Cis–Trans Isomerization of Olefins

Synthesis, structural analysis, and properties of highly twisted alkenes 13,13'-bis(dibenzo[a,i]fluorenylidene) and its derivatives

The rotation of a C = C bond in an alkene can be efficiently accelerated by creating the high-strain ground state and stabilizing the transition state of the process. Herein, the synthesis, structures, and properties of several highly twisted alkenes are comprehensively explored. A facile and practical synthetic approach to target molecules is developed. The twist angles and lengths of the central C = C bonds in these molecules are 36-58° and 1.40-1.43 Å, respectively, and confirmed by X-ray crystallography and DFT calculations. A quasi-planar molecular half with the π-extended substituents delivers a shallow rotational barrier (down to 2.35 kcal/mol), indicating that the rotation of the C = C bond is as facile as that of the aryl-aryl bond in 2-flourobiphenyl. Other versatile and unique properties of the studied compounds include a broad photoabsorption range (from 250 up to 1100 nm), a reduced HOMO-LUMO gap (1.26-1.68 eV), and a small singlet-triplet energy gap (3.65-5.68 kcal/mol).

Determination of the pKa Values of trans-Resveratrol, a Triphenolic Stilbene, by Singular Value Decomposition. Comparison with Theory

Several independent determinations of the pK_a values of trans-resveratrol in water have led to conflicting results. Singular value decomposition analysis of UV absorption spectra of trans-resveratrol (t-Resv) in N₂-outgased aqueous solutions buffered to pH values in the 7.0-13.6 range yielded the UV spectra of the three anionic forms and the corresponding pK_a values: pK_a1 = 9.1₆, pK_a2 = 9.7₇, and pK_a3 = 10.5₅ in very good agreement with calculated theoretical values. The analysis of the absorption spectra guided the assignment of the fluorescence spectrum of each anionic form. With the resolved spectra on hand, we applied the Förster equation to estimate pK_a* values of 2.5 and 0, respectively, for the p- and m-OH substituents of t-Resv in S₁. Theory supports a proposed mechanism for the reaction of t-Resv anions with O₂.

Water-Assisted/Water-Accelerated Photoreaction of trans-2,3,4',5-Tetrahydroxystilbene-2-O-β-d-glucoside from the Roots of Polygonum multiflorum

The photoreaction of 2,3,4',5-tetrahydroxystilbene-2-O-β-d-glucoside (TSG) has been investigated. Water-assisted/water-accelerated photodimerization of trans-TSG favored the formation of syn-head-to-tail [2 + 2] photocyclobutane under 365 nm irradiation as a result of hydrophobic association and a fluorescent solute-solute aggregate from their excited singlet states. In contrast, irradiation with 254 nm led to [2 + 2] photocycloreversion. The two cyclobutane dimers were first obtained through straightforward photoreaction and identified as multiflorumiside A and multiflorumiside C through the detailed analysis of high-resolution electrospray ionization mass spectrometry and one- and two-dimensional nuclear magnetic resonance. Therefore, trans-TSG should be protected from light and water.

Origin of the Preferential Formation of Helicenes in Mallory Photocyclizations. Temperature as a Tool to Influence Reaction Regiochemistry

The regiochemistry of four bis-Mallory photocyclization substrates has been examined from experimental and computational perspectives. Formation of all three possible regioisomers was only observed in the reaction of one of the substrates. In the other three substrates, only the two C₂-symmetric products, but not the C₁ product, were formed. In the three reactions that only formed two products, the photocyclization temperature could be used to select for exclusive formation of one or the other regioisomer. The use of temperature to select between two regioisomers also worked in the photocyclization of the substrate that formed three products. However, no temperature was located for exclusive formation of the third component, one of the C₂-symmetric products, which always formed alongside either one or both of its regioisomers. B3LYP/6-311+G(2d,p) calculations were used to determine the energies of all of the dihydrophenanthrene (DHP), tetrahydrophenanthrene (THP), and mono-Mallory photocyclization intermediates. The oscillator strengths of the DHP precursors to the helicene products were a factor of 4.8-9.2 smaller than those of competitively formed DHPs. This observation suggests that establishment of a photostationary state is responsible for the preferential formation of helicenes that has been observed as a unique and useful feature of many Mallory photocyclizations.

