Singlet Pathway Research Articles

Distance Dependence of Singlet and Triplet Charge Recombination Pathways in a Series of Rigid Bichromophoric Systems

The recombination process of the charge-separated state in a series of rigid donor−bridge−acceptor compounds 1[n] follows two pathways, viz. recombination to the singlet ground state and recombination to the local triplet donor state. The rates of both these processes are shown to depend exponentially on distance, but with a significantly different damping factor. Thus at short distances singlet recombination is dominant, while at longer distances the triplet pathway eventually becomes the major process.

Photoprocesses in Spiropyran-Derived Merocyanines

The photophysical and photochemical properties of a series of spiro[2H-1-benzopyran-2,2‘-indoline] compounds (Sp-n)five of them containing a nitro group in the 6-position of the benzopyran portion (n = 1−5) while two (n = 6, 7) do notwere studied by nanosecond laser photolysis in solution. An intermediate, which is attributed to a photoisomer with cis structure, was observed for 1−5 upon excitation of either the spiropyran form or the trans isomer. The ground state properties of the spiropyran/trans isomer couple are characterized in a variety of solvents, and the activation parameters (e.g., activation energies of 38 and 49 kJ mol-1 for 1 and 5 in acetone, respectively) of the thermal cis → trans isomerization were determined. Relative quantum yields of the spiropyran → trans-merocyanine photocoloration and trans → cis photoisomerization (ΦSp→t and Φt→c, respectively) were obtained. ΦSp→t is substantial for 1−5 in solvents of low polarity but reduced in polar solvents. For 1−5 a short-lived transient (maximum at 420−440 and 560−590 nm, lifetime ≤ 10 μs) was observed and identified as a triplet state. It is suggested that this is the triplet of the trans isomer in equilibrium with the perpendicular triplet conformation. The nitro group strongly enhances the quantum yield of intersystem crossing for the spiropyran, thereby changing the singlet pathway of photocoloration for 6 or 7 into a triplet pathway for 1−5. The trans → cis photoisomerization for 1−5, despite the presence of the triplet state, is suggested to occur essentially via the singlet state.

New Aspects of the Photophysical Properties of 1′H‐2′,3′‐Dimethoxycarbonylspiro[fluoren‐9,1′‐pyrrolo[2,1‐a]quinoline

AbstractIn this paper we describe photophysical and mechanistic studies of spiro[fluorene‐9,1′‐pyrrolo[2,1‐a]quinolines] (DHI) 1–4 and of their styryl homologues 5–11 (styryl DHI). DHIs 1–11 have low fluorescence quantum yields (approximately ϕf = 10‐3). The position of the fluorescence emission band is temperature‐dependent. The ring‐opened betaines 1′–11′ do not fluoresce at all. DHIs 1–11 exhibit phosphorescence, which is hypsochromically shifted by NO2 substitution of the fluorene ring. The triplet can be populated only by sensitization (with benzophenone as sensitizer) and, therefore, the reaction that occurs under direct excitation of the molecule involves the singlet excited state only. On irradiation under time‐resolved conditions, two transients were seen to be formed in the picosecond and microsecond domains. Slightly tilted educt or product‐like geometries are attributed to these transients. An energy diagram is proposed for the photoreaction of the model compound spiro[pyrroloquinoline] 1 → 1′, which takes into account the singlet pathway and the different transients of ca. 100 ps lifetimes formed from the singlet excited state of 1–11.

Effect of substituents on the photochromism of a spiro[indoline-napthoxazine] under laser excitation

Spirooxazines have been extensively investigated in the last few years because of their remarkable photostability. Laser flash photolysis was used to study the mechanisms of the photocoloration reaction of a spiro[indoline-naphthoxazine] substituted either with an electron-withdrawing group, the 8′-nitro derivative (compound I), or with electron-donating groups, the 6′-morpholino and 6′-piperidino derivatives (compounds II and III), in toluene solution at 297 K; the unsubstituted compound ( IV) was also studied for comparison. The photomerocyanine is formed in the photocoloration process via a triplet pathway in the case of I with a high quantum yield (0.7), as was previously observed for homologous nitroindolinospiropyrans. In the case of electron-donating compounds ( II and III) the photomerocyanine is formed exclusively via an excited singlet pathway; however, the quantum yields of photocoloration are much higher for compounds II and III (0.5) than for the unsubstituted one (0.2), but lower than for nitro derivative.

