Saturable absorption dynamics of mode-locking dye DODCI

0 –S₁–S_m–S_n multilevel system. An S_m→S₁ absorption recovery time of τ^S1 _A=(600±100) fs is determined. The picosecond and femtosecond pulse excitation leads to S₀→S₁ ground-state absorption bleaching and S₁→S_m first excited-state absorption bleaching. The excited-state absorption cross-sections σ^S1 _ex(S₁→S_m) and σ^Sm _ex(S_m→S_n) are determined. Received: 3 June 1996     

Intersystem crossing in oligothiophenes studied by fs time-resolved spectroscopy

High triplet quantum yields of more than 90% for bithiophene and terthiophene have to be connected with very fast and effective formation of triplets after excitation. We studied these processes with fs pump–probe spectroscopy. The time behaviour of transient optical spectra within the singlet and triplet manifold was examined for bi- and terthiophene (2T and 3T) in solution. For 2T we used two-photon absorption for excitation. We found transient spectra of the excited singlet state, the triplet state and that of radical cations. The kinetics of the excited-state absorption was described by a bi-exponential function. Additionally we observed formation and recombination of radical cations. The recombination is connected with triplet formation. Both processes could be described by a time constant of 62 ps±9 ps. For 3T we found a dependence of the processes on excitation energy using one-photon absorption. The triplet quantum yield increased with higher excitation energy. The kinetics becomes bi-exponential with increasing amplitude of the short time constant of 2 ps at increasing excitation energy. The main reasons for the effective intersystem crossing (ISC) in both oligothiophenes are – besides the high spin-orbit coupling factor introduced by the sulphur atom – the almost isoenergetic positions of the S ₁ and T ₂ states, detected by PD-PES [1]. At higher photon excitation energy for 3T above the band gap an additional channel for ISC was detected. We believe that during the geometric change from the non-relaxed non-planar to the relaxed planar excited state S ₁, ultrafast intersystem crossing takes place. Received: 6 December 1999 / Published online: 2 August 2000     

Femtosecond time-resolved difference absorption spectroscopy of all-<Emphasis Type="Italic">trans</Emphasis>-β-Apo-8′-carotenal

The femtosecond time-resolved difference absorption spectra of all-trans-β-Apo-8′-carotenal have been recorded and analyzed by the singular-value decomposition (SVD) method followed by global fitting using a sequential model for the excited-state energy relaxation. With this model, we have obtained the excited-state absorption spectra and the lifetimes of the corresponding excited states both in nonpolar solvent n-hexane and polar solvent methanol. Three excited states, namely S₃(170fs), S₂(2.32ps) and S₁(26ps) in n-hexane, and two excited states S₂(190fs) and S₁(9.4ps) in methanol have been observed. The excited-state absorption spectra of all-trans-β-Apo-8′-carotenal in methanol display a red shift and broadeness, while the lifetime of S₁ state becomes shorter. It is proposed that these effects are related to the presence of a carbonyl functional group that leads to the solvent effect on the excited-state energy level. At the same time, it is shown that the SVD method is a useful tool in resolving the time-resolved absorption spectra.     

Molecular thermometers based on phosphorescent porphin metal complexes

We study the temperature dependence of the radiative deactivation of triplet states of Pdand Pt-porphin molecules in n-alkane matrices in the temperature range 4.2–210 K. The nature of the thermally activated “hot” lines that are observed in phosphorescence spectra of Pdand Pt-porphin is discussed in detail. We show that, because of the degeneracy lifting of the triplet state T _{1, 2} in the crystal field of n-alkane matrices, lines of the transitions T1 → S ₀ and T ₂ → S ₀ are spectrally spaced and, in all cases, the T ₂ state is the first state that is activated with increasing temperature. We analyze the dependences of the T ₂-T ₁ splitting on the chelated metal ion and the type of the matrix. The possibility of measuring cryogenic temperatures with molecular thermometers that use the thermochromic properties of Pdand Pt-porphin in n-alkane matrices is discussed.     