The intramolecular hydrogen bond as a unit of molecular electronics: Molecular switching controlled by overcrowded intramolecular three-centered hydrogen bond

The equilibrium geometry of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1[Formula: see text]-pyrrole calculated at the MP2/6-311[Formula: see text]G([Formula: see text],[Formula: see text]) level of theory evidences the breaking of one of the components in the three-centered intramolecular hydrogen bond due to the steric strain. For this reason, the three-centered intramolecular hydrogen bonding turns out to be asymmetric interaction involving the major and minor components. However, the reversible switching between these components under an external impact is also possible. Two different stable states with unequal geometric and electronic structure are observed in the derivatives of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1[Formula: see text]-pyrrole. These molecules represent novel molecular switches operating due to the pendulum-like transition between the nonequivalent two-centered components of the overcrowded three-centered intramolecular hydrogen bond. Implantation of hydrogen bond as a unit of the molecular scale device enhances potential of molecular electronics and could serve as a step towards the construction of artificial biological ensembles.

Photochemistry and Photophysics of the 3-Styrylidenebenz[e]indanes

The photochemical and photophysical properties of the extended conformers of trans- and cis-1-(2-naphthyl)-2-phenylethenes (t- and c-NPE_B ) are strikingly different than those of their rigid analogues, trans- and cis-3-styrylidenebenz[e]indane (t- and c-BPE). The fluorescence quantum yield and lifetime at 25°C in methylcyclohexane drop from 0.76 and 22.2 ns in t-NPE_B to 0.0051 and 0.25 ns in t-BPE, and there are complementary changes in the photoisomerization quantum yields. In both cases, photoisomerization occurs in S₂ , a stilbenic excited state. The differences in behavior are traced to the diminished S₁ /S₂ energy gap on the alkyl substitution afforded by the five-membered ring in BPE. The effect of viscosity on the torsional relaxation of t-BPE, evaluated in glycerol/methanol mixtures at 25°C and in 95/5 and 99.9/0.1 glycerol/methanol (% v/v) as a function of T, is well accounted for by the medium-enhanced barrier model. Absorption and fluorescence spectra of t-BPE in pure glycerol reveal aggregate formation. Quenching of t-BPE fluorescence in methylcyclohexane by tri-n-butylamine (TBA) leads to exciplex fluorescence. Comparison with analogous results for t-NPE_B suggests that charge transfer involves primarily the naphthalenic S₁ state in t-NPE_B and the stilbenic S₂ state in t-BPE.

Volume Conserving Geometric Isomerization of Encapsulated Azobenzenes in Ground and Excited States and as Radical Ion

To probe the role of the supramolecular steric effects and free volume on photoreactions, geometric isomerization of neutral azobenzenes (ABs) and their radical ions, generated by electron transfer with gold nanoparticles, included within an octa acid capsule, was investigated. A comparison of the isomerization of ABs that proceed by volume conserving pyramidalization and stilbene analogues that proceed by volume demanding one bond flip has indicated the differing influence of 4-alkyl groups on these two processes.

Cooperativity and Site-Selectivity of Intramolecular Hydrogen Bonds on the Fluorescence Quenching of Modified GFP Chromophores

This paper provides the first example of experimentally characterized hydrogen-bond cooperativity on fluorescence quenching with a modified green fluorescence protein (GFP) chromophore that contains a 6-membered C═N···H-O and a 7-membered C═O···H-O intramolecular H-bonds. Variable-temperature (1)H NMR and electronic absorption and emission spectroscopies were used to elucidate the preference of intra- vs intermolecular H-bonding at different concentrations (1 mM and 10 μM), and X-ray crystal structures provide clues of possible intermolecular H-bonding modes. In the ground state, the 6-membered H-bond is significant but the 7-membered one is rather weak. However, fluorescence quenching is dominated by the 7-membered H-bond, indicating a strengthening of the H-bond in the excited state. The H-bonding effect is more pronounced in more polar solvents, and no intermediates were observed from femtosecond fluorescence decays. The fluorescence quenching is attributed to the occurrence of diabatic excited-state proton transfer. Cooperativity of the two intramolecular H-bonds on spectral shifts and fluorescence quenching is evidenced by comparing with both the single H-bonded and the non-H-bonded counterparts. The H-bond cooperativity does not belong to the conventional patterns of σ- and π-cooperativity but a new type of polarization interactions, which demonstrates the significant interplay of H-bonds for multiple H-bonding systems in the electronically excited states.

