Enantioselective Protonation of Radical Anion Intermediates in Photoallylation and Photoreduction Reactions of 3,3-Diaryl-1,1-dicyano-2-methylprop-1-ene with Allyltrimethylsilane.

Photoreactions of acetonitrile solutions of 3,3-diaryl-1,1-dicyano-2-methylprop-1-enes (1a-c) with allyltrimethylsilane (2) in the presence of phenanthrene as a photoredox catalyst and acetic acid as a proton source formed photoallylation (3) and photoreduction (4) products via photoinduced electron transfer pathways. When (S)-mandelic acid was used as the proton source, the reactions proceeded with 3.4 and 4.8 %ee for formation of 3 and 4, respectively. The results of studies of the effect of aryl ring substituents and several chiral carboxylic acids suggested that the enantioselectivities of the reactions are governed by steric controlled proton transfer in intermediate complexes formed by π-π and OH-π interactions of anion radicals derived from 1a-c and chiral carboxylic acids.

Cooperative effects of o- and m-methyl groups on the intramolecular charge-transfer emission properties of dibenzoylmethanatoboron difluorides.

The photophysical properties of o-tolyl-, m-tolyl-, and p-xylyl-substituted asymmetric diaroylmethanatoboron difluorides in a mixture of CH2Cl2 and c-C6H12, and in the crystalline state were determined. In solution, the fluorescence (FL) properties of these substances are controlled by the position and number of methyl groups on the phenyl rings. An especially interesting finding is that FL from the p-xylyl derivative occurs from an excited state which possesses intramolecular charge-transfer character caused by the o- and m-methyl groups cooperatively. The results of X-ray crystallographic analysis reveal that these asymmetric diaroylmethanatoboron difluorides form dyads through orbital overlap of neighboring molecules. This phenomenon governs the unique FL properties of these substances in the solid state.

Synthesis and fluorescence properties of dioxa-, dithia-, and diselena-[3.3](1,3)pyrenophanes.

[3.3](1,3)Pyrenophanes tethered by oxygen (1), sulfur (2) and selenium (3) atoms were synthesized and the structural and physical properties of these substances were determined. The absorption maxima of the [3.3](1,3)pyrenophanes were observed to shift to longer wavelengths in the order of 1 < 2 < 3. The fluorescence spectra of 1-3 contained both monomer and intramolecular excimer emissions, which correspond to anti and syn conformers, respectively. The ratios of the intensities of intramolecular excimer to monomer emission were observed to increase with the increasing solvent polarity. The intensity ratios also depend on temperature. For example, an increase in temperature results in an increase of the ratio of intensities of the intramolecular excimer to monomer fluorescence of 2. The results of 1H NMR spectroscopic investigations show that resonances for the methylene and aromatic hydrogens in these substances coalesce at low temperatures with coalescence temperatures (Tc) that decrease in the order of 1 > 2 > 3. The results of geometry optimization studies using B3LYP/6-31G(d,p) demonstrate that the syn conformers of 1-3 have lower enthalpies than their anti counterparts, but the syn conformer of 1 and the anti conformers of 2-3 are entropically more favorable. These findings suggest that an equilibrium exists between the syn and anti conformers of the [3.3](1,3)pyrenophanes and that the conformer ratios are dependent on both the solvent polarity and temperature in a manner that can be explained in terms of a combination of enthalpies, dipole moments and entropies. The combined results show that the pyrenophanes are interesting substances that emit different fluorescence colors in a manner that is controlled by the surrounding environment.

Intermolecular hydrogen bonding controlled stereoselective photocycloaddition of vinyl ethers to 1-cyanonaphthalenes.

