A theoretical characterization of the photoisomerization channels of 1,2-cyclononadienes on both singlet and triplet potential-energy surfaces

The ground-, (1)(pipi*)-, and (3)(pipi*)-state potential-energy surfaces of 1,2-cyclononadiene and isomeric C(9)H(14) species, as well as 1-methyl-1,2-cyclononadiene and isomeric C(10)H(16) species were all mapped using CASSCF and the 6-31G(d) basis set. Theoretical results were found to be in good agreement with the available experimental observations for both 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene isomerization reactions under singlet and triplet direct or sensitized irradiation. Extremely efficient decay occurs from the first singlet excited state to the ground state through at least three different conical intersections (surface crossings). The first of these crossing points is accessed by a one-bond ring closure. From this conical intersection point (CI-A or CI-C), some possible subsequent ground-state reaction paths have been identified: 1) intramolecular C--H bond insertion to form the bicyclic photoproduct and 2) intramolecular C--H bond insertion to form tricyclic photoproducts. An excited state [1,3]-sigmatropic shift leads to the second conical intersection (CI-B or CI-E), which can give a three-bond cyclononyne species. Besides these, in the singlet photochemical reactions of 1-methyl-1,2-cyclononadiene, excited-state, one allenic C--H bond insertion leads to a third conical intersection (CI-D). Possible ground-state reaction pathways from this structure lead to the formation of a diene photoproduct or to transannular insertion photoproducts. Moreover, in the case of triplet 1,2-cyclononadiene and 1-methyl-1,2-cyclononadiene photoisomerization reactions, both chemical reactions will adopt a 1,3-biradical (T(1)/S(0)-1, T(1)/S(0)-2, and T(1)/S(0)-3), which may undergo intersystem crossings leading to the formation of tricyclic or bicyclic photoproducts. The results obtained allow a number of predictions to be made.     

Photochemical reactions, 135th communication Photochemistry of Homoconjugated Cyclobutanones. II. Decisive Effect of gem- Dimethyl Substitution on the Course of the Oxa-di-π-methane Rearrangement

The synthesis and photolysis of the spirocyclobutanones 4–7 incorporating a cyclohexa-, cyclohepta- and cyclooctadiene moiety, respectively, is described. On triplet excitation, these compounds undergo isomerization via a 1,2-acyl shift involving one or both double bonds of the diene system. The presence of a gem-dimethyl group as in 1 , 4 and 7 dramatically changes the photoproduct distribution, since only these substrates leads to the products 3, 29 and 34 resulting from vinylogous ring closure (Scheme 5). Those substrates without methyl substitution ( 5 and 6 ) give only products of a rearrangement involving one double bond.     

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.     

ELECTRONIC PROPERTIES AND PHOTOCHEMICAL MONOMOLECULAR REACTIONS IN ORGANIC CONJUGATED COMPOUNDS: PHOTOCYCLIZATIONS, PHOTOISOMERIZATIONS AND PHOTOCHEMICAL REARRANGEMENTS—I

Abstract— It is assumed that quantum yields of monomolecular reactions are mainly determined by the probability of de-excitation from the excited state to the ground state of the photoproduct and that a reaction will occur only if the stable molecular configuration of the excited state is close enough the molecular configuration of the photoproduct. In order to evaluate the stabilizing deformation of the excited states, the different methods of representing the excited states wave-functions are compared. Though very different in their ability to reproduce spin densities in the triplet state, these methods appear to give very close results for the electronic repartition; particularly, unrestricted Hartree-Fock calculations seem to give nearly the same net charges and bond indices for corresponding singlet and triplet states. A simple method for a rough evaluation of the direction and of the amplitude of the stabilizing deformation is proposed. In the Hiickel scheme, the variations in bond orders may be taken as an approximate index of that deformation.     