The Missing C1-C5 Cycloaromatization Reaction: Triplet State Antiaromaticity Relief and Self-Terminating Photorelease of Formaldehyde for Synthesis of Fulvenes from Enynes

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.

Time-resolved emission spectra of 4-dimethylamino-4'-cyano-stilbene and resveratrol in high viscosity solvents and silica matrices

Time-resolved emission spectra of 4-dimethylamino-4'-cyano-stilbene (DMACS) and 3,5,4'-trihydroxy-stilbene (resveratrol, RSV) in propylene glycol and in rigid silica xerogel matrix at 23°C were studied. For the polar molecule DMACS in propylene glycol, a 66nm shift of maximum wavelength of emission spectra was observed within 1ns after excitation, and most of the shift occurred during the first 200ps. For resveratrol in propylene glycol no such a shift was observed. The rigid silica environment eliminates some deactivation pathways and stabilizes spectroscopic properties of both molecules. Spectral properties of nonpolar and high dipole moment molecules in viscous liquids and rigid environments are compared. Results are explained on the basis of intramolecular processes and solute-solvent relaxation, as well.

Controlling the stereospecificity of a volume-conserving adiabatic photoisomerization within a nanotubular self-assembled cage: a reversible light-heat torque converter

We present herein a host-guest supramolecular system by which we were able to obtain precise control of the stereospecificity of a new and unusual adiabatic photoisomerization reaction capable of restoring reversibly the original configuration. The host-guest system is composed of (a) a naphthalene ring linked centrosymmetrically-via sp(2) hybridized oxygen atoms-with methoxytriethyleneglycol chains (1) and (b) a nanotubular cage formed by four self-assembled face-to-face β-cyclodextrins threaded onto the long "axle" of 1. The compound 1 can exist in distinct cis,cis, cis,trans, and trans,trans conformations that are spectrally distinguishable (see Scheme 1 ). Spectroscopic and kinetic manifestations of the torsional isomerization of 1 in the lowest excited singlet state both in solution and within the tubular cage were investigated. The results provide clear evidence that the compact cavity completely blocks the photoisomerization pathway manifested in common solution (cis,cis* → cis,trans*), allowing observation of stereospecific, volume-conserving turning of the naphthalene ring about the two "quasidouble" bonds C(Naph)-O by φ ≈ 180° (cis,cis* → trans,trans*). The photoisomerization is purely adiabatic, and the encaged molecule restores its original configuration by generating torque thermally, when relaxing to the ground state.

Meta conjugation effect on the torsional motion of aminostilbenes in the photoinduced intramolecular charge-transfer state

The photochemical behavior of a series of trans-3-(N-arylamino)stilbenes (m1, aryl = 4-substituted phenyl with a substituent of cyano (CN), hydrogen (H), methyl (Me), or methoxy (OM)) in both nonpolar and polar solvents is reported and compared to that of the corresponding para isomers (p1CN, p1H, p1Me, and p1OM). The distinct propensity of torsional motion toward a low-lying twisted intramolecular charge-transfer (TICT) state from the planar ICT (PICT) precursor between the meta and para isomers of 1CN and 1Me reveals the intriguing meta conjugation effect and the importance of the reaction kinetics. Whereas the poor charge-redistribution (delocalization) ability through the meta-phenylene bridge accounts for the unfavorable TICT-forming process for m1CN, it is such a property that slows down the decay processes of fluorescence and photoisomerization for m1Me, facilitating the competition of the single-bond torsional reaction. In contrast, the quinoidal character for p1Me in the PICT state kinetically favors both fluorescence and photoisomerization but disfavors the single-bond torsion. The resulting concept of thermodynamically allowed but kinetically inhibited TICT formation could also apply to understanding the other D-A systems, including trans-4-cyano-4'-(N,N-dimethylamino)stilbene (DCS) and 3-(N,N-dimethylamino)benzonitrile (3DMABN).

