cw Two-photon transitions in K2 molecular beams

We report on the two-photon visible excitation spectra of potassium molecules in a supersonic beam. A dye laser excites K₂ to different (7s ¹Σ_g, 9s ¹Σ_g, 8d ¹Δ_g) Rydberg states. Two-photon transitions are detected by monitoring the resulting (third-photon) ionization. We have made detailed studies, including rotational assignment of 92 such transitions, ¹Δ_g(v¹, J¹) ← X ¹Σ_g(v = 0, J), where the B ¹Π_u state forms the intermediate enhancing state.     

Primary photoprocesses in retinol

Pulsed laser photolysis techniques, with nanosecond time-resolution, are applied to solutions of retinol (vitamin A) and retinyl acetate, under varying conditions of solvent polarity and oxygen content. The observed absorbance and conductivity changes are identified as due to: (a) the excited fluorescent state, λ_max = 435 nm; (b) the lowest excited triplet state, λ_max = 405 nm; (c) the retinylic cation (λ_max = 590 nm), formed via: ROH→^hv R⁺ + OH^?. The results constitute the first experimental evidence for ionic photodissociation and for spontaneous, as well as oxygen-induced, intersystem crossing in retinol. Both processes may provide routes for isomerization, being thus relevant to the visual process.     

Micellar effects on photoprocesses in retinyl polyenes

The effects of micellar solubilization on excited-state properties of several retinyl polyenes have been examined primarily by nanosecond laser flash photolysis. The relative intensity of band system III (254–256 nm) in the ground state absorption spectrum of 11-cis retinal decreases significantly on going from methanol to micellar solutions, suggesting that the 12-s-trans form of 11-cis retinal is relatively favored in the organized media. In addition to microsecond transient phenomena due to triplets, the laser flash photolysis of all-trans and 11-cis retinal and all-trans retinyl Schiff base incorporated into micelles leads to ‘permanent’ absorption changes attributable to photoisomerization (in the case of retinals) and protonation and/or complexation with water (in the case of Schiff base). All-trans retinol and retinyl acetate in micellar solutions undergo ionic photodissociation leading to long-lived retinyl carbocation (λ_max = 585–600 nm), the process being monophotonic in the case of retinyl acetate and predominantly bipho-tonic in the case of retinol. The trends in the location of ground-state absorption maxima (^IB_u^+*←^IA_g) and triplet yield of retinals, and photodissociation yield of retinyl acetate suggest that the polarity of the environment probed by the polyene systems increases in the order: Triton X-100 < CTAB < NaLS.     

Dynamics of photoprocesses in novel bifunctional salicylideneiminospironaphthoxazines

The dynamics of photochemical processes in a number of novel photobifunctional compounds, whose molecules combine a photochromic spironaphthoxazine fragment with the salicylideneimine moiety containing different substituents, was studied by the flash photolysis technique. Three products of photolysis of these compounds were detected, and the effects of the substituents and the solvent nature (toluene and methanol) on their spectral and kinetic characteristics and quantum yields were studied.     

Primary photoprocesses in oxyblepharismin interacting with its native protein partner

The primary photoprocesses in the photoreceptor for the step-up photophobic response of the light-adapted cells of Blepharisma japonicum (OBIP, OxyBlepharismin bInding Protein) have been studied by ultrafast UV–vis transient spectroscopy. The results are rationalized in terms of heterogeneity of the OBIP sample. Two independent classes of chromoprotein are proposed: a “reactive” species, which presents a specific 680-nm band decaying in 4 and 56 ps and a “non-reactive” one, which behaves like the free chromophore (OxyBP) in solution. A bimolecular photooxidation of OxyBP in the presence of 1,4-benzoquinone was performed to record the absorption spectrum of the OxyBP radical cation. Comparison with reactive OBIP suggests that an electron transfer could be involved in the primary photoprocesses of OBIP and possibly trigger the sensory transduction chain of B. japonicum. In addition, the specificity of the chromophore–protein interaction was investigated by studying the artificial complex that OxyBP forms with human serum albumin (HSA). OxyBP–HSA happens to be spectroscopically much closer to free OxyBP than to OBIP. This highlights the specific nature of the interaction between OxyBP and its native protein partner and further supports the proposal that OBIP is the actual photoreceptor for the photophobic response of B. japonicum.     

