Femtosecond Up-conversion Spectroscopy Research Articles

Photoinduced carrier dynamics in colloidal CdSe/CdS nanoparticles

We have studied picosecond dynamics of photo-induced charge carriers in CdSe/CdS nanoparticles with sizes of 2.0 and 2.9 nm in toluene colloidal solutions using femtosecond up-conversion spectroscopy. The kinetics of nanoparticles luminescence excited by laser pulses of 50 fs duration at 350 nm were observed. The simulation of experimental data showed that electron thermalization occurs within 0.5 ps, while recombination processes have characteristic times of 5 and 20 ns, although these cannot be unequivocally identified using the simplest recombination model with direct transitions and single type of traps. However, a comparative analysis indicates a fundamental difference in the mechanisms responsible for the dynamics of charge carriers in the time interval up to 5 ps after laser excitation, which we associate with different structures of traps in 2.0 and 2.9 nm particles.

Ultrafast photoinduced charge dynamics in colloidal CdSe/CdS quantum dots in toluene

We studied the dynamics of photoinduced charge carriers in core/shell CdSe/CdS quantum dots (QDs) of two sizes, 2.0 and 2.9 nm, in a toluene colloid by femtosecond up-conversion spectroscopy. We observed the luminescence kinetics of QDs excited by 50 fs laser pulses at a wavelength of 385 nm. The modeling of experimental data showed that a 0.5 ps rise of the luminescence intensity corresponds to the process of electron thermalization. The two-exponential luminescence decay with characteristic times in the picosecond range corresponds to populating of two types of traps, which we associate with core and shell defects. Electron–hole recombination process does not affect the shape of the luminescence kinetics in the picosecond range. A comparative analysis indicates that the concentration of defects in 2.0 nm QDs is greater than that in 2.9 nm QDs, while potential wells, which charge carriers eventually escape due to thermal activation, are deeper for 2.9 nm QDs.

Presence of two emissive minima in the lowest excited state of a push-pull cationic dye unequivocally proved by femtosecond up-conversion spectroscopy and vibronic quantum-mechanical computations.

The long-standing controversy about the presence of two different emissive minima in the lowest excited state of the cationic push-pull dye o-(p-dimethylamino-styryl)-methylpyridinium (DASPMI) was definitively proved through the observation of dual emission, evidenced by both experimental (femtosecond up-conversion measurements) and theoretical (density functional theory calculations) approaches. From the fluorescence up-conversion data of DASPMI in water, the time resolved area normalized spectra (TRANES) were calculated, showing one isoemissive point and therefore revealing the presence of two distinct emissive minima of the excited state potential energy hypersurface with lifetimes of 0.51 and 4.8 ps. These spectroscopic techniques combined with proper data analysis allowed us to discriminate the sub-picosecond emitting state from the occurrence of ultrafast solvation dynamics and to disentangle the overlapping fluorescence (very close in energy) of the two components. Vibronic computations based on TD-DFT potential energy surfaces fully confirm those results and provide deeper insights about the key factors playing a role in determining the overall result. The two emissive minima have different structural and electronic characteristics: on one hand, the locally excited (LE) minimum has a flat geometry and an electric dipole moment smaller than the ground state; on the other hand, the twisted-intramolecular-charge-transfer (TICT) minimum shows a rotation of the methylpyridinium moiety with respect to the rest of the structure, and has an electric dipole moment significantly larger than the ground state.

A time resolved fluorescence and quantum chemical study of the solar cell sensitizer D149

The steady-state spectroscopic properties and ultrafast relaxation dynamics of the very promising indoline-based sensitizer dye D149 are reported in various organic solvents. For the study of relaxation dynamics of both S2 and S1 states, time resolved femtosecond upconversion spectroscopy was used. Quantum chemical calculations of excitation energies and dipole moments at the DFT level as well as using high-level correlated second-order approximate coupled-cluster model with singles and doubles (CC2) have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be ∼13D in agreement with calculations. According to our time resolved results, the population of the S2 state has shown a rapid internal conversion to S1, taking place within 0.15–0.45 ps, depending on the solvent. This is followed by a slower decay component indicating that some S2 species relax on a long timescale. As the detection is shifted to the transition from the S1 state, the overall decay dynamics is retarded, while a slow fluorescence rise is observed at long wavelengths, indicating a time-dependent spectral shift. The lifetime of S1 was found to depend on the solvent used, being 630 ps in toluene, 720 ps in chloroform, 670 ps in THF, 700 ps in dichloromethane, 220 ps in acetonitrile and 190 ps in EtOH. The time resolved emission spectra as well as the solvent response function, C(t), obtained in the aprotic solvents, have shown that as the solvent polarity increases a more rapid spectral shift is observed. An exception is found in the case of the protic solvent EtOH due to specific H-bonding effects. The time resolved emission spectra in all solvents experience no significant change of shape and width with time, except from THF, where the initially formed Intramolecular Charge Transfer state is observed at early times.

