Thermalization of photoexcited molecules in solution

Analysis of the steady-state absorption/fluorescence spectra of several laser dyes in room temperature solution suggests that the fluorescing molecules, if initially formed in a vibrationally excited state, lose their excess energy surprisingly slowly, remaining significantly warm on a nanosecond time scale. A similar analysis of the steady-state absorption/fluorescence spectra of GaAs confirms that in this case the carriers are fully thermalized, possessing no excess energy when they recombine on a microsecond time scale. A classical model which accounts qualitatively for slow molecular cooling is presented. We conclude that rapid molecular photoconversion processes are likely to involve incompletely cooled (thermalized) excited states.     

Proton transfer to photoexcited aromatic compounds in solution

This review paper will summarize work carried out during the last thirty years in the area of proton transfer to the aromatic ring of photoexcited aromatic compounds. The aromatic ring has been predicted to be much more basic in the singlet excited state than in the ground state. However, only recently has there been systematic and quantitative studies in this area. This review will highlight the significant new insights now available and demonstrate that there is much more to be learned in the excited state acid-base chemistry of aromatic rings.     

Endor on photoexcited triplets randomly oriented in solid solution

It is shown that hyperfine interactions in photoexcited triplets randomly oriented in solid solution can be measured with the aid of ENDOR. Results of measurements on naphtalene and zinc tetraphenylporphyrin triplets in polymethyl-methacrylate are presented. Preliminary results obtained with Zn(CN)₄TPP show that ENDOR spectra of triplets with short lifetimes (less than 5 ms) can also be obtained due to the signal enhancement resulting from electron spin alignment.     

Direct versus indirect H atom elimination from photoexcited phenol molecules

The active role of the optically dark pi sigma* state, following UV absorption, has been implicated in the photochemistry of a number of biomolecules. This work focuses on the role of the pi sigma* state in the photochemistry of phenol upon excitation at 200 nm. By probing the neutral hydrogen following UV excitation, we show that hydrogen elimination along the dissociative pi sigma* potential energy surface occurs within 103 +/- 30 fs, indicating efficient coupling at the S1/S2 and S0/S2 conical intersections, with no identifiable role of statistical unimolecular decay of vibronically excited (S0) phenol in the timeframe of our measurements.     

Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.     

Kinetics and mechanism of reactions of photoexcited kynurenine with molecules of some natural compounds

Photochemical reactions involving kynurenines, viz., molecules present in the eye lens, can result in modifications of the lens proteins and cause a development of a cataract. The rate constants of the reactions of photoexcited kynurenine with several amino acids and antioxidants contained in the lens were measured. The most efficient quenchers of triplet kynurenine are amino acids tryptophan and tyrosine, as well as antioxidant ascorbate. In all cases, the quenching reaction proceeds by the electron transfer mechanism, except for the reaction with oxygen where transfer of the triplet energy to the oxygen molecule occurs. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 704–710, April, 2007.     

Identification of unavoided crossings in nonadiabatic photoexcited dynamics involving multiple electronic states in polyatomic conjugated molecules

Radiationless transitions between electronic excited states in polyatomic molecules take place through unavoided crossings of the potential energy surfaces with substantial non-adiabatic coupling between the respective adiabatic states. While the extent in time of these couplings are large enough, these transitions can be reasonably well simulated through quantum transitions using trajectory surface hopping-like methods. In addition, complex molecular systems may have multiple "trivial" unavoided crossings between noninteracting states. In these cases, the non-adiabatic couplings are described as sharp peaks strongly localized in time. Therefore, their modeling is commonly subjected to the identification of regions close to the particular instantaneous nuclear configurations for which the energy surfaces actually cross each other. Here, we present a novel procedure to identify and treat these regions of unavoided crossings between non-interacting states using the so-called Min-Cost algorithm. The method differentiates between unavoided crossings between interacting states (simulated by quantum hops), and trivial unavoided crossings between non-interacting states (detected by tracking the states in time with Min-Cost procedure). We discuss its implementation within our recently developed non-adiabatic excited state molecular dynamics framework. Fragments of two- and four-ring linear polyphenylene ethynylene chromophore units at various separations have been used as a representative molecular system to test the algorithm. Our results enable us to distinguish and analyze the main features of these different types of radiationless transitions the molecular system undertakes during internal conversion.     

Effect of adsorbed concentration on the radiative rate enhancement of photoexcited molecules embedded in single microspheres

The variation of the molecular density in a single microcavity and its influence on the radiative rate enhancement (RRE) are reported here. The quality factors of the observed morphology-dependent resonances (MDRs) of the microcavity remain unchanged in the absence of any absorbing effects. In contrast, the MDRs tend to disappear in the presence of strong absorption even due to the self-absorption by the molecule. Time-resolved fluorescence studies reveal the fact that the value of RRE decreases with an increase in the adsorbed concentration of the molecules. The results have been explained in terms of a detuning parameter, which is a function of the refractive index of the microcavity. The increased dispersing capability of the microsphere upon increasing its molecular density has been found to be responsible for the observed decrease in RRE.     

Aggregation of rose bengal molecules in solution

Absorption, emission and excitation spectral data support the thesis that rose bengal forms H-type aggregates in water and polar, protic solvents.     

