PHOTOPHYSICAL STUDIES ON ORGANIC COMPOUNDS——TIME-RESOLVED TRANSIENT ABSORPTION SPECTRA STUDIES ON TRIPHENYLAMINE IN VARIOUS SOLVENTS

The transient absorption spectra of triphenylamine (TPA) in various solvents have beeninvestigated by the methods of nanosecond laser photolysis. The lifetimes of TPA were meas-ured. In hexane and ethanol, the transient absorptions of triplet state and triplet excimer ofTPA were observed. However, the transient absorptions of the TPA cation radical was alsoobserved in acetonitrile besides that of the triplet TPA monomer and excimer. According tothe experimental facts, the transient photophysical mechanism of TPA in various solvents hasbeen proposed.     

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).     

Large substituent effect on the photochemical rearrangement of 1,6-(N-Aryl)aza-[60]fulleroids to 1,2-(N-Arylaziridino)-[60]fullerenes

Large substituent effects were observed in the rates and reaction mechanisms of the photochemical rearrangement of N-arylaza-[60]fulleroid 1 to N-arylaziridino-[60]fullerene 2, in which the difference of the rates between the fastest and the slowest (>2160-fold) was attained only by changing the aryl group from 1-naphthyl to 2-naphthyl. The decreasing order of the reaction rates in relation to the substituents was 1-naphthyl (1b) > 1-pyrenyl (1d) > phenyl (1a) > 2-naphthyl (1c). The reactions proceeded via triplet states of the fulleroids and a triplet sensitization of the reaction by rearranged product 2b was observed in the case of 1b. The slow reactions of 1a,c were interpretated by the participation of charge-separated species in the excited triplet states, which was supported by nanosecond transient absorption spectra.     

Photochemistry and photopolymerization activity of novel perester derivatives of fluorenone: a conventional and laser flash photolysis study

The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed.     

Triplet formation of 4-thiothymidine and its photosensitization to oxygen studied by time-resolved thermal lensing technique

Excited-state dynamics of 4-thiothymidine (S4-TdR) and its photosensitization to molecular oxygen in solution with UVA irradiation were investigated. Absorption and emission spectra measurements revealed that UVA photolysis of S4-TdR gives rise to a population of T1(pipi*), following S2(pipi*) --> S1(npi*) internal conversion. In transient absorption measurement, the 355 nm laser photolysis gave broad absorption (380-600 nm) bands of triplet S4-TdR. The time-resolved thermal lensing (TRTL) signal of S4-TdR containing the thermal component due to decay of triplet S4-TdR was clearly observed by the 355 nm laser excitation. The quantum yield for S1 --> T1 intersystem crossing was estimated to be unity by a triplet quenching experiment with potassium iodide. In the presence of molecular oxygen, the photosensitization from triplet S4-TdR gave rise to singlet oxygen O2 (1Deltag) with a quantum yield of 0.50. Therapeutic implications of such singlet oxygen formation are discussed.     

Photoinduced Electron Transfer Reaction between Poly-guanylic Acid (5`) with Anthraquinone-2-sulfonate

TheinteractionofquinonephotonucleasewithDNAhasbeenwidelystUdied.Anthraquinonederivatives,inparticularthatofanhraquinone-2-sulfonatehasbeenusedascleavingagentforduPlexDNA1-5.Howevef,directobservationofexcitedionpairsofbiomoleculesespeciallytheStabilizedradicalcationofbiomoleculeishamPeredbytheoverwhelmingtransientabsorPtionofhydrogenbondedradicalanionofquinone.lnthiswork,theinteractionofpolylG]withtripletanthraquinone-2-sulfonateinCH,CN-H:O(97f3)viaelectrontransferreactionhasbeenachieved…     

The reactive state in the photo-rearrangement of o-nitrobenzaldehyde

The primayy step of the o-nitrobenzaldehyde-o-nitrosobenzoic acid photorearrangement in solution has been studied by flash absorption with 35 ps 355 nm light pulses. Flash photolysis of o-nitrobenzaldehyde in acetonitrile or THF solutions produces a transient absorption with a maximum at ca. 440 nm. Formation of the transient was < 35 ps, the laser pulse width, and within experimental error, no furthrr buildup was observed. The transient which decayed at nanosccond times is attributed to a remarkably reactive ketene intermediate formed by H abstraction of the aldehydic hydrogen by the excited state of the nitro group. Decay of the ketene was more rapid in water-acetonitrile, methanol-acetonitrile, tert-butyl alcohol and in THF than in acetonitrile solution. It is suggested that the intramolecular reaction of the ketene intermediate is enhanced in THF relative to acetonitrile because of the ability of THF to faciliaate proton transfer associated with the reaction. The addition of the triplet quencher cis-piperylene to a solution of o-nitrobenzaldehyde in THF did not accelerate decay of the transient nor reduce its yield. The n,π* triplet excited state band observed in the 625–650 nm region for a number of the nitroaromatic compounds was not observed in the case of o-nitrobenzaldehyde. The results provide evidence that in the direct irradiation on o-nitrobenzaldehyde in THF or acetonitrile solutions, the intramolecular reaction occurs from the singlet rather than the triplet excited state.     

