Femtosecond Fluorescence Up-conversion Technique Research Articles

Femtosecond Fluorescence Anisotropy Studies of Solvation-Induced Intraligand Charge Transfer in Photoexcited Aluminum(III) Tris(8-hydroxyquinoline)

For the organic light emitting diode compound aluminum(III) tris(8-hydroxyquinoline) (Alq3), dissolved in dimethylformamide (DMF), the time dependence of the fluorescence anisotropy has been studied using the femtosecond fluorescence upconversion technique. Upon excitation within the first absorption band, near 364 nm, with polarized laser pulses of duration less than 150 fs, an initial fluorescence anisotropy of about 0.2 is found to rapidly decay with a time constant of 2.0 ± 0.2 ps. The observed fast anisotropy decay component is concomitant with the solvation-induced dynamic Stokes shift of about 1000 cm-1. When excitation is at wavelengths below 330 nm, the fluorescence of Alq3 in liquid solution does not show any anisotropy effect. It is discussed that the emissive lowest excited electronic state of Alq3 is ligand localized and that its electronic wave function is affected by solvation. More specifically, the electronic wave function is considered to be an admixture of wave functions belonging to se...

Vibrational Motion of Electron Donor-Acceptor Complexes Time-Resolved by Femtosecond Fluorescence Spectroscopy

The excited-state dynamics of several electron donor-acceptor complexes were studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence decay of the complexes contains an oscillatory component superimposed with an ultrafast decay component. The oscillatory component was assigned to the out-of-plane vibration of the acceptors (tetracyanoethylene (TCNE), chloranil, fluoranil) having b3u symmetry. The time-dependent fluorescence spectrum for the TCNE - hexamethylbenzene (HMB) complex was reconstructed from the fluorescence dynamics at different wavelengths and the fluorescence peak-shift correlation function was obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mainly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory behavior of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The time dependence of the mean transition moment was constructed. It oscillates with the same frequency (155 cm−1), which is an indication of the non-Condon transition. Thus, two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.

Femtosecond fluorescence upconversion spectroscopy of vapor-deposited tris(8-hydroxyquinoline) aluminum films

Vapor-deposited Alq 3 is used as the green emitting layer in a class of organic light-emitting diodes. In this paper, the time dependence of the fluorescence from thin Alq 3 films has been studied by means of the femtosecond fluorescence upconversion technique. From the temporally resolved emission spectra, obtained after spectral reconstruction, the existence of different emissive sites in the Alq 3 film is concluded. They are identified as shallow and deep trapping sites. The average shallow trap excitation energy is located about 160 cm −1 below the (quasi-) exciton band, while the deep trap excitation energy is lower by approximately 2000 cm −1. Above 75 K, the shallow trap excitation is thermally depleted, via the (quasi-) exciton band, to fast-trapping non-emissive sites. At liquid helium temperatures, the depletion of the highest-lying shallow traps is frozen out. At room temperature, the lifetime of the shallow traps varies from about 1 to 20 ps; the lifetime of the deep traps is about 10 ns.

Vibrational Coherence in Electron Donor−Acceptor Complexes

The excited-state dynamics of several electron donor−acceptor complexes were studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence decay of the complexes contains an oscillatory component superimposed with a ultrafast decay component. The oscillations in fluorescence are observed for the complexes with three different electron acceptors, tetracyanoethylene (TCNE), chloranil, and fluoranil, with various electron donors, hexamethylbenzene, carbazole, 1-chloronaphthalene, N-methylaniline, and some others. The similar oscillatory frequencies are observed for the complexes with different donors and a common acceptor: ∼155 cm-1 (period of ∼215 fs) for complexes with TCNE, 178 cm-1 (187 fs) for the complexes with chloranil, and 212 cm-1 (157 fs) for the complex with fluoranil. The assignment of the oscillatory component to a particular vibration is proposed. It is concluded that the out-of-plane vibrational mode of the acceptor, having b3u symmetry, is responsible for the observed oscillations in all three cases (three acceptors). The time-dependent fluorescence spectrum for the TCNE−HMB complex is reconstructed from the fluorescence dynamics at different wavelengths, and the fluorescence peak shift correlation function is obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mostly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory component of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The modulation of the mean transition moment is observed as well. Thus, two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.