Interfacial electron transfer dynamics and photovoltaic performance of TiO2 and ZnO solar cells sensitized with Coumarin 343

Abstract In this work, the photovoltaic performance and interfacial electron transfer (IET) dynamics of Coumarin 343 (C343) on ZnO and TiO 2 nanocrystalline semiconductors (sensitized films) were investigated using femtosecond upconversion spectroscopy. Inert Al 2 O 3 nanostructured films were also used for comparison. In addition, electron transfer dynamics were studied in complete solar cells in the presence of a redox electrolyte in order to investigate the relation between electron transfer and solar cell performance. For the sensitized films, a more rapid IET was observed from C343 towards ZnO compared to TiO 2 . An IET efficiency of 0.90 and 0.81 from C343 to ZnO and TiO 2 films respectively, was found at a transition corresponding to an emission wavelength of 480 nm. In the case of complete cells, electron transfer dynamics showed some retardation compared to the sensitized films. It is noteworthy that, in complete TiO 2 cells the injection dynamics were much faster than in ZnO ones, in contrast to the results obtained for sensitized semiconductor films. However, TiO 2 cells sensitized with C343 exhibited lower energy conversion efficiency than ZnO ones, indicating that faster injection is not always associated with better solar cell efficiency and that other factors, such as electron recombination, affect significantly the performance of the cells. Indeed, dark current measurements for ZnO and TiO 2 cells have shown that the recombination of the injected electrons with accepting sites in the electrolyte occurs more rapidly in TiO 2 cells, thus reducing their efficiency.

Femtosecond Decay and Electron Transfer Dynamics of the Organic Sensitizer D149 and Photovoltaic Performance in Quasi-Solid-State Dye-Sensitized Solar Cells

In this work, the femtosecond upconversion spectroscopy was used for the study of ultrafast decay and electron transfer dynamics of the very promising all-organic sensitizer indoline dye, D149, in inert environments as well as on TiO2 substrates. The inert environments were toluene and acetonitrile solutions as well as poly(methyl methacrylate) (PMMA) films and Al2O3 nanostructured substrates. These were used to investigate relaxation and aggregation induced decays. TiO2 substrates were used to probe interfacial electron transfer dynamics. The excitation was at the S2 band while the dynamics of both S2 and S1 bands were detected. The dynamics of the S2 band in solutions and PMMA film, show a rapid S2–S1 internal conversion, ranging from 0.20 to 0.45 ps depending on the environment. This component becomes <0.10 ps in Al2O3 and in TiO2. The dynamics of the S1 band probe both hot and cold S1 species. In solutions and PMMA, the hot species relax to cold ones within a few tens of picoseconds while the cold ones decay to the ground state within a few hundreds of picoseconds. In Al2O3, the dynamics become much faster because of aggregation induced quenching. In TiO2, the S1 dynamics exhibit three mechanisms of 0.1, 1.3, and 8.6 ps time constants indicating a multiphasic injection mechanism. The electron transfer quantum yield and rate from the S1 band were calculated, using the time-resolved data, and were found to be 0.90 and 3.82 × 1011 s–1, respectively. Finally, quasi-solid-state dye-sensitized solar cells, with D149 as sensitizer, were constructed, and their efficiency was found 5.66%. Similar cells, using the ruthenium complexes N719 and black dye as sensitizers, were also constructed for comparison. It was found that D149 showed higher efficiency than black dye but lower than N719.

Energy and electron transfer in the photosynthetic reaction center complex of Acidiphilium rubrum containing Zn-bacteriochlorophyll a studied by femtosecond up-conversion spectroscopy

A photosynthetic reaction center (RC) complex was isolated from a purple bacterium, Acidiphilium rubrum. The RC contains bacteriochlorophyll a containing Zn as a central metal (Zn-BChl a) and bacteriopheophytin a (BPhe a) but no Mg-BChl a. The absorption peaks of the Zn-BChl a dimer (P Zn), the accessory Zn-BChl a (B Zn), and BPhe a (H) at 4 K in the RC showed peaks at 875, 792, and 753 nm, respectively. These peaks were shorter than the corresponding peaks in Rhodobacter sphaeroides RC that has Mg-BChl a. The kinetics of fluorescence from P Zn *, measured by fluorescence up-conversion, showed the rise and the major decay with time constants of 0.16 and 3.3 ps, respectively. The former represents the energy transfer from B Zn * to P Zn, and the latter, the electron transfer from P Zn to H. The angle between the transition dipoles of B Zn and P Zn was estimated to be 36° based on the fluorescence anisotropy. The time constants and the angle are almost equal to those in the Rb. sphaeroides RC. The high efficiency of A. rubrum RC seems to be enabled by the chemical property of Zn-BChl a and by the L168HE modification of the RC protein that modifies P Zn.