Photocycloaddition of 9,10-Dichloroanthracene to 2,5-Dimethyl-2,4-hexadiene. The Singlet Pathway for [4 + 2]-Adduct Formation in Benzene

Irradiation of 9,10-dichloroanthracene (DCA) in the presence of 2,5-dimethyl-2,4-hexadiene (DMHD) in benzene at 25{degree}C gives a single adduct (Ad) corresponding to [4 + 2] addition of DMHD to the 9,10 positions of DCA. Quantum yields for Ad formation are reported as a function of [DMHD], [DCA], and methyl iodide concentration. [MeI]. Fluorescence quantum yields and lifetimes of DCA and (DCA/DMHD) singlet exciplex as a function of [MeI] are also reported. Transient absorption measurements reveal the time evolution of excited DCA singlet, {sup 1}DCA, and triplet, {sup 3}DCA, states and of the (DCA/DMHD) singlet exciplex, {sup 1}(DCA-DMHD). They show that the dominant decay path from {sup 1}(DCA-DMHD) gives {sup 3}DCA. Quenching of {sup 1}DCA and exciplex fluorescence and transient absorption by MeI are observed. The dependence of Ad quantum yields on [DMHD] and the strict proportionally between product quantum yields and exciplex fluorescence quantum yields at different [MeI] establish singlet mechanism for adduct formation. Assuming that Ad is a primary photoproduct, the results are consistent with Yang`s proposal of stepwise collapse of polar exciplexes to seemingly forbidden cycloadducts. A singlet biradical could be the intermediate, but intersystem crossing of the singlet exciplex to a triplet biradical as the initial step cannotmore » be ruled out. 41 refs., 8 figs., 5 tabs.« less

Effect of nitro substituents on the photochromism of some spiro[indoline-naphthopyrans] under laser excitation

Nanosecond laser flash photolysis has been used to study the photochromic reaction of three spiro(indoline-naphthopyrans) substituted by one or two nitro groups on the two moieties of the molecule: either on the naphthopyran ring (compound I), or on the indoline ring (compound II), and the third compound ( III) with a nitro group on both rings; the unsubstituted compound ( IV) was also studied for comparison. In the present work it was found for the three nitro-substituted compounds that the photomerocyanine (the opened all-trans isomer of the spironaphthopyran) is formed partially via a triplet pathway and also to a less extent via an excited singlet pathway, according to the position of the nitro group. In contrast, the photomerocyanine of the unsubstituted compound is formed only via an excited singlet state. The quantum yields of photomerocyanine formation were determined in toluene solution: substitution by NO 2 on the naphthopyran moiety leads to a very high quantum yield (∼0.9) and proceeds essentially via the triplet pathway, while for the other two compounds ( II and III) the yield obtained is around 0.5 and both triplet and singlet pathways are involved; in the unsubstituted compound in which only the singlet pathway is present, the coloration yield is much lower (∼0.2). The results show that the triplet channel induced by the presence of the nitro group is much more efficient than the singlet one for CO bond rupture in the indolinospironaphthopyrans. Moreover semi-empirical calculations were performed using the AM1 method; the theoretical results show the importance of the S 1—T 1 energy gap in the closed form for coloration efficiency.

Photophysics and reversible photochemistry of 9-anthrylazomethines

9-Anthrylanil (AA) and two 4-phenyl substituted derivatives (BrAA, NO 2AA) were investigated. The E isomers undergo efficient direct photoisomerization to the respective Z isomers down to −180 °C. Sensitized photoisomerization is observed with acridine orange, and partly with rhodamine B, similar to the naphthylazomethines. The activation energies of the thermal back reactions are in the range 65 – 81 kJ mol −1 and the rates at 25 °C are 10 −2 s −1 and 15 s −1 for BrAA and NO 2AA respectively. All three compounds efficiently quench the phosphorescence of biacetyl and the TT absorption of several energy donors. The triplet states are observed in flash photolysis by direct photoexcitation of NO 2AA and by triplet energy transfer of all three compounds (λ max ≈ 480 – 570 nm). The triplet energy of the three compounds is estimated to be around 168 kJ mol −1. The triplet lifetime of NO 2AA increases from a few microseconds at 25 °C up to 20 ms at −185 °C. With regard to the mechanism, a singlet pathway leading to the Z isomer is most likely for the parent compound and BrAA, whereas for NO 2AA a contribution of the triplet pathway is established.

