The discrimination between triplet state and radical cation in the pulse radiolysis of bithiophene in CCl4

The triplet state (³2T) and the radical cation (2T⁺√) of 2,2′-bithiophene (2T) are characterized by pulse radiolysis in CCl₄. Two main absorption bands at 360 and 420 nm are respectively attributed to ³2T* and to 2T⁺√. The triplet, induced in an excited state through a Förster mechanism, undergoes a conformational rearrangement (k₆=(6.8±0.9)×10⁶ s⁻¹). The radical cation is produced both through a resonance charge transfer and a second diffusional process; the two oxidizing species are respectively CCl₄⁺√ and (CCl⁺₃Cl⁻)_solv through the mediation of a singlet excited state, ¹2T*.     

SOLVENT EFFECTS ON THE ELECTRONIC ABSORPTION AND FLUORESCENCE SPECTRA OF INDOLE-4-CARBOXYLIC ACID: PROTOTROPIC EQUILIBRIA IN AQUEOUS SOLUTIONS

Abstract— The absorption and fluorescence spectra of indole-4-carboxylic acid in various solvents have indicated that the -COOH group is more planar with respect to the indole ring in the first excited singlet state (S₁) than in the ground (S₀) state. Relatively large Stokes' shifts indicate that polarisability and dipole moment of the molecule are increased predominantly upon excitation. Prototropic reactions in the S₀ and S₁ states are the same. The -COO^- and -COOH+₂ groups are not coplanar in the S₀, but coplanar in the S₁ state. pH-dependent fluorescence spectra have revealed that both protonation and deprotonation of the -COOH group increase the basicity of the molecule upon excitation.     

ANALYSIS OF PRIMARY PHOTOCHEMICAL PROCESSES IN BACTERIORHODOPSIN

Abstract— The sequence of primary events following light absorption by light adapted bacteriorhodopsin (bR₅₇₀) is considered by analyzing recent picosecond absorption and emission data. The analysis is facilitated by theoretical calculations which allow us to characterize the properties of the first excited singlet state. It is concluded that excitation leads to the eventual population of a photochemically important nonfluorescent excited state (I) which decays into a photoproduct (J₆₂₅)- In J₆₂₅, which is most probably a ground state molecule, the chromophore has undergone a structural change, presumably trans → 13- cis isomerization. It is suggested that the subsequent process

reflects a relaxation of the protein environment involving proton transfer.     

Ground-and Excited-State Characterization of an Electrostatic Complex between Tetrakis-(4-Sulfonatophenyl)porphyrin and 16-Pyrimidinium Crown-4

Abstract— We report the formation of an electrostatic complex between (16-pyrimidinium crown-4)tetranitrate (16PC4) and tetrakis-(4-sulfonatophenyl)porphyrin (4SP) in aqueous solution. Ground-state complex formation results in a red shift of the 4SP visible absorption bands and a decrease in absorbance of the Soret band. The equilibrium constant for complex formation (determined from optical titrations) is found to be (2.0 ± 0.2) × 10⁵ M ⁻¹. In addition, the data fit to an expression describing a 1:1 stoichiometry. Excitation of the complex results in quenching of both the excited singlet and triplet states of the associated porphyrin. The singlet-state lifetime decreases from 10 ns for the free porphyrin to 1.5 ns in the presence of 16PC4 at low solution ionic strengths. In addition, evidence is presented for triplet-state quenching within the complex with k _q= (1.1 ± 0.1) × 10⁴ s⁻¹. The mechanism of quenching is tentatively assigned to electron transfer from either the excited singlet or excited triplet state of the porphyrin to the ground state of the 16PC4.     

极性敏感的BDP分子溶剂化效应的光谱性质

合成了具有分子内电荷转移(ICT)性质的三重态光敏剂分子BDP, 研究了其稳态吸收光谱、 荧光光谱、 荧光寿命、 飞秒/纳秒瞬态吸收光谱及诱导产生单线态氧的能力等性质, 发现强极性溶剂对BDP分子的溶剂化效应降低了其ICT态和第一激发三重态(T₁态)的能量, 从而降低了BDP分子单线态氧的产量.     

4-硝基咪唑A-带激发态结构动力学

通过共振拉曼光谱实验和量子化学计算的方法研究了4-硝基咪唑(4NI)A-带激发态衰变动力学.对4NI的振动光谱、紫外电子吸收光谱、荧光光谱和共振拉曼光谱进行了指认.在全活化空间自洽场法(CASSCF)/6-31G(d)计算水平下获得了单重激发态S1(nOπ*)和S2(ππ*)和势能面交叉点S1(nOπ*)/S2(ππ*)的优化几何结构和能量,分析了A-带共振拉曼光谱的强度模式特征,获得了短时结构动力学,并结合全活化空间自洽场法(CASSCF)理论计算结果确定了4NI在S2(ππ*)态衰变通道主要是S2,FC→S2,min(ππ*)→S0辐射弛豫.     