The behavior of the singlet and triplet spin states of methylene-bridged 1,8-naphthoquinodimethane with O2

It has been shown that thermolysis of naphthocyclopropane 8 yields the singlet 1,8-naphthoquinodimethane 7 by disrotatory ring opening. In fluid solution the singlet biradical prefers to ring close to regenerate the starting material rather than under a 1,2-hydrogen shift to yield phenalene (9). It has been demonstrated that the singlet 7 does not react with O₂ or undergo intersystem crossing to ground state triplet 7. It has also been shown by an intricate set of experiments that triplet 7 is produced on photolysis of 8 and that it reacts with O₂. Unfortunately, it was not possible to elucidate unequivocally the mechanism by which triplet 7 reacts with O₂.     

Solvent effects on the singlet - triplet equilibrium and reactivity of a ground triplet state arylalkyl carbene

Results from intramolecular singlet and triplet specific reactivity in solvents of different Polarity suggest that the spin state equilibrium of 1,2-diphenyl-1-butylidene, a triplet ground state carbene. is largely susceptible to solvent polarity. The results are consistent with stabilization of the zwitterionic singlet state in solvents of high polarity.     

Ultrafast observation of a solvent dependent spin state equilibrium in CpCo(CO)

We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct.     

Photochemistry of dienones-VIII: The low temperature photochemistry of (z)-β-ionone and its photo-isomers

The isomerizations of (E)-β-ionone 1, and of mixtures of the isomeric pyran 2 and (Z)-β-ionone 3 in CD₃OD as solvent on direct irradiation with λ 254 nm and on triplet photo-sensitization have been studied at temperatures ? - 50°, where the thermal isomerization between 2 and 3 is fully inhibited. The direct irradiation of 1 at -60° leads to 3 and (Z)-retro-γ-ionone 4 as primary products; 3 is subsequently rapidly photo-converted into mainly 2. Evidence is presented that 4 is also a primary photoproduct from both 2 and 3. The quantum yield ratio φ_2→4:φ_3→4 ?0.50. On starting with either 1 or mixtures of 2 and 3 the same photo-stationary equilibrium ratio of 1-3 is eventually obtained, viz 1:2:3 ?17:72:11. 4 is photostable relative to 1–3.The perdeuterobenzophenone triplet photo-sensitization with λ 366 nm at -50° of 1 leads to 3 as the sole primary product, which isomer on triplet sensitization yeilds both 1 and 2. The triplet sensitized conversion is much faster for (Z)- than (E)-β-ionone. On starting with either 1 or mixtures of 2 and 3, eventually the same photo-stationary state is obtained, viz 1:2:3?39:46:15. (Z)-retro-λ-ionone 4 is not formed in the triplet sensitized irradiations of 1,2 and 3 and in the direct irradiation it apparently results from the singlet excited state of the three substrates.The UV spectrum of the (unstable) (Z)-β-ionone 3 has been indirectly determined; its absorption occurs at lower wavelength and is of lower intensity than that of the (E)-isomer 1.     

Oxa-di-pi-methane photochemical rearrangement of quinuclidinones. Synthesis of pyrrolizidinones

[reaction: see text] The oxa-di-pi-methane (ODPM) photochemical rearrangement, a triplet-sensitized sigmatropic 1,2-acyl shift of beta,gamma-enones, was successful utilizing methyl and heptyl 1-aza-3-carboalkoxybicyclo[2.2.2]oct-2-en-5-ones (quinuclidinones) as the photoprecursors. The cyclopropane of the heptyl ester tricyclic photoproduct could be opened with lithium dimethylcuprate or via hydrogenolysis to produce the corresponding pyrrolizidinone skeletons.     

The gold porphyrin first excited singlet state

Gold porphyrins are often used as electron-accepting chromophores in artificial photosynthetic constructs. Because of the heavy atom effect, the gold porphyrin first-excited singlet state undergoes rapid intersystem crossing to form the triplet state. The lowest triplet state can undergo a reduction by electron donation from a nearby porphyrin or another moiety. In addition, it can be involved in triplet-triplet energy transfer interactions with other chromophores. In contrast, little has been known about the short-lived singlet excited state. In this work, ultrafast time-resolved absorption spectroscopy has been used to investigate the singlet excited state of Au(III) 5,15-bis(3,5-di-t-butylphenyl)-2,8,12,18,-tetraethyl-3,7,13,17-tetramethylporphyrin in ethanol solution. The excited singlet state is found to form with the laser pulse and decay with a time constant of 240 fs to give the triplet state. The triplet returns to the ground state with a life-time of 400 ps. The lifetime of the singlet state is comparable with the time constants for energy and photoinduced electron transfer in some model and natural photosynthetic systems. Thus, it is kinetically competent to take part in such processes in suitably designed supermolecular systems.     