Optical properties of new indotricarbocyanine dye as a limiter of laser radiation power

We present results of experimental and theoretical studies of the optical characteristics of a new indotricarbocyanine dye that is capable of effectively limiting the power of laser radiation in the visible spectral range. The spectral-luminescent and energy characteristics of the dye molecules and their absorption spectra from the excited state with nanosecond resolution are investigated experimentally. Quantum-chemical methods are used to calculate electronic absorption spectra from the ground (S₀ → S_n) and excited (S₁ → S_n) states and to determine the nature of electronic states of the molecule and the rate constants of intramolecular photophysical processes. The results of the theoretical research agree with experimental data. It is shown that the investigated dye has singlet-singlet absorption at 400–600 nm. Nonlinear absorption of the dye upon excitation by radiation of the second harmonic of a Nd:YAG laser is studied by z-scanning with an open diaphragm. The ratio of dye absorption cross sections from the excited and ground states at 532 nm is determined in the framework of a three-level model. The results are compared with those for previously studied compounds. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 473–480, July–August, 2007.     

Ultrafast S1 and ICT state dynamics of a marine carotenoid probed by femtosecond one- and two-photon pump-probe spectroscopy

Ultrafast relaxation kinetics of fucoxanthin in polar and non-polar solvents have been studied by femtosecond pump-probe spectroscopy. Transient absorption associated with S₁ or intramolecular charge transfer (ICT) excited state has been observed following either one-photon excitation to the optically allowed S₂ state or two-photon excitation to the symmetry-forbidden S₁ state. The results suggest that the ICT state formed after excitation of fucoxanthin in a polar solvent is a distinct excited state from S₁.     

Induced Intersystem Crossing at the Fluorescence Quenching of Laser Dye 7-Amino-1,3-Naphthalenedisulfonic Acid by Paramagnetic Metal Ions

The fluorescence and triplet state quenching of 7-amino-1,3-naphthalenesulfonic acid by paramagnetic metal ions have been investigated in an aqueous medium. The basic mechanism of the fluorescence quenching involves the static and dynamic electron transfer to the paramagnetic cation. The induced S₁→T₁ intersystem crossing at fluorescence quenching of the fluorophore by Cu²⁺ cation has been found. There is a correlation between triplet state quenching rate constants and values of the efficient paramagnetic susceptibility and spin of the cations. The rate constants for the quenching pathways have been calculated.     

Correlations of polarizations and entangled states in the two-photon system

The correlations of the linear and circular polarizations in the system of two photons have been theoretically investigated. The polarization of a two-photon state is described by the one-photon Stokes parameters and by the components of the correlation “tensor” in the Stokes space. It is shown that in the case of two-photon decays π⁰ → 2γ, η → 2γ, K _L ⁰ → 2γ, K _S ⁰ → 2γ and the cascade process |0〉 → |1〉 + γ → |0〉 + 2γ(|0〉 and |1〉 are states with the spin 0 and 1, respectively) the final two-photon state represents a characteristic example of the entangled (nonfactorizable) state, and the correlations between the Stokes parameters in all these decays have the purely quantum character: the incoherence inequalities of the Bell type for the components of the correlation “tensor”, established previously for the case of classical “mixtures”, are violated. The general analysis of the registration procedure for two correlated photons by two one-photon detectors is performed.     

Spectroscopic investigations of a novel push-pull azo compound embedded in rigid polymer

A new heterocyclic push-pull azo compound-in-poly(methymethacrylate) (PMMA) film has been made by means of the spin-coating method. The spectroscopic properties of the films have been investigated with the steady-state absorption spectra, and steady-state fluorescence and femtosecond time-resolved fluorescence spectra in the first time, which is an important characteristic for the application of the film. The excited singlet (S₁) state lifetimes for trans and cis isomers of the film at room temperature have been measured. The excited triplet (T₁) state lifetime of cis isomer of the film has been obtained. The electronic structure of the film has been explained. The results show that the aggregate state of the azo molecules greatly influences its absorption spectra.     

Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in β-Cyclodextrin

The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated β-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in β-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in β-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T ₁ and T ₂ of triplet BP at 30 K measured by pulse EPR are found to be different in β-CD compared to nonpolar toluene glass. The observed increase of T ₂ by up to a factor of four in β-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in β-CD. The influence of β-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.     

