Excited States In Solution Research Articles

Relaxation Dynamics of 2,7- and 3,6-Distyrylcarbazoles in Solutions and in Solid Films: Mechanism for Efficient Nonradiative Deactivation in the 3,6-Linked Carbazole

We performed time-resolved spectral investigations of two distyrylcarbazole derivatives, 2,7- and 3,6-distyrylcarbazole (2,7-DPVTCz and 3,6-DPVTCz, respectively), in dilute toluene solution and in solid films mixed with poly(methyl methacrylate) (PMMA). The lifetime of 2,7-DPVTCz in its excited state in solution is approximately 100 times as great as that of 3,6-DPVTCz, consistent with their photophysical nature. The former shows intense emission, but the latter is nearly nonfluorescent in a free environment. Moreover, the lifetime of 3,6-DPVTCz in its excited state increased also approximately 100 times when the molecule was encapsulated in a 3,6-DPVTCz/PMMA solid film, indicating that intramolecular motion of the molecule significantly affects the observed relaxation dynamics in a confined environment. Calculations on the excited states indicate that an efficient intersystem crossing is activated upon twisting of the bridged C-C single bond in a free 3,6-linked carbazole; such efficient deactivation is impractical in 2,7-linked carbazole or for 3,6-linked carbazole in a PMMA matrix. Information obtained from experiments on femtosecond fluorescence enables us to distinguish crucial relaxation processes in the excited state for a profound understanding of the details of vibrational and electronic relaxations of 3,6-DPVTCz in solution.

A Trimetallic Mixed Ru(II)/Fe(II) Terpyridyl Complex with A Long-Lived Excited State in Solution at Room Temperature

The photophysical behavior of a series of mono- and trimetallic Ru(II) and mixed Ru(II)/Fe(II) bis-terpyridyl complexes was examined. The complexes have bridging terpyridyl ligands linked by phenylene-vinylene substituents on the terpyridyl. For the complexes bridged by a single phenylene-vinylene, the lowest-energy excited state is metal-to-ligand charge transfer (MLCT), and excited-state decay is on the 1-10 ns time scale. The complexes with two phenylene-vinylene groups have thermally equilibrated excited states that are localized on the phenylene-vinylene bridge and have much longer lifetimes (>200 ns). Remarkably, the trimetallic complex having an Fe(II) terpyridyl center also has a long-lived excited state, despite the fact that low-energy iron-localized excited states exist on the complex.

Nonstationary rotational diffusion in room temperature liquids measured by femtosecond three-pulse transient anisotropy.

A femtosecond three-pulse chi((5)) polarization anisotropy experiment is used to examine the time dependence of the rotational diffusion of coumarin 153 in polar liquids. By probing the polarization anisotropy decay at various points during the molecule's excited state lifetime, a time-dependent diffusion coefficient is found in several solvents. This anomalous behavior is consistent with the relaxation of the solvent friction to accommodate the solute's excited state charge distribution. Rotational diffusion times measured immediately after photoexcitation by two-pulse chi((3)) experiments may reflect a nonstationary bath dynamics, rather than the equilibrium friction of the solvent.

Study of Nonpolar Solvation Dynamics in Supercritical Lennard−Jones Fluids in Terms of the Solvent Dynamic Structure Factor

The nonpolar solvation dynamics in Lennard−Jones fluids is discussed in terms of the relationship with the dynamic structure factor of neat solvent, using the theoretical expression that describes the solvation correlation function as the superposition of solvent dynamic structure factors at various wavenumbers. Several expressions for the coupling strength between the state transition of the solute and the solvent density modes are examined with respect to their abilities to predict the static fluctuation. In the present theoretical model, it is found that the difference between the ground- and excited-state solute−solvent interactions can be adequately taken as the coupling strength with the solvent density mode. Employing this expression for the coupling, the solvent fluctuation around k ≅ σ-1 (σ stands for the diameter of the solvent) contributes dominantly to the static fluctuation in all the densities investigated. This corresponds to the feature of the solvation dynamics in mixed solvents, in which...

Dynamic Solvation of Aminophthalimides in Solvent Mixtures

The solvatochromism and thermochromism of 4-aminophthalimide and 4-amino-N-methylphthalimide were studied by absorption and steady state and time-resolved fluorescence emission in solvent mixtures of toluene−ethanol and toluene−acetonitrile in the temperature range 5−70 °C. The wavelengths of the maximum of absorption and of fluorescence emission shift to the red with the increase of the proportion of the polar component in the mixture. The greater affinity for the polar component of the mixture of the excited state compared to the ground state enhances preferential solvation, which is the origin of this red shift. On the other hand, a spectral shift to the blue is found upon temperature increase in solvent mixtures. This fact can be explained by considering that association of the polar component with the excited-state solute is exothermic and decreases with temperature. The appointed solvation change is attained by diffusion-controlled exchange of solvent molecules between the bulk and the solvation sphere. This process leads to a time-dependent emission spectrum in the nanosecond time domain. In this work a kinetic scheme is developed to describe this exchange and explain the time-dependent fluorescence emission spectra. The description is based on a Langmuir type association of the solvent molecules with the solute. The solvation equilibrium is attained by stepwise solvent exchange. The kinetic data and the spectral information are integrated in a thermodynamic cycle that can describe the solvation of excited and ground states at any solvent composition.

