A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen (¹Δ_g), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with ¹Δ_g in benzene is predominantly physical in character with only a small (6%) contribution from chemical reaction.     

TRIPLET STATES OF CAROTENOIDS BOUND TO REACTION CENTERS OF PHOTOSYNTHETIC BACTERIA: TIME-RESOLVED RESONANCE RAMAN SPECTROSCOPY

Time-resolved, low-temperature resonance Raman spectra of triplet states of the carotenoids specifically present in bacterial reaction centers in a strained cis conformation have been obtained, thus demonstrating the possibility of studying intermediate transient states of these structures using resonance Raman spectroscopy. Resonance Raman spectra of triplet cis spheroidene and cis methoxyneurosporene present in reaction centers of Rhodopseudomonas spheroides, (strains 2.4.1. and Ga, respectively) exhibit marked differences with those of triplet, all- trans carotenoids previously studied in vitro. These differences, together with the frequency shifts measured for the v ₁ modes, indicate that triplet carotenoids bound to reaction centers retain a cis conformation, and that probably no isomerization occurs to all- trans carotenoids upon T ← S₀ excitation. P_i electron distributions along the polyene backbone are probably less regular in the triplet state than in the singlet ground state, although probably not to the extent suggested by previous theoretical calculations. The apparently anomalous behaviour of the v ₂ bands of all- trans carotenoids upon T ← S₀ excitation is shown to result largely from the actual complexity of this region of the Raman spectra, together with a weak participation of the v _c—–c internal coordinate in the corresponding modes. Finally, the Raman scattering efficiency of triplet spheroidene bound to reaction centers is lower than that of the singlet, ground state form, under equivalent excitation conditions.     

Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and 1700 cm⁻¹ in room-temperature acetonitrile-d₃ solution. These bands and additional ones observed between 1300 and 1450 cm⁻¹ are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4O carbonyl exhibits substantial single-bond character, explaining the large (70 cm⁻¹) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed ¹nπ^* state as the triplet precursor.     

Parametrically polarization shaped pulses guided via a hollow core photonic crystal fiber for coherent control

We present ultrafast polarization pulse shaping through a micro structured hollow core photonic crystal fiber. The pulses are shaped in pulse sequences in which the energy, distance, phases, and chirps as well as the state of polarization of each individual sub-pulse can be independently controlled. The application of these pulses for coherent control is demonstrated for feedback loop optimization of the multi-photon ionization of potassium dimers. In a second experiment, this process is investigated by shaper-assisted pump-probe spectroscopy which is likewise performed with pulses that are transmitted through the fiber. Both techniques reveal the excitation pathway including the dynamics in the participating electronic states and expose the relevance of the polarization. These methods will be valuable for endoscopic applications.     

Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.     

Spiro[4.4]nonatetraene and its Positive and Negative Radical Ions: Molectronic Structure Investigations

The electronic structure of spiro[4.4]nonatetraene 1 as well as that of its radical anion and cation were studied by different spectroscopies. The electron‐energy‐loss spectrum in the gas phase revealed the lowest triplet state at 2.98 eV and a group of three overlapping triplet states in the 4.5 – 5.0 eV range, as well as a number of valence and Rydberg singlet excited states. Electron‐impact excitation functions of pure vibrational and triplet states identified various states of the negative ion, in particular the ground state with an attachment energy of 0.8 eV, an excited state corresponding to a temporary electron attachment to the 2b₁ MO at an attachment energy of 2.7 eV, and a core excited state at 4.0 eV. Electronic‐absorption spectroscopy in cryogenic matrices revealed several states of the positive ion, in particular a richly structured first band at 1.27 eV, and the first electronic transition of the radical anion. Vibrations of the ground state of the cation were probed by IR spectroscopy in a cryogenic matrix. The results are discussed on the basis of density‐functional and CASSCF/CASPT2 quantum‐chemical calculations. In their various forms, the calculations successfully rationalized the triplet and the singlet (valence and Rydberg) excitation energies of the neutral molecule, the excitation energies of the radical cation, its IR spectrum, the vibrations excited in the first electronic absorption band, and the energies of the ground and the first excited states of the anion. The difference of the anion excitation energies in the gas and condensed phases was rationalized by a calculation of the Jahn‐Teller distortion of the anion ground state. Contrary to expectations based on a single‐configuration model for the electronic states of 1 , it is found that the gap between the first two excited states is different in the singlet and the triplet manifold. This finding can be traced to the different importance of configuration interaction in the two multiplicity manifolds.     