Determination of decay constants with a sampling flash apparatus. The triplet state lifetimes of anthracene and pyrene in fluid solutions

A sampling flash apparatus for recording weak transient absorptions in the microsecond range is described. Two methods for the evaluation of first- and second-order rate constants from mixed-order plots are briefly discussed. The concentration dependence of the lifetime of anthracene in its excited triplet state has been investigated in cyclohexane at room temperature. It is assumed that the observed decrease in triplet lifetime with increasing anthracene concentration is due to impurity quenching and not due to triplet excimer formation.     

Combined Transient Spectra and Semiempirical Calculation of [60]Fullerenes Attached with Piperidinodithiocarboxylate and 7-Chloro-phenazine

[60]Fullerenes attached with piperidinodithiocarboxylate dyad （1） and 7-chloro-1,2,3,4-tetrahydrophenazine （2） were efficiently synthesized through Diels-Atder cycloaddition with dienes. The physical properties of the triplet states of these compounds, in which strong electron acceptor moieties were covalently attached to C60 cores, were investigated by nanosecond laser flash photolysis. The excited triplet states in benzonitrite have been evaluated by observing the transient absorption bands in the near-IR region. The HOMO and LUMO were calculated by semiempirical methods AM1, which could predict the intramolecular photoinduced electron transfer in 1 and 2, and the nanosecond transient absorption spectra observed experimentally in solution were in excellent agreement with the calculated ones.     

Room-temperature long-lived triplet excited states of naphthalenediimides and their applications as organic triplet photosensitizers for photooxidation and triplet-triplet annihilation upconversions

Naphthalenediimide (NDI) derivatives with 2,6- or 2,3,6,7-tetrabromo or amino substituents were prepared. N,N'-dialkyl-2,6-dibromo NDI (compound 2) and N,N'-dialkyl-2,3,6,7-tetrabromo NDI (compound 4) show phosphorescence emission at 610 or 667 nm, respectively. Phosphorescence was never observed for NDI derivatives. Conversely, N,N'-dialkyl-2,6-dibromo-3,7-diamino NDI (compound 5) shows strong absorption at 526 nm and fluorescence at 551 nm, and no phosphorescence was observed. However, nanosecond time-resolved transient difference absorption spectroscopy confirmed that the triplet excited state of 5 was populated upon photoexcitation. 2,3,6,7-Tetraamino NDI (6) shows fluorescence, and no triplet excited state was populated upon excitation. The compounds were used as singlet oxygen ((1)O(2)) photosensitizers for the photooxidation of 1,5-dihydroxylnaphthalene (DHN). We found that 5 is more efficient than the conventional photosensitizer, such as Ir(ppy)(2)(bpy)[PF(6)]. The compounds were also used as organic triplet photosensitizers for triplet-triplet annihilation based upconversions. An upconversion quantum yield up to 18.5% was observed.     

Photophysical properties and photoreduction of N-acetyl- and N-benzoylphthalimides

The photophysical properties and photoreduction of N-acetylphthalimide (AcP) and N-benzoylphthalimide (BzP), N-3,4,5-trimethoxybenzoylphthalimide (trimethoxyBzP) and N-4-nitrobenzoylphthalimide (nitroBzP) were studied by steady-state and transient techniques. Radicals and their precursor triplet states were detected by flash photolysis. The triplet state properties of AcP and BzP were characterized. In contrast, no triplet absorption was observed with ns-detection for trimethoxyBzP and nitroBzP. Specific products are formed upon electron transfer from triethylamine to the photoexcited acylphthalimides. In addition, H-atom transfer from 2-propanol or other alcohols to the triplet state takes place. The properties of several radical intermediates involved in photoreduction of the acylphthalimides as well as some structure-function relationships are described.     