The cis—trans isomerization of nitrostilbenes XV: mixed singlet and triplet mechanism for trans → cis photoisomerization of 4-nitro-4′-dialkylaminostilbenes in non-polar solvents

The quantum yields of direct cis ⇌ trans photoisomerization (φc → t and φt → c) and of fluorescence of the trans isomers (φf) of three 4-nitro-4′-R-stilbenes (R  amino (1), dimethylamino (2) and diethylamino (3)) were measured in several saturated hydrocarbons. Formation and decay of the lowest triplet state was observed by nanosecond laser flash photolysis. The triplet yield (φT), the triplet lifetime (τT), φt → c and φf were measured as a function of temperature and of the concentration of the quenchers ferrocene, azulene (Q) and oxygen. Twisting in the triplet, involving a 3t* ⇌ 3p* equilibrium, analogous to that in other 4-nitrostilbenes, is suggested on the basis of the effects of temperature and quenchers on φT and τT. The trans → cis photoisomerization of 1 follows the triplet route almost completely. The existence of a singlet pathway (20% – 30% contribution) for 2 and 3 in non-polar solvents at room temperature is concluded from the non-linear dependence of the φ0t → c/φt → c ratio on the concentration of Q. For these two nitrostilbenes a mixed singlet—triplet mechanism for the trans → cis photoisomerization is suggested.

The mechanism of photolysis of some benzyltrimethylammonium salts in water and in alcohols

Photolysis of a range of benzyltrimethylammonium and 3,5-dimethoxybenzyltrimethylammonium salts in water or methanol at 253.7 nm gave typically benzyl alcohols (or benzyl methyl ethers), toluenes, bibenzyls and isomers, and di- and tri-methylammonium salts. By detailed sensitisation and quenching experiments it was established that, in the photolysis of benzyltrimethylammonium bromide in aqueous t-butyl alcohol, benzyl t-butyl ether, benzyl alcohol, and some toluene were produced by a singlet pathway and bibenzyl by a triplet pathway. A general mechanistic scheme for all the photolyses studied is thus suggested.

Laser-induced two-photon ionization of acridine in aqueous solution

The present work reports the formation of hydrated electrons upon excitation of a'queous solutions of acridine using the thud harmonic of a Nd-glass laser. The experimental results indicate that acridine is photoionized via a consecutive two-photon mechanism in which the intermediate states are both the fust excited singlet and the lowest triplet. However the triplet pathway is shown to be less efficient than the singlet pathway.

Singlet and triplet states in the cis—trans photoisomerization of 4-cyanostilbenes in solution

The quantum yields of fluorescence φ f, cis→trans photoisomerization φ c→ t and trans→cis photoisomerization φ t→ c , and photostationary trans/cis ratios of 4-cyanostilbene, 4,4′-dicyanostilbene, 4-cyano-4′-methoxystilbene and 4-cyano-4′-dimethylaminostilbene were measured in several solvents as functions of quencher (ferrocene, azulene and oxygen) concentration, temperature and viscosity. The results suggest a singlet pathway for the direct trans→cis photoisomerization of the cyanostilbenes. Photosensitized cis—trans isomerization and triplet energy transfer measurements were performed at room temperature for 4-cyano-4′-methoxystilbene. A transient was observed be laser flash photolysis for trans isomers of 4-cyanostilbene, 4,4′-dicyanostilbene and 4-cyano-4′-methoxystilbene at high viscosities. On decreasing the temperature and thus increasing the viscosity the yield φ T and lifetime τ T of the transient increase, reaching constant values in rigid media where φ t→ c is almost zero. The viscosity independence of the transient spectrum and the temperature dependences of φ T and τ T lead to the identification of the transient as the planar trans configuration of the lowest triplet state. Contribution of this triplet state and possibly of a higher excited triplet state to the trans→cis photoisomerization is suggested to be very small even at low temperatures.