Chemically Initiated Electron Exchange Luminescence of Silyloxyaryl-Substituted Spiroadamantyl Dioxetanes: Kinetics and Excited State Yields

Abstract— The kinetics of the fluoride-induced decomposition of the thermally stable silyloxyaryl-substituted spiroadamantyl dioxetanes 1a,b and the excited state formation of this chemically initiated electron exchange luminescence (CIEEL) have been investigated. Two limiting kinetic regimes flash and glow have been identified, which depend on the fluoride concentration, the first at high, the second at low [F^-] triggering, whose detailed kinetic analysis affords the rate constants for the deprotected dioxetanes 2a,b cleavage in acetonitrile and dimethyl sulfoxide and chemiluminescence measurements the CIEEL and phen-olate 4 (CIEEL emitter) excitation yields. Chloro-substi-tution in the spiroadamantyl dioxetane does not affect the deprotection step k ₂ but leads to a ca five-fold faster cleavage of the deprotected dioxetane 2, while the chemiexcitation yield is the same for both dioxetanes. The energies of the first excited singlet and triplet states of the emitting phenolate 4 were estimated by AM1 configuration interaction calculations with explicit consideration of acetonitrile as solvent (self-consistent reaction field approach). The first excited singlet and triplet state of the CIEEL emitter phenolate 4 possess π,π* character, as suggested by the π-type molecular orbitals and the large singlet-triplet energy gap. The chemiexcitation of both singlet and triplet states of the excited phenolate 4 is feasible during the dioxetanes 1a,b cleavage, but the experimentally determined high singlet excitation yields suggest that preferentially the phenolate 4 singlet state is populated in the fluoride ion-triggered CIEEL process.     

PHOTOCHEMISTRY OF FLAVINS. II. PHOTOPHYSICAL PROPERTIES OF ALLOXAZINES AND ISOALLOXAZINES

Abstract. Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φ_ISC) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H⁺ ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φ_ISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φ_ISC˜ 0.45) and lumichrome (φ_ISC˜ 0.7) the results are more difficult to interpret owing to interconversion between alloxazine and isoalloxazine structures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φ_ISC for lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet-triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in equilibrium.     

A study of the B2 excited state geometries of the metal-metal quadruply bonded compounds Mo2X4(PMe3)4 (X = Cl, Br or I)

The excited state geometries of the metal-metal quadruply bonded compounds Mo₂X₄(PMe₃)₄ (X = Cl, Br or I) have been studied by means of resonance Raman and absorption spectroscopy. A fit of the parameters of a simple theoretical model to the experimental data indicates that the metal-metal bond increases some 10 pm on excitation to the ¹B₂ (δδ^*) state, whereas other geometric changes are small. Furthermore, the phenomenological lifetime factor of the excited state, Γ, is found to be dependent on the vibrational quantum number, ν, of this state.     

Geometry relaxation effects in the 1 Bu and 2 Ag states of trans-1,3-butadiene

MCSCF and MRCI calculations on the first three singlet states of trans-1,3-butadiene are presented. Flexible basis sets were applied and full geometry optimization was carried out at the MCSCF level for planar and selected non-planar structures including twisting and pyramidalization of terminal CH₂-groups. Geometry relaxations in and excitation energies to 1 ¹B_u and 2 ¹A_g states are discussed in detail. For planar structures the covalent 2 ¹A_g state is lower in energy than the 1 ¹B_u state. If non-planar geometry relaxations are allowed, the lowest lying non-planar excited singlet state turns out to be ionic with one terminal CH₂ group rotated by 90°. Limitations of the current investigations due to restrictions in the MRCI treatment and because of incomplete scanning of excited state surfaces are pointed out.     

Theoretical studies of electronic absorption and emission spectra of Pt(saloph)

The performance of ab initio calculations for the ground and excited states of the Pt(saloph) complex is examined in detail. The S₀–S_i and T₁–T_i absorption spectra are calculated, and the transition between the ground S₀ state and the excited S₁ state involves the HOMO-2, HOMO-1, HOMO and LUMO. Moreover, calculations show that the emissive singlet is of mixed MLCT/LLCT characteristic. On the other hand, the molecular geometry of the complex is nearly planar in the ground state while the geometry is obviously nonplanar in the excited state of S₁(π, π^*) in the gas phase.     