A Terminal,Fluxional η4‐Benzene Complex with a Thermally Accessible Triplet State is the Primary Photoproduct in the Intercyclobutadiene Haptotropism of (CpCo)phenylenes

Low‐temperature irradiation of linear [3]‐ and [4]phenylene cyclopentadienylcobalt complexes generates labile, fluxional η⁴‐arene complexes, in which the metal resides on the terminal ring. Warming induces a haptotropic shift to the neighboring cyclobutadiene rings, followed by the previously reported intercyclobutadiene migration. NMR scrutiny of the primary photoproduct reveals a thermally accessible 16‐electron cobalt η²‐triplet species, which, according to DFT computations, is responsible for the rapid symmetrization of the molecules along their long axes. Calculations indicate that the entire haptotropic manifold along the phenylene frame is governed by dual‐state reactivity of alternating 18‐electron singlets and 16‐electron triplets.     

Effect of N-pyridyl substitution and hydrogen bonding on the deactivation of singlet excited 1,2-naphthalimide

Photophysical properties of 1,2-naphthalimide (1) and N-(4-pyridyl)-1,2-naphthalimide (2) as well as the effect of their hydrogen bonding with phenols have been studied in toluene. Fluorescence emission is the dominant energy dissipation pathway of the singlet excited 1. Introduction of the 4-pyridyl substituent into the imide moiety significantly accelerates the internal conversion due to the efficient vibronic coupling between close-lying S1 and S2 excited states, however, the rate of triplet formation exhibits negligible change. In contrast with the behavior of the corresponding substituted phenyl derivatives, 2 does not emit dual fluorescence because of the less extensive conjugation within the molecule. Fluorescence quenching with phenols takes place both in dynamic and static processes. Electron transfer is slower in the hydrogen bonded complex where phenols are linked to the pyridyl moiety due to the larger distance between the electron donor and acceptor components.     

Photophysics and photochemistry of naphthoxazinone derivatives

The photophysics and photochemistry of a series of naphthoxazinones have been studied using a combination of methods ranging from steady-state and time-resolved spectroscopic techniques to product analysis. The photophysics of naphthoxazinone derivatives is very dependent on the structure: phenanthrene-like compounds exhibit higher fluorescence quantum yield than the less aromatic anthracene-like homologous. The latter, exhibit a substantial degree of charge transfer in the excited singlet state. These compounds are fairly photostable in the absence of additives, yielding a single photoproduct arising from the triplet state. The presence of electron donors such as amines increases the photoconsumption quantum yield and changes the product distribution, the primary photoproduct being a dihydronaphthoxazinone that photoreacts further yielding ultimately an oxazoline derivative.     

Iodine(III)-promoted ring contraction of 1,2-dihydronaphthalenes: a diastereoselective total synthesis of (+/-)-indatraline

[reaction: see text] A new approach for the synthesis of (+/-)-indatraline, which is a 3-phenyl-1-indanamine that displays several biological activities, is described. The strategy features as the key step a diastereoselective ring contraction of a 1,2-dihydronaphthalene promoted by PhI(OTs)OH, to construct the indan ring system. The oxidative rearrangement of other 1,2-dihydronaphthalenes was also investigated, generalizing this method to obtain indans.     

Triplet-state formation along the ultrafast decay of excited singlet cytosine

We address the possibility of populating the lowest triplet state of cytosine by an "intrinsic"mechanism, namely, intersystem crossing (ISC) along the ultrafast internal conversion pathway of the electronically excited singlet species. For this purpose, we present a discussion of the ISC process and triplet-state reactivity based on theoretical analysis of the spin-orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine. High-level ab initio computations show that ISC is possible in wide regions of the singlet manifold along the reaction coordinate that controls the ultrafast internal conversion to the ground state. Thus, the ISC mechanism documented here provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases.     