With the aim of developing photoreactions that use intermolecular hydrogen bonding interactions to control the efficiency and stereochemistry, the 1,2-[2 + 2] photocycloaddition reactions of 1-cyanonaphthalene derivatives with vinyl ethers possessing hydroxy groups were examined. The photoreaction of 1-cyano-4-(hydroxymethyl)naphthalene (1a) with 2-hydroxyethyl vinyl ether (2a) at room temperature was found to produce cycloadducts, endo-3aa and exo-3aa, in a non-stereoselective manner (56 : 44). However, this photoreaction at -40 °C displays a high endo-selectivity (81 : 19). In the reaction of 1a with ethyl vinyl ether (2b), high endo selectivity was observed both at room temperature and at -40 °C. The endo selectivity in the [2 + 2] photocycloaddition process is attributed to the intermolecular hydrogen bonding interactions between the reacting partners in the ground and excited states. Evidence to support this conclusion comes from the results of VT NMR and fluorescence quenching experiments, as well as the photoreactions of deuterated substrates.

Intramolecular Photocycloaddition Reactions of Arylcyclopropane Tethered 1-Cyanonaphthalenes.

Intramolecular photocycloaddition reactions of 1-cyanonaphthalenes bearing an arylcyclopropane containing side chain were investigated. Photoreactions of members of this family in which the arylcyclopropane moiety is bonded at the 2-position of the 1-cyanonaphthalene ring produce head-to-head and head-to-tail 1,2-[3+2] photocycloadducts. On the other hand, substances in this family containing an arylcyclopropane side chain linked to the 4-position of the cyanoarene ring undergo photoreactions to form [4+3] photocycloadducts along with novel nine-membered ring products, which are produced by photochemically induced 10π conrotatory ring opening of the initially formed intramolecular [3+2] cycloadducts. The results of solvent effects and fluorescence investigations along with those focusing on corresponding intermolecular photoreactions demonstrate that the photocycloadducts are formed predominantly through an intramolecular exciplex mechanism and that a photoinduced intramolecular electron transfer pathway via zwitterionic biradicals might be partly responsible for the process when CH3CN is the solvent.

New Fluorescence Domain "Excited Multimer" Formed upon Photoexcitation of Continuously Stacked Diaroylmethanatoboron Difluoride Molecules with Fused π-Orbitals in Crystals.

The crystal-packing structures of seven derivatives of diaroylmethanatoboron difluoride (1 a-gBF2 ) are characterized by no overlap of the π-conjugated main units of two adjacent molecules (type I), overlap of the benzene ring π-orbitals of two adjacent molecules (type II), and overlap of the benzene and dihydrodioxaborinine rings π-orbitals of adjacent molecules (type III). The crystal-packing structures govern the fluorescence (FL) properties in the crystalline states. The FL domain that is present in type I crystals, in which intermolecular orbital interactions are absent, leads to excited monomer-like FL properties. In the case of the type II crystals, the presence of intermolecular overlap of the benzene rings π-orbitals generates new FL domains, referred to as "excited multimers", which possess allowed S0 -S1 electronic transitions and, as a result, similar FL lifetimes at longer wavelengths than the FL of the type I crystals. Finally, intermolecular overlap of the benzene and dihydrodioxaborinine ring π-orbitals in the type III crystals leads to "excited multimer" domains with forbidden S0 -S1 electronic transitions and longer FL lifetimes at similar wavelengths as that in type I crystals.

Formation of a new benzotriquinane skeleton via intramolecular photocycloaddition reactions of a phenylethynes moiety to a 1-cyanonaphthalene ring system.

Photoirradiation of 1-cyano-2-(2,2-dicyano-5-phenylpentyn-4-yl)naphthalene and its analogues promotes sequential intramolecular [2π + 2π] photocycloaddition and cyclobutene ring opening to form benzocyclooctatetraenes, which are then transformed to benzotriquinanes in good yields via a photochemical transannular cyclization process.

3,14-Bis(p-nitrophenyl)-17,17-dipentyltetrabenzo[a,c,g,i]-fluorene: a new fluorophore displaying both remarkable solvatochromism and crystalline-induced emission.