N_2O分子C~1Ⅱ态的吸收光谱及解离动力学

通过四波混频差频的方法产生高分辨的真空紫外激光,用以测量143.6至146.9 nm范围内的射流冷却N_2O分子吸收光谱,对应于C~1Ⅱ←Ⅹ~1∑~+的吸收跃迁.谱图显示出三个分立的振动谱峰叠加在宽吸收背景上,谱峰间隔分别是521和482 cm~(-1).前人的高精度量子化学计算表明C~1Ⅱ态在N-O键长方向表现为无势垒的排斥态.而在N-N键伸缩及N_2O弯曲振动方向则表现为束缚态,因此观测到的振动谱峰被归属为激发态的Feshbach共振.通过反Fourier变换可以得到Feshbach共振对应的非稳定周期轨道的特征周期为61 fs,相应的振动频率为546 cm~(-1).鉴于这一频率与弯曲振动频率非常接近,非稳定周期轨道被认为是由C~1Ⅱ态的弯曲振动与解离运动相互作用而形成的,N-N伸缩振动没有参与形成非稳定周期轨道.由此,N_2O分子C~1Ⅱ态光激发-解离过程得以清晰地阐述.     

Ligand-Localized Triplet-State Photophysics in a Platinum(II) Terpyridyl Perylenediimideacetylide

The synthesis, electrochemistry, and photophysical behavior of a Pt(II) terpyridyl perylenediimide (PDI) acetylide (1) charge-transfer complex is reported. The title compound exhibits strong (ε ≈ 5 × 10(4) M(-1)cm(-1)) low-energy PDI acetylide-based π-π* absorption bands in the visible range extending to 600 nm, producing highly quenched singlet fluorescence (Φ = 0.014 ± 0.001, τ = 109 ps) with respect to a nonmetalated PDI model chromophore. Nanosecond transient absorption spectroscopy revealed the presence of a long excited-state lifetime (372 ns in 2-methyltetrahydrofuran) with transient features consistent with the PDI-acetylide triplet state, ascertained by direct comparison to a model Pt(II) PDI-acetylide complex lacking low-energy charge-transfer transitions. For the first time, time-resolved step-scan FT-IR spectroscopy was used to characterize the triplet excited state of the PDI-acetylide sensitized in the title compound and its associated model complex. The observed red shifts (～30-50 cm(-1)) in the C═O and C≡C vibrations of the two Pt(II) complexes in the long-lived excited state are consistent with formation of the (3)PDI acetylide state and found to be in excellent agreement with the expected change in the relevant DFT-calculated IR frequencies in the nonmetalated PDI model chromophore (ground singlet state and lowest triplet excited state). Formation of the PDI triplet excited state in the title chromophore was also supported by sensitization of the singlet oxygen photoluminescence centered at ～1275 nm in air-saturated acetonitrile solution, Φ((1)O(2)) = 0.52. In terms of light emission, only residual PDI-based red fluorescence could be detected and no corresponding PDI-based phosphorescence was observed in the visible or NIR region at 298 or 77 K in the Pt(II) terpyridyl perylenediimideacetylide.     

Long-lived room temperature deep-red/near-IR emissive intraligand triplet excited state (3IL) of naphthalimide in cyclometalated platinum(II) complexes and its application in upconversion

[C(^)NPt(acac)] (C(^)N = cyclometalating ligand; acac = acetylacetonato) complexes in which the naphthalimide (NI) moiety is directly cyclometalated (NI as the C donor of the C-Pt bond) were synthesized. With 4-pyrazolylnaphthalimide, isomers with five-membered (Pt-2) and six-membered (Pt-3) chelate rings were obtained. With 4-pyridinylnaphthalimide, only the complex with a five-membered chelate ring (Pt-4) was isolated. A model complex with 1-phenylpyrazole as the C(^)N ligand was prepared (Pt-1). Strong absorption of visible light (ε = 21,900 M(-1) cm(-1) at 443 nm for Pt-3) and room temperature (RT) phosphorescence at 630 nm (Pt-2 and Pt-3) or 674 nm (Pt-4) were observed. Long-lived phosphorescences were observed for Pt-2 (τ(P) = 12.8 μs) and Pt-3 (τ(P) = 61.9 μs). Pt-1 is nonphosphorescent at RT in solution because of the acac-localized T(1) excited state [based on density functional theory (DFT) calculations and spin density analysis], but a structured emission band centered at 415 nm was observed at 77 K. Time-resolved transient absorption spectra and spin density analysis indicated a NI-localized intraligand triplet excited state ((3)IL) for complexes Pt-2, Pt-3, and Pt-4. DFT calculations on the transient absorption spectra (T(1) → T(n) transitions, n > 1) also support the (3)IL assignment of the T(1) excited states of Pt-2, Pt-3, and Pt-4. The complexes were used as triplet sensitizers for triplet-triplet-annihilation (TTA) based upconversion, and the results show that Pt-3 is an efficient sensitizer with an upconversion quantum yield of up to 14.1%, despite its low phosphorescence quantum yield of 5.2%. Thus, we propose that the sensitizer molecules at the triplet excited state that are otherwise nonphosphorescent were involved in the TTA upconversion process, indicating that weakly phosphorescent or nonphosphorescent transition-metal complexes can be used as triplet sensitizers for TTA upconversion.     