PhotochemicalZ→E Isomerization of a Hemithioindigo/Hemistilbene ω-Amino Acid

The molecule HTI, which combines hemithioindigo and hemistilbene molecular parts, allows reversible switching between two isomeric states. Photochromic behaviour of the HTI molecule is observed by irradiation with UV/Vis light. The photochemical reaction, a Z/E isomerization around the central double bond connecting the two molecular parts, is investigated by transient absorption and emission spectroscopy. For a special HTI molecule, namely, an omega-amino acid, the Z-->E isomerization process occurs on a timescale of 30 ps. In the course of the reaction fast processes on the 1-10 ps timescale are observed which point to motions of the molecule on the potential-energy surface of the excited state. The combination of transient absorption experiments in the visible spectral range with time-resolved fluorescence and infrared measurements reveal a photochemical pathway with three intermediate states. Together with a theoretical modelling procedure the experiments point to a sequential reaction scheme and give indications of the nature of the involved intermediates.

Solvent Effect on the Photochemical Properties of Symmetrically Substituted trans-3,3‘,5,5‘-Tetramethoxystilbene

Photochemical properties of trans-3,3',5,5'-tetramethoxystilbene (TMST) have been studied in various polar solvents. The Stokes shift of trans-TMST was found to be increased with increasing solvent polarity. The fluorescence lifetime of trans-TMST experienced a large solvent effect changing from 2.3 ns in cyclohexane to 16.6 ns in acetonitrile. These results indicate that the excited singlet state of trans-TMST has a charge-transfer (CT) character. On the basis of the obtained results, the interior polar environment of a water-soluble TMST dendrimer is discussed.

Photoinduced Single- versus Double-Bond Torsion in Donor−Acceptor−Substituted trans-Stilbenes

The electronic absorption and fluorescence spectra, quantum yields for fluorescence (Phi(f)) and trans --> cis photoisomerization (Phi(tc)), and fluorescence lifetimes of trans-4-(N-arylamino)-4'-cyanostilbenes (2H, 2Me, 2OM, 2CN, and 2Xy with aryl = phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-cyanophenyl, and 2,5-dimethylphenyl, respectively), trans-4-(N-methyl-N-phenylamino)-4'-cyanostilbene (2MP), trans-4-(N,N-diphenylamino)-4'-cyanostilbene (2PP), trans-4-(N-methyl-N-phenylamino)-4'-nitrostilbene (3MP), and three ring-bridged analogues 2OMB, 2MPB, and 3MPB are reported. Whereas fluorescence and torsion of the central double bond account for the excited decay of the majority of these donor-acceptor substituted stilbenes in both nonpolar and polar solvents (i.e., Phi(f) + 2Phi(tc) approximately 1), exceptions are observed for 2OM, 3MP, and 3MPB in solvents more polar than THF and for 2Me and 2MP in acetonitrile as a result of the formation of a weakly fluorescent and isomerization-free twisted intramolecular charge transfer (TICT) state (i.e., Phi(f) + 2Phi(tc) << 1). The TICT state for 2OM, 2Me, and 2MP results from the torsion of the stilbenyl-anilino C-N single bond, but the torsion of the styryl-anilino C-C bond is more likely to be responsible for the TICT state formation of 3MP and 3MPB. In conjunction with the behavior of aminostilbenes 1, a guideline based on the values of Phi(f) and Phi(tc) for judging the importance of a TICT state for trans-stilbenes is provided. Accordingly, the TICT state formation is unimportant for the excited decay of trans-4-(N,N-dimethylamino)-4'-cyanostilbene (DCS). In contrast, our results support the previously proposed TICT state for trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS).