A femtosecond study of photoinduced electron transfer from dimethylaniline to coumarin dyes in a cetyltrimethylammonium bromide micelle

Ultrafast photoinduced electron transfer (PET) from N,N-dimethylaniline to coumarin dyes in cetyltrimethylammonium bromide (CTAB) micelle is studied using femtosecond upconversion spectroscopy. The rate of PET in a CTAB micelle is found to be highly nonexponential with components much faster (approximately 10 ps) than the slow components of solvation dynamics. The ultrafast components of electron transfer exhibits a bell-shaped dependence on the free energy change which is similar to the Marcus inversion.

Ultrafast Relaxation Dynamics from the S2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy

We have studied the relaxation dynamics in a triphenyl methane (TPM) dye, malachite green (MG), following S2-state excitation using fluorescence up-conversion measurements in ethanol and ethylene glycol solutions. Information on the mechanisms and dynamics of radiationless transitions from higher excited states in TPM dyes is interesting from the torsional dynamic aspects of the phenyl rings and for the applications of TPM dyes involving knowledge of the excited-state relaxation channels. Kinetic measurements at different wavelengths suggested a cascade population relaxation along the S2 state with a time constant of ∼130 fs that is almost independent of solvent viscosity in the two solvents used, contrary to the relaxation dynamics of the S1 state. Wavelength-dependent time-resolved anisotropy measurements along the S2 and S1 emission bands displayed successive reductions in the anisotropy values, indicating a continuous evolution to the S1 surface. A detailed analysis led us to propose a relaxation path...

Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA–ANL) and 280 fs (TCA–DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kCS) determined from the decay of the acceptor fluorescence lie between 3.6×1012–6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kCS was observed in the region of ΔE of 0.36–1.47 eV. This relation between kCS and ΔE is quite different from the charge recombination rate (kCR) vs ΔE relation in the charge transfer (CT) complexes, which has been reported by Mataga et al. The present result can be explained by a model which takes into account the interaction between the zeroth-order D⋅A* and D+⋅A− states. Ultrafast charge separation would occur from the D⋅A* to exciplex state on an adiabatic potential curve without barrier along the coordinate of the intermolecular stretching vibration. This is considered to be induced by the strong interaction between the donor and acceptor at short distances.

Ultrafast fluorescence quenching by electron transfer and fluorescence from the second excited state of a charge transfer complex as studied by femtosecond up-conversion spectroscopy

Photoinduced electron transfer dynamics between fluorescer (acceptor, A) and quencher (donor, D) was investigated by measuring the fluorescence quenching using femtosecond up-conversion spectroscopy. The measurements were made in a quencher concentration range of 0.15 M–1 M and also in a neat quencher solvent. Fluorescence decay at times longer than 5 ps can be explained by combining the diffusion equation with the Marcus equation of electron transfer. At higher quencher concentrations (&amp;gt;0.3 M), an additional component with a time constant of ∼250 fs appears. At these concentrations, the fluorescers (9-cyanoanthracene, CA and 9,10-dicyanoanthracene) and the quenchers (N,N-dimethylaniline, DMA) were found to form “weak” CT complexes. Fluorescence from the S1 state of the CA-DMA complex was detected by steady state spectroscopy. The excitation spectrum observed at the maximum intensity of this fluorescence indicates the existence of an excited S2 state of the CT complex near the energy of D⋅A* (the locally excited state of the pair). Excitation of CA at 400 nm leads to simultaneous excitation of the CT complex to the S2 state. It was concluded that the fast component is the fluorescence from the S2 state of the complex. This was confirmed by the concurrent rise of the S1 fluorescence of the CA-DMA complex. The fast decay of ∼250 fs is caused by the competition between the radiative transition S2→S0 and the nonradiative internal conversion S2→S1. The fast S2→S1 nonradiative transition can be regarded as a charge separation process.

Femtosecond dynamics of secondary radiation formation from quantum well excitons

The time-resolved secondary emission of resonantly created excitons in GaAs quantum wells is studied using femtosecond up-conversion spectroscopy. The behaviour of the rise and decay of the secondary emission and reflectivity in quantum wells is strongly dependent upon the disorder at the interfaces, the exciton density and the temperature. In the case of low densities and temperatures the emission is independent of the exciton density and rises quadratically in time, in excellent agreement with recent theory for Rayleigh scattering from two-dimensional excitons subjected to disorder. These rise times are compared directly with T 2 times measured by time-integrated four-wave mixing (FWM). The comparison of the dynamics displayed in time-resolved secondary radiation and time-integrated FWM provide a clear understanding of the coherence properties of QW excitons in the first few picoseconds after excitation. High-contrast oscillations that are due to quantum beats between the heavy- and light-hole 1s-states are seen. The visibility decay at very low densities is long τ HL=25 ps and is related to the action of potential fluctuations on the scattering of heavy-hole and light-hole excitons.