Two-photon absorption and photoluminescence of europium based emissive probes for bioactive systems

Observation of two-photon excitation (760 nm) and emission of two responsive water soluble europium complexes is reported with cross-sections of up to 2 GM. Two-photon excitation spectra have also been measured, acquisition being achieved by the use of a cavity-dumped mode locked Ti-sapphire laser. Time-gated detection is used to differentiate the ligand fluorescence and metal centred emission in these europium complexes.     

Organized molecular systems in catalysis

Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2407–2424, October, 1990.     

Target analysis of primary photoprocesses involved in the oxyblepharismin-binding protein

Target analysis is performed on previously published transient absorption spectra of the 200-kDa oxyblepharismin-binding protein (OBIP) thought to trigger the photophobic response of the ciliate Blepharisma japonicum. The OBIP sample is considered as heterogeneous and made of two distinct classes of chromophore-protein complexes. A so-called nonreactive class is seen to be comparable to free oxyblepharismin in organic solution. Another, reactive, class is shown to undergo a fast picosecond photocycle involving the formation in 4 ps of an intermediate state noted Y1. The spectrum associated to Y1 bears striking similarities with that of the oxyblepharismin radical cation. This element favors the hypothesis that an excited-state intermolecular electron-transfer could be the primary step of the sensory transduction chain of B. japonicum. Proton release is also considered as a possible secondary step. These possibilities support the idea that reactive OBIP functions like an electron or proton pump. We alternatively propose a new hypothesis stating that the fast photocycle of reactive OBIP actually does not generate any photoproduct or protein change of conformation but is involved in another biological function. It would act as a kind of solar screen, providing additional protection to the light-adapted form of B. japonicum in case of excessive illumination.     

Two-photon absorption dependence on the temperature for azoaromatic compounds: effect of molecular conformation

This work reports on the effect of temperature on the two-photon absorption cross section of azoaromatic chromophores. A linear decrease in the two-photon absorption cross section with the temperature was observed for several azochromophores. This process was characterized by introducing a two-photon absorption thermal coefficient (ddelta/dT), whose typical values are approximately 2GM/degrees C for all the azochromophores studied here. Such an effect was attributed to thermal induced molecular conformation changes, described by the sum-over-states model and semiempirical calculations, which affect the molecular dipole moments. The characterization of the phenomenon reported here for other nonlinear materials can help in the design of specific applications using two-photon absorption.     

Quantum theory of chemical bond formation processes in condensed systems: studies towards exploration of electrochemical dark and photoprocesses on semiconductors

Quantum theoretical models of chemical bond formation and partial charge transfer processes in condensed systems, and their extension to processes in electronic non-equilibrium are investigated in this paper with a view to further exploration of electrochemical and photoelectrochemical kinetics on semiconductors. Electrochemical dark and photocurrents on n-III-V-semiconductors are correlated with calculated transition probabilities for atom-group transfer over larger distances (80 to 240 pm), leading to a first estimate of potential surface shapes compatible with experiment. Some specific problems connected with transition probability calculations for heavy-particle transfer in strong anharmonic potentials are considered in detail, including approximation of Franck-Condon transitions in arbitrary potentials by their classical limit.     

Primary photoprocesses in molecules of carbocyanine dyes in binary solvent mixtures

The effect of the composition of the dimethyl sulfoxide (DMSO)-toluene mixture on the photophysical processes of thia-, indo-, and imidacarbocyanine dyes was studied. In the mixtures with the DMSO content of more than 20 vol %, the dyes representing solvated cations are characterized by high efficiency of trans → cis photoisomerization and fluorescence, in contrast to the extremely low efficiency of intersystem crossing to the triplet state. With growing the toluene content in the mixture, ion pairs between the dye cation and Cl⁻, Br⁻, I⁻, or BF⁴⁻ anion are formed. In the cases when the dye counterions are Br⁻ or I⁻, a sharp increase in the yield of the triplet molecules and a decrease in their lifetime take place. The results are discussed in terms of “the external heavy atom effect” in ion pairs.     