KINETIC AND SPECTROSCOPIC FEATURES OF SOME CAROTENOID TRIPLET STATES: SENSITIZATION BY SINGLET OXYGEN

Abstract— Triplet-triplet absorption spectra of a series of carotenoid pigments in benzene solution have been determined by pulse radiolysis experiments. The natural lifetimes in deaerated solution have also been measured. They fall in the range 2–10 µ s as found for other carotenoids under similar conditions. Pulsed laser (337 nm) excitation of benzene solutions containing oxygen, carotenoid and a photosensitized molecule (anthracene) showed the generation of absorption spectra of the triplet states. These absorptions decayed by first order kinetics in such a way as to indicate that they were formed in reactions with singlet oxygen, itself generated by interaction with the anthracene triplet state. Bimolecular rate constants for energy transfer from O*₂ (¹△_g), to carotenoid have been evaluated.     

Direct observation of a 1MLCT state by ultrafast transient absorption spectroscopy in Mo2(O2C-9-anthracene)4

We have observed the excited-state dynamics of Mo2(O2C-9-anthracene)4 in THF using ultrafast transient UV/vis absorption spectroscopy. Following excitation at 514.5 nm into the metal-to-ligand charge-transfer (MLCT) state, transient absorption bands of the 1MLCT state appear at 410 and 610 nm. We assign these features to the 1MLCT state, which has a lifetime of approximately 10 ps. The decay of 1MLCT is accompanied by the formation of the triplet 3MLCT state, with an absorption band peaking at 410 nm. Excitation at 347 nm populates directly the ligand-centered (LC) state. From the rise of the 1MLCT band, the lifetime of the 1LC state is estimated to be less than 1 ps.     

The Photobiological Differences of Gilvocarcins V and M Are not Related to their Transient Intermediates and Triplet Yields

Abstract— The transient absorption spectra of the intermediates produced by the 355 nm laser excitation of gilvocarcin derivatives have been investigated in various solvents. The spectra consist of a triplet-triplet absorption in the visible region and a residual absorption observed between 340 and 700 nm due to a long-lived species, assigned to the radical cation. A broad-fast decaying band with a maximum at around 700 nm attributed to the solvated electron is also seen in solutions containing a low DMSO/water volume ratio and at 266 nm irradiation of a 50% methanol/water solvent mixture. The molar absorption coefficient of the triplet state of gilvocarcin V (GV) and gilvocarcin M (GM), determined by the energy transfer method, is independent of the solvent properties and has a value of 3.0 × 10⁴/ M cm. The triplet decay rate constants for both drugs are between 1 and 5 × 10⁴/s. A similar initial yield and triplet decay rate constant of GV were observed in the presence of 3.4 m M thymine. Thus, a quenching rate constant of the GV's triplet state by thymine is estimated to be lower than 10⁶/Ms. The triplet quantum yields of both antibiotics determined by using the comparative method are higher in dimethylsulfoxide (DMSO) (0.18) than are those corresponding to 25% DMSO/water (0.06). The decrease in φ_T in the presence of water could be attributed to an enhanced internal conversion rate constant from the S₁ state or to an increase in the photoionization yield. The similarity of the transient intermediates and their yields for GV and GM suggest that their photobiological differences are due to other factors such as DNA binding constants, preferential localization of the drugs in the cell or the enhanced reactivity of the vinyl group toward cellular components.     