Effect of extra liganding on spectral and kinetic characteristics and on quenching of excited states of multiporphyrin complexes by molecular oxygen

Based on studies of spectral and kinetic parameters of dimers of Zn porphyrins and of multiporphyrin self-assembling complexes formed on their basis, we find that extra liganding of dimers by pyridine inappreciably lowers the energy of the triplet level E(T ₁). We show that, in this case, the nonradiative deactivation T ₁ ↝ S ₀ of the electronic excitation energy of dimers Zn porphyrins increases not only due to increasing Franck-Condon factor. We discuss mechanisms of the quenching action of an extra ligand related to an accepting role played by high-frequency overtones of vibrations of extra ligand molecules, to an enhancement of the spin-orbit interaction due to energy lowering of σπ* states, and to out-of-plane distortion of dimers caused by the displacement of the Zn²⁺ ion out of the plane of the tetrapyrrole macrocycle. Quenching of triplet states of extra liganded dimers of Zn porphyrins by molecular oxygen in liquid solutions at 295 K depends on the character of donor-acceptor interactions with pyridine and rigidity of a linking molecular fragment. We find that the rate constants of oxygen quenching of the excited electronic states S ₁ and T ₁ of multiporphyrin complexes depend on their structure and composition, as well as steric hindrances, created by dimers (screening effects) for contact interactions of a π-conjugated system of the free base (extra ligand) with molecular oxygen. Screening effects of extra ligands by dimer molecules of Zn porphyrins, which reduce the oxygen quenching rate constants k _S and k _T , are found to barely affect the singlet-oxygen generation quantum yield γΔ.     

Sub-natural linewidth resonances in coherently-driven double Λ system

We investigate theoretically the pump-probe spectroscopy of coherently-driven four-level Λ system with two closely spaced excited common levels, thereby forming a double Λ system. Using the master equation approach, analytical results are obtained for the absorption spectrum of a weak probe in the presence of a strong pump. The model is applied to the double Λ system 5² S _1/2 F = 1, 2 → 5² P _3/2 F′ = 1,2 of ⁸⁷Rb atom. It is shown that the absorption spectrum consists of a triplet, of which one resonance is of sub-natural linewidth depending on the atom-field interaction parameters. The effect of Doppler broadening on the absorption spectrum is also investigated.     

Excited singlet and triplet states of aromatic hydrocarbons determined by a laser photolysis technique

A kinetic spectrophotometric technique, employing a Q-switched, frequency-doubled ruby laser as the excitation source, has been used to determine the short-lived (S₁ → S_p), and long-lived (T₁ → T_q) transient absorption spectra of several benzenoid aromatic hydrocarbons. The excited singlet states determined in this way are compared with the singlet states observed by absorption from the ground state (S₀ → S_n). Some previously assigned triplet states are reassigned as singlet levels.     

Fast photoprocesses in a symmetric indotricarbocyanine dye (HITC) in solutions

Spectral-kinetic and photochemical properties of HITC dye with iodide and perchlorate counterions have been studied in environments where the dye molecules exist in different ionic forms. In ethanol, the dye molecules exist as free ions; in dichlorobenzene, as contact ion pairs. Superfast transformation of non-stationary spectra in an HITC dye bleaching band is found. The observed effects are interpreted within the framework of concepts on “burning out” a notch in the contour of a non-uniformly widened vibronic band of S ₀ → S ₁-absorption. Qualitative differences in recorded absorption spectra from the dye excited electronic states for weakly and highly polar solvents are found. It is shown that the observed differences are caused by superfast charge transfer in the contact ion pairs that results in the formation of free radicals.     