Continuous molecular enrichment in microfluidic systems

Highly efficient molecular extractions in continuous flow microfluidic systems are demonstrated utilising the rapid mixing properties of biphasic segmented flow in conjunction with suspended micro-particulate adsorbents. A continuous flow technique providing potential for continual on-line sample enrichment, purification and clean-up in chemical synthesis, and sample preparation.     

Electrostatic molecular potentials in hydrogen-bonded systems

A study is made of the modifications of the electrostatic molecular potential brought about by hydrogen bonding both in the hydrogen-bond region itself and in the external regions of the proton-acceptor and proton-donor molecules. Systems up to four successive units in a chain of donor-acceptors are considered. The possibility of obtaining a satisfactory picture of the global potential by a simple superposition of the potentials of the individual units is evaluated.     

Timeresolved studies of photoprocesses involving transition metal polypyridyl complexes

Applications of laser flash photolysis techniques are illustrated using three specific cases among many photoprocesses involving transition metal polypyridyl complexes (LL = bpy, phen or related ligands): dynamics of photoinduced formation of complexes of the type M(CO)₄(LL), where M = Mo⁰, Cr⁰ or W⁰; dynamics of photosubstitution reactions of Ru(LL) ₃ ^{2 +} and photoredox reactions of MLCT excited states of Ru^II(LL) ₃ ²⁺ and [Re^I(CO)₃(Cl)(LL)].     

Two-photon dispersion in conjugated polymers

The frequency dependence of the dispersive and absorptive parts of a strongly allowed two-photon transition is determined in a series of conjugated polymer solutions. The data analysis yields the energy and width of the two-photon transition, symmetry assignment for the two-photon transition (A_g → A_g), and oscillator strengths for both the one-photon and two-photon transitions.     

Effect of counterions on photoprocesses of thiacarbocyanine in a binary solvent blend

A model describing the effect of counterion X⁻ (X = Cl, I) on the deactivation kinetics of the S ₁ state of thiacarbocyanine Cy⁺X⁻ is presented. According to the model, the ion pair Cy⁺X⁻ in a binary solution is characterized by a distribution function f(r) over interatomic distances r, which depends on the composition of the mixture. The assumption of kinetically independent local states of the ion pair, which decay with the rate constants k _i(r)(i = 1–4 is the index of the decay channel), is made. The statistic analysis of the experimental data in terms of the model permitted us to find the functions f(r) and to estimate the parameters of the constants k _i(r).     

Degree of molecular self-sorting in multicomponent systems

Self-sorting represents the spontaneous and high fidelity self and/or non-self-recognition of two or more related components within a complex mixture. While the effective management of self-sorting principles perceptibly requires some key expertise in molecular programming, at a higher stage of operation it is of supreme interest to guide the process to increasingly higher degrees of self-sorting. In this article, we present the emerging principles of how to guide several components toward formation of self-sorted multicomponent architectures. To provide further guidance in denominating such systems, we suggest to utilise a systematic classification as well as a formula to evaluate their degree of self-sorting (M).     

Vibrational modes in partially optimized molecular systems

In this paper the authors develop a method to accurately calculate localized vibrational modes for partially optimized molecular structures or for structures containing link atoms. The method avoids artificially introduced imaginary frequencies and keeps track of the invariance under global translations and rotations. Only a subblock of the Hessian matrix has to be constructed and diagonalized, leading to a serious reduction of the computational time for the frequency analysis. The mobile block Hessian approach (MBH) proposed in this work can be regarded as an extension of the partial Hessian vibrational analysis approach proposed by Head [Int. J. Quantum Chem. 65, 827 (1997)]. Instead of giving the nonoptimized region of the system an infinite mass, it is allowed to move as a rigid body with respect to the optimized region of the system. The MBH approach is then extended to the case where several parts of the molecule can move as independent multiple rigid blocks in combination with single atoms. The merits of both models are extensively tested on ethanol and di-n-octyl-ether.