Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

Femtosecond absorption spectroscopy of cytochrome <Emphasis Type="Italic">c</Emphasis> oxidase: Excited electronic states and relaxation processes in heme <Emphasis Type="Italic">a</Emphasis> and heme <Emphasis Type="Italic">a</Emphasis><Subscript>3</Subscript> centers

Cytochrome c oxidase, the key bioenergetic protein, was studied by femtosecond absorption spectroscopy. Time-resolved spectral characteristics of the difference spectra recorded in the timescale from 80 fs to 20 ps were analyzed. Electronic relaxation of the excitation energy in heme a occurs in three successive steps. After completion of these steps, heme a is in the excited vibrational state of the ground state. Vibrational relaxation, cooling of the heme, occurs for several picoseconds.     

Complexation of bis‐crown stilbene with alkali and alkaline‐earth metal cations. Ultrafast excited state dynamics of the stilbene‐viologen analogue charge transfer complex

The complex formation of bis(18‐crown‐6)stilbene ( 1 ) and its supramolecular donor‐acceptor complex with N,N′‐bis(ammonioethyl) 1,2‐di(4‐pyridyl)ethylene derivative ( 2 ) with alkali and alkaline‐earth metal perchlorates has been studied using absorption, steady‐state fluorescence, and femtosecond transient absorption spectroscopy. The formation of 1 ?Mⁿ⁺ and 1 ?(Mⁿ⁺)₂ complexes in acetonitrile was demonstrated. The weak long‐wavelength charge‐transfer absorption band of 1 · 2 completely vanishes upon complexation with metal cations because of disruption of the pseudocyclic structure. The spectroscopic and luminescence parameters, stability constants, and 2‐stage dissociation constants were calculated. The initial stage of a recoordination process was found in the excited complexes 1 ?M⁺ and 1 ?(M⁺)₂ (M = Li, Na). The pronounced fluorescence quenching of 1 · 2 is explained by very fast back electron transfer (τ_et = 0.397 ps). The structure of complex 1 · 2 was studied by X‐ray diffraction; stacked ( 1 · 2 )_m polymer in which the components were connected by hydrogen bonding and stacking was found in the crystal. These compounds can be considered as novel optical molecular sensors for alkali and alkaline‐earth metal cations.     

Femtosecond Absorption Spectroscopy of Reduced and Oxidized Forms of Cytochrome c Oxidase: Excited States and Relaxation Processes in Heme a and a3 Centers

Optics and Spectroscopy - Excited electronic states and intraheme relaxation processes in the oxidized and reduced forms of mitochondrial cytochrome c oxidase extracted from a beef heart have been...     

Femtosecond dynamics of excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

The dynamics of excited-state intramolecular proton transfer (ESIPT) and of relaxation processes in o-tosylaminobenzoic acid (TAC) and o-acetylaminobenzoic acid (AAC) have been studied by femtosecond absorption spectroscopy with a time resolution of 30 fs. The ESIPT characteristic time in the TAC dimer and monomer and in AAC monomer is 50 fs. The excited product of photoinduced proton transfer in the monomer undergoes effective radiationless deactivation with a characteristic time of 30 ps, one of the channels of which is internal rotation followed by intersystem crossing and internal conversion. The product of ESIPT in the TAC dimer deactivates preferentially into the ground state via radiative transition with a time of 291 ps. ESIPT in the AAC dimer is thermodynamically unfavorable and occurs with a low yield.     

P680 (P(D1)P(D2)) and Chl(D1) as alternative electron donors in photosystem II core complexes and isolated reaction centers

Low temperature (77-90 K) measurements of absorption spectral changes induced by red light illumination in isolated photosystem II (PSII) reaction centers (RCs, D1/D2/Cyt b559 complex) with different external acceptors and in PSII core complexes have shown that two different electron donors can alternatively function in PSII: chlorophyll (Chl) dimer P(680) absorbing at 684 nm and Chl monomer Chl(D1) absorbing at 674 nm. Under physiological conditions (278 K) transient absorption difference spectroscopy with 20-fs resolution was applied to study primary charge separation in spinach PSII core complexes excited at 710 nm. It was shown that the initial electron transfer reaction takes place with a time constant of ~0.9 ps. This kinetics was ascribed to charge separation between P(680)* and Chl(D1) absorbing at 670 nm accompanied by the formation of the primary charge-separated state P(680)(+)Chl(DI)(-), as indicated by 0.9-ps transient bleaching at 670 nm. The subsequent electron transfer from Chl(D1)(-) occurred within 13-14 ps and was accompanied by relaxation of the 670-nm band, bleaching of the Pheo(D1) Q(x) absorption band at 545 nm, and development of the anion-radical band of Pheo(D1)(-) at 450-460 nm, the latter two attributable to formation of the secondary radical pair P(680)(+)Pheo(D1)(-). The 14-ps relaxation of the 670-nm band was previously assigned to the Chl(D1) absorption in isolated PSII RCs [Shelaev, Gostev, Nadtochenko, Shkuropatov, Zabelin, Mamedov, Semenov, Sarkisov and Shuvalov, Photosynth. Res. 98 (2008) 95-103]. We suggest that the longer wavelength position of P(680) (near 680 nm) as a primary electron donor and the shorter wavelength position of Chl(D1) (near 670 nm) as a primary acceptor within the Q(y) transitions in RC allow an effective competition with an energy transfer and stabilization of separated charges. Although an alternative mechanism of charge separation with Chl(D1)* as the primary electron donor and Pheo(D1) as the primary acceptor cannot be ruled out, the 20-fs excitation at the far-red tail of the PSII core complex absorption spectrum at 710 nm appears to induce a transition to a low-energy state P(680)* with charge-transfer character (probably P(D1)(δ+)P(D2)(δ-)) which results in an effective electron transfer from P(680)* (the primary electron donor) to Chl(D1) as the intermediary acceptor.     

Femtosecond dynamics of photocyclization of 1-[(4-{5-[4-chloromethyl-2,5-dimethyl-3-thienyl]-2-oxo-1,3-dioxol-4-yl}-2,5-dimethyl-3-thienyl)methyl]pyridinium chloride

The photochromic ring closure in diarylethylene was studied by femtosecond laser spectroscopy. The absorption spectrum of the initial excited state under pulse excitation at 305 nm was observed. The kinetic scheme of transitions from the initial excited state to the closed-ring isomer as the final product is proposed.     

Femto-picosecond relaxation of triazole-bridged bis(zinc porphyrin)

The femtosecond dynamics of excitation relaxation has been revealed for the zinc porphyrin dimer using the pump-probe technique. The data obtained have been analyzed with the use of quantum-chemical calculations. The excitation relaxation dynamics shows that systems of this kind hold promise as models for investigation of photosystems and development of artificial analogues of natural photosynthetic centers. A model has been proposed to explain the found coherent dynamics of exciton bands.     

Femtosecond dynamics of excited-state intramolecular proton transfer in <Emphasis Type="Italic">o</Emphasis>-tosylaminobenzaldehyde

Dynamics of excited-state intramolecular proton transfer (ESIPT) in o-tosylaminobenzaldehyde has been studied by femtosecond absorption spectroscopy with a time resolution of 30 fs. The characteristic time of this process is ∼100 fs. Differential absorption rate curves exhibit oscillations that are consistent with theoretically predicted ESIPT-promoting vibrational modes. Efficient nonradiative deactivation with a rate constant of 6.25 × 10¹⁰ s⁻¹ occurs in the excited product of proton transfer, with internal rotation followed by intersystem crossing being one of the feasible deactivation pathways.