One- and Two-Photon Fluorescence Anisotropy of Selected Fluorene Derivatives

The steady-state excitation anisotropy spectra of fluorene derivatives were measured in viscous solvents, under the one- and two-photon excitation, over a broad spectral range (UV–Visible). The orientation of their absorption transition moments for the first, S₀S₁, and second, S₀S₂, excited states were determined. It was shown experimentally that a decrease in the angle between S₀S₁and S₀S₂ transitions corresponded to an increased value of two-photon absorption (2PA) cross section for these molecules. Two-photon excitation anisotropy was nearly constant over the spectral region investigated (in contrast to one-photon excitation anisotropy spectra) and can be roughly explained by a simple model of 2PA based on the single intermediate state approximation. For comparison, the same trend in two-photon excitation anisotropy was observed for Rhodamine B inglycerol.     

Role of charge-transfer complexes in oxygen quenching of excited states of anthracene derivatives in the gas phase

Oxygen quenching of excited triplet and singlet states of gas-phase anthracene and its derivatives that have similar energies of the lower triplet levels but widely different oxidation potentials (0.44 < E_ox < 1.89 V) was studied. Quenching rate constants for singlet (k_S^O2) and triplet (k_T^O2) states in addition to the fraction of oxygen-quenched singlet and triplet states q_{S 1}(T₁^O2 were determined from the decay rates, fluorescence intensities, and delayed fluorescence as functions of oxygen pressure. It was found that k_S^O2 values vary from 2·10⁴ (9,10-dicyanoanthracene) to 1.2·10⁷ sec⁻¹·torr⁻¹ (anthracene, 9-methylanthracene, 2-aminoanthracene) and k_S^O2 values from 5·10² to 1·10⁵ sec⁻¹·torr⁻¹. The k_S^O2 values for anthracene, 9-methylanthracene, and 2-aminoanthracene, which have fast rates of interconversion from S₁ to T₁, are close to the rate constants for gas-kinetic collisions and are independent of the oxidation potentials (E_ox). The quenching rate constants k_S^O2 for the other anthracene derivatives and k_T^O2 for all studied compounds decrease with increasing free energy of electron transfer ΔG_ET, which indicates the important role of charge-transfer interactions in the oxygen quenching of singlet S_1- and triplet T₁ states. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 36–42, January–February, 2008.     

Determination of intramolecular energy-transfer efficiency in ytterbium chelates

The quantum yield of²F_5/2 →²F_7/2 luminescence of the Yb³⁺ ion has been measured for ytterbium tris-thenoyltrifluoroacetonate upon excitation in the UV S₀ → S₁ absorption band of a ligand and in the IR²F_7/2 →²F_5/2 absorption band of the Yb³⁺ ion. It has been established that the quantum efficiency of intramolecular transfer of electron excitation energy from the lowest triplet state of the ligand to the excited²F_5/2 level of Yb³⁺ is equal to unity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 539–542, July–August, 2000.     

A theoretical analysis of the excited states in 2-benzoylthiophene

The lower excited states of 2-benzoylthiophene have been studied using ab initio quantum chemical methods based on multiconfigurational wave functions. Six singlet and six triplet excited states have been characterized. The geometry has been optimized for the two lowest triplet states, which are responsible for the photoreactivity of the chromophore in the photosensitizing drug tiaprofenic acid. The T¹(π → π?) and T₂(n → π?) states have been found to be close in energy with the π → π? state slightly lower. The excited states have been characterized using density difference and spin density plots. The different photochemical behaviour of the two triplet states can be rationalized from the theoretical data.     

Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T ₁ formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S ₁ occurs by two channels: S ₁ ⤳ T ₁ and S ₁ ⤳ S ₀, the latter being predominant. As the S ₁ level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S ₁ ⤳ S ₀ channel in the dissipation of the electronic excitation energy.     

Nonlinear absorption of CS2 at the wavelength of 400 nm with femtosecond pulses

Nonlinear absorption of carbon disulfide (CS₂) was investigated by Z-scan technique and time-resolved pump-probe technique with femtosecond pulses at 400 nm wavelength. By the two techniques, we confirmed that the nonlinear absorption of CS₂ arise from a combination of two-photon absorption (TPA) and the excited state absorption induced by TPA under the incident laser pulses with 400 nm wavelength. The coefficient of TPA, the absorption cross-section of low excited state and lifetime of low excited state were obtained by theoretical fitting the experimental results. The results indicated that the CS₂ has good optical limiting capability at 400 nm wavelength.