Excitation-energy dependence of phosphorescence quantum yield of benzaldehyde vapour at low pressure

The phosphorescence quantum yield of benzaldehyde vapour was measured at low pressure (down to 2 mtorr) as a function of excitation wavelength. The quantum yield is essentially constant in the range of excitation energy corresponding to the S₁(n,π^*) state, but it decreases very rapidly as the excitation energy is raised to the value corresponding to S₂(π,π^*), indicating that the phosphorescence property of the benzaldehyde molecule varies, depending on the nature of the singlet state to which the molecule is initially excited.     

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.     

Theoretical study of the singlet electronically excited states of N4

Vertical excitation energies for the lowest eleven singlet states of T_d N₄ were calculated using the TD-DFT method with the B3LYP functional, and at the EOM-CCSD level of theory. The vertical excitation energies for the five lowest-lying excited states were also obtained using the state-averaged CASSCF, CASPT2, CASPT3, and MRCI + Q methods. Our results show that the five lowest-lying states are of valence character. EOM-CCSD/d-aug-cc-pVTZ calculations predict that there are two weakly allowed optical transitions of T₂ symmetry at 10.44 and 10.82 eV. The transition to the third T₂ state, which is predicted to be at 10.89 eV, has an oscillator strength about one order of magnitude higher.     

Triplet excited-state characterization and determination of the photoionization mechanism of the antitumoral drug pazelliptine

The triplet properties of the excited triplet state of pazelliptine (PZE), an antitumoral drug derived from ellipticine, were investigated in dioxane, ethanol and buffer aqueous solutions using the laser flash photolysis technique. The triplet absorption spectra and the kinetic parameters associated with the excited state decay were quite similar in the different solvents. ³PZE reacted with unexcited PZE in deaerated solutions (k = 6 × 10¹⁰ M⁻¹ s⁻¹) and was quenched by oxygen (k ≈ 2 × 10⁷ s⁻¹). The extinction coefficients of the triplet transition were estimated and used to calculate the singlet-triplet intersystem crossing quantum yields of about 5%.

A biphotonic ionization of PZE in buffer aqueous solution has been demonstrated in a previous work. This process was also observed in ethanol but not in dioxane. Mixed yttrium aluminum garnet laser harmonics (355 nm + 532 nm) and delayed-pulse experiments were carried out in order to determine the intermediate excited state involved in this photoionization process. The results indicate that pazelliptine radical cation and e⁻_s are formed via a consecutive two-photon absoprtion in which the first excited singlet state is the only intermediate.     

Radical cation generation from singlet and triplet excited states of all-trans-lycopene in chloroform

On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1B(u)-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-beta-carotene in generating the radical cation. The time constant of radical cation generation from the 1B(u)-type state was found to be approximately 0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for beta-carotene. A slow rising phase (20-30 micros) in the bleaching of ground-state absorption was common for both lycopene and beta-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.     

Singlet versus triplet dynamics of β-carotene studied by quantum control spectroscopy

Biomolecules very often present complex energy deactivation networks with overlapping electronic absorption bands, making their study a difficult task. This can be especially true in transient absorption spectroscopy when signals from bleach, excited state absorption and stimulated emission contribute to the signal. However, quantum control spectroscopy can be used to discriminate specific electronic states of interest by applying specifically designed laser pulses. Recently, we have shown the control of energy flow in bacterial light-harvesting using shaped pump pulses in the visible and the selective population of pathways in carotenoids using an additional depletion pulse in the transient absorption technique. Here, we apply a closed-loop optimization approach to β-carotene using a spatial light modulator to decipher the energy flow network after a multiphoton excitation with a shaped ultrashort pulse in the near-IR. After excitation, two overlapping bands were detected and identified as the S₁ state and the first triplet state T₁. Using the transient absorption signal at a specific probe delay as feedback, the triplet signal could be optimized over the singlet contribution.     

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.     

WAVELENGTH EFFECTS ON THE PHOTOPROCESSES OF INDOLE AND DERIVATIVES IN SOLUTION

Abstract— The two main primary photoprocesses (electron ejection and H-atom release) for indole, 5-methoxyindole and N-methylindole in various polar and nonpolar solvents were studied as a function of the excitation energy and were correlated with the corresponding fluorescence quantum yields. In hydrocarbon solvents, N–H bond cleavage is the main primary photoprocess from the ¹B_b band of the substrates with the exception of N-methylindole. In alcohols, both processes are of negligible importance. Hydrated electrons (e⁻_aq) are ejected from the relaxed singlet states of all three compounds in aqueous solutions with a similar yield for excitation at 280 and 254 nm (¹L_a and ¹L_b states). The yield increases when the excitation is into the ¹B_b band. The quantum yields of the two primary processes from the higher excited states are generally lower than the fraction of molecules not converting to the fluorescent state. This is explained by an efficient back reaction in competition with a thermally activated radical release from an intermediate state or radical pair formed from the S₂ (¹B_b) state. The non-occurrence of a photoionization energy threshold is discussed.