Comparative kinetic study of the reactivity of molecules i n their excited singlet and triplet states in the cases of an electron abstraction and an intramolecular photocyclization reaction

Summary Two photochemical reactions are considered, illustrating quantitatively the differences in rate constants for chemical and physical deactivating processes from excited singlet and triplet states, in each case, for the identical photochemical reaction. Firstly we have determined the rate constants of electron abstraction, ke, and quenching, kG, of oxonine singlet and triplet states by EDTA in aqueous solution. The rate kSe for the singlet state, is found to be about ten times larger than kTe for the triplet state, whereas kSG is 103 to 104 faster than kTG. Secondly, in the case of the intramolecular photocyclization of 1-(orthodiphenyl) −1 phenyl ethylene in cyclohexane, a common intermediate X is found for the singlet and triplet pathways. The rate constant kSX is found to be 150 times faster than an apparent rate kTX whereas the non radiative (nr) rate constant kSnr is about 103 times faster than kTnr. In conclusion it is cleared that the measurement of rate constants can really provide the necessary information concerning the difference of reactivity between singlet and triplet pathways of photochemical reactions. However kinetics only yields the values of the rate constant for each step considered in the scheme. This means that we can only have confidence in elementary rate constant for reaction scheme that have been established very carefully.

ChemInform Abstract: THE PHOTOLYSIS OF BENZYL AMINOACETATES

AbstractAus den Aminoessigsäurebenzylestern (I) werden bei einer Photolyse die Folgeprodukte (II)‐(IV) erhalten.

THE PHOTOLYSIS OF BENZYL AMINOACETATES

Abstract The photolysis of benzyl aminoacetates (1a–1d) in benzene gave β-aminoethylbenzenes (2a–2d) and N,N-disubstituted glycines (3a–3d). Benzyl acetate (4) was also obtained in low yield. Solvent and quenching effect suggest the singlet pathway for this photoreaction.

Mechanism of photolysis of benzyltrimethylammonium salts in hydroxylic solvents

Photolysis of benzyltrimethylammonium bromide in aqueous t-butyl alcohol gives benzyl t-butyl ether and some toluene by a singlet pathway with more toluene and bibenzyl by a triplet pathway, in each case via a reactive intermediate geminate pair (or pairs); analogous mechanisms are suggested for photolysis of a range of benzyltrimethylammonium salts in water or methanol.

Separation of viscosity and temperature effects on the singlet pathway to stilbene photoisomerization

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSeparation of viscosity and temperature effects on the singlet pathway to stilbene photoisomerizationJack Saltiel and Joan T. D'AgostinoCite this: J. Am. Chem. Soc. 1972, 94, 18, 6445–6456Publication Date (Print):September 1, 1972Publication History Published online1 May 2002Published inissue 1 September 1972https://doi.org/10.1021/ja00773a031RIGHTS & PERMISSIONSArticle Views546Altmetric-Citations141LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

The photo-addition of 3-phenylcyclohex-2-enone and tetramethylethylene. Evidence for singlet and triplet pathways leading to a common intermediate

The photo-addition of 3-phenylcyclohex-2-enone and tetramethylethylene occurs from S1 on direct irradiation, and from T1 on sensitization, to afford the same cyclobutane adduct.

The photochemical Beckmann rearrangement

The direct irradiation of aryl aldoximes gives the amide (the photochemical Beckmann rearrangement) by intramolecular oxygen migration. This has been shown by the rearrangement of 18O-labelled benzaldoxime in the presence of p-tolualdoxime: no exchange of label was observed.The rearrangement usually gives the amide, but in the case of the anisyl derivative the anilide was also obtained. The highest quantum yield for benzamide formation noted (in acetic acid) was 0.034, but the figure is dependent on irradiation time. The results of low-temperature irradiation suggest that an oxazirane is an intermediate. Phenyl-N-methyl oxazirane is converted into the amide on irradiation (though not thermally) and it seems likely that the reaction is induced, under the conditions of the reaction, by benzylic hydrogen abstraction by thermally produced small amounts of benzaldehyde; a fact which may explain the variation in quantum yield of amide formation with extent of irradiation. An additional route from oxazirane to amide may also be available.The activation of the oxime by high energy sensitizers under the most favorable conditions gives Φamide = 0.002, and so a singlet pathway for oxazirane formation is preferred. The main route for energy degradation appears to be by syn–anti isomerism. This can be induced by a variety of sensitizers and so can be a triplet process. The direct irradiation of benzaldoxime leads to isomerism with Φαβ = 0.40 and Φβα = 0.38. From these figures, together with the composition of the stationary state found, the composition of the sensitized stationary state can be calculated in fair agreement with the experimentally found value. This suggests that the isomerization following direct irradiation is also a triplet process.In a single example of an aralkyl ketoxime (p-anisylmethyl ketoxime) irradiation products of both aryl and alkyl migration, were found.