A series of 17,17-dialkyl-3,14-diaryltetrabenzofluorenes were efficiently prepared by using Suzuki-Miyaura cross-coupling reactions of the corresponding 3,14-dibromo derivatives. Studies of the unique fluorescence properties of these compounds showed that they display intense blue to yellow fluorescence with high quantum yields in the solution state and blue to orange fluorescence with moderate quantum yields in the solid state. In addition, the fluorescence wavelength of the bis(p-nitrophenyl) derivative is remarkably solvent-dependent in a manner that correlates with the solvent polarity parameter E(T)(30). The results of density function theory calculations suggest that the intramolecular charge-transfer character of the HOMO-LUMO transition is responsible for the large solvent effect. Moreover, addition of water to a tetrahydrofuran (THF) solution of this compound leads to quenching of the yellow fluorescence owing to an increase in the solvent polarity. However, when the amount of water fraction exceeds 70%, a new fluorescence band appears at the same orange-red emission wavelength as that of the solid-state fluorescence. This observation suggests the occurrence of a crystallization-induced emission (CIE) phenomenon in highly aqueous THF.

Absorption and fluorescence spectroscopic properties of 1- and 1,4-silyl-substituted naphthalene derivatives.

Silyl-substituted naphthalene derivatives at the 1- and 1,4-positions were synthesized and their UV absorption, fluorescence spectroscopic properties, and fluorescence lifetimes were determined. Analysis of the results shows that the introduction of silyl groups at these positions of the naphthalene chromophore/fluorophore causes shifts of the absorption maxima to longer wavelengths and increases in fluorescence intensities. Bathochromic shifts of the absorption maxima and increases in fluorescence intensities are also promoted by the introduction of methoxy and cyano groups at the naphthalene 4- and 5-positions. In addition, the fluorescence of 9,10-dicyanoanthracene is efficiently quenched by these naphthalene derivatives with Stern-Volmer plot calculated rate constants that depend on the steric bulk of the silyl groups.

A probable hydrogen-bonded Meisenheimer complex: an unusually high S(N)Ar reactivity of nitroaniline derivatives with hydroxide ion in aqueous media.

Observations show that nitroanilines exhibit an unusually high S(N)Ar reactivity with OH(-) in aqueous media in reactions that produce nitrophenols. S(N)Ar reaction of 4-nitroaniline (2a) in aqueous NaOH for 16 h yields 4-nitrophenol (4a) quantitatively, whereas a similar reaction of 4-nitrochlorobenzene (1a) gave 4a in 2% yield together with recovered 1a in 97%, suggesting that the leaving ability of the NH(2) group far surpasses that of Cl under these conditions. An essential feature of S(N)Ar reactions of nitroanilines is probably that the NH(2) leaving group participates in a hydrogen-bonding interaction with H(2)O. Density functional theory (DFT) calculations for a set of 4-nitroaniline, OH(-), and H(2)O suggest a possible formation of a Meisenheimer complex stabilized by hydrogen-bonding interactions and a six-membered ring structure. The results obtained here contrast with conventional S(N)Ar reactivity profiles in which nitroanilines are nearly unreactive with nucleophiles in organic solvents.

Hydrogen-bonding directed, regioselective photocycloaddition reactions of cyanonaphthalenes with furanmethanols.

1-Cyanonaphthalene and 2-cyanonaphthalene react with 2-furanmethanol and 3-furanmethanol, respectively, under photoirradiation conditions to give [4 + 4] photocycloadducts and caged compounds with high degrees of regioselectivity. Analysis of data obtained from variable temperature (1)H NMR and fluorescence quenching experiments indicates that hydrogen bonding between cyano and hydroxy groups in the excited states of the corresponding naphthalene and furanmethanol substrates is the source of the observed regioselectivities.

Contrasting intermolecular and intramolecular exciplex formation of a 1,4-dicyano-2-methylnaphthalene-N,N-dimethyl-p-toluidine dyad.

An intramolecular exciplex is formed upon excitation of the cyclohexane solution of the 1,4-dicyano-2-methylnaphthalene-N,N-dimethyl-p-toluidine dyad, but little if any intramolecular CT complex exists in the ground state of this substance in solution. In contrast, in the crystalline state, the dyad forms an intermolecular mixed-stack CT complex in the ground state and an intermolecular exciplex when it is photoexcited.

Multi-input/multi-output molecular response system based on the dynamic redox behavior of 3,3,4,4-tetraaryldihydro[5]helicene derivatives: reversible formation/destruction of chiral fluorophore and modulation of chiroptical properties by solvent polarity.