Femtosecond Dynamics of Double Proton Transfer in a Model DNA Base Pair: 7-Azaindole Dimers in the Condensed Phase

The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams: Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.

Energy transfer in a porphyrin chelate assembly

Energy transfer among isoenergetic pigments, three oxinyl-substituted free-base porphyrins in an octahedral chelate Ga(III) complex, has been studied by the femtosecond fluorescence up-conversion technique. The anisotropy decay of the complex has a fast component of ∼10 ps which is assigned to the energy transfer between the nearest porphyrin units in the complex. The anisotropy decay of the monomer (oxinyl-substituted porphyrin) is much slower and reflects monomer rotational dynamics in solution with decay times of 50 and ∼200 ps in dichloromethane.

Blocking effect of TiO 2 colloids on the excited state isomerization of a cyanine dye

Fluorescence dynamics of a cyanine dye 3,3′-di(3-sulfopropyl)-thiacyanine triethylaminium salt in water with and without doping TiO 2 colloids has been investigated using the femtosecond time-resolved fluorescence up-conversion technique. A new component with a decay time larger than 30 ps for the dye molecules associated with TiO 2 was observed and a blocking effect of TiO 2 on the conformational isomerization process of the cyanine dye is suggested.

Ultrafast intermolecular electron transfer from orthomethoxyaniline to excited coumarin dyes

Ultrafast intermolecular electron transfer (ET) from orthomethoxyaniline (orthoanisidine, ANS) to a number of excited (S1) 4-trifluoromethyl-1,2-benzopyrones (coumarins) having differently substituted 7-amino group has been investigated by femtosecond fluorescence up-conversion technique. The ET dynamics in the present systems are nonsingle-exponential and occur faster than the diffusive solvation dynamics. The ET rates are largely dependent on the nature of the substituents at the 7-amino group of the coumarins. This dependence is well correlated with the free energy changes (ΔG0) for the ET reactions. The ET dynamics become slower on using deuterated ANS as the donor, where the amino group hydrogens of ANS are substituted by deuterium. The deuterium isotope effect, however, gradually reduces as the ET dynamics becomes faster. Conventional ET theories can not explain all the observations. The results are explained on the basis of the two-dimensional ET model, which considers the solvent coordinate and the intramolecular coordinate separately to depict the ET process. It is seen that in coumarin-ANS systems the ET occurs much faster than the coumarin-aniline systems. It is indicated that the electronic coupling matrix element, a parameter which determines the extent of interaction between the reactant and the product states in the ET process, is much larger in the present systems than for the coumarin-aniline systems. The deuterium isotope effect on the ET dynamics is explained in terms of the changes in the ΔG0 values on isotopic substitution of the solvent donors.

Solvation dynamics of methanol investigated by femtosecond time-resolved fluorescence up-conversion technique

The solvation dynamics of methanol has been investigated using a femtosecond time-resolved fluorescence up-conversion technique. Transient fluorescence spectra of Coumarin 152A dissolved in methanol at different time were reconstructed from the measured fluorescence decays. The Stokes shift was obtained and the solvent response function Sv(t) was fitted using a bimodal function with a fast Gaussian component and a slow bi-exponential component. The Gaussian component was attributed to the effect of free streaming motion of the solvent molecules, whereas the bi-exponential component was caused by the rotational diffusion motion of the solvent molecules. A comparison of our results with reported data was made.

Ultrafast Photoinduced Solute−Solvent Electron Transfer: Configuration Dependence

Ultrafast photoinduced electron transfer between excited oxazine-1 (OX1) dye and electron donating solvents aniline (AN) and N,N-dimethylaniline (DMA) has been studied with the femtosecond fluorescence up-conversion technique. Ultrafast and highly nonexponential fluorescence decay of OX1 is observed in AN and DMA. The fastest component observed is about 60 fs in DMA and assigned to fluorescence quenching due to an electron transfer reaction. Fluorescence decays are observed to be strongly dependent on the wavelength of observation in DMA and moderately dependent in AN. The wavelength dependence is assigned to fluorescence from different configurations of OX1 and solvent molecules. The energy parameters for electron transfer are estimated and discussed.