Photochemical elimination of leaving groups from zwitterionic intermediates generated via electrocyclic ring closure of alpha,beta-unsaturated anilides

Methacrylanilides, ArN(CH3)COC(CH2LG)=CH2, with allylic leaving groups (LG(-) = BocAla, PhCO2(-), PhCH2CO2(-), PhO(-)) undergo photochemical electrocyclic ring closure to produce a zwitterionic intermediate. Further reaction of the intermediate results in expulsion of the leaving group to give an alpha-methylene lactam as the major product. In addition, a lactam product that retains the leaving group is formed via a 1,5-H shift in the intermediate. Elimination of the leaving group is generally preferred, even for LG(-) = PhO(-), although in benzene as the solvent the lactam retaining the phenolate group becomes the sole photoproduct. The electrocyclic ring closure occurs in the singlet excited-state for the para-COPh-substituted anilide derivative and is not quenched by 0.15 M piperylene or 0.01 M sodium 2-naphthalenesulfonate (2-NPS) as triplet quenchers. Comparable concentrations of 2-NPS strongly quench the transient absorption of the triplet excited state observed at 450-700 nm according to laser flash photolysis experiments. In aqueous media, quantum yields for total products are insensitive to leaving group ability, and Phi(tot)(para-CO2CH3) = 0.04-0.06 at 310 nm and Phi(tot)(para-COPh) = 0.08-0.1 at 365 nm, for which Phi(isc) = 0.15.     

The Irreversible Photochemistry of Bianthrone

A 4a,4b-dihydrophenanthrene-type cyclic photoisomer, the C isomer, is the major primary photoproduct of bianthrone in protic and aprotic polar solvents, and undergoes solvent-dependent secondary reactions, including the formation of dihydrohelianthrone in protic solvents. The C isomer was shown to be formed through the singlet excited state while the B isomer is formed via the triplet manifold.     

Revised structure of a purported 1,2-dioxin: a combined experimental and theoretical study

3,6-Bis(p-tolyl)-1,2-dioxin (1g) was suggested by Shine and Zhao as a product in an electron-transfer (ET) photochemical reaction. This photoproduct is instead shown to be (E)-1,4-di-p-tolylbut-2-ene-1,4-dione ((E)-4a). Ab initio and DFT calculations indicate that ring-closed 1,2-dioxin is thermodynamically far less stable than open-chain but-2-ene-1,3-dione. These calculations indicate that (E)-4a is formed via the cation radical of 1g, which sequentially isomerizes to a novel sigma-radical with an O,O 3e bond [(Z)-4a](+)(*), undergoes ET to give (Z)-4a, and then photoisomerizes to (E)-4a.     

Fluorescence quenching of photoreaction products in the excited singlet state

The properties of steady-state spontaneous luminescence of a quantum system with a photoproduct with recordable fluorescence under the conditions of dynamic quenching of excited states by extraneous substances were considered. It was shown that the dependence of photoproduct fluorescence intensity and yield on quencher concentration was nontrivial and could not be conveniently used to determine the Stern-Volmer constant. At the same time, the initial form of the luminophore and its photoproduct produced in a kinetically controlled reaction are quenched in such a way that the ratio of their fluorescence intensities increases linearly as the quencher concentration grows. The corresponding equation was used to determine the constant of bimolecular quenching of reaction product excited states. The results were used in an analysis of the experimental fluorescence spectra of flavone (3-hydroxiflavone), whose fluorescence was excited under the conditions of dynamic quenching of the S ₁ state. Our analysis was shown to be applicable to a wide range of compounds with photoreactions accompanied by two-band fluorescence (charge transfer, proton transfer, phosphorescence, complex formation, etc.). It could be used to accurately determine bimolecular contact constants for excited states of photoreaction product molecules. Original Russian Text ? V.I. Tomin, 2009, published in Zhurnal Fizicheskoi Khimii, 2009, Vol. 83, No. 3, pp. 580–585.