3,3,4,4-Tetaaryldihydro[5]helicenes (1) and 1,1'-binaphthyl-2,2'-diylbis(diarylcarbenium)s (2(2+)) can be reversibly interconverted upon electron transfer, which is accompanied by a vivid color change (electrochromism) as well as by the formation/cleavage of a C-C bond ("dynamic redox behavior"). Because only the neutral donor 1 exhibits strong fluorescence, electrochemical input can further modify the fluorescent properties of the pair. Due to the configurational stability of the helicity in 1 and axial chirality in 2(2+), the redox reaction of optically pure material proceeds stereospecifically, which induces a chiroptical change such as circular dichroism (CD) as an additional output. The CD spectra of dications 2(2+) exhibit solvent dependency (chiro-solvatochromism), which is accompanied by solvatochromic behavior based on the pi-pi interaction of the two cationic chromophores as well as coordinative interaction of the Lewis basic solvent to the Lewis acidic triarylcarbenium moieties. Thus, the present system is endowed with multi-input functionality for modifying multiple output signals.

Photoinduced tandem three-component coupling of propanedinitrile, 2,5-dimethylhexa-2,4-diene, and cyanoarenes.

A tandem three-component coupling photoreaction proceeds upon photoirradiation of MeCN/H2O solutions containing propanedinitrile (1, malononitrile), 2,5-dimethylhexa-2,4-diene (2), and polycyanoarenes in the presence of phenanthrene and carbonate, leading to selective alpha-monoalkylation of 1. The reaction proceeds via photo-NOCAS (Nucleophile-Olefin Combination, Aromatic Substitution) type mechanism: nucleophilic attack of the anion of 1 to photogenerated 2(*+) is followed by ipso-substitution on the radical anion of the polycyanoarene. It advances under mild, safe, and environmentally friendly conditions such as proceeding at ambient temperature without metals and halogens, and in the presence of weak base. The reaction also represents a novel and metal-free cross-coupling reaction that leads to ipso-substitution on polycyanoarene via aryl-cyano bond cleavage. In addition, the reaction is a rare example of introducing carbon nucleophile in the photoinduced electron transfer reaction, except that of cyanide ion.

Photochemical monoalkylation of propanedinitrile by electron-rich alkenes.

The synthesis of monoalkylated propanedinitriles was achieved upon photoirradiation of MeCN/H(2)O solutions containing propanedinitrile (1; malononitrile) and electron-rich alkenes in the presence of lithium carbonate and a catalytic amount of 9-cyanophenanthrene or redox-type photosensitizers (electron-mediating photosensitizers), through regioselective anti-Markovnikov photochemical polar addition of 1 into electron-rich alkenes. With 2,5-dimethyl-2,4-hexadiene (2g) as an electron-rich alkene, propanedinitrile-incorporated dimer 4g was obtained.

The facile generation of a tetramethyleneethane type radical cation and biradical utilizing a 3,4-di(alpha-styryl)furan and a photoinduced ET and back ET sequence.

Product analyses and nanosecond time-resolved spectroscopy on laser flash photolysis were studied for the photoinduced electron-transfer reaction of 3,4-di(alpha-styryl)furan (6a). A combination of these results, kinetic, density functional theoretical (DFT), and time-dependent DFT analyses enabled assignment of the absorption to the tetramethyleneethane (TME)-type radical cation (7a*+, lambda(max) = 392 nm) and the corresponding singlet biradical ((1)7a**, lambda(max) = 661 nm). These two intermediates were mechanistically linked to each other with a facile back electron-transfer reaction. The present studies provide a new method for the generation of aryl-substituted TME-type intermediates.

Photooxygenation of alkynylperylenes. Formation of dibenzo[jk,mn]phenanthrene-4,5-diones.

3-(1-Alkynyl)perylenes undergo oxygenation when subjected to irradiation with visible light under aerated conditions. The structures of novel oxygenated products formed in this manner are assigned as regioisomeric dibenzo[jk,mn]phenanthrene-4,5-diones.

Evidence for significant through-space and through-bond electronic coupling in the 1,4-diphenylcyclohexane-1,4-diyl radical cation gained by absorption spectroscopy and DFT calculations.

Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.