Femtosecond Hyper-Rayleigh Scattering Measurement System for Organic Dyes in Solutions at the Excited State

Nonlinear optical properties can be enhanced at the excited state due to changes of electronic distribution. Femtosecond (fs) hyper-Rayleigh scattering measurement systems were constructed in order to quantitatively evaluate hyperpolarizability both at the ground and the excited states in solutions. Excitation of nitrostyrylpyridinium or crystal violet at 400 nm or 550 nm by fs laser resulted in one-photon or two-photon fluorescence at the wavelength to probe the HRS signals.

Ab Initio Study of the Effect of Solvation on the Electronic Spectra of Formamide and N-Methylacetamide

Calculations on the electronic spectra of formamide and N-methylacetamide in solution are presented. Solvents are modeled by macroscopic continua characterized by their dielectric constant and refractive index. The complete-active-space self-consistent-field model implemented within a self-consistent reaction field model is used. Dynamical correlation is accounted for using multiconfigurational perturbation theory. The Rydberg states are shown to be destabilized by the Pauli repulsion of the solvent. An advantageous side effect of this destabilization is the reduction of mixing between valence and Rydberg states encountered in gas-phase calculations on amides. A red-shift of 0.5−1.0 eV relative to comparable gas-phase calculations is seen for the ππ* transitions. For formamide in cyclohexane, nπ* and ππ* vertical transitions are computed to lie at 5.56 eV (oscillator strength f = 0.0006) and 6.94 eV (f = 0.274). For N-methylacetamide, the corresponding transitions are found at 5.55 eV (f = 0.001) and 6.59 eV (f = 0.256). In addition, higher energy excitations have been studied, including excitations from the lower-lying oxygen lone-pair orbital. Good agreement with available experimental data is achieved throughout, indicating the method used provides an accurate treatment of valence excited states in solution.

Time-Resolved Vibrational Spectroscopy of Electronically Excited Inorganic Complexes in Solution.

Time-resolved vibrational spectroscopic studies of electronic excited states in solution were reviewed by Morris and Woodruff in 1987.1 This review focused on time-resolved resonance Raman (TR3) experiments. While these efforts continue to be vital, transient Raman spectroscopy, and more recently transient infrared spectroscopy, are being increasingly applied to molecular assemblies to study the intermediate excited states formed following photoinduced electron or energy transfer. The ability to obtain structural information following the absorption of a photon is an important component of photochemical and photophysical research, which includes research related to artificial photosynthesis, photoinitiated reactions, energy transduction, energy conversion, photocatalysis, and photoremediation. Time-resolved infrared (TRIR) spectroscopy complements the Raman method, and with the advances made recently in TRIR spectroscopy, infrared spectra can now be measured on the time scales previously obtained only by the transient Raman method. This technique is particularly valuable in, but not limited to, the study of metal complexes containing CO or CN-. These ligands are useful because the ν(CO) and ν(CN) bands have large oscillator strengths and the sensitivities of their frequencies and bandwidths to electronic and molecular structure are well established. Electronic excitation generally produces transient changes in the infrared absorption that are characteristic of the changes in electron density within the complex. A potentially valuable approach to transient infrared studies of excited states on the nanosecond time scale uses step-scan interferometry. This approach has the ability to supply vibrational information throughout the middle-infrared region.2 Some studies utilizing time-resolved infrared spectroscopy of excited states of transition-metal complexes have been reviewed by Turner and Ford.3

Molecular theory of electronic spectroscopy in nonpolar fluids: Ultrafast solvation dynamics and absorption and emission line shapes

We present a theory of time- and frequency-domain spectroscopy of a dilute nonpolar solute in a nonpolar liquid or supercritical fluid solvent. The solute and solvent molecules are assumed to interact with isotropic pair potentials. These potentials, together with the solute and solvent masses, are the only input in the theory. We arrive at expressions for the absorption and emission line shapes, which include the possibility of motional narrowing, and for the time-resolved fluorescence and transient hole-burning observables, by assuming that the solute’s fluctuating transition frequency describes a Gaussian process. These expressions depend only on the average and variance of the transition frequency distributions in absorption and emission and on the normalized frequency fluctuation time-correlation functions. Within our formalism the former are obtained from the solute-solvent and solvent-solvent radial distribution functions, which are calculated using integral equations. The time-correlation functions involve the time-dependent solute-solvent Green’s function. Its solution depends upon the solute and solvent diffusion constants, which in turn are determined from the radial distribution functions. The theory compares favorably with computer simulation results of the same model. We then investigate the dependence of the various spectroscopic observables on the solvent density, the temperature, and the difference between the ground- and excited-state solute’s pair interaction with the solvent molecules. For example, since our theory for the time-correlation functions captures both their short- and long-time behavior, we can see how the crossover from inertial to diffusive dynamics depends on these variables. Our results are similar to a variety of experiments on solutes in both nonpolar and polar solvents.