Femtosecond Double Proton-Transfer Dynamics in [2,2‘-Bipyridyl]-3,3‘-diol in Sol−Gel Glasses

Intramolecular excited state double proton-transfer dynamics has been studied for [2,2‘-bipyridyl]-3,3‘-diol (BP(OH)2) in sol−gel glass. By means of the femtosecond fluorescence up-conversion technique, the spectral dependence of the fluorescence transients obtained for BP(OH)2 in a few sol−gel glasses has been followed. From the temporal behavior of the reconstructed spectra, two concurrent double proton-transfer pathways are concluded to occur in the sol−gel hosts: the first is a concerted double proton-transfer process (within 100 fs after the excitation pulse) and the second is a two-step process involving the reaction from the excited dienol-to-monoketo tautomer (<100 fs), followed by the monoketo-to-diketo step (with a time constant of a few ps). The proton-transfer dynamics has been studied in the sol−gels TMSPM and TMOS, both as a function of the temperature and as a function of the wavelength of the excitation pump pulses. In the sol−gel TMSPM, the proton-transfer dynamics in photoexcited BP(OH)...

Ultrafast intermolecular electron transfer in coumarin–hydrazine system

Ultrafast intermolecular electron transfer (ET) from electron-donating solvents to excited coumarin dyes has been investigated by the femtosecond fluorescence up-conversion technique. In this work we have used hydrazine derivatives as an electron-donating solvent to investigate the effects of very low oxidation potentials of electron-donating solvents on the ET dynamics. The ET rate with the aromatic hydrazines is much faster (sub-picoseconds) than that with the aliphatic hydrazine (a few tens of picoseconds), although the oxidation potential of the aliphatic hydrazine is lower than that of the aromatic hydrazine. However, the coumarin–hydrazine systems used in this study are not in the so-called Marcus inverted region. The results are discussed in light of the theoretical prediction from the two-dimensional ET model. We find that the electronic matrix element should be largely different between the aromatic and aliphatic hydrazine cases, which may result from the different molecular orbitals of these molecules.

Deuterium Isotope Effect on the Solvation Dynamics of Methanol: CH3OH, CH3OD, CD3OH, and CD3OD

Deuterium isotope effects on the solvation dynamics of methanol have been investigated using the femtosecond fluorescence up-conversion technique. Deuterated methanols (CH3OD, CD3OH, and CD3OD) show slower solvation dynamics than normal methanol. The isotope effect arising from the OH group is about 10% and that arising from the CH3 group is about 5%, which are explained by the effects of hydrogen bonding and the frictional constant, respectively.

Deuterium Isotope Effect on Ultrafast Intermolecular Electron Transfer

Deuterium isotope effects on ultrafast intermolecular electron transfer (ET) from electron-donating solvents such as aniline (AN) and N,N-dimethylaniline (DMA) to a number of excited (S1) coumarin dyes have been investigated by a femtosecond fluorescence up-conversion technique. We carry out investigations using both normal and deuterated solvents and normal and deuterated coumarin dyes. With perdeuterated AN (AN-d7) or amino-deuterated AN (AN-d2), we observe almost the same extent of reduction in the ET rate constants as in normal AN. In the case of DMA, we do not observe any isotope effect upon substituting all of the N-methyl group hydrogens by deuterium (DMA-d6). Using deuterated coumarins, where the 7-amino group hydrogens are substituted by deuterium, we do not observe any change in the ET dynamics. The effect of isotopic substitution of AN on the ET dynamics has been discussed in terms of the several ET parameters. Cyclic voltammetric measurements indicated that the isotopic substitution of the solvent AN causes a reduction in the driving force for the ET processes. It is inferred that the differences in the driving force for the normal and the deuterated AN as the donor solvent mainly arise from the solvent structural effects, caused by the intermolecular hydrogen bonding in this solvent. The present results also correlate well with the theoretical predictions of the two-dimensional ET model.

Direct measurements of energy transfer between identical chromophores in solution

The rate of intramolecular electronic excitation energy transfer in 9,9′ bifluorene has been investigated in a variety of solvents, using both time-correlated single photon counting and femtosecond fluorescence upconversion technique. The kinetics of energy transfer were determined in both cases by time dependent fluorescence anisotropy measurements. The energy transfer dynamics between fluorene moieties has been found to occur on a time scale of approximately 600 fs in different solvents and has been correlated with the T2 value calculated from the absorption linewidth and the β value obtained from jet measurements. The dihedral angle between the fluorene moieties was also calculated from the anisotropy measurement and compared with the values obtained from a solution phase NMR determination.