Chirp dependence of wave packet motion in oxazine 1

The motion of vibrational wave packets in the system oxazine 1 in methanol is investigated by spectrally resolved transient absorption spectroscopy. The spectral properties of the probe pulse from 600 to 700 nm were chosen to cover the overlap region where ground-state bleach and stimulated emission signals are detected. The spectral phase of the pump pulse was manipulated by a liquid crystal display based pulse-shaping setup. Chirped excitation pulses of negative and positive chirp can be used to excite vibrational modes predominantly in the ground or excited state, respectively. To distinguish the observed wave packets in oxazine 1 moving in the ground or excited state, spectrally resolved transient absorption experiments are performed for various values of the linear chirp of the pump pulses. The amplitudes of the wave packet motion show an asymmetric behavior with an optimum signal for a negative chirp of -0.75 +/- 0.2 fs/nm, which indicates that predominantly ground-state wave packets are observed.     

Efficient and robust strong-field control of population transfer in sensitizer dyes with designed femtosecond laser pulses

We demonstrate control of electronic population transfer in molecules with the help of appropriately shaped femtosecond laser pulses. To this end we investigate two photosensitizer dyes in solution being prepared in the triplet ground state. Excitation within the triplet system is followed by intersystem crossing and the corresponding singlet fluorescence is monitored as a measure of population transfer in the triplet system. We record control landscapes with respect to the fluorescence intensity on both dyes by a systematic variation of laser pulse shapes combining second order and third order dispersion. In the strong-field regime we find highly structured topologies with large areas of maximum or minimum population transfer being insensitive over a certain range of applied laser intensities thus demonstrating robustness. We then compare our experimental results with simulations on generic molecular potentials by solving the time-dependent Schr?dinger equation for excitation with shaped pulses. Control landscapes with respect to population transfer confirm the general trends from experiments. An analysis of regions with maximum or minimum population transfer indicates that coherent processes are responsible for the outcome of our excitation process. The physical mechanisms of joint motion of ground and excited state wave packets or population of a vibrational eigenstate in the excited state permit us to discuss the molecular dynamics in an atom-like picture.     

HYPERICIN DERIVED TRIPLET STATES and TRANSIENTS IN ALCOHOLS and WATER

Abstract— Direct pulse photoexcitation of an antivirally active compound, hypericin sodium salt in ethanol, results in a short-lived transient, attributed to a triplet state. In the presence of reducing agents, a long-lived transient is observed, which indicates a radical anion species. In isopropanol solution, an identical triplet state is formed, accompanied by a long-lived intermediate that consists of a semiquinone-type radical. Laser excitation of hypericin sodium salt aggregates dispersed in water produces a very short-lived transient, also assigned to a triplet state, which decays, leaving an absorption spectrum, indicating a radical anion species. The latter reacts with oxygen with a rate constant of k ∼ 6 × 10⁷ M^-1 s^-1, suggesting the formation of superoxide.     

A Unified Experimental/Theoretical Description of the Ultrafast Photophysics of Single and Double Thionated Uracils

Photoinduced processes in thiouracil derivatives have lately attracted considerable attention due to their suitability for innovative biological and pharmacological applications. Here, sub-20 fs broadband transient absorption spectroscopy in the near-UV are combined with CASPT2/MM decay path calculations to unravel the excited-state decay channels of water solvated 2-thio and 2,4-dithiouracil. These molecules feature linear absorption spectra with overlapping ππ* bands, leading to parallel decay routes which we systematically track for the first time. The results reveal that different processes lead to the triplet states population, both directly from the ππ* absorbing state and via the intermediate nπ* dark state. Moreover, the 2,4-dithiouracil decay pathways is shown to be strongly correlated either to those of 2- or 4-thiouracil, depending on the sulfur atom on which the electronic transition localizes.     