Ultrafast rise of translational temperature after photoexcitation to electronic excited state in solution: Transient lens study of Ni2+ aqueous solution

An increase of the translational temperature after photoexcitation to the electronically excited state of Ni2+ in aqueous solution is detected using a time-resolved transient lens method from the picosecond to millisecond time scale. Besides a large lens signal due to the volume expansion of water in the submicrosecond time range, two other lens signals are observed at an initial part of the signal. During the pump laser pulse, a convex lens signal is observed, which is attributed to the optical Kerr lens signal. After the pulse, a concave lens is created, which remains a nearly constant amplitude until the density lens signal appears. The signal is attributed to the temperature lens signal, which originates from the heat energy by the nonradiative transition of the excited state. It is found that the signal rises within 3 ps after the photoexcitation, which indicates that the temperature rise is very fast. After the fast rise, another slower dynamics (500 ps) is observed and the deactivation processes of Ni2+ in aqueous solution are discussed based on these results.

Metal–oxo photooxidants. Crystal structure, spectroscopy and photophysical properties of neutral luminescent trans-dioxoosmium(VI) complexes with cyanide and mesityl ligands

A series of neutral osmium(VI)–oxo complexes having mesityl (mes) and cyanide ligands have been prepared and the crystal structures of [OsVI(py)2(CN)2(O)2](py = pyridine), [OsVI(tmen)(CN)2(O)2](tmen =N,N,N′,N′-tetramethylethylenediamine), [OsVI(mes)2(phen = 1,10-phenanthroline)] and [OsVI(mes)2(tmen)(O)2] determined. While [Os(mes)2(L–L)(O)2](L–L = chelating aromatic diimines and diamines) are non-emissive, the related [Os(L–L)(CN)2(O)2], complexes have long-lived and emissive excited states in solution at room temperature. The [Os(4,4′-Me2bipy)(CN)2(O)2](4,4′-Me2bipy = 4,4′-dimethyl-2,2′-bipyridine) complex has a E°(OsVI*–OsV) value of 1.77 V vs. normal hydrogen electrode and is capable of oxidizing alkenes to the corresponding epoxides upon irradiation with UV/VIS light at room temperature.

Picosecond pulse radiolysis on geminate ion recombination and formation of solute excited state in liquid cyclohexane

The geminate ion recombination and the formation process of solute excited states in solutions of biphenyl were studied in cyclohexane by means of picosecond pulse radiolysis with absorption and emission spectroscopies. The time-dependent behavior of the biphenyl anion was analyzed with the diffusion theory in the solute-solvent system. By applying this analysis to the time-dependent fluorescence from excited states of biphenyl, yields of the excited states through various formation processes were deduced. Also, yield of excited states of cyclohexane in the absence of solutes was estimated.

E.S.R. studies of excited states in solution

The observation of electron spin polarized spectra from free radicals using flash-photolysis electron spin resonance methods is described. The origins of polarization are summarized and a critical account of their present understanding provided. The phases of the polarized signals carry information on the multiplicity of the radical precursor. Examples are provided from observations of nitrogen heterocyclic radicals of the use of the spectra to obtain such information, and to identify reactive states precisely.

Bis(bipyridine)ruthenium(II) cyanobridge polymeric cations

Polymeric cations made up of Ru(bpy) 2 units linked by cyanide bridges can be prepared by reacting Ru(bpy) 2(CN) 2 with Ru(bpy) 2C 2O 4. The reaction yields a mixture of polymeric cations of various chain lengths. Polymeric cations of this type have emitting excited states in solution, with lifetimes in the 50–100 ns range.

Laser photodetachment from aromatic anions in nonpolar solvents

A combined x-ray visible-dye-laser double-pulse technique is used to determine the photodetachment (action) spectra of the anions of biphenyl, trans-stilbene, pyrene, perylene, benzperylene, coronene, and nitrobenzene in nonpolar solvents. Quantum yields of electrons (phi/sub e/) are reported for the visible transitions, and photodetachment is shown to be a major process for visible light. For pyrene and trans-stilbene anions the quantum yields are 0.47 and 0.31 independent of solvent. For perylene and benzperylene anions phi/sub e/ is solvent dependent. The results are interpreted in terms of autoionizing excited states which can also internally convert to ground state anions. The mechanism of autoionization and the lifetime of the excited states in solution are discussed.