Dipoles-dipole association of mesogenic molecules in solution

The dielectric polarization has been used to study dipolar association of 4-n-pentyl-4'-cyanobiphenyl in benzene solution. The results have been interpreted with the assumption of a monomer-dimer equilibrium. To explain the relatively high effective dipole moment of the dimers, a new structure has been proposed for them.     

Modeling absorption spectra of molecules in solution

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute–solvent interactions are key to understanding solution-phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute–solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute–solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.     

Elucidating ultrafast nonradiative decay of photoexcited uracil in aqueous solution by ab initio molecular dynamics

Nonradiative decay of the photoexcited RNA base uracil has been studied in fully explicit aqueous solution using nonadiabatic ab initio molecular dynamics. Detailed comparison of the time-dependent nonadiabatic transition probability with specific molecular vibrational motions provides insight into the mechanism of the ultrafast internal conversion. From a monoexponential fit to the excited state ensemble population, the lifetime of the first electronically excited ππ^* singlet state has been determined to be 359 fs. Additional, reference, nonadiabatic simulations have been carried out in the gas phase, pinpointing the effects of the solvent on the photophysics of uracil. The gas phase excited state lifetime is calculated to be 608 fs, somewhat longer than in solution. In terms of excitation energies and geometrical parameters, the differences between gas phase and aqueous solution are found to be generally small. A notable exception is the excited state out-of-plane torsional motion about the CC double bond, which appears severely damped by the solvent. Moreover, hydrogen bond interactions between the uracil oxygens and the solvent hydrogens are seen to enhance internal conversion.     

Sub-picosecond relaxation of large organic molecules in solution

Using sub-picosecond ultraviolet and visible pulses we have studied relaxations from highly-excited molecular vibronic states with a resolution of 2 × 10^?13 s.     

Dynamics of branched polymer molecules in dilute solution

Theoretical formulas for the intrinsic viscosity and viscoelastic properties of some model branched molecules in dilute solution are calculated by means of the normal coordinate method of Rouse modified to include hydrodynamic interactions. The calculations are exact except for the usual approximation of the hydrodynamic interactions by the Kirkwood-Riseman formula. The ratio of the intrinsic viscosity of a branched molecule to that of a linear molecule of the same weight is found to vary almost as the square root of the ratio of the mean square radii, instead of as the latter ratio to three-halves power, as has been postulated before. It is proposed that this square root relation is applicable in general to branched molecules of all types. Several sets of experimental data in the literature are shown to agree well with this hypothesis.     

Anisotrophy of rotational diffusion of excited molecules in solution

The principal values of the rotational diffusion tensor of perylene and 9,10-dimethylanthracene in ethanol are evaluated from temperature-dependence measurements of the degree of fluorescence polarization upon exciting the molecules into two perpendicularly polarized transitions.     

The Stokes-shifted fluorescence risetime of dye molecules in solution

We have measured the rise of Stokes-shifted spontaneous fluorescence intensity during a 10 ps 530 nm actinic light pulse for rhodamine B, rhodamine 6G and erythrosin B dissolved in water, ethanol and methanol. The best numerical fit to our data corresponds to a time delay of less than 1 ps between excitation and fluorescence emission.     

Supramolecular structures of polyoxomolybdate-based giant molecules in aqueous solution

A combination of static and dynamic laser light-scattering techniques was employed to study the supramolecular structures of polyoxomolybdate-based giant molecules in aqueous solution. Convincing evidences indicate that the spherical aggregates have unique vesicle-like structures. A model is built up based on solid experimental results. It is the first time that a clear explanation is presented for the long-time enigma in inorganic chemistry: the nature of the polyoxomolybdate aggregates in "molybdenum blue solution".     

Fluctuations in absorbance observed for photoexcited ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules as impurities

The time-dependent changes in absorbance in the case of ferrocene-doped poly(methyl methacrylate) thin films containing chloroform molecules have been investigated after photoexcitation in nitrogen atmosphere. Photoexcitations have been made by the monochromatic light (using Xe-source and a monochromator) in the UV range. An increase in absorbance associated with spectacular fluctuations in its value, has been noticed. The nature of the variation in the fluctuations has been studied as a function of time (duration) of photoexcitation, photoexcitation wavelength and concentration of ferrocene in the film. The observed results have been discussed considering the role of secondary reaction after photoexcitation as well as the diffusion of small chemical species in the polymer matrix.     

Relationship between free molecules and molecules involved in various heteroassociates in HF-Me2CO solution

The experimental and computational methods to acquire the data on the relationship between free molecules and molecules involved in various heteroassociates (HA) in the HF—organic solvent binary liquid system (BLS) were developed and applied to the series of HF solutions in acetone. The first method is appropriate at the component molar ratios from 0 : 1 to 6 : 1 under the condition that the IR spectrum of the solvent molecule contains a band, whose frequency and intensity can be measured as the molecule passes from the free state to the composition of all HA formed in the BLS. The second method is based on the consideration of the balance between free molecules and molecules involved in HA of the solvent and HF. This method requires the knowledge of molar ratios of the components of the solution at which each HA appears in the solution. It is shown that in an HF-Me₂CO solution the main part of the molecules is involved in the HA composition starting from the HF content ～0.25 molar fraction, whereas 90–100% molecules are associated at an HF content of ～0.50–0.93 molar fraction.