Optimal control methods applied on the ionization processes of alkali dimers

We present two novel optimization methods by employing shaped fs-laser pulses in a closed feedback loop. The first describes control pulse cleaning where extraneous features were removed by applying genetic pressure on certain pulse components. The second reports parametric optimization with intuitive parameters such as subpulse distances, chirps, phase differences, and spectral peak patterns. These methods were conducted on the ionization process of alkali dimers produced in a molecular beam and improved the performances of the optimized pulses compared with short pulses at the same pulse energy. Moreover, we attempt to analyze the obtained pulse shapes regarding the underlying optimized processes. Further investigations concerning isotope selective fragmentation and optimal control of excitation processes of ultracold rubidium dimers in a magneto-optical trap (MOT) are also shown.     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

Detection of benzaldehyde and formaldehyde in the UV photolysis of gas-phase methyl benzoate

The excitation of gas-phase methyl benzoate at 240 nm leads to the observation of phosphorescence. The dispersed phosphorescence spectrum has an assigned origin of 25 270 cm⁻¹ and a prominent C=O progression of 1720 cm⁻¹, consistent with literature reports of gas-phase benzaldehyde spectroscopy. Weaker bands, which correspond to formaldehyde ν₁₇ and ν₂₅, are also evident. Time-resolved IR diode laser absorption spectroscopy has been used to probe formaldehyde. Excitation of methyl benzoate at 222 nm clearly indicates the generation of formaldehyde as a photoproduct. The temporal profile of the formaldehyde signal is consistent with significant nascent vibrational excitation in this product. The ratio of formaldehyde initially in the ground vibrational state to that in the excited vibrational states is estimated to be 0.6 ± 0.1. The proposed elimination mechanisms are analogous to those postulated for the formation of CO₂ and acetaldehyde from pyruvic acid.     

Observation of triplet intraligand excited states through nanosecond step-scan Fourier transform infrared spectroscopy

Nanosecond step-scan Fourier transform infrared spectroscopy permits the observation of triplet intraligand ((3)IL) character in the excited states of [Ru(bpy)2(PNI-phen)]2+ and [Ru(PNI-phen)3]2+ where PNI is 4-piperidinyl-1,8-naphthalimide. After pulsed 355-nm laser excitation, the two ground-state imide C=O bands in each compound are bleached and two substantially lower energy vibrations are produced; the lower energy feature appears as two distinct bands split by an overlapping transient bleach. Model studies confirm that the time-resolved vibrational data are consistent with photoinduced sensitization of the 3IL excited state. Density functional theory calculations also support these assignments because localization of triplet electron density on the PNI moiety is expected to lead to red-shifted C=O vibrations of magnitude similar to those measured experimentally. The current results illustrate that triplet electron density can be directly tracked by time-resolved infrared measurements in metal-organic chromophores and that frequency shifts comparable to those observed in charge-transfer systems can be realized.     

Ultrafast dynamics of exciton fission in polycrystalline pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ～80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.     

LUMINESCENCE AND PHOTOCHEMISTRY OF 4-THIOURIDINE IN AQUEOUS SOLUTION

Abstract— In bidistilled water, 4-thiouridine (4TU) exhibits a weak unusual luminescence, the quantum yield of which is 3 × 10^-4 at 25°C. The excitation spectrum corresponds well to the 4TU absorption spectrum. The emission lies at longer wavelengths (Λ_max 550 nm) than the 4TU phosphorescence observed at 77 K (Λ_max, 470–480 nm). From the emission signal obtained after an excitation flash of 3 ns half-width, an "apparent" rate constant for the radiative deactivation process, shorter than 5 × 10⁶ s, can be inferred. The 300 K emission is efficiently quenched by halides and by oxygen: quenching involves a long-lived intermediate (⋍ 200 ns).
Clearly the emissive state X is populated through the S₀-S₁ electronic transition π→π* of 4TU. The nature of X cannot be unambiguously determined: it cannot be an excimer but can be either the 4TU triplet state or another chemical state distinct from the 4TU excited singlet or triplet states.
An interesting finding is that the 300 K emission and the ability of 4TU to photoreact are related: they are quenched with the same efficiency by halide anions. This indicates that quenching occurs at the same long-lived intermediate species , which is either a precursor of the emitter or the emitter itself.