Methoxy-Substituted Stilbenes, Styrenes, and 1-Arylpropenes: Photophysical Properties and Photoadditions of Alcohols

The photochemistry of trans-stilbene and four methoxy-substituted stilbene derivatives has been investigated in a variety of solvents. The fluorescence of all five trans isomers was quenched by 2,2,2-trifluoroethanol (TFE). Upon irradiation of the five substrates in TFE, the products derived from photoaddition of the solvent were detected. Nuclear magnetic resonance spectroscopy of the products formed by irradiation in TFE-OD indicated that the proton and nucleophile are attached to two adjacent atoms of the original alkene double bond. Irradiation of the corresponding methoxy-substituted styrenes and trans-1-arylpropenes in TFE produced the analogous solvent adducts. The photoaddition of TFE proceeded with the general order of reactivity: styrenes > trans-1-arylpropenes > trans-stilbenes. Transient carbocation intermediates were observed following laser flash photolysis of the stilbenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The results are consistent with a mechanism that involves photoprotonation of the substrates by TFE or HFIP, followed by nucleophilic trapping of short-lived carbocation intermediates. Compared to the other stilbene derivatives, trans-3,5-dimethoxystilbene displayed a large quantum yield of fluorescence and a low quantum yield of trans-cis isomerization in polar organic solvents. The unique photophysical properties of trans-3,5-dimethoxystilbene are attributed to formation of a highly polarized charge-transfer excited state (mu(e) = 13.2 D).

How stable is trans-cycloheptene?

There is a discrepancy between the observed and calculated stability of trans-cycloheptene (t-CHP). Generation of t-CHP has always led to its low-temperature (-40 degrees C) isomerization to cis-cycloheptene (c-CHP). However, force field and semiempirical calculations on the energy difference between the two isomers have suggested that t-CHP should be stable at room temperature. We performed a series of ab initio calculations, which predicted that the simple process of double bond rotation leading from t-CHP to c-CHP would have an activation barrier too high to permit isomerization below 100 degrees C (35 kcal/mol). The validity of our calculation method on this very strained system was supported by the agreement between the calculation and the dynamics of the ring flip of the unsymmetrical t-CHP ring and the observed NMR shifts and coupling constants for the system. This incompatibility between the experimental behavior of t-CHP and our calculations led to our reexamining the decay kinetics of t-CHP. We find that this decay is second order and represents an "interrupted" dimerization, where an initially formed 1,4-biradical rapidly changes its geometry and cleaves back to produce two c-CHP molecules. This mechanism was supported by calculations of the 1,4-biradical potential energy surface.

Excited-State Behavior of N-Phenyl-Substituted trans-3-Aminostilbenes: Where the “m-Amino Effect” Meets the “Amino-Conjugation Effect”

The electronic spectroscopy and photochemistry of the trans isomers of 3-(N-phenylamino)stilbene (m1c), 3-(N-methyl-N-phenylamino)stilbene (m1d), 3-(N,N-diphenylamino)stilbene (m1e), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (m1f) and their double-bond constrained analogues m2a-m2c and m2e are reported. When compared with trans-3-aminostilbene (m1a), m1c-m1e display a red shift of the S0 --> S1 absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more efficient S1 --> T1 intersystem crossing. Consequently, the N-phenyl derivatives m1c-m1e have lower fluorescence quantum yields and higher photoisomerization quantum yields. The corresponding N-phenyl substituent effect in m2a-m2e is similar in cyclohexane but smaller in acetonitrile. This is attributed to the weaker intramolecular charge transfer character for the S1 state of m2 so that the rates for intersystem crossing are less sensitive to solvent polarity. It is also concluded that N-phenyl substitutions do not change the triplet mechanism of photoisomerization for m1 in both nonpolar and polar solvents. Therefore, the "m-amino conjugation effect" reinforces the "m-amino effect" on fluorescence by further reducing its rate constants and highlights the N-phenyl-enhanced intersystem crossing from the "amino-conjugation effect" by making S1 --> T1 the predominant nonradiative decay pathway.

Resolution of Three Fluorescence Components in the Spectra of all-trans-1,6-Diphenyl-1,3,5-hexatriene under Isopolarizability Conditions

all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm(-1), is consistent with the 1615 cm(-1) difference between the lowest energy bands in the 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T.

Substituent-Dependent Photoinduced Intramolecular Charge Transfer in N-Aryl-Substituted trans-4-Aminostilbenes

The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes.