present a new design for multi-chromophoric molecules where the non-linear absorption properties of a central electron-donating core are used to confer indirect biphotonic absorption features to molecular actuators. This design allows for a general way to provide two-photon reactivity to molecular systems which, by themselves, could not be controlled by a non-linear optical input. The compounds are based on a two-photon-active N,N′-1,4-dihydropyrrolo[3,2-b]pyrrole central unit. This core is symmetrically substituted at the ends of its long axis by nitrophenyl groups defining a pseudo-centrosymmetric acceptor–donor–acceptor (A–D–A) antenna-unit. For this first proof-of-concept study, azo-isomerizable actuators were bonded to the central unit through the nitrogen atoms of the heterocyclic core. This design allows for the pyrrolo-pyrrole chromophore to act as a non-linear absorbing unit, so that E–Z isomerization of the actuators can be induced after excitation into higher states localized at the central unit, followed by energy transfer into acceptor-localized excited states. The non-linear isomerization of the actuators was observed using pulsed NIR light from a Ti:Sapphire laser in a setup that allows for significant accumulation of the actuator-isomerized product in macroscopic samples. We demonstrate that the two-photon photochemistry of the actuators is due to the coupling with the central core unit since models of the azo-actuator compounds alone do not undergo these transformations at the same irradiation levels. Time-resolved emission studies indicate that the population associated with higher core-centered 1ππ* states evolve towards actuator-localized in a time-scale of picoseconds. These signals and the overall photochemistry are consistent with a mechanism where the formation of the actuator-localized 1nπ* states is directly followed by the azo-isomerization reactions. Computational studies at the TD-DFT level of theory, together with the use of a quadratic response method were used to characterize the states responsible for the two-photon absorption properties. This kind of molecular assembly can be extended to induce non-linear photochemistry in other types of actuators, including dissociative systems or uncaging ligands, or to induce catalytic activity in highly localized environments. [ABSTRACT FROM AUTHOR]

We report studies of hydration dynamics at the surface of the enzyme protein bovine pancreatic [alpha]-chymotrypsin. The probe is the well known 1-anilinonaphthalene-8-sulfonate, which binds selectively in the native state of the protein, not the molten globule, as shown by x-ray crystallography. With femtosecond time resolution, we examined the hydration dynamics at two pHs, when the protein is physiologically in the inactive state (pH 3.6) or the active state (pH 6.7); the global structure and the binding site remain the same. The hydration correlation function, C(t), whose decay is governed by the rotational and translational motions of water molecules at the site, shows the behavior observed in this laboratory for other proteins, Subtilisin Carlsberg and Monellin, using the intrinsic amino acid tryptophan as a probe for surface hydration. However, the time scales and amplitudes vary drastically at the two pHs. For the inactive protein state, C(t) decays with an ultrafast component, close to bulk-type behavior, but 50% of the C(t) decays at a much slower rate, [tau] = 43 ps. In contrast, for the active state, the ultrafast component becomes dominant (90%) and the slow component changes to a faster decay, [tau] = 28 ps. These results indicate that in the active state water molecules in the hydration layer around the site have a high degree of mobility, whereas in the inactive state the water is more rigidly structured. For the substrate--enzyme complex, the function and dynamics at the probe site are correlated, and the relevance to the enzymatic action is clear.

Singlet diphenylcarbene generated by photodissociation of diphenyldiazomethane with a UV pulse at 266 nm is examined by femtosecond transient absorption spectroscopy. Study of rate of protonation of Ph(sub 2)(super 1)C in several neat alcohol solvents depicted that Ph(sub 2)(super 1)C protonation is among the fastest intermolecular proton-transfer reactions known.

The way the femtosecond pump-probe measurements made at very low pump intensities are used to isolate the lowest excited singlet (S(sub 1)) dynamics from signals arising from multiphoton processes such as water photoionization is described. The measurements presented here provide a more complete and richly detailed picture of nucleobase photophysics by directly observing the ultrafast internal conversion and vibrational cooling.

A study of photochemical deactivation pathways of electronically excited methyl viologen (1,1'-dimethyl-4,4'-bipyridinium, MV2) in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques is done. Unexpectedly diverse behavior of methyl viologen in different polar solvents is reported.

A study was conducted to analyze the ultrafast photoionization characteristics of indole in water. A white-light continuum probe pulse was utilized to obtain transient absorption measurements. Pump and probe beams were crossed in a free-flowing jet of the liquid sample. Results indicated that the bimolecular reaction between indole radical cations and solvated electrons were slower than the diffusion limit.

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field//configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T2 electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T1→T3 and T1→T5 transitions, supporting that the intermediate triplet state (T2) decays by internal conversion to T1. [ABSTRACT FROM AUTHOR]

Excess electrons were studied in liquid acetonitrile at room temperature by femtosecond pumpprobe spectroscopy. Using ~ 200 fs, 265 nm laser pulses, electrons were ejected into the liquid by photodetachment from iodide ions and, in separate experiments, by photoionization of indole. A strong and broad absorption band with a maximum near 1400 nm was observed in both systems. A second absorption band was observed at wavelengths below 620 nm for iodide solutions, but was not seen in photoexcited indole due to overlapping excited state absorption. The bands are in good agreement with ones seen previously in nanosecond pulse radiolysis experiments [I. P. Bell, M. A. J. Rodgers, and H. D. Burrows, J. Chem. Soc., Faraday Trans. 1 73, 315 (1977)]. Bell, Rodgers, and Burrows assigned the visible and IR bands to absorption by acetonitrile dimer and monomer anions, respectively. Our results strongly question this interpretation. Instead, we assign the short-wavelength absorption band to a solvent-bound valence anion consisting of one or two acetonitrile molecules and the IR band to a solvated or cavity electron. Low-level quantum chemical calculations indicate that valence anion formation is strongly correlated with CCN bending, but do not provide a clear indication of whether a monomer or dimer valence anion is favored. The highly mobile cavity electron is scavenged by added chloroform ata bimolecular reaction rate of (1.02 &pusmin; 0.03)x 10[SUP11] M[SUP-1] s[SUP-1]. The appearance of both absorption bands within our time resolution suggests that the two forms of the excess electron are produced by prompt reaction with the iodide charge-transfer-to-solvent (CTTS) excited state. In support of this mechanism, strong static scavenging by chloroform was observed at both visible and IR wavelengths. For iodide in acetonitrile, the signal in the IR decays biexponentially due to competition between geminate recombination of the cavity electron with the parent iodine atom and... [ABSTRACT FROM AUTHOR]

The dynamics of the first singlet excited state decay of I-nitronaphthalene through intersystem crossing to the triplet manifold in a series of solvents is demonstrated. Both the excited state of the I-NN molecule is found to be [pi]-[pi]* type with some aromatic-to-nitro charge-transfer character.

The femtosecond fluorescence up-conversion studies which were carried out to characterize the excited state enol to cis-keto tautomerization through measurements of the transient molecular emission are reported.

The resolved emission measurements of the singlet excited states are presented which are the precursors in the electronically exited nitrated polycyclic aromatic hydrocarbons (NPAHs). Results show that NPAHs have the largest intersystem crossing rates in an organic molecule.

Copper(I) complexes (CICs) are of great interest due to their applications as redox mediators and molecular switches. CICs present drastic geometrical change in their excited states, which interferes with their luminescence properties. The photophysical process has been extensively studied by several time‐resolved methods to gain an understanding of the dynamics and mechanism of the torsion, which has been explained in terms of a Jahn–Teller effect. Here, we propose an alternative explanation for the photoinduced structural change of CICs, based on electron density redistribution. After photoexcitation of a CIC (S0→S1), a metal‐to‐ligand charge transfer stabilizes the ligand and destabilizes the metal. A subsequent electron transfer, through an intersystem crossing process, followed by an internal conversion (S1→T2→T1), intensifies the energetic differences between the metal and ligand within the complex. The energy profile of each state is the result of the balance between metal and ligand energy changes. The loss of electrons originates an increase in the attractive potential energy within the copper basin, which is not compensated by the associated reduction of the repulsive atomic potential. To counterbalance the atomic destabilization, the valence shell of the copper center is polarized (defined by ∇2ρ(r) and ∇2Vne(r)) during the deactivation path. This polarization increases the magnitude of the intra‐atomic nuclear–electron interactions within the copper atom and provokes the flattening of the structure to obtain the geometry with the maximum interaction between the charge depletions of the metal and the charge concentrations of the ligand. That's exciting! We propose an alternative explanation for the photoinduced structural change of a copper(I) complex during the deactivation process following photoexcitation, where the metal valence shell polarizes to increase intra‐metal nuclear‐electron interactions, which provokes the flattening of the structure to obtain the maximum interaction between the charge depletions of the metal with the charge concentrations of the ligand. [ABSTRACT FROM AUTHOR]

Key issues macromolecular hydration with emphasis on proteins in the native and dematured state, and micelles. A simple theory, which relates the observations of solvation to the residence times on the protein surface and address the influence of dielectric relaxations, by rotational and translational motions, on the dynamics of bound-to-free water exchange.