聚乙烯醇缩醛膜的加湿透气性

前曾报道聚乙烯醇(PVA)与脂肪醛缩合后亲水性的羟基减少,影响到其薄膜的透湿性和透气性,本文考察PVA及其三种脂肪族缩醛物膜在加湿条件下的透气性。 PVA膜为10％PVA水溶液用流涎法制成。缩醛膜为PVA缩醛衍生     

V2O5/ACF催化剂低温下选择性催化还原NO的机理

将V2O5担载在活性炭纤维(ACF)上制得V2O5/ACF催化剂,并采用暂态响应实验和NH3吸附氧化实验等考察了影响V2O5/ACF催化剂上选择性催化还原(SCR)反应的关键因素.结果表明,NH3在催化剂表面的吸附是必要的,而且该吸附是一个快速过程;气相O2的存在有利于形成催化剂中所需的活性氧化态物种.NH3吸附.脱附与原位质谱相结合的实验表明,V2O5/ACF催化剂具有吸附NH3和将NH3氧化为N2H2的能力,N2H2为NH3氧化的一种中间体.     

Bodipy–C60 triple hydrogen bonding assemblies as heavy atom-free triplet photosensitizers: preparation and study of the singlet/triplet energy transfer

Supramolecular triplet photosensitizers based on hydrogen bonding-mediated molecular assemblies were prepared. Three thymine-containing visible light-harvesting Bodipy derivatives (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, respectively) were used as H-bonding modules, and 1,6-diaminopyridine-appended C₆₀ was used as the complementary hydrogen bonding module (C-1), in which the C₆₀ part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepared as references (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C₆₀ module (C-1). The photophysical properties of the H-bonding assemblies and the reference non-hydrogen bond-forming mixtures were studied using steady state UV/vis absorption spectroscopy, fluorescence emission spectroscopy, electrochemical characterization, and nanosecond transient absorption spectroscopy. Singlet energy transfer from the Bodipy antenna to the C₆₀ module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C₆₀ module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). Intra-assembly forward–backward (ping-pong) singlet/triplet energy transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was observed for the non-hydrogen bonding mixtures. As a proof of concept, these supramolecular assemblies were used as triplet photosensitizers for triplet–triplet annihilation upconversion.     

TRIPLET EXCITED STATE OF THE 4'5' PHOTOADDUCT OF PSORALEN AND THYMINE

Abstract— The triplet-triplet absorption spectrum of the 4'5' psoralen-thymine mono-adduct has been determined in water and methanol using the technique of laser flash photolysis. The extinction coefficient of the triplet was measured by the energy-transfer method with retinol triplet as standard, and used to determine the singlet → triplet intersystem crossing quantum yield for 353 nm excitation. Reaction rate constants for mono-adduct triplet with thymine and tryptophan were measured in water. Long-lived transient absorptions detected after quenching the mono-adduct triplet with thymine and tryptophan are assigned mainly to the corresponding mono-adduct radical anion, whose spectrum was established in separate pulse radiolysis studies of the mono-adduct in aqueous formate.
The significant singlet → triplet quantum yields found for the mono-adduct might be consistent with the involvement of triplet excited mono-adduct in DNA cross-link formation, as also may be the high reactivity obtained for the triplet with thymine. The initial quenching products observed resulted from a charge-transfer reaction.     

Ionic photodissociation and ionic photosensitized dissociation of charge-transfer complexes

Flash spectroscopic and kinetic studies have been carried out on the charge-transfer complex of tetracyanoethylene with tetrahydrofuran in a liquid paraffin solution at room temperature. The rise and decay curves in transient electronic absorption intensity have been observed with a common rate constant, corresponding to the anion radical of tetracyanoethylene and the triplet state of the charge-transfer complex, respectively. From the kinetic analysis it has been concluded that the ionic photodissociation of this complex takes place in its lowest excited triplet state. This dissociation mechanism has also been confirmed by employing a triplet energy transfer technique with which “ionic photosensitized-dissociation” phenomenon is observed. Furthermore, a few other examples of ionic photosensitized-dissociation are demonstrated in rigid glasses with typical weak charge-transfer complexes whose photodissociation processes are well-known.     

N2键合在固氮酶M簇笼内的一个可能证据

研究了重水固氮酶体系中Ｎ２对Ｃ２Ｈ２的还原活性和立体化学产物的影响，发现Ｎ２的存在不仅抑制了Ｃ２Ｈ２的还原活性，而且提高了产物乙烯中反式１，２－二氘代乙烯的相对含量，为证实Ｎ２在固氮酶Ｍ簇合物笼内的还原氢化提供了一个有力的证据。