Triplet state resonance Raman spectroscopy

AN important objective of theoretical photochemistry is to describe the interplay between the different decay channels of molecules in electronic excited states1,2. Detailed information on the potential energy hypersurface is required to specify the molecular dynamics in terms of saddlepoints and activation energies along the reaction coordinate. Excited electronic states differ from the ground state because of the changes in the electronic distribution within the molecular framework. These changes are accompanied by changes in bond orders, force constants of vibration and dissociation energies. Vibrational spectra of excited states could be expected to provide important information on changes in frequencies of particular normal modes relative to those of the ground state molecule. Triplet excited states in solution have been studied by many techniques, the most common being flash photolysis in combination with optical absorption spectroscopy. Vibrational spectroscopy has to our knowledge only been reported in a single case, where one vibrational band of the lowest state of deuterated naphthalene generated by steady-state illumination was detected by infrared spectroscopy3. Recently, resonance Raman spectroscopy has been established as a powerful technique for studying shortlived species in solution4. This technique combines high sensitivity and time resolution with selectivity and structural information. It has been used successfully to study free radicals in solution generated by stopped flow methods5, electrochemically6, by pulse radiolysis4 and recently by laser flash photolysis7. Here we make the first report on the resonance Raman spectrum of a molecule in its triplet state generated by pulse radiolysis.

Nanoseconds UV laser study of radiationless deactivation from upper electronic excited states in solution: Tb 3+

Using 25 ns half width pulses of a frequency-quadrupled Nd laser at 265 nm Tb 3+ perchlorate in D 2O solution, or in borate glass, was excited to high electronic states and the fluorescence on transition from the upper 5D 3 and lower 5D 4 excited states to various sub-levels of the lowest 7F state were measured. The appropriate energy gap spacing of this rare earth ion results in lifetimes which enabled us to observe the decay of 3D 3 as well as the growing in and decay of 5D 4. Radiationless processes from 5D 3 direct to 7F or cascading to 5D 4 are discussed and correlated with previous theoretical and experimental results on related systems involving the interaction of the non-complexed solvated ion with the medium, and compared with related phenomena in glassy solids.

Organic photochemistry. XIV. Intra- vs. intermolecular olefin interactions with the phenyl singlet and triplet excited states in solution. 1-Phenyl-2-butene photoisomerization, toluene photosensitized isomerization of 2-heptene, and quenching of the fluorescence of benzenoid aromatics by olefins

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOrganic photochemistry. XIV. Intra- vs. intermolecular olefin interactions with the phenyl singlet and triplet excited states in solution. 1-Phenyl-2-butene photoisomerization, toluene photosensitized isomerization of 2-heptene, and quenching of the fluorescence of benzenoid aromatics by olefinsHarry Morrison, John Pajak, and Robert PeifferCite this: J. Am. Chem. Soc. 1971, 93, 16, 3978–3985Publication Date (Print):August 1, 1971Publication History Published online1 May 2002Published inissue 1 August 1971https://doi.org/10.1021/ja00745a027RIGHTS & PERMISSIONSArticle Views53Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (963 KB) Get e-Alerts Get e-Alerts

Exciplex Studies. II. Indole and Indole Derivatives

An excited-state solute—solvent complex (exciplex) has been shown to be responsible for a large red shift and loss of vibrational structure in the fluorescence spectra of indole and indole derivatives in polar solvents. Solute—solvent stoichiometry of 1:2 and 1:1 is observed with associating and nonassociating solvents, respectively. Hydrogen bonding between the indole >N–H group and solvent is shown not to be responsible for the interaction. It is suggested that the exciplex state is a charge-transfer state and is an intermediate in the process of electron transfer from the solute to the solvent.

Solvent effects in vibrational spectroscopy

The effects of solute-solvent interactions on the frequencies, intensities and shapes of the infra-red absorption bands of dissolved molecules are discussed. The interaction potential energy U between a single solute molecule and its environment is treated as a variable depending upon the solvent configuration and on the normal co-ordinates of the isolated vibrating solute. Formulae relating the mean solvent shifts Δω to the derivatives of U with respect to the normal co-ordinates and to the anharmonic constants of the free solute are deduced for diatomic and polyatomic molecules. Invariants on isotopic substitution, such as Δω/ω, are discussed and it is shown that the effects of isotopic changes on the solvent shift, and hence on the probability of a particular solvent configuration occurring, are normally small. The different solvent environments around ground-state and excited-state solute molecules are found to lead to a small difference between the displacements of the Stokes and anti-Stokes lines in the Raman spectra of dissolved substances; the difference has not yet been established experimentally, and it may prove hard to measure, as it is proportional to the square of the half-width of the band, and is therefore